Anal. Chem. 2002, 74, 1031-1037
Extending the Solid-Phase Microextraction Technique To High Analyte Concentrations: Measurements and Thermodynamic Analysis Kathleen Gill and Wayne A. Brown*
Department of Chemical Engineering, McGill University, 3610 University Street, Montre´ al, Que´ bec, Canada
The solid-phase microextraction (SPME) technique has been used historically to quantify analytes present at the parts per million level. However, the nonintrusive nature of SPME lends itself to other applications involving analytes at higher concentration. In the current work, the possibility of using the SPME technique to measure concentrated gaseous samples was examined. Pentane concentrations between 0 and 100% saturation were studied, over a temperature range of 20-45 °C. The results showed that, up to a critical mole fraction in the solid phase, the concentrations of pentane in the polymeric extracting solid and vapor phases were related by a constant, equal to Henry’s constant. The temperature dependence of Henry’s constant was shown to follow the predicted trend with temperature, as determined from rigorous thermodynamic calculations. Above the pentane concentration in the polymeric phase, the response deviated from linearity. The nonideality was captured in an activity coefficient. An activity coefficient model developed to describe the nonideality was found to be a function of the swollen volume of the SPME polymer phase. The results indicate that the SPME technique can be applied to high analyte concentrations, although difficulties may be encountered when multiple analytes are absorbed. Solid-phase microextraction (SPME) is a solvent-free analytical technique developed for dilute systems. In this method, a polymercoated fiber is exposed to the gas or liquid phase containing the analytes of interest. Generally, a minimum analysis time is imposed so that equilibrium conditions are established between the solid and fluid phases, with respect to the measured species. However, this is not necessary, provided the exposure time and conditions are kept constant.1-3 Following exposure, the contents of the fiber are desorbed and quantified using gas chromatography. Through knowledge of the equilibrium relationship between the polymer and fluid phases, the concentration of analytes in the original fluid can be determined. * Corresponding author: (e-mail)
[email protected]; (fax) (514) 3986678. (1) Ai, J. Anal. Chem. 1997, 69, 1230-1236. (2) Ai, J. Anal. Chem. 1997, 69, 3260-3266. (3) Bartelt, R. J.; Zilkowski, B. W. Anal. Chem. 2000, 72, 3949-3955. 10.1021/ac010920f CCC: $22.00 Published on Web 01/30/2002
© 2002 American Chemical Society
Application of the SPME technique has focused exclusively on quantifying dilute solutions of the compound of interest.4,5 The method has been, and continues to be, focused on systems where the upper concentration of the range of interest is on the parts per million level.5-8 However, many of the attributes of the technique make it an attractive option for quantifying systems of higher concentration. The miniscule sample size required, the lack of a need for an internal standard in many instances, and the elimination of extracting solvents from the methodology are a few of the obvious benefits afforded by SPME.9 Furthermore, the SPME technique has been automated and provides the basis for a number of analyzers and integrated analysis techniques.10-13 There are numerous applications involving concentrated solutions that could make use of these benefits. For example, in equilibrium studies, concentrations of the compounds of interest are typically varied from 0 to 100% saturation. In these studies, a minimum disturbance of the system is required, making the SPME method an obvious candidate. These benefits have already been realized in equilibrium studies of systems at low concentrations but have yet to be applied to vapor-liquid equilibrium problems where concentrations on the order of grams per liter are generated.14-16 The focus of the current study was to determine the applicability of the solid-phase microextraction technique to concentrated systems. A gaseous pentane-air system was used for this purpose. EXPERIMENTAL SECTION Chemicals. HPLC grade pentane (99.4% purity) was obtained from Fisher Chemicals (Fisher Scientific, Fair Lawn, NJ). Materials. SPME fibers were obtained from Supelco Inc. (Oakville, ON, Canada). The fibers consisted of a solid core, (4) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1996, 68, 144-155. (5) Martos, P. A.; Pawliszyn, J. Anal. Chem. 1997, 69, 206-215. (6) Achen, C.; Puttmann, W. Environ. Sci. Technol. 2000, 34, 1359-1364. (7) Koziel, J. A.; Pawliszyn, J. J. Air Waste Manage. Assoc. 2001, 51, 173-184. (8) Hernandez, F.; Beltran, J.; Lopez, F. J. Anal. Chem. 2000, 72, 2313-2322. (9) Boyd-Boland, A. A.; Chai., M.; Luo, Y. Z. Environ. Sci. Technol. 1994, 28, 569A-574A. (10) Yang, J.; Her, J.-W. Anal. Chem. 2000, 72, 878-884. (11) Kataoka, H.; Narimatsu, S.; Lord, H. L. Anal. Chem. 1999, 71, 4237-4244. (12) Martos, P. A.; Pawliszyn, J. Anal. Chem. 1999, 71, 1513-1520. (13) Bartelt, R. J. Anal. Chem. 1997, 69, 364-372. (14) Poerschmann, J.; Kopinke, F.-D.; Pawliszyn, J. Environ. Sci. Technol. 1997, 31, 3629-3636. (15) Urrestarazu, R. E.; Meijer, S. N.; Vaes, W. H. J. Environ. Sci. Technol. 1998, 32, 3430-3435. (16) Dean, J. R.; Tomlinson, W. R.; Makovskaya, V. Anal. Chem. 1996, 68, 130133.
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coated with a polymer. The polymer phase used in this study was poly(dimethylsiloxane) (PDMS). The majority of the experiments were performed with fibers having a 7-µm polymer thickness. However, Polymer phases of 30- and 100-µm thickness were also used, where indicated. The polymer phase extended 1 cm in the axial direction, for all fibers used. Analytical Methods. All analyses were carried out using a Varian 3800 gas chromatograph (GC), coupled with a Varian Star workstation. The GC was equipped with a 30-m Supleco SPB-5 column, with an internal diameter of 0.32 mm, and a film thickness of 0.25 µm (Supelco Inc.). Samples were detected using a flame ionization detector (FID). The injector (Varian 1177 type 1 EFC) was equipped with a low-volume insert designed specifically for analysis by SPME (Supelco Inc.). Preparation of Gaseous Standard. Experiments were performed using air-pentane systems. Glass bottles were used in all cases. The volume of each bottle was determined by weight using liquid water of a known density. The average volume was determined to be 161.0 mL, with a standard deviation of 0.7 mL (n ) 6). The bottles were first heated to the experimental temperature in an incubator. The equilibration process was carried out in the presence of ambient air. Once a uniform temperature was attained, each bottle was capped with a septum. All septa were equilibrated at the experimental temperature prior to use. Bottles were then removed from the incubator, and a volume of pentane (
