Precise analysis of calcium stable isotope variations in biological apatites using laser ablation MC-ICPMS

Abstract

Laser ablation (LA) is potentially an interesting technique to measure natural variations (δ^44/42Ca) of calcium isotopes in calcium-rich minerals because it allows spatial resolution and avoids micro-sampling and consecutive wet chemistry. We developed a matrix-match sample/standard normalization method and used an Excite 193 nm Photon Machines LA system coupled to a Neptune plus MC-ICPMS to measure δ^44/42Ca variations in enamel apatite. First, high precision δ^44/42Ca solution mode (SOL) analyses were performed on a series of 5 crystalline igneous apatite and 6 modern tooth enamel samples, which were micro-sampled using a MicroMill device. The δ^44/42Ca isotopic values ranged evenly between −0.60 and +0.60‰ (per amu). Second, we sintered by means of a spark plasma sintering technique the bone ash SRM1400 standard and two synthetic apatites (doped or not with Sr). The Ca isotope compositions using LA were measured in the samples in the raster mode along 600 × 85 μm profiles and bracketed with the SRM1400 standard. We obtained very good agreement between SOL and LA measurements, i.e. δ^44/42Ca_LAvs. δ^44/42Ca_SOL slope of 0.960 ± 0.091 (2SE, R² = 0.971) and null offset at origin (0.012 ± 0.084, 2SE). For all samples, residual values to the 1 : 1 slope were ≤0.1‰ (per amu). However, an unexplained and constant 0.13‰ offset occurred when considering the ^43/42Ca ratio, suggesting an uncorrected isobaric interference on ⁴³Ca in the LA mode. We also noticed that the doubly charged strontium (Sr) interference correction is of crucial importance for accurate matching between LA and SOL measurements. In the SOL mode, Sr is discarded by ion chromatography leading to typical ⁸⁷Sr²⁺/⁴⁴Ca⁺ ratios of 10⁻⁵ to 10⁻⁶. In the LA mode, this ratio can exceed 10⁻³. We show that the value set for the ⁸⁷Sr/⁸⁶Sr ratio is of importance to correct the Sr interference, and that optimized residuals to the 1 : 1 slope are obtained using a Sr correction that takes into account a mass fractionation factor for doubly charged Sr distinct from that of Ca. We found that deciduous teeth enamel is depleted of Ca heavy isotopes by about 0.35–0.40‰ (per amu) compared to wisdom teeth enamel, a shift compatible with a transition from a milk based diet to a plant and meat based diet.