High spatial resolution quantitative elemental imaging of foraminifer by laser ablation-inductively coupled plasma-mass spectrometry
Quantitative determination of the concentrations of elements and systematical characterization of their distribution in foraminifer is of vital significance. Quantitative elemental imaging by LA-ICP-MS is a useful alternative method; however, a methodology for high spatial resolution elemental imaging of foraminifer has not been obtained. In this work, a laser ablation spot size of 16 μm and a line scan speed of 8 μm s−1 were selected for elemental imaging after optimization; then, a two-point calibration strategy (TPCS) was established by combining NIST SRM 610 and NIST SRM 612 glasses with MACS-3 as external standards. The concentrations of Mg and Sr in four carbonate reference materials obtained by TPCS were close to the reference values with relative errors less than 10%. TPCS can avoid incorrect calibration caused by inhomogeneously distributed internal standards (e.g., 43Ca) between foraminifer shells and holes. A methodology for quantitative LA-ICP-MS elemental imaging of foraminifer was then developed, and high spatial resolution elemental images of Mg, Sr, and Ba were obtained. The spatial resolution of these images was calculated to be 16 × 0.40 μm per pixel. Elemental imaging of the Mg/Ca, Sr/Ca, and Ba/Ca ratios of a second foraminifer further confirmed the reproducibility of the elemental imaging methodology. The Mg/Ca ratio and the calcification temperature were found to gradually increase from the inner chambers (f-0) to the final chamber (f-1), while Sr was distributed more homogeneously and Ba showed little uptake in foraminifer shells. All these results demonstrate that this elemental imaging methodology is applicable to providing visual evidence to distinguish the elemental distributional differences in foraminifer.