Automated methods for multi-isotopic analysis of major cations in biological samples: application to chronic kidney...

Natural variability in stable isotope ratios provides critical constraints on elemental cycling in nature without the need for the introduction of artificial tracers. While such data are widely used in environmental studies, they are not as widely employed in biomedical research, despite vast potential. One critical hurdle to the adoption of such techniques in biomedical studies is sample throughput. Elemental purification via ion-exchange chromatography and isotopic analysis via multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) are time-consuming, requiring long hours from experienced researchers to generate datasets. Here we present new methods to improve the throughput of both elemental purification and sample introduction to mass spectrometers. We use an automated, low-pressure ion exchange chromatography system to isolate purified fractions of potassium, magnesium, and calcium from one sample in a single sequence with high yields (80-100%) and low blanks (<0.5% carryover). Modification of flow rates and column volumes also enables recovery of purified strontium, lithium, and sodium in the same routine. Solutions are introduced to the MC-ICP-MS via syringe injection and with automated removal of vial caps to minimize evaporation. We find that syringe injection from capped vials gives a >10 × more stable signal (0.7% RSD) over a 9-h sequence than self-aspirated, uncapped solutions (8.0% RSD). Syringe injection also enables modification of signal intensity by changing the injection rate, with a linear response of signal to flow rate. We demonstrate the potential of these methods by analyzing calcium, magnesium, and potassium isotope ratios at high precision (<0.1 ‰) from single 0.5 mL aliquots of urine samples from individuals with chronic kidney disease. These data show a change in calcium reabsorption, highlighting avenues for further research as well as the value of these multi-isotopic analysis methods.