Iron Isotope Ratios in Human Whole Blood by PlasmaQuant MS Application Note · PlasmaQuant MS

Introduction

Inductively coupled plasma mass spectrometry (ICP-MS) is now a well-established technique for use in human studies because it can determine both the total elemental content and isotopic ratios with sufficient precision and accuracy in a single analytical run. Stable Fe isotope ratio determinations by ICP-MS in blood have attracted much interest in recent years, especially in human nutrition research where Fe stable isotopes are used to trace metabolic fate as a safer alternative to previously used radioisotopes. Because the precision of isotope analysis and mass spectrometric sensitivity are key issues that determine isotope dosage and the capacity to obtain meaningful isotopic enrichment after isotope administration, high precision and accurate isotopic measurements of at least three Fe isotopes are required.

Fe isotopic analysis by ICP-MS is a real analytical challenge because of the numerous and severe isobaric and polyatomic interferences on the four stable isotopes. In addition to Ar based molecular ions having the same nominal mass-to-charge ratio as the target isotopes (54Fe, 56Fe, 57Fe, and 58Fe) including 40Ar14N, 38Ar16O, 40Ar16O, 38Ar18OH+, 40Ar16OH+, and 40Ar18O+, the presence of high concentrations of concomitant elements in blood samples, such as calcium, can affect the precision and accuracy of Fe isotopic ratios by the formation of 40Ca16O+, 40Ca16OH+, 40Ca18O+, and 42Ca16O+ in the ICP plasma. The presence of Cr and Ni in relatively low concentrations (μg.L−1) in blood samples have also been found to significantly affect the accuracy of ratios involving 54Fe+ and 58Fe+ isotopes, unless additional corrections are applied or matrix separation performed before analysis. Several approaches have been applied for the determination of Fe isotopes in organic matrices.

Cool plasma conditions and membrane desolvation were reported by Vanhaecke et al. to offer insufficient reduction in the intensity of the interfering ions. However, the precision obtained was found sufficient only for single tracer experiments or for isotope dilution purposes. Instrumental approaches to resolve most of the spectral interferences on Fe isotopes such as the use of ion-molecule chemistry in collision/reaction cell constitute the most effective approaches.

This study combines the techniques of cool plasma and collision/reaction cell to completely eliminate polyatomic interferences and achieve the necessary precision and accuracy in the determination of Fe isotopes to distinguish between male and female whole blood samples.