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Microextraction under vacuum conditions

Giantzi Evaggelia

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Summary

The present thesis investigates the possibility of sampling semi – volatile analytes from the headspace of aqueous or solid samples using headspace solid – phase microextraction (HSSPME) under reduced pressure conditions. The new procedure was termed vacuum assisted headspace solid phase microextraction (Vac – HSSPME).In Chapter 1 sample preparation techniques are presented. A comprehensive review on solvent – free sample preparation techniques is given with emphasis to SPME principles and the parameters affecting the two sampling modes (direct and headspace).Chapter 2 presents the published report entitled: Vacuum-assisted headspace solid phase microextraction: Improved extraction of semivolatiles by non-equilibrium headspace sampling under reduced pressure conditions. In this report, a new headspace solid-phase microextraction (HSSPME) procedure carried out under vacuum conditions was proposed 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%.Chapter 3 presents the published report entitled: Effect of Henry’s law constant and operating parameters on vacuum-assisted headspace solid phase microextraction. This paper investigated the effects of organic analyte properties and sampling parameters (headspace volume and sample agitation) on vacuum-assisted HSSPME (Vac-HSSPME). The results revealed that at room temperature, acceleration effects on extraction rates induced by reducing the total pressure of the sample container are important for those compounds where the Henry’s law constant, KH, is close or below the reported threshold values for low KH solutes. For these compounds evaporation rate is controlled by mass transfer resistance in the thin gas-film adjacent to the gas/sample interface and reducing the total pressure will increase evaporation rates and result in a faster overall extraction process. Conversely, for analytes with an intermediate KH value, Vac-HSSPME is not expected to improve extraction rates compared to regular HSSPME given that mass transfer resistance in the liquid-film becomes important. In accordance with the theory, at equilibrium, the amount of analyte extracted by the SPME fiber is not affected by the pressure conditions inside the sample container. Furthermore, Vac-HSSPME extraction kinetics for low KH analytes were marginally affected by the tested change in headspace volume as evaporation rates dramatically increase under reduced pressure conditions and the sample responds much faster to the concentration drops in the headspace when compared to regular HSSPME. At equilibrium however, increasing the headspace volume may result in a loss of sensitivity for Vac-HSSPME similar to that observed for regular HSSPME. As expected, stirring the liquid sample was found to improve Vac-HSSPME. Finally, the method yielded a linearity of 0.998, detection limits in the ppt level and precision varying between 1.8% and 8.4 %.Chapter 4 presents the published report entitled: Downsizing vacuum-assisted headspace solid phase microextraction. In this study, we downsized the extraction device to a 22 mL modified sample vial and concluded that changes in the final total pressure of the pre-evacuated vial following sample introduction were sufficiently low to allow efficient Vac-HSSPME sampling. The downsized extraction device was used to extract five low molecular weight polycyclic aromatic hydrocarbons and several experimental parameters were controlled and optimized. For those compounds whose mass transfer resistance in the thin gas-film adjacent to the gas/sample interface controls evaporation rates, reducing the total pressure during HSSPME sampling dramatically enhanced extraction kinetics in the 22 mL modified vial. Humidity was found to affect the amount of naphthalene (intermediate KH compound) extracted by the fiber at equilibrium as well as impair extraction of all analytes at elevated sampling temperatures. All the same, the high extraction efficiency and very good sensitivity achieved at room temperature and within short sampling times comprised the most important features of Vac-HSSPME in this downsized extraction device. Analytically, the developed method was found to yield linear calibration curves with limits of detection in the low ng L-1 level and relative standard deviations ranging between 3.1 and 6.4 %. Matrix was found not to affect extraction.Chapter 5 investigates the possibility of using Vac-HSSPME for extracting polycyclic aromatic hydrocarbons from soil samples. Several parameters were controlled and optimized. The optimum conditions found were: sampling the headspace of a 2 g spiked sandy sample and 2 mL of water for 30 min while stirring the sample at a 1400 rpm agitation rate. The application of Vac – HSSPME yielded good linearity in the range 1 to 400 ng g-1 (r2>0.9478) and precision ranging between 4.3 to 10 % (expressed as RSD). The detection limits were in the low ng g-1 levels (0.003 – 0.233 ng g-1). Overall, Vac-HSSPME method confirmed that very good sensitivity and precision could be attained within short sampling times and under mild sampling temperatures.In Chapter 6 the results of the present study are summarized and conclusions are drawn. The parameters affecting Vac-HSSPME procedure as well as the overall performance of the method are evaluated. Future directions in the field are also given.

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