ИСТИНА

On-line coupling of solid-phase extraction (SPE) is widely used to enhance the capabilities of reversed phase HPLC analysis. Porous graphitic carbon (PGC) is often used for SPE when increased retention of analytes is required compared to octadecylsilica. Careful selection of desorption conditions of analytes from SPE column is crucial for effective coupling of SPE and HPLC. The usual approach is to use the same eluent for both desorption and reversed phase HPLC separation. However, effective desorption requires the use of the eluent with high organic content to minimize band broadening; but reversed phase HPLC separation is usually carried out with low or medium organic content in the eluent. This contradiction is usually resolved in a compromise manner – by choosing a medium strength eluent, which hampers effective desorption and results in excessive peak broadening. In the recent years there is a considerable interest in replacing organic eluents in HPLC separations with so-called subcritical water. Pressurized hot (liquid) water at 150-250 °C was shown to be similar in elution strength to ambient temperature water-acetonitrile mixtures of various compositions. The present report is devoted to study and development of the on-line coupling of SPE and HPLC via subcritical water desorption. We have used PGC column (Hypercarb 5 µm porous graphitic carbon HPLC column, 30 mm × 2.1 mm i.d., Thermo Scientific, USA) for solid-phase extraction of phenols from water samples and octadecylsilica (ODS) column for HPLC separation. HPLC separation was carried out at ambient temperature with water-acetonitrile eluent. Phenol and its chloro- and nitro- derivatives were used as model substances. The developed procedure includes the following steps. First, solid-phase extraction of analytes on PGC column is carried out. Then, analytes are eluted by subcritical water at 150-250 °C and 30-60 bar; the effluent flow is cooled and passed through HPLC column, where peak focusing takes place. Finally, HPLC separation by water-acetonitrile mixture at ambient temperature is carried out. Extraction and desorption conditions were optimized. Desorption was carried out with 3-5 ml of subcritical water. Chromatographic peak widths were compared for three cases: direct injection HPLC analysis; desorption with water-acetonitrile mixture; desorption with subcritical water with subsequent focusing. It was found out that the third approach resulted in narrow peaks comparable to peaks obtained by direct injection. Desorption with water-acetonitrile resulted in excessive peak broadening. The developed procedure was applied to analysis of natural, potable and tap water samples. Detection limits were 0.3-2 ng/ml for UV detection, accuracy was verified by method of spikes.