Design and Validation of Portable SPME Devices for Rapid Field Air

Dec 30, 2000 - In these conditions, a simple math- ematical model allows quantification without the need of calibration curves. This work describes tw...
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Anal. Chem. 2001, 73, 481-486

Design and Validation of Portable SPME Devices for Rapid Field Air Sampling and Diffusion-Based Calibration Fabio Augusto,† Jacek Koziel,‡ and Janusz Pawliszyn*

Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

The use of SPME fibers coated with porous polymer solid phases for quantitative purposes is limited due to effects such as interanalyte displacement and competitive adsorption. For air analysis, these problems can be averted by employing short exposure times to air samples flowing around the fiber. In these conditions, a simple mathematical model allows quantification without the need of calibration curves. This work describes two portable dynamic air sampling (PDAS) devices designed for application of this approach to nonequilibrium SPME sampling and determination of airborne volatile organic compounds (VOCs). The use of a PDAS device resulted in greater adsorbed VOC mass compared to the conventional SPME extraction in static air for qualitative screening of live plant aromas and contaminants in indoor air. For all studied air samples, an increase in the number of detected compounds and sensitivity was also observed. Quantification of aromatic VOCs in indoor air was also carried out using this approach and the PDAS/SPME device. Measured VOC concentrations were in low partsper-billion by volume range using only 30-s SPME fiber exposure and were comparable to those obtained with a standard NIOSH method 1501. The use of PDAS/SPME devices reduced the total air sampling and analysis time by several orders of magnitude compared to the NIOSH 1501 method. Sampling of air and related gas mixtures for chromatographic analysis of contaminants has been performed using a broad range of techniques. Sorbent adsorption, cryotrapping, and canister sampling, followed by thermal desorption/cryofocusing or solvent desorption are the most employed procedures for air analysis.1 However, most of these procedures have several serious drawbacks, such as production of artifacts2 and retention of large amounts of water.3 Techniques such as membrane extraction with sorbent interface (MESI),4 where the analytes present in a sample * Corresponding author: (fax) (519) 746-0435; (e-mail) [email protected]. † Current address: Instituto de Quı´mica, Unicamp, CP 6154-13083-970 Campinas, SP, Brazil. ‡ Current address: Texas Agricultural Experiment Station, Amarillo, TX 79106. (1) Dewulf, J.; Van Langenhove, H. J. Chromatogr., A 1999, 843, 163-177. (2) Clausen, P. A.; Wolkoff, P. Atmos. Environ. 1997, 31, 715-725. (3) Helmig, D.; Vierling, L. Anal. Chem. 1995, 67, 4380-4386. (4) Yang, M. J.; Harms, S.; Luo, Y. Z.; Pawliszyn, J. Anal. Chem. 1994, 66, 1339-1346. 10.1021/ac000629k CCC: $20.00 Published on Web 12/30/2000

© 2001 American Chemical Society

selectively permeate through a polymeric membrane and are trapped in a sorbent interface for further desorption into a chromatographic system, also had been suggested.5 SPME is an attractive alternative to the aforementioned techniques, considering features such as accuracy, cost, simplicity, and speed,6 and has been widely used in analysis of several contaminants in air.7-10 Many of these SPME methods reported in the literature employ fibers coated with liquid polymeric phases, such as poly(dimethylsiloxane) (PDMS) and polyacrylate. However, the use of fibers covered with mixed porous solid adsorptive coatings, such as Carboxen/PDMS and PDMS/divinylbenzene (PDMS/DVB), seems to be especially interesting for analysis of air contaminants. They are more efficient than the liquid-coated fibers,11 especially for extraction of analytes with low molecular weight.12 Both quantitative and qualitative applications of solidphase coated fibers have been described for analysis of food contaminants,13 fruit pulp volatiles,14 and flavor compounds in milk.15 Adsorption is the physicochemical mechanism involved in extractions using fibers coated with solid phases. Both the theoretical foundations of the equilibrium16 and the kinetics17 of adsorption by solid-phase coated fibers had already been addressed. These studies point to problems, e.g., competition between the analytes for the adsorptive sites available in the fiber and interanalyte displacement, as severe drawbacks to the application of solid-phase coated fibers to quantitative analysis and limiting the accuracy and precision of the results. However, in recent work, Koziel et al.18 presented an alternate methodological approach to overcome these deleterious effects. According to the (5) Luo, Y. Z.; Pawliszyn, J. Anal. Chem. 2000, 72, 1064-1071. (6) Pawliszyn, J. TrAC, Trends Anal. Chem. 1995, 14, 113-122. (7) Chai, M.; Pawliszyn, J. Environ. Sci. Technol. 1995, 29, 693-701. (8) Grote, C.; Pawliszyn, J. Anal. Chem. 1997, 69, 587-597. (9) Martos, P. A.; Pawliszyn, J. Anal. Chem. 1997, 69, 206-215. (10) Eisert, R.; Pawliszyn, J.; Barinshteyn, G.; Chambers, D. Anal. Commun. 1998, 35, 187-190. (11) Mani, V. Properties of Commercial SPME Coatings. In Applications of SolidPhase Microextraction; Pawliszyn, J., Ed.; RSC.: Cornwall, UK, 1999; Chapter 5, pp 63-67. (12) Gorecki, T. Solid versus Liquid Coatings. In Applications of Solid-Phase Microextraction; Pawliszyn, J., Ed.; RSC.: Cornwall, UK, 1999; Chapter 7, pp 92-108. (13) Page, D. B.; Lacroix, G. J. Chromatogr., A 2000, 873, 79-94. (14) Augusto, F.; Valente, A. L. P.; Tada, E. S.; Rivellino, S. R. J. Chromatogr., A 2000, 873, 117-127. (15) Marsili, R. T. J. Chromatogr. Sci. 1999, 37, 17-26. (16) Go´recki, T.; Yu, X.; Pawliszyn, J. Analyst 1999, 124, 643-649. (17) Semenov, S.; Koziel, J.; Pawliszyn, J. J. Chromatogr., A 2000, 873, 39-51.

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authors, when a fiber is exposed to a gaseous sample moving perpendicularly to the fiber axis for a period of time much smaller than the equilibration time, the coating behaves as a perfect sink and all analyte molecules reaching the fiber surface are immediately adsorbed. As a large number of nonoccupied adsorptive sites are available in these conditions, interanalyte competition and displacement are minimized and can be disregarded. It can be demonstrated that the extracted amount of an analyte n depends only on its concentration in the gaseous matrix Cg, its diffusion coefficient in air Dg, the fiber’s length and radius L and b, respectively, the thickness of the effective static boundary layer surrounding the fiber δ, and the sampling time t:

Cg ) n ln

(b +b δ)/2πD Lt g

(1)

The extracted amount, n, can be calculated from the peak area and from the detector response factor. Equation 1 holds true for air speeds up to values between 4 and 10 cm s-1, depending on the analyte. Further increase in the air velocity shows no effect on the mass uptake rate, which becomes nearly constant and limited by the diffusion of the analyte in the coating.18 For practical reasons, devices that allow extractions using air speeds superior to this critical limit would be desirable because variations in air velocity would not affect the mass uptake rate, ensuring better analytical precision and accuracy. Several models are available to estimate diffusion coefficients in air needed for the use of eq 1 with the Fuller-SchettlerGiddings19 model being the most adequate for a large number of analytes in normal air sampling conditions:

0.001T1.75 Dg ) p[(

∑

x

1

Mair

Vair)1/3 + (

1

+

Mvoc

∑

(2)

Vvoc)1/3]2

where T is the absolute temperature, Mair is the air apparent molecular weight (i.e., the weighted average of the molecular weights of the components of air), Mvoc is the molecular weight of the analyte, p is the ambient pressure, and Vair and Vvoc are respectively the molar volumes of air and of the analyte. The thickness δ of the effective static boundary layer surrounding the fiber can be calculated from eq 3, where Re refers

δ ) 9.52b/Re0.62Sc0.38

(3)

to the Reynolds number (Re ) 2ub/νj; u is the linear velocity of the air and ν is the air kinematic viscosity) and Sc to the Schmidt number (Sc ) ν/Dg). Using these equations, the concentration of an analyte can be directly estimated from the chromatographic peak area, given that the sampling conditions (sampling time, air velocity, temperature, and pressure) and constants (diffusion coefficient and fiber dimensions) are known. For that reason, apart from the suppres(18) Koziel, J.; Jia, M.; Pawliszyn, J. Anal. Chem. 2000, 72, 5178-5186. (19) Fuller, E. N.; Schettler, P. D.; Giddings, J. C. Ind. Eng. Chem. 1966, 58, 19-27.

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sion of interanalyte effects, this methodology also allows quantitation of analytes in air without construction of calibration curves. Another benefit would be the increase of the extracted amounts (and, therefore, of the sensitivity), when this approach is compared to the traditional static SPME sampling (simple exposure of the fiber to the air). It can be proved, from eqs 1 and 3, that increasing the air speed also increases the fiber’s mass uptake, due to the decrease of the boundary layer thickness. Under static conditions, the extraction would depend on transport of the mass through a boundary layer which would turn progressively thicker during the process, due to the depletion of the analyte.20 This, in turn, would limit the amount extracted in short periods of fiber exposure to the sample. This work describes several portable devices designed to apply the dynamic nonequilibrium sampling concept to analysis of airborne chemicals. The suitability of this approach both for qualitative analysis of living plant aroma compounds and for volatile organic contaminants in indoor air was examined. Also, quantitation of air contaminants using dynamic SPME sampling was compared to results obtained using a standard air analysis method. EXPERIMENTAL SECTION Materials. Chemicals and Supplies. All chemicals were of analytical grade and used as supplied: benzene, toluene, ethylbenzene, o-xylene, p-xylene, and mesitylene (Sigma-Aldrich, Mississauga, ON, Canada) and carbon disulfide (BDH, Toronto, ON, Canada). The SPME holder and 65-µm PDMS/DVB fibers were obtained from Supelco (Oakville, ON, Canada); the fibers were conditioned at 210 °C for 8 h prior to their use. Supelco ORBO32 charcoal tubes and a model I.H. portable air pump (A.P. Buck, Orlando, FL) were employed for the validation quantitative analysis according to NIOSH method 1501.21 All preparations involving CS2 (flammable and toxic) and benzene (suspect carcinogen) were carried out in a ventilated hood. Gas Chromatography. Qualitative chromatographic analyses of aromas were carried out in a Saturn IV GC-ITMS system (Varian Associates, Sunnyvale, CA) fitted with a 30 m × 0.25 mm × 0.25 µm HP-5 column (Hewlett-Packard, Avondale, PA) and a septumpurged injector (SPI). The carrier gas was 1.5 mL min-1 helium at 12 psi. The SPI was kept at 210 °C, and the column oven temperature was ramped from 60 to 280 °C at 5 °C/min. Profiles of indoor air contaminants and quantitative data were obtained using a Varian Star 3400 GC-FID chromatograph equipped with a 30 m × 0.25 mm × 0.25 µm Supelco SPB-5 column and SPI; 2.0 mL min-1 helium at 20 psi was used as carrier gas. The temperatures were set at 250 °C for the FID and 210 °C for the SPI, and the column oven program for all injections was as follows: 1 min hold at 60 °C, followed by a 15 °C/min ramp until ramped to 180 °C, and hold there for 3 min. Plant Sample. Aroma from juniper bushes (Juniperus communis) from the University of Waterloo campus gardens was used as a sample for the qualitative application in this work. Portable Dynamic Air Sampling Devices (PDAS) for SPME. Two devices to perform air sampling under dynamic conditions were (20) Pawliszyn, J. Solid-Phase Microextraction: Theory and Practice; Wiley-VCH: New York, 1997; pp 67-69. (21) National Institute of Occupational Safety and Health. Manual of Analytical Methods, 4th ed.; U.S. Department of Health and Human Services: Cincinnati, OH. 1994; Vol. I (Method 1501 (“Hydrocarbons, Aromatic”).

Figure 1. Side view (A) and front view (B) of the portable dynamic air sampling device for SPME. (1) modified hair-dryer; (2) aluminum tube; (3) 18 VDC power supply cable; (4) fixing brace; (5) cardboard pieces; (6) 3-mm slit; (7) SPME holder and fiber.

Figure 2. “Sandwich” PDAS-SPME; (A) unassembled and (B) assembled device. (1) 1-mm-thick stainless steel sheets; (2) 3-mmthick Teflon spacer; (3) 0.6-mm hole; (4) ) SPME holder and fiber; (5) silicone tube.

projected and built; the design concepts for these apparatus are discussed in Results and Discussion. Figure 1 shows the schematics of the first device, built using a VS-513F household hair-dryer (Helen of Troy, El Paso, TX) modified to revert the air flow direction and to disable the internal heating coil. An aluminum tube was machined and adapted to the front part of the modified hair-dryer. Two plain cardboard sheets fixed to the opposite side of the aluminum tubing creating a 3-mm slit; the modified hairdryer suction forces the passage of the ambient air through this slit. PDAS-SPME sampling is performed by exposing the fiber to the flowing air in front of the slit. The average air speed in front of the slit was measured to be 1.5 m s-1 with an HHF51 digital wire anemometer (Omega Engineering, Stamford, CT). This value is greater than the critical air speeds mentioned in the introduction. All data presented in this work were collected using this apparatus. A different PDAS-SPME (“sandwich” design), shown in Figure 2, was also projected and assembled. A portable air sampling pump was used to force ambient air through the rectangular orifice of the device; a small hole (diameter 0.6 mm)

in the Teflon spacer allowed exposure of the SPME fiber to the ambient air flowing through the orifice. Using a Buck I.H. air pump, it is possible to sample air flowing with controllable speeds up to 1.38 m s-1. This device is presented here as an alternative to that described above, and its use is currently being evaluated. Methods. Screening of Living Plant Aromas. PDAS-SPME was compared with conventional (static) SPME sampling for identification of compounds found in the fragrance released by an aromatic plant (juniper). Both for static SPME and for PDAS-SPME, the SPME fiber was exposed to the air surrounding the living specimen for 30 s, and the approximate distance between the SPME fiber and the specimen was 5 cm. The extracted materials were separated and identified by GC-ITMS. The desorption time was 5 min. The time between sampling and chromatographic analysis was kept lower than 20 min for all extractions performed here and in the subsequent essays; under these conditions, loss of sorbed materials can be assumed as negligible.22,23 Qualitative Profiles of Contaminants in Indoor Air. Qualitative profiles of the contaminants present in the air of the Motor Vehicle Maintenance Shop of the University of Waterloo were obtained using PDAS-SPME and conventional SPME. Extractions with a fiber exposure time of 30 s were carried out simultaneously by both methods. The SPME fibers were kept refrigerated under dry ice and capped during their transportation to the laboratory and storage. Several samples were collected during one workday to show the variation of the air contamination profile during this time span. Quantitative Analysis of Aromatic Hydrocarbons in Indoor Air. PDAS-SPME was employed to quantify aromatic hydrocarbons present in the air of several sites in the University of Waterloo. These sites included two different locations in a chemical laboratory (close to a solvent storage cabinet and in an analytical instrument room), the Motor Vehicle Maintenance Shop, and in the Engineering Mechanical Shop. Replicate measurements exposing the SPME fiber to the flowing air for 30 s were made. Uncertainties were expressed as estimates of standard deviation of replicates, i.e., vehicle and mechanical shop air analysis, three replicates, and laboratory air analysis, eight replicates. Concentrations of the aromatic hydrocarbons were calculated using eq 1 for nonequilibrium dynamic SPME extraction. It was assumed that the thickness of the boundary layer did not significantly change when air velocity was greater than 10 cm s-1. Thus, the threshold air velocity of 10 cm s-1 was used to estimate the thickness of the boundary layer in eq 3. For comparison purposes, vehicle and mechanical shop samples were simultaneously analyzed using the NIOSH method 1501 for aromatic hydrocarbons.21 Air was pumped through ORBO-32 charcoal adsorption tubes with sampling times and flow rates adjusted according to the level of contamination of each sample (see Results below). Immediately after the sampling, both the charcoal portion of the tube containing the extracted analytes and the breakthrough control portion were transferred to separate 4-mL glass vials sealed with Teflon-coated silicone septa. Two milliliters of CS2 was added to each vial. After 1 h, 1 µL of the (22) Mu ¨ ller, L. Field Analysis by SPME. In Applications of Solid-Phase Microextraction; Pawliszyn, J., Ed.; RSC.: Cornwall, UK, 1999; Chapter 20, pp 269283. (23) Koziel, J.; Pawliszyn, J. J. Air Waste Manag. Assoc., in press.

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Figure 3. Section of GC-ITMS chromatogram of living juniper aroma. (A) PDAS-SPME; (B) static SPME; (C) fiber blank). Peak identification: (1) limonene; (2) 3-nonen-1-ol; (3) 2-decen-1-ol.

CS2 phase in the vials was injected into the GC-FID system using the same operational conditions employed for the PDAS-SPME analysis. RESULTS AND DISCUSSION Design Aspects of the PDAS-SPME. The PDAS-SPME complements previously described devices for field SPME sampling.22 These already reported devices and the techniques used extractions under analyte/fiber equilibrium conditions, which demands the use of calibration curves or quantitation based on chromatographic retention data.24 However, quantitation in preequilibrium conditions, where the analyte uptake depends only on its diffusion through the static boundary layer,18 has several advantages for field use. Since no calibration procedures are needed for well-defined flow rates, the analytical process is simplified. Also, the sampling time is noticeably shorter when compared to the typical equilibrium times for airborne analytes, resulting in faster analysis. The main design feature of the PDASSPME project is to ensure a constant and uniform air flow around the fiber, consistent with the demands of diffusion-based extraction. These devices should also provide flow rates high enough to have air speeds higher than the critical values mentioned in the introduction, where the extraction rate is dependent mainly on diffusion of the analyte through the adsorbent pores or through the liquid coating film. For the device shown in Figure 1, this was achieved by using a modified dc-powered hair-dryer. The reversion of the direction of the air flow was made to avoid contact of the fiber with potential artifacts originated from the dryer body and motor. The device shown in Figure 2 was intended to use with industrial hygiene air sampling pumps, a resource already existent in several laboratories, as a source of air motion. In addition to these features, characteristics such as weight, cost, and handiness of use were taken in account. An alternate version of this device without the two cardboard sheets allows sampling of large volume air samples, i.e., indoor air, under higher flow rates. In this case, a special 1-in.-O.D. short tube is mounted perpendicularly to the main aluminum tube to position the SPME holder and to allow one-hand operation. A 1-mm hole was made in the aluminum tube for insertion of the SPME needle and fiber in the sampled air stream. (24) Martos, P.; Pawliszyn, J. Anal. Chem. 1997, 69, 206-215.

484 Analytical Chemistry, Vol. 73, No. 3, February 1, 2001

Figure 4. Variation of the GC-FID chromatographic profiles of contaminants in the UW Vehicle Maintenance Shop monitored in a workday using conventional SPME (A) and PDAS-SPME (B) collected during a work shift. Chromatograms: (1) 9:15 a.m.; (2) 9:55 a.m.; (3) 10:20 a.m.; (4) 11:10 a.m.; (5) 12:15 p.m.; (6) 12:55 p.m.; (7) 1:55 p.m.; (8) 3:05 p.m. For peak identification see Results and Discussion.

Screening of Living Plant Aroma. Figure 3 allows the comparison of a selected section of GC-ITMS chromatograms obtained with PDAS-SPME and conventional SPME for juniper aroma. Three compoundsslimonene, 3-nonen-1-ol, and 2-decen1-olswere identified in this section of the PDAS-SPME chromatogram (Figure 3A). Peaks corresponding to the same compounds in the static SPME chromatogram section were considerably lower (Figure 3B). In addition, the 3-nonen-1-ol peak is not distinct from the baseline noise and not detected with static SPME sampling. As shown by these results, the application of PDAS-SPME produced a significant increase in the number of detectable compounds in the analyzed samples, when compared to conventional SPME. This observation agrees with the theory; i.e., the air flow around the fiber increases the extracted amount of analytes per unit of time due to the reduction of the effective boundary layer thickness. The increase in the analyte uptake is also reflected in the increase of the number of detected compounds. Qualitative Profiles of Contaminants in Indoor Air. Figure 4 shows the chromatographic data profiles obtained after extraction of the indoor air at the UW Vehicle Maintenance Shop, using both PDAS-SPME and static conventional SPME sampling, respectively. The signal scale for the chromatograms in both sets was adjusted to the same value. Both the number of detectable peaks and the peak intensities are considerably greater in the chromatograms in the PDAS-SPME profile (Figure 4B), allowing easier visual assessment of the correlation between the pattern of air contamination in this environment and the activities taking place there. For example, it can be seen that the intensity of the peak attributed to toluene (large peak with tR ) 2.6 min) decays during the period between 9:15 a.m. (just after the beginning of the work shift) and 12:15 p.m., becoming roughly constant after

Table 1. Concentrations in ppb v/v of Some Aromatic Hydrocarbons in Indoor Air Measured by PDAS-SPME and NIOSH Standard Method 1501 vehicle shop

benzene toluene ethylbenzene p-xylene o-xylene mesitylene

mechanical shop

SPME

NIOSH

SPME

NIOSH

48 ( 10a 212 ( 43 60 ( 8 189 ( 43 249 ( 35 202 ( 28

b 215 48 222 137 75

17 ( 4 62 ( 9 ndc 25 ( 5 18 ( 5 nd

b 73 nd nd nd nd

a Uncertainties expressed as estimates of standard deviation of triplicates. b Not quantifiable (see text). c nd, not detected.

this time. This was credited to the residues of degreasing solvents containing toluene that were left for overnight cleanup of motor parts. The group of peaks with retention times between 4 and 8 min were found to be volatile hydrocarbons present in gasoline and diesel fuels. The intensity of these peaks was found to be at maximum around 12:55 p.m., which correlated to the admission of several vehicles to the shop. Quantitative Analysis of Aromatic Hydrocarbons in Indoor Air. Table 1 compares concentrations of several aromatic hydrocarbons found after PDAS-SPME sampling, combined with nonequilibrium diffusion-based quantification, with concentrations obtained after simultaneous application of NIOSH 1501 standard method to the same samples. For PDAS-SPME calculations, the values for the needed constants were as follows: b ) 0.0120 cm; L ) 1 cm (both previously measured in the laboratory); Mair ) 28.97 g mol-1; Vair ) 20.1 mL, and ν ) 0.15 cm2 s-1.25 Values for Vvoc needed for estimation of Dg (eq 2) were calculated according to the literature.25 The sampling time and air flow rate for NIOSH analysis was adjusted according to the expected concentrations of contaminants in each sample, based on preliminary exploratory extractions: 91 min for the vehicle shop air and 215 min for the mechanical shop air, with a flow rate of 138 mL min-1 (sampled air volumes: 12.6 L for vehicle shop and 29.7 L for mechanical shop). Under these conditions, no analyte breakthrough was observed when the NIOSH method was applied. It should be emphasized that the sampling time for the NIOSH-based sampling was a few orders of magnitude greater than the sampling time associated with PDAS-SPME. However, none of the existing standard methods could be compared with the 30-s PDAS-SPME sampling time. The PDAS-SPME results obtained for the vehicle and mechanical shops were similar to those from NIOSH analysis, except for the hydrocarbons with higher molecular weight in the set (oxylene and mesitylene), which are underestimated by the NIOSH method. A possible cause for this could be associated with the incomplete desorption of these analytes from the charcoal tubes employed in the NIOSH method, when the recommended desorption procedure was used. Another reason for the observed discrepancies in measured concentrations could be due to the widely different sampling times used in both methods. The (25) Tucker, W. A.; Nelken, L. H. Diffusion Coefficients in Air and Water. In Handbook of Chemical Property Estimation Methods: Environmental Behaviour of Organic Compounds; Lyman, W. J., Reehl, W. F., Rosenblatt, D. H. Eds.; McGraw-Hill: New York, 1982; Chapter 17, pp 17-1-17-25.

NIOSH-based concentration can be considered as a time-weighted average sample over a long sampling period. In contrast, the PDAS-SPME concentrations can be associated with spot or grab 30-s sampling. In addition, it was not possible to measure benzene concentration in the evaluated samples using this method. Benzene is a common and significant contaminant in the CS2 solvent recommended for the desorption step in the NIOSH method. The method precision can be estimated from the uncertainties presented in Table 1. VOC concentration levels can be considered typical of indoor air in occupational environments. Expressed as estimates of relative standard deviations (sR), the precision of PDAS-SPME results ranged from 13 to 28%, with an average value of 20%. These results can be compared to those presented in an extensive study of NIOSH charcoal tube collection methods for airborne organics.26 The sR values calculated from the data presented in this study ranged from 0.4% to as much as 69%, with an average of 15% (for xylene sR ranged from 5.2 to 22%, with an average of 10%, and for benzene, from 4.3% to as much as 43%, with an 15% average). Therefore, precision for the PDAS-SPME method can be considered in the same order of magnitude (if not better for some analytes) to the range of precision reported for the NIOSH standard method. An estimate of the detection limits of PDAS-SPME was provided by the laboratory air samples analysis. For sampling close to the solvent cabinet 18 ( 6 ppbv benzene, 6 ( 3 ppbv toluene, and 2 ( 1 ppbv p-xylene were detected, and for the air in the instrument room, 3 ( 1 ppbv toluene and 2 ( 1 ppbv p-xylene; other analytes were not detected. Those results show that the detection limits for PDAS-SPME are in the low-ppbv range. Comparison with the NIOSH method was not considered valid here, since for the same samples no aromatic hydrocarbons were detected with this method even extending the sampling volumes to values up to 50 L, except for toluene in one of the samples. For the sampling volumes employed in the vehicle and mechanical shops analysis, the detection limits calculated according to data provided in method 1501 would be in the range between 6 and 100 ppbv for mechanical shop air sampling and 15-230 ppbv for vehicle shop air sampling, depending on the analyte in consideration. Therefore, PDAS-SPME can be considered as more sensitive than the standard NIOSH 1501 method. CONCLUSIONS This work demonstrated that the combination of SPME and the simple and inexpensive (∼U.S. $10) PDAS-SPME device was a powerful tool for both qualitative screening and quantitative analysis of varied samples as aromas from living plants to occupational air. When compared to SPME extraction with simple static exposure of the fiber to the air, the application of PDASSPME increased significantly the number of detectable analytes, the adsorbed amounts, and the method sensitivities. Findings in this work suggest that PDAS-SPME can provide more accurate qualitative profiles of extremely diluted samples such as natural aromas. A few remarks should be made on the herein proposed methodology. As it involves short sampling times, the assessment (26) Larkin, R. L.; Crable, J. V.; Catlett, L. R.; Seymour, M. J. Am. Ind. Hyg. Assoc. J. 1977, 38, 543-554.

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of long-term exposure of contaminants in indoor air, which is frequently necessary, would require averaging several measurements made during a period of time. For these cases, procedures such as standard NIOSH methods, time-weighted average SPME mode, or similar alternatives would be more adequate. Since the sampling time is one of the variables needed to calculate the concentration, errors in its measurement would reflect in the accuracy and precision of results. Such errors could be significant considering that these short exposure times should be manually measured. Another possible handicap of the method is the dependence of the results on dimensional parameters of the fibers (their radius and length, which are constants in the model’s equations). Fibers should be checked in respect to their real dimensions to ensure accurate measurements, as well as the integrity of the coating. The use of PDAS-SPME also allowed the application of nonequilibrium diffusion-based quantification to air samples using fibers coated with solid (porous) polymers. The use of short sampling time minimized the effects of interanalyte displacement that in the past prevented the use of these fibers for accurate

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quantitative air analysis. Also, this nonequilibrium model can result in quantitative analysis without need of calibration curves, provided that some constants, e.g., analyte diffusion coefficient in air and the detector response factor, are known. When compared to standard methodologies, a 30-s sampling using PDAS-SPME allowed measurement of VOC concentrations that where not detected by the NIOSH standard method, even after several hours of extraction using expensive (air sampling pumps) and nonreusable (charcoal tubes) materials. ACKNOWLEDGMENT The authors acknowledge the Fundac¸ ˜ao de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) for the scholarship provided to F.A. and also thank NSERC (Natural Sciences and Engineering Research Council of Canada) for funding this study.

Received for review June 1, 2000. Accepted November 8, 2000. AC000629K