Elements mass-independent isotope fractionation

Malinovsky, D. and Vanhaecke, F. (2011) Mass-independent isotope fractionation of heavy elements measured by MC-ICPMS a unique probe in environmental sciences. Anal. Bioanal. Chem., 400 (6), 1619-1624. 

As PGE-containing minerals are difficult to digest, the temperature at which the digestion step is carried through has a marked influence [9]. Blanks are important at a target element level lower than 1 pg kg . Spectral interferences in ICP-MS cannot be neglected and in most natural matrices they can cause severe overestimations of concentrations. Hence this influence quantity has to be considered and its contribution to the budget estimated when mass-dependent and mass-independent isotope ratio fractionation need to be studied. 

For many decades, TIMS was the isotope analytical technique of choice, but due to instrumental developments in ICP-MS, especially with multiple ion collectors (MC-ICP-SFMS), and the advantages of ICP-MS in comparison to TIMS (e.g., higher element sensitivities, faster isotope ratio measurements, comparable precision and accuracy, practically no restriction on the ionization potential of chemical elements, time independent mass fractionation and the possibility of additional multi-element analysis at trace and ultratrace level and fewer, less time-consuming sample preparation steps75), TIMS will be replaced in future by powerful ICP-MS to an ever greater extent. 

Since isotopic fractionation is mass dependent, there will be a smooth relationship between effects observed in various isotopic ratios. In most cases of interest, the magnitude of fractionation and the fractional mass range are both small in an absolute sense, and to first order the degree of fractionation is approximately linear in mass 8m is proportional to dm, where 8m is a fractional measure (e.g., in per mil) of the fractionation effect between isotopes of mass m and m0 [Equation (1.1)] and 8m=m - m0. In elements of three or more isotopes, this is the signature by which fractionation is distinguished from specific isotope effects. If two or more isotopic ratios are observed to vary in this fashion appropriate for fractionation, it is usually considered that this is not an accident and that fractionation has indeed occurred (rather than two or more independent effects that depend on the nuclear identity of the isotopes in question). 

Mass-independent fiactionation Most cases of isotope fractionation are characterized by a hnear relationship between the magnitude of the effect estabhshed and the difference in mass between the isotopes considered. For an increasing number of elements, however, an apparently aberrant behavior is estabhshed for some of their isotopes. This is currently a hot topic of research... 

Although currently very much an academic topic, the study of mass-independent fractionation of metals and metalloids is also believed to have practical applications. Mass-independent fractionation provides the element with a very specific isotopic signature. For environmentally important elements, such as Hg and Sn, this can be exploited to reveal their sources and understand conversions, thereby enhancing our understanding of their biogeochemical cycles. Such fingerprints have already been demonstrated for Hg in real-life samples [52-54]. 

Among the benefits from using isotopic analysis to study trace element movement across the geosphere and biosphere is the realization that Hg shows mass-independent fractionation that enhances our ability to define the sources of this toxic element [13]. Isotopes of Ca and Mg in biogenic carbonates indicate that there are both equilibrium mineralogical controls and kinetic controls on how these elements are incorporated by organisms from the geosphere and... 

As research into absolute isotope ratio measurements by MC-ICP-MS continues, some new limitations have been identified recently. Although in MC-ICP-MS, mass bias is generally considered to be mass-dependent fractionation (MDF) and is corrected by various mass-dependent correction models, mass-independent fractionation (MIF) in MC-ICP-MS was described by Lu Yang et al., of the National Research Council Canada, at the 2013 European Winter Conference on Plasma Spectrochemistry. Their study looked at numerous elements and concluded that this appears to be a common phenomenon, and consequently has serious implications on the absolute isotope amount ratio measurements. 

It is thus the motion of the electrons that generates the potential surface for the motion of the nuclei. The mass of the nuclei does not appear in H and hence PES must be independent of isotope mass. The most common chemical properties depend only on the PES, and are therefore conserved under isotope substitution. In fact, the basis for nuclear chemical analysis is that the isotopes, including the radioactive ones, behave chemically in exactly the same way as the natural ones. One may carry out reactions that are specific for a certain element and check in which fraction radioactivity remains (radioactivity may appear in the precipitation). 

As discussed in Chapter 2, many elements have multiple stable isotopes of varying abundance. The abundances for stable isotopes of the elements are listed in Appendix 2. The ratio of two stable isotopes of a given element can be determined by independently measuring their ion currents.The isotope ratio is then computed by dividing one m/z isotope ion current by the other m/z isotope ion current. If no isobaric mass spectral interferences are present, and the sample has a natural isotopic abundance for the element in question (i.e., no fractionation or radiogenic processes have affected the abundances), the calculated ratio should be close to the theoretical value computed from the Appendix 2 table entries. This ability to perform isotope ratio measurements provides an opportunity to carry out isotope dilution analysis. 

---