SPACE-CHARGE EVALUATION IN A PLASMA-SOURCE MASS SPECTROGRAPH

Space charge effects, and the matrix interferences they cause, are pro blems in inductively coupled plasma mass spectrometry (ICPMS). It has previously been observed that these deleterious space charge effects a re not significantly present in sector-field instruments, a fact that has been attributed, but not demonstrated, to the high accelerating po tentials they commonly employ. To examine the significance of space ch arge in our plasma-source mass spectrograph (which operates at only mo derate accelerating potentials) and in other sector instruments, a gra phite disk was placed -7 similar to cm behind the skimmer. An inductiv ely coupled plasma was operated for 17 h while a 0.01 mM multielement solution was introduced. This disk was then analyzed by spatially reso lved laser ablation ICP time-of-flight MS, Second vacuum-stage acceler ation appears to be an important factor that governs the elemental dis tribution within the ion beam. The ion beam width at m/z 208 is one-th ird of its width at nit 7 using an accelerating potential of 800 V; at an accelerating potential of 4000 V, the ion beam width does not vary with mass.