A simple CO2 equilibration method for measuring blood oxygen isotope compositions

Rationale Blood water oxygen isotope compositions can provide valuable insights into physiological processes and ecological patterns. While blood samples are commonly drawn for medical or scientific purposes, blood fractions are infrequently measured for oxygen isotopic compositions (delta O-18) because such measurements are time consuming and expensive. Methods We sampled blood from sheep, goats, and iguanas raised in field and animal laboratories into serum, EDTA, heparin, and uncoated plastic vials commonly used in medical and scientific research, then separated red blood cell (RBC) and plasma or serum blood fractions. These were injected into helium-flushed Exetainer tubes where they naturally outgassed endogenous CO2 (goat blood), or into He- and CO2-flushed tubes (iguana blood). The CO2 gas was sampled on a GasBench II system, and delta O-18 was measured by an isotope ratio mass spectrometer (IRMS). Results Repeated delta O-18 measurements were stable over multiple days. The addition of desiccated blood solids to water standards had little impact on their delta O-18 measurements, suggesting that organic molecular constituents within blood serum and plasma do not interfere with blood water delta O-18 values. We observed slight but statistically significant delta O-18 offsets between plasma, serum and RBC fractions. Mass-dependent body water turnover times for iguanas were derived from the data. Conclusions We demonstrate that a simple blood-CO2 equilibration method using the GasBench can quickly, reliably and accurately characterize water delta O-18 in the plasma, RBC, and whole blood fractions of mammalian and reptilian blood samples (precision <= 0.1 parts per thousand). This method will expand the application of blood stable isotope analysis in physiological and medical research.