Evaluation of a 42Ca– 43Ca double-spike for high precision Ca isotope analysis

Abstract

Use of a 42Ca– 43Ca double-spike for Ca isotopic analysis on Thermal Ionization Mass Spectrometers (TIMS) offers two important advantages over the 42Ca– 48Ca or 43Ca– 48Ca double-spikes that are currently used to correct for instrumental mass fractionation according to the exponential law. First, since the normalizing ( 42Ca/ 43Ca) and the corrected ( 44Ca/ 40Ca) ratios differ in their average mass by only 0.5 mass unit, they will tend to fractionate coherently thereby minimizing errors in the corrected ratio due to any deviation of the actual machine fractionation from an exponential law. Secondly, all four isotopes (40–44) required for the analysis can be measured simultaneously in modern multi-collector TIMS. This will eliminate errors in mass fractionation correction caused by rapid fluctuations or drifts in fractionation and will also reduce analysis time substantially. We report here initial data for laboratory standards and seawater using a 42Ca– 43Ca double-spike that show how these advantages lead to more consistent and faster Ca isotopic analysis than hitherto possible.