Calcium isotope determination in urine samples via the monitoring of 44CaF and 40CaF molecules by high-resolution continuum source graphite furnace molecular absorption spectrometry

Abstract

This study investigates the possibility of obtaining Ca isotopic information by monitoring the absorption spectrum of a diatomic molecule generated in a graphite furnace using a high-resolution continuum source spectrometer (HR CS GFMAS). The method proposed enables calcium isotopic analysis using CaF as the target species. The difference in wavelength between ⁴⁰CaF (maximum at 628.534 nm) and ⁴⁴CaF (maximum at 628.191 nm) absorption signals was found to be 337.8 pm, evidencing that both species can be determined simultaneously. The method developed relies on the determination of ⁴⁰Ca and ⁴⁴Ca concentrations based on calibration functions specific to each isotope and makes use of 12 μg of F as a molecule forming element without further chemical modifiers. The sensitivity of the method depends on the pH. It increases at pH values close to 2, and decreases until reaching a plateau at pH 6. The characteristic mass found for ⁴⁰CaF was 0.11 μg for solutions with pH 2 and 0.18 μg for alkaline solutions. The respective limits of detection were 1 and 2 mg L⁻¹. The method was applied to the analysis of three urine samples, spiked with different ⁴⁰Ca and ⁴⁴Ca amounts. Due to chlorine interference, before the analysis of urine, calcium must be separated from the samples by precipitation with ammonium oxalate. Recovery tests provided results between 95 and 109%, highlighting the potential of the method for ⁴⁰Ca and ⁴⁴Ca determination in tracer experiments.