OCCURRENCE AND REDUCTION OF NOISE IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY FOR ENHANCED PRECISION IN ISOTOPE RATIO MEASUREMENT

The limitations imposed upon measurement precision by instrumental noise sources present within the inductively coupled plasma mass spectrometer used have been established using noise spectral analysis. Based upon the spectral information gained, a methodology for the sequential measurement of isotopes has been developed to minimize the deleterious influence of non-random instrumental noise. Efficient use of the noise reduction techniques available within the peak-jumping mode have been found to remove the majority of the instrumental noise associated with sample introduction and excitation facilitating a measurement precision of approximately 0.05% relative standard deviation, with respect to the Ag-107 : Ag-109 isotope ratio. The precision of isotope ratios has been found to be limited by inaccuracies associated with the operation of the quadrupole mass analyser and the statistical error arising from the random arrival of ions at the detector.