Reproducibility assessment for a broad spectrum drug screening method from urine using liquid chromatography time-of-flight mass spectrometry

During the reproducibility validation for a time-of-flight (TOF) high-resolution mass spectrometry (HRMS) method set up to detect 61 drugs of abuse commonly encountered in the toxicology laboratory, it was noticed that, a number of compounds were not identified correctly during the between run analysis; the most difficult compounds to identify were norpropoxyphene, morphine, norbuprenorphine, nortriptyline, EDDP and tramadol. In subsequent patient comparison studies, screening a panel of 338 analytes, the TOF-HRMS method correctly identified 211 analytes over two runs, but did not identify 127. A total of 11 false positive results were identified by manual review of the data to be the result of confirmation ion signal-to-noise ratio(s) < 3, although one false positive that was difficult to resolve (i.e., identification of maprotiline as amitriptyline) was due to similar fragment ions and retention times. The TOF-HRMS method showed reasonable agreement with LC-MS/MS results, but there were a number of discrepant results. Additionally, the TOF-HRMS did detect five compounds missed by the LC-MS/MS methods. This extensive validation effort highlights the difficulty of analysis for certain compounds that are likely to require additional follow up prior to reporting a positive result, especially at low and high concentrations, regardless of the type of instrumentation involved.