Comparison of an ion-trap and a quadrupole mass spectrometer using diazepam as a model compound

Recent innovations in mass spectrometry (MS) have led to the development of instruments with increased capabilities, smaller footprints, and relatively low cost. The traditional MS in most toxicology laboratories is a quadrupole system equipped with electron impact ionization. Recently, an ion trap with electron impact, positive chemical ionization, negative chemical ionization, and tandem MS capabilities was introduced by Finnigan MAT. This paper compares the sensitivity and precision of ion-ratio measurements between a Finnigan GCQ ion-trap mass spectrometer (ITMS) and a Hewlett Packard quadrupole mass spectrometer (QMS) using electron impact ionization with diazepam as the model compound. Additionally, the sensitivity and precision of ion ratio measurements are evaluated for the ITMS using positive chemical ionization, negative chemical ionization and tandem MS modes of analysis. In the full scan mode (m/z 50-650, 1 Hz), the ITMS had an average signal-to-noise ratio (S/N) of 1400 for a 2-ng injection of diazepam (10 injections per day for 5 days), within-run ion ratio precision had coefficients of variation from 5 to 11%. Using similar full scan conditions, a 10-ng injection of diazepam on the QMS had an average S/N ratio of 160, and precision of ion ratio measurements varied from 5 to 13%. In the selected ion mode (SIM) of analysis (three ions, 2 Hz), the ITMS had an average S/N of 14,000 for a 2-ng injection and ion-ratio precision ranging from 6 to 15%. Using similar SIM conditions, a 2-ng injection in the QMS had an average S/N of 3000 with ion ratio standard deviations of 0.67 to 2.9%. Overall, the ITMS provided at greater S/N, equivalent precision in full scan, but was 5- to 10-fold less precise in measuring ion ratios in the SIM mode as compared with the QMS.