Evaluation of the vacuum effect on headspace solid-phase microextraction of volatile organic compounds from aqueous samples using finite element analysis modeling

Headspace solid-phase microextraction (HSSPME) of volatile organic compounds (VOCs) from aqueous samples under vacuum conditions (Vac-HSSPME) allows increasing extraction rates and decreasing detection limits compared to HSSPME under atmospheric pressure. The positive effect of the vacuum on HSSPME of an analyte can be quickly estimated using its Henry's law constant (HLC). According to the two-layer model of evaporation, substantial positive effect of the vacuum can be expected for analytes with HLC lower than 1.6·10⁻⁴ atm m³ mol⁻¹ (0.0065 at 25 °C), but the model does not consider possible effects of other important extraction parameters. This research was aimed at the evaluation of the possible effect of vacuum on the equilibration time and extracted amounts of analytes with various HLC and fiber coating-headspace distribution constants (K_fh) using the computational model recently developed in COMSOL Multiphysics® software. It has been proven that HSSPME under vacuum provides faster equilibration of VOCs with all studied K_fh and HLC. The largest vacuum effect on the extracted analyte amount was 3.9–4.0 times at logK_fh = 6 and HLC = 10⁻⁶–10⁻³. The substantial (≥1.5 times) vacuum impact should not be expected for analytes with logK_fh < 5 for 15-min extraction and logK_fh < 5.5 for 30-min extraction. The vacuum impact should be more pronounced when using fiber coatings with a stronger affinity to analytes. The obtained results will be useful for the development of new methods based on Vac-HSSPME and evaluation whether HSSPME is reasonable to conduct under vacuum conditions for faster equilibration and/or lower detection limits.