Precise determination of element/calcium ratios in calcareous samples using sector field inductively coupled plasma mass spectrometry

A new method was developed for rapid and precise simultaneous determination of Mg/Ca, Sr/Ca, Mn/Ca, Cd/Ca, Ba/Ca and U/Ca ratios in foraminiferal shel ls using sector field inductively coupled plasma mass spectrometry (ICPMS). Element/calcium ratios were determined directly from intensity ratios usin g external, matrix-matched standard to correct for instrumental mass discri mination, Because of large differences in the abundance of chemical constit uents of the foraminiferal shell, major elemental ratios were determined in analog mode (using Mg-24, Ca-43, Ca-44, Mn-55, and Sr-88) whereas trace el emental ratios were determined in pulse-counting mode (using Cd-111, Ba-138 , U-238, and the low-abundance Ca-46 isotope). Matrix-induced variations in mass discrimination over a calcium concentration range of 2.0-24.5 mM were observed only for Mg/Ca and Cd/Ca ratios. However, these effects are negli gible if the samples and standard calcium concentration are within a factor of 2-3. Multi-ratio method reproducibility was better than previously repo rted for other ICPMS methods yielding precision (1 sigma) of Sr/Ca = 0.45%; Mg/Ca = 0.45%, Mn/Ca = 0.8%, Cd/Ca = 1.7%, Ba/Ca = 0.7%, and U/Ca = 1.4% f or foraminifera samples as small as 25 mu g. Using this approach for a sing le-ratio analysis, Sr/Ca ratios were determined with precision of 0.06% (1 sigma) on carbonate samples as small as a single foraminifera shell (<10 mu g). The new method is more sensitive, more precise, and simpler to use tha n previously available ICPMS techniques. It provides an efficient tool for simultaneous determination of several elemental ratios of paleoceanographic interest in a single foraminiferal sample, thereby reducing overall sample size requirement and analysis time.