Direct measurement of 44Ca/40 Ca ratios by MC-ICP-MS using the cool plasma technique

Here, we present a new technique for the direct measurement of Ca-44/Ca-40 isotope ratios on a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS, AXIOM) using the cool plasma technique. By reducing the plasma energy to about 400 W, the isobaric effect resulting from Ar-40(+) can be significantly reduced, enabling the simultaneous and precise measurement of Ca-44 and Ca-40 beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a Ca-43/Ca-48 double spike, the isotope measurements on MC-ICP-MS can be performed by bracketing standards. We express the calcium isotope variation relative to NIST SRM 915a (delta(44/40)Ca [parts per thousand] = [((Ca-44/Ca-40)(sample)/(Ca-44/Ca-40)(NIST SRM 915a)) - 1]*1000). Isobaric effects of (MgO+)-Mg-24-O-16 and (NaOH+)-Na-23-O-16 interfering With Ca-40 and (MgOH2+)-Mg-26-O-16 with Ca-44 can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of Sr-87(2+) monitored on 43.5 atomic mass units (arnu) was found. Repeated measurements of two Johnson Matthey CaCO3 standards (lot No. 4064 and lot No. 9912) revealed values of about -11.29(parts per thousand SRM915a) and 0.57(parts per thousand SRM 915a). These values are in accordance with previous values published by Russell et a]. [Geochim. Cosmochim. Acta 42 (1978) 1075], Heuser et a]. [Int. J. Mass Spectrom. 220 (2002) 385], Hippler et a]. [Geostand. Newsl. 27 (2003) 267] and Schmitt et a]. [Geochim. Cosmochim. Acta 67 (2003) 2607]. Repeated measurement of the NIST SRM 915a CaCO3 standard showed that the variance of a single delta(44/40)Ca measurement is about 0.14parts per thousand RSD being comparable with TIMS. MC-ICP-MS-based delta(44/40)Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of delta(44/40)Ca and temperature. MC-ICP-MS-based delta(44/40)Ca measurements on cultured Orbulina universa showed a slope of about 0.026 parts per thousand/degreesC being similar to the TIMS-based delta(44/40)Ca measurements showing a slope of about 0.019 parts per thousand/degreesC. The large offset of about 5 parts per thousand between the two techniques is shown to be caused by a matrix effect, indicating that any delta(44/40)Ca measurements on MC-ICP-MS are sensitively controlled by the Ca concentration and the acidity of the solution. Our study demonstrates the possibility to measure the whole dispersion of calcium isotopes with MC-ICP-MS, showing that Ca-40 can be used for normalization of Ca-44. (C) 2004 Elsevier B.V All rights reserved.