INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY WITH AN ENLARGED SAMPLING ORIFICE AND OFFSET ION LENS .1. ION TRAJECTORIES AND DETECTOR PERFORMANCE

A new inductively coupled plasma mass spectrometer (ICP-MS) with four stages of differential pumping is described. The relatively large sampling orifice (1.31-mm dia.) improves signals for metal ions and resists plugging from deposited solids. A new ion lens is described that deflects ions off center and then back on center into the differential pumping orifice; there is no photon stop in the center of the beam. Calculations of ion trajectories using SIMION show that only those ions that leave the skimmer on center are transmitted, whereas most other lenses used in ICP-MS transmit only ions that leave the skimmer off axis. The performance of a Channeltron electron multiplier is compared to that of a Daly detector. Both detectors yield similar sensitivities of approximately 10(6) counts s-1 per ppm and detection limits of approximately 1 pptr. The background with a Channeltron electron multiplier is only 0.4 counts s-1 and is only slightly higher than the dark current count rate. Presumably the offset ion lens used in the present work efficiently screens the detector from photons emitted by the plasma. The background with the Daly detector is 4 counts s-1, which represents a substantial improvement over the background obtained in pervious use of the Daly detector with ICP-MS.