Automated In-Tube Solid-Phase Microextraction Coupled to High-Performance Liquid Chromatography

Abstract

Recently, solid-phase microextraction (SPME) was successfully coupled to high-performance liquid chromatography. However, the efficiency of this analytical method, in terms of manpower, still suffers from its manual operation technique. Furthermore, the selectivity obtained for the analysis of very polar compounds is still poor because of a limited selection of commercially available fiber coatings that can withstand the aggressive HPLC conditions (solvents). This paper describes the first approach to developing an automated SPME-HPLC system. A mixture of polar thermally labile analytes, phenylurea pesticides, was selected for microextraction directly from an aqueous sample. A piece of a ordinary capillary GC column with its coating (Omegawax 250) was used for the absorption of analytes from the aqueous sample (in-tube solid-phase microextraction). A needle hosts the capillary when it is pierced through the septum of the vial containing the spiked aqueous sample. The aqueous samples were stored in 2 mL vials on the tray of a commercial autosampler. A sample of 25 μL was aspirated and dispensed several times from the sample into the capillary using a syringe. After the extraction the absorbed analytes were released from the coating by aspiring methanol into the column and then dispensing the methanol into the HPLC injector loop. The absorption−time profiles, the amounts absorbed by different coatings, linearity, and precision were studied under different sampling conditions using spiked aqueous samples. SPME selectivity for polar compounds, which represent an important compound class for water analysis, can be improved by using more polar column coatings such as Carbowax instead of poly(dimethylsiloxane)-coated columns. Compared to the manual version this automated SPME-HPLC system could increase productivity and reproducibility. Furthermore, the desorption step is quantitative; i.e., no carryover could be detected. This entire method for automated SPME sample preparation is simple and controlled by a commercial autosampler from LC Packings which was modified to operate in-tube SPME. The automated SPME-HPLC device obtains RSD for all investigated compounds below 6%. A simple mathematical model was used to calculate the concentrations vs length profiles in the column for any time. The model was in good agreement with experimental data which was obtained for benzene as a model compound. Thus, the main parameters affecting the partitioning process were determined and the amount absorbed by the coating could be predicted.