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

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 9 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. (author)