Multiple stable isotopes

Elements with multiple stable isotopes may produce several radioisotopes that can be measured to assure the accuracy of the analysis. For example, Zn has five stable isotopes. The isotope Zn will produce the radioisotope Zn, and Zn will produce the radioisotope Zn. Both of these radioisotopes can provide an independent measurement of the Zn concentration and therefore can be used to check the consistency and quality of the analysis. On the other hand, Zn will produce Zn, which is nonradioactive and therefore cannot be used in NAA. 

Phillips DL. 2001. Mixing models in analyses of diet using multiple stable isotopes a critique. Oecologia 127 166-170. 

Subsequent marine studies have shown that there are often several important plant sources of carbon, and a multiple stable-isotope approach has been widely adopted to resolve this more complex situation. Stable isotopes of sulfur, successfully combined with carbon measurements, have been particularly valuable for tracing the importance of detrital foods that are difficult to identify or quantify visually (41). The information gained in these isotopic studies has allowed ecologists to powerfully verify or refute hypotheses about the importance to consumers of certain food sources (42). 

Note that fossils may contain several chemical components that allow measurement of multiple stable isotopes, e.g., H in apatite, C or N in collagen, C in carbonate, but these either are not preserved on timescales of interest to tectonics, or are usually investigated for biological or ecological reasons. Thus, we focus our discussion of stable isotopes in biogenic tissues on oxygen isotopes, with some additional consideration of carbon isotopes, when they help to interpret oxygen isotope trends. 

Currin, C. A., S. Y. Newell, and H. W. Paerl. 1995. The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs Considerations based on multiple stable isotope analysis. Marine Ecology Progress Series 121 99-116. 

Kwak, T.J., and Zedler, J. (1997) Food web analysis of southern California coastal wetlands using multiple stable isotopes. Oecologia 110, 262-277. 

Peterson, B.J., Howarth, R.W., and Garritt, R.H. (1985) Multiple stable isotopes used to trace the flow of organic matter in estuarine food webs. Science 227, 1361-1363. 

Sullivan, M.J., and Moncreiff, C.A. (1990) Edaphic algae are an important component of salt marsh food-webs evidence from multiple stable isotope analyses. Mar. Ecol. Prog. Ser. 62, 149-159. 

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. 

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