Isotope ratios of calcium determined in calcium-46 enriched samples from infants by automated multiple-collector thermal ionization mass spectrometry

Abstract

A high-precision method was developed for automated multiple-collector determination of ⁴⁶Ca enrichment using magnetic sector, thermal ionization mass spectrometry (TIMS). Calcium was separated from biological samples by precipitation as calcium oxalate. The calcium oxalate was washed, then dissolved in nitric acid, and isotope ratios were determined in 10 µg samples. The ⁴⁴Ca, ⁴⁶Ca and ⁴⁸Ca isotopes were collected simultaneously. The ⁴⁶Ca:⁴⁸Ca ratios were measured and corrected for fractionation by iterative normalization, using the ⁴⁴Ca:⁴⁸Ca ratio, in order to achieve the required precision. Blocks of ten ratios were measured with an internal or within-run precision of 0.14% relative standard deviation (RSD) in urine samples and 0.10% RSD in faecal samples. The external or between-run precision for nine replicates was 0.07% RSD for urine and 0.09% RSD for faecal samples. Enrichment of samples collected following the feeding of ⁴⁶Ca to pre-term infants ranged from 8 to 179 Δ% excess in faecal samples and from 19 to 91 Δ% excess in urine samples. If another isotope in addition to ⁴⁶Ca is to be enriched, the identical analytical method can be applied by collecting ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca simultaneously, using the two unenriched isotopes to correct for fractionation.