Membrane Inlets for Mass Spectrometry

2.24

Membrane Inlets for Mass Spectrometry

RA Ketola, University of Helsinki, Helsinki, Finland RT Short, SRI International, St Petersburg, FL, USA RJ Bell, SRI International, St Petersburg, FL, USA Ó 2012 Elsevier Inc. All rights reserved.

2.24.1 2.24.2 2.24.2.1 2.24.2.2 2.24.3 2.24.3.1 2.24.3.1.1 2.24.3.1.2 2.24.3.1.3 2.24.3.2 2.24.3.2.1 2.24.3.2.2 2.24.3.2.3 2.24.3.2.4 2.24.3.3 2.24.3.4 2.24.3.5 2.24.3.5.1 2.24.3.5.2 2.24.3.6 2.24.3.6.1 2.24.3.6.2 2.24.3.6.3 2.24.3.6.4 2.24.3.6.5 2.24.4 References

2.24.1

Introduction Fundamentals of Membrane Inlet Mass Spectrometry Theory Membrane Inlet Designs Principles and Applications of Analysis Gaseous Samples Analysis of Volatile Organic Compounds Analysis of Semivolatile and Polar Organic Compounds in Air Online/on-site/in situ Applications Water Samples Characteristics of Water Analysis Bioreactor and Chemical Reactor Monitoring Underwater MIMS Other online/on-site Applications Biological Matrices Organic Matrices Solid Samples Soil, Sediment, and Peat Other Solid Materials Calibration and Quantification Air Samples Aqueous Standards Sediment/Soil Matrix Standards Calibration Mathematics Mixture Analysis Conclusions and Future Perspectives

497 498 498 500 503 503 503 505 505 506 506 513 516 520 520 522 522 522 524 525 525 526 528 529 529 531 531

Introduction

Membrane introduction (inlet) mass spectrometry (MIMS) can be used for a direct analysis of both volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) from soils, liquids, and gases. This can be achieved through an inert polymer membrane that is used as the only interface between the sample and a mass spectrometer. The membrane is brought into contact with the sample, whereafter VOCs diffuse through it and evaporate into the mass spectrometer, where they are ionized and analyzed. In this manner, most volatile organic compounds can be analyzed directly and, in most cases, a changing chemical environment in the sample can be monitored online. MIMS is a relatively old technique. It was introduced in 1963 by Hoch and Kok1 as a method for the continuous monitoring of gases dissolved in water. A special characteristic of the membrane inlet system is a very high selectivity in the transport through the membrane. The membranes normally used in MIMS are very hydrophobic and hydrophobic compounds dissolve very well in the membrane. They have low detection limits (ng l
1 in water and z1 mg m
3 in air), whereas hydrophilic compounds have detection limits in the high mg l
1 range in water and mg m
3 in air. The membrane inlet causes the concentration of organic compounds with respect to water molecules or atmospheric gases (air samples) and the concentration effect can vary from 10 to 100 but even a concentration factor of 105 has been achieved using a three-membrane inlet configuration. The first applications of the MIMS technique were for monitoring of dissolved gases and small organic compounds in fermentors and in vivo monitoring of blood gases. In these applications, a probe with the membrane mounted at the end was inserted into the sample and VOCs were transported through an evacuated tube, where chromatographic effects at the surfaces can take place. Later on, a direct insertion membrane probe was invented. In this design, the membrane probe was inserted into the ion source of a mass spectrometer and the sample as such was transported to the membrane inlet. In the 1980s, the direct insertion membrane probe clearly increased the number of MIMS applications, especially in the detection of VOCs in both environmental samples and

Comprehensive Sampling and Sample Preparation, Volume 2

doi:10.1016/B978-0-12-381373-2.10152-8

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Theory of Extraction Techniques

biological matrixes. In the 1990s, the applicability of MIMS was extended to the analysis of complex organic solutions, such as gasoline and olive oil, as well as of gaseous samples and residual volatiles in solid samples. Today, the detection of VOCs (boiling point < 200  C) with MIMS has become a routine analysis. Detection of semivolatile and nonvolatile organic compounds has also become feasible with new MIMS techniques. For example, a desorption chemical ionization MIMS method is currently the most versatile one, allowing the detection of biomolecules and polar compounds, such as steroid hormones, atrazine, polychlorinated biphenyls (PCBs), and phthalates. In addition to basic MIMS techniques and methods, several modifications of simple MIMS inlets have been presented where various mass spectrometers have been applied to various matrices and analytes, showing the tremendous flexibility of the MIMS technique. This chapter focuses on the theory of MIMS, different inlet designs, principles of analysis of gaseous, liquid, and solid samples, as well as calibration and quantitation.

2.24.2

Fundamentals of Membrane Inlet Mass Spectrometry

2.24.2.1

Theory

The basic principle in MIMS is a process called pervaporation, which involves dissolution of organic compounds in the membrane, transport through the membrane, and evaporation from the membrane surface into the ion source of the mass spectrometer. Each process is highly selective toward organic compounds, the selectivity being dependent on the structure (polarity and hydrophobicity) of the compound and the type of membrane. A hydrophobic and inert silicone (polydimethylsiloxane) membrane favors nonpolar organic compounds, which dissolve very well into the membrane. Silicone has a nonporous structure, and the transport process is a simple diffusion process. More polar membranes favor more polar compounds but the diffusion step might be slow, reducing its sensitivity. The evaporation step of the compounds from the membrane surface can limit the applicability of standard MIMS to the detection of compounds with a relatively low boiling point (<200  C). The pervaporation process can be described by a set of simple equations. First of all, the steady-state flow (ISS) through a flat-sheet membrane is given by Iss ¼ A  D  K 

C l

(1)

where A is the surface area of the membrane, D is the diffusion constant, K is the distribution ratio of the analyte between the concentration in the membrane and in the sample, C is the analyte concentration in the sample, and l is the thickness of the membrane. This means that the flow through the membrane is directly proportional to the surface area of the membrane, the diffusion constant, the distribution ratio (a parameter called permeability), and the concentration gradient across the membrane. Depending on the type and thickness of the membrane as well as the type of the analyte, it might take a long time before a steady-state flow could be achieved. For that reason, a response time (t10–90%) is defined as the time it takes the signal to increase from 10 to 90% following a step change in concentration given by t10
90% ¼ 0:237 

l2 D

(2)

or t50% ¼ 0:14 

l2 D

(3)

As can be seen from Equation (2), the response time increases with the square of the membrane thickness and decreases proportionally to the diffusion constant. Thus, the response time does not depend on the distribution ratio as the steady-state flow does. These expressions describe the experimental behavior and are useful in measuring the diffusivity of an analyte in a particular membrane. In general, analyte/membrane solubilities vary much more than molecular diffusivities, as illustrated in Table 1.2 The distribution ratio K is probably the most important parameter, which affects the sensitivity of an MIMS system. In most MIMS systems, a very hydrophobic silicone membrane is used; therefore, compounds containing polar groups are not easily dissolved into the membrane. The effect of polar substituents in the organic molecules effectively increases the limits of detection (LODs) obtained; e.g., the LOD for toluene is in the ng l
1 range, whereas that for anisole (methoxybenzene) is 10 times higher, for benzaldehyde 10–100 times higher, for benzyl alcohol more than 100 times higher, and for benzoic acid (the most polar compound) it is more than 1000 times higher. Therefore, the signals obtained for polar compounds are much lower than those for nonpolar compounds per moles in the sample, and cannot be directly used for the estimation of their relative concentrations. Polar compounds can even remain undetected if the concentrations are low while the concentrations of nonpolar compounds are high. The same applies when hydrophilic membranes are used; with them, the polar compounds give higher signals. That is why a proper calibration of the MIMS system is very crucial, as with other analytical systems in general. The diffusion constant influences both the steady-state flow through the membrane and the response time. In general, highmolecular-weight compounds have longer response times than low-molecular-weight compounds. However, a small diffusion constant does not necessarily mean that the steady-state flow through the membrane becomes small because the steady-state flow through the membrane depends on the product between the diffusion constant and the distribution ratio as stated before. The

Membrane Inlets for Mass Spectrometry

Table 1

499

Typical diffusivity and solubility data for different compounds at 25  Ca

Analyte Aromatic hydrocarbons Toluene Benzene Ethylbenzene Gases Nitrogen Carbon Dioxide Chloromethanes Methylene chloride Chloroform Alcohols Methanol Ethanol

Permeation coefficient b (10 6)

27 13 42

Diffusivity c (10 6)

3.5 4.9 1.7

0.028 0.33

Solubility d

7.6 2.8 25

21 13

9.7 12

9.1 4.9

5.3 11

0.42 0.40

0.0013 0.026 1.2 2.4 13 28

a

Experimentally measured in a silicone capillary membrane composed of 69% (w/w) poly(dimethylsiloxane) and 31% (w/w) fumed silica. Product of diffusivity and solubility: Units of permeation coefficient are cm3 cm s
1 cm
2 cm Hg
1. c Units of diffusivity are cm2 s
1. d Units for solubility are cm3(STP) cm
3 membrane cm Hg
1. b

Adapted from J. Memb. Sci., 86, LaPack, M. A.; Tou, J. C.; McGuffin, V. L.; Enke, C. G., The correlation of membrane permselectivity with Hildebrand solubility parameters, 263–280, Copyright (1994), with permission from Elsevier.

distribution ratio quite often overcomes the diffusion constant, resulting in a higher flow through the membrane for highmolecular-weight compounds. For example, the steady-state flow of octanol is about 50 times larger than that of ethanol. Hydrophobic compounds, such as hexane, at high concentration levels can cause swelling of the membrane and therefore nonlinearity in the signal detected; e.g., 3-chloro-4-methoxybenzaldehyde gives a linear signal from the detection limit (10 mg l
1) and up to about 10 mg l
1, whereafter the signal increases rapidly until it becomes linear again. This behavior can be explained by the fact that at medium concentration levels the membrane is only partly swollen, causing an unexpected high flow through the membrane. At very high concentration levels, the membrane is fully swollen, producing again a steady-state flow with high permeability. It is not easy to observe the effect of membrane swelling because of common saturation effects in the mass spectrometric system. The membrane swelling affects not only the signal of the compound causing the effect but also the signals of other compounds (hydrophobic or hydrophilic) present in the sample. Thus, it can cause serious bias in the concentrations of the analytes. A similar effect can be caused also by a high salt concentration (at tens of grams per liter levels) because the salt can change the distribution ratio of the analytes between the aqueous sample and the membrane. The evaporation of VOCs from the membrane surface into the mass spectrometer for ionization is the final part of pervaporation process. The vapor pressure is high and the flow through the membrane so low that heat conduction can compensate for the energy used in the evaporation process. Analytes in the gas phase can interact with the surfaces in the vacuum system, causing prolonged response times or even a complete suppression of the signal. Therefore, it is important that the distance from the membrane to the ion source of the mass spectrometer is as short as possible, although for the analysis of very volatile compounds also longer distances have been successfully utilized. The membrane temperature is an important factor for the performance characteristics of membrane inlets. The diffusion constant, the distribution ratio, and the permeability (equal to the product of the diffusion constant and the distribution ratio) all depend on the temperature according to the Arrhenius equations:    1 1 D ¼ D0  exp
Ed 
(4) RT RT0    1 1
K ¼ K0  exp
DHs  RT RT0

(5)

   1 1
P ¼ P0  exp
Ep  RT RT0    1 1
¼ D0  K0  exp
ðEd þ DHs Þ  RT RT0

(6)

where D0, K0, and P0 represent the diffusion constant, the distribution ratio, and the permeability, respectively, at some initial temperature, T0. Ed is the energy of activation for diffusion, DHs is the difference in heats of solution between the membrane and the sample matrix, and EP is the activation energy for permeation. In general, the activation energy for diffusion, Ed, is larger than zero,

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Table 2

Types of membranes commonly used in various MIMS applications

Membrane

Applications

Affinity Cellulose dialysis Latex Liquid membranes Microchannel filled with silicone Microporous (polypropene and Teflon) Polyethene Polyimide Polyurethane Silicone (sheet and capillary) Track membranes Ultra-thin membranes Zeolite membranes

Selective analysis of compounds with appropriate functionality Enrichment and specificity Similar to silicone VOCs Spatially resolved sampling Water chemical ionization Selectivity over water Higher water permeability Similar to silicone VOCs in water and air Ionic compounds from water SVOCs Selectivity for isomers

which means that the diffusion constant increases with temperature, whereas for most organic compounds the difference in heats of solution between the membrane and the matrix, DHs, is less than zero, which means that the concentration of analytes inside the membrane decreases with temperature. Under all circumstances, the response time, which is inversely proportional to the diffusion constant, becomes shorter at higher temperatures. The steady-state flow through the membrane can either increase or decrease when the temperature is increased, depending on the sign of the sum of Ed and DHs. In gas analysis DHs is often larger than Ed, whereas in water analysis DHs is usually smaller than Ed. As a result, in gas analysis the steady-state flow through the membrane decreases with an increase in temperature, whereas it increases in water analysis. Therefore, a gas analysis is carried out at a temperature that is chosen as a compromise between a maximum signal (at low temperature) and a short response time (at high temperature). A water analysis is carried out at a maximum practical temperature, which is normally 70–80  C. At higher temperatures, bubble formation in front of the membrane can cause serious instability in the signals. Most semivolatile compounds (boiling point > 250  C) cannot be detected by MIMS if the temperature of the membrane is below 100  C. They cannot evaporate from the membrane to the ion source without some sort of stimulated evaporation. Therefore, several versions of MIMS with increased evaporation efficiency have been designed, e.g., trap-and-release MIMS with a sudden thermal heating of the membrane up to 300  C, laser desorption MIMS where the vacuum side of the membrane is irradiated by laser, and desorption chemical ionization MIMS with a plasma-assisted evaporation. The main limitation of traditional MIMS techniques has been the inability to analyze polar compounds, such as many biological metabolites and pharmaceuticals. Polar compounds do not permeate well through commonly used membranes, such as silicone; thus, they have high LODs in MIMS analysis. However, detection of polar compounds can be enhanced using a membrane inlet system, where the membrane has a donor liquid (the sample) on one side and an acceptor liquid on the other side. This has two major advantages: first, polar membranes can be used for improved permeation of hydrophilic compounds and, second, ionization techniques other than electron ionization (EI) can be utilized, such as atmospheric pressure chemical ionization (APCI)3 and electrospray ionization (ESI),4 for the analysis of polar compounds. This approach might give the opportunity of analyzing any type of organic compounds from an aqueous sample. A wide selection of hydrophobic and hydrophilic membranes has later enabled different types of analytes and applications (Table 2). Another disadvantage of the membrane inlets is caused by possible memory effects, i.e., analytes or matrix components from one run are carried over to the following analysis. The main reason for this memory effect is that molecules can remain in the membrane or surfaces of the analytical system longer than the measurement of one sample is run. The memory effect can last for minutes or even hours, as in the case of extremely hydrophobic compounds, such as higher-molecular-weight polyaromatic hydrocarbons (PAHs). The memory effect is less when a higher temperature of the membrane can be used, i.e., it is largest with aqueous samples, where the inlet temperature has to be kept below 80  C, and smallest with air samples, where high temperatures above 150  C can be used. To prevent or decrease the memory effect, several procedures can be applied, e.g., dilution of the sample, calibration and analysis using non-steady state, the use of thinner membranes and elevated membrane temperature, as well as the analysis of blank samples between the real samples.

2.24.2.2

Membrane Inlet Designs

Several different membrane inlets have been designed over the years for different purposes. Most of the designs can be divided into seven main categories (Figure 1): (a) a membrane probe, where the membrane is mounted at the end of a probe which is inserted directly into the sample matrix, (b) a direct insertion membrane probe, where a sample (gas or liquid) has to be transported to the membrane system, typically a capillary membrane, which is mounted inside the mass spectrometer, (c) a flow-over membrane design, where the sample is flushed over a flat membrane, the other side of which is exposed to the vacuum of the mass spectrometer, (d) a measuring cell, where a sample (liquid or solid) is transferred to a small measuring cell that has a membrane on one

Membrane Inlets for Mass Spectrometry

501

Figure 1 The most common membrane inlets used. (a) a membrane probe, (b) a direct insertion membrane probe (DIMP), (c) a flow-over design using a flat-sheet membrane, (d) a measuring cell, (e) a helium-purged inlet, (f) stimulated desorption device (a trap-and-release design), and (g) a two-stage inlet (helium-purged with a jet separator). Copyright (2010) Wiley. Adapted with permission from Ketola, R. A.; Kotiaho, T.; Lauritsen, F. R. Sample preparation in membrane introduction mass spectrometry, In Handbook of Sample Preparation, John Wiley & Sons Limited.

side in front of the ion source of the mass spectrometer, (e) a helium-purged inlet, where molecules permeate through the membrane, and the inside of the membrane is purged by helium, to transport analytes to the mass spectrometric ion source, (f) a two-stage inlet system, where two different systems (one being a membrane inlet) for separation and/or preconcentration of the sample are used in series, and (g) a membrane inlet using a stimulated desorption for the analysis of semi- and/or nonvolatile organic compounds. The membrane probe consists of a perforated capillary (commonly of steel) that is covered by a polymeric membrane at one end, whereas the other end is connected via an evacuated tube to the mass spectrometer. The probe can be inserted directly into almost any sample, e.g., into arteria of humans, plants, sediments, and chemical and biological reactors of all types. One disadvantage of the membrane probe is that it can be easily plugged if it is inserted into a complex sample matrix, which contains particles, such as dust and organic matter in soil or cells in biological samples. On the other hand, the membrane probe can be easily changed for another clean one. Another drawback of the membrane probe is that it can only be used for the analysis of atmospheric gases and highly volatile organic compounds (boiling point < 100  C). Compounds of lower volatility can interact with the surfaces of the evacuated tube that connects the inlet to the mass spectrometer; thus, very long response times and poor sensitivity can result.

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The direct insertion membrane probe (DIMP) overcomes the problem with condensation of sample molecules in the tube of the membrane probe because in this inlet the membrane is mounted at the end of a probe that is inserted directly into the ion source of the mass spectrometry.5 The probe is continuously flushed with the liquid or gas sample and it is easily cleaned by flushing with a blank sample, enabling the use of flow injection analysis (FIA) for sampling and measurement. The advantage of the inlet is that VOCs with a boiling point up to around 200  C can be easily measured. Furthermore, the inlet has two advantages: (1) the sample can be chemically modified online in the connecting tubing before it reaches the membrane inlet and (2) calibration using an external standard is straightforward, e.g., by using the FIA technique. Typical applications of the direct insertion membrane probe are detection of volatile contaminants in water and monitoring of growing microbial cultures. A flow-over design uses a flat-sheet membrane, where one side of the membrane is exposed to the sample and the other to the vacuum. In the optimal design, the membrane directly constitutes a part of the wall in the ion source of the mass spectrometer. Otherwise, the working principle and also the advantages of this design are the same as with the direct insertion membrane probe. An additional advantage is that the membrane can be heated to a desired temperature by adding a suitable heating element to the flange where the membrane is mounted. This way it is possible to reach a constant temperature, e.g., 80  C, for water samples and even 200–300  C for air samples if the membrane can tolerate such high temperatures. The measuring cell is a small vessel (typically 1–5 ml) mounted as close as possible to the ion source of the mass spectrometer. It is interfaced directly to the mass spectrometer via a flat-sheet membrane that constitutes a part of one of the walls in a similar way as in the flat-sheet membrane inlet. The sample, liquid or solid, is transferred to the vessel and after a few minutes a mass spectrum of the volatile constituents can be measured. The advantage of the measuring cell is that it can be used for monitoring changes in the chemical composition of the sample for minutes, hours, or even days. The helium-purged membrane inlet is a more universal sample introduction system than the previous ones.6 In this inlet, a liquid or gaseous sample is flushed across the outer surface of a capillary membrane, whereas the other (inner) surface is continuously purged by a helium stream that carries the permeated molecules to the ion source of the mass spectrometer via a short silica capillary. Similarly, a sheet membrane can be utilized: one surface is exposed to the sample and the other surface is flushed with helium. The purging of molecules from the membrane to the ion source reduces the problem with condensation effects in connection tubes, and the inlet has almost the same applications as the direct insertion membrane probe. Because the membrane inlet is a separate system, it can be connected to almost any type of mass spectrometer without a modification to the ion source. Thus, it can be easily connected to a closed electron ionization (EI) source of commercial MS instruments while the membrane inlet is placed inside a gas chromatograph (GC) where the temperature of the inlet can be accurately controlled. The drawback of the system is the need for an extra gas for flushing. The above-mentioned two-stage membrane inlets have also been combined with other analytical techniques to improve either selectivity or sensitivity by additional separation or concentration of the analytes. They can be divided into three different systems: (1) two steps of membrane separation, (2) a preseparation or preconcentrating device in front of the membrane inlet, and (3) systems using a postconcentrating device after the membrane inlet. The systems in the first category combine two or more membranes together in the same inlet, having shorter response times than membranes with double thickness and resulting in higher concentration factors for the analytes. However, the system is more complicated and more difficult to use than a single-membrane inlet. Therefore, their use in real applications has been limited. The most common inlet category of the three systems is the combination of the helium-purged membrane inlet and a jet separator7 where the jet separator is used for selective removal of helium from the purge stream before it reaches the mass spectrometer. Other advantages of the membrane inlet/jet separator combination are practically unlimited surface area of the membrane and the fact that the separator acts as a barrier between the membrane and the vacuum system of the MS. Other systems which have been used for postconcentration are a sorbent (Tenax) or a cold trap. While resulting in a large concentration factor (very low detection limits), these systems suffer from the loss of online nature of single-membrane inlets. The sorbent trap can also be used prior to the membrane inlet in connection with gas analysis. Both a preseparation and preconcentration of related compounds can even be achieved if the adsorbed molecules are released from the trap using a controlled temperature gradient. The above-mentioned six inlet designs (a–f) are suitable for the measurement of gases and/or VOCs and they are of only limited use for the measurement of less volatile (boiling point > 200  C) organic compounds. To be able to measure compounds with low volatility with a membrane inlet, it is necessary to stimulate their evaporation from the membrane surface (design g). The most common techniques for stimulated desorption in MIMS are a trap-and-release (T&R) technique,8 laser desorption,9 and desorption chemical ionization (DCI)10 techniques. In the T&R technique, the sample is passed through a capillary membrane and organic compounds diffuse into the membrane, but do not evaporate from it. During aqueous sampling, a slit in the source parallel to both the membrane and the filament allows heat radiation from the filament to continuously bombard the membrane surface of the membrane but organic compounds are not released because the membrane is cooled by the liquid flowing through the inside of the tube. During a short interruption of the liquid flow, the membrane is rapidly heated to more than 300  C and SVOCs dissolved in the membrane are released into the ion source. In this manner, a desorption peak is obtained. In T&R-MIMS, the trapping can also be done with an external interface. The DCI method is an improved version of the T&R method, where the membrane is positioned in the center of a chemical ionization plasma. The plasma assists in desorption of less volatile compounds from the membrane during the heating process by ionizing the analyte molecules directly at/on the membrane surface. With this method, it is possible to measure high-molecular-weight compounds, such as PAHs, and more polar compounds, such as phthalates, at ng l
1 concentrations in water, whereas many steroid hormones and pesticides can be detected at low mg l
1 concentrations. In laser desorption

Membrane Inlets for Mass Spectrometry

503

MIMS, the evaporation of less volatile organic compounds from the membrane is stimulated by irradiation of the membrane surface with laser light. This technique has successfully been applied to measure PAHs in water at low ng l
1 concentrations.

2.24.3

Principles and Applications of Analysis

2.24.3.1

Gaseous Samples

2.24.3.1.1

Analysis of Volatile Organic Compounds

One of the main applications of MIMS has been the measurement of VOCs directly from air.11,12 Air is an easier sample matrix than water for MIMS analysis because the analytes are already in the gas phase and the air samples usually contain only small amounts of particles; thus, sampling is easy. By far the most important application in MIMS analysis of air samples is the measurement of volatile organic compounds (VOCs). Most of the different inlet types have been utilized in the analysis of VOCs from air samples, showing that gaseous samples are easy and suitable for various MIMS configurations and instruments (Table 3). The most common membrane used is PDMS since, as with water samples, it has one of the highest concentration factors and it is very durable, also at high temperature and high pressure. In addition to variation of concentrations of different VOCs, the parameters which mostly affect the signals and thus calibration with MIMS are temperature, humidity, and pressure of the air sample. Very high concentrations have previously been shown to cause memory effects with water samples. The memory effect is similar with air samples; however, the magnitude of memory effect is much smaller, due to fast response times with air samples compared to those with water samples, being approximately one order of magnitude faster. With air samples, the memory effect can be further diminished using elevated membrane temperatures up to 200  C, which increases the desorption of analytes from the membrane. If the temperature of the original sample is higher than that of the transfer lines and the membrane inlet, there is a risk of condensation of water and analytes with high boiling point inside the membrane inlet, thus contaminating the transfer lines and the inlet. Table 4 shows the performance characteristics of selected VOCs from air using a flow-over design of MIMS inlet with a single quadrupole MS instrument, showing detection limits at low mg m
3 levels or below. Lower detection limits at ng m
3 levels have been reached with more complicated membrane inlets and MS instruments. Importantly, the response times that can be obtained using an MIMS system (a few seconds) are very short when measurements are done from air using a thin membrane (25 mm), still wide linear dynamic ranges are obtainable, typically 3–5 orders of magnitude.

Table 3 Compound

Selected applications of membrane introduction mass spectrometry in air analysis Membrane inlet

Application, characteristics

Aromatic VOCs including benzene, Sheet toluene, and ethylbenzenes Atmospheric isoprene Hollow fiber

Online measurement of exhaust gases (engine test stand, dynamometer, on the road); 1-s cycle rate Vinyl methyl ether as alkene-selective CI reagent; 0.5–10 ppb calibration curve; membrane transfer efficiency measured at 10% Dynamite effluent Llewellyn separator Characterization of prototype portable MS for explosives detection VOCs Hollow fiber Investigation of flow geometry, dimensions, temperature, flow rate; steadystate response with flow-through inlet 50 greater than flow-over inlet Hollow fiber Online analysis at wastewater treatment plant Hollow fiber Response time <1–3 s; comparison of silicone with other materials Hollow fiber, jet separator Response times 60–90 s; minimal memory effects; membrane transfer efficiency measured at 10–20% Hollow fiber, sorbent tube/Peltier cell Cryotrapping inside Peltier cell Llewellyn separator Portable monitor for explosives, degreasing operations, pesticides, chemical agent monitoring Llewellyn separator Portable MS (150 lbs.) for explosives, degreasing operations, pesticides, chemical agent monitoring; 106 sample enrichment; heated separator enhanced detection of less volatile compounds Sheet Transportable MS for direct breath monitoring; 2 s response time; response inhibited by water vapor Sheet Mobile MS using 500 W power; very fast rise times Sheet Comparison of MIMS analysis of paintshop air with FID; characterization of membrane thickness, temperature, sample flow rate Sheet Passive sampling of room air on sorbent tubes followed by thermal desorption; good agreement with continuous monitoring method Sheet Portable battery-powered instrument using 40 W power Sheet, sorbent tube Temperature-programmed desorption to achieve fast separation of VOCs; 6–10-min analysis cycle Volatile organic sulfur compounds Sheet Custom-made gas calibrator used to produce gas standards; 4 orders of magnitude dynamic range Copyright (2002) Wiley. Adapted with permission from Ketola, R. A.; Kotiaho, T.; Cisper, M. E.; Allen, T. M. J. Mass Spectrom., John Wiley & Sons Limited.

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Table 4 Detection limits, response times, and linear ranges measured by MIMS for a selected set of environmentally significant compounds from air samples Compound

Response time (s) a

Detection limit (mg m
3) b

Linear range (mg m
3)

Benzene Toluene Xylenes Chlorobenzene Chloroform Carbon tetrachloride 1,2-Dichloroethene 1,1-Dichloroethane 1,1,1-Trichloroethane 1,1,2,2-Tetrachloroethane Tetrachloroethene Dimethyl sulfide Dimethyl disulfide Ethanethiol

1 1 2 2 1 1 1 1 2 3 1 1 2 2

1 0.5 2 0.5 5 3 1 1 4 1 0.5 1 2 5

1–10 000 1–10 000 2–10 000 2–10 000 5–30 000 10–20 000 5–10 000 1–5000 10–30 000 1–5000 1–10 000 1–40 000 2–40 000 5–50 000

Membrane (polydimethylsiloxane) thickness 25 mm. Response time is defined as the time it takes for the signal to rise from 10 to 90%. S/N ¼ 3.

a

b

Adapted from Ketola et al. Anal. Chim. Acta 1997, 349, 363 and Ketola, R. A. et al. Anal. Chem. 1997, 69, 4539.

The majority of applications have used silicone membranes, quadrupole mass spectrometers, and electron ionization. Different membrane configurations and materials other than silicone have also been investigated for air analysis. In studies using single or multiple hollow fiber probes, it was demonstrated that the performance of silicone exceeded that of the other polymers with respect to response time.13 A dramatic response time difference for VOCs between air and water samples reflected the degree of hydrogen bonding occurring for polar compounds in water. In addition to flowing sample over the outside, the sample can be pumped through the inside of the fiber. The flow-through configuration was found to be more efficient, since higher linear sample velocities could be obtained and the surface area was used more effectively. Very low LODs in air were obtained with an ion trap mass spectrometer using a membrane inlet design incorporating a jet separator for a second enrichment stage, e.g., 20 pptv for toluene.14 The helium flow through a silicone hollow fiber was countercurrent to the sample flow, and served as both the sample transport and the buffer gas in the ion trap. No evidence of long-term memory effects after membrane exposure to high concentrations was observed. With a high surface area membrane that caused long rise and fall times for organic compounds in water, ppt limits for VOCs and SVOCs in air with brief rise and fall times (10 and 48 s for toluene) were achieved. In methanol analysis, an ultra-thin allyl alcohol membrane generated linear responses extending beyond two orders of magnitude for air samples typically measured for methanol by Environmental Protection Agency (EPA) methods TO-15 and TO-17. Ionization by electron impact (EI) has been typically employed in the majority of MIMS experiments. With another technique, a charge exchange (CE) ionization-MIMS using membrane-diffused oxygen as the reagent gas, enhanced signals over EI were obtained. The CE/EI response ratio ranged from 2 for benzene to 22 for cyclohexanol. CE ionization proved suitable for SVOCs with reduced mass spectral fragmentation. A dedicated trace gas analyzer was developed which interfaced to a quadrupole MS system with a membrane inlet made of stainless steel and polytetrafluoroethylene (PTFE) parts to minimize sorption of organic compounds. Chlorinated hydrocarbons diffused very quickly with a 90% signal rise time of 0.5 s, whereas alcohols diffused more slowly. A linear calibration for toluene from the ppb level to 100 ppm was obtained, above which saturation occurred. Using ion trap mass spectrometer (ITMS), membrane introduction was more sensitive than direct air sampling by orders of magnitude with detection limits in the low-ppb range. The membrane inlet discriminated against air background that reduced sensitivity and affected fragmentation mechanisms in the ion trap when direct sampling was employed. In order to increase selectivity in the VOC analysis, a MIMS technique called temperature-programmed desorption (TPD-MIMS) was used to obtain analyte separation.15 The gaseous sample was flushed through a solid adsorbent in a steel tube, which was then heated at a controlled rate releasing analytes into a helium stream entering a membrane inlet, resulting in good peak separation with 10-s desorption profiles at half height. Figure 2 shows desorption profiles of nine compounds obtained from analysis of a mixture containing a few micrograms of each contaminant. Industrial sources, including the pulp and paper industry, and natural sources, such as oceans, soil, and volcanoes, release toxic volatile organic sulfur compounds (VOSCs) into the environment, contaminants typically analyzed by GC and GC/MS methods. MIMS provided sensitivity high enough for real-time monitoring of many VOSCs, including carbon disulfide, ethanethiol, dimethyl sulfide, thiophene, and dimethyl sulfoxide. Mixed VOSC samples containing three compounds were analyzed using a deconvolution program. The average of the absolute differences between calculated and measured concentrations was 6.1%. In addition to EI and CI, vacuum UV ionization has been applied to the measurement of VOCs in air.16 In this method, a singlephoton ionization is produced by a krypton discharge lamp with photon energy of 10.6 eV, i.e., the same lamp which is commonly used in atmospheric pressure photoionization (APPI) sources. A detection limit of 25 ppbv for benzene was achieved with this ionization method when a time-of-flight-MS (TOF-MS) instrument was used.

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Figure 2 Desorption profiles of nine compounds obtained from analysis of a mixture containing a few micrograms of each contaminant with TPD-MIMS. 1 trans-1,2-dichloroethene, 2 chloroform, 3 carbon tetrachloride, 4 trichloroethene, 5 toluene, 6 tetrachloroethene, 7 xylenes, 8 styrene and 9 1,2dibromo-1,2-dichloroethene in SIM mode using HayeSep D adsorbent. Copyright (1998) Wiley. Used with permission from Ketola, R. A.; Grøn, C.; Lauritsen, F. R. Temperature-programmed desorption for membrane inlet mass spectrometry, Rapid Comm. Mass Spectrom., John Wiley & Sons Limited.

2.24.3.1.2

Analysis of Semivolatile and Polar Organic Compounds in Air

The analysis of polar or semivolatile organic compounds (SVOCs) with MIMS is not as straightforward as that of VOCs either because the membrane commonly used (silicone) is hydrophobic or because the analyte vapor pressure is too low. To improve sensitivity, especially for polar VOCs, chemical ionization (CI) using water or air as the reagent gas proved to be useful.17 Taking advantage of the presence of water vapor in the samples or using supplemental water with CI-MIMS, it has been possible to obtain detection limits for airborne polar VOCs at 50% relative humidity from 31 ppbv for methyl-tert-butylether (MTBE) to 104 ppbv for methyl ethyl ketone. Also, polyaromatic hydrocarbons (PAHs) can be analyzed using a membrane inlet connected to a GC-MS. After sampling into a glass fiber filter tape sampler, the compounds adsorbed were then desorbed into a membrane inlet through a GC column. In tests at an oil combustion plant, a production plant for coal tar-based electrodes, and a motor test site, good correlations were found between the on-site method and literature-derived data. In addition to PAHs, sulfur-containing compounds were detected. To increase the permeation rates of airborne SVOCs through the membrane, ultra-thin composite membranes have been employed because they reduced the amount of membranedissolved analyte present, an advantage for compounds with slow diffusion rates and high solubilities. SVOCs from several compound classes were tested including nitrobenzene, methyl salicylate, 2-chlorophenol, and malathion, an organophosphate pesticide. Several other SVOCs, including lindane (a largely banned but environmentally persistent pesticide), dimethylmethyl phosphonate (DMMP, a flame retardant), naphthalene, and hexahydro-1,3,5-trinitro-1,3,4-triazine (RDX, an explosive), in air have been analyzed using a technique called single-sided membrane introduction mass spectrometry (SS-MIMS). To overcome the slow diffusion through PDMS, the method utilized adsorption and desorption from the same side of the membrane. Rise times of 4–7 s and fall times of 12–36 s were achieved with cycle times from sampling to data acquisition of approximately 1 min, limits of detection were in the parts per trillion in volume (pptrv) range with linear response over 4 orders of magnitude. MIMS is suitable also for the analysis of organometallic compounds, such as dimethyl mercury with a hollow fiber probe or ferrocene and molybdenum hexacarbonyl vapors using an ultra-thin membrane fiber and ITMS. A charge exchange ionization reaction with O
2 resulted in intense molecular ion fragments for both ferrocene and molybdenum hexacarbonyl. To increase selectivity in air analysis, an automatic MIMS apparatus utilizing a membrane and cryogenic concentration unit has been used for monitoring isoprene from greenhouse air below ppb levels. The necessary selectivity for the measurement was obtained using chemical ionization with vinyl methyl ether (VME) reagent gas and tandem mass spectrometry. VME reacts with isoprene to form an adduct ion at m/z 94 corresponding to [isoprene þ VME
MeOH]þ, which under collision-induced dissociation produces a product ion at m/z 79 by methyl radical loss. Online monitoring of this selected reaction monitoring (SRM) pair gave the necessary selectivity and sensitivity for the online greenhouse air monitoring.

2.24.3.1.3

Online/on-site/in situ Applications

Due to the minimal need for sample treatment, MIMS is very suitable for online and on-site applications. This is also possible due to the recent development of low-power-consumption portable MS instruments. A transportable TOF-MS incorporating a membrane inlet system was tested for environmental analysis.18 Converging annular geometry in the MS allowed for lower source pressure, smaller pumps, low power consumption, and a more efficient membrane inlet design in which two membranes operated in series, providing a secondary enrichment stage with enrichment factors as high as 15 600. Another instrument, a Tenax cryotrap inside a Peltier cell, inserted between a silicone membrane and an ITMS system, showed the detection of 0.1 mg m
3 of aromatic compounds in air. Another portable vapor detection quadrupole analyzer, incorporating a silicone membrane inlet for pollution monitoring, solvent detection in degreasing operations, identification of pesticides, and monitoring of cholinesterase-inhibiting chemicals, was tested in the 1970s.19 The membrane separator was pumped, using evacuated canisters containing zeolite, thus eliminating

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mechanical pumping of this stage in the field. Detection limits for VOCs in ambient air ranged from 0.1 ppbv tetrachloroethene to 5 ppbv phosdrin, an organophosphate compound. With an optimized MIMS method, paintshop exhaust air was analyzed using a quadrupole MS and a silicone sheet membrane inlet.20 Linear dynamic ranges for 14 VOCs extended over 4 orders of magnitude, and limits of detection ranged from 0.5 to 5 mg m
3. The analytical results obtained with MIMS were compared to those obtained with an online flame ionization (FID) analyzer, and the agreement of the results was good as the differences in total VOC concentrations ranged from
6.7 to 15.0%. Throughput in MIMS as high as 150 samples per hour was possible. A multimembrane inlet system with a quadrupole ion trap was used in conjunction with chemical ionization for a fastresponse detection method for the determination of atmospheric isoprene.21 Figure 3(a) shows a schematic diagram of the sampling system. Ambient air was preconcentrated with three tubular silicone membranes in a helium-purged inlet. The analytes in the helium stream were further concentrated with a cryogenic loop with a 5-min sampling time to achieve sufficiently low detection limits. In this method, vinyl methyl ether was utilized as a selective reagent for alkenes with which the reagent undergoes a [2 þ 4] cycloaddition reaction, followed by loss of methanol, to generate an adduct ion at m/z 94 [M þ 58
32]þ. Upon collision-induced dissociation (CID), this ion fragmented to an ion at m/z 79 by loss of methyl radical which was then used for quantitation. The method was applied to monitoring isoprene emissions from velvet bean plants (Figure 3(b)). It can be seen that isoprene emissions are released only when the plants are irradiated by UV lamps or at least the concentration of isoprene is below the LOD (0.1 ppb) when the lamps are turned off. A membrane inlet and a quadrupole MS instrument were used for the analysis of air in five laboratory rooms.22 Samples were collected twice a day in passive sampling tubes, which were then desorbed into a silicone sheet membrane interface. The results obtained with the continuous monitoring MIMS technique over 7 days were in good agreement with those obtained from passive sampling tubes. MIMS also provided sensitivity high enough for real-time monitoring of many volatile organic sulfur compounds (VOSCs), including carbon disulfide, ethanethiol, dimethyl sulfide, thiophene, and dimethyl sulfoxide. Mixed VOSC samples containing three compounds were analyzed using a deconvolution program. The average of the absolute differences between calculated and measured concentrations was 6.1%. Another interesting demonstration of the wide applicability of MIMS is online measurement of industrial solvents (e.g., styrene, 1,1,1-trichloroethane, and toluene) in breath to noninvasively monitor solvent exposure in workers.23 A silicone membrane inlet was combined with a gas handling system with a transportable quadrupole MS for direct analysis of VOCs in breath. Selected ion monitoring tracked the decay of a chlorinated solvent in a volunteer’s breath for 168 h, as shown in Figure 4. The calibration must be performed carefully as it was noticed that water vapor in breath air affects the signals obtained with MIMS, i.e., the amount of water vapor typically found in breath lowered the signals of solvents 24–27%. This is due to the fact that water molecules hinder the transport of other molecules across the membrane, probably blocking the active sites. Thus, gaseous standards must contain approximately the same amount of water vapor as the real breath samples contain. This also applies to other air samples with high humidity. Time-resolved measurements of auto exhaust are critical for correlating engine state with auto emissions. For this purpose, aromatic compounds in automobile exhaust gases were measured using a membrane inlet quadrupole MS system with a cycle rate of one analysis per second for on-site evaluation on an engine test stand, a dynamometer, and in automobile traffic.24 PAHs up to anthracene and phenanthrene were detected online. A membrane inlet with a quadrupole MS system was used for the analysis of air in five laboratory rooms where the activities included ion source cleaning, vacuum pumping, glassware cleaning, and MS operation. Samples were collected twice a day in passive sampling tubes, which were then desorbed into a silicone sheet membrane interface. A continuous monitoring method providing averaged concentrations over 7 days resulted in good agreement with the MIMS technique. The room containing cleaning and washing operations was found to be the most contaminated with a total VOC concentration measured at 158 mg m
3 (158 pptrv) with acetone as the major constituent.

2.24.3.2 2.24.3.2.1

Water Samples Characteristics of Water Analysis

Measurement of volatile organic compounds directly from aqueous phase is a very common application of MIMS but many semivolatile organic compounds can also be reliably measured. Table 5 summarizes the typical performance characteristics of MIMS in direct measurement of VOCs and SVOCs from water samples. The detection limits given for VOCs are those, which are easily achievable with most MIMS systems using a flow-through membrane inlet and a single quadrupole mass spectrometer. Table 6 shows typical applications with detection limits of specified MIMS methods and/or membrane inlets in water analysis. Fast and direct detection of contaminants in water samples is very useful for characterization of drinking-water supply systems and high sensitivity is required because many contaminants can be toxic or harmful at very low concentration levels. It took almost 30 years of development of the technique before the goal of detection limits in the 1–10 pptr (yng l
1) range for solvents such as chloroethylenes and gasoline residuals in drinking water was reached. Today, the analysis of solvents in aqueous samples is straightforward with response times between 30 and 60 s and detection limits in the upper ng l
1 level are routine but detection limits at even high pg l
1 (ppq, parts per quadrillion) have been demonstrated as shown in Figure 5.25 Detection limits of SVOCs are normally much higher than those of VOCs with standard MIMS methods, but with the trap-and-release methods (T&R-MIMS) SVOCs can be measured at mg l
1 levels (ppb, parts per billion). Table 5 shows also that with standard MIMS a wide linear

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507

Figure 3 (a) Schematic diagram of membrane preconcentration sample inlet. (b) Online monitoring of isoprene by examining using tandem mass spectrometry the product of a DielsAlder ion/molecule reaction with vinyl methyl ether. This selective reactivity allows isoprene to be monitored as it is emitted from velvet bean plants during irradiation by ultraviolet lamps. Isoprene emission drops below 0.1 ppb when the lamps are turned off. Reprinted with permission from Colorado, A., Barket Jr., D. J., Hurst, J. M., Shepson, P. B., Anal. Chem., 1998, 70, 5129–5135. Copyright 1998 American Chemical Society.

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Figure 4 1,1,1-Trichloroethane was tracked in alveolar breath (m/z 61) for 168 h after a volunteer had been exposed to 750 mg m
3 of solvent for 4 h. Copyright (1981) Wiley. Used with permission from Wilson, H.K., Ottley, T.W. The use of a transportable mass spectrometer for the direct measurement of industrial solvents in breath. Biomed. Mass Spectrom., John Wiley & Sons Limited.

dynamical range is obtained for VOC analysis, typically 3–5 orders of magnitude, whereas the trap-and-release method only has a linear dynamical range of 2–3 orders of magnitude. One reason for this is that all molecules cannot be released at the same time from the membrane; therefore, several cleaning steps are needed. The reliability of the quantitative results obtained by MIMS has been confirmed by comparing MIMS, purge-and-trap gas chromatography-mass spectrometry, and static headspace gas chromatography for analysis of VOCs from water samples.26 Agreement of the results obtained by the various methods was very good. MIMS, with a hollow fiber probe membrane, was first used by Westover et al.13 to monitor reaction systems containing VOCs such as chloroform, hexane, and methanol in aqueous solution and air. Permeation of organic compounds across different membrane materials was studied; it was found that silicone rubber was the most useful membrane material for the analysis of VOCs from aqueous solution. Hydrogen bonding between a compound molecule and the aqueous matrix was believed to be the most important reason for methanol’s higher detection limit (1 ppm) vs. that for chloroform (10 ppb). Low ppb levels of VOCs were detected with response times of a few seconds. An enrichment factor of 1.1  104 was determined for chloroform. Tetler et al.27 introduced the first sheet membrane system for detecting VOCs in estuarine water. A silicone rubber membrane supported by wire mesh and a sealed Teflon collar was interfaced directly to the vacuum chamber of a mass spectrometer. Minimum detection limits of 1–7 mg l
1 and a few hundred mg l
1 were achieved, respectively, for nonpolar and polar compounds. A high concentration of sodium chloride (35 g l
1, i.e., sea concentration) reduced the toluene signal only slightly (6%). While the MIMS analysis of VOCs in aqueous samples has become routine, the analysis of SVOCs (boiling point above 250  C) has not. In water analysis, the standard membrane inlets cannot be operated at temperatures much higher than 70  C before bubble formation in front of the membrane causes highly unstable signals. At temperatures above 100  C, signals of organic compounds are lost due to water’s large volumetric expansion as it starts to boil. The low inlet temperature limits the vaporization of the semivolatiles from the membrane surface and results in long membrane response times (>5 min). In the first applications of detection of SVOCs, direct insertion membrane probes (DIMPs), in which the membrane is mounted inside or in the immediate vicinity of the ion source region, were used. Due to the short distance between the membrane and the ion source, adsorption of molecules to vacuum surfaces was negligible. In this system, the whole membrane is in the vacuum, thus promoting the desorption of SVOCs into the gas phase. The measurement of polar compounds is also limited with standard MIMS because of the widely used hydrophobic polydimethylsiloxane membrane. The trap-and-release-MIMS (T&R-MIMS) technique was introduced for the analysis of semivolatiles in water samples. This technique allows the detection of both VOCs and SVOCs in the same run as exemplified in Figure 6 which shows the desorption profiles of toluene (volatile), caffeine, and 1-naphthalenemethanol (semivolatiles) after trapping them from a water sample and

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509

Table 5 Detection limits, response times and linear ranges measured by MIMS for a selected set of environmentally significant compounds from aqueous samples Compound

Response time (min) a

VOCs Benzene Toluene Xylenes Chlorobenzene Chloroform Carbon tetrachloride 1,2-Dichloroethene 1,1-Dichloroethane 1,1,1-Trichloroethane Dimethyl sulfide Dimethyl disulfide Ethanethiol

2.0 1.5 2.0 1.5 1.5 2.0 1.0 1.0 1.5 ND ND ND

SVOCs DDT Phenoxyacetic acid Fluoranthene Phenanthrene Caffeine

NA NA NA NA NA

Testosterone Testosterone acetate Progesterone Levonorgestrel Ethynylestradiol Fluoranthene Phenanthrene Atrazine Aldrine

NA NA NA NA NA NA NA NA NA

Detection limit (mg l
1) b

0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.4 0.1 0.5 0.5 0.1 Standard 1000 10 000 25 4 NA

Linear range (mg l
1)

0.1–1000 0.3–2000 0.1–5000 ND 0.1–1000 0.1–1000 ND ND 0.4–1000 ND ND ND T&R-MIMS 25 100 5 0.5 600 DCI-MIMS 30 3 16 30 150 0.2 0.02 5 5

ND ND ND ND 1000–1 000 000 ND ND 30–6000 ND ND ND ND ND ND

For VOCs membrane thickness 100 mm. Response time is defined as the time it takes for the signal to rise from 10 to 90%; S/N ¼ 3; ND ¼ Not determined; NA ¼ Not applicable.

a

b

Adapted from Lauritsen, F. R.; Rose, J. Analyst 2000, 125, 1577–1581; Aggerholm, T.; Lauritsen, F. R. Rapid Commun. Mass Spectrom. 2001, 15, 1826–1831; Kotiaho, T.; Ketola, R. A.; Ojala, M.; Mansikka, T.; Kostiainen, R. Am. Environ. Lab. 1997, No. 3, 19–21, and Lauritsen, F. R.; Ketola, R. A. Anal. Chem. 1997, 69, 4917–4922.

release with a 50-s air plug.28 Typically, response times in the MIMS method are in the range of 1–3 min from water using a 100-mm-thick silicone membrane. In the trap-and-release method, a preset concentration time is used before the analytes are desorbed from the membrane. Therefore, the total analysis time (5–30 min) is more important than membrane response times. The T&R-MIMS method has been applied, e.g., to a direct analysis of caffeine from water extracts of ground coffee and tea leaves. A similar MIMS system, a thermal membrane desorption application (TMDA) for the online GC-MS analysis of organics in water or fermentation suspension, has been developed. With TMDA, a membrane separator was used to extract VOCs from the sample for direct analysis by GC-MS. Compounds which did not diffuse through the membrane during sampling but which accumulated in the membrane were then thermally desorbed and transported to a GC-MS for the analysis. The major difference between TMDA and T&R-MIMS is that in T&R-MIMS the inlet is an integral part of the ion source, whereas in TMDA the inlet forms a separate unit mounted at a short distance from the ion source. Other means of assisted desorption of SVOCs from the membrane include laser desorption and ambient sonic spray desorption/ionization. Laser desorption-membrane introduction mass spectrometry (LD-MIMS) has been used to monitor ppb concentrations of naphthalene (bp. 218  C), anthracene (bp. 340  C), pyrene (bp. 393  C), chrysene (bp. 448  C), indeno(1,2,3-cd)pyrene (bp. 530  C), and benzo(b)-fluoranthene (bp. 480  C).9 An attenuated continuous-wave CO laser (10.4 mm wavelength, <5 W) desorbed semivolatile compounds that were 2 subsequently ionized in an electron impact source. The LD-MIMS system was subsequently modified by adding a KrF excimer laser to allow ionization by resonance-enhanced multiphoton ionization (REMPI). The combination allowed the monitoring of pptr concentrations of polyaromatic hydrocarbons (PAHs) in aqueous solutions. REMPI acted as a highly selective, soft ionization technique. REMPI is limited to molecules that are resonantly excited at a specific wavelength and that have a sufficiently long lifetime for a second photon absorption (these requirements significantly reduce background signals). The absorption cross section directly influences sensitivity, as shown by the increased response of pyrene over naphthalene. Compared to EI, REMPI decreased the LODs by two orders of magnitude, i.e., 100 pptr vs. 10 ppb, and increased also the linear dynamic range of the

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Table 6

Typical applications of membrane introduction mass spectrometry in water analysis

Compound

Method/inlet

Detection limit

Acrolein, acrylonitrile, aldehydes Aldehydes Benzene, ethanol BTEX, ethanol BTEX BTEX, VOC, polar compounds Chemical agents Chlorine, chloramines Deuterated water Dicarboxylic acids Biomolecules Steroid hormones Disinfection by-products, VOC Dissolved gases Dissolved gases, VOC Gases Inorganic chloramines, chlorobenzenes Kathon CG Methanol Naphthalene Nitrogen trichloride Organic compounds Organohalogen and BTEX compounds Organohalogen compounds PAH, estrogenic compounds, pesticides p-Cresol, 2-methyl-2-pentanol, trichloroethene Phenolic compounds Semivolatiles Terpenes Trihalomethanes VOC VOC VOC VOC VOC VOC VOC VOC VOC, nitrogen VOC, polar compounds VOC, semivolatiles VOC, semivolatiles VOC, semivolatiles VOC, semivolatiles, organometallics Volatile sulfur compounds

Sheet membrane Affinity liquid membrane Sheet membrane Sheet membrane DIMP PEI/S membrane Flow-through Flow-through Sheet membrane Flow-through

10 ppb 50 ppb 10–50 ppb

Flow-through Sheet membrane DIMP Flow-through Trap-and-release Membrane probe Helium-purged Hollow fiber membrane extraction Hollow fiber membrane DIMP Helium-purged Helium-purged Flow-through Flow-through Sheet membrane Trap-and-release Sheet membrane Helium-purged Cryotrap DIMP Flow-through Helium-purged Membrane probe Microporous Purge-and-membrane Sheet membrane Sheet membrane DIMP Flow-through Trap-and-release DIMP Helium-purged Sheet membrane

0.05 mg l
1

0.02–0.1 mg l
1 0.1 M High ppb

0.1 ppb Low or sub ppb 2–5 ppm Concentration factor 166 10
6 M 8 ppt 0.5–1 ppb 20 ppt–300 ppb Low ppm 1–1000 mg l
1 0.5–600 mg l
1 0.2–2 mg l
1 2–8 ppt 10–20 ppt 0.5 ppt–10 ppb 190 ppt–100 ppb 0.1–0.7 mg l
1 250 mg l
1 100 ppb 0.1–10 mg l
1 0.1–100 ppb 3 mg l
1 10 mg l
1 0.04 ppb–6 ppm

0.1–0.5 mg l
1

Copyright (2002) Wiley. Adapted with permission from Ketola, R. A.; Kotiaho, T.; Cisper, M. E.; Allen, T. M. J. Mass Spectrom., John Wiley & Sons Limited.

method. In easy ambient sonic-spray-MIMS (EASI-MIMS), a cellulose dialysis membrane is used as an interface between a circulating aqueous sample and an ionization region in ambient atmosphere. A supersonic spray of charged droplets desorbs molecules from the outer surface of the membrane, ionizes them, and the ions are directed to MS for the analysis. The method was shown to be applicable to the analysis of drugs from aqueous solutions suspended from tablets. In sorption MIMS, the analytes permeating the membrane inlet are adsorbed on a trap prior to MS detection. In all cases, a relatively long trapping period combined with a rapid release of the analytes resulted in analyte enrichment. A disadvantage of the sorption MIMS technique is that real-time monitoring can no longer be conducted. Rivlin introduced a system in which the trap was mounted in the vacuum between the membrane inlet and the ion source.29 A heating wire mounted inside the trapping material allowed the trapped sample to be thermally released into the mass spectrometer. Major drawbacks were degradation of the sorbent material (Tenax) during the thermal desorption step and a considerable pressure increase inside the mass spectrometer when the large amount of water trapped by the sorbent was released.

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Figure 5 Direct detection of toluene at parts-per-trillion (pptr, ng l
1) and parts-per-quadrillion (ppq, pg l
1) levels from water by MIMS. (a) Ion chromatogram measured with a direct insertion membrane probe and an ion trap, (b) ion chromatogram from (a) after postacquisition signal averaging performed using a five-point moving average. Reprinted with permission from Soni, M.; Bauer, S.; Amy, J. W.; Wong, P.; Cooks, R. G. Anal. Chem. 1995, 67, 1409–1412. Copyright 1995 American Chemical Society.

Figure 6 Trap-and-Release-MIMS desorption profiles of toluene (m/z 91 monitored), 1-naphthalenemethanol (m/z 158 monitored), and caffeine (m/z 194 monitored) obtained during the passage of a 50-s air plug. Reprinted with permission from Lauritsen, F. R.; Ketola, R. A. Anal. Chem. 1997, 69, 4917–4922. Copyright 1997 American Chemical Society.

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Detection limits at parts-per-quadrillion levels have been achieved by using stored waveform inverse Fourier transform (SWIFT) and the ITMS.46 In this technique broadband waveforms, notched at the resonance frequencies of analyte ions, are applied during long ionization periods to eject unwanted ions and store only analyte ions. Similarly, an improvement factor of 100 is achieved by a cryotrap-MIMS (CT-MIMS) method compared to a standard MIMS method, and typical limits of detection for VOCs are 10–20 pptr.47 In this technique, analytes are cryotrapped after crossing the membrane; rapid heating of the trapped compounds releases them into the ion source of a quadrupole MS. The purging technique has been applied in purge-and-membrane (PAM) methods, where the detector is either an electron capture detector (PAM/ECD) or a mass spectrometer (PAM/MS). In these methods, the sample (water or soil) is purged with an inert gas and the purged compounds are collected from the gas phase through a silicone membrane inlet to the analytical system. Detection limits are sub-mg l
1 for water samples and at the low mg kg
1 level for soil samples. The measurement of polar compounds can be enhanced by the analysis of derivatized products using MIMS and PDMS membrane. An example of this procedure is the acetylation of phenolic compounds in the aqueous phase and subsequent analysis with MIMS.30 The results obtained showed that with acetylation phenolic compounds, such as phenol and 4-nitrophenol, can be detected at one to two orders of magnitude lower levels compared with the direct analysis. A coaxially heated flow-over inlet was designed for simultaneous detection of both VOCs and SVOCs. In this design, the interior of the capillary hollow fiber membrane was heated via a coaxial nichrome wire, establishing a thermal gradient counter to the analyte concentration gradient in the membrane. This design offers improved sensitivity for SVOCs while retaining good sensitivity for VOCs as well, e.g., detection limits of 37 and 200 pptr are obtained for toluene and trichloroanisole, respectively. The method allows also a continuous sampling mode for SVOCs. Figure 7(a) shows the signal of naphthalene in aqueous solution with and without internal membrane heating and Figure 7(b) shows the signal of guaiacol using internal vs. external membrane heating.31 Reliability and accuracy of MIMS methods have sometimes been proven by comparisons with other analytical methods. For example, Harland and Nicholson32 compared the MIMS method with two purge-and-trap methods (GC-FID and GC-MS), in which six volatile halogenated hydrocarbons were analyzed from five environmental samples using the selected ion-monitoring mode. The concentration levels of the hydrocarbons ranged from less than 0.1–90 mg l
1 in the samples, and the analytical results were in good agreement. MIMS has also been compared with other analytical methods, such as static headspace gas chromatography and purge-and-trap (P&T) GC/MS, with HSGC and GC using a Hall electrolytic conductivity detector in the analysis of volatile organic sulfur compounds, with P&T and solid phase microextraction (SPME), with P&T-GC/MS in the analysis of MTBE, with HPLC in the analysis of phenolic compounds, with DPD/FAS titration, and UV–visible spectroscopy. In general, agreement between MIMS and the other methods was good, indicating the suitability of MIMS for analyzing organic compounds in water.

Figure 7 (a) Signal comparison obtained for a 1.6-ppb aqueous NA solution (1 l, recirculated) using continuous internal heating (rise time ¼ 19.4 min) vs. no internal heating (rise time > 63 min). In each case, the sample was maintained at 30  C (signals offset by 125 units). (b) Signal comparison for aqueous GU (1 l, recirculated at 30  C) using continuous internal vs. external membrane heating. At 30  C external heating a 1-ppm GU sample yielded a signal intensity of ~400 a.u. (rise time ¼ 22.4 min). At 50  C, the same concentration gave 25% less signal (rise time ¼ 32.5 min). With continuous internal heating, a 500-ppb GU sample resulted in an observed signal of 800 a.u. (rise time ¼ 11.8 min). Copyright (2006) Wiley. Used with permission from Thompson, A. J.; Creba A. S.; Ferguson, R. M.; Krogh, E. T.; Gill, C. G. Rapid Commun. Mass Spectrom., John Wiley & Sons Limited.

Membrane Inlets for Mass Spectrometry

2.24.3.2.2 l

513

Bioreactor and Chemical Reactor Monitoring

Bioreactors

Biological reactions can be characterized by MIMS analysis of gaseous, liquid-phase, or solid-phase samples. Common applications are monitoring of the production (or consumption) of gaseous or dissolved organic compounds, which can include various organic acids and other semivolatiles. The use of MIMS to analyze these reacting mixtures is driven by its simplicity, the associated speed of analysis, and the ability to perform continuous or closely spaced periodic measurements with fluctuating matrices. Traditionally, data from reaction mixtures are measured by GC or HPLC, which often requires filtration of the biomass prior to analysis; a step that is generally not required for MIMS. Bioreactor monitoring experiments have generally relied on sheet silicone membranes, but the capillary configuration is equally suitable. Many experiments have used external standards, often using FIA to alternatively examine standard and sample solutions, with quantification based on the ratio of the associated peak intensities. Alternate injections are convenient and correct for instrumental drift when reactions must be monitored over several hours or even days. More rugged and efficient flow injection techniques with fed-batch fermentations have also been studied. Gradual addition of substrate to a fermentation vessel is often needed in order to produce a higher reaction yield than would be expected from a single substrate infusion, a process known as fed-batch fermentation. The ideal rate at which the substrate should be added to a bioreactor is variable and is dependent on experimental conditions such as the reactor temperature, the nature of the microorganism, and the substrate type and concentration. The state of a reactor is easily monitored by using MIMS to analyze volatile components such as ethanol or CO2. The rate of increase in ion abundance, which is proportional to the concentration of these chemical indicators, can be used as a parameter in triggering a feedback control action. MIMS has become an established method of monitoring metabolism and it is an ideal sensor for closed-loop control of oxygen and carbon dioxide in fermentation vessels. In anaerobic digesters, which produce methane from farm water, control of dissolved hydrogen using MIMS provides long-term stability. Likewise, MIMS monitoring of malolactic fermentation during wine and cider production can help optimize these processes. Several intelligent interfaces between the bioreactor and the mass spectrometer have been reported. In one system, a Cþþ-based program is used to follow ethanol ion abundance (m/z 47) in a chemical ionization mass spectrum, and the rate of its increase is calculated. If the rate of change of product formation drops below a preselected level, then an electronic pulse is used to actuate a metering valve, which delivers additional glucose substrate to the fermentation medium. The recorded mass spectra initially display a decrease in ion intensity, due to dilution upon glucose solution addition, followed by the desired increase in production rate. Acetic acid, lactic acid, acetal, and acetoin are minor bioreactor products that provide valuable information related to the status of the reactor and the probable ultimate product yield. These by-products are often indicators of competing reactions that can be enhanced by nonoptimal reactor conditions, including possible contamination processes. These trace products can be used to alert the operator to the need for corrective action. With MIMS it is possible to simultaneously monitor both major (g l
1) and minor (mg l
1) reaction products. This dynamic sensitivity arises because a single scan provides information on all ions produced from a mixture (total ion monitoring) or on a select subset (selected ion monitoring). For example, this was shown in monitoring ethanol at the percent level while simultaneously monitoring low ppm levels of other by-products of fermentations. Other typical examples in bioreactor monitoring with MIMS are cellulose degradation to form ethanol for use as an alternative fuel source, pharmaceutical production, medical research, mechanistic studies involving enzymes, bacterial research, metabolism of halogenated organic compounds, enzymatic conversion of CO2, microbial oxidation of dichloroethene, and aqueous denitrification. Monitoring of fermentation products/off-gases (e.g., N2, O2, CO2, and ethanol) from the outlet gas of fermentors is a very important application field of MIMS. MIMS offers a higher sampling frequency than conventional methods. One practical example is online monitoring of an ethanol-producing pilot plant. The mass spectrometer was packaged to withstand the high temperature, humidity, and dust level of the plant. An appropriate tangential filter, capable of handling a 10-gallon min
1 sample flow, was designed and constructed for use with MIMS sampling. Reaction data were gathered in six-minute cycles, with sample and standard both being analyzed continually during a four-day period. Using water-CI conditions, the ethanol concentration in this sample was quantified as 2.97  0.07%. These data were consistent with that obtained with offline HPLC which was the accepted method in this fermentation plant. The major benefit of the MIMS over the HPLC method was the capability of continuous monitoring, whereas with the HPLC only eight measurements a day were possible. Another advantage of MIMS in monitoring is long-term stability of the system. This is exemplified by online measurement of beer fermentation with an automatic MIMS system (Figure 8).33 The sample from a fermenter was online filtered prior to acidification and MIMS analysis using a flow-over sheet membrane inlet and a quadrupole instrument (Figure 8(a)). Not only the concentration of ethanol but also concentrations of by-products were continuously monitored for two weeks during which the MIMS system did not need any maintenance (Figure 8(b)). Penicillin-V production has been optimized in another experiment that uses feedback control, through reference to the ion intensity due to the precursor compound phenoxyacetic acid (POAA, m/z 152).34 To avoid toxic effects and increase fermentation yield, standard solutions of POAA were prepared and added in small quantities to the bioreactor. A negative deviation from a linear increase in penicillin concentration with time formed the basis for triggering the feedback mechanism and delivering additional POAA to the reactor during a 150-h reaction period. Monitoring POAA by MIMS was challenging due to its high boiling point (285  C vs. 78  C for ethanol), whereas the weakly acidic character of this analyte necessitated the acidification of sample and standard streams prior to analysis. Nevertheless, the online MIMS data on POAA agreed well with an offline HPLC data. Biofilms that contain methanotrophic bacteria are of practical significance to the environment because they are known to degrade halogenated pollutants such as cis- and trans-1,2-dichloroethylene. However, analytes of interest in these reactors are present in very

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Figure 8 Online measurement of beer fermentation with an automatic MIMS system. Different compounds are marked with the following symbols: () propanol, ()) ethyl acetate, (A) 3-methylbutanol, (:) 2-methylpropanol, (C) 2-methylbutanol and (-) ethanol. 3-Methylbutyl acetate and ethyl caproate were also measured but their concentrations were below 1-mg l
1 level. Reprinted from Talanta, 65, Tarkiainen, V.; Mattila, I.; Kotiaho, T.; Virkaja¨rvi, I.; Aristidou, A.; Ketola, R. A. On-line monitoring of continuous beer fermentation process using automatic membrane inlet mass spectrometric system, 1254–1263, Copyright (2005), with permission from Elsevier.

low ppb concentrations. The biodegradation of dichloroethylene (DCE) has been kinetically modeled against a reference compound, trichloroethylene (TCE), and it has been shown that the degradation rate depends on the isomer involved. The mass spectrum of a proposed reaction intermediate was directly measured, and was consistent with a DCE-epoxide structure. Another complex biological system in which MIMS was successfully used is the white rot fungus, Bjerkandera adusta (B. adusta). The metabolism of halogenated aromatic compounds was studied in this fungus, which is known to produce halogenated aromatic compounds from a chloride or bromide source and glucose. An understanding of the metabolic pathways of this fungus might be useful in the synthesis of other compounds, and in the understanding of the behavior of related systems. The chemical information was obtained through the use of fluorine-‘labeled’ phenylalanine, tyrosine, and benzaldehyde, which are known to be precursors in the formation of chloromethoxybenzaldehydes. A specific meta-halogenation mechanism was observed in the production of fluorodichloro compounds, which was only observed with o-fluorophenylalanine. In this work, electron capture negative chemical ionization (ECNCI) coupled with tandem mass spectrometric MIMS analysis was found to be a highly sensitive and selective method for the detection and structural elucidation of these halogenated metabolites.35 The ECNCI method provides complementary structural information to isobutane CI, but is chemically specific to compounds containing halogenated substituents. The chemical specificity of ECNCI also reduces the background signal intensity, and this increases the sensitivity of the ionization method. Figure 9 contrasts these two ionization techniques, using an identical sample of B. adusta in both spectra. Isobutane CI of the B. adusta synthesized chloromethoxybenzaldehydes provided the protonated molecules, whereas ECNCI yielded [M
CH3]$
ions. Monitoring dissolved gases in reaction media is common with MIMS. The conversion of 13CO2 to H13CO3 has been followed in a study of the enzyme bovine carbonic anhydrase II.36 The concentration of 13CO2 was monitored in the reaction broth with a membrane probe, and this information was used to deduce the mechanism of this enzymatic reaction. A similar study has been

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Figure 9 Comparison of two techniques for the ionization of chloromethoxybenzaldehydes in a complex mixture from the culture supernatant of a bioreactor solution of Bjerkandera adusta sampled by MIMS using a triple quadrupole mass spectrometer. (a) Isobutane CI shows the protonated molecular ion [M þ H]þ of 3-chloromethoxybenzaldehyde at m/z 171/173 and (b) electron capture negative chemical ionization (ECNCI) shows a prominent [M
CH3]
ion at m/z 189, due to the dichloromethoxybenzaldehyde. Copyright (1996) Wiley. Used with permission from Beck, H. C.; Lauritsen, F. R.; Patrick, J. S.; Cooks, R. G. Metabolism of halogenated compounds in the white rot fungus Bjerkandera adusta studied by membrane inlet mass spectrometry and tandem mass spectrometry, Biotechnol. Bioeng., John Wiley & Sons Limited.

reported on the denitrification of water, using Pseudomonas nautica. This work involved the monitoring of isotopically labeled nitrate compounds during their conversion to molecular nitrogen (14N, 15N, and 15N2). MIMS in monitoring of diagnostic molecules in complex matrices could be utilized also in the production of pharmaceuticals as well as alternative fuel sources. l

Chemical reactors

MIMS is applicable to real-time, in situ monitoring of chemical reactors.37 Quantitative data are provided rapidly and structural information on unknown reaction products can be obtained. One example is studies of aqueous phase reactions used to convert chlorine to chloramines. These reactions are very suitable for MIMS because they occur in water phase, which is an ideal medium for MIMS measurements using a silicone membrane. Note also that in the fundamental investigation of a particular area of chemistry, e.g., that of the chloramines, further chemical information may be gathered through gas phase ion/molecule reactions by using MIMS as the sample introduction technique. Differentiation and quantification of free chlorine and the chloramines by MIMS have been developed into a rugged analytical method. Mechanistic studies have been reported on the chlorination by hypochlorite of organics, including phenol and related compounds, which are considered as models for humic substances. The successive steps in phenol chlorination, the probable intermediates leading to the formation of chloroform, the reaction rates, and the chlorination mechanism are all readily accessible through a simple MIMS experiment. Fenton’s reagent (Fe2þ/H2O2) can be used to oxidize organic compounds in aqueous environmental matrices by a process known as mineralization. Online, kinetic monitoring of this environmentally significant reaction has been reported in a study on the mechanism of action of Fenton’s reagent. The reagent produces in situ hydroxyl radicals that oxidize substituted aromatic compounds, as shown in Figure 10.38 Chlorobenzene is sequentially oxidized, forming the intermediate products chlorophenol, chlorohydroxyquinone, and chloroquinone. The rate of oxidation was found to follow the order C6H5Cl > C6H5Br > C6H6 > C6H5CH3 > C6H5OCH3 > C6H5NO2 > C6H5OH. The underlying electronic substituent effect was confirmed by a good linear correlation between the ratio of the rate constant (kx) relative to that of a reference compound, benzene (rate constant kH), and the Hammett substituent parameter. In a related work with Fenton’s reagent, phenol and trichloroethylene were mineralized to CO2, using three separate photocatalytic processes. The efficiencies of UV-catalyzed photooxidation using TiO2, Fenton’s reagent and ferrioxalate/H2O2 were calculated by comparing the decrease in concentration of phenol and dichloroethylene with the corresponding increase in concentration of CO2. Aryl methyl ester photolyses, such as the photolysis of benzyl acetate and 3,5-dimethoxybenzyl acetate in aqueous and aqueous-methanolic solution, have been shown to occur by two competing mechanisms. The first mechanism proceeds via homolytic cleavage with radical recombination to yield bibenzyl and ethylbenzene. The competitive process is a heterolytic cleavage reaction in which out-of-cage recombination occurs to yield benzyl methyl ether (methanol solvolysis) and benzyl alcohol (solvolysis by water). MIMS was applied to quantify in situ production and to provide information relevant to the mechanism. An industrially significant compound in the polymer industry, epichlorohydrin, is known for its reactivity under acidic conditions. The reaction of HCl with epichlorohydrin in water to produce 3-chloro-1,2-propanediol and 1,3-dichloro-2-propanol

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Figure 10 Online monitoring of the chemical degradation of chlorobenzene by Fenton’s reagent (Fe2þ/H2O2). The top panels show the reactions involved, and the plot shows the time variation of the abundances of characteristic ions recorded by MIMS while the reaction was monitored online. Intermediate products are chlorophenol (m/z 128), chlorohydroquinone (m/z 144), and chloroquinone (m/z 142). Reprinted with permission from Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. J. Phys. Chem. A 1998, 102, 10723–10727. Copyright 1998 American Chemical Society.

(or 2,3-dichloro-1-propanol) was monitored by MIMS. Confirmation of these reaction products was achieved through additional experiments with HBr, which leads to analogous bromide-addition products, and with HClO4, which favors the net addition of water to the oxirane. This experiment was the first application of a liquid membrane to reaction monitoring.39 A cross-linked silicone and a Teflon membrane were also used to monitor epichlorohydrin; both membranes gave similar response times and low ppm sensitivities. Other examples of monitoring of aqueous phase reactions are measurement of gas-exchange rates in the Belousov–Zhabotinskii reaction,40 online monitoring of reactions of epichlorohydrin,41 studies of photocatalytic reactions of phenol and trichloroethylene,42 and benzyl acetate and 3,5-dimethoxybenzyl acetate.43

2.24.3.2.3

Underwater MIMS

Traditional methods for chemical analyses of water bodies, such as oceans, lakes, and rivers, have typically involved collection of water samples and delivery to a laboratory for analysis, in many cases by mass spectrometry. Inherent in this practice is the possibility of contamination of the samples, or loss of analytes by atmosphere exchange or some other means. This is particularly true for highly reactive and volatile species. Degradation of collected samples is actually a problem for any type of environmental or field sampling strategy. For this reason and others, there has been a significant amount of effort over the last 20 years or so to create field-portable instrumentation, including mass spectrometers. This development effort has been enabled in part by ruggedization of vacuum pumping components, such as turbomolecular/molecular drag pumps and small-diaphragm roughing pumps, as well as advances in computer technology. Many of these portable mass spectrometer systems use membrane inlets to minimize vacuum pumping requirements, and therefore size and power consumption as well. Even if discrete samples are collected for analysis, taking the mass spectrometer to a field site helps to significantly mitigate sample contamination and loss of analytes by reducing

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turnaround time between sample collection and analysis. On-site analysis improves sample throughput times and provides the possibility of adaptive sampling, whereby intelligent decisions can be made in near real-time regarding where subsequent samples should be collected, rather than merely following a set sampling scheme. This capability can possibly allow gradients in chemical concentrations to be followed, e.g., to track environmental pollutants to a source. Despite the improvements gained by analyzing collected samples on site, there is still a limitation on the number of discrete samples that can be processed in a reasonable amount of time. Therefore, it is desirable to have a method that provides continuous real-time analyses in order to have sampling densities and frequencies sufficient to properly characterize and resolve very heterogeneous chemical distributions, as well as capture highly transient chemical events and signatures. MIMS is an ideal technique for continuous analysis, and, with the proper sampling interface, an MIMS instrument can obviate the need for collection of discrete samples. An example of this concept is a shipboard MIMS system that pumps water from near the surface while a research vessel (R/V) is underway. An instrument of this type was used to measure dissolved gas and dimethyl sulfide (DMS) concentrations with high spatial resolution transects in the subarctic Pacific Ocean.44 Although the availability of shipboard systems that can perform continuous real-time measurements represents a significant advancement for chemical oceanography and related fields, it is often not practical to pump water from extreme depths (in particular, for analysis of volatile compounds, which may degas as the hydrostatic pressure decreases as the sample approaches the surface). Consequently, shipboard analyses are primarily limited to surface and near-surface waters, which excludes the bulk of the ocean water. There is a desire and need, however, to study chemical phenomena at depth in oceans and lakes for basic scientific purposes, as well as more practical reasons as the recent Deepwater Horizon oil spill in the Gulf of Mexico has demonstrated. As a consequence, since the late 1990s, several groups have undertaken the challenge of developing underwater mass spectrometers (UMSs) that can perform continuous in situ analysis in aqueous environments.45,46 To date, all of these UMS systems have used membrane inlets of various types. In order to perform in-water analysis, all mass spectrometer components in a UMS are contained inside a watertight pressure housing. Typically, components that generate a significant amount of heat (e.g., vacuum pumps and electronics) are ‘heat-sinked’ to the pressure housing walls in order to dissipate heat into the naturally cooling underwater environment. The membrane interface is usually the only component of the mass spectrometer that is exposed to the water column. Since pressure increases approximately 1 atmosphere (bar) for every 10-m depth, both the pressure housing and membrane interface must be designed to withstand the increased hydrostatic pressure at depth. Consequently, the membrane material (which is often an elastic polymer) is normally mechanically supported. Often, a porous metal frit is used to provide support against the high-pressure differential between the water column and the mass spectrometer vacuum, and still allows permeates from the membrane to pass into the ion source. The elastic membrane compresses under increased hydrostatic pressure (which may affect transmission, see Calibration section), but does not rupture if properly supported. Figure 11 shows a design of a variation on the flow-over type membrane interface that has been used for analysis down to 1500-m depth.

Figure 11 Schematic representation of a flow-over deep-water membrane interface for an underwater mass spectrometer. Calibration of an in situ membrane inlet mass spectrometer for measurements of dissolved gases and volatile organics in seawater. Reprinted with permission from Bell, R. J.; Short, R. T.; van Amerom, F. H. W.; Byrne, R. H. Environ. Sci. Technol. 2007, 41, 8123–8128 (supporting info). Copyright 2007 American Chemical Society.

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UMSs are normally designed to be completely autonomous if needed, but can be operated in a tethered mode for communication and if longer-term power is needed. An embedded computer or microprocessor is used to control mass spectrometer functions, as well as collect and store data. The possibility for real-time communication often exists through a hardwired connection, wireless system, or acoustic modem. Communication can be either high bandwidth or at serial data rates depending on the type of connection and requirements for the experiments. The duration of a deployment is normally limited by battery lifetime when a continuous power source is not available. Typical power requirements for a UMS are 30–150 W, depending on the particular instrument and mode of operation; so, commercially available batteries can often be used for deployments up to around 8 h. If continuous power is provided through a tether, then other parameters may limit deployment durations. The UMS pressure housing is a closed system and the only interface the UMS has with the water column is the membrane inlet, which acts as a slow leak into the UMS vacuum system. If mechanical vacuum pumps (e.g., turbomolecular and diaphragm) are used, then the exhaust is normally contained inside the pressure housing, or within a dedicated volume within the pressure housing. Since the main gas load on the vacuum system is water vapor, a desiccant inside the pressure housing can extend deployment times. Ultimately, the pressure inside this volume will increase to a level that inhibits vacuum pumping operation. If entrainment pumps (e.g., ion getter pumps) are used, these need to periodically be regenerated to maintain efficient pumping operation. With either of these approaches, however, deployments of weeks to months are possible. Biofouling of the sampling interface will likely become the factor that ultimately determines the achievable length of deployment. Drift in mass spectrometer performance may also limit deployment time, if precise calibration is critical for the long-term measurements. Both of these issues are highly variable, depending on the environment and particular instrumentation used, and further discussion is beyond the scope of this chapter. Several types of mass analyzers have been used as the basis for UMS systems. The most common are small linear quadrupole mass filters, such as those used in residual gas analyzers.45,47 But other types of analyzers have also been used. For example, other groups have used small magnetic sector-based cycloidal mass spectrometers,46,48 and two groups have constructed and used a UMS based on an ion trap mass spectrometer with the same design.45 Since the membrane interface is typically impermeable for analyte masses greater than 300 Da, the mass range for these analyzers is a good match for MIMS analyses. In addition, they all are relatively small, portable, and consume fairly low power. All of these underwater instruments have employed electron impact ionization using a hot filament as the electron source, which is one of the components that is most likely to fail during deployment. Some analyzers have two filaments for redundancy, which saves valuable time in the field if one filament were to fail. In order to quantify in situ MIMS measurements using a UMS, the sampling and environmental parameters must be carefully controlled or measured. The physical parameters of the water being sampled should be measured in order to properly interpret the MIMS results. A conductivity, temperature, and depth (CTD) sensor is typically deployed in conjunction with the UMS to measure ambient water salinity, temperature, and pressure. As noted above, increased hydrostatic pressure will likely compress the membrane and change its permeation properties (see Calibration section); thus, it is important to record the pressure during deployments. It is also important to control or measure the sample flow rate past the membrane interface, in order to understand or control the boundary layer conditions at the membrane interface. A non-negligible analyte-depleted boundary layer forms in the water sample at the membrane interface under laminar and low-flow conditions. It is often difficult to measure the flow rate at a membrane interface that is simply exposed to the ambient water column; so, the easiest way to control the boundary layer is to use a sample pumping system to achieve a steady flow. It is also very important to measure or control the water temperature at the membrane interface, since permeation rates through most membranes are temperature dependent. If a sample pumping system is used, and a relatively moderate flow rate (~10–20 ml min
1) is used, then heater cartridges and a feedback controller can be used to maintain a constant temperature of the water and membrane regardless of the ambient temperature of the water being sampled. This makes calibration of the MIMS measurements easier, but if temperature control is not practical (e.g., if very large flow rates are used) then a measurement of the water temperature should allow corrections of the MIMS results. UMS systems have been deployed on a variety of platforms for a wide range of applications. A common oceanographic procedure is to perform vertical profiles from an R/V at fixed locations in the ocean. Traditionally, a CTD is used for continuous physical measurements, and water samples are collected at discrete predetermined depths. The use of a UMS for vertical profiles provides continuous measurements of dissolved gas and volatile organic concentrations in parallel with the standard CTD measurements, thereby providing high spatial resolution chemical data in conjunction with the physical data.49 Figure 12 shows a UMS being deployed from an R/V for measurements of these types and examples of the high-resolution MIMS profile data obtained. For shortduration vertical profiles (<2 h), UMS power is typically provided by batteries on the profiling frame (in a separate pressure housing) and serial communication is established through the deployment sea cable using in-water and shipboard modems. Other deployment schemes have included long-term (up to 1 month) deployments on the seafloor for time-series measurements (power provided by a tethered source), and shorter-term deployments (<2 days) on autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), and deep-water or shallow-water towed platforms, for investigations of distributions of volatile organic compounds (VOCs), light hydrocarbons, and dissolved gases. UMS systems have also been used on scuba diver deployable frames, e.g., for investigations of shallow-water seeps. See Table 7 for examples of UMS deployments in lakes, rivers, and oceans. Permeation of analytes through a membrane interface is not instantaneous, and this effect can distort interpretation of the MIMS analyses using a portable UMS. If the permeation time for an analyte is longer than the time scale that analyte concentration is exposed to the UMS, then the UMS will not faithfully capture the event. The UMS can respond to changes in dissolved gas concentrations in 5–10 s, but volatile organic compounds can take several minutes to permeate through the membrane. For time-series measurements in a fixed location, these permeation times usually do not present a problem. A UMS that is rapidly moving through the water, however, can encounter changes in chemical concentration that are more rapid than the response time of

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Figure 12 Vertical profiling using a UMS. (a) High-resolution vertical profiles of dissolved gases: (black) MIMS data; (blue) calculated saturation profiles; (red) DO sensor data. (b) Photograph of UMS being deployed from an R/V along with a CTD, dissolved oxygen (DO) sensor and battery pack.

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Table 7

Examples of UMS deployments and applications

Deployment platform

Application

Unmanned surface vehicle Remotely operated vehicle (ROV) Autonomous underwater vehicle (AUV)

2-dimensional mapping of chemical distributions in lakes and rivers Investigations of vents in Lake Yellowstone Subsurface plumes of light hydrocarbons Study of lake water chemistry 2-dimensional mapping of shallow hydrocarbon seeps Mapping of subsurface plume signatures Vertical concentration profiles in the ocean Gas concentrations emanating from shallow seeps Time-series measurements

Surface/shallow tow platform Deep-tow platform Vertical profiling frame Neutrally buoyant frame for diver deployment Mooring

the membrane interface system. In other words, a slow time response due to membrane permeation limits the spatial resolution of a moving UMS. This situation is encountered at times for vertical profiling experiments, in which the descent and ascent rate of the UMS is typically 0.5 m s
1. It becomes an even more serious issue for AUVs and towed platforms in which UMS speed is often 1.5–3 m s
1. Methods are being developed to mathematically correct UMS data to account for membrane permeation response times by deconvoluting the membrane response function, and have been shown to help in some cases to correct field data for plume characterization that were distorted by membrane response times.

2.24.3.2.4

Other online/on-site Applications

One of the most interesting applications of MIMS is on-site and online monitoring of environmentally significant compounds from water. A custom-made MIMS apparatus to be used in a mobile laboratory has been built for on-site measurements,50 a specialized MIMS system has been designed for online measurement of VOCs of river water in a boat,51 and a comprehensive kit for analytical field work utilizing a GC/MS instrument equipped with a membrane inlet has been constructed.52 Other interesting instrumental developments have, e.g., been the development of a cryotrap-MIMS for measurement of VOCs at low pptr levels53 and demonstration that polar organic compounds can be measured at ppb levels using a microporous membrane combined with glow discharge ionization.17 Some nonconventional MIMS applications published are measurement of volatile active principles in tisanes of vegetable drugs,54 studies of aroma fraction of wines,55 and classification of cola beverages.56 The latter study clearly demonstrated that MIMS is an excellent alternative or complementary method to the electronic nose sensors used for classification of foodstuffs. Real-time or near-real-time analytical techniques allow the full characterization of environmental chemical reactions or waste streams. A single analyzer incorporating a silicone membrane fiber and a quadrupole MS system was utilized for the analysis of gas streams from a wastewater-treatment biodegradation process.37 Twelve compounds, including chlorinated compounds and substituted benzenes, were detected in air effluent at concentrations ranging from 0.003 to 0.07 mg l
1. Detection of organic vapor emissions in the air effluent tracked the influx of solids in the wastewater stream. Online monitoring and quantification can be performed by injecting aliquots of the sample mixture into the continuous water stream supplied by a peristaltic pump. The FIA technique reduces the amount of sample used. A simple FIA-related system was developed to allow pH adjustment of the sample solution online before MIMS measurement.57 For example, acetic acid, which cannot be analyzed at pH 6, can be easily measured with MIMS when the pH is lowered below 2. In the FIA-MIMS system the membrane is exposed to the sample for a short period, i.e., sample flow is interrupted by pure water before steady-state permeation is reached. The height of the FIA peak can be used for quantification because the total flux through the membrane at any time is linearly dependent on the sample concentration in the feed liquid. Due to short sampling times, the FIA-MIMS technique is a very rapid method for analyzing organic compounds in aqueous solutions, and it can be automated very easily. Harland and Nicholson32 utilized MIMS for continuous monitoring of volatile compounds in canal and estuarine water. A transportable MIMS system was constructed and mounted in a boat, and measurements were done continuously while the boat was traveling along the canal. MIMS has also been used on-site for analysis of groundwater and during wastewater monitoring. Figure 13 presents results of continuous online monitoring of a wastewater stream of an oil refinery,58 clearly showing the advantage of MIMS in online measurements when the compounds are well known beforehand. A hollow fiber silicone membrane with a metal jet separator and a quadrupole ion trap mass spectrometer has been used for monitoring TiO2 photocatalyzed destruction of aqueous environmental compounds at pptr levels.42 A continuous monitoring of an aqueous sample containing 244 pptr of toluene for 3 h showed loss of the toluene signal, while a signal of the photocatalyzed reaction product (methylphenol) appeared. Later in the experiment, also the signal of methylphenol decreased as it reacted to form further degradation products. This study showed that the formation of both intermediate and final reaction products can be monitored by MIMS.

2.24.3.3

Biological Matrices

MIMS is very suitable for the measurement of dissolved gases and organic compounds from biological matrices, such as blood, bone, urine, saliva, stomach fluid, and plants. The main advantage of the MIMS technique in these measurements is that,

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Figure 13 Measurements made over an 18-day period of the wastewater stream of an oil refinery. The ions measured were m/z 78 for benzene, m/z 92 for toluene, and m/z 120 for cumene (isopropylbenzene). The results show daily variations in concentrations of these compounds, which correlated well with the offline gas chromatographic measurements, which were performed once a day. Reprinted with permission from Kotiaho, T.; Kostiainen, R.; Ketola, R. A.; Mansikka, T.; Mattila, I.; Komppa, V.; Honkanen, T.; Wickstro¨m, K.; Waldvogel, J.; Pilvio¨, O. Development of a fully automatic membrane inlet mass spectrometric measurement system for on-line industrial waste water monitoring. Process Contr. Qual. 1998, 11, 71. Copyright 1998 VSP International Science Publishers.

typically, no sample treatment is needed or the need is minimal. As a few examples, oxygen (O2) and carbon dioxide (CO2) were measured in vivo from circulating blood of animals already in 1966 and O2 was measured in vivo and in vitro from rabbit blood. Partial pressures of O2 and CO2 have been measured from bone tissue, and haloethanes from dog blood using a stainless steel intra-arterial cannula; VOCs have been measured from rat blood, and oxygen from tissue samples of rabbits, pigs, and humans; NO has been measured with a special miniaturized membrane inlet from human plasma samples; and an MIMS technique was developed to measure ventilation-to-perfusion ratios for several gases (SF6, Kr, desfluorane, enflurane, diethyl ether, and acetone). In most of the above-mentioned applications, a small cannula (e.g., i.d. 0.4 mm, o.d. 1.1 mm, length 1 m) type of membrane inlet was used for the monitoring of gases and the membrane often used was a silicone membrane, even though other polymers can also be applied (e.g., polyethylene and Teflon). There are some parameters, which have to be taken into account when measurements from biological samples are made. The membrane probe should be designed and constructed in a way that the signal is independent from the sample flow rate or a sample stirring rate. To minimize the effect of the flow or stirring rate, the membrane area should be smaller than the area available for diffusion in the liquid sample (the inlet should be larger than the membrane itself) or a proper mixing should be used. In most biological MIMS applications, small sample volumes are used for in vivo measurements; therefore, small membrane areas are suitable, but in some cases a membrane with lower permeability has alternatively been selected (e.g., Teflon instead of silicon). The small sampling area ensures that the response of the MIMS probe is insensitive to changes in gas solubility in the sample. This is important since gas solubilities in blood are known to vary from sample to sample. Only a slight dependence on gas solubility was observed with a stainless steel probe (i.d. 1 mm, o.d. 1.6 mm, length 220 mm) with a silicone membrane when comparing the responses of the gases in water, rabbit blood, and 20% intralipid solution. Reliability of the oxygen measurements made by MIMS has been demonstrated, e.g., a very good correlation (correlation coefficient of 0.997) was shown between in vivo MIMS measurement and in vitro oxygen electrode measurement. It was also noticed that the measurement cannulas are reusable and stable at least for half a year and that argon signal can be used as an indicator of the reliability of the measurements. MIMS has also been used to measure a human toluene metabolite, p-cresol, in urine, using hydrolysis of the urine sample, extraction with an organic solvent, and reconstitution into 1-propanol prior to MIMS measurement.59 Similarly, a trap-and-release MIMS method was used to quantify total cysteine and homocysteine in human plasma, after a sample pretreatment including disulfide bond reduction with dithiothreitol, protein precipitation with trichloroacetic acid, and derivatization with ethyl chloroformate.60 In this way, an LOD of 2 mM for cysteine was obtained, which was well below the mean concentration in plasma. The time persistence of monochloramine in human saliva and stomach fluid was measured using a direct insertion membrane probe and continuously monitoring the decay of monochloramine signal when an aliquot of a saliva or stomach fluid sample was added to the monochloramine solution circulating through the membrane probe. In vivo monitoring of gases in plants (e.g., wheat, corn, sugar beet, and reed) has also been performed with MIMS using the same type of small cannula or a capillary type of membrane probe as used for blood gas analysis. If possible, the membrane probe should be inserted into a cavity in the plant (i.e., in the gas phase); therefore, the stirring effect observable in the liquid-phase measurements is lacking.

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A special characteristic of the use of MIMS in anesthesiology is the need for high spatial resolution; therefore, an MIMS system using a micropore membrane probe with 30-mm spatial resolution has been developed. Nitric oxide (NO) concentrations in blood plasma have physiological significance, and are readily monitored by MIMS. A small membrane inlet allows one to monitor 1–60-mM NO concentrations in sample volumes as small as 10–300 ml.61 In situ NO concentrations, measured during its binding to/release from plasma proteins, indicate that the technique could be useful for providing diagnostic physiological data. MIMS with a measuring cell is applicable to in situ monitoring of volatile products from incubation medium. For example, when rat hepatocytes were incubated with methylseleninic acid, volatile reaction products or metabolites, such as dimethyl didelenide, were directly detected from the medium. In this study, a measuring cell of 6 ml and a silicone membrane with a thickness of 125 mm were used with a single quadrupole instrument using an EI source. The hepatocyte medium gave very little background in the measurement, thus enabling sensitive analysis of reaction products.

2.24.3.4

Organic Matrices

In most MIMS applications, organic compounds are measured from aqueous or air samples, but water or organic compounds can be measured also from an organic matrix with MIMS, using reversed-phase membrane inlet mass spectrometry (RP-MIMS). RP-MIMS has been performed with many different membranes such as polyethylene terephthalate, polyimide, silicone, hydrophilic Nafion, microporous polypropylene, microporous polyvinylidene fluoride, microporous cellulose, and microporous polyether sulfone. In general, the performance characteristics of RP-MIMS are similar to those of normal-phase MIMS and no special sample treatment methods prior to the MIMS measurement are needed. However, when microporous membranes are used the mass spectrometer itself has to tolerate higher pressure, due to a much higher flow of molecules into the mass spectrometer. In the first application of RP-MIMS, a hydrophilic polyethylene terephthalate membrane was used to measure water activity in organic solvents. With the terephthalate membrane, the permeation of organic molecules was restricted and only water passed through it; therefore, the amount of dissolved water in organic solvents could be directly detected at ppm (parts per million) levels. In one example, the rate of hydrolysis of diphenyl carbonate by porcine liver esterase as a function of water activity in di-isopropyl ether was monitored online. The restrictive nature of the polyethylene terephthalate membrane made it possible to replace the mass spectrometer with a simple vacuum gauge to create a very simple method for water analysis in organic liquids. RP-MIMS, using a hydrophilic Nafion membrane, demonstrated a linear detection of low-molecular-weight alcohols in chloroform at concentration levels of 0.1–2.5%. The RP-MIMS system was used for online monitoring of the alcohols during distillation of chloroform and the online RP-MIMS results were found to agree well with offline GC/MS data. There have also been applications that can be considered as RP-MIMS, in which a silicone membrane was used. For example, styrene and tetrachloroethene residues were detected directly from olive oil and Kathon CG (an antimicrobial agent) was measured from cosmetic emulsions. In addition, it has been demonstrated that a thin (25 mm) silicone membrane with a supporting matrix can be used for RP-MIMS. As mentioned above, the requirements for the mass spectrometer are more demanding when microporous membranes are used in RP-MIMS, because of the 100- to 1000-fold increase in the number of molecules that enter the instrument. This problem can be solved by using chemical ionization (CI), where the pressure in the ion source is typically 1000 times higher than that in the electron ionization source. In this system, a missing enrichment of the sample is compensated by the higher sample flux into the mass spectrometer and in its most advanced form the solvent itself can be used as reagent gas in the CI process. High selectivity in this type of RP-MIMS can be obtained if tandem mass spectrometry is used and even highly complex matrices, such as gasoline, can be analyzed directly. An RP-MIMS method based on a microporous polyvinylidene fluoride membrane has been developed for real-time monitoring of pharmaceutical processes, the Michael addition reaction of phenylethylamine being the model reaction in the study. In this experiment, the microporous membrane was used to transfer analytes of interest from an ethanol solution to a water–acetonitrile solution, with subsequent analysis by atmospheric pressure chemical ionization (APCI) mass spectrometry. Four membrane materials (cellulose, polyether sulfone, microporous polypropylene, and silicone membrane) were characterized for their potential use for measurement of organic compounds from air, water, and organic solvents using a miniaturized mass spectrometer. With the standard silicone membrane, the mini-MIMS apparatus behaved like a standard MIMS system, i.e., VOCs could be easily measured from water. The microporous polypropylene membrane was not useful for any MIMS modes; in the RPMIMS mode, the limiting factor was the very high pressure caused by the solvents. Both cellulose and polyether sulfone membranes were suitable for RP-MIMS, but cellulose gave clearly better performance and organic contaminants and water could be measured in organic solvents at 10–100-ppm levels by weight.

2.24.3.5 2.24.3.5.1

Solid Samples Soil, Sediment, and Peat

MIMS can be applied to soil analysis in two separate fields, namely the analysis of organic compounds, both volatile and semivolatile, and the analysis of atmospheric gases and other gaseous components. Commonly, in the analysis of organic compounds, the soil sample is transferred to a vessel that is attached to the mass spectrometer in various ways, whereas the gas analysis is performed using a membrane probe that is inserted directly into the soil. The latter technique works very well with gas analysis, since the gases do not stick to the soil and the design prevents interferences from the atmosphere during the sampling step. Organic compounds can bind to the soil and need to be thermally released prior to analysis. The sample vessels used for analysis of organics

Membrane Inlets for Mass Spectrometry

Table 8

523

Common applications of membrane introduction mass spectrometry in soil analysis

Compound

Method

Permanent gases N2, O2, Ar Permanent gases N2, N2O, O2 Permanent gases N2O, O2, Ar, CO2 Permanent gases CH4, CO2, O2

Capillary

Dissolved gases in sediment core water

Inlet probe

Dissolved gases in sediment cores

PTFE (30 mm) on silicone

Dissolved gases in sediment core water

Inlet probe Inlet probe Inlet probe Inlet probe Custom sheet membrane inlet

1 ppb

Custom sheet membrane inlet Hollow fiber, ion trap MS Sheet

1 ppb

Sheet

50–100 ppt

Dissolved gases in peat cores Dissolved gases in freshwater sediment and peat cores Gases in peat cores Dissolved gases in peat cores Purge-and-membrane technique, commercial and authentic soil samples; moisture content effect preheating, desorption temp, MeOH, purge gas effects Improved version of purge-and-membrane device Proof-of-principle, spiked soil sample Soil, water: purge-and-membrane technique, ‘authentic soil sample,’ LODs below guidelines Soil, dry or wet solids: headspace technique

VOCs

Detection limit

Notes

Copyright (2002) Wiley. Adapted with permission from Ketola, R. A.; Kotiaho, T.; Cisper, M. E.; Allen, T. M. J. Mass Spectrom., John Wiley & Sons Limited.

in soil samples are specially made for the different techniques and they normally operate with small sample amounts (<10 g). Table 8 summarizes the applications of MIMS in soil analysis. One of the first examples of soil analysis involving MIMS was the online detection of 1 ppb of toluene spiked in air, water, and soil samples.14 The experiment demonstrated the potential of using MIMS for the rapid and direct determination of VOCs in different matrices, including soils. Since that time, improvements and alterations have been made to membrane introduction methods for soil analysis. In situ soil, peat, and sediment techniques provide environmental and ecological data related to gas exchange and bacterial growth without disturbing natural processes. VOCs, such as terpenes, can also be measured directly from soil atmosphere (microair) with a membrane probe. With this system, sample collection is avoided, but it is uncertain from how large a volume sample is collected and possible matrix effects are not well understood. However, good correlation between an EPA method 8260 and the penetrometer MIMS method was observed for some samples (r2 ¼ 0.9). In another study, with a similar membrane probe, monoterpenes and monoterpenealcohols were detected and identified from forest soil atmosphere.62 The LODs for monoterpenes and monoterpenealcohols were approximately 10 mg m
3. The method was compared to a chamber–GC/FID method and the results show that the chamber–GC/FID method gave higher concentrations, most probably due to more efficient sampling of terpenes from the soil atmosphere with a closed chamber. Therefore, the chamber method could give the total concentrations of terpenes (free and adsorbed fractions), whereas the MIMS method could give the concentrations of free terpenes in the soil microair. An MIMS technique called purge-and-membrane mass spectrometry (PAM/MS) utilizes both a headspace and an MIMS technique for the analysis of soil samples.63 In this technique, a soil sample is mounted to a sample vessel and after a certain sample preheating time (e.g., 10 min at 80  C) a four-port valve is switched to the sampling position and simultaneous sampling line needles are punctured through the sample vessel septa and finally the purge gas with the desorbed VOCs is directed via the membrane inlet into the mass spectrometer for analysis. Figure 14 shows in detail the apparatus used in the PAM/MS technique; all the materials used in the device were selected so that possible memory effects would be minimized.64 One of the main points studied in the PAM/MS method development was the effect of soil type and the moisture content on the signal observed. Some general observations were: (1) desorption times increased as the organic matter content was increased; (2) desorption times normally decreased as the water content was increased – however, for sand samples moisture content did not have much effect; (3) the highest peak areas were observed for dry sand and lowest for dry garden soil; and (4) peak areas for sand and garden soil at moisture content of 10 and 20%, respectively, were about the same. As a conclusion, it was stated that the results are essentially independent of soil type, especially if the moisture is higher than 10% as is normally for authentic samples. This means that quantification can be done using only one standard soil sample. With a headspace (HS) MIMS method, responses at the same level are obtained for different types of soil samples if the moisture content is adjusted to 25% level. Both ex situ techniques (PAM/MS and HS-MIMS) are characterized by short analysis times, no pretreatment of samples, high sensitivity and selectivity, and elimination of solvent extraction. The reliability of the PAM/MS method was demonstrated by comparing results obtained for 79 samples made by a vapor fortification method and 35 authentic samples to those measured with a headspace gas chromatographic method. The results obtained with the different methods are in relatively good agreement (coefficient of regression (r) is 0.953). Depending on the sample and analyte type, the PAM-MS method allows analysis of chlorinated and aromatic solvents with detection limits at the level of mg kg
1 and with linearities of 3–4 orders of magnitude and with relative standard deviations of 5–10%. The internal standard’s

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Figure 14 A schematic picture of the purge-and-membrane mass spectrometry device. Reprinted with permission from Ojala, M.; Mattila, I.; Tarkiainen, V.; Sa¨rme, T.; Ketola, R. A.; Ma¨a¨tta¨nen, A.; Kostiainen, R.; Kotiaho, T. Anal. Chem. 2001, 73, 3624–3631. Copyright 2001 American Chemical Society.

more rapid signal decline compared with that of the VOCs is an indication that it was less tightly bound to the matrix than the contamination. Another similar technique is a hot cell MIMS which is a modification of a measuring cell inlet.65 In this technique, any solid sample is mounted into a cell, which can be heated up to 150–200  C, thus releasing volatile and semivolatile components from the sample. This technique was successfully applied to the detection of PAHs from contaminated sand and degradation products of 2,4dichlorophenoxyacetic acid and 2,4,5-tripchlorohenoxyacetic acid from contaminated soil. The hot cell MIMS was attached to a miniaturized mass spectrometer, thus producing a hand-held instrument for on-site applications. Measurement of gases from soil samples and especially measurement of dissolved gases from sediments and peat samples to produce information about the biological and microbiological processes (e.g., denitrification and characteristics of life in the soil and of the quality of the soil) are easily achieved with MIMS. Concentration profiles of dissolved gases indicate the point of detection rather than the site of production, and are subject to change with upward and lateral diffusion. Gases, which are easily measurable with MIMS, are, e.g., methane (CH4), N2, N2O, O2, and CO2. One difficulty while measuring these gases is that there can be overlapping m/z values in the mass spectra, e.g., methane is measured using a fragment ion at m/z 15 and not at m/z 16, due to overlapping oxygen signal. The possible interference caused by water vapor and carbon dioxide can be prevented by using a liquid nitrogen trap. In addition, in many cases, when measuring metabolically significant gases it is better to estimate the amount of a particular gas in relation to a chemically and biologically inert standard such as argon. Most of the gas measurement studies are performed with membrane inlet probes, which can be directly inserted into the samples. For example, gas concentrations can be measured at 1-mM levels with spatial resolution of the order of 1 mm using a miniprobe, which has a 4-cm piece of a 0.3-mm (o.d.) hypodermic needle with a 0.07-mm hole covered with silicone rubber membrane at the end of a 1-m-long stainless steel tubing. A very important advantage of the MIMS technique compared to microelectrodes used for gas measurements, even though MIMS does not provide the same spatial resolution, is the possibility for simultaneous measurement of multiple species and to measure gases for which no electrode method exists. An additional advantage of the MIMS method is the robustness, which is needed since sediment or peat samples are often firm and contain many roots. A membrane probe does not break easily when moved in the sample. An example of the measured gas concentration in a peat sample is shown in Figure 15.66 A relatively good agreement between the oxygen signal measured by the mass spectrometric method and the oxygen electrode method was observed. A spatial resolution of 1 mm (vertical) has been demonstrated in the determination of dissolved O2 and CO2. Needles capped with silicone served as in situ membrane inlets for quadrupole mass filters when monitoring dissolved gases (e.g., N2, O2, CO2, Ar, CH4, and N2O) in peat and sediment samples.

2.24.3.5.2

Other Solid Materials

Solid samples mounted in various forms of vessels attached to an MIMS system have formed the basis for many interesting studies. For example, volatile pyrolysis products have been monitored from Kraton 1107 (an isoprene–styrene co-polymer), peptides, pathogenic bacteria, and even free radicals from aromatic diazoamino compounds. Total organic contamination on a silicon wafer can be detected by using a heated chamber, a pyrolyzing carrier gas containing oxygen and online MIMS. The volatile organic compounds can be directly purged at lower temperatures from the surface of the wafer into the carrier gas, which is directly analyzed by online mass spectrometry. Semivolatile and nonvolatile organic contamination can be converted into volatile species using

Membrane Inlets for Mass Spectrometry

525

Figure 15 Dissolved gas profiles (CH4 and O2) for a peat sample measured using a mass spectrometer equipped with a miniprobe and with an oxygen electrode; CH4 mass spectrometer (:), O2 mass spectrometer (-), O2 electrode (C). Reprinted from J. Microbiol. Methods, 24, Thomas, K. L.; Price, D.; Lloyd, D. A comparison of different methods for the measurement of dissolved gas gradients in waterlogged peat cores, 191–198, Copyright (1995), with permission from Elsevier.

pyrolysis in oxygen-containing atmosphere (synthetic air) and subsequently analyzed by mass spectrometry. Typical stripping solutions and photoresists used in silicon wafer production contain both volatile and nonvolatile compounds; thus, one analytical method has usually not been enough for the detection of both these components from a single wafer as a whole. The presented analytical setup is very sensitive, and, at the same time, it is selective as individual components can be detected and identified according to their mass spectra. No special sample handling is needed because a single silicon wafer can be measured directly. A temperature of 600–700  C and an oxygen-containing atmosphere are needed for the complete pyrolysis of all nonvolatile components in order to identify photoresists reliably as a source of contamination. This analytical instrumentation can be used for daily quality control and troubleshooting in wafer production. The PAM/MS method was successfully used to detect residual solvents in pharmaceutical products. The dry solid pharmaceutical material was mounted inside the PAM sample vessel for the analysis, and solvents such as benzene, toluene, and chloroform could be analyzed at around 0.1-mg kg
1 levels.67 When combined with a custom-made data interpretation program, the PAM/MS identified solvents almost as well as purge-and-trap GC/MS. Water slurries of birth control pills have been analyzed for their content of steroid hormones at ppb levels using desorption chemical ionization MIMS.68 Hot cell MIMS was shown to detect phthalates and other additives from polymers and flavor compounds from tea leaves.69 In each case, the sample did not need any sample treatment, and analytical turnover was eight to ten samples per hour. Furthermore, selectivity could be enhanced by varying the hot cell temperature or monitoring the time trend of individual ions. The hot cell MIMS technique can also be applied for direct detection of common ingredients, such as antidepressives and antihistamines in tablets.70 Simply, the tablet is placed as such into the hot cell and after a few minutes a mass spectrum is acquired. The analytes are identified from the mass spectra measured. The other chemicals, such as fillers and polymer coating, did not interfere with the detection of analytes. The hot cell MIMS system is very simple and small; thus, it is easily adaptable to portable instruments. In a similar way, the hot cell MIMS can be applied for the analysis of semivolatile pharmaceuticals and environmental pollutants from organic microextracts.71 Permeation properties of several membranes has been efficiently studied by using MIMS. In these studies, MIMS provided fundamental knowledge of permeation parameters, such as diffusion coefficients and relative permeation coefficients for organic compounds diffusing through different types of membrane materials.

2.24.3.6 2.24.3.6.1

Calibration and Quantification Air Samples

Quantitative measurements from air samples are commonly made using external air standards, which are used for preparing a calibration curve. The measurement range or even the linear dynamic range of MIMS is normally large; thus, the standards have to be made in a large concentration range. The calibration is normally performed using either gasbags, static dilution bottles, permeation tubes or different gas calibration devices for VOC or gas analysis, e.g., a gas calibrator. The calibration is normally performed using steady-state conditions, because the response times are short in air analysis and the steady state is reached very rapidly, in contrast to water analysis. Calibrants can also be produced via the online diffusion of an internal standard using permeation tubes as demonstrated by Etzkorn et al.72 In this method, isotopically labeled internal standard was continuously introduced to an MIMS system without the

526

Table 9

Theory of Extraction Techniques

Gaseous sample parameters that may affect MIMS signal intensity or stability

Parameter

Comments

Membrane temperature

l l

Total pressure

l

l

Sample Matrix

l

Unless a active desorption technique is under use, this will be defined by the sample temperature and surrounding materials Increased membrane temperature will shorten response times and but may increase or decrease steady-state analyte permeability Increased total pressure may lead to nonlinear MIMS effects resulting from bulging of the membrane, changes in gas diffusion paths due membrane reseating, reduced membrane permeability due to membrane compression, or increased membrane permeability due to swelling. The balance of these to competing artifacts will dictate the degree and direction of nonlinearity Multivariate statistics may be necessary for multicomponent spectra.

need for offline external calibration. A variation of signal stability of less than 7% was obtained. Continuous infusion of the internal standard can correct the signal fluctuation due to changes in measurement environment. Therefore, the method can be easily deployed in the field. Table 9 shows the main parameters of gaseous samples which can affect MIMS signal intensity or stability and which have to be taken into account when planning MIMS calibration and measurements.

2.24.3.6.2

Aqueous Standards

Calibrating MIMS systems for aqueous sample analysis is more complicated than gas samples for several reasons, most of which are a result of how the sample matrix can affect instrument response. Sample matrix composition and conditions (e.g., a sample flow rate) during calibration should be as close as possible to the expected ones during sample analysis. If the sample matrix conditions are not controlled or otherwise accounted for, systematic and random errors will result. Generally, it is best to regulate as many variables as possible; in this way, factors affecting instrument response are identical during analysis of calibrants and the unknown sample. For sources of error that are not practical to control, the user must be able to account for how these fluctuations impact the instrument response through corrective terms in the calibration mathematics. Corrective terms may be based on directly measured conditions such as temperature fluctuations or based on measurements of internal standards that potentially fluctuate as a result several varied conditions. When analyzing aqueous samples, there are many conditions that may affect instrument response (Table 10). As discussed above, analyte concentration and membrane temperature are obvious factors, but aqueous systems are complicated by several factors. The main complication is the depletion if sample is in contact with the membrane, which produces a boundary depletion layer that reduces the concentration of analyte immediately adjacent to the membrane. This is significant because instrument response becomes dependent on sample flushing rate and turbulence. In strong boundary layer conditions (low or zero flow past the membrane), analytes must diffuse through a thick depletion layer before they are analyzed, if the sample contains particles (e.g., membrane probes in soil) the analyte must diffuse around the particles, creating a dependence on sample porosity and tortuosity.73 Further, the analyte depletion can change sample chemistry. Two examples of this artifact are CO2 depletion in the boundary layer will be replenished if carbonates are present in solution and, similarly, analytes adsorbed to particulates in the sample matrix will be desorbed in the depleted layer. Regardless of the conditions of the MIMS system, instrumental calibrations are dependent on the production of a calibration curve to relate instrument response to analyte concentrations. Calibration curves can be created by the analysis of an unknown sample by MIMS and comparing results to an alternate method such as gas chromatography, UV–Vis, or IR and directly comparing results. This is sometimes a popular route because the alternate method may have a more established calibration methodology. Certified aqueous reference standards are available for some analytes from various organizations for direct analysis by MIMS. Three methods for directly producing aqueous standards for MIMS analysis have been demonstrated for calibration of MIMS instrumentation: (1) an open system under steady-state equilibrium with a headspace, (2) an open system undergoing dynamic exchange with the headspace, or (3) a closed system generated by quantitative additions. In principle, aqueous standards may be produced inline using a flow-through infusion system analogous to the methods outlined by Etzkorn et al.72 Steady-State Open System Standards: Dissolved gas concentrations in equilibrated gas–liquid systems can be predicted with precisions better than 0.4% using well-known gas standards.74 This precision is achieved by flushing the headspace of a temperature-controlled stirred solution of known ionic strength until equilibrium between the two phases is achieved through gas exchange across the system interface. Henry’s law and established functions based on solution temperature and ionic strength are then used to calculate gas concentrations in solution. Since the rate of gas exchange is proportional to the contact area between gas and solution, equilibrium can be sped up through vigorous sparging and equilibration times are significantly improved (i.e., less than 15 min). However, small errors (generally less than 1%) may be introduced by bubble phenomena that increase total gas pressure (1) through surface tension effects at the bubble–solution interface75 and (2) through forced dissolution of gases due to increased hydrostatic pressure on bubbles at the bottom of the sample (i.e., bubble injection). For example, a 10-cm-tall sample will have about 0.01 atm of additional hydrostatic pressure applied to the bottom-most bubbles. Suitable certified gas standards containing specified analyte partial pressures can be obtained in cylinders through several suppliers. Using several tanks containing different partial pressures, one can allow for several data points on the calibration curve. Alternatively, gas partial pressure can be accurately controlled by mixing several gases in real time using mass flow controllers or

Membrane Inlets for Mass Spectrometry

Table 10

Aqueous sample parameters that may affect MIMS signal intensity or stability during analysis

Parameter

Comments

Membrane temperature

l l

Sample temperature

l l

Ionic strength (Dissolved sample matrix)

l l l

Solid sample matrix species

l l

Hydrostatic pressure

l l

Flow rate

l l l

Contributing chemistry

l

Interactions with fluidic handling surfaces

l l

Bubbles

527

l l l

Unless a active desorption technique is under use, this will be defined by the sample temperature Increased membrane temperature will shorten response times and increase steady-state membrane permeability Sample temperature should ideally be regulated, alternatively it can be closely monitored and corrections can be applied to the data Changing sample temperature can change equilibrium constants and therefore may shift equilibria (e.g., CO2 system) Though particles will not clog the membrane pores they do pose a risk to clogging sample flow and abrading components Dissolved ionic species (e.g., salinity) will affect both the gas solubility in solution and the partition coefficient between the water and membrane Membrane swelling species in high concentrations will result nonlinear signals for coexisting analytes During analysis of soils and other solid matrices, boundary condition can become severe and matrix porosity and tortuosity can have a dramatic effect on signal intensity Adsorption of analyte to particles complications determinations. Free analyte vs. total analyte Hydrostatic pressure will result in the compression of the membrane, changing its diffusion and partition coefficients Nonideal elastic rebound will result in hysteresis Flow rate will determine the degree of depletion in the boundary layer by influencing sample replacement rate and degree of turbulence Too high a rate will make thermal regulation difficult and power intensive, while slow flow rate will have significantly reduced signal intensity via boundary layer depletion Strong boundary layers can be impacted by flow rates. Alternative species may contribute to signal intensity via shifts in chemical equilibria. Drivers of these shifts include the removal of the analytical species by the membrane (i.e., a strong boundary layer) and increased sample temperature by thermal regulation (i.e., membrane temperature regulation) Dissimilar metals may undergo galvanic corrosion, affecting sample chemistry Surface interactions can detectably alter parameters like pH or significantly adsorb surfaceactive analytes such as H2S or O2. Sample alteration will be especially severe after switching between chemically dissimilar samples Bubbles in the region of the membrane will result in spurious signals by affecting local flow and diffusion patterns (especially in laminar flow environments). Thermal regulation to an elevated set point will result in bubble formation. Faster flow rates and addition of surfactants will aid the flushing of the bubbles. Formation of volatile species will also result in bubble formation. E.g. acidification of carbonate containing solutions

related mixing techniques.76 In a similar vein, two or more equilibrated aqueous solutions can be mixed inline real time via a well-timed multiposition valve or pumping system capable of accurate volumetric mixing (e.g., an HPLC pump capable of gradient elution) just prior to MIMS analysis49 (Figure 16). Schlüter et al.47 found that a volumetric mixing technique using peristaltic pumps produced the most stable results while avoids the availability issues with calibrated gas mixtures. Open systems under constant equilibrium can be convenient to use because atmospheric contamination is unlikely, provided the headspace is well flushed and the sample tubing is well constructed. Further, users of field-portable instrumentation benefit from this method because there is no need for precision measurement techniques (i.e., volumetric or gravimetric). If sample temperature is stable during equilibrium, and is accurately determined, it does not necessarily need to be controlled in a constant temperature bath. However, when done accurately (i.e., without sparging and in a constant temperature bath), equilibration times can be rather slow, and significant quantities of calibrated gases might be used. This is especially the case for carbon system measurement in circumneutral solutions, where equilibrium is delayed by the rather slow reaction kinetics of the carbonate system.77 In such cases, one of the following two methods is preferred. Dynamic Open System Standards: A dynamic calibration procedure has been developed for the analysis of dissolved gases with MIMS. Yang et al.78 describe a gaseous–aqueous system where complete equilibrium does not have to be attained and therefore calibrations are performed in 10–15 min. The dissolution of a carbon dioxide is a first-order process, as are the subsequent reactions (at constant pH) forming carbonic acid and carbonates. This enables a rather elegant form of standard addition calibration where it is possible to measure the mass signal–concentration relationship, the mass transfer coefficients, and equilibrium of all dissolved species of carbon dioxide. The dynamic calibration was shown to give accurate calibration results for cell respiration rates of a bacterial culture in a small-scale reactor for both oxygen and carbon dioxide.

528

Theory of Extraction Techniques

Figure 16 This is a schematic of a system in which sample concentration is controlled by mixing two or more aqueous solutions inline. Concentrations between the end-member concentrations can be obtained by rapidly switching the multiposition valve with specific duty cycle.

Where Yang et al.78 used the mass transfer of CO2 into solution, Andersen et al.79 have demonstrated a dynamic calibration where the mass transfer of CO2 out of solution is used. The model allows pH to vary drastically and rapidly though the use of a pHdependent term that relates signal response to total dissolved inorganic carbon, CT. The method uses additions of known quantities of NaHCO3 that cause sudden pH-dependent changes in [CO2](aq) that were used to calibrate the instrumentation during the subsequent evaporation into an argon-flushed headspace via mass transfer. Closed System Standards: Common for VOC and semivolatile analyses, external standards can be generated via the addition of known quantities of analyte into a known quantity of sample matrix. This procedure is most accurately done gravimetrically, though volumetric additions are not uncommon, as well as in-line volumetric additions. As most detectable VOCs are not soluble in aqueous solutions, they cannot be added directly to aqueous sample matrices. For this reason, stock solutions are often produced using an intermediate solvent in which the analyte is soluble, but is also miscible with water. Many polar protic solvents (e.g., methanol) are suitable, as well as some polar aprotic solvents (e.g., acetonitrile). The stock solution is then used to initiate serial dilution with aqueous solutions using ratios selected to obtain a target concentration. Extremely dilute concentrations can be obtained through serial dilutions. Due to the volatility of many analytes, these standards should be analyzed promptly. Outside of VOC analysis standards, some ionic salts can be used to produce closed system standards for dissolved gases when added to an appropriate solution. For example, sodium carbonate can be added to aqueous solutions that are subsequently acidified to produce CO2 standards as demonstrated by Bell et al.80 Similarly, Hansen et al.81 demonstrated the analysis of carboxylic acids by inline acidification of fermentation samples. The acidification of both ensures that the analyte is in its volatile and undissociated form. Hansen et al.81 observed multiple pH-dependent species during fermentation analysis of phenoxyacetic acid. Direct Baseline Determinations: Although, in principle, good calibrations can be performed using only external standards, performing baseline measurements is a worthy procedure that can expose sample contamination or interfering ions that are otherwise difficult to detect. Baseline measurements should be made using a sample matrix that matches the unknown sample as closely as possible as to observe possible interfering signals. For dissolved gases (e.g., CH4, N2, O2, Ar, and CO2), zero analyte solutions can usually be obtained for baseline measurements by sparging analyte with a gas that simulates the experimental atmosphere, but does not contain analyte. Alternatively, sampling boiling water, which is condensed inline prior to analysis, can be a convenient and less wasteful method to completely degas a sample for baseline measurements. Baseline values for VOC analyses are straightforward to obtain by filtering the sample matrix through activated carbon; this will ensure removal of most dissolved organic molecules from solution.

2.24.3.6.3

Sediment/Soil Matrix Standards

Calibration of the gas measurements in sediment or soil matrices is typically performed using a soil matrix submerged in aqueous solutions with known gas concentrations. The soil matrix should closely resemble the sample matrix as the physical environment experienced will impact instrument response. Measuring gases relative to a biologically inert internal standard such as argon produces more meaningful data. Due to the boundary layer that may develop at the membrane interface, the characteristics (porosity, tortuosity, and air content) of the matrix in the immediate vicinity of the membrane will affect instrument response. Further, analytes with chemical equilibria in solution are difficult to characterize due to the subsequent Le Chatelier shift occurring in the analyte-depleted boundary layer. This is expected to work well for peat and sediment samples since they are typically saturated with water, but for soil samples this does not necessarily hold. Sheppard and Lloyd82 made a study by comparing various calibration methods in order to find the best method for soil gas analysis. The MIMS output was calibrated using gas phase calibration, with a gas-equilibrated aqueous solution with stirring, with a gas-equilibrated aqueous solution without stirring or with a water-glass bead slurry equilibrated with a gas. From these methods, the last one clearly deviated from the others. Additionally, the mass spectrometer output was calibrated using a sterile soil with known amount of water (reflecting the natural rainfall level) and by introducing a known gas concentration through the soil until a constant signal was obtained. A correlation between the sterile soil calibration method and the calibration

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with stirred water column was reported, i.e., the most useful calibration method for soil analysis could be the use of a gasequilibrated aqueous solution with stirring.

2.24.3.6.4

Calibration Mathematics

Gas–Liquid Equilibrium Mathematics: Compressed calibrated gases are supplied as dry gases (i.e., containing no water vapor). As such, a simple dilution expression should be used to account for the addition of water vapor into the calibration gas during equilibration with an aqueous sample. To ensure complete hydration and avoid evaporative changes in sample salinity and temperature, the streaming calibration gas is sometimes passed through water prior to sample equilibration. Water vapor partial pressure, pH2O, can be calculated using formulas such as those developed by Millero and Leung83 and the partial pressure of gas, G, in a hydrated mixture can be calculated as follows: pGwet ¼ pGdry ð1
pH2 OÞ dry

(7)

wet

where pG is the original dry gas partial pressure and pG is the diluted wet gas partial pressure. Further, nonideal gases such as carbon dioxide must have their fugacity (fG, expressed in atmospheres) calculated from partial pressure (expressed as a mole ratio), usually via a virial expression. Corrections for weather-related variations in atmospheric pressure may also be applied. Once analyte fugacity is properly established, the analyte concentration in solution concentration ([G]aq) can be calculated using the Henry’s law ([G]aq ¼ KHfG). The Henry’s law constant (KH) can be calculated from empirical functions of temperature (i.e., the integrated van’t Hoff equation) and ionic strength (i.e., the Setchénow equation). Because various forms of Henry’s law are present in the literature, careful attention should be paid to units during calculations to ensure proper usage. During dynamic calibrations, solution concentration during mass transfer between the gas and aqueous phase is important, and is reviewed by Andersen et al.79 as follows:   d½GðaqÞ þ KMT ½GðaqÞ
E ½GðaqÞ Vr ¼ (8) dt where EGaq is the aqueous gas concentration when in equilibrium with the gas phase, KMT is the mass transfer coefficient between gas and aqueous phases, and Vr is the rate of production or consumption. Relation between analyte concentration and MIMS output: MIMS measurements at specific mass to charge ratios (m/z) are generally reported as an ion current, Im/z. For most mass spectrometers, ion currents are linearly proportional to analyte partial pressure in the operating vacuum,84 and which are governed by the permeation of gases through the membrane interface, which, in turn, is linearly dependent on analyte concentrations. Consequentially, ion current can be related to sample concentration using a linear calibration curve. The electronic baseline (Iel) of the mass spectrometer is usually measured at every scan and is subtracted from the ion current, often automatically by system software, making the relation between ion current and analyte concentration as follows: Im=z
Iel ¼

G

b q ½G þ G b0

(9)

where Gb1 is the calibration sensitivity or slope of the calibration curve and Gb0 is the background or y-intercept for a given gas. Each can be determined as coefficient estimates through a least-squares regression. Direct measurements of Gb0 via blank standards often contain far less error than measurements of spiked standards and as such, may be directly subtracted from subsequent measurements to simplify calibration. Instead, however, it is statistically proper to emphasize the most well-known calibration points (e.g., the blank reading) by using a weighted least-squares regression fit; this method will provide the most realistic error estimates.85 As the ion current is baseline subtracted, Gb0, it should be negligible; if it is not, then an interfering ion may be contributing to the current intensity. To account for this, additional terms are included to create a multivariate system. Interfering ions can be derived from undesirable species in the sample matrix or other instrumental effects including dissociative ionization, isotopes, multiple ionization, and ion–molecule reactions.86 Relative calibration sensitivity between similar analytes, or response factor (i.e., G1b1/G2b1), is very generally very stable. This fact enables the use of various internal standards to correct for variations in sampling conditions. For example, Kana et al.87 demonstrated MIMS measurements of N2, O2, and Ar with a precision better than 0.5%; however, when N2 and O2 were measured as a ratio to argon, precision was improved to better than 0.05%. Alternatively, an internal standard may be intentionally infused into the sample at a predictable rate. The intentional infusion of internal standard, demonstrated by Etzkorn et al.72 to account for instrumental drift in VOCs, is particularly effective because isotopically labeled internal standards are used.

2.24.3.6.5

Mixture Analysis

The main advantage of MIMS is the rapid, continuous measurement of the analytes from samples streams. The simultaneous detection of all analytes at the same time can also be a disadvantage because the analytes can interfere with the detection of each other and identification of single analytes from a complicated sample matrix, producing a complicated mixture mass spectrum, can be difficult or even impossible. For universal online monitoring, it is desirable to utilize electron ionization, which efficiently ionizes most compounds and provides structural information through extensive fragmentation. Unfortunately, structurally similar compounds, such as ethylbenzene and p-xylene, produce overlapping fragment ions and identification or quantitation of this kind of compounds is not easy. Therefore, several instrumental and mathematical techniques have been developed and utilized for efficient multicomponent analysis, such as separation of individual compounds from complicated mixtures.

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One efficient method for identification is the use of tandem MS; e.g., the MS/MS technique has been used in the identification of fermentation products of Bacillus polymyxa organism, metabolites of white rot fungus Bjerkandera adusta, and metabolites of parasitic flagellates Trichomonos vaginalis. On the other hand, MS/MS makes the MIMS system more complex and expensive and not feasible for on-site experiments. Other means of enhancing multicomponent analysis with MIMS include selective membranes, mathematical methods, sample modulation, and combination with other techniques. For example, the use of chemically modified affinity membranes facilitated a selective adsorption and concentration of analytes, bearing a particular functional group, from solution. Release of the bound analyte results in its transfer across the membrane and allows it to be monitored mass spectrometrically. Alkylamine-modified cellulose membranes were used to bind substituted benzaldehydes through imine formation at high pH. Release of the bound aldehyde was achieved by acid hydrolysis of the surface-bound imine. Benzaldehyde was detected with excellent specificity at 10 ppm in a complex mixture using this method. Using the enrichment capability of the membrane, a full mass spectrum of benzaldehyde can be measured at a concentration of 10 ppb. CI with tandem mass spectrometry is a very powerful tool for analyzing mixtures. CI simplifies the mass spectrum of a mixture when compared to the one obtained with EI. Analyte identification is enhanced with mass spectrometry/mass spectrometry (MS/MS). When a microporous polypropylene membrane is used, solvent flux is high enough for the vaporized solvent to be used as a chemical ionization gas.39 Instead of a filament, a glow discharge was used to ionize the reagent gas, since the high water pressure was found to shorten filament lifetime. Response times were short, about 10 s, but limits of detection (low or sub-ppm) were higher than with a nonporous silicone membrane because the enrichment step is missing. On the other hand, polar compounds were easily analyzed. Multivariant techniques have been used to differentiate co-eluting analytes that produce similar mass spectra. In one study,88 multivariant and univariant calibrations were compared for a solution containing noninterfering components of benzene (m/z 78), toluene (m/z 91), and p-xylene (m/z 105), all of which were measured with similar accuracy. Once established, the multivariant method was used to monitor a solution that contained a mixture of benzene, toluene, ethylbenzene, and p-xylene. Interferences included identical characteristic ion masses of m/z 105 for ethylbenzene and p-xylene, as well as contributions to the intensity of m/z 92 from several components. The univariant technique could not differentiate these components, whereas the multivariant technique achieved a relative error of 3.4% (benzene), 14.1% (toluene), and 17.6% (ethylbenzene) when several test solutions that contained varying concentrations (ranging between 19 and 456 ppb) of the analytes were analyzed. Problems with the multivariant technique were noted when quantifying p-xylene in the presence of high concentrations of ethylbenzene (48% error). Principal component analysis (PCA) was utilized to confirm these results, as it showed that multivariant analysis allows one to differentiate concentrations of p-xylene and ethylbenzene, where a univariant calibration method could not be applied. There have been other applications of multivariant techniques with MIMS, especially a nonlinear asymmetric error least-squares method (SPECTACS), which resolves a measured mass spectrum of an unknown mixture utilizing the complete EI library spectrum of analytes used for calibration.89 Excellent qualitative accuracy and quantitative precision have been obtained, e.g., the analytical results obtained from spiked water samples (five compounds at 20–500-mg l
1 levels) show quantitative precision typically less than 20% and RSD between replicate samples less than 10%. Differences in the permeation rates of different analytes result in small temporal differences in analyte response, which can be monitored in a non-steady-state experiment, sample-modulated MIMS.90 The experiment is readily performed by modulating the sample exposure to the membrane at a rate to operate in the non-steady-state regime for all components of interest. There is no net separation of the components of the sample mixture, but their different permittivities result in phase shifts in the output signals at various m/z ratios. The responses that correspond to a single component will appear in the ions derived from that compound and will be directly related to the diffusivity of that compound, whereas responses that correspond to multiple components will result in many ions producing a mixture of time-varying signals. The total response of a mixture is a function of the concentrations of permeating components, each producing responses (ion abundances) in a number of detector channels (m/z ratios), which are fewer than the total number of such channels. Each component contributes to the total instrument response as a function of sample concentration and its response factor, the latter being calculated from data taken with a standard solution. In a demonstration of these capabilities, a mixture of benzene and ethylbenzene was quantified by using data only for the ion at m/z 91. This approach is also best applied for process monitoring, where the identity of a mixture of components is known and the primary interest is in gathering online concentration data. In another study, time-dependent permeation behavior of binary gas mixtures through a ZSM5 zeolite membrane was studied. Although steady-state permeation rates were indistinguishable for CO2 and N2 or for cis- and trans2-butene in binary mixtures, differences in the rate of approach to steady state allowed component distinction. In traditional MIMS systems, one component is initially enriched in the permeate by following application of a pulse of analyte gas to the membrane, and then disappears more quickly upon termination of the pulse. Mixtures of cis- and trans-2-butene exhibit qualitatively different behavior; the permeate is enriched in cis-2-butene during both the leading and trailing edges of a sample pulse (though not at steady state). These differences in permeation behavior reflect different balances among multiple transport mechanisms through the zeolite membrane, thought to reflect a combination of selective component sorption and intracrystalline diffusion; in the case of cis- and trans-2-butene, these two factors oppose one another. Therefore, when time-dependent selectivities of cis- and trans-2-butene were plotted against a sampling time, different curves were obtained for each compound, facilitating the identification of both.91 As described in Section 2.24.3.1.1, temperature-programmed desorption (TPD) can be utilized to obtain further separation of analytes prior to MIMS measurement. A complete sampling cycle, including adsorbent cooling, required from 6 to 10 min. During the sampling of one liter of air, detection limits (at the ppb level or lower) were comparable to published MIMS data, with the added advantage of component separation for VOCs.

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The coupling of MIMS with a chromatographic technique provides improved separation, but it makes online monitoring impossible. An advantage of combinations of MIMS with chromatography, e.g., HPLC/MIMS or GC/MIMS, is a cost-efficient, offline alternative to the use of MS/MS or multivariant techniques for mixture analysis. For example, HPLC/MIMS was applied to drinking-water analysis of 18 interfering components in 28 min.92 Silicone membranes can be used to trap and monitor diagnostic gases, VOCs, and SVOCs from automobile exhausts, air, and bioreactors. This type of experiment can include the following parts: (1) membrane extraction, (2) thermal release, (3) chromatographic separation, and (4) mass spectrometric detection. In an interesting application, a PDMS membrane was used to form a membrane introduction/GC-MS instrument for industrial wastewater and bioreactor studies (see Section 3.2.2). The membrane was placed in the bioreactor and used for dissolved gas and VOC analysis and to accumulate SVOCs. After a suitable sampling period, the membrane was removed from the bioreactor and heated (180  C) to desorb the SVOCs that were analyzed by GC/MS. The total analysis time periods were less than 15 min for monitoring dissolved gases, VOCs, and SVOCs at ppm concentrations. MIMS has been combined with Fourier transform infrared spectroscopy (FTIR) for simultaneous detection and analysis of VOCs and gases from air samples. In this combination, a gaseous sample was directed simultaneously to FTIR and MIMS instruments, both of which are capable of online monitoring of VOCs, and in the case of MIMS, also ambient gases. The techniques could be used independently, i.e., separate quantitative results are calculated from both instruments and if the results differ from each other, then the reliability of results is judged from compound to compound, as it is known that FTIR measurement can be more reliable for isomeric compounds, whereas MIMS is more suitable for detection of similar compounds with different molecular weights. In both techniques, a deconvolution algorithm (SPECTACS, see above) was utilized in the identification of individual components from multicomponent IR spectra or mass spectra. A further developed version of SPECTACS was capable of combining both IR spectra and mass spectra to a single combined spectrum, and solving the combined spectrum gave more reliable results than the spectra separately.

2.24.4

Conclusions and Future Perspectives

The ability to analyze practically any type of sample for its content of volatile organic compounds without or with very little sample preparation makes MIMS a remarkable technique with applications as diverse as in vivo analysis of drugs in blood, online monitoring of chemical and biological processes, analysis of contaminants in environmental and food samples, surveillance of air quality, analysis of gas distributions in peat with 1-mm spatial resolution, and studies of membrane permeation properties. The common element in all these applications is a membrane that separates the sample from the vacuum in the mass spectrometer. This simple feature encompasses a large number of specially designed inlets that are optimized and dedicated to a particular application. MIMS has been shown to be a very versatile method for environmental monitoring of water, air, and soil samples with low detection limits. Proper calibration to obtain reliable concentrations for the analytes is a challenge for different applications. In many cases, a simple external standard can be used, but it is important to ensure that the physical conditions regarding the exposure of the membrane to the sample and to the standard are identical and to keep in mind that matrix effects are common. Despite the obvious possibilities of MIMS for field applications due to simple sample preparation in many applications, most MIMS applications in the past have been performed inside a laboratory. A major reason for this is the size and complexity of the complete mass spectrometric system. The membrane inlet may be very simple to operate, but the rest of the mass spectrometric system is not usually fieldportable. However, there has been an intense focus upon the development of miniature mass spectrometers, and the first fieldportable instruments (10–20 kg everything included) have become commercially available. Therefore, we expect that MIMS continues to be one of the analytical methods for rapid online analysis and monitoring of organic compounds, especially VOCs, from various sample matrices.

See also: Seawater Organic Contaminants; Membrane Extraction: General Overview and Basic Techniques; Solid-Phase Microextraction; Membrane-based Extraction for Environmental Analysis

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