K, Mg, and Ca isotopic analysis from a single aliquot of thirteen biological certified reference materials

Combining different but related isotopic systems is key to understanding biological systems as biological functions involving metals in the body generally interact. Potassium, magnesium, and calcium in the ionic form are such elements with related properties. The present work aims to demonstrate a new method to isolate these three metals from a single sample aliquot using AG50W-X12 cation exchange resin, custom-made quartz columns and HCl at different molarities for their subsequent isotopic analysis using MC-ICP-MS. The accuracy of the method was assessed using thirteen certified reference materials with different biological matrices, seven of which have not been analyzed for either their potassium, magnesium, or calcium isotopic compositions before. The procedure allows for an efficient isolation of potassium using AG50W-X12 resin only. However, for matrices characterized by high concentrations of copper, zinc, and/or iron, a prior chromatographic treatment is required, as copper and zinc co-elute with magnesium and iron with calcium, causing matrix effects during the subsequent isotopic analysis. Furthermore, with AG50W-X12 resin, strontium co-elutes with calcium, thus requiring another purification step to separate these elements using a Sr-Spec resin. The efficiency of this method is confirmed by quantitative chromatographic recoveries, low procedural blanks and isotopic compositions corresponding to literature values. This protocol should greatly enhance sample processing with lower volumes of acid, resin, and sample. From a single aliquot, and three consecutive steps, it is possible to isolate seven target elements. Additionally, we provide new values for the isotopic compositions of K, Mg, and Ca for diverse biological certified reference materials enabling improved future method validation of biological applications by using reference materials that show closer resemblance to the investigated samples.