Vacuum-assisted headspace solid phase microextraction: improved extraction of semivolatiles by non-equilibrium headspace sampling under reduced pressure conditions.

A new headspace solid-phase microextraction (HSSPME) procedure carried out under vacuum conditions is proposed here where sample volumes commonly used in HSSPME (9 mL) were introduced into pre-evacuated commercially available large sampling chambers (1000 mL) prior to HSSPME sampling. The proposed procedure ensured reproducible conditions for HSSPME and excluded the possibility of analyte losses. A theoretical model was formulated demonstrating for the first time the pressure dependence of HSSPME sampling procedure under non equilibrium conditions. Although reduced pressure conditions during HSSPME sampling are not expected to increase the amount of analytes extracted at equilibrium, they greatly increase extraction rates compared to HSSPME under atmospheric pressure due to the enhancement of evaporation rates in the presence of an air-evacuated headspace. The effect is larger for semivolatiles whose evaporation rates are controlled by mass transfer resistance in the thin gas film adjacent to the sample/headspace interface. Parameters that affect HSSPME extraction were investigated under both vacuum and atmospheric conditions and the experimental data obtained were used to discuss and verify the theory. The use of an excessively large headspace volume was also considered. The applicability of Vac-HSSPME was assessed using chlorophenols as model compounds yielding linearities better than 0.9915 and detection limits in the low-ppt level. The repeatability was found to vary from 3.1 to 8.6%.