Solution to data integration problems during isotope ratio measurements by magnetic sector inductively coupled plasma mass spectrometer at medium mass resolution: application to the certification of an enriched 53Cr material by isotope dilution

Abstract

The suitability of a single-detector magnetic sector inductively coupled plasma mass spectrometer for low uncertainty Cr isotope ratio measurements was evaluated. Operation at medium mass resolution ( m/Δ m⩾4000) was required to eliminate the interferences from polyatomic ions commonly observed on Cr isotope masses. However, the repeatability of the ratios appeared to be far worse than expected and extremely unstable. The mass calibration was found to drift by up to 0.0016 amu on peak center (i.e. ∼12.5% of the peak width) for the duration of a measurement (i.e. 675 s). Moreover, for individual peak signals (0.12–0.36 s duration depending on isotopes) the instabilities observed, particularly for low abundant isotopes, lead to multiple maxima that could potentially complicate the data integration step. However, the major problem turned out to be the instrument software, failing to integrate the data in a reproducible and predictable manner. An ‘off line’ method of data integration was developed to overcome these problems that led to a nearly tenfold improvement in the repeatability of natural n( 52Cr)/ n( 50Cr) isotope ratio measurements. The stability of the repeatability over 45 min improved by a factor of 2.6, the reproducibility of the ratios improved by more than a factor of 4 and the average ratio changed by ∼0.75% (and by up to 1.5% in the worst case). Under these stabilized conditions, direct isotope dilution could be applied as a primary method of measurement for the certification of the Cr amount content in a 53Cr enriched material. The isotope ratio measurements, whose repeatability varied from 0.1 to 0.7% depending on the value of the ratio, were calibrated (corrected for mass discrimination effects) using the IRMM-625 certified isotopic reference material. Combined uncertainties were estimated for all results following the ISO guide to the expression of uncertainty in measurements. A combined uncertainty (expanded, with k=2) on the Cr amount content of less than 0.6% relative was achieved, where the repeatability of the isotope ratio measurements accounted for less than 1% of this value.