Measuring gas-liquid partition coefficients of aroma compounds by solid phase microextraction, sampling either headspace or liquid

Abstract

Hydrophobic compounds are important odorants and nutrients in foods and beverages, as well as environmental contaminants and pharmaceuticals. Factors influencing their partitioning within multi-component systems and/or from the bulk liquid phase to the air are critical for understanding aroma quality and nutrient bioavailability. The equilibrium partitioning of hydrophobic analytes between air and water was analyzed using solid phase microextraction (SPME) in the headspace (HS-SPME) and via direct immersion in the liquid (DI-SPME). The compounds studied serve as models for hydrophobic aroma compounds covering a range of air–water partition coefficients that extends over four orders of magnitude. By varying the total amount of analyte as well as the ratio of vapor to liquid in the closed, static system, the partition coefficient, K_v, can be determined without the need for an external calibration, eliminating many potential systematic errors. K_v determination using DI-SPME in this manner has not been demonstrated before. There was good agreement between results determined by DI-SPME and by HS-SPME over the wide range of partitioning behavior studied. This shows that these two methods are capable of providing accurate, complementary measurements. Precision in K_v determination depends strongly on K_v magnitude and the ratio of the air and liquid phases.