Estimation of Partition Coefficients of Benzene, Toluene, Ethylbenzene, and p-Xylene by Consecutive Extraction with Solid Phase Microextraction

Abstract

It is very important to use partition coefficients of organic pollutants to predict their fate in the environment. A liquidliquid extraction technique was used to determine the partition coefficients of organic compounds between water and organic solvent. The concentration of the target compounds must be determined after equilibrium is established between the two phases. The partition coefficients can be estimated using the capacity factors of HPLC and GC. The solid phase microextraction (SPME) technique has been widely used for the analysis of volatile organic compounds (VOCs) in water and air; fragrances in perfume, food, and cosmetics; and many other emerging pollutants. In addition, the fact that SPME extraction is based on equilibrium drove scientists to develop a new approach to determine partition coefficient (Kd) values. Vaes et al. used the kinetics of partitioning to SPME fibers to estimate the Kd values for 19 organic compounds. Nardi used the “depletion technique” and a capillary tubing coated with siloxane to estimate the Kow values of BTEX. 6 The Kd values of hydrophobic compounds are generally known to have a close relationship with Kow. 7 Dean and colleagues showed a close correlation between water-fiber coefficients (Kfs) and octanol-water partition coefficients (Kow). 8 All these approaches use an absolute amount of analyte extracted from the SPME fiber. The liquid injection of a known amount of standard solution is a common technique to calibrate GC for quantitative analysis with the SPME fiber. This technique assumes that the sample transfer efficiency by the SPME fiber is the same as that of liquid injection. However, this study shows that several factors need to be considered in order to achieve higher sample transfer efficiency in the case of liquid injection, i.e., the liner, glass wool, and the temperature of the injector. The difference in sample transfer efficiency between liquid injection and the SPME fiber may result in errors in the estimation of Kfs values, i.e., Kow values. In this study, water-fiber partition coefficients (Kfs) were determined from consecutive extraction data by a SPME fiber. Instead of using the absolute mass of the analyte, the relative GC signal ratio was used to estimate the partition coefficients. An SPME fiber extracts target compounds multiple times from the same sample vial and inserts the fiber into a hot GC injector. The extracted amount on the fiber coating decreases exponentially after each extraction; Kfs of an analyte can be derived from the resulting GC responses. Theory. The following is the equation showing the relationship among the extracted amount (n, the number of moles of the analyte with an initial concentration of Co), the extraction phase volume, (Vf, typically 0.6 μL ), and the sample volume (Vs) when an SPME fiber has reached equilibrium with the analyte. Eq. (1) shows that the extracted amount is governed by the partition coefficient (Kfs) of the target analyte: