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United States Department of Agriculture

Agricultural Research Service

Title: Non-Equalibrium Quantitation of Volatiles in Air Streams by Solid-Phase Microextraction

item Bartelt, Robert
item Zilkowski, Bruce

Submitted to: Analytical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 19, 1998
Publication Date: N/A

Interpretive Summary: A chemical-measurement technique was improved so that the analysis of particular agriculturally important compounds would be simpler and more rapid than before. Solid-phase microextraction (SPME) is a rather new, commercially available method for sampling organic compounds from the air. The technique is sensitive, flexible, easy to do, and does not involve potentially dangerous solvents. SPME would be an especially useful method for measuring air-borne concentrations of pheromones and other important insect attractants, either from living organisms or from synthetically prepared dispensers. Such measurements are vital for understanding insect biology and also for evaluating pest management techniques involving behavioral chemicals. Unfortunately, relatively heavy organic compounds, including pheromones, interact so slowly with the SPME device that the "standard" approach to quantitation by SPME doesn't work (which assumes that the device equilibrates or "stabilizes" with the air being sampled). Therefore, a method was developed so that SPME can be used for quantitation even when the device has not equilibrated with the air. The method involves a mathematical formula that allows sampling time and extent of equilibration to be taken into account. The research broadens the scope of this simple and powerful tool, and the information will be of use to scientists interested in quantitating chemicals in the air in an effort to control agricultural pests via environmentally friendly (natural) methods.

Technical Abstract: Solid phase microextraction (SPME) is useful for measuring analyte concentrations in the air; calculations normally assume equilibration between the SPME fiber and the sample medium. Unfortunately, relatively heavy compounds often require an impractical amount of time to equilibrate, and this study focused on non-equilibrium SPME quantitation. The kinetics of absorption by polydimethylsiloxane fibers was studied for 33 moderately heavy alkanes, alcohols, and methyl esters. Sampling was from air streams with constant analyte concentrations, and sampling times were 30 min to 3 da. Other experimental variables included sampling temperature, fiber coating thickness, air flow rate, and tubing diameter where SPME sampling took place. Over 1900 data points were acquired. A simple kinetic equation was formulated that described the data very well. Non-linear regression was then used to estimate the equation parameters from known properties of the analyte and to certain sampling conditions. The regression model allows the absolute concentration of the analyte in the air stream to be calculated directly from the amount absorbed by the SPME fiber (which is easily measured by GC), regardless of whether equilibrium has been established or not, as long as the 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 (about 5%). Practical considerations for SPME sampling from air are also discussed.

Last Modified: 4/16/2014
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