Quantitation of small molecules using high-resolution accurate mass spectrometers - a different approach for analysis of biological samples.

The quantitative capabilities of a linear ion trap high-resolution mass spectrometer (LTQ-Orbitrap) were investigated using full scan mode bracketing the m/z range of the ions of interest and utilizing a mass resolution (mass/FWHM) of 15000. Extracted ion chromatograms using a mass window of +/-5-10 mmicro centering on the theoretical m/z of each analyte were generated and used for quantitation. The quantitative performance of the LTQ-Orbitrap was compared with that of a triple quadrupole (API 4000) operating using selected reaction monitoring (SRM) detection. Comparable assay precision, accuracy, linearity and sensitivity were observed for both approaches. The concentrations of actual study samples from 15 Merck drug candidates reported by the two methods were statistically equivalent. Unlike SRM being a tandem mass spectrometric (MS/MS)-based detection method, a high resolution mass spectrometer operated in full scan does not need MS/MS optimization. This approach not only provides quantitative results for compounds of interest, but also will afford data on other analytes present in the sample. An example of the identification of a major circulating metabolite for a preclinical development study is demonstrated.