Vacuum-assisted headspace single-drop microextraction: Eliminating interfacial gas-phase limitations

Gas-phase limitations have been neglected in headspace single-drop microextraction (HS-SDME) and rate control has been assumed to primarily reside in the liquid water and/or organic phases, but not in the headspace. Herein we demonstrate the presence of interfacial gas constraints and propose using reduced headspace pressures to remove them. To describe the pressure dependence of HS-SDME, the system was decoupled into two interfacial steps: (i) the evaporation step (water-headspace interface) formulated using the two-film theory and (ii) the analyte uptake by the microdrop (headspace-microdrop interface) formulated using the resistance model. Naphthalene, acenaphthene, and pyrene were chosen as model analytes for their large Henry's law solubility constants in n-octanol -1 (H-OA > 10(3) M atm(-1)), and their low to moderate Henry's law volatility constants in water as a solvent (K-H). We have found that extraction times were significantly shortened for all analytes by sampling at pressures well below the 1 atm used in the standard HS-SDME procedure. The acceleration of naphthalene extraction, whose facile evaporation into the headspace had been assumed to be practically pressure independent, highlighted the role of mass transfer through the interfacial gas layer on the organic solvent drop. The larger accelerations observed for acenaphthene and (especially) pyrene upon reducing the sampling pressure, suggested that gas-sided constraints were important during both the evaporation and uptake steps. Model calculations incorporating mass transfers at the headspace-microdrop interface confirmed that gas-phase resistance is largely eliminated (>96%) when reducing the sampling pressure from 1 to 0.04 atm, an effect that is nearly independent of analyte molecular mass. The relative importance of the two interfacial steps and their gas- and liquid-phase limitations are discussed, next to the use of K-H and H-OA to predict the positive effect of vacuum on HS-SDME. (C) 2019 Elsevier B.V. All rights reserved.