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 diam.) improves signals for metal ions and resists plugging from deposited solids. The ion lens 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 with 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 ~10[superscript]6 counts s[superscript]-1 per ppm and detection limits of ~1 pptr. The background with a Channeltron electron multiplier is only 0.4 counts s[superscript]-1 and is only slightly higher than the dark current count rate. The background with the Daly detector is 4 counts s[superscript]-1, which represents a substantial improvement over the background obtained in previous use of the Daly detector with ICP-MS;This new ICP-MS yields very low levels of many troublesome polyatomic ions such as ArO[superscript]+, ArN[superscript]+, Ar[subscript]2[superscript]+, ClO[superscript]+, and ArCl[superscript]+. The signals from refractory metal oxide ions are ~1% of the corresponding metal ion signals, which is typical of most ICP-MS devices. Grounding the first electrode of the ion lens greatly reduces the severity of matrix effects to [subscript]sp~<20% loss in signal for Co[superscript]+, Y[superscript]+ or Cs[superscript]+ in the presence of 10 mM Sr, Tm or Pb. This latter lens setting causes only a modest loss (30%) in sensitivity for analyte elements compared to the best sensitivity obtainable by biassing the first lens;Conventionally, the metal cones through which the ions are extracted are connected to ground potential. Floating one or both of these cones at various potentials can improve the ion transmission by a factor of at least four to six. The net result is better sensitivity and higher ion beam intensity than are obtained with a grounded skimmer and sampler. (Abstract shortened with permission of author.)