Expression of enrichment

Expression of enrichment. As indicated previously an isotope mass spectrometer is used to compare the relative isotopic enrichment of a sample gas with a reference gas. Absolute isotopic content cannot be obtained from these measurements. 

The reference gas of choice is dependent on the type of investigation undertaken. Environmentahsts such as geochemists, petrologists or oceanographers are involved in the determination of naturally occurring variations of a specific isotopic species. In order that interlaboratory results should be strictly comparable it is essential that the reference gas used should be identical in all laboratories. To this end international standards of known isotopic content are available from the International Atomic Energy Agency in Vienna. 

These investigators have always expressed their results in the 6%o notation as the deviation per mil (parts per thousand) of the isotopic ratio of the sample compared to that of the reference gas. Thus in terms of measurements  

In biological and clinical investigations the position is somewhat different. In both instances the enrichments measured are often large and it is normal practice to express the enrichment of a particular fraction as compared to the same fraction prior to the introduction of the labelled compound to the system. In such cases most workers express enrichment of isotopic content as either atom% or atom% excess which have been defined earlier (p. 5). Where is used at low enrichments and measured in an isotopic ratio instrument normally equipped to monitor the m/e 28 and m/e 29 ion beams, the following formula is applicable  

At higher enrichments modification of this formula is required to account for the increasing contribution of the mje 30 ion species. 

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