SIMULTANEOUS MEASUREMENT OF ION RATIOS BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY WITH A TWIN-QUADRUPOLE INSTRUMENT

The ion beam from the inductively coupled plasma (ICP) is split into two parts by a device comprised of two electrostatic analyzers in a back-to-back arrangement. Each ion beam is then monitored by a separate quadrupole mass analyzer and electron multiplier. Thus, ion ratios can be measured simultaneously without scanning. Isotope ratios for copper are measured with a precision of 0.14% relative standard deviation (RSD). Internal standard ratios for Co+/La+ are measured with a precision of 1% RSD. Ion signals in the two channels are highly correlated, and the magnitude of flicker noise is reduced in the resulting ratios. Bias in the measured isotope ratios is substantial but can be corrected by calibration.