Enhancement of ion transmission and reduction of background and interferences in inductively coupled plasma mass spectrometry

摘要

An inductively coupled plasma - mass spectrometer (ICP-MS) (four stages of differential pumping) is described. The large sampling orifice (1.31 mm dia.) improves signals for metal ions and resists plugging. 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. Ion trajectories calculations 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. Background with the Daly detector is 4 counts s(sup (minus)1). This ICP-MS yields low levels of many troublesome polyatomic ions. Signals from refractory metal oxide ions are about 1% of the corresponding metal ion signals. Grounding the first electrode of the ion lens reduces matrix effects to (approx lt) 20% loss in signal for Co(sup +), Y(sup +) or Cs(sup +) in presence of 10 mM Sr, Tm or Pb. This latter lens setting causes only 30% loss in sensitivity compared to biassing the first lens. Matrix effects can also be mitigated by re-adjusting the voltage on the first lens with matrix present. Floating the metal cones at various potentials can improve the ion transmission by a factor of at least four to six. Also, floating the cones extends the upper end of linearity. Net result is more sensitivity and higher ion beam intensity than with a grounded skimmer and sampler. Furthermore, mass discrimination can be reduced.