Improved signal-to-noise ratio in glow discharge ion trap mass spectrometry via pulsed discharge operation

An improvement in the S/N ratio is reported for the analysis of trace elements in brass by glow discharge ion trap mass spectrometry. This was achieved by synchronizing the pulsed discharge voltage with the ion injection and acquisition events; 'on' during ion injection and 'off' during data acquisition. Two modes of operation were evaluated: (1) a high duty cycle pulse, which allowed a continuous injection over the duration of the pulse; and (2) a low duty cycle pulse with multiple data gates, which allowed gated injections of ions at selected regions of the pulse profile. The latter afforded a means of selective ion injection since discharge and residual gas species are formed at different times in the pulse event than analyte ions. Improvements in S/N ratios greater than 40-fold were observed, primarily due to a reduction in background and background noise after the discharge was extinguished. Evidence is presented which suggests that electrons emanating from the ion source are the precursors of most of the noise. Detection limits for various elements were 0.2-0.5 ppm.