Optimisation of instrumental parameters for the precise measurement of isotope ratios with inductively coupled plasma mass spectrometry

The effect of several instrumental operating parameters (IOPs) on the precise and accurate measurement of isotope ratios was investigated. The IOPs of interest were: dwell-time (DT) and cycle time (CT) of the software control parameters, the r.f. power level, the analyte solution flow-rate and the argon gas plasma, auxiliary and nebuliser flow-rates. Two measurement criteria were evaluated for each isotope pair (a,b) in relation to these independent variables: the precision (RSD) of the isotope ratio measurements and the deviation of the measured isotope ratio from a reference value. The isotope pairs of interest were: 6Li:7Li, 57Fe:54Fe, 58Fe:54Fe, 65Cu:63Cu, 67Zn:68Zn and 70Zn:68Zn. It was shown that the best precision (lowest RSD) and most stable measurements were generally obtained over a relatively wide range of r.f. powers, while the range of r.f. power corresponding to the maximum ion-beam intensity was much narrower. The optimum r.f. power for the best results was Li, 1170; Fe, 1240; Cu, 1250; and Zn, 1290 W. Employing the optimum settings of the IOPs, the long-term stability (5 h of continuous operation) of the measurements was investigated. It was shown that the ion-beam intensities were stable to better than 8%. In contrast values of isotope ratios always stayed within the expected measurement precision [RSD<1% for all except Li (<1.5%)]. It was concluded that inductively coupled plasma mass spectrometry (ICP-MS) permitted the precise and accurate measurement of stable isotope ratios for routine application in the studies of trace-element metabolism in man employing the concept of stable isotope tracers. (author)