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 44 Ca/ 40 Ca 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 40 Ar + can be significantly reduced, enabling the simultaneous and precise measurement of 44 Ca and 40 Ca beam intensities in different Faraday cups. In contrast to the TIMS technique requiring a 43 Ca/ 48 Ca 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 44/40 Ca [‰]=[(( 44 Ca/ 40 Ca) sample /( 44 Ca/ 40 Ca) NIST SRM 915a )−1]*1000). Isobaric effects of 24 Mg 16 O + and 23 Na 16 OH + interfering with 40 Ca and 26 Mg 16 OH 2 + with 44 Ca can be neglected by measuring calcium isotopes near the low-mass edge of the peaks. No influence of 87 Sr 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 44/40 Ca measurement is about 0.14‰ RSD being comparable with TIMS. MC–ICP–MS-based  δ 44/40 Ca values measured on inorganically precipitated aragonite samples are indistinguishable from earlier measurements based on TIMS, confirming the positive correlation of δ 44/40 Ca and temperature. MC–ICP–MS-based δ 44/40 Ca measurements on cultured Orbulina universa showed a slope of about 0.026‰/°C being similar to the TIMS-based δ 44/40 Ca 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 44/40 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 40 Ca can be used for normalization of 44 Ca.