On-site calibration method based on stepwise solid-phase microextraction

A stepwise solid-phase microextraction (SPME) method was developed for on-site calibration of SPME for volatile organic compounds analysis. In this approach, a 75-μm Carboxen–polydimethylsiloxane coated fibre was loaded with a standard prior to exposure to samples of interest. Extraction time for the target analytes can be controlled independently from that of the standard, and the response factors for the target analytes can be adjusted accordingly. A good reproducibility of the response factors for BTEXs (benzene, toluene, ethylbenzene and xylenes) was obtained with stepwise SPME. Satisfactory results were obtained by using this method for quantitative analysis of BTEXs in the air of a gas station when tetrachloroethylene was used as a standard. The introduction of standard via the stepwise SPME procedure makes SPME more useful in field applications. It can be used to detect leaks, contaminations and losses from loading of a standard onto a fibre to introduction of the fibre to an analytical instrument. However, this method cannot be used for compensation of sample matrix effects.