Nonequilbrium quantitation of volatiles in air streams by solid-phase microextraction

Solid-phase microextraction (SPME) is a valuable technique for analyzing air-borne organic compounds; one important application is measuring concentrations when these are constant over time. Quantitation normally relies on the SPME fiber being fully equilibrated with the sample medium. Unfortunately, relatively heavy compounds do not equilibrate within a reasonable amount of time, and this has limited the scope of SPME. The ability to quantitate during equilibration was needed and was the focus of this investigation. This entailed having an accurate description of SPME kinetics, and the kinetics of extraction by poly(dimethylsiloxane) fibers was studied for alkanes of 9-22 carbons, primary alcohols of 6-13 carbons, and methyl esters of 6-16-carbon acids. Sampling was from air streams in which analyte concentrations were effectively constant, and sampling times ranged from 30 min to 3 days. Other experimental variables included sampling temperature, fiber coating thickness, air flow rate, and tubing diameter in which the SPME sampling took place. Over 1900 data points were acquired. Previous theoretical kinetic models were not applicable to the present experimental conditions, but a simple kinetic equation was formulated that described the data very well; its key property is an explicit relationship between fiber sensitivity and equilibration time. Using nonlinear regression, the equation parameters were linked to known properties of the analyte (the functional group and GC retention index on a nonpolar column) and to certain sampling conditions (temperature, sampling duration, air flow rate, tubing diameter). The regression equation serves as a practical quantitation formula and allows the absolute concentration of the analyte in the air stream to be calculated directly from the amount extracted by the SPME fiber (which is easily measured by GC), regardless of whether equilibrium has been established or not, as long as the above analyte properties and sampling conditions are known. The residual variability for the model (RSD = 9.4%) was only slightly larger than the variability inherent in SPME alone (∼5%). Considerations for SPME sampling from air are discussed, and new fiber calibration information is presented for the larger hydrocarbons, alcohols, and methyl esters.