The ability to rapidly detect and characterize drug metabolites in biological fluids often relies on a combination of a high quality chromatographic separation and sensitive high resolution mass spectrometry. Here, the performance of two high throughput LC/MS approaches, namely monolith columns and sub-2 mu m particle Ultra Performance Liquid Chromatography (UPLC) columns is compared for the detection and identification of the human metabolites of acetaminophen in urine. The UPLC system produced approximately three times the sensitivity and detected more metabolites than the monolithic column approach. The sharp peaks produced by UPLC seem to be particularly advantageous when coupled to electrospray mass spectrometry, apparently reducing ion suppression leading to superior sensitivity and hence lower limits of detection. (c) 2005 Elsevier B.V. All rights reserved.