Membrane introduction mass spectrometry

ANALYTICA

CHIMICA ACTA

ELSEVIER

Analytica

Chimica

Acta 350 (1997) 257-27 1

Membrane introduction

mass spectrometry

N. Srinivasan, R.C. Johnson, N. Kasthurikrishnan, Department

l? Wong, R.G. Cooks*

of Chemistry. Purdue University, West Lafayette, IN 47907-1393,

Received in revised form 19 February

1997; accepted

19 February

USA

1997

Abstract An overview of membrane introduction mass spectrometry (MIMS) is presented and comparisons are made with other direct sample introduction techniques. Special attention is given to the unique advantages and the limitations of newer variants on the MIMS technique, including affinity MIMS, reverse-phase and trap MIMS. The salient features of the interfaces used in MIMS are summarized and the various membrane materials commonly used are delineated. The applicability of MIMS is illustrated via discussion of (i) bioreactor monitoring (represented by yeast fermentation), (ii) environmental monitoring (illustrated by analysis of contaminated ground water samples) and (iii) on-line chemical reaction monitoring (exemplified by the photolysis of aryl esters). The applicability of MIMS to the analysis of environmental samples, including complex mixtures in water, air and soil, is noted. Keywork

Sample introduction; Bioreactor monitoring; Environmental analysis; In situ analysis

1. Introduction 1.1. Description

of the MIMS process

Membrane introduction mass spectrometry (MIMS) is a simple method of sampling and performing mass analysis which is applicable to on-line monitoring. The analyte is introduced into the ion source of the mass spectrometer via a semi-permeable membrane in a process known as pervaporation. The subsequent ionization step is usually performed by electron ionization (EI) or by positive or negative ion chemical ionization (CI). The membrane, usually a silicone polymer, acts as an interface between the sample solution and the vacuum of the mass spectrometer. *Corresponding 494 942 1.

author. E-mail: [email protected];

fax: +1 317

0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(97)00212-2

Hydrophobic membranes are permeable to relatively non-polar and low molecular weight compounds (400 Da) while the aqueous matrix is largely excluded. The high resistance to fouling, mechanical strength and chemical inertness of silicone membranes add to their usefulness. Pervaporation involves three steps (i) adsorption of the analyte(s) onto the membrane, (ii) diffusion through the membrane and (iii) desorption from the inner membrane surface into the gas phase. The permeation process can be described by Fick’s equations of diffusion (Eqs. (1) and (2)). I&t)

= -AD{ac$“‘}

(1) (2)

258

N. Srinivasan et al./Analytica

where Z&J) is the analyte flow inside the membrane (mol/s), C&J) is the concentration inside the membrane (mol/cm3), D is diffusion coefficient (cm’/s), A the membrane surface area (cm’), x the depth in the membrane (cm), and r time (s). Eq. (1) helps one to understand the analyte flow in the membrane, while Eq. (2) reveals how the concentration within the membrane changes with time. At steady-state, X,(X, t)/& is a constant through the membrane. Therefore,

where C, is the concentration of the analyte on the solution side, C, is the concentration of the analyte on the vacuum side, and L membrane thickness (cm); i.e.,

Chimica Acra 350 (1997) 257-271

by Eq. (5):

where Y,IY6 is the mole fraction of a vs. b in the gas phase (after passage through the membrane) and &/& is the mole fraction of a vs. b in the liquid phase. The relative transfer efficiency or permselectivity of MIMS is another measure of performance [4] and is defined as

where DilDj is the ratio of the diffusivities, and SilSj is the ratio of the Henry’s law solubility coefficients of components i and j. 1.2. Brief history of membrane spectrometry

Z,, =AD(+) where C,,,=C,-C,. Applying Henry’s law (C,,,=SxP,) to express concentrations in terms of partial pressures, the steadystate flow rate during pervaporation, represented by Eq. (3), is obtained. (3) where Z,, is the steady-state flow through the membrane (moYs), S the solubility constant of the analyte in membrane (moYtorr cm3), and P, the vapor pressure of analyte in the sample (torr). It is evident from Eq. (3) that larger and thinner membranes are preferable for maximum analyte flow. The standard technique of separation of variables [ 1,2] can be applied to Eq. (2) to obtain the concentrations inside the membrane as a function of time [3], when there is a small change in concentration. The concentration inside the membrane thus calculated can then be substituted into Eq. (1) and solved for the flow through the membrane. The rise time, tle_gO%, can be then calculated using Eq. (4). (4) The selectivity factor of MIMS, s, also known as the enrichment factor or the separation factor, is defined

introduction

mass

As early as 1963, MIMS was used by Hoch and Kok [5] to follow the kinetics of photosynthesis, in situ, through measurements of oxygen and carbon dioxide. This study was followed by several other photochemica1 studies by MIMS [6-81. This type of application later developed to include physiological measurements of dissolved organic compounds such as dichloromethane and the anaesthetic methoxyflurane in blood [9-l l] and p-cresol in urine. Another early application of membranes in mass spectrometry was their utilization as interfaces between chromatographic devices and mass spectrometers [12-151. Applications to the kinetics of biological reactions include the pioneering work of Calvo et al. [ 16,171 and Degn and Kristenson [18]. Applications to the monitoring of reacting systems were introduced by Westover and Tou who studied volatile organic compounds such as chloroform, hexane and methanol [ 191. Reuss et al. [20] adapted this approach to on-line fermentation monitoring, an area in which MIMS now sees substantial application [21-241. In this laboratory we introduced [25] the use of direct insertion membrane probe, which adds the advantages of flow injection analysis of sample handling to the short response times, sensitivity and chemical specificity of membrane introduction mass spectrometry. This approach was found to eliminate

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et al. /Analytica

memory effects due to condensation of analytes on the walls of the transfer tubes used in earlier experiments. MIMS has also been applied to the on-line monitoring of volatile organic compounds (VOCs) in water and air [26-281 and to monitor such processes as waste water treatment [29]. 1.3. Characteristics of MIMS MIMS plays an increasingly important role in fields such as environmental [28,30,31], bioreactor [22-241 and reaction monitoring [32-351 due to its high chemical specificity, high sensitivity for VOCs and convenience of use. It is particularly useful for online monitoring experiments and is compared in Table 1 with other methods of on-line monitoring of gaseous and solution phase samples by mass spectrometry. Although purge and trap methods [36] coupled with GC-MS have been successfully

Table 1 Different

sample introduction

methods used for on-line monitoring

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259

employed to separate and determine VOCs in drinking water, the chromatographic separation requires a significant amount of time and is labor intensive. In addition, short term fluctuations in contaminants may be missed in periodic sampling methods. MIMS by contrast, is a rugged analytical technique which allows rapid analyses and continuous on-line, in situ monitoring using comparably priced instrumentation and much lower operator costs than the more traditional GC-MS methods which are less readily adapted to continuous monitoring. It allows quantitation with external samples in approximately 5 min compared to the approximately 40 min purge and trap analysis time. Some membranes allow selective introduction of VOCs into the mass spectrometer and when coupled with a flow injection analysis (FIA) system, allow quantitation using external standards which can be processed in an automated fashion. Thus a standard

by mass spectrometry

(adapted from [37])

Method

Characteristics

Disadvantages

Sample

Reference

Direct vapor sampling

Short response times, simplicity and ruggedness

Not suitable for non-volatile liquids and solids. Difficult to analyze reactive species

Vapor

]381

Atmospheric ionization

High sensitivity (pptr), short response times and small memory effects

Significant matrix effects, sensitivity is high only for compounds having high proton/electron affinities, low ionization potentials or high gas-phase acidities

Vapor or gas

]391

MIMS

Simple and rugged high sensitivity (ppb), small matrix effects

Analysis of polar molecules and larger molecules (>5OClDa) is difficult

Vapor or solution

[401

Direct liquid introduction valves

Simple and easy automation

Evaporation of liquid hampers the speed of analysis and method is inherently slow

Vaporizable

[411

LC-MS

Separation on column and multicomponent analysis

Method development consuming

Liquid

1421

GC-MS

Concentration, separation on column and multicomponent analysis

Not as fast as direct methods; more complicated

Gas and volatile liquid

[43]

pressure

is time

liquid

N. Srinivasan et al./Analytica

260 Table 2 General characteristics

of membrane

introduction

Chimica Acta 350 (1997) 257-271

mass spectrometly

Characteristics Simple, rugged technique, many days operation Can be used together with quadrupole, magnetic sector, or ion trap instrument, On-line monitoring with flow injection analysis (FIA) High specificity, when used with tandem mass spectrometry (MS/MS) Simultaneous multi-component analysis

Table 3 Analytical

characteristics

with EI or CI

of MIMS

Characteristics

Representative

Response times (t1046_908) Detection limits Linear dynamic range Quantitative accuracy Matrix effects Molecular weight range Specificity

15-120 s ppb levels Several orders of magnitude (f5%) with external standards Small, except in some CI ~500 Da High (when used with tandem mass spectrometty

Table 4 Types of membranes

commonly

values

or derivatization)

used in MIMS

Membrane

Applications

Representative

Silicone (Sheet) Silicone (Tubular) Microporous (Polypropylene) Microporous (Teflon) Cellulose dialysis Liquid membranes Polyurethane (polyester) Polyethylene Polyimide Latex Polyurethane (polyether)

Volatile organics Volatile organics Water CI Water CI Enrichment and specificity Volatile compounds Similar to silicone Selectivity over water Higher water permeability Similar to silicone Similar to silicone

P51

FIA system for sampling, membrane introduction for selectively introducing the VOCs into the mass spectrometer, and an ion trap for determination of the individual components of the sample mixture can provide all of the desired information. This methodology, amenable to both automation and process control using feedback algorithms [23], is also portable and inexpensive. The advantages of MIMS are summarized in Tables 2 and 3. In addition, a wide variety of membrane materials can be chosen for different applica-

tions, which extends Table 4).

reference

~271 r4.41 [451 [461 [471 t4u [481 [481 [481 [481

the usefulness

of MIMS

(see

2. Devices 2. I. Sample introduction Several sample introduction systems have been successfully employed in MIMS. A brief description of systems that have been widely used is presented here.

N. Srinivasnn et al./Analytica

2.1.1. Sheet direct insertion membrane probe (S-DIMP) This type of probe [25], used for flow injection analysis sampling, is readily fabricated and permits the use of a sheet membranes of varying thickness. The maximum surface area of the membrane is approximately 30 mm*, and is limited by the diameter of the probe inlet. The solution passing through the probe is heated to a desired temperature which is typically measured with a platinum resistance thermometer detector (RTD). The elevated temperature enhances analyte permeation, improving response time and decreasing memory effects, but often decreasing the selectivity factor. Capillary direct insertion membrane probe (C-DIMP) C-DIMP [49] closely resembles the S-DIMP described above in design and applications. However, the C-DIMP permits the use of tubular membranes which typically have more surface area than the sheet membranes (for a given probe diameter) and this results in enhanced analyte(s) permeation and sensitivity. Unfortunately, many membranes are not available in capillary form.

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261

are transported pneumatically to the mass spectrometer, with the help of a helium carrier gas (typically helium with a GC line being used for the transport). The direction of flow of the sample stream through the membrane is usually opposite to that of the helium carrier gas. A jet separator [5 l] is used to remove most of the helium and much of the water before the permeates reach the GC transfer line. This removal of small molecules with the selective permeation of analyte through the membrane results in a two-step analyte enrichment.

3. A brief summary experiments

of variants

on MIMS

2.1.2.

2.1.3. Dual membrane probe This probe is much like the C-DIMP described above, but is equipped with two pairs of inlet and outlet lines, and hence it can be used with two capillary membranes (which may differ in their properties). The membranes can be mounted either in parallel or series. This also allows concurrent analysis of two sample streams, differing in composition. In addition to extending the MIMS methodology, a dual capillary membrane probe offers advantages of flexibility in the choice of the combination of membrane materials. These capabilities should facilitate the analysis of reactive intermediates, aid in simultaneous monitoring of the equilibrium between vapor and solution phase of the analyte, and allow one to monitor kinetics of formation of transient species.

3.1. Afinity

MIMS

In the technique of affinity MIMS [46], which has parallels with affinity chromatography, a chemically modified membrane is used to selectively adsorb analytes bearing a particular functional group and therefore to selectively concentrate them from solution. Chemical release of the bound analyte from the membrane allows its transfer across the membrane and into the ion source where it can be monitored mass spectrometrically. In the first application of this method, alkylamine-modified cellulose membranes were used to bind substituted benzaldehydes through imine formation at high pH (Fig. 1). After preconcentration, release of the bound analyte was achieved by acid hydrolysis of the surface-bound imine. Using the enrichment capability of the membrane, a mass spectrum of benzaldehyde could be measured at a concentration of 10 ppm in a complex mixture 1461 (Fig. 1 inset). The experiment can presumably be generalized to selectively adsorb analytes with other functional groups and its sensitivity is capable of significant improvement. It must be emphasized that the method sacrifices the on-line monitoring capability for enhanced chemical selectivity. 3.2. Thin membranes

2.1.4. Membrane/jet separator system Slivon and Budde [50] introduced the idea of pneumatically assisted transport of the membrane permeate. In this system the membrane is normally housed outside the mass spectrometer, and permeates

Eqs. (l)-(3) suggest that the membrane thickness and the nature of the membrane material play important roles in the performance of a MIMS system. Reverse phase MIMS experiments (vide infra) with

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Cellulose-Matrix-OH

+,

Chimica Acta 350 (1997) 257-271

IsI’- Carbonyldiimidazoie

y=&d=J

H,N

I

I

8

1

Loading

High pH

Elution

Low pH

)

I

66:46

3320

Time (Minutes)

.

Schiff base

ME&% -0X8-NH(CH,&N=CH

Fig. 1. Chemistry used in the modification of cellulose membrane and adsorption/desorption of the analyte. Reconstructed ion chromatogram due to the characteristic benzaldehyde fragment (m/z 105) for repetitive injections of a solution of 10 ppm benzaldehyde using a 20 s loading time. Loaded ( l ) benzaldehyde is eluted (*) by washing with 1% aqueous acetic acid or hydrochloric acid.

Methyl Ethyl Ketone

500 ppb

1 npm

1 wm I

10 pm

10 pm

lZb0

500 wb

24;O

Scan Number Fig. 2. Reconstructed ion chromatogram of methyl ethyl ketone (mlz 72) obtained using 25 pm thick polydimethylsiloxane 7O”C, 1 ml sample volume. These data were collected using an ion trap mass spectrometer (Finnigan ITS-40).

the thin membranes showed an improvement in rise times and analysis times [52]. Thus, thin membranes may be suitable for chemical and petrochemical online process monitoring where the frequency of sampling is of great importance. Fig. 2 shows an example

membrane kept at

of the reproducibility and dynamic range that is obtainable with a thin membrane, using EI, in the case of methyl ethyl ketone. Thin hydrophobic membranes allow permeation of enough water for chemical ionization to be performed as an alternative to electron

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Chimica Acta 350 (1997) 257-271

impact. The detection limits in typical experiments are in the lower parts-per-billion range [52]. Thus the thinner hydrophobic membranes offer the unique combination of rapid permeation and low detection limits in chemical ionization experiments. Circulation Ports for Analyte Solution

3.3. Liquid MIMS Liquids can be used instead of solid polymers for MIMS, for example, using a sheet-type probe [53]. Such a membrane can be positioned either internally or externally, relative to the ion source, and its surface area can be varied as can the nature of the liquid. Liquids used include but are not limited to Santovac@ (polyphenyl ether), Krytox@ (perfluoroalkyl ether), Penzane (alkylated cyclopentane) and silicone oil. Ideal characteristics of these liquids are high molecular weights and low vapor pressures so they make little contribution to the mass spectrum. Among the many advantages of using liquids as membrane materials is the ease of their chemical modification by addition of appropriate analytes. Such a means of modification might serve to improve the selectivity and might prove a more useful method of preparing membranes suitable for affinity MIMS (vide supra). Fig. 3 shows the preliminary results obtained for toluene using polyphenyl ether as the liquid; the mass spectrum is inset. Fig. 4 describes the experimental liquid membrane configuration. Recently using thinner liquid membranes, lower detection limits (5 ppb) were achieved.

100% R.A. 1

cap

Prnb

c

500 ppb

lo:oo

moo

IS’

jil!

263

’ ’ Celgard

Stainless Steel Support Mesh

Fig. 4. Schematic representation of a direct insertion liquid MIMS probe showing the liquid membrane sandwiched between two Celgard membranes, and the supporting stainless steel mesh.

3.4. Cold-trap MIMS (CT-MIMS) In this method [54] a membrane (of relatively large area compared to those used in conventional MIMS) is located outside the ion source (about 15 cm away) and the membrane interface is connected to the ion source via a U-shaped stainless steel tube which serves as cryo-trap. Analytes passing through the membrane are cryo-trapped in this U-shaped tubing, using external liquid nitrogen cooling. The trapped analytes are subsequently released into the ion source by ballistic heating. The method increases sensitivity but at the expense of response time. 3.5. Reverse phase MIMS This technique is generally used for the direct detection of volatile organic compounds dissolved in organic solvents. The original description used a porous polypropylene membrane which allowed both analyte and solvent to pass into the ion source where the solvent was employed as the reagent gas for chemical ionization (CI). With this variant of MIMS it has been demonstrated that a wide range of volatile organic compounds dissolved in organic solvents can be quantified in the sub-ppm range [55].

92

65

30:oo

Fig. 3. Ion abundance for toluene (m/z 91+92) polyphenyl ether as the liquid membrane.

40~00

3.6. Trap-and-release

min.

obtained

using

MIMS

In this variation of the methodology, a tubular silicone membrane is kept cold by the continuous flow of flush (typically water) or sample liquid. The liquid flow is briefly interrupted and during this period the membrane is heated causing the evaporation of semi-volatile organic compounds which have been accumulated in the membrane. The heat is provided

N. Srinivasan

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et al. /Analytica

Chimica Acta 350 (1997) 257-271

Sample In

ource

.:

To Pump Fig. 5. Schematic diagram of the trap-MIMS inlet. Diagram is adapted from [56]. The position of the membrane micrometer screw, relative to the filament. This feature is depicted by upward-pointing arrows.

by radiation from the filament used for ionization of the analytes [56,57] as illustrated in Fig. 5. The method has shown excellent detection limits and has the advantage of being applicable to relatively non-volatile compounds.

4. Applications 4.1. Bioreactor

monitoring

Analytical instruments and techniques which continually monitor the state of a dynamic chemical system, in particular chemical processes which are industrially important, are undergoing significant development [58J. A knowledge of the chemical composition of a system enables one to maintain near optimal conditions, thereby allowing product yield enhancement and/or maximization of process economy. Furthermore, such on-line analyses provide useful information regarding the nature of transients in the system. Analytical methods applied to achieve the above goals should possess fast response times relative to the rate at which changes occur in the system, and MIMS meets this criterion for some systems [23,25]. One of the major applications of MIMS is fermenta-

can be adjusted, using a

tion monitoring [59] since it provides a much higher sampling frequency than a number of other on-line methods, and one in which the cycle time is relatively long. As an example of this application, consider on-line monitoring data obtained during batch and fed-batch mode (vide infra) of operations involving yeast fermentations. A typical measurement cycle involves the injection of a plug of sample into a Ha0 stream, followed by the injection of a plug of standard solution. An FIA sampling system was used for on-line quantitation. Yeast strain 1400, a fusion product of Saccharomyces uvarum and Saccharomyces diastaticus, was used for the fermentation. 4.1. I. Batch and fed-batch fermentations The fermentation experiments, in the batch mode were carried out in approximately 1.5 1 of growth medium under microaerobic conditions. A typical batch mode fermentation involves continuous on-line monitoring for about 12-18 h. In general, the batch mode of operation is not optimum for obtaining ethanol concentrations above 100 g/l because of substrate (glucose) inhibition [60]. In order to reduce the effects of substrate inhibition, fed-batch fermentations are preferred. In this mode glucose (the substrate) is

N. Srinivasan et al./Analytica

added step-wise every time when it is nearly depleted. This step is repeated, using a feedback control, until the end of the fermentation which lasted for some 60 hours in the case described and so allowed maximization of the product yield and minimization the substrate inhibition. The monitoring process and feed addition are both automated. 4.1.2. Feedback control The change in the calculated concentration of an analyte in a fermentation as a function of time (the local slope of a portion of a concentration profile) can often be used to trigger one or more feedback actions. Typical feedback actions include the addition of a fixed volume of substrate (e.g., glucose) and/or opening a value for adding a gas such as oxygen to the fermentor (especially in the case of aerobic fermentations), adding an acid/base to control the pH, and/or changing the mode of analysis (e.g., from EI to CI). In the fed-batch mode of operation, the lack of an increase in ethanol production was used as the decision parameter in deciding whether a feedback action be taken or not. The local slope of the ethanol concentration versus scan number (time) profile was used as the control action parameter and the feedback control signal was used to automatically add glucose. Fig. 6 demonstrates that on-line process monitoring by MIMS can be used to generate data for use in automated decision making and process monitoring [23]. 4.2. Environmental

monitoring

Various environmentally important VOCs have been studied, both individually and in mixtures, by employing MIMS. They include priority pollutants at ppb and lower levels [40,61], contaminated ground water samples [31], carbon tetrachloride, trichloro ethane [3 11, acrolein, acrylonitrile, acrylamide and aldehydes [62-64], and both aliphatic and aromatic chloramines at low levels [51,65,66]. 4.2.1. Monitoring of water treatment plants On-line environmental monitoring using MIMS is an alternative to laboratory analysis of aqueous solutions such as drinking water. Drinking water purification is typically performed using chlorine and/or chloramine for disinfection in spite of the fact that

Chimica Acta 350 (1997) 257-271

265

many of the organic compounds present in water react with chlorine, producing toxic and carcinogenic byproducts (chloramines, cyanogen chloride, trihalomethanes, etc.) [67]. One of the byproducts, cyanogen chloride (CNCl), is usually found in chloramine-treated drinking water and is listed in the environmental protection agency’s drinking water priority list (U.S. EPA 1988). The analytical method recommended by EPA for the quantification of CNCl, method 542.2, employs purge and trap GC-MS. Fig. 7 shows information obtained from the analysis of aqueous cyanogen chloride solutions [68] using MIMS. As can be discerned from the data, MIMS reduces the analysis time to a considerable extent (compared to purge and trap) without significantly compromising detection limits. Furthermore, a feedback control algorithm can be used to control the dosage of the disinfection reagent. 4.2.2. On-site analysis of contaminated ground water samples Virkki et al. [31] have successfully demonstrated that on-site environmental analysis can be effectively implemented using an advanced mobile analytical laboratory (AMAL). This laboratory uses a custombuilt, helium purge type membrane inlet. The researchers have clearly demonstrated that environmental pollutants in contaminated ground water can be rapidly analyzed (response times l-2 min), using MIMS. Fig. 8 shows the reconstructed, backgroundsubtracted mass spectrum of a contaminated ground water sample [31], measured on-site. Contaminated ground water samples were found to contain toluene (at ppm level), benzene (at ppm level), xylenes, and a number minor contaminants including trichloroethene (at ppb level), dichloroethene, monochlorbenzene among several other pollutants. 4.2.3. Air monitoring The growing interest in MIMS for environmental monitoring is evident based on recent reviews [26,69]. In particular, Hemberger and coworkers [28] have demonstrated the use of a two stage membrane sampling system for analyzing VOCs in air at ppt levels. Many of the same issues are encountered as in water analysis, although the compounds present in air are generally more volatile and thus better suited to MIMS than those in water. As an example of some of the

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140 130

Automatic Substrate

(Glucose) Addition

120

52 Hour Period

110 100 i-

90

B

80

=e

70

(II

60

5

50

w,

40

Third Addition Second Addition

30 20

First Addition

10 0

4

‘r~~“l”“l”‘~l”~~l”“l”“l”“l”“l”“l””l”~ 0

5

10

15

20

25

30

35

40

45

50

Time /Hours Fig. 6. Top: a windows environment showing the data acquired in a strip chart recorder, other relevant information and possible actions that the user can take in an on-line bioreactor monitoring experiment. Bottom: representative plot of ethanol concentration versus time data, from an automated fed-batch yeast fermentation monitored over a period of 52 h. The arrows indicate the successive automatic addition of approximately 200 ml of the substrate (glucose) into the fermentor, using a feedback control algorithm.

issues of current interest, the analysis of peroxyacetyl nitrate (PAN) (Structure I) is considered here. PAN is a product of photochemical reactions of hydrocarbons and although it has a short lifetime (~42 min) at

ambient temperature, it has a longer lifetime in the colder troposphere. This longer lifetime permits substantial transport and redistribution of PAN in the atmosphere. Though the potential human health

N. Srinivasan et al. /Analytica

Chimica Acta 350 (1997) 257-271

267

Cl-C=N(m/z61)

J

36 rwb

36 ppb

Cl-C=N(m/z61) i t

1”

164

6:20

19:oo

UI

30

*m W:W

UI

11:40

44%

40

50

60

70

80

90

100

110

120

130

mh

Time (Minutes)

Fig. 7. Analysis of cyanogen chloride (CNCI) at ppb level using a jet separator-silicone membrane-ion separator and GC transfer line were kept at 110°C while the silicone membrane was kept at 90°C.

trap mass spectrometer

PAN--d

100

PAN-Hf-Ref %

80

d:

60

I

% a

199

Ref--H+

90

.$

Tar9etGa.:Ar9on

PAN-H 20

1

i22

B 4

40

system. The jet

77

‘*20-

0 50

70

w

110

Ii0

Ii0 60

100

140

190

220

260

m/z

Fig. 8. Reconstructed, background-subtracted mass spectrum of a contaminated ground water sample analyzed using MIMS [31]. A Fisons MD-800 mass spectrometer was used for this on-site measurement. Note that the relative abundances are multiplied by a factor of 20 after the ion m/z 93.

effects are not known, several studies have indicated that PAN even in the ppb level can lead to damage to vegetation [70].

In

order

to understand

the

chemistry

of this

Fig.

9. CID spectrum of the proton bound dimer of peroxyacetyl nitrate (PAN, m/z 121) and monofluoroacetone (mlz 76) where argon was used as the target gas. A triple quadrupole mass spectrometer (Finnigan TSQ-700) was used for this experiment.

highly functionalized small molecule in the atmosphere, thermochemical properties such as its proton affinity are of interest. MIMS has proved to be a successful method of introducing PAN into the mass spectrometer and has been used with the kinetic method [71] to determine the proton affinity of PAN [72] as x795 k.I mol-’ (~190 kcal mol-I). Fig. 9 shows the CID (collision-induced dissociation) spectrum of the proton bound dimer of PAN and monofluoroacetone used to make this measurement.

268

4.3. Reactor monitoring

N. Srinivasan et al. /Analytica

by MIMS

The standard method used to follow the kinetics of photochemistry of organic compounds involves collection of aliquots at chosen time intervals during the photolysis reaction followed by their off-line analysis by the use of gas chromatography or other spectroscopic techniques [73]. These methods, however, are time-consuming and do not allow real time on-line monitoring of the photolysis process. By contrast, MIMS can be used for continuously monitoring such reactions. The use of the direct insertion membrane probe [25], used in conjunction with FIA procedures of sampling, provide the short response times which are desirable for on-line reaction monitoring. The importance of the experiment is that the method of detection, mass spectrometry, is a universal method, and hence a suite of compounds can be examined simultaneously over the entire reaction period.

Chimica Acta 350 (1997) 257-271

The high selectivity of the experiment makes it a promising technique in studying relative rates of product formation, although improvements are needed in precision and sensitivity. These improvements are likely to come through the development of a wider range of membrane materials to supplement current performance of silicone membranes. In order to test its ability to detect reaction intermediates, MIMS has been applied to the on-line monitoring of the photolysis (254 nm) of benzyl acetate in 1 : 1 methanol-water and 3,Sdimethoxybenzyl acetate in water [33]. The reaction mixture is continuously exposed to a silicone membrane through which the reactant and products diffuse into a triple quadrupole mass spectrometer (Fig. 10) for quantitative and qualitative analysis. Fig. 11 shows the MIMS ion abundances of the reactant and the products formed during the photolysis of benzyl acetate in 1 : 1 methanol-water using ammonia as a chemical

mperature

Controller

Fig. 10. A schematic of the photolysis monitoring experiment using MIMS. A triple quadrupole mass spectrometer (Finnigan TSQ-4500). Rayonet photoreactor, and an 8 ml quartz reaction vessel were used for this experiment. Photolysis was carried out at 254 nm.

a

N. Srinivasan et al. /Analytica

Chimica Acta 350 (1997) 257-271

l

l

269

combined air, soil and water analyses using MIMS and monitoring of reactive intermediates (e.g., species analogous to free radicals).

6. Conclusions

0

loo

200

300

400

500

600

700

Scan Number (2 see/scan) Photolysisstarts Fig. Il. The ion abundance of benzyl acetate (m/z 168, [M + NH:]), benzyl methyl ether (mlz 140, [M + NH,+]), bibenzyl (mlz 200, [M + NH:]) and ethylbenzene (m/z 106, M+) are plotted as a function of time under ammonia CI condition. Note that the ion abundance at m/z 106 is expanded five times. The photolysis started at scan number 50. Ion abundances have not been corrected for losses through the membrane; this procedure changes the ion counts but has an insignificant effect on the shapes of the curves.

ionization reagent gas. Information on the relative rates of formation of the products can be obtained either by the initial rate method or from a plot of the concentration of one product versus the concentration of the other. Quantitation was achieved using external standard solutions.

MIMS has found applications in a variety of fields. Methods of improving selectivity (e.g., by fabrication of new membranes), and sensitivity (e.g., on-line derivatization) should make MIMS more widely applicable. The results given above show that an FIA/MIMS system can provide valuable information by allowing one to carry out real time on-line monitoring, with automated feedback control. MIMS can be used not only to monitor and quantify major products in industrially and environmentally important processes, but also to maximize the product yield and/or minimize pollution.

Acknowledgements The authors acknowledge support from Phillips Corporation (for Fellowship support to N. Kasthurikrishnan) and from the Office of Naval Research,

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