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

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 (MR_a/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 (MR_a/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.