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Calibration of Solid Phase Microextraction for Air Analyses Based on Physical Chemical Properties of the Coating

by Perry A. Martos, Janusz Pawliszyn
Analytical Chemistry ()

Abstract

Solid phase microextraction (SPME) with poly(dimethylsiloxane) (PDMS) is used to sample dynamic hydrocarbon standard gas mixtures with calibration for temperature, relative humidity, gas velocities, and gas concentrations. Equilibration times were found to vary from 15 to 450 s depending on the compound. Relative humidity greater than 90% at various temperatures was found to decrease analyte mass loading by 10%. Investigation of linearity with PDMS showed all of the analytes had a linear relationship between mass loaded at equilibrium and the gas concentrations studied. Analyte detection limits are better than those for conventional grab sampling (concentration with adsorbent tubes and analysis by gas chromatography and flame ionization detection). Partition coefficients (K) were established at various temperatures, yielding a linear relationship between log K and T-1. The linear expression has a slope that is a function of the gas constant and the analyte heat of vaporization (?H?v) and a y-intercept that is a combination of terms among which include the gas constant, analyte ?H?v, and activity coefficient; therefore, K values can be estimated using literature ?H?v and ascertained at temperatures for which a K was not determined. This removes the restriction that calibration and air sampling must be carried out at the same temperature. A comparison of PDMS (100 ?m) fiber lengths (n = 13) revealed an average fiber length of 1.02 ± 0.03 cm. An interfiber estimation of method precision (n = 10) yielded less than 9% RSD for all the test analytes. When a correction for fiber lengths was applied, the differences between the means of area counts for the individual fibers was statistically insignificant. SPME was successfully field tested as a grab sampler and an integrated sampler to analyze air at an industrial site, with excellent agreement to traditional air sampling methods. This work suggests that ambient and industrial air sampling with SPME is possible with essentially no calibration of the sampling device. It is only necessary to know the analyte's K to the fiber at various temperatures which is based on defined and empirically determinable physical constants.\nSolid phase microextraction (SPME) with poly(dimethylsiloxane) (PDMS) is used to sample dynamic hydrocarbon standard gas mixtures with calibration for temperature, relative humidity, gas velocities, and gas concentrations. Equilibration times were found to vary from 15 to 450 s depending on the compound. Relative humidity greater than 90% at various temperatures was found to decrease analyte mass loading by 10%. Investigation of linearity with PDMS showed all of the analytes had a linear relationship between mass loaded at equilibrium and the gas concentrations studied. Analyte detection limits are better than those for conventional grab sampling (concentration with adsorbent tubes and analysis by gas chromatography and flame ionization detection). Partition coefficients (K) were established at various temperatures, yielding a linear relationship between log K and T-1. The linear expression has a slope that is a function of the gas constant and the analyte heat of vaporization (?H?v) and a y-intercept that is a combination of terms among which include the gas constant, analyte ?H?v, and activity coefficient; therefore, K values can be estimated using literature ?H?v and ascertained at temperatures for which a K was not determined. This removes the restriction that calibration and air sampling must be carried out at the same temperature. A comparison of PDMS (100 ?m) fiber lengths (n = 13) revealed an average fiber length of 1.02 ± 0.03 cm. An interfiber estimation of method precision (n = 10) yielded less than 9% RSD for all the test analytes. When a correction for fiber lengths was applied, the differences between the means of area counts for the individual fibers was statistically insignificant. SPME was successfully field tested as a grab sampler and an integrated sampler to analyze air at an industrial site, with excellent agreement to traditional air sampling methods. This work suggests that ambient and industrial air sampling with SPME is possible with essentially no calibration of the sampling device. It is only necessary to know the analyte's K to the fiber at various temperatures which is based on defined and empirically determinable physical constants.

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