The LC/ESI-SRM chromatograms obtained at unit mass-resolution for compound A in plasma at a concentration of 250 ng/mL, are shown in Figure 3. Background interference to the signal of compound A at this concentration is clearly evident in the chromatographic trace. A calculated signal to noise (S/N) ratio of 8 is determined for this peak (Figure 3). Hence, the LOQ for compound A at unit mass-resolution can be assumed to be of the order of 250 ng/mL.
When the same sample of compound A (conc. 250 ng/mL) was analyzed at enhanced mass-resolution, a much cleaner SRM peak was observed, as shown in Figure 4. This is a consequence of the removal of a considerable amount of isobaric chemical interference from the analyte through the improved mass selectivity at enhanced mass-resolution. In addition to a more uniform SRM peak, an improved S/N ratio of 31 was obtained for compound A at enhanced mass-resolution, relative to unit mass-resolution (Figures 3,4). The dramatic decrease in background noise at the higher mass-resolution setting is responsible for the significant improvement in S/N ratio, despite the loss of a factor of 3-4 in peak height/area for compound A (Figures 3,4). An increase in analyte sensitivity at enhanced mass-resolution, relative to unit, has been reported in previous LC/ESI-SRM and LC/APCI-SRM quantitative studies on the Finnigan TSQ Quantum Discovery.[2-6] Indeed, one of the prior quantitative studies was based on the monitoring of SRM transitions involving small molecule losses, analogous to the transition used in this work (m/z 473456, loss of 17 u). Since small molecule losses through collisionally-induced dissociation processes are common-place and the potential for matrix interference to the analyte SRM is much greater, analysis at enhanced mass-resolution should be an extremely viable method for improved quantitative performance in