APPLICATIONS
Isotopic dilution analysis (IDA)
Today, enriched isotopes are used for two main purposes:
- To accurately quantify elements or chemical species when losses or interconversions between species occur during sample preparation (example: speciation of Hg or Cr).
- To investigate the bioaccumulation, reactivity, metabolic pathways and ultimate fate of previously added isotopically enriched chemical species.
What is isotope dilution analysis?
Isotopic dilution analysis (IDA) is an alternative form of measurement in analytical chemistry that does not require a methodological calibration chart. It is a primary measurement method, since it is directly traceable to the international system of units (high metrological quality) used for the certification of reference materials and in the validation of analytical methods. IDA consists of modifying the isotopic composition of an element or compound within a sample by adding that compound or element enriched in one of its isotopes. The added enriched compound is called tracer or spike.
Advantages and requirements of isotope dilution
IDA provides several advantages compared to other methodological calibrations.
Advantages
- Any variation in sensitivity due to instrumental instabilities will not influence the final concentration in the sample.
- Once isotopic equilibrium is achieved between the natural element present in the sample and the added enriched tracer, any losses during sample preparation will not affect the final results.
Requirements
- Any loss of the analyte from the natural sample or the fortified species prior to isotopic equilibration will be a major source of error.
- Once isotopic equilibrium has been achieved, the enriched tracer must behave identically to the natural tracer throughout the analytical process (sample preparation).
- The measurement of isotopic peak areas in ICP-MS must be free of spectral interferences.
- The amount of the enriched compound added should be within a certain range compared to the natural compound.
In summary, this calibration technique compensates for systematic errors in sample preparation and improves the uncertainty of quantification. Only contamination and spectral overlap should be evaluated. A disadvantage of isotopic dilution is that it is only applicable to elements with more than one isotope.
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