When attempting to collect volatile analytes, the Carboxen/PDMS fiber is shown to be ideal for adsorption of volatile compounds. Adsorption type fibers are also better for extracting analytes present at low concentration levels and often provide lower MDLs for many analytes. To improve sampling of volatiles in complex matrices, place the sample in a closed vial with 2/3 of the vial filled with the sample and 1/3 remaining as headspace. Note that excessive headspace volume reduces the analyte concentration in the headspace and reduces the extraction efficiency. For the majority of volatile analytes evaluated in this study, the linear concentration range was 1ppb to 10ppm; however, this range will vary depending upon the analyte polarity.
If you are attempting to extract semi-volatile compounds in the headspace, use an absorptive fiber coating such as PDMS or polyacrylate. Heating the sample slightly will increase the volatility of the analytes. To insure reproducibility, the sample must be stabilized at the extraction temperature before inserting the fiber and initiating extraction of the sample. Even analytes with high boiling points can be extracted from the headspace if they have sufficient vapor pressure. For example, PCBs can be extracted from soils into the headspace by heating the sample at only 50 °C. Absorptive fibers have greater capacity and linear concentration ranges than adsorptive fibers, since they utilize partitioning for the extraction.
For some polar analytes that have low vapor pressure, polar fibers such as Carbowax DVB and polyacrylate may be the better choice. Immersion into solution may be necessary. In general, Absorption fibers or adsorptive fibers with large pores (DVB for example) are better for extracting and releasing large analytes (>150 amu).
SPME Application Guide (PDF)