Simultaneous determination of five quinoxaline-1,4-dioxides and two major metabolites in surface water by on-line solid phase extraction coupled to high-performance liquid chromatography

Veterinary antibiotics are environmental contaminants of recent concern, so this study was designed to develop an analytical method of simultaneous determination for quinoxaline-1,4-dioxides (QdNOs) and their metabolites in the aqueous environment at trace levels. The new method is based on on-line solid phase extraction (SPE) using cigarette filter (CGFR) as the sorbent coupled to high-performance liquid chromatography (HPLC). Five QdNOs (carbadox, olaquindox, cyadox, mequindox, quinocetone) and two major metabolites (quinoxaline-2-carboxylic acid and methyl-3-quinoxaline-2-carboxylic acid) were detected in the surface water. The cleaned CGFR precolumn selectively retained target analytes when untreated water samples were upload by a flow-inject pump. Then the enriched extracts were subsequently eluted by HPLC mobile phase to the analytical column for chromatographic analysis. The on-line setup was uncomplicated and automated. The on-line SPE conditions were optimized in detail including sample pH, sample loading flow rate, sample volume, eluent, elution time and sorbents. Under the optimal experimental conditions, the enrichment factors were 33.3-630.0 by preconcentrating 25.0 mL of water samples. Limits of detection (S/N = 3) ranged from 1.6 to 24.3 ng L-1. Satisfactory recoveries were obtained and ranged from 87.1 to 107.5% at two spiked levels in real water samples with high precision (RSD, 1.3-3.8%). The on-line SPE-HPLC method is simple, rapid, reliable, sensitive and could be applied for multiresidue determination of the QdNOs and their metabolites in water samples.