Fidelity of radially viewed ICP-OES and magnetic-sector ICP-MS measurement of Mg/Ca and Sr/Ca ratios in marine biogenic carbonates: Are they trustworthy together?

Improving interlaboratory reproducibility (in both precision and accuracy) of Mg/Ca and Sr/Ca determination in marine biogenic carbonates is critical in optimizing their utility as paleothermometers. Coupled with a need for uniform sample cleaning practices, there is a need for more exacting methods and procedures across laboratories using varied instrumentation. Here we employ an intensity ratio/matrix-effect correction methodology to a suite of solution standards and biogenic carbonates (foraminifera tests and a gastropod shell) to investigate short-term and long-term Mg/Ca and Sr/Ca precision and accuracy by different instruments: a magnetic-sector inductively coupled plasma-mass spectrometer (ICP-MS) and a radially viewed inductively coupled plasma-optical emission spectrophotometer (ICP-OES). Over an extended 1.0-24.5 mM Ca concentration range, both instruments have significant Ca matrix effects for Mg/Ca and somewhat less for Sr/Ca. Over our working Ca range (1-8 mM Ca), Mg/Ca matrix effects are significant, requiring correction, and Sr/Ca matrix effects are small to negligible, occasionally requiring correction (linear or logarithmic fit) using a suite of matrix standards for both instruments. The short-term (intrarun) precision for a suite of solution standards is <0.2% (1 sigma %RSD) for Mg/Ca and Sr/Ca for both instruments. A long-term (interrun) precision of <0.9% is demonstrated for Mg/Ca and <0.6% for Sr/Ca on both instruments. The accuracy of measured Mg/Ca and Sr/Ca values for short- and long-term standards is similar on both instruments at better than 1 +/- 0.5%, on par with our long-term precision. An interinstrument comparison of the same measured suite of biogenic carbonates demonstrates that after accounting for matrix effects, data generated on either instrument are essentially interchangeable (within analytical precision) to a high degree of fidelity.