IMPROVING ION-TRAP GC-MS QUANTITATION LIMITS FOR THERAPEUTIC AGENTS EXTRACTED FROM RAT PLASMA

Insufficient quantitation limits using ion-trap gas-chromatography mas s-spectrometry (GC-MS) prevented the assay of some samples during a pr eliminary screening of preclinical rat plasma samples (50 mu l) contai ning novel, polar therapeutic agents. Few options were available for i mproving the lower limit of quantitation. THe limited amount of sample available precluded the extraction additional plasma. Liquid-liquid e xtraction recoveries were greater than 90% throughout the range of the standard curve (500-2000 ng ml(-1)). Chromatography was optimized and multiple, equivalent sites for analyte fragmentation were precluded, using MS-MS to improve assay sensitivity. Quantitation limits were dec reased 10-fold however: by using a larger syringe to increase the inje ction volume from 5 to 50 mu l, in combination with a universal progra mmable injector. These large injection volumes required changes in the injector events program and in column plumbing. Additionally, evaluat ion of injection liner packing material demonstrated a 2-fold improvem ent in sensitivity, using carbofrit, relative to silanized glass wool. Converting to inert ion-trap electrodes did not appear to affect the detection limit. perhaps due to over-riding peak broadening during gas chromatography. The changes described produced a 20-fold improvement in the lower limit of quantitation. (C) 1998 Elsevier Science B.V. All rights reserved.