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

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Abstract
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 (Δ) of the measured isotope ratio (MRa/b) from a reference value (MR0 a/b). 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 (lowest Δ) 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. Argon gas flow-rates corresponding to the optimum values for %RSD and %Δ were: plasma flow-rate 15 for Li and 12 l min–1 for the other elements, auxiliary flow-rate 1.7 for Li and 1.8–1.9 l min–1 for the other elements, and nebuliser pressure 42 lb in–2 for all elements. The analyte solution flow-rate giving the best results was investigated for Cu only (best value 0.91 ml min–1). 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 (MRa/b) 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.

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