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). Substantial positive effect of the vacuum can be expected for analytes with HLC lower than 1.6·10-4 atm m3 mol−1 (0.065 at 25 °C). This conclusion was made using a two-layer model of evaporation, which does not consider possible effects of other parameters of HSSPME. This research was aimed at a detailed evaluation of the possible effect of vacuum on the equilibration time and extracted amounts of analytes with various HLC and coating-headspace distribution constants (Kfh) 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 Kfh and HLC. Drop in the equilibration time can reach 33.7 times. The largest vacuum impact on the extracted analyte was 3.9-4.0 times at Kfh = 106 and HLC = 10-6-10-3. The substantial (<1.5 times) vacuum impact should not be expected for analytes with Kfh < 105 for 15-min extraction and Kfh < 105.5 for 30-min extraction. For greater Kfh, HLC values should be lower than 0.1. The obtained results and the developed model can be recommended for the evaluation whether HSSPME is reasonable to conduct under vacuum conditions.