On-line trace enrichment in hyphenated liquid chromatography-nuclear magnetic resonance spectroscopy.

The potential of on-line trace enrichment combined with column liquid chromatography-nuclear magnetic resonance detection (LC-NMR) is demonstrated for the identification of hop bitter acids in hop extracts. By performing on-line solid-phase extraction on 1-5 ml of hop extract (which contains the six major hop bitter acids) the detection/analysis time in the stopped-flow detection mode could be reduced 4-fold, from 60 min to 15 min per individual peak. The applicability of combined trace enrichment and separation using a single short (12.5 mmx4.6 mm I.D.) high-pressure-packed column combined on-line with NMR was also studied. Sample enrichment of 5 ml of a solution spiked with three drugs at 5 μg/ml was sufficient for the acquisition of high-quality spectra both in the stopped-flow and even in the continuous-flow detection mode. The examples demonstrate that analyte detectability can be significantly improved by performing on-line sample enrichment prior to LC-NMR analysis. Copyright (C) 1998 Elsevier Science B.V.