In-tube solid phase micro-extraction–gas chromatography of volatile compounds in aqueous solution

Abstract

This paper describes the use of conventional coated capillary gas chromatography columns for sorption of organic solutes from aqueous solution, with subsequent gas chromatographic analysis. The essential principles are similar to those of solid phase extraction (SPE) and solid phase micro-extraction (SPME); this approach may be referred to as in-tube solid phase micro-extraction (ITSPME). The technique was evaluated using toluene in water as the initial test solute, and a mixture of BTEX solutes (benzene, toluene, ethylbenzene, xylenes) in Milli-Q water was used to further characterise ITSPME. A 1 m length of capillary GC column was used for sorption of analytes from aqueous solution passed through the capillary by using nitrogen pressure. Collection of small fractions of aqueous solution issuing from the capillary enabled a sorption profile to be generated, with initial fractions depleted in analyte. A Boltzmann curve could be fitted to the sorption profile data, exhibiting good agreement with experimental data. For recovery of sorbed toluene, a single 100 µL aliquot of hexane was passed through the column as a stripping solvent. The back-extraction step was quantitative. Equilibrium extraction of solutes shows that the total amount of recovered solute is proportional to its initial concentration in the extracted aqueous solution and allows distribution constants to be readily estimated. For BTEX solutes, K values were similar to those reported for SPME and literature K_ow values. For toluene, log K decreases from 2.47 to 1.48 when the sorption column temperature increases from 20 to 30 °C; adding salt or reducing the pH of the aqueous solution increases the degree of extraction of phenols, agreeing with general considerations on solute partitioning behaviour.