In this presentation we will detail our efforts to optimize methods for high-precision calcium and iron isotope ratio analysis of urine and blood using single-collector high-resolution ICP-MS. Stable isotope tracers are gaining acceptance as the approach of choice for human health and nutrition studies, and application to calcium and iron pharmacokinetics are particularly valuable. Many common and debilitating diseases result from clinical deficiencies of calcium and iron (e.g. osteoporosis and anemia, respectively) and understanding factors controlling and/or influencing the absorption/uptake of these elements is critical to managing and preventing illness. Additionally, therapies used to treat other disease processes many impact calcium and iron metabolism (e.g. proton pump inhibitor (PPI) therapy for acid reflux disease).
Dual isotope tracer techniques (one stable isotope given orally, the other intravenously) are recognized as the gold-standard for assessing absorption and re-absorption of calcium and iron in human and animal studies. However, the high-purity stable isotope required (44 and 42 for calcium; and 58 and 57 for iron) are expensive and the amount required per dose is directly related to the precision of the isotope ratio measurement. Therefore improving routine isotope ratio precision can significantly reduce the quantity (and cost) of isotope required. We present here a summary of a study where we examined several key factors impacting precision and accuracy of the stable isotope ratio measurements of calcium in urine and blood, and iron in blood. Our studies addressed the following factors in terms of impact on ratio precision: (a) the efficacy of digestion/matrix-removal approaches; (b) spectral interference management strategies (the interplay of mass resolution and isotope concentration); (c) peak-hopping data acquisition tactics; and (d) mass-bias normalization techniques.
With the optimized methods developed, routine external isotope ratio precision has been improved from 0.5-0.6% to 0.1-0.3%. We will present examples and outcomes of studies where we have applied these optimized isotope ratio techniques, including the role of Vitamin D insufficiency and PPI use on fractional calcium absorption, and fright anemia (Fe-absorption) in primates.
Support for this work was provided by NIH (K23 AR050995), the Jackson Foundation (NCRR MO1 RR03186), and the Wisconsin State Laboratory of Hygiene.
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