Fundamentals and Applications of Needle Trap Devices

2.30

Fundamentals and Applications of Needle Trap Devices

HL Lord, W Zhan, and J Pawliszyn, University of Waterloo, Waterloo, ON, Canada Ó 2012 Elsevier Inc. All rights reserved.

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2.30.1 Introduction 2.30.2 Theory 2.30.2.1 Exhaustive Active Sampling 2.30.2.1.1 Calibration 2.30.2.2 Passive Sampling 2.30.2.2.1 Calibration 2.30.2.3 Particle Trapping 2.30.3 Evolution of Needle Trap Technologies 2.30.3.1 Devices 2.30.3.2 Sorbent Immobilization 2.30.3.3 Device Configurations 2.30.3.4 Sorbents 2.30.3.5 Desorption 2.30.3.5.1 Air-Assisted Desorption 2.30.3.5.2 Thermal Expansion Desorption 2.30.3.5.3 Water Vapor-Assisted Desorption 2.30.3.5.4 Inert Gas-Assisted Desorption 2.30.3.5.5 Solvent Desorption 2.30.4 Applications 2.30.4.1 Breath 2.30.4.2 Distinguishing Free/Total 2.30.4.3 Passive (Integrated) Sampling 2.30.5 Automation 2.30.6 Future Directions Acknowledgment References Relevant Websites

2.30.1

Introduction

Miniaturization and simplification of the steps in sample preparation hold promise for reducing cost and error in sampling and sample preparation, as well as providing better solutions for handling and processing small sample volumes. To this end, several technologies have been introduced that take advantage of syringe needle configurations for sampling, sample preparation, and sample introduction to analytical instrumentation. The needle trap devices exploiting exhaustive extraction are appearing as an interesting and distinct subset of these technologies. In this chapter, we aim to provide a broad overview of the subject, detailed descriptions of needle trap-type devices, and an updated status of the technology. There are five types of needle-based extraction methods (Figure 1). A number of reviews summarize the developments in these relatively new technologies.1–11 The most commercially successful of the needle-based extraction methods to date is solid-phase microextraction (SPME) in the coated microfiber format12–15 (Figure 1(a)). This technology introduced the concept of an extraction phase within a small 22–23-gauge needle and demonstrated the high sensitivity of the microextraction approach. Sensitivity is comparable to traditional large-volume extraction methods due to high enrichment onto the small volume of extraction phase and introduction of all extracted analytes to instrumental analysis. A very important feature of this technique has been the solvent-free nature of this extraction format and convenient instrument introduction resulting in a high-level automation using autosamplers and robotic workstations. Following the success of SPME other small-needle extraction methods were proposed16 and developed16–18 which used a piece of gas chromatography (GC) capillary column inside the needle (in-tube SPME or in-needle capillary adsorption trap (INCAT), Figure 1(b)). Also described were the internally coated needles (Figure 1(c)) named solid-phase dynamic extraction (SPDE),19,20 a needle microliter volume liquid-phase microextraction (Figure 1(d)) named single-drop microextraction (SDME),21–23 and, finally, a packed needle format (Figure 1(e)) named needle trap device (NTD), which is the primary subject of this review. The strengths of the NTD technique, similar to other needle-based techniques, are its potential for laboratory automation and on-site sampling as well as convenient coupling to analytical instrumentation. The main difference between NTD and the first four is its exhaustive extraction nature, which simplifies the calibration and allows particle trapping, resulting in total concentration information as

Comprehensive Sampling and Sample Preparation, Volume 2

doi:10.1016/B978-0-12-381373-2.10056-0

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Theory of Extraction Techniques

Figure 1 Schematics of: (a) SPME fiber located in a needle; (b) in-tube SPME or INCAT; (c) SPDE; (d) SDME (e) Scheme of a sorbent-packed NTD. From Lord, H. L.; Zhan, W.; Pawliszyn, J. Anal. Chim. Acta 2010, 677, 3–18, reprinted with permission.

compared to the free concentration information provided by the other techniques illustrated in Figure 1. In that regard it is similar to the much larger sorbent traps commonly used in numerous analytical applications.24 As NT is an exhaustive technique rather than a true microextraction, for air analysis it may be compared to other exhaustive methods where pumps, filters, and sorbents are employed to interrogate the total sample. Such methods require back-extraction from the sorbent or filter followed by subsequent analysis. For liquid samples, SPE methods are comparable. These techniques are typically applied to large sample volumes, and consequently require lengthy and large-volume cleanup and desorption steps, with only a small proportion of the final purified analyte being applied to the analytical instrument. By miniaturizing all components, the method is perhaps more comparable to micro-SPE, although the NTD design permits the sampling, sample preparation, and sample introduction to be combined, with quantitative transfer of analyte to the instrumental analysis. The goal is to simplify the sample preparation by eliminating the time and expense associated with processing unnecessarily large volumes, while retaining the required method sensitivity. In NTD, quantification is performed by simply determining the amount of compound exhaustively extracted by reference to instrument detector response calibration, and expressing this per volume of original sample. The first four techniques are microextraction techniques and so extraction depends on the kinetic and thermodynamic properties of the extraction phase and sample. In these cases, calibration by either internal or external standard is used to account for both the kinetics/thermodynamics of the extraction and the instrument detector response. Below we summarize the fundamental principles behind the NTD technique that could be useful in optimization of the needle trap design, and also summarize progress and applications investigated to date.

2.30.2

Theory

2.30.2.1

Exhaustive Active Sampling

In NT sampling, since the sample is introduced continuously into the needle, the process of extraction in the needle can be described as frontal (gas–solid) chromatography. In frontal chromatography, the capacity of column – the packed needle in this case – is an important parameter since it determines when the needle is saturated by the analytes and consequently unable to adsorb any more analytes. The capacity, described by the breakthrough volume in needle trap, is affected by gas pressure, temperature, humidity, flow rate, and sorbent bed geometry,25 and is closely related to the shape of the eluting front which can be described as the integral of a Gaussian peak. Based on the frontal chromatography assumption, many attempts have been made to find a mathematical relationship between sampling capacity and chromatographic parameters such as the retention volume and number of theoretical plates. With such attempts, several models have been developed26–28 among which the model of Per Lovkvist et al.28 is the most appropriate for needle trap. In this model, the theoretical plate number (n) is expressed as n ¼

uL 2D

(1)

where L is the length of the packed sorbent bed, D is the apparent diffusion constant which is intended to include all mechanisms of dispersion, and u is the linear gas velocity. Ignoring the gas compressibility and assuming that the flow rate is constant, the linear flow rate (u) of the gas sample through the needle can be calculated by considering the packing density and the pressure drop using a typical equation applicable for porous media: u ¼

Q Af

(2)

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679

where Q is the volume flow rate in the needle, A is the cross-sectional area of the needle, and f is the porosity of the sorbent bed. The volumetric flow rate can be defined by29 Q ¼ ðkp A=mÞðDp=LÞ

(3)

where 6p is the hydrostatic pressure drop, m is the viscosity of the fluid, and kp is the permeability of the sorbent bed, which is defined as 2

kp ¼ dp2 f5:5 =5:6 K

(4)

where dp2 is the average sphere diameter which equals to the particle diameter, dp, when the particles are assumed spherical, K is a constant, and K ¼ 1 for a narrow particle distribution. Equation (3) shows that the sampling rate is determined by the particle size and packing density for constant pressure drop along the bed. Considering that sampling is typically done by a pump with which the pumping pressure is controlled, then at one end of the sorbent bed the pressure is equal to environmental pressure (1 atm), while the other end of sorbent bed is the inner pressure produced by the pump. The linear and volume flow rates for air sampling may be calculated by Eqn (2) and Eqn (3), respectively. The time of sampling at a given volume Q0 of the air sample then can be calculated to be ts ¼

Q0 Q

(5)

In the frontal chromatography arrangement, the concentration profile along the axis x, of the tubing containing the extracting phase, as a function of time t, can be described by adopting and deriving the expression for dispersion of a concentration front:28 1 0 0 ut 1 ut x x þ2 1 B Lð1 þ kÞC 1 Lð1 þ kÞ C B pffiffiffi A  cs  expð2nÞ  @1  erf pffiffiffi (6) Cðx; tÞ ¼ cs @1  erf A 2 2 sL 2 sL 2 where cs is the concentration of analyte in the sample, k is the retention factor, which is defined as k ¼ Kes

Ve Vv

(7)

and Kes is the extraction phase/sample matrix distribution constant, Ve is the volume of the extracting phase, and Vv is the void volume of the tubing containing the extracting phase. s is the root mean square dispersion of the front, which is defined as rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u L ut 2Dt ¼ s ¼ Ht ¼ (8) 1þk n1þ k 1þk The difference between Eqn (6) and other frontal equations is that it is more applicable when n is very small. n is usually small in needle trap, due to the short sorbent bed.28 Figure 2 illustrates the normalized concentration profiles produced in the bed during extraction30 based on an equation similar to Eqn (6). Full breakthrough is obtained for the right-most curve, which corresponds to the appropriate volume of the sample matrix for extraction. The time required to pass this volume through the extraction system corresponds to the equilibration time of

Figure 2 (a) Schematic representation of a packed needle; (b) theoretical concentration profiles in the sorbent bed assuming, cs is the concentration of the analyte in the sample, L is the length of the sorbent bed, x is the relative position along L. From Lord, H. L.; Zhan, W.; Pawliszyn, J., Anal. Chim. Acta 2010, 677, 3–18, reprinted with permission.

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the compound with the bed and the equilibration time can be assumed to be the time required for the center of the front to reach the end of the sorbent: ts ¼

Lð1 þ kÞ u

(9)

During sampling, before breakthrough, the sorbent bed may be treated as a ‘perfect sink’ for analyte. In this case, the mass of a certain analyte loaded in the sorbent bed can be described as the total mass flow through the beginning of the sorbent bed (when x ¼ 0 in Eqn (6)): Z t Cð0; tÞdt ¼ AfucS t (10) nðtÞ ¼ Auf 0

The breakthrough level can then be defined as the percentage of mass exiting the end of the sorbent bed compared with the initial mass passing through the beginning of the sorbent bed: Rt Rt Auf 0 CðL; tÞdt CðL; tÞdt b ¼ ¼ 0 (11) AfucS t cS t The approximated solution of b may be found in the work of Per Lovkvist et al.,28 from which the breakthrough time may be obtained by derivation: tb ¼

h Lð1 þ kÞ a1 a2 i1=2 ¼ ð1  bÞ2 þ þ 2 n n u

(12)

a1 and a2 are complicated functions of b, and their values corresponding to b were provided in the work of Per Lovkvist et al.28 as well. We assume breakthrough occurs when b  5%, when b ¼ 5%, a1 ¼ 5.360, a2 ¼ 4.603. As a result, Eqn (11) could be rewritten as   Lð1 þ kÞ 5:360 4:603 1=2 tb ¼ ¼ 0:903 þ þ 2 (13) u n n By converting the breakthrough from the time scale to volume scale, we obtain the breakthrough volume:   5:360 4:603 1=2 þ 2 Vb ¼ AfLð1 þ kÞ 0:903 þ n n

(14)

Comparing Eqn (9) and Eqn (13), we find that te is very close to tb at high plate numbers (n > 10). Therefore, at high plate numbers we can use Eqn (9) to calculate the breakthrough time as an approximation. The foregoing gives clear guidance on how to construct the needles for chemical trapping to avoid breakthrough (i.e., exhaustive sampling). In order to get a higher sampling rate and reduce the sampling time, the linear flow rate should not be too low, while for obtaining larger capacity, the linear flow rate should not exceed a certain value. Therefore, the porosity should be kept in a defined range. A large particle size would be helpful to decrease the resistance but disadvantageous for increasing the capacity since a large particle size would decrease the surface area as well. For obtaining higher capacity, a longer sorbent bed can be used; however, this would result in an increase in resistance. As a result, a larger diameter of needle can be used to increase the capacity without suffering lower sampling rate. Moreover, a higher retention adsorbent can be used when desorption efficiency is not greatly affected. However, due to limited surface area, the adsorbent is easily saturated at high concentrations long before an equilibrium condition has been achieved, and since the above model was based on a linear distribution isotherm assumption, it is more applicable at sufficiently low concentrations. Nevertheless, it could be quite useful to predict the maximum sampling time or breakthrough volume in on-site sampling since, in such places, the concentrations are usually low and a much longer sampling time is required.

2.30.2.1.1

Calibration

So long as the needle trap is designed to perform exhaustive sampling, calibration is conducted identically to other exhaustive sampling techniques such as SPE or sorbent trapping. Sample volume must be controlled and known. Amount extracted is determined from a predetermined instrument detector response calibration, and then sample concentration is calculated directly from the ratio of amount extracted and sample volume.

2.30.2.2

Passive Sampling

An NTD packed with a strong sorbent at a defined distance from the needle opening is a very simple arrangement for use in passive time-weighted average (TWA) sampling. The sampling device, as shown in Figure 3(a), is able to generate a response proportional to the integral of the analyte concentration over time and space (when the needle is moved through space).31 Under these conditions, the only mechanism of analyte transport to the extracting phase is diffusion through the matrix contained in the tip of the needle. During this process, a linear concentration profile (shown in Figure 3(b)) is established in the tubing between the small

Fundamentals and Applications of Needle Trap Devices

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Figure 3 Scheme of a TWA sampling. (a) scheme of a packed needle with a distance Z between the needle opening and the position of the sorbent bed; (b) concentration profile along the distance Z as a function of time t. From Lord, H. L.; Zhan, W.; Pawliszyn, J., Anal. Chim. Acta 2010, 677, 3–18, reprinted with permission.

needle opening, characterized by a surface area (A), and the position of the extracting phase from the opening (distance, Z). The amount of analyte extracted, dn, during time interval, dt, can be calculated by considering Fick’s first law of diffusion:32 dn ¼ ADm

dc DCðtÞ dt ¼ ADm dt dz Z

(15)

where DC(t)/Z is an expression of the gradient established in the needle between the needle opening and the position of the extracting phase; DC(t) ¼ C(t)  CZ, where C(t) is the time-dependent concentration of analyte in the sample in the vicinity of the needle opening, and CZ is the concentration of the analyte in the vicinity of the sorbent bed. CZ is close to zero for a high extraction phase/ matrix distribution constant (‘zero’ sink). In this case, DC(t) ¼ C(t). The concentration of analyte, CZ, at the beginning of sorbent position in the needle will increase with integration time, but it will remain low compared with the sample concentration outside the needle, because of the presence of the extraction phase. The amount of analyte accumulated over time can therefore be calculated as Z A n ¼ Dm (16) Cs ðtÞdx Z As expected, the amount of analyte extracted is proportional to the integral of sample concentration over time, the diffusion coefficient of analyte in the matrix filling the needle, Dm, and the area of the needle opening, A, and inversely proportional to the distance, Z, of the sorbent from the needle opening. It should be emphasized that Eqns 15 and 16 are valid only when the amount of analyte extracted onto the sorbent is a small fraction (typically about 10%) of the equilibrium amount for a given analyte in the sample, in order to meet the ‘zero’ sink requirement. To extend integration times the sorbent bed can be placed further into the needle (larger Z), and the opening of the needle can be reduced, for instance, by placing an additional orifice over the needle (smaller A), or a higher-capacity sorbent can be used. The first two solutions will result in low measurement sensitivity. Increasing the sorbent capacity is a more attractive proposition. It can be achieved either by increasing sorbent volume or by changing its affinity for the analyte. Because increasing the sorbent volume would require an increase in the size of the device, the optimum approach to increasing the integration time is to use sorbents characterized by large sorbent/gas distribution constants, like Carboxen. If the matrix filling the needle is something other than the sample matrix, an appropriate diffusion coefficient should be used in Eqn (16). In the system described, the length of the diffusion channel can be adjusted to ensure that mass transfer in the narrow channel of the needle controls overall mass transfer to the extraction phase, irrespective of convection conditions outside the needle.33 This is a very desirable feature of TWA sampling, because the performance of this device is independent of the flow conditions in the system investigated. This is difficult to ensure for high surface area membrane permeation-based TWA devices, e.g., passive diffusive badges34 and semipermeable membrane devices (SPMDs).35 For analytes characterized by moderate to high distribution constants, mass transport is controlled by diffusive transport in the boundary layer. The performance of these devices therefore depends on the convection conditions in the investigated system.36

2.30.2.2.1

Calibration

In the case of passive sampling, so long as the appropriate conditions are met, calibration is conducted similarly for other passive samplers. This involves determining the total amount adsorbed over the sampling time and converting this to the time-weighted average sample concentration from a previously determined response factor. The conditions that must be met for the device to perform properly include that the sorbent responds as a zero-sink with a fast response to variable ambient analyte concentrations throughout the sampling time, and that the tip of the sampler is always exposed to a sample that is representative of the bulk analyte concentration in the sample. This process is described in more detail in a recent publication.37

2.30.2.3

Particle Trapping

The NTD is able to act as a filter to trap particulate matter in a sample. There are four mechanical collection mechanisms by which aerosol particles can be trapped by an NTD: interception, inertial impaction, diffusion, and gravitational settling.38 Interception

682

Theory of Extraction Techniques

Figure 4 Schematic explanation of interception, impaction, and diffusion. From William C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 1st ed.; John Wiley & Sons, Inc., New York, 1982, reprinted with permission.

(see Figure 4(a)) occurs when a particle follows a gas stream that happens to come within one particle radius of the surface of a sorbent particle.38 The single sorbent efficiency for interception is closely related to the ratio of particle diameter to sorbent diameter, and the packing density.38 Inertial impaction (see Figure 4(b)) occurs when a particle, because of its inertia, is unable to adjust quickly enough to the abruptly changing streamlines in the vicinity of a sorbent particle and crosses those streamlines to hit the sorbent particle.38 Its efficiency is governed by the value of Stokes number and related to the ratio of particle diameter to sorbent diameter as well as the packing density.38 Diffusion (see Figure 4(c)) is caused by the Brownian motion of small particles, which is sufficient to greatly enhance the probability of their hitting a sorbent particle while traveling past it on a nonintercepting streamline.38 The efficiency of diffusion is related to the particle size of the sorbent, the linear flow rate, and the diffusion coefficient of the particle.38 The gravitational settling is negligible compared with the other three mechanisms.38 Mathematical equations for the efficiencies of the above mechanisms for the traditional fibrous filters have already been investigated and it was found that the total collection efficiency might not reach 100% for some range of particle sizes under a certain sorbent size, porosity, and linear flow rate for a fibrous filter. When the same equations are applied to NTDs packed with different sorbents, penetrations might also occur for the sample particles with particle size at certain ranges, as indicated in Figure 5. As seen from Figure 5, the conventional NTDs (NTD 1) packed with particles having sizes of about 150 mm are only able to completely collect the particles with a diameter larger than 0.5 mm. Even when the needle trap devices are packed with a sorbent with smaller particle sizes, the penetrations of the sample particles might decrease but still be significant (see NTD 2). One solution to this penetration problem might be packing the needles with glass wools, which have not only a much smaller sorbent diameter but also a large porosity (see NTD 3 and 4). During the sampling process, smaller sorbent diameter would help to decrease the penetration significantly, while the large porosity would help to reduce the resistance. By packing the glass wool, especially the silanized glass wool, in the front of a needle, to trap the particles, while packing another sorbent afterwards to extract free molecules, the NTD would be able to extract the free molecules and particles simultaneously.

2.30.3

Evolution of Needle Trap Technologies

2.30.3.1

Devices

The first reported use of a syringe needle packed with a sorbent bed for trapping organic compounds in air was described in the 1970s when a large Tenax-filled needle was used for fragrance collection.39 More recently, in 1996, a similar approach was developed for pre-concentration of gaseous trace organic compounds on solid sorbents including charcoal and silica gel for determination of the analytes in natural and industrial atmospheres and their monitoring in human breath.40 The sampling procedure was found to be rapid and sensitive with detection limits approaching a few ppb, standard deviation of 10%, and linear range over five orders of magnitude, which was similar in performance to typical sorbent traps. The major limitation of these earlier applications was the large size of the needle, requiring modification of the standard inlet systems, which eliminated their advantages

Fundamentals and Applications of Needle Trap Devices

683

Figure 5 Extraction efficiencies for the fibrous filter and the needle trap devices. (Fibrous filter: thickness ¼ 1 mm, solidity ¼ 0.05, sorbent diameter ¼ 2 mm, and the linear flow rate of sampling is 10 cm s1; NTD 1: packed with sorbent particles, the length of sorbent bed ¼ 10 mm, solidity ¼ 0.35, sorbent particle diameter ¼ 150 mm, and the linear flow rate of sampling is 100 cm s1; NTD 2: packed with sorbent particles, the length of sorbent bed ¼ 10 mm, solidity ¼ 0.35, sorbent particle diameter ¼ 50 mm, and the linear flow rate of sampling is 100 cm s1; NTD 3: packed with glass wool, the length of sorbent bed ¼ 10 mm, solidity ¼ 0.10, sorbent particle diameter ¼ 10 mm, and the linear flow rate of sampling is 100 cm s1; NTD 4: packed with thinner glass wool, the length of sorbent bed ¼ 10 mm, solidity ¼ 0.10, sorbent particle diameter ¼ 5 mm, and the linear flow rate of sampling is 50 cm s1.) Adapted from William C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 1st ed.; John Wiley & Sons, Inc., New York, 1982.

over well-accepted sorbent traps. Typically, a dedicated carrier gas purge line or an additional volume of clean air was needed to aid the introduction of desorbed analytes to the column. It was primarily used as a fast screening tool for qualitative purposes. The needle trap approach became more practical for automation and on-site application when smaller diameter needles of 22 or 23 gauge were used, which fit conveniently into common GC inlets. In 2001, a needle trap device for trapping large particles was described that consisted of a 40-mm-long 23-gauge stainless steel needle (0.53 mm o.d.) containing 5 mm of quartz wool packing.41,42 The analytes associated with aerosols in an air sample were collected by drawing air across the NTD with a Luer-lock gas-tight syringe. The mass loading of particulate matter was performed by adjusting the volume of air pulled through the NTD and the particulates were then trapped by the filter plug inside the needle. The device was introduced via a conventional inlet to a gas chromatograph with FID or mass spectrometer detector by delivering 10 ml of clean air or a carrier gas with a help of a gas-tight syringe, to aid the transfer of the desorbed analytes from the hot needle. The results showed that the NTD can be used for airborne aerosol sampling and determination of the volatiles and semi-volatiles collected on the trapped material. No carryover was observed. The NTD performed well in extracting diesel exhaust particulates containing polycyclic aromatic hydrocarbons (PAHs), triamcinolone acetonide in an aerosol dose of asthma drug, and DEET in an insect repellent spray. Since the early 2000s, several groups have worked in parallel on the development of sorbent-packed needles or related devices. In 2003, Berezkin at al.43 described two approaches: Tenax-packed hypodermic needles (0.5 mm ID and 0.8 mm OD) (Figure 6(a)) and an enlarged sorbent bed approach, using a 2-mm ID and 100–150-mm-long cartridge having a small-diameter needle at its end to effectively puncture the septum for introduction to an analytical instrument (e.g., GC) (Figure 6(b)). A dedicated external desorption system was used to facilitate desorption from the large cartridge. The second approach has a similar construction to two high-capacity commercial systems: the CTC ITEX44 developed by Shilling (BGB Analytik AG, Switzerland) for gaseous and aqueous headspace samples and the SGE microextraction in packed syringe (MEPS) developed by Bloomberg and Abdel-Rehim45 for liquid samples with analysis by either LC or GC applications. The devices described by Berezkin were applied to the determination of volatile aromatic hydrocarbons in cigarette smoke. The obtained results correlated well with previously published data. Berezkin and his co-workers subsequently developed a needlebased direct water extraction system.46,47 In this approach, they used Porapak Q as a sorbent material and wet alumina as a water reservoir for desorptive water vapor flow in a closed analytical system. The analytical characteristics of the developed device and of a compared purge-and-trap device for BTEX compounds are similar. The same authors also developed a purge-and-trap system using Carbopack X as a sorbent material, which facilitated pre-concentration and desorption of volatile benzene, toluene, ethylbenzene,

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Theory of Extraction Techniques

Figure 6 (a) Needle microconcentrator: 1-needle stem (0.5 mm I.D.30.8 mm O.D.); 2-plugs, limiting the sorbent layer, 3-sorbent layer (Tenax); 4- holder, 5- capillary line for carrier gas. (b) Cylindrical microconcentrator: 1- needle stem, 2- tube stem (1 mm I.D.32 mm O.D.); 3- plugs, limiting the sorbent layer, 4- sorbent layer; 5- holder, 6- capillary line for carrier gas. From Berezkin, V.G.; Makarov, E. D.; Stolyarov, B.V., J. Chromatogr. A 985 (2003) 63–65, reprinted with permission.

and xylene (BTEX) into a gas chromatograph.48 A closed system for stripping the analytes from water samples was used. An injection port with a modified metal liner was used to desorb analytes trapped in the NTD. An alternative purge-and-trap approach using a bidirectional syringe pump has been published recently.49 This approach has proven very versatile for developing a sequential purge-and-trap sampling technique for needle trap devices. The sequential sampling/purging method with recycled headspace gas offered higher efficiency of extraction, resulting in an effective extraction in less time and with minimized dilution effect compared with the continuous (simultaneous) purge-and-trap technique with clean nitrogen gas. The main advantage of the NTD lies in the simple methodology and speed of the analysis.48 Although needle-based sample preparation strategies relying on microextraction have a limitation in the additional method optimization required for appropriate calibration, the exhaustive NTD does not. The main limitations of the NTD are the additional care that must go into device design to ensure that it is capable of exhaustive extraction with simplified methodology, and for the user to ensure that breakthrough does not occur during sampling. As an exhaustive technique, it also has the limitation that it extracts less selectively than does a microextraction. The various devices and techniques introduced over the years by various groups are presented and compared in Table 1 along with their typical applications and most appropriate references. Table 1

Overview of needle trap and related techniques

Technique name

Description

Application

Reference

Microextraction Techniques INCAT SPDE

section of GC capillary inside needle sorbent coating inside wall of needle

VOCs, BTEX, gas or liquid samples pesticides and hydrocarbons in water

16–18 20,21

Exhaustive Techniques ITEX MEPS NeedleEx Fiber in Needle Needle Trap

sorbent packed cartridge attached to needle sorbent packed cartridge attached to needle quartz wool and packed sorbent in a 21-ga needle fiber bundle sorbent in needle packed particulate sorbent in needle

hydrocarbons from aqueous headspace liquid analysis, drugs in plasma VOCs from gaseous samples phthalates in wastewater, drugs in urine gaseous sample analysis

44 45 50,60 4 50

Fundamentals and Applications of Needle Trap Devices

2.30.3.2

685

Sorbent Immobilization

The immobilization of sorbents has been accomplished with the use of side-hole needles,50 glue combined with inert wire49 or frits.46 To retain sorbent with frits, Kubinec et al. employed a 1.1-mm ID stainless steel tube and fashioned two stainless steel o-rings from 1.1-mm OD tubing to retain the frits at each end of a multilayered sorbent bed.46 Three stainless steel frits with 20-mm porosity and 0.16-mm depth were fitted inside the tubing to separate and retain aluminum oxide and Porapaq Q sorbents. For the side-hole needle approach, Wang et al. explored two possibilities.50 First, they described a 21-gauge needle (Hamilton) with a sealed pointed tip and side hole near the point. Quartz wool was packed into the tip of the needle up to the level of the side hole. Separate layers of PDMS, DVB, and Carboxen were subsequently added. No additional mechanism to retain the sorbent at the Luer-lock end of the needle was employed. NTDs based on this construction are presently available from Shinwa as NeedlEX (Shinwa Chemical Industries Ltd, Kyoto, Japan). In the second option, a 22-gauge open needle with an open blunt tip was packed with Carboxen 1000. A side hole was incorporated 3 cm back from the tip. The sorbent was packed either right to the tip for exhaustive grab sampling or at a set distance back to act as a diffusion barrier for TWA sampling. To immobilize the sorbent, a mixture of rapid curing (5 min) epoxy glue and the sorbent was packed into the needle. Before the glue cured, a syringe was connected to the needle. Air was moved in and out of the sorbent bed to prevent the epoxy from curing as a solid block and plugging the needle trap. After full cure of the epoxy, the needle trap was conditioned in a GC injector at 300  C for 5 h to remove impurities. In the inert wire method,49,51,52 a spring was first fashioned by wrapping a 0.002"-diameter stainless steel wire in six revolutions around a 0.006"-diameter support wire. The ends of the thin wire were trimmed and set flush with the support wire. The spring was then positioned into a 22-gauge needle at the Luer-lock end of the desired packing length, by temporarily inserting a support wire into the Luer-lock end of the needle, after which the support wire is removed. Sufficient packing is added with the aid of a water aspirator attached to the Luer-lock end of the needle and finally a small drop of fast cure (5 min) epoxy is aspirated into the packing through the tip of the needle. After an initial cure at room temperature while attached to the aspirator, the needle was moved to a stand-alone heater or GC injector (300  C) and nitrogen was passed through the sorbent bed for 2 h. The choice of incorporating a side hole with the latter means of retaining sorbent was optional, and dependent on the operator’s choice of desorption method.

2.30.3.3

Device Configurations

In 2003, Pawliszyn and Bloomberg reported the designs and applications of sorbent-packed needles (needle trap) and syringes (MEPS), respectively, at ExTech 2003 in Tampa and they discussed initial results corresponding to spot and TWA gas and automated liquid sampling/enrichment, which were later published.45,50,53,54 In MEPS, a small amount of a sorbent is packed in the barrel of a gas-tight syringe, or in a special container positioned between the barrel and the needle. The goal was to provide an automated high-throughput solution for conventional SPE, where sample preparation is fully integrated with other components in the analytical system. An additional goal was to miniaturize the process such that a minimum volume of solvent was required for elution, with quantitative introduction of eluent into the instrumental analysis system, typically LC, GC, or directly to mass spectrometry (MS). Full automation was enabled by the use of a CTC-PAL autosampler. The MEPS method has much in common with typical syringe cartridge SPE as the same sorbents are used in both techniques; therefore, transferring a method from traditional SPE to MEPS is relatively straightforward. A number of automated pharmaceutical and clinical applications have been developed based on this technique.6 The system is now available commercially from SGE (Ringwood, Australia). Two types of needle trap devices were proposed to address convenience in packing and desorption, in order to improve manufacture and performance of the NT technology, particularly in desorption modes.50 One type of NTD, shown in Figure 7, was designed with a sealed tip in which particles of PDMS, DVB, and Carboxen were packed in sequential discrete layers. The lengths of the layers were 3, 2, and 2 mm, respectively, and quartz wool was packed between the tip of the needle and the side port. Packing in this way was easier as the sorbent cannot fall out. In this approach, simple desorption is accomplished by considering the fact that

Figure 7 Schematic of the NTD packed with PDMS, DVB and Carboxen particles. From Wang, A.; Fang, F.; Pawliszyn, J., J. Chromatogr. A 1072 (2005) 127–135, reprinted with permission.

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Theory of Extraction Techniques

the flow of the gas is naturally reversible. Thermal desorption can be simplified if the structure of the sorbent in the micro-cartridge consists of layers of with different sorbing characteristics divided into segments (as shown in Figure 7) by locating the weakest sorbent close to the entrance passage (left end) and the strongest sorbent deepest into the cartridge near the closure (right end). In this arrangement, the middle sorbent is preferably of medium effectiveness. This will result in the analytes that are the most difficult to desorb being adsorbed on the third sorbent from the entrance, the analytes of medium difficulty will be adsorbed on the second sorbent from the entrance, and the easiest analytes to desorb will be adsorbed on the first sorbent from the entrance. Desorption may be accomplished by applying inert gas to the Luer-lock end. Also, water vapor46 and expanded gas in a needle49 can facilitate desorption, as is described in detail below (Section 2.30.3.5). If the strongest adsorbing sorbent is located nearest the entrance or if only one strong sorbent is used, the sorbent will adsorb strongly binding analytes so tightly that it would be difficult to desorb analytes from the sorbent bed. This configuration of NTD has been further investigated more recently for the analysis of a wide range of VOCs (b.p. 80–200  C) using single beds (1 cm) of either Carbopak X or Tenax TA.55 After extraction, the Luer-lock end was sealed with a push-button syringe valve from SGE Europe (Milton Keynes, UK). Desorption was by thermal expansion with no make-up gas to aid desorption. The authors reported that carryover was not a problem for the range of compounds tested but did not provide data to demonstrate this claim. The authors also found that the use of epoxy to fix the packing material was problematic for ultra-trace analysis. The second type of NTD in which Carboxen 1000 was packed near the blunt tip of the needle was designed with a side hole (i.d. 0.016 in.) positioned 3 cm from the tip of the needle, as illustrated in Figure 8(a).50 For desorption the carrier gas enters the needle through side hole, passes through the sorbent, and aids the delivery of desorbed analytes into the GC column. This desorption method was convenient and no carryover was observed. This type of NTD, used to quantitatively detect BTEX, was simpler and more convenient than the design associated with the modification of the carrier gas line or an additional volume of clean air injection. In this case, desorption was carried out by flow created as the carrier gas was diverted through the needle. Figure 8(b) illustrates this desorption approach, which does not require a valve, but rather uses an injection liner containing a restriction to divert the carrier gas flow through the sorbent bed when the needle is introduced through the septum (not shown) into the liner, producing very low dead volume. Figure 8(c) shows a schematic side view of the NTD located in the liner. The side-hole needle desorption approach has recently been demonstrated as effective in largersized needles.56

Figure 8 (a) Side-hole needle with inert spring to retain sorbent; (b) preferred desorption approach which does not require a valve, but rather uses a liner containing a restriction to divert the carrier gas flow through the cartridge when the needle is introduced through the septum (not shown) into the liner. From Gong et al. Anal. Chem. 80 (2008) 7275–7282, reprinted with permission; (c) Schematic side view of the cartridge located in the liner [from Wang et al. Journal of Chromatography A, 1072 (2005) 127–135, reprinted with permission].

Fundamentals and Applications of Needle Trap Devices

2.30.3.4

687

Sorbents

The bulk of the efforts to date in developing NTD have employed conventional sorbents for extraction. For internally coated needles (INCAT and SPDE), sorbents conventionally used for wall coating have been applied, such as PDMS, carbowax, or carbon particles.2,16,57 For packed needles and syringes, bare silica particles, silica particles coated with PDMS or alkylsilanes, cation exchange particles, divinylbenzene co-polymers, Tenax, Carbopak, and Carboxen have been employed, in either continuous or segmented beds.6,48,50,58,59 The performance characteristics of the sorbents generally mirrored the experience with their use in SPE, so method transfer from SPE to needle trap was straightforward. In addition to conventional sorbents, more polar ones have been proposed and introduced by Saito et al.60 They proposed the use of a newly synthesized polymeric extraction medium consisting of a copolymer of methacrylic acid and ethylene glycol dimethacrylate. The results clearly demonstrated the excellent extraction performance for typical organic solvents and also suggested future possibilities such as in the applications for the analysis of work environments. These materials have also been commercialized by Shinwa (Kyoto, Japan) and the needles have been used for extraction of very polar compounds including formic acid. A combined method of dynamic headspace-needle trap sample preparation and GC for the determination of formic and acetic acids in aqueous solution was developed. A needle extraction device coupled with a gas aspirating pump was used to perform sampling and pre-concentration of target compounds from aqueous sample before GC analysis. The needle trap extraction technique also allows for the successful sampling of short-chain fatty acids under dynamic conditions while keeping the headspace (HS) volume constant.61 Two important parameters, including extraction temperature and effect of acidification, have been optimized and evaluated using the NTD. The method detection limits for the compounds estimated were 87 g l1 for acetic acid and 235 g l1 for formic acid in spite of the low flame ionization detection response for formic acid and its low Henry’s law constant in aqueous solution. Precision was determined based on two real samples and ranged between 4.7 and 11%. The validated headspace-needle trap extraction method was also successfully applied to several environmental samples.61 Around the same time, the group of K. Jinno also introduced the fiber in needle concept and subsequently developed devices with a range of polarity characteristics for their application in GC and SPE.4,62–64 The technology was based on advances made previously in the use of a fiber-packed capillary as the extraction phase for sample preparation for liquid-phase separations, and as the stationary phase in GC. Polymeric fiber filaments were used as the extraction medium for microscale liquid-phase and gas-phase separations and a miniaturized sample preparation technique consisting of a fine fiber-packed needle as the extraction medium was described. The primary application was to the analysis of volatile organic compounds by GC. When the needle was packed longitudinally with a bundle of fine filaments (12 mm o.d.), which were also surface-coated with polymeric materials, successful sample pre-concentration was obtained. The storage performance of the needle clearly demonstrated the potential of the technique for typical on-site sampling during environmental analysis. A comprehensive review on the subject appeared recently.7 The authors’ previous work with extraction devices constructed from fiber-packed needles for on-line coupling of miniaturized sample preparation and chromatographic separation and for hyphenation of LC-GC also bears mention.65 A fiber-packed needle was developed on the basis of several successful studies on the use of a fiber-packed capillary as a miniaturized extraction and separation medium.4 The same group enhanced sensitivity for the method by introduction of a derivatization reagent in the needle, similar to the way this was accomplished previously in SPE and SPME. The resulting simultaneous extraction and derivatization enhanced the sensitivity and selectivity of the detection and determination of target analyte(s).63 The authors described a needle device designed for GC analysis of aldehydes and ketones found in typical in-house environments. A bundle of polymer-coated filaments was packed longitudinally into a specially designed needle. Derivatization reactions were facilitated by 2,4-dinitrophenylhydrazine (NDPH) preloaded in the needle, to convert the aldehydes and ketones to their corresponding hydrazones during extraction. Although some additional studies targeting a more systematic optimization of the extraction and determination procedures might be necessary along with additional investigations into the derivatization reaction and isomerization, the results suggest the practical applicability of the method to the analysis of other volatile compounds in typical in-house air. The results highlighted potential future benefit in the further development of the technology, particularly in relation to the development of other fibrous materials and the design and synthesis of additional novel polymeric extraction media utilizing surface derivatization reactions,66 and its application to biological gas or liquid analysis with high selectivity. A needle trap (NT) technique for simultaneous sampling and analysis of vapor and particle mercury in ambient air using gold wire filled in a syringe needle has been developed.67 This NT technique relies on gold amalgamation rather than adsorption/ absorption to achieve traditional solid-phase microextraction. Mercury trapped by gold amalgamation in the NT device is thermally desorbed in a hot injection port of a gas chromatograph. Desorbed mercury is then determined by the coupled mass spectrometer. This simultaneous sampling and analysis technique was optimized, tested, and used for the collection and accurate determination of elemental mercury in ambient air. Linear calibration curves spanning over 4 orders of magnitude were obtained for mercury sampling by NT when mass spectrometry (MS) was used for detection. MS offered excellent sensitivity and selectivity. Selected ion monitoring (SIM) mode was used for the linear calibration curves. The selected quantification ion was m/z 202, as this was the strongest isotope of the mercury mass spectrum. The method was verified with HgCl2-spiked solution samples. An excellent agreement was found between the results obtained for the mercury-saturated air samples and HgCl2-spiked solution samples. Molecularly imprinted polymers have seen extensive use in extractions for selective sample preparation, and these so-called plastic antibodies have also been employed in needle trap format.68 The authors described a MEPS application where approximately 1 mg of a solid MIP sorbent was inserted into a syringe (100–250 ml) as a plug inside the syringe, or between the barrel and the

688

Theory of Extraction Techniques

needle. An autosampler was used to draw 20–250 ml of plasma through the sorbent, which was subsequently washed with 50 ml of water and the analytes eluted with 20–50 ml of solvent directly into the injector of an LC. A bupivacaine imprinted polymer was used for the analysis of ropivacaine in plasma in the range of 2–2000 nM with high correlation coefficients (R2 0.999). The assembly could be used more than 100 times with extraction recovery of 60%. The accuracy, given as a percentage deviation from the nominal concentration values, ranged from 26 to 3%. The precision at three different concentrations in QC samples was 3–10% and the limit of quantification was 2 nM. Monolithic materials were first described for use in separations69 and have also seen application to sample preparation.70 In 2006, Zhang et al. described a monolith prepared in fused silica capillary, incorporated in a needle trap device.71 A 2-cm section of the monolith-filled capillary was used in place of the steel barrel in a hypodermic needle assembly. The device was assembled with a disposable plastic syringe and used to extract angiotensin II receptor agonists from 2 ml of human urine, with the extracted analytes subsequently desorbed in 50 ml of acetonitrile and analyzed by CZE. Recoveries of 80% were achieved with precisions of 1–3% for analysis of samples at concentrations between 0.08 and 3 mg ml1. More recently, Blomberg has described a monolithic acrylamide plug as sorbent in polypropylene tips primarily intended for use with 96-well plate systems.6 The devices were used for quantitative analysis of local anesthetics in human plasma with a limit of quantification (LOQ) of 2 nmol l1. Accuracy for qualitycontrol samples was between 101 and 118% and precision ranged from 4 to 17%. Sae-Khow and Mitra recently reported the implementation of micro-solid-phase extraction (m-SPE) employing nanomaterials in the needle of a syringe for integrating sampling, analyte enrichment, and sample introduction into a single device.72 Both singleand multiwalled self-assembled carbon nanotubes (CNTs) were explored as high-performance sorbents for m-SPE in packed and self-assembled formats. The need for such a sorbent was critical because the needle probe could hold only a small amount of material (around 300 mg). Conventional C-18 and self-assembled CNTs were found to be ineffective with enrichment factors less than one. However, packed beds of CNTs were found to be excellent sorbent phases, where high extraction efficiencies (as high as 27%) as well as enrichment factors close to seven could be achieved. The overall method showed excellent linearity, reproducibility, and low method detection limit (0.1–3 ng ml1 for MWNTs). The sorption on CNTs followed Freundlich isotherms and functionalized CNTs were more effective for enriching the polar compounds. The group of H. Bagheri has also investigated the use of nanomaterials as sorbents for needle trap devices for the analysis of PAHs in aqueous samples. In one example, short CNTs (length 1–10 mm) were prepared in a sol–gel polymeric network.73 In a second example, nano-silica particles with an oleic acid grafted surface were employed.74 15-mm and 10-mm bed lengths and 22- and 24ga. needles were used, respectively. In both cases, a side hole above the sorbent bed allowed for desorption by directing carrier gas flow through the sorbent bed during injection (Section 2.30.3.5.5).

2.30.3.5

Desorption

Achieving efficient desorption with needle trap devices has been a focus of attention for many authors. Several approaches have been investigated to date, including filling the syringe with air or inert gas to aid desorption in a heated injector, diverting carrier gas flow at the injector to the inlet of the needle, incorporating a side hole in the needle and employing a narrow-neck liner to seal the tip of the syringe to enable desorption in an unmodified injector, simply heating the sealed needle in a GC injector and relying on thermal expansion of the contained gases to sweep analytes to the column, the addition of a small amount of water inside the needle to facilitate desorption with heated water vapor, or the application of an external heater to the sorbent outside of an instrument injector to effect external thermal desorption.

2.30.3.5.1

Air-Assisted Desorption

2.30.3.5.2

Thermal Expansion Desorption

In this simplest and first implemented desorption technique,41 clean air is drawn into the syringe barrel and expelled to aid desorption when the needle is inserted into the GC injector. Koziel et al. used just 10 ml of air to desorb diesel exhaust compounds from a needle trap consisting of a 5-mm-long bed of quart wool packed into a 23-gauge needle. Later Lipinski used 2.5 ml of air to desorb analytes from an SPDE device packed with PDMS for the analysis of pesticides in water.19 The authors compared desorption aided by air injection with carrier gas-aided injection and found that carryover was higher with the air injection. Subsequently, Wang et al. also compared air injection with carrier gas for desorption.50 These authors observed split injection peaks for analysis of VOCs (alkanes and BTEX). The first peak was found to be due to desorption aided by thermal expansion of gases within the needle and the second due to analytes desorbed when the air plug was pushed through.

Several authors have conducted needle trap or related desorptions by thermal expansion only. In one of the earliest demonstrations of the technology,16 McComb et al. describe the preparation of needles with internal diameters of 0.25 or 0.4 mm, either coated internally with carbon or containing a section of GC capillary coated with DB-5 (INCAT). After either active or passive sampling of gas, liquid, or headspace samples containing VOCs, the Luer-lock end of the needle was plugged with septum and the needle desorbed at 175  C in a GC injector. While a DB-5 extraction phase resulted in extensive peak tailing, analytes desorbed efficiently from the thinner carbon sorbent, resulting in good chromatographic peak shape. The authors suggested this was due to both the difference in sorbent thickness as well as the difference in thermal conductivity between the DB-5-coated fused silica and the carbon. These authors subsequently published advances in the INCAT technique and application to a broader range of analytes.17 Optimization of the thermal expansion desorption technique resulted in a reduction of carryover. The method compared favorably to purge-and-trap.

Fundamentals and Applications of Needle Trap Devices

689

Eom et al. subsequently made a study of desorption characteristics for thermal desorption of an alkane mixture (C6–C15) from a packed DVB sorbent bed.51 Desorption time and temperature, along with the optimal dead space volume in the needle above the sorbent, were investigated. Thirty seconds of desorption at 250  C was determined to be optimal. Peak shape deteriorated noticeably with increasing dead space volume. Zero dead space was determined as optimal. Wang et al. also investigated the effect of the GC injector temperature profile for thermal expansion desorption.50 These authors determined that the length and profile of the temperature variation within the injector were critical parameters in determining the optimal sorbent bed length. Carryover was significantly worse if part of the sorbent bed was located outside of the optimal heated zone in the injector during desorption. Although very simple in design and use, direct thermal expansion desorption has not been widely applied for needle trap technologies, likely due to the limitations of desorption carryover and high dependence on analyte type, device configuration, and GC injector set-up for desorption efficiency. Where these variables can be adequately controlled, the method proves simple and effective.

2.30.3.5.3

Water Vapor-Assisted Desorption

Several authors have noted improved desorption efficiency and broader application when water vapor expansion is incorporated in thermal expansion desorption. Prikryl et al. demonstrated the use of water vapor to aid thermal expansion desorption, to desorb VOCs from a DVB sorbent bed.48 The ’needle concentrator’ was packed with separate layers of DVB (divinylbenzene) and alumina: DVB was used for enriching VOCs and alumina was used to adsorb water. After enrichment of BTEX from water by dynamic headspace sampling, the alumina layer of the concentrator was loaded with water and then the device was inserted into a GC injector for thermal desorption. The water on the alumina layer was vaporized quickly due to the hot injector temperature, which desorbed and flushed the BTEX compounds into the separation column. A clean and sharp BTEX chromatogram was achieved but the use of water for aiding desorption was problematic. The massive water injection produced a long tailing water peak with high noise and which co-eluted with the early eluting benzene and toluene. As a result, poorer LODs for benzene and toluene were reported compared to those of ethylbenzene and xylenes. Also slightly worse detection and quantification limits were observed for all compounds in comparison with the purge-and-trap method. In addition, bulk injection of water can shorten column lifetime and cause possible interferences with detectors.

2.30.3.5.4

Inert Gas-Assisted Desorption

Numerous authors have recognized that superior desorption performance may be obtained by using inert mobile phase gas passed through the sorbent bed during thermal desorption. The major disadvantage of using a syringe filled with air during desorption is the introduction of oxygen to the sorbent and resulting potential for sorbent degradation. Soon after the appearance of the INCAT and SPDE techniques, authors described the use of inert carrier gases to aid desorption. Jochmann et al. described a popular desorption technique whereby a volume of inert gas (nitrogen in this case) is withdrawn through the needle trap into the syringe, after extraction and just prior to desorption.57 In this work, commercially available SPDE needles (Chromtech, Germany), coated with carbowax, cyanopropylphenyl/PDMS, PDMS, and PDMS with 10% embedded activated carbon (AC) were compared for the extraction of polar VOCs. The target compounds included 3 ethers and 12 alcohols. Efficient desorption for a range of the target compounds was achieved by withdrawing 1 ml of nitrogen and desorbing over 20 s at 200  C in splitless mode. The lowest method detection limits were obtained with the carbowax and the PDMS/AC phase. Following desorption the needle was removed from the injector and flushed with nitrogen for 5 min at 200  C to eliminate carryover. Joachmann et al. have also described desorption processes for their work on ITEX (see Figure 6(b)) where the larger diameter of the needle body is packed with Tenax TA.44 In this configuration, the packed sorbent bed is located external to the GC injector during desorption with an external heater to control desorption temperature. VOCs are extracted from sample headspace by dynamic extraction. After extraction, 700 ml of helium was withdrawn from the injector into the syringe. Subsequently, the desorber around the needle body was heated in a few seconds to 170  C and the sorbed analytes were transferred with a desorption flow rate of 10 ml s1 into the hot injector. After desorption, the device was flushed with nitrogen gas at 210  C for 20 min to prevent carryover and to condition the sorbent for the next sample. Slower desorption flow rates were found to be more effective, with 4 to 26 higher peak areas observed for desorption flows of 10 ml s1 relative to 100 ml s1. The effect of desorption volume in the range of 500–1000 ml was less important. Ueta et al. have described a needle packed with a particulate sorbent consisting of copolymer of methacrylic acid and ethylene glycol dimethacrylate, applied to breath acetone analysis.75 The sorbent was packed into a sealed tip needle with a side hole adjacent to the tip. On completion of the sampling, the needle was removed from the vacuum sampler and attached to an injection syringe. N2 gas (0.5 ml) gas was withdrawn, the needle was inserted to a heated GC injection port, and the extracted analyte was injected by the N2 gas in the syringe after a preheating time of 10 s in the injector. The desorption was efficient and quantitative recovery of acetone was maintained over all the concentration range between 10 ppmv and the limit of quantification (0.001–0.01 ppmv). A limitation of using a syringe filled with inert gas to aid desorption is the potential of having an insufficient amount of desorption gas to quantitatively move all analytes from the sorbent to the analytical device. As a result, several authors have investigated means of directing carrier gas through the sorbent bed during desorption as an alternative to withdrawing inert gas into the needle and injecting this to aid thermal desorption. Two general means of achieving this have been described.50 Where a standard needle is employed, a secondary carrier gas split valve is used. In the separation mode, the carrier gas flows to the injector and through the column as normal. For desorption a line from the split valve is connected to the back of the needle trap and carrier

690

Theory of Extraction Techniques

gas is directed to flow through the needle during desorption. The other option described utilizes a needle with a side hole positioned about 3 cm back from the tip of the needle, with sorbent packing located between the side hole and the tip as previously described in this Section (2.30.3.2). When a narrow-neck liner is used, the needle trap can be injected to the resistance point where it is sealed against the narrow neck (Figure 8(c)). Carrier gas is then automatically directed to flow through the side hole and sorbent before entering the column. In the carrier gas bypass mode, any standard GC injector liner can be employed but the system is a bit more difficult to automate. In the side-hole approach, the needles are somewhat more cumbersome to prepare and the injector liner must be changed, but the technique is the simplest to automate. Wang et al. compared desorption efficiencies either by applying the desorption gas to the Luer-lock end of the needle trap through a divert valve, when the needle was in a conventional GC injector liner, or by employing a side hole in the needle with a narrow-neck GC liner and diverting the injector carrier gas through the sorbent during desorption.50 The latter method, where injector carrier gas was automatically diverted through the needle side hole during desorption, was convenient and no memory effect was observed. As described previously (Section 2.30.3.3), the side-hole approach for diverting carrier gas through the sorbent bed for desorption has also been demonstrated for a large-diameter packed needle device (INCAT).56 The same group has recently demonstrated an improvement to the injector design to reduce the potential of contaminants adsorbed on the outside of the needle from entering the GC column during desorption.76 Impurities originating from the needle surface are intended to be flushed out through the split vent during desorption as the analytes desorbed from the sorbent are transported to the column, although the magnitude of the benefit of this modification was not investigated. To summarize all of the various gas-assisted desorption regimes evaluated to date, it seems that a consensus is developing that desorptions assisted by inert gas flushing of the sorbent bed generally produce the most efficient desorptions with the least amounts of carryover, although the techniques are more challenging for device design and more cumbersome and difficult to automate than thermal expansion desorption options. Further efforts to simplify these processes would aid considerably in the general acceptance of the technology.

2.30.3.5.5

Solvent Desorption

Solvent desorption has also been frequently applied, particularly where the aim is to inject desorbed analytes to LC (or possibly GC) for analysis. Saito et al. have reported a sample preparation strategy termed ‘fiber-in-tube’ for the direct coupling of aqueous extraction to LC for the analysis of n-butylphthalate in wastewater.77 To prepare the extraction tube, a heterocyclic polymer (ZylonÒ) multi-filament fiber was cut to 10 cm and packed longitudinally into the same length of PEEK tubing (0.25 mm i.d.). The diameter of each filament in the fiber was about 11.5 mm with about 280 total filaments packed in the PEEK tubing. The observed preconcentration factor for phthalate was about 160 with a 20-min extraction at 16 ml min1. Solvent desorption involved pumping pure methanol through the tube at 2 ml min1 for 3.5 min. The extracted analytes were desorbed and directly transferred into the loop of the LC injection valve. The volume of the desorption solvent and the flow rate were optimized to ensure that the analytes were quantitatively transferred to the injection loop during the desorption and to eliminate carryover. Limits of quantification were estimated at sub-ppb levels for various organic analytes in an aqueous sample matrix. MEPS has been coupled to GC with solvent desorption.53 In this report, 1 mg of sorbent (C2 silica) was packed into a syringe for the extraction of local anesthetics from plasma. Plasma (50 ml) was drawn through the sorbent bed followed by a wash with 50 ml of water. The analytes were then eluted with 30 ml of methanol directly into the GC injector. The authors reported that it was sometimes necessary to dilute the plasma with water 1:1 to avoid clogging of the sorbent bed. Extraction recoveries of 60% were reported with LOQ of 10 nM. To reduce carryover between samples to the range of 0.2%, the sorbent bed was washed four times with methanol and four times with water after every injection. Altun et al. followed up on this work with a study of MEPS sample preparation for LC.45 The sorbent was switched to 1 mg of silica-based benzenesulfonic acid cation exchanger packed in a 250-ml syringe. Again, the application was the analysis of local anesthetics and their metabolites from plasma. The goals of the effort were a simplified sample preparation with a low quantification limit, high recovery, and the possibility to automate. Plasma (25 ml) was extracted at 20 ml s1. After a water wash (100 ml), desorption was accomplished with 50 ml of methanol/water 95:5 with 50% recovery reported. The sorbent was cleaned and reconditioned between analyses with 5  50 ml of elution solution followed by 5  50 ml of the washing solution, which resulted in 0.5% carryover. The LOQ for the method was 2 nmol l1 with an accuracy of 97–105% and precision ranging between 8% and 11%. To date, numerous sorbents have been investigated with the MEPS technology based on the nature of the target analyte, in order to achieve acceptable cleanup and recovery.6 Relatively large particles are used to avoid high backpressures, with generally 1 mg packed into a 100–250-ml syringe. A desorption solvent of 50 ml is typically employed, with the advantage that the entire elution volume may be injected into the chromatograph. Heating of the syringe during elution improves desorption efficiency. An advantage is that the technique may be fully automated with carryover being the primary limitation. More recently, the technology has been applied to polypropylene pipette tips packed with monolithic sorbent. A significant advantage with this approach is that the price per unit is reduced and so the sorbent may be replaced for each extraction, effectively eliminating any possibility of carryover while maintaining a high degree of automation. Sorbent amount was increased to about 3 mg and sample volume increased to 100–150 ml. Elution was with 100–150 ml of 60% methanol/water. Limits of quantification for local anesthetics in plasma were reported as 2 nmol l1 with an accuracy between 101 and 118% and precision between 4 and 17%.

Fundamentals and Applications of Needle Trap Devices

2.30.4

691

Applications

Numerous applications have been introduced in the preceding text describing the development and evolution of the various needle trap-based techniques. These have been summarized in Table 2 according to the device and method used to provide the reader with a convenient comparison of the types of samples, sorbents, techniques, and method parameters described by the various authors. From these, some broad categories of applications warrant more detailed discussion, which is presented below.

2.30.4.1

Breath

To facilitate their use in trace gas analysis, the adsorption capacity of needle trap devices (NTDs) was increased by combining three adsorbent materials and increasing total adsorbent amount.58 The use of 22-gauge needles, application of thermal expansion desorptive flow without cryofocusing, and a new on-site alveolar sampling method for NTDs provided sensitivity in the parts per trillion range of VOC concentrations without losing precision or linearity. LODs were 0.4 ng l1 for isoprene, 0.5 ng l1 for dimethyl sulfide, 0.9 ng l1 for 2-butenal, 1.0 ng l1 for hexane, 1.2 ng l1 for pentane, 2.3 ng l1 for hexanal, 5.3 ng l1 for pentanal, and 8.3 ng l1 for acetone. Calibration curve linear correlation coefficients (r2) were consistently >0.98. Loss of volatile aldehydes during storage for 7 days was less than 10%. NTDs packed with more than one adsorbent material represent a promising alternative to SPE and SPME for analysis of volatile organic compounds in the low parts per billion/parts per trillion range. Crucial problems of clinical breath analysis concerning sensitivity of analytical methods, limited stability, and decomposition of breath compounds during sampling and storage could be solved. The authors recently extended this work to GC  GC characterization of breath samples extracted by a multibed sorbent NTD.78 NTDs used with high-throughput automatic desorption and separation systems were tested in a study with patients undergoing cardiac surgery for analysis of blood-based biomarkers, intravenous drugs, and clinical contaminants. The use of heart-cut GC/MS allowed the linearity of analyte response to be conserved even in the presence of high concentrations of contaminants such as anesthetic gases. Another approach to the determination of human breath acetone with particle-packed sample preparation needle was developed by the group of K. Jinno.75 The extraction needle was packed with a copolymer of methacrylic acid and ethylene glycol dimethacrylate as the extraction medium. For the analysis of breath samples, exhaled breath was collected in a sampling bag, and 50 ml of the breath sample was extracted with the needle-type sample preparation device followed by analysis using GC/MS. After the optimization of several basic extraction conditions for standard acetone samples, breath acetone concentration taken from controlled type-2 diabetic patients was determined. Furthermore, time variations of breath and urine acetone of four healthy individuals under fasting conditions were measured. Urine samples were collected in glass vials and urine acetone concentration was determined with the extraction needle by analyzing the corresponding headspace gas. The results demonstrated that the particle-packed extraction needle showed an excellent extraction performance for acetone in both breath and urine headspace samples, and that there is a clear correlation between the concentration of breath acetone and glycosylated hemoglobin levels in controlled type-2 diabetic patients. The breath acetone levels in controlled diabetic patients were in a range between 0.19 and 0.66 ppmv, where its concentration in medically untreated type-2 patients was between 0.92 and 1.20 ppmv. The breath acetone concentration in healthy males was increased to 5.66 ppmv under the 24-h fasting test and a high correlation between the breath and urine acetone concentration was also observed. On the basis of the above results, the potential applications of the proposed method to the diagnosis of diabetes and/or ketoacidosis were suggested.

2.30.4.2

Distinguishing Free/Total

For many matrixes, a significant portion of the analyte of interest is found closely associated with one or more discontinuous phases, with an equilibrium condition determining the proportion of free vs. bound analyte. Depending on the configuration of the extraction device and the nature of the sample preparation, either the free, bound, or total analyte concentration may be determined. Whereas a filter will physically retain particulate matter, sorbent extracts only freely dissolved analyte. However, where sample is percolated through a packed sorbent bed, the bed may act as both a filter and a sorbent, thereby retaining total analyte. Koziel et al. described the use of filter media (quartz wool) inside an NTD, to determine analyte bound to particulate matter after trapping of the particulate and thermal desorption of the bound analyte.41 Simultaneous extraction of the gas sample by SPME provided free analyte concentration. The utility of the technique was demonstrated through the analysis of polycyclic aromatic hydrocarbons (PAHs) in diesel exhaust, triamcinolone acetonide in a dose of aerosolized asthma drug, and DEET in an application of insect repellent spray. The authors pointed out that it should be possible to combine the two samplers into one device for simplified combined sample preparation, with the option of performing thermal desorption either separately or together, depending on whether free, total, or particle-bound concentrations were desired. The authors also pointed out the potential utility of NTD with packed sorbent. The simultaneous determination of total (particle-bound plus free) concentrations of analytes with packed sorbent bed NTD and free concentrations with SPME was later described for the analysis of smoke generated by a mosquito coil.79 Allethrin as the active ingredient in mosquito coils was chosen as the target analyte. Under the same sampling conditions, the amount of allethrin extracted from the mosquito-coil smoke was higher for the NTD compared to the SPME fiber, while the extracted amounts were almost the same for both devices when sampling gaseous samples of allethrin. Semivolatiles were extracted more efficiently by the

692 Theory of Extraction Techniques

Table 2 Summary of applications for needle trap and related techniques. TD: thermal desorption; N/A: the information is not provided or investigated; part per billion (ppb) and part per trillion (ppt) are based on weight/volume Parameters Method

Instrumentation

Application

Sampling mode

Desorption

LOD

Publication year Reference

GC-FID

BTEX in both air and water

Active and passive

Thermal expansion

N/A

1997

16

GC-FID GC-FID GC-FID GC-FID GC-FID GC-ECD/NPD GC-MS GC-MS

BTEX in aqueous solution BTEX in air BTEX in aqueous sample BTEX in aqueous sample Both liquid and gaseous BTEX sample Pesticides in water Polar VOCs in water Volatile organic hydrocarbons in water

Active Active and passive Active Active Active Active Active Active

Thermal expansion Thermal expansion Water vapor flow enabled TD Water vapor flow enabled TD Carrier gas-assisted TD External air-assisted TD External N2-assisted TD External N2-assisted TD

65 ppb–2 ppm N/A 0.23–0.36 ppb 0.059–0.125 ppb N/A 1–100 ppt 0.004–4.9 ppb 12–870 ppt

1999 1999 2004 2006 2009 2001 2006 2007

17 18 46 47 56 19 57 20

Exhaustive extraction methods Cation-exchange sorbent Silica C2 particle and other materials Molecularly imprinted polymer Polymer monolith

HPLC-MS GC-MS LC-MS-MS CZE-UV/VIS

Active Active Active Active

Solvent desorption Solvent desorption Solvent desorption Solvent desorption

N/A N/A 2 nmol l1 15–20 ppb

2004 2004 2006 2006

45 53 68 71

Charcoal and silica gel

GC-FID

Active

Carrier gas-assisted TD

A few ppb

1997

40

Quartz wool Tenax Multilayer (PDMS/DVB/Carboxen) and Carboxen

GC-MS N/A GC-FID

Anesthetics in human plasma Anesthetics in plasma samples Ropivacaine in plasma Angiotensin II receptor antagonists in human urine Gaseous trace organic compounds in human breath and air Airborne particulate matter and aerosol Tobacco smoke VOCs in air

Active Active Active and passive

Air-assisted TD Carrier gas-assisted TD External air or carrier gas-assisted TD

N/A N/A 0.23–2.1 ppt

2001 2003 2005

41 43 50

Sorbents

Microextraction methods INCAT DB-5TM or coating of carbon in needle Carbon blacks Carbon coating Porapak Q and alumina Porapak Q and alumina Tenax TA SPDE PDMS WAX, 1701, PDMS, PDMS/AC PDMS with embedded activated carbon MEPS

Needle Trap

GC-FID

VOCs in air

Active

External N2-assisted TD

N/A

2006

60

GC-MS

Active

External N2-assisted TD

1.0–3.6 ppt

2007

62

Passive Active Active Active Active

Carrier gas-assisted TD Carrier gas-assisted TD Thermal expansion Thermal expansion External N2-assisted TD

37 48 49 51 61

GC-MS GC-MS

Active Active

Carrier gas-assisted TD Thermal expansion

N/A 0.05–0.07 ppb 1 ppb N/A 87.2 ppb and 234.8 ppb 0.23 ppt 0.4–8.3 ppt

2008 2008 2008 2008 2008

Gold wire Multibed (Carboxen 1000, Carbopack X, Tenax) Carbon nanotubes

Smoking-related compounds in hair and air samples VOCs in air BTEX in aqueous samples BTEX in aqueous samples VOCs in air Formic and acetic acids in aqueous solution Vapor mercury in ambient air Volatile breath biomarkers

2008 2009

67 58

Active

Solvent desorption

0.1–3 ppb

2009

72

Active

External N2-assisted TD

N/A

2009

75

Active

Carrier gas-assisted TD

N/A

2009

79

Active Active Active

Solvent desorption External N2-assisted TD Solvent desorption

Sub-ppb level 1.2–11.7 ppt 0.2–2 ppb

2000 2006 2007

77 63 64

Active

Solvent desorption

N/A

2009

65

Active

External He-assisted TD

28–799 ppt

2008

44

Copolymer of methacrylic acid and ethylene glycol dimethacrylate Divinylbenzene particle Fiber in needle Polymer-coated fiber Polymer-coated fiber Polymer-coated fiber Novel polymercoated fibrous phase ITEX

Tenax TA

GC-MS GC-FID GC-FID GC-MS GC-FID

LC-UV GC-MS GC-MS

2-Nitrophenol, 2,6-dichloroaniline, and naphthalene in water Breath acetone

Free and particle-bound compounds in mosquito-coil smoke LC-UV/VIS n-Butylphthalate in wastewater GC-MS Volatile aldehydes in air sample GC-MS Aromatic compounds in aqueous samples 2D LC-UV/VIS- Aliphatic and aromatic hydrocarbons GC-FID in aqueous sample GC/MS Volatile organic hydrocarbons in aqueous sample

Note: TD is abbreviated for thermal desorption; N/A stands for the information is not provided or investigated; part per billion (ppb) and part per trillion (ppt) are based on weight/volume scale.

Fundamentals and Applications of Needle Trap Devices

Copolymer of methacrylic acid and ethylene glycol dimethacrylate Polymer based beads and polymercoated fiber Carboxen 1000 Carbopack X Divinylbenzene Divinylbenzene particle Divinylbenzene particle

693

694

Theory of Extraction Techniques

PDMS-coated SPME fiber, likely due to their higher partition coefficients, whereas highly volatile compounds such as benzene, which have lower partition coefficients for PDMS, and particle-bound analytes were better extracted by the NTD. Breakthrough for NTD and carryover for both NTD and SPME were negligible under the given sampling and desorption conditions. The technique was further validated for analysis of a range of PAHs in various aerosol samples (sea salt, barbeque, and cigarette smoke).80 The discrimination of free and bound analyte concentrations is beneficial in several areas including assessments of human and environmental health impacts as well as for product development and quality control. Simpler, quantitative tools will be beneficial in expanding applications in this area, as will be the development of similar tools for liquid sample analysis.

2.30.4.3

Passive (Integrated) Sampling

A simple, cost-effective NT analysis combining solventless extraction, thermal desorption, and determination of volatile organic compounds (VOCs) was recently developed and validated for TWA analysis.37 In the method, an NTD packed with the sorbent Carboxen 1000 was used as a TWA diffusive sampler to collect target compounds by molecular diffusion and adsorption to the packed sorbent. This process can be described with derivations of Fick’s first law of diffusion (Section 2.30.2.2), which expresses the relation between the TWA concentrations to which the passive sampler is exposed and the mass of analytes adsorbed to the packed sorbent in the sampler. The effects of experimental factors such as temperature, pressure, humidity, and face velocity were taken into account in applying diffusive sampling under nonideal conditions. This study demonstrated that this NTD configuration is effective for air analysis of benzene, toluene, ethylbenzene, and o-xylene (BTEX), due to the good adsorption/desorption quality of Carboxen 1000 and to the special geometric shape of the needle with a small cross section avoiding the need for calibration. Storage tests showed good storage stability for BTEX. Verification of the theoretical model showed good agreement between theoretical and experimental sampling rates. Method validation done against NIOSH method 1501, SPME, and NTD active sampling revealed good agreement between those methods. Automated NTD sample introduction to a GC facilitates the use of this technology for industrial hygiene applications. More recently, NTD prepared with DVB particles were evaluated for TWA sampling for analysis of toluene, ethylbenzene, and o-xylene, and also compared with Carboxen/PDMS SPME (retracted fiber).81 A 3-mm diffusion path length was used in both cases. TWA sampling periods of 4–12 h were evaluated. Uptake rates for the DVB–NTD were significantly higher (ng VOC min1) than those observed for the retracted SPME fiber device.

2.30.5

Automation

The simplicity of needle trap designs and their ready incorporation with standard injectors and autosampler equipment have permitted a wide variety of options for automating the entire sample preparation and instrument injection processes for many of the techniques. Although INCAT was the first needle trap device to be reported, to the best of our knowledge no automated versions of this technology have been reported. However, SPDE, which was introduced soon after and is a closely related technology, has been automated for numerous applications. The reported applications all make use of the CTC Analytics (Zwingen, Switzerland) CombiPAL autosampler that was also first used for SPME analyses at about the same time. One of the first examples was the automated SPDE extraction and GC analysis of pesticides and organics from water using needles coated internally with 7 mm of PDMS.19 The extraction and desorption steps were automated using a CTC CombiPAL supplied by Chromtech. All movements of the syringe, the sample vials, and the plunger of the syringe were controlled by macros written with the CTC Cycle Composer software. Soon after, Musshoff et al. described the automated SPDE analysis of both amphetamines82 and cannabinoids83 from hydrolyzed hair samples for clinical and forensic toxicology. The SPDE needles for these analyses were coated with PDMS containing 10% activated carbon. In both of these analyses, an on-needle derivatization was conducted from a separate vial containing derivatization agent after extraction and before injection. Again, all of the SPDE method steps were fully automated, controlled by a CTC-CombiPAL autosampler and software with custom-made macros. The authors identified the strengths of the technology as robustness, capacity, reproducibility, low detection limits, and simple automation. Ridgway et al. later described the use of PDMS-coated SPDE devices and a CombiPAL autosampler for the automated GC analysis of aromatic VOCs from water.2 The technique was evaluated for the determination of furan, benzene, and toluene. The system was used successfully for both liquid and headspace extraction and results were compared to static headspace extraction at the same temperature. The sensitivity for toluene was greatly improved on using SPDE compared to static headspace. A slight increase in sensitivity was observed for benzene but none for determination of furan. Estimated limits of detection ranged from 0.2 to 2 mg l1. Jochmann et al. used SPDE needles coated with various phases for the extraction of polar VOCs from water.57 The desorption process used was described in detail in Section 2.30.3.5.4. For this work, the autosampler was supplied with a heatable CTC agitator for incubation and agitation, an additional gas station to withdraw desorption gas prior to injection, and a heated flushing station for conditioning of the SPDE needles and reconditioning after each analysis to prevent carryover. The same group, in 2008, reported on the automation of ITEX for analysis of VOCs from water.44 In this technique, the analytes were extracted from sample headspace by dynamic extraction (multiple draw and eject cycles with a 2.5-ml headspace syringe plunger) into a Tenax-packed cartridge located between the syringe and the needle. The Tenax cartridge was surrounded by a separate heater for efficient thermal desorption of analytes into the GC injection port. For the automation, a modified CombiPAL autosampler head was used that was able to hold the 2.5-ml headspace syringe with a longer interchangeable ITEX needle to

Fundamentals and Applications of Needle Trap Devices

695

accommodate the cartridge. The entire ITEX extraction and desorption procedure was fully automated by the autosampler, controlled by the PAL Cycle composer software and homemade macros. The control of the thermal desorber around the needle body was integrated in the CombiPal software and allowed reproducible heating to desorption temperature within a few seconds. The autosampler was equipped with a single magnet mixer and a temperature-controlled (45  C) sample holder tray. Before extraction, the sample was stirred for 15 min in the single magnet mixer at an incubation temperature of 50  C to establish equilibrium distribution of the analytes between the aqueous and gas phases in the vial before extraction. Another example of packed-syringe NTD automation is the MEPS technology introduced by Abdel-Rehim in 2004 for the analysis of local anesthetics in plasma and commercialized by SGE.53 The authors employed solvent desorption, as was described in Section 2.30.3.5.5. Again, automated sample preparation was accomplished with a CTC Analytics CombiPAL and a PTV injector was used to allow GC injection of 30 ml of methanol containing the desorbed analytes. More recently, Blomberg has reported a related technique involving sorbent-packed pipette tips.6 Here, automation was achieved by means of commercially available systems using 96-well extraction plates and a robot. The utility of the method was demonstrated again by extraction of local anesthetics from plasma and desorption, this time for LC-MS/MS analysis, in 100–150 ml of 60% methanol. The observed method limit of quantification was 2 nmol l1. Accuracy for quality-control samples was between 101 and 118% and precision ranged from 4 to 17%. The concept of automation employed in the packed-pipette tip approach is a significant departure from the other automation strategies described to this point. A set of replaceable extraction devices offers both the option of conducting multiple extractions off-line with automated desorption and analysis, as well as the important feature of eliminating the most significant source of carryover from one analysis to the next. The concept was also explored by Gong et al. in 2008 with the introduction of the PAS Technology Concept autosampler (Magdala, Germany) capable of sequential desorption and analysis of a set of NTD previously used for off-line extractions, stored in a rack on the autosampler.37 In the Gong paper, the NTDs were designed for off-line TWA analysis of BTEX in gaseous samples. After extraction, the NTDs were assembled manually with Luer-lock adapters on an NTD sampler tray. The NTDs were transferred one by one to the GC injector by the magnetic arm, and then the closure (pneumatic arm) closed the NTD to supply carrier gas through a Luer-lock needle head. After a ready signal was received from the GC, the control software (CONCEPT 1.1) started the GC and data acquisition simultaneously. The GC was equipped with a programmable GC injector (OPTIC 3, ATAS GL) which kept the carrier gas flow at 3.5 ml min1 and controlled the injector temperature as programmed. A similar automation system was later evaluated for automated sampling with NTD designed for clinical breath analysis,58 and subsequently employed for automated desorption and GC analysis as well, also for clinical breath analysis.78 Clinical studies usually require large numbers of analyses in a short time. The NTD autosampler was a critical tool to allow needle traps to be employed for such high-throughput purposes. It is encouraging that such a high number of publications have appeared describing various options for automating NTD sampling and analysis. The potential for automated high-throughput analysis will be a critical feature of the technology going forward to enable a broader acceptance in a variety of analytical applications.

2.30.6

Future Directions

Interesting future avenues of investigation for this rapidly progressing technology are envisioned in the areas of additional sorbent technologies, the incorporation of focusing technologies, automation to enable high-throughput analysis, and on-site application. In the area of sorbent advances, incorporation of monolithic sorbents in a rugged needle would appear particularly interesting due to the high porosity and good flow characteristics (low backpressure) of monoliths and their high extraction efficiencies. Although monolithic sorbents for sample preparation have been successfully incorporated in wide bore (4–5 mm id) stainless steel tubes,84–86 silica-based capillaries71,87 pipette tips,6 and microfluidic devices,71,88 technical challenges to date have prevented their deposition in narrow stainless steel capillaries. The coating of the interior surface of a stainless steel capillary with a thin silica layer may provide a solution here. Sorbent modifications to better tailor the selectivity of NTD extraction to the target analyte(s) are also of interest. This can be envisioned through either developing a nonselective general sorbent (or mixed-bed sorbent) where the distribution of a broad range of extracted target analytes closely matches their distribution in the sample matrix, which could be beneficial in proteomic and metabolomic applications, or the broader use of selective derivatizations to stabilize unstable or shortlived species in the NTD, provide better extraction efficiency of analytes with low partitioning or affinity, or to provide better sensitivity for compounds that do not generate strong signals in current detectors. New options for applying focusing to the sample preparation step could also be considered. For instance, van der Vlis et al. have described the application of electrofocusing to liquid–liquid extraction in a needle pre-concentration device, for sample preparation prior to HPLC.89 The method was applied for the analysis of a variety of compounds in the mmol l1 range. Although the authors pointed out that the technique could be applied in conjunction with SPE prior to HPLC, it appears that this has not yet been reported. To date, needle trap technologies have not been broadly accepted as mainstream analytical techniques. We see the primary impediment to this as the current limited supply of commercial devices and lack of convenient technical solutions to many of the somewhat cumbersome handling requirements. In particular, more efficient and simplified desorption regimes would be beneficial. Finally, we envision the potential for broad application of the technology to on-site analysis. The NTD technology is applicable to a range of analytical strategies. It is simple and can be used in both active and TWA sampling, as well as for the analysis of both total and free concentrations in combination with SPME devices, in air and water matrixes. To date, field application has focused on field sampling with laboratory analysis of the extracted samples,50 but the capabilities of on-site instrumentation are progressing rapidly. Future application of needle trap technology to conducting the entire analytical method in the field is a distinct possibility.

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Theory of Extraction Techniques

Acknowledgment This article was originally published with the title Fundamentals and Applications of Needle Trap Devices, in Analytica Chimica Acta, vol 677, issue 1 pp 3–18 (2010), Copyright Elsevier B.V. (2010) and is reprinted with permission. The article has been updated since the original publication.

See also: Principles and Practice of Solid-Phase Extraction; Solid-Phase Microextraction; Needle-Trap Devices for Environmental Sample Preparation; Preparative Gas Chromatography as a Sample Preparation Approach

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Fundamentals and Applications of Needle Trap Devices

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Relevant Websites BGB Analytik: www.bgb-analytik.de Chromtech: www.chromtech.com/ CTC Analytics: www.ctc.ch PAS Technology: www.pas-tec.com SGE Analytical Science: www.sge.com Shinwa Chemical Industries Ltd: shinwa-cpc.co.jp/eng/

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