Isotope ratio measurements IDMS applications

The fundamentals and several applications of isotope dilution mass spectrometry requiring accurate isotope ratio measurements are reviewed by Heumann.50,51 Today isotope dilution mass spectrometry (IDMS) is recognized as a primary measurement method, by means of which accurate results with sufficiently small uncertainties can be achieved and therefore it has been used in certifying the composition of reference materials. A requirement of isotope dilution analysis in mass spectrometry is to achieve equilibration of spike and sample so that very careful sample preparation steps, especially in solid mass spectrometry, are necessary when a homogeneous sample spike mixture is to be prepared. 

Murozumi et al. (9) determined the concentration of Pb in Greenland and Antarctic snow and ice by Isotope Dilution Mass Spectrometry (IDMS) (27). They added pure Pb tracer to their samples before chemically processing them. Both the mass of tracer and the water sample were measured. After thoroughly mixing the tracer with the sample Pb was isolated in a chemically pure form and the isotopic ratios were measured in a mass spectrometer. Simple comparison of the ° Pb abundance with the abundance of other isotopes in the mass spectrum allowed the amount of Pb in the sample to be determined. The technique is not limited to Pb, but is applicable to any element with two or more stable or... 

The typical Rx values presented here are 0, 0.1/99.9,0.5/99.5,1/99, 4/96, 10/90, 20/80, 50/50. The magnification factors indicated by the graphs, allow to compute the uncertainty to be expected on a given quantitative determination of an element by IDMS. The assumption a R) = eR (Eq. (13)) is not applicable to very enriched and depleted samples, and in those cases the uncertainties are worse than indicated by the graphs. However, good mass spectrometers and measurement procedures achieve a constant uncertainty on R measurements for values from 0.01 to 100, i.e. over four useful orders of magnitude. Attention is drawn on the fact that an isotope ratio can be defined as either Nz/N-t or N N2 and therefore it is always possible to choose fix 1-... 

---