Evaluation of axial DC offsets during scanning of a quadrupole ion trap for sensitivity improvements

In the normal operation of quadrupole ion trap mass spectrometers, approximately half of the trapped ions are ejected through the source endcap during a mass-selective instability scan. This reduces the sensitivity of the instrument by approximately 50%. In this preliminary study, a circuit was constructed that produced a dipolar DC offset on the axial modulation waveform to recover this lost ion current. A variable (0 to 10 V DC), positive and negative offset was applied to the source and detector endcap, respectively. This DC offset axially displaced the ion cloud toward the detector endcap increasing the probability of detection. Several compounds, including 11 pesticides, were evaluated. Sensitivity enhancements ranged from 13 to 97% (theoretical 100%). No spectral resolution problems were observed; however, a compound-dependent mass discrimination was observed in several cases. This mass discrimination problem is currently under investigation.