Continuum Flow Sampling Mass Spectrometer for Elemental Analysis with an Inductively Coupled Plasma Ion Source

摘要

The sampling of ions from an atmospheric pressure inductively coupled plasma for mass spectrometry (ICP-MS) with a supersonic nozzle and skimmer is shown to follow similar behavior found for neutral beam studies and for ion extraction from other plasmas and flames. In particular, highest ion beam intensity is found if the skimmer tip is close to the Mach disk and at a calculated skimming Knudsen number close to the recommended value of 1. Our ICP-MS instrument with an off-axis detector and conventional cylindrical electrostatic ion focusing in the transition flow regime gives intense count rates of 1 to 5 MHz per mg L exp -1 of analyte superimposed on a background of 1 to 10 kHz. The dependence of count rates for metal oxide and doubly charged ions on ICP operating parameters, and sampling interface configuration are discussed for this instrument. A simple method is described for the approximate measurement of the ion energy distribution in ICP-MS. The average ion kinetic energy, kinetic energy spread, and maximum kinetic energy are evaluated from a plot of ion signal as a function of retarding voltage applied to the quadrupole mass analyzer. The effects of plasma operating parameters on ion signals and energies are described. In particular, kinetic energy is a sensitive function of aerosol gas flow rate. This behavior is attributed to a non-thermal, possibly electrical, interaction between the plasma and the sampling interface, which is induced by the presence of the axial channel in the ICP. The interference on the ionization of cobalt by five salts, NaCl, MgCl sub 2 , NH sub 4 I, NH sub 4 Br and NH sub 4 Cl, in an ICP is first considered theoretically and subsequently the theoretical trends are established experimentally by ICP-MS. The interference trends are found to be in the order of the most easily ionized element in the matrix salt, i.e., Na > Mg > I > Br > Cl. (ERA citation 10:032052)