Solid-phase Microextraction in Analysis of Pollutants in the Field

Transcription

1 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD 1 Solid-phase Microextraction in Analysis of Pollutants in the Field L. Müller, T. Górecki, and J. Pawliszyn University of Waterloo, Canada 1 Introduction Suitability of Solid-phase Microextraction for Field Analysis 3 2 Solid-phase Microextraction Coupled to Field Portable Fast Gas Chromatography 3 3 Selected Field Applications of Solidphase Microextraction Field Analysis by Solid-phase Microextraction Coupled to Gas Chromatography Field Applications for Indoor Air Monitoring In Situ Analysis of Groundwater and Soil Gas 8 4 New advances in Solid-phase Microextraction Field Samplers Storage Investigations Field Applications 13 5 Conclusions 14 Acknowledgments 15 Abbreviations and Acronyms 15 Related Articles 15 References 15 Solid-phase microextraction (SPME) is a modern sampling/sample preparation method, used for isolation and preconcentration of organic molecules from a variety of matrices. SPME uses a short piece of a fused silica fiber coated with a polymeric stationary phase. The fiber is mounted in a device resembling a syringe. During transport, storage and manipulation, the fiber is retracted into the needle of the device. During extraction and desorption of the analytes, the fiber is exposed. Analytes present in a sample partition into or onto the coating, depending on its type. The process continues until equilibrium is reached between the coating and the sample. From then on, longer extraction times do not result in larger amounts of analyte extracted. Once the extraction is finished, the fiber is retracted back into the needle, and the device is transferred to a gas or liquid chromatograph for analyte separation and determination. When gas chromatography (GC) is used, the analytes are thermally desorbed from the fiber in a GC injector. Coupling of SPME with high-performance liquid chromatography (HPLC) requires a special interface. Two distinct SPME coating types are available commercially. Coatings of the first type, including poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA), extract analytes by absorption. This process is noncompetitive, therefore in most cases the amount of an analyte extracted by such coatings from a sample is independent of the matrix composition. No saturation or displacement effects occur. The amount of an analyte extracted from a sample is linearly dependent on its initial concentration, provided that several important variables, including (but not limited to) temperature, extraction time and mass transfer conditions, are kept constant. Coatings of this type usually perform very well for compounds of medium to low volatility. Coatings of the second type, including poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB), Carbowax /divinylbenzene (CW/DVB) and Carboxen /poly(dimethylsiloxane) (CX/PDMS), extract analytes by adsorption. This process is limited to the surface of the coating. It is competitive, which means that a molecule with higher affinity to the coating can displace a molecule with lower affinity. Since the number of active sites on the surface of any coating is limited, a linear response for those coatings can be expected only when the concentrations of all the compounds that can be extracted by the coating from a sample are low. Adsorption-type coatings are particularly suitable for volatile analytes, for which they offer much better sensitivity than PDMS or PA. SPME is very well suited for field applications, especially when the analysis is carried out on site. The fiber can be exposed directly to the medium analyzed, for example lake water or ambient air, without the need to collect a sample and without knowing the exact volume of the sample the fiber is exposed to. Analysis can then be performed using field portable instrumentation. Manual operation of the device is very simple. The fibers are reusable, which makes the cost of analysis low. Separation of volatile components sampled by SPME can be very fast when a dedicated system (available commercially from SRI Instruments) is used. Alternatively,SPMEcanbeusedtosampleinthefield, and then transported for the analysis to the laboratory. Modified devices have to be used for this purpose to avoid analyte losses during transport and storage, as well as contamination of the samples. Transporting fibers is much easier than transporting glass or metal containers with water or air samples. The main disadvantage of SPME in the field is its lack of robustness. The needle can be easily bent, and the fiber can be broken when handled without sufficient care. New designs of field portable SPME devices address those Encyclopedia of Analytical Chemistry Edited by Robert A. Meyers. John Wiley & Sons Ltd, Chichester. ISBN

2 2 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT issues. Also, it might be difficult to accurately control in the field all the experimental parameters that affect the amount of analyte extracted. 1 INTRODUCTION Field analysis is rapidly gaining more and more importance, responding to the need for immediate results in environmental monitoring, as reviewed by V. Lopez- Avila et al. 1 Immediate remediation for industrial spillages or environmental accidents is possible only if the results are provided in real time. Performing an interactive sampling, where only relevant locations are sampled, can save time and resources. Also, the costs of a field investigation are lower compared to a traditional procedure, by cutting the expenses of transport and storage of bulky and heavy environmental samples, by shortening the time of the entire analysis and by focusing the investigation only on significant sampling sites. Results are more representative of the real sample composition if the analysis is performed directly in the field, so that volatile losses or sample degradation are minimized. To date, several types of analytical instrumentation can be transported to the field for on-site investigations. The promise of cheaper and faster analysis is pushing the optimization of field analytical methods, from immunochemical assays and chemical sensors to GC. 1 Even the sensitivity and identification capabilities of mass spectrometry (MS) can be applied directly in the field with the recent introduction of portable instruments. 2 For field investigations of volatile compounds, portable high-speed gas chromatographs can be utilized. Fast GC, which utilizes short capillary columns and high carrier gas flow-rate, can reduce the time of the instrumental analysis by one order of magnitude, producing therefore even more immediate results and reducing the cost of analysis. 1 SPME is a powerful tool for the extraction/sample preparation of environmental samples, widely described in literature. 3 5 A microextraction, defined as extraction of a very small portion of analytes, is performed by a polymeric coating immobilized on a silica fiber and exposed to the sample. For this purpose, the fiber is mounted at the end of a small diameter stainless steel tube, which, in turn, is located inside a fine steel needle. The fiber can be retracted into the needle or exposed from it by manipulating the plunger of a syringe-like device. When the fiber does not need to be exposed, it can be protected inside the needle. The device is commercially available from Supelco (Bellefonte, PA), together with a variety of fiber assemblies with different coatings. The SPME extraction process is based on partitioning of an analyte between the sample matrix (gas, liquid or solid) and the fiber. Typically, only a fraction of the total amount of the analyte present in the sample is extracted. Exhaustive or near-exhaustive extraction can be achieved only under specific conditions, when the volume of the sample is small and the affinity of the compound towards the coating (measured by the fiber/sample partition coefficient K fs ) is high. The amount of an analyte n extracted by absorption-type SPME fibers at equilibrium in a two-phase system can be described by Equation (1). 3 n D K fsv s V f C 0 1 V s C K fs V f where V f is the fiber coating volume, V s is the sample volume, and C 0 is the initial concentration of the analyte of interest in the sample. This equation indicates that in a two-phase system there is a direct relationship between the initial analyte concentration in the sample and the amount extracted, even though other factors influence the efficiency of extraction at equilibrium, such as the affinity of the fiber for the target compound, and the sample volume. As in all equilibrium processes, several factors can affect the amount of an analyte extracted by the fiber, from the temperature and the ph to the organic and salt content in the sample. 3 If an adsorption process is applied for extraction, a different theory is necessary to describe the extraction process. 6 The following considerations pertain to PDMS/DVB, CW/DVB and Carbowax /template resin coatings. No comprehensive theory has been developed yet for the CX coating, for which capillary condensation plays an important role. Based on Langmuir s theory of the adsorption process, 6 Equation (2) can be used to describe the processes involved in SPME extraction with solid coatings: n D CfA 1 V f D K AC 0A V s V f C f max CfA 1 V s C K A V f C f max CfA 1 2 where n is the amount of analyte adsorbed by the porous polymeric coating at equilibrium, CfA 1 is the concentration of analyte A on the fiber at steady state; V f is the volume of the fiber coating; K A is the adsorption equilibrium constant of analyte A; C 0A is the initial concentration of analyte A in the sample; V s, V f are the volumes of the sample and of the fiber, respectively; C f max is the concentration of active sites on the surface (corresponding to the maximum achievable analyte concentration on the surface). The general form of Equation (2) is very similar to that of Equation (1), which describes the extraction process

3 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD 3 in a two-phase system when absorption is the extraction mechanism. The main difference between Equations (1) and (2) is the presence of the fiber concentration term (C f max CfA 1 ) in the numerator and denominator of Equation (2) (also, note that the meaning of K A is entirely different than that of K fs : K A is adsorption equilibrium constant, while K fs is the partition coefficient). For very low analyte concentrations on the fiber, it can be assumed that C f max CfA 1. For this condition to be fulfilled, analyte concentration in the sample and/or its affinity towards the coating must be very low. When these requirement(s) are met, a linear dependence should be observed. If, however, the amount of an analyte on the fiber is not negligible compared to the total number of active sites, the dependence cannot be linear any more. 6 SPME fibers can be exposed to solid, liquid or gaseous matrices, using three different extraction modes: ž ž ž direct immersion of the fiber in the sample matrix (liquid or gaseous matrices); extraction of the headspace above the sample (liquid or solid samples); direct immersion with the fiber protected with a high-molecular weight cutoff membrane eliminating adverse effects when the sample is very dirty (liquid samples). 3 A variety of available polymeric coatings with different chemical properties allow the technique to be used for a broad range of compounds widely differing in polarity and volatility (from amines and phenols to volatile organic compounds (VOCs) and inorganic substances). 7 To date, hundreds of applications have been developed with SPME, 8 even though the power of this technique in areas including industrial hygiene and environmental and process monitoring has not been completely explored yet. 1.1 Suitability of Solid-phase Microextraction for Field Analysis Several features of SPME make this technique very well suited for field investigations. The feature that differentiates SPME from all the other sampling techniques can be explained from the principle of the extractionpartitioning process described by Equation (1). If a large volume sample is extracted, e.g. atmospheric air or lake water, or K fs is very small (low affinity of the fiber coating toward the analyte), the term V s in the denominator of Equation (1) is much larger than the K fs V f term, and the latter can be neglected. The final relationship that describes the amount absorbed by the SPME fiber in such a case is described by Equation (3): n D K fs V f C 0 3 It follows from Equation (3) that under such conditions the amount of analyte extracted by the fiber at equilibrium, in a two-phase system, is not dependent on the sample volume. This means that collection of a known volume of a sample is not necessary in the sampling procedure, and the fiber can be exposed directly to the sample in the field, without removing it from the environment. The amount of analyte extracted is negligible in comparison to the total amount present in a large volume of a sample. The system is not disturbed by the SPME extraction. For instance, the scents released by a flower can be sampled by simply exposing the fiber to the inside of the flower bulb without removing it from the field, and a real field sample can be analyzed. The above is not always true: if the amount of scents removed from the headspace of the flower is not negligible, the flower will react by increasing their production and the normal situation will be disturbed. Other features make SPME very suitable for field use. It is pen-shaped, easy to handle and to operate by any operator. It can be exposed even to locations difficult to reach for sample collection (e.g. groundwater or deep soil). It is reusable and the fiber can be easily changed. If a field portable instrument is available, the fiber can be immediately desorbed in the hot injector of a GC for chromatographic investigation, since the extraction and sample preparation processes are combined into a single step. SPME can also be coupled to fast GC: the lack of solvent in the whole process is an important requirement for high-speed GC. 9,10 The possibility of automation of the extraction technique makes it potentially useful for semi-continuous monitoring of industrial processes 11 and for industrial hygiene purposes. 12 The advantages of SPME for field sampling can also be exploited when the use of portable instrumentation is not possible. After field sampling, the SPME device, with the analytes stored on the fiber, can be transported to the laboratory for further investigation. A special sampler is required for this purpose in order to prevent analyte losses during storage and transportation. Since the SPME fiber is fragile, care should taken during sampling in field conditions. New SPME field samplers were designed in order to satisfy all the features required by field applications, such as robustness of the device, protection of the fiber during sampling and minimization of analyte losses during storage. 2 SOLID-PHASE MICROEXTRACTION COUPLED TO FIELD PORTABLE FAST GAS CHROMATOGRAPHY High-speed GC is a powerful analytical technique that dramatically reduces the time of chromatographic

4 4 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT separation, by up to one order of magnitude. Fast GC typically utilizes short narrow-bore capillary columns and high carrier-gas flow rates. 10 Good separation can be achieved by fast GC if narrow injection bands are produced. Band broadening can be minimized by avoiding the use of solvents, by reducing the detector inner volume and by introducing the sample to the column very rapidly. SPME coupled to high-speed GC is a good combination to perform rapid and cost-effective investigations in the field, even of complex organic samples. SPME is particularly suited for fast GC, as it is solvent-free, and the thin coatings can provide very fast desorption of analytes at high temperatures. Some instrumental modifications were performed recently in order to achieve successful fast separations. 9,10 A portable system was optimized for field investigations using SPME coupled to fast GC and was commercialized by SRI Instruments (model 8610C, SRI Instruments, Torrance, CA). The instrument was tested in combination with a flame ionization detector (FID), a photoionization detector (PID), and a dry electrolytic conductivity detector (DELCD). A dedicated injector, presented in Figure 1, was mounted on the portable system in order to use SPME for high-speed separation. The injector guarantees very fast thermal desorption of the analytes from the SPME fiber. The injector for high-speed GC should produce as narrow an injection band as possible. Regular injector ports have too large internal volumes (e.g. split/splitless injector), since they usually have to accommodate large volumes of gaseous samples or vapors produced by solvent injection. Thermal focusing for separation improvement is not convenient for fast separations, since temperature programming is impractical for high-speed GC. Hence, an injector port with a small internal volume was required for this application. Also, very fast thermal desorption from the SPME fiber was required to produce a narrow injection band and achieve effective separation. In the dedicated injector for SPME coupled to fast GC, the injector port was maintained cold during needle introduction and was rapidly heated only when the fiber was exposed to the carrier gas stream. The desorption area of the injector was heated by capacitive discharge that allowed heating rates as fast as 4000 Cs 1, and very narrow injection bands were observed, as required by fast GC. The injector (Figure 1) was based on a modified Swagelok fitting (1) which was drilled through in the middle, and the carrier gas was supplied by a piece of 1/16 inch tubing (2), soldered to it. The inlet of the injector was closed with a molded septum (3) and the original SRI nut with needle guide (4). The hole inside the fitting was enlarged to accommodate the brass needle guide (5). The back end of the injector was closed with a nut (6) and drilled-thru plug (7). The plug had a Figure 1 Dedicated injector to couple SPME with fast GC, allowing heating rates of up to 4000 Cs 1. 1: modified Swagelok fitting; 2: stainless steel tubing; 3: molded septum; 4: nut; 5: needle guide; 6: nut; 7: drilled-thru plug; 8: stainless steel tubing; 9: 0.53-mm i.d. fused silica capillary; 10: electrical contact. piece of gauge 19 steel tubing (8) soldered into it. Inside the tubing was a segment of 0.53 mm i.d. fused silica capillary (9). The tubing (8) assured mechanical stability and durability, and served as the heating element at the same time. The capacitative discharge power supply was connected to the injector body and to the end of tubing (8) through contact (10). The GC column was connected to the injector with a zero-dead volume butt connector. Tubing (8) was thermally insulated along its entire length (not shown). Neither the FID nor the DELCD detector required any modifications before being used for fast GC. On the other hand, the relatively large internal volume of the PID caused a very significant peak broadening. To overcome this problem, a simple insert was designed and introduced into the internal cavity of the detector, reducing the internal volume of the PID from approximately 130 µl to 25 µl. The insert was tightly fitted into the detector cavity to avoid creation of void volumes. An excellent improvement in resolution for separation of BTEX (benzene, toluene, ethylbenzene, xylenes) was achieved in this way

5 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD 5 The system was optimized in the laboratory with standards of BTEX (benzene, toluene, ethylbenzene and o,m,p-xylenes; 100 µgl 1 each compound in water) and a complete separation was performed within 15 s. Purgeables A (trichlorofluoromethane, 1,1-dichloroethene, dichloromethane, 1,1-dichloroethane, trichloromethane, tetrachloromethane, trichloroethene, 1,2-dichloropropane, 2-chloroethyl vinyl ether, 1,1,2-trichloroethane, tetrachloroethene, dibromochloromethane, chlorobenzene; 200 µgl 1 each compound in water) were separated in 2 minutes with good precision using the DELCD. 9,10 The SRI system was then carried to the field for trace analysis of trichloroethylene (TCE) in soil (see section 3.1) and airborne formaldehyde for industrial hygiene monitoring (see section 3.2). 3 SELECTED FIELD APPLICATIONS OF SOLID-PHASE MICROEXTRACTION 3.1 Field Analysis by Solid-phase Microextraction Coupled to Gas Chromatography Several applications of SPME and high-speed GC for field analysis have been reported. The portable SRI model 9300B instrument, described in the previous section and now commercially available as model 8610C, was applied in the field for various matrices and different pollutants: field determination of traces of TCE in soil samples, 13 VOC in groundwater 14 and indoor measurements of airborne formaldehyde 15 (see section 3.2). SPME/fast GC field determinations were performed by quantifying TCE in soil (clay) samples. By collecting core samples from depths up to 5 m, the migration of the pollutant was investigated. Soil samples were collected and extracted with methanol. An aliquot of the extract obtained was used to spike pure water, and the aqueous solution was then vigorously stirred in a sealed vial to equilibrate with the headspace above the water. A PDMS/DVB 65 µm SPME fiber was exposed to the headspace of the static water sample for the extraction. The described procedure was performed directly in the field. After extraction, the SPME fiber was desorbed in the portable fast GC coupled to PID system for onsite instrumental investigation. The entire process of SPME extraction of spiked water samples, desorption and instrumental analysis took 3 min to complete, allowing over 500 samples to be quantified in 10 days. This successful study is an effective application of SPME coupled to fast GC for rapid field investigations. Groundwater samples were analyzed by SPME/fast GC in the field to evaluate VOC content. 14 The commercially available instrumentation (SRI model 8610C) was used for this purpose. Of great interest is the validation performed by comparison with traditional liquid liquid extraction (LLE) followed by instrumental analysis by gas chromatography/electron capture detector (GC/ECD) with a Hewlett Packard model HP 5890 GC in the laboratory. A comparison of the quantitative results obtained by the two methods showed a scattered difference between 3% and 5%, demonstrating the reliability of field investigations performed by SPME/high-speed GC. 3.2 Field Applications for Indoor Air Monitoring Monitoring of air is becoming more and more important not only for environmental pollution evaluations, but also for industrial hygiene applications. 12 SPME can be a useful tool for industrial hygiene studies since it is suitable for field evaluations and is easy to automate. It can potentially be used therefore for semi-continuous monitoring of indoor/outdoor air pollutant levels. An automated procedure is necessary to monitor levels of hazardous and undesirable exposures workers are subjected to in their working day. Proper monitoring equipment for this purpose should be portable, should give immediate and continuous results and should alert to excessive exposure. 12 When sampling air, SPME can be used in two ways. In the conventional approach, the fiber is simply exposed to the atmosphere analyzed by depressing the plunger of the SPME device. The analytes reach the fiber coating mainly via advection and (to a lesser extent) diffusion. The fiber is usually exposed for a time long enough for the analytes to reach equilibrium. Analyte equilibration times depend on their partition coefficients and mass transfer conditions. Alternatively, SPME can be used as a timeweighted average (TWA) sampler for gas phase analytes. In this approach, the fiber is retracted a known distance into the needle during the sampling period. In contrast to the conventional approach, the analytes are not allowed to reach equilibrium with the fiber. Analyte sampling rate is controlled by a number of factors, including the diffusion coefficient and the concentration gradient inside the needle. Analysis by this method yields the concentration of an analyte averaged over the entire sampling period (e.g. eight hours in industrial hygiene). TWA concentration is defined according to Equation (4): C tn D C 1t 1 C C 2 t 2 C C 3 t 3 CÐÐÐCC n t n 4 t 1 C t 2 C t 3 CÐÐÐCt n where C tn is the TWA concentration, C 1 is the analyte concentration observed for time t 1, and so on, until time t n. This definition can be used to determine TWA concentration from discrete measurements (grab sampling). Such measurements should cover the entire period of interest. Conventional SPME can be relatively

6 6 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT easily utilized for this type of measurement provided field portable instrumentation is used. The fiber has to be exposed to the outside atmosphere and equilibrated with the analytes, followed by immediate desorption and GC analysis. Once the analytes are removed from the fiber, the entire cycle has to be repeated for as many times as required to cover the period of interest. An alternative approach to determining TWA concentration is with one sampling session where t 1 D t n.forthis approach to be successful, one has to make sure that the amount of analyte reaching the trapping medium at any given moment is directly proportional to the analyte concentration outside of the sampler at the same moment. It is also very important that analyte molecules trapped inside the sampler are not re-released to the surrounding atmosphere and do not affect further uptake of analyte molecules. When the latter conditions are fulfilled, the trapping medium is called a zero sink. 16,17,18 The following presentation is based on a paper by Martos and Pawliszyn. 19 TWAsamplingbySPMEcanbe accomplished by leaving the fiber inside the needle during the sampling session. Figure 2(a) shows the SPME fiber position for TWA sampling, while Figure 2(b) presents analyte concentration gradient inside the needle. In Figure 2, C BULK is the analyte concentration in the bulk of the atmosphere examined, C FACE is the concentration (a) C BULK = C FACE C FACE (b) A L Concentration gradient when C SORBENT = 0 Needle C SORBENT L Concentration gradient when C SORBENT 0 C SORBENT Needle SPME coating Sorbent (SPME coating) Figure 2 (a) Positioning of the fiber inside the needle used for TWA sampling with SPME. (b) Schematic representation of the concentration gradient of the analyte between the opening of the needle and the fiber coating for a zero and a non-zero sink situation. L is the path length and A is the surface area of the opening. at the face of the sampling device, and C SORBENT is the concentration at the gas/sorbent interface. Since the dimensions of the SPME needle are very small, analytes can reach the fiber only by diffusion. From Fick s law, analyte transport in such a system can be described by Equation (5): dj dt D DA L dc 5 where J is the weight (ng) of analyte passing through a cross section A (cm 2 ) during a time t (min); D is analyte diffusion coefficient in air (cm 2 min 1 ), L is the diffusion path length (cm), and C is analyte concentration (ng cm 3 ). We can now define the sampling rate R (cm 3 min 1 ) by Equation (6): ( ) A R D D 6 L Hence (Equation 7) dj D Rdc dt 7 Equation (7) can be integrated with the following limits to concentration and time to give Equation (8): t2 CS CS J D R dc dt D Rt dc 8 t 1 C F C F where C F D C FACE,andC S D C SORBENT. After integration, Equation (9) is obtained: J t D R C F C S 9 where J/t is the average sampling rate over the period of time from t 1 to t 2. For a zero sink, C S D 0, and J D M (mass of analyte trapped by the coating), so that Equation (10) has the following form: M D R Ð C F 10 t where C F is the average concentration of an analyte at the face of the SPME needle during the sampling period. It follows from the above equation that the rate of analyte uptake is directly proportional to the sampling rate (being a function of the geometric dimensions of the sampler and analyte diffusion coefficient) and analyte concentration. Equation (10) can be rearranged to allow the determination of TWA concentration C t of an analyte from the mass of the analyte extracted by the SPME fiber (Equation 11): M Rt D C t 11 Sampling rate R can be easily determined by exposing the SPME device with the fiber retracted into the needle

7 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD 7 to an atmosphere containing a constant concentration of the analyte of interest. In such a case, from Equation (9) we can derive Equation (12): R D M 12 tc F Once R is known, TWA concentration of an analyte in air can be easily determined by SPME from the amount of analyte extracted by the retracted fiber in a given time t. SPME fiber assembly is very flexible as a passive sampler, since the length of the diffusion path L can be easily changed by repositioning the fiber inside the needle. When high concentrations of an analyte are to be analyzed for prolonged periods of time, the fiber can be retracted deeper into the needle, and vice versa. If the sampling rate R is determined with the fiber at L and the sampling took place with the fiber at L 0, the left side of Equation (10) has to be multiplied by a factor L 0 /L to obtain correct results. The assumption that SPME coating acts as a zero sink is not always fulfilled. Analytes with relatively low affinity to the coating may reveal certain non-negligible vapor pressure at the gas coating interface, according to Equation (13): C S D X A P P t MW T 13 where X A is the mole fraction of analyte in the PDMS coating, P is the pure analyte vapor pressure, P t is the total gas pressure at the interface, MW is the analyte molecular weight, T is the temperature in Kelvin and is the molar volume of gas at 298K. When C S 6D 0, a reduction in the concentration gradient occurs, as depicted by the dotted line in Figure 2(b). The zero sink assumption is usually fulfilled for highly efficient sorbent materials and for chemisorption, as is the case with on-fiber analyte derivatization. Figure 3 presents the results of an experiment in which continuous exposure of the SPME fiber assembly with retracted needle for a certain time to a standard gas atmosphere was compared to intermittent exposure, in which the fiber assembly was alternately exposed to the standard gas and pure air. In the latter case, the total time for which the assembly was exposed to the standard gas was equal to the continuous exposure time. Should the zero sink assumption be fulfilled, the amount of analyte from the intermittent exposure should be the same as from continuous exposure. It follows from Figure 3 that PDMS coating does not act as a zero sink for analytes with relatively low affinity to this coating. Assuming that a 10% loss is acceptable, the useful range starts with compounds characterized by fiber/gas partition coefficients (K fg ) greater than Short-term (<30 min) TWA sampling is possible for such compounds. 100% % of Intermittent versus continuous exposure % of Equilibrium 90% 80% 70% 60% 61% 50% 40% 38% 30% 31% 20% 15% 10% 7.2% 6.6% 5.7% 3.4% 2.9% 1.7% 0% 0.8% n-pentane n-hexane benzene toluene ethylbenzene p-xylene o-xylene a-pinene mesitylene d-limonene n-undecane Figure 3 Amounts of analytes trapped by the 100 µm PDMS fiber after intermittent exposure relative to continuous exposure (black bars; total exposure time 15 min in both cases) and the fraction of equilibrium amount trapped by the fiber after the continuous 15 min exposure (gray bars).

8 8 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT Figure 3 also presents what fraction of equilibrium amount was loaded on the fiber after 15 min continuous retracted fiber exposure. The data indicate that when the amount extracted is greater than ¾5% of the equilibrium amount, analyte mass uptake rate becomes significantly affected. With this in mind, it is possible to use this method to sample n-undecane for 75 min with L D 0.3cm, and proportionally longer with larger L. Applicability of this method for field use was proven in a study where styrene (K fg ³ 3100) was determined in an industrial facility. The fiber was retracted 3 mm into the needle, and the sampling time was 30 min. The results were compared to those obtained using 30 min charcoal tube samples. Concentrations of 54 µgl 1 (25 C) versus 56 µgl 1 (25 C) were determined for the charcoal tubes and SPME, respectively. 15 PDMS coating is not an ideal sorbent for passive sampling of volatile and/or polar analytes. Sorbent capacity for such compounds is low, and the non-zero analyte vapor pressure at the surface of the coating affects the mass uptake rate. One way to remediate problems of this nature is to derivatize the analyte on the coating to a stable product with low vapor pressure and high affinity to the coating. Such an approach was verified with on-fiber derivatization of formaldehyde using o- (2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) with the PDMS/DVB fiber in the retracted position. 20 Experiments in which the fiber doped with PFBHA was alternately exposed to a standard formaldehyde gas mixture and pure air indicated that a cumulative amount of the derivatization product (oxime) was formed with no measurable losses caused by intermittent exposure to pure air. Thus, the system fulfilled the zero sink criteria. No significant losses of derivatizing agent were observed when the doped fiber was stored inside the needle of the SPME device, whose tip was inside an empty 1.8 ml vial sealed with a Teflon-lined septum during transportation to the field. After sampling, the fibers were carried to the laboratory for instrumental analysis. In another experiment, the doped fiber was exposed to a standard mixture containing 636 ppbv formaldehyde for 1007 min (L D 3.0 cm). TWA concentration found with this system was within 7% of the expected concentration. The device was tested in the field against the standard of the National Institute for Occupational Safety and Health (NIOSH) Method The fiber was retracted to 3.0 cm, and the sampling time was 420 min. Sampling was carried out in three separate locations. The TWA concentrations found for the SPME sampler were 109, 152 and 57 ppbv, compared to 102, 160 and 51 ppbv, respectively, for the reference method. The excellent agreement between the results obtained by the two methods indicates that passive sampling of formaldehyde using SPME can be a very successful alternative to currently used techniques, as it offers a number of unique advantages: the devices are small, lightweight, reusable and cheap, and the manipulations are minimal and very simple. Once the sampling is finished, the results can be obtained within minutes by thermally desorbing the derivatization product in a portable GC injector for chromatographic analysis. On-fiber derivatization combined with portable fast- GC, previously described in section 3.1, can also be used for grab sampling of formaldehyde. In this case, the fiber is directly exposed to the atmosphere examined for sampling. The sampling time is selected according to formaldehyde concentration in the air. Very short sampling times (a few seconds) are used for high concentrations. When the expected formaldehyde concentration is low, the sampling time can be extended to a few minutes. In general, the sampling time is selected in such a way that the amount of analyte trapped is within the linear part of the uptake curve. Grab sampling was carried out simultaneously with TWA measurements described above. The doped fiber was exposed for 30 s to the air and then immediately desorbed in the GC port. The entire procedure of fiber loading/sampling/sample preparation/separation and detection took 3 min with the portable system. Formaldehyde concentrations found in the same locations were 106, 140 and 93 ppbv, respectively. The agreement with the results of TWA sampling was therefore excellent taking into account the completely different nature of the two types of measurements: grab sampling represents momentary concentrations, while TWA sampling reflects concentrations averaged over a long period of time. The limit of detection for the grab sampling method was of the order of 10 ppbv (30 s sampling), which is well below the limits of detection of other formaldehyde grab sampling methods. 3.3 In Situ Analysis of Groundwater and Soil Gas Volatile content in groundwater and soil should be measured on-site, because composition changes can occur during sampling, pumping or excavation. SPME can be successfully applied to monitor underground pollution as an alternative to other methods that showed some limitations in field applications. 21 A device designed for this purpose (Figure 4) was developed by Nilsson et al. 21 and tested in the field for the determination of VOCs in groundwater and soil gases. The SPME sampling probe was introduced into the well, to reach groundwater for direct sampling. Monitoring of groundwater headspace or underground gases at different depths was accomplished by placing the SPME sampler at different depths, or over the headspace of the water. After extraction by exposure of the fiber, the probe was retracted to the

9 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD 9 Attachment links Probe endcap the volatile content of the soil, during excavation, an alternative SPME probe was designed to fit directly inside the head of a cone penetrometer. In this application of SPME, different ground levels can be monitored. Plunger 32.5 cm 2.5 cm Septum Barrel Z-slot Connection Connection ring O-ring Plunger retaining screw Adjustable needle guide/depth gauge Septum piercing needle Fiber attached tubing Fused silica fiber Holes for liquid exchange Probe head for fiber protection Figure 4 Design of a prototype SPME probe for in situ sampling of groundwater and soil gas. (Adapted by permission of Gordon and Breach Publishers from Nilsson et al. 21 ) surface and the SPME fiber was immediately desorbed in the GC of a mobile laboratory. Results obtained by underground sampling with the SPME probe were compared to concentrations found for the same samples collected with traditional procedures and analyzed by SPME in a mobile laboratory. VOC levels were evaluated in the underground waters of several wells. The results provided by in situ SPME extraction of groundwater were, in general, higher than those obtained with traditional sampling methods, where the pumping of the sample to the surface causes volatile compound losses. Toluene and naphthalene were detected by underground soil gas investigation and their presence was confirmed by extracting the same samples with Tenax tubes. To sample 4 NEW ADVANCES IN SOLID-PHASE MICROEXTRACTION FIELD SAMPLERS If a portable instrument is available, the SPME fiber can be immediately desorbed after sampling in the GC injector, and storage of the sample on the fiber is not necessary. If the instrument is not movable to the field, the sample has to be carried to the laboratory. Collection and transportation of bulky and/or heavy samples of water, soil or air can be avoided by using SPME for field extractions. The fiber can be exposed to the matrix directly where the sample is located, and only the SPME sampler, with the fiber sealed in an appropriate way, has to be transported to the laboratory. In this case the minimization of sample losses from the SPME fiber is crucial for the reliability of the results. A dedicated SPME sampler for field applications has to be designed to preserve the integrity of the sample during the transportation between the sampling location and the laboratory. The purpose of a recent study on the optimization of SPME field samplers was to achieve no losses during storage, even of volatile compounds, adsorbed/absorbed onto the SPME fiber by the time the sample is analyzed. 22,23 An SPME device for field sampling should also be simple, easy to handle and to transport, robust, inexpensive and, in some cases, disposable, in order to avoid any cross-contamination due to multiple extractions of dirty matrices. It is important that the fiber is protected during the sampling procedure, so that it does not break when it is exposed to the matrix in field. To date, several approaches have been studied to prevent analyte losses from the SPME fiber during storage. 24 Various silicone rubber septa were tested as sealing materials for the needle during storage in the SPME sampler. By sealing the tip of the needle, the fiber was supposed to be stored in a closed environment. However, this method did not take into consideration the possible partitioning of the analytes released from the fiber during storage into the septum itself. Taking into account that a septum is made of nearly the same material as the PDMS fiber, the significance of such a phenomenon should not be underestimated, especially for volatile analytes. Also, losses due to permeation of volatiles through the septum are possible. A field SPME sampler, commercialized by Supelco, is presented in Figure The fiber is exposed to the sample by pushing the plunger (1) of the device during the extraction process, as presented in Figure 5(a). Sealing the device for sample

10 10 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT (a) 4 (b) 5 Figure 5 Schematic diagram of an SPME field sampler commercialized by Supelco. (a) Sampling position with fiber exposed to the sample. (b) Storage position with the fiber retracted inside the needle and the tip of the needle sealed by a septum located inside the nosepiece. 1: plunger; 2: aluminum nosepiece; 3: needle; 4: SPME fiber; 5: thermogreen LB-2 septum (Supelco). storage is achieved by retracting the tip of the needle (3) into the sealing septum (5) (Thermogreen Septum LB- 2, Supelco) placed in the aluminum nosepiece (2) of the device (Figure 5b). In this way, the SPME fiber (4), carrying the analytes, is stored inside the sealed needle. The lifetime of this device is related to the longevity of the fiber, which is not replaceable. Some alternative devices were designed and built for field sampling (Figures 6 and 7). 23 The samplers were designed with the goal to avoid the use of rubber septa as the sealing material, to eliminate or minimize contamination or losses during storage. Figure 6 presents a field sampler that does not use a septum to seal the needle. Instead, the tip of the needle is closed by squeezing a two-leaf closure. The body of the sampler is made of aluminum. Moving the barrel (3), whose position can be fixed by thumbscrew (5), can change the length to which the needle is exposed. The SPME fiber (8) can be exposed from the needle by depressing plunger (1), guided by a Z-slot. In the storage position, the tip of the needle is sealed by a nylon two-leaf closure (9) squeezed around it. The closure is sealed by tightening a fingertight nut (10). Commercial fiber assembly (7) was used in Figure 6 SPME field sampler with a two-leaf closure. 1: plunger; 2: aluminum body; 3: movable barrel; 4: set screw; 5: thumb screw; 6: inner tubing; 7: SPME fiber assembly; 8: SPME fiber; 9: two-leaf closure; 10: finger-tight nut; 11: nylon shield. this prototype after removing the colored hub from the internal tubing (6). Should the fiber need replacement, it can be done by introducing the inner tubing from a new fiber assembly through the needle opening and fastening it to the plunger with screw (4). Between-fiber reproducibility is strictly related to the volume of the stationary phase on the fibers. The dependence of the volume on the length of the fiber is linear, while the dependence on the thickness of the coating is quadratic. Therefore, it is very important that the stationary phase thickness and length are reproducible. This feature is guaranteed by the manufacturer. To sample, the opening between the two halves of the closure (9) is enlarged by unscrewing the nut (10), and the needle is pushed out. To protect the fiber during sampling, a removable nylon shield (11), with large holes through which air or water can circulate, can be optionally mounted on the nut (10). With this device, no cap has to be removed from the body of the sampler to expose the fiber, so the sampling procedure is more rapid and simple. More advanced versions of the prototype were subsequently prepared, with two new leaf closures made of different materials (Delrin and KEL-F) with a cleaner

11 SOLID-PHASE MICROEXTRACTION IN ANALYSIS OF POLLUTANTS IN THE FIELD µl µl Figure 7 Disposable SPME field sampler with Teflon cap. 1: plunger; 2: nylon body; 3: set screw; 4: SPME fiber assembly; 5: nylon shield; 6: SPME fiber; 7: Teflon cap. 3 1 cut between the two halves. The choice of the material is very important because it cannot be too hard so that a good seal between the two surfaces is achieved (which excludes any metal), but also cannot be too soft (e.g. Teflon) to prevent premature wear caused by the needle movement. Any kind of coating commercially available can be used with this device. The device was tested for storage with CX/PDMS 75 µm, PDMS 100 µm and PDMS/DVB 65 µm fibers. 23 Figure 7 shows a prototype of a rugged and disposable field sampler made of nylon. The needle tip is sealed during transport with a Teflon cap (7). The opening in the cap fits tightly the outer diameter of the needle. A nylon shield (5) can be mounted to protect the SPME fiber (6) during sampling. The plunger (1), guided by a small screw (3) that moves in a Z-slot, regulates the exposure of the SPME fiber during sampling. Any commercial SPME fiber assembly can be mounted on this device. Since Teflon is inert, it is preferred over silicone rubber as the sealing material for the SPME needle during storage. No interactions between the analytes desorbed from the fiber and the cap should occur during typical storage times. On the other hand, Teflon is soft, therefore the tightness of the cap fit can decreases with use. Besides, the cap can get lost when it is removed from the needle. The device was tested with PDMS 100 µm, PDMS/DVB 65 µm and CX/PDMS 65 µm coatings. 23 (a) 2 1 Figure 8 Gas-tight valve syringe modified for SPME field applications. (a) Sampling, (b) Storage with the valve in the closed position. 1: SPME fiber; 2: inner tubing of the commercial fiber assembly; 3: metal gas-tight valve; 4: Teflon tip of the plunger; 5: plunger. A different concept was also explored for field analysis purposes. An innovative field sampler (Figure 8) was designed and built by modification of a commercially available gas-tight syringe with a special valve for gas sampling (SampleLock 50 µl, Hamilton, model 80956). The syringe has a metal valve (3) that, when it is in the closed position, seals the air sample inside the barrel even under pressure, as specified by the manufacturer. Some modifications of the valve syringe were performed in order to adapt it for use with SPME fibers. A small hole, of a diameter closely matching that of the inner tubing (2) of the commercial fiber assembly, was drilled through the Teflon tip (4) of plunger (5). The stainless steel inner tubing of the fiber assembly was cut at a right length so (b)

12 12 FIELD PORTABLE AND TRANSPORTABLE AIR AND VAPOR MEASUREMENT that it was completely contained inside the barrel when the plunger indicated 50 µl. The tubing was mounted by pressing it into the hole in the plunger tip. Since no glue was used, the fiber could be easily changed or replaced. When the SPME fiber (1) was exposed to the sample for extraction (Figure 8a), the valve was open, the hole inside the valve was aligned with the needle opening, and the inner tubing with the fiber could easily go through. To store the analytes during transport to the laboratory (Figure 8b), the plunger was withdrawn up to the 50 µl mark, so that a fixed amount of air was collected inside the barrel, and the SPME fiber (1) was maintained in the sample environment during storage. Should losses of analytes from the fiber occur via equilibration between the fiber and the air inside the syringe barrel, all the analytes should be injected anyway, since also the air inside the barrel is introduced into the GC column by pushing the plunger. To minimize losses due to desorption of volatiles from the fiber the volume of air in the syringe should be as small as possible. The danger of adsorption of less volatile compounds onto the glass walls of the barrel potentially exists during prolonged storage, but the interaction for the most volatile compounds should be negligible and deactivation (by silanization) of the glass surface of the barrel should prevent any undesirable adsorption. 4.1 Storage Investigations A very important feature for an SPME field sampler is the sealing capacity during storage and transport of the sampler to the laboratory. Storage capacity of the new field samplers, described in section 4, was investigated by storing several VOCs at different temperatures on selected coatings, as described by L. Müller et al. 23 Gaseous standards of methylene chloride, chloroform, 1,1,1-trichloroethane, benzene, toluene, tetrachloroethylene, 1,1,2,2-tetrachloroethane and 1,3-dichlorobenzene covered a wide range of volatility of the investigated compounds. Air standards (30 µg L 1 ), prepared freshly for each experiment in 1 L glass bulbs, were extracted by exposing the SPME fiber for 3 minutes under static conditions. The following fibers were tested for storage capacity: PDMS 100 µm, PDMS/DVB 65 µm and CX/PDMS 75 µm. The 100 µm PDMS fiber is particularly suitable for investigations on the sealing capacity of field samplers, because it has the lowest affinity towards very volatile analytes. (Standards of 60 µg L 1 were used for experiments with PDMS 100 µm, due to a lower response of this fiber toward the analytes investigated.) The PDMS/DVB mixed coating is a new generation fiber characterized by high affinity towards volatile and certain polar compounds. Analytes are extracted by this coating via adsorption, therefore they do not have to diffuse into the bulk of the coating material, as is the case with 100 µm PDMS fibers that extract analytes by absorption. 6 CX/PDMS 75 µm is a relatively new fiber created specifically for volatiles. The molecular sieve Carboxen fraction of the coating acts as a trap for volatiles, which can be desorbed only at high temperatures. From the physicochemical characteristics, the best storage efficiency should be expected for this fiber, which in theory is the best for field investigations. However, its applications are limited to very volatile compounds only. At the scheduled time (š3 s), after storage, the fibers were desorbed in the GC injector and the analysis by GC coupled to FID (Varian 3400) was carried out. An SPB-5 column (30 m ð 0.25 mm ð 1 µm film thickness) from Supelco was used for separation of the analytes with the following GC parameters: oven temperature program 40 C for2min,15 C min 1 to 150 C hold for 1 min; injector temperature 250 C; carrier gas pressure (H 2 ) 30 psi. Increasingly longer storage times were investigated, from 5 min to 24 h. The dependence of storage efficiency on temperature was evaluated by keeping the samplers at different temperatures: room temperature (24 C), fridge temperature (4 8 C) and dry ice temperature ( 70 C). Results presented in Table 1 show the percentage of the analytes retained by the SPME fiber after 24 hours storage, evaluated by comparison with the amount extracted and immediately desorbed in the GC injector. Two values are given, one for the most volatile, and one for the least volatile analyte. If not otherwise specified, the first number corresponds to chloroform and the second number to 1,1,2,2-tetrachloroethane. As expected, cold storage significantly reduces losses of volatile analytes from the fiber. However, certain drawbacks occur due to the low temperature of the device when storing the field sampler in dry ice ( 70 C). They include mechanical problems with the movement of the inner tubing through the septum of the fiber assembly, as the septum becomes very hard at low temperatures. For this reason, after storing any sampler in dry ice, a few minute warm-up at room temperature was required before injection, especially with metal devices. It can be noticed that refrigerated storage (4 8 C) can generate some unexpected results, e.g. storage percentages of more volatile compounds higher that those of less volatile analytes. This behavior can be explained assuming that significant contamination could have occurred in the refrigerator, where high concentrations of solvent vapors were present. This means that good sealing during storage is important not only to avoid losses of analytes from the fiber, but also to prevent contamination from the outer environment where the sampler is kept during transport. This effect is especially noticeable for the CX/PDMS fiber, with the highest affinity towards the most volatile compounds. As expected, the most volatile analytes are retained to a lesser extent by the SPME fiber during storage.