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

Abstract

Here, we present a new technique for the direct measurement of 44Ca/ 40Ca 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 40Ar + can be significantly reduced, enabling the simultaneous and precise measurement of 44Ca and 40Ca beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a 43Ca/ 48Ca 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 ( δ 44/40Ca [‰]=[(( 44Ca/ 40Ca) sample/( 44Ca/ 40Ca) NIST SRM 915a)−1]*1000). Isobaric effects of 24Mg 16O + and 23Na 16OH + interfering with 40Ca and 26Mg 16OH 2 + with 44Ca can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of 87Sr 2+ monitored on 43.5 atomic mass units (amu) was found. Repeated measurements of two Johnson Matthey CaCO 3 standards (lot No. 4064 and lot No. 9912) revealed values of about −11.29 (‰ SRM 915a) and 0.57 (‰ SRM 915a). These values are in accordance with previous values published by Russell et al. [Geochim. Cosmochim. Acta 42 (1978) 1075], Heuser et al. [Int. J. Mass Spectrom. 220 (2002) 385], Hippler et al. [Geostand. Newsl. 27 (2003) 267] and Schmitt et al. [Geochim. Cosmochim. Acta 67 (2003) 2607]. Repeated measurement of the NIST SRM 915a CaCO 3 standard showed that the variance of a single δ 44/40Ca measurement is about 0.14‰ RSD being comparable with TIMS. MC–ICP–MS-based δ 44/40Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of δ 44/40Ca and temperature. MC–ICP–MS-based δ 44/40Ca measurements on cultured Orbulina universa showed a slope of about 0.026‰/°C being similar to the TIMS-based δ 44/40Ca measurements showing a slope of about 0.019‰/°C. The large offset of about 5‰ between the two techniques is shown to be caused by a “matrix” effect, indicating that any δ 44/40Ca 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 40Ca can be used for normalization of 44Ca.