Page - (000117) - in Biomedical Chemistry: Current Trends and Developments

detection, reducing analysis time. Even though both LC/MS and LC/MS/MS are very expensive and not available to all research laboratories, bioanalysts have been devoted to improve MS and MS/MS hardware and software, not only to identify and quantify metabolites in a variety of in vitro and in vivo assays during all stages of DDD, but also to allow users to benefit from lower LLOQs and ease of use. In fact, reduced LLOQ values are essential in order to guarantee strong characterization during the elimination phase of a compound. Besides their high sensitivity and specificity, MS and MS/MS detections are based on a combination of the unique parent and fragment masses of each compound, eliminating the need for baseline separation and achieving fast analyses. Prior to MS detection, analyte ionization is required and can be achieved by electrospray ionization (ESI), atmospheric pressure ionization (API) or atmospheric pressure chemical ionization (APCI) modes. MS detection of ionized analytes has been carried out in a linear scan mode in which a range of m/z values is constantly monitored. In a more selective and sensitive mode called SIM, one specific m/z value (or more) is selected for the monitoring. Single-reaction monitoring (SRM) or multiple-reaction monitoring (MRM) modes can also be used: in SRM mode, a specific product ion of a specific parent ion is detected, producing very simple plots (usually containing only a single peak). MRM delivers a unique fragment ion that can be monitored and quantified in a very complicated matrix, making the MRM plot ideal for sensitive and specific quantifications. Commonly, several kinds of tandem mass spectrometers including the triple stage quadrupole, the three-dimensional ion-trap, linear ion- traps/quadrupole coupled with time-of-flight (Q-TOF) and hybrid triple quadrupole linear ion trap mass spectrometers have been used in DDD. Nevertheless, classical detection modes such as UV and fluorescence, although less employed in current practice than MS or MS/MS detections, continue to be applied in the early in vitro bioavailability or metabolic stability screening and remain valuable in cases where sensitivity is not paramount or where LC/MS is not economically viable. 2.1.4.1 Bioanalytic Support of In Vitro ADME Studies