A simplified protocol for measurement of Ca isotopes in biological samples

Abstract

We describe a chemical separation protocol of calcium from biological materials for isotopic measurement by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). The method was tested using elution profiles along with HCl and HNO₃ acids only, on human urine, sheep serum and red blood cells (RBC), seawater and herbaceous plants. It allows the elimination of all interfering species (including K, Sr, Mg) and the remaining matrix (including Fe, P, Na and S) beyond required levels. In order to further test this protocol and better understand the Ca isotopic signatures of mammalian fluids and organs, we purified and analyzed a wide range of materials from sheep, i.e. serum, RBC, muscle, liver, kidneys, enamel, bone, urine and feces. The data show a wide range of variations, expressed as δ, over 1‰ per amu, with a precision of 0.1‰ or better, spanning most of the variability reported so far. Red blood cells appeared to be heavier than serum by 0.3‰ per amu. This isotopic difference between serum and red blood cells was not taken into account in previous studies and it provides further information on Ca isotopic cycling in organisms. The Ca isotopic compositions of organs are correlated with concentrations, bone and RBC representing the two end-members, bone being Ca rich and ⁴⁴Ca-depleted and RBC Ca poor and ⁴⁴Ca-enriched. The trend is compatible with a distillation process by which Ca is extruded from cells along with a kinetic fractionation process favoring lighter Ca isotopes.