Determination of Mg/Ca and Sr/Ca ratios in microgram-quantity coral and foraminiferal samples with permil-level precision by cold plasma inductively coupled plasma sector field mass spectrometry

A permil-level analytical technique for determination of coral and foraminiferal Mg/Ca and Sr/Ca ratios using inductively coupled plasma sector-field mass spectrometry (ICP-SF-MS, Thermo Electron ELEMENT2) has been established. Cold plasma technique with an 800-W RF power was employed to diminish spectral interference. A wet introduction system with Scott-type double-pass spray chamber and an ESI micro- nebulizer with a sample uptake rate of 50 microliter per minute were used, which can offer low blank and high stability. Intensities of ion beams, ^{24}Mg, ^{43}Ca and 86Sr, were detected in pulse-counting mode. Mg/Ca and Sr/Ca ratios were determined directly from intensity ratios using external, matrix-matched standard to correct for mass discrimination and low-frequency ratio drift. Other elemental ratios, such as Al/Ca, Mn/Ca, Fe/Ca, Cd/Ca, and Ba/Ca were also determined. Three different matrix effects were observed in this system. (1) Acid effect: Elemental intensities and ratios vary with increasing HNO3 from 1-6%. 5% HNO3 was determined to be ideal for routine measurement. (2) Ca effect: Mg/Ca ratio decreases 2.5% with [Ca] from 0.5-2.5 ppm and less than 0.5% with [Ca] from 3-5 ppm. (3) Sr/Ca effect on Mg/Ca: Sr/Ca slightly affects Mg/Ca determination with a slope of 0.18% per mmol/mol. This effect causes only 0.1 s or less for Mg/Ca ratio in both foraminiferal and coral samples. Replicate measurements made on standards and samples show that our method can give an internal precision of 1-2‰ (2RSD) and reproducibility of 4.5‰% for Mg/Ca and 3.5‰ for Sr/Ca. Corresponding errors are 0.05 °C for foraminiferal Mg thermometer and 0.5 °C for coral Sr thermometer. Advantages of this new method include (1) short measurement time of 3 minutes and (2) only 2-mircogram carbonate consumed for each sample, and (3) throughput of 10 samples per hour.