Sample preparation for isotopic analysis

It seems appropriate therefore to consider briefly the preparation of hydrogen, carbon dioxide and nitrogen required for the isotopic analysis of H, and content respectively. The preparation of carbon dioxide and sulphur dioxide for analysis of and content will not be considered in detail as these isotopes have as yet very limited application in a clinical context. 

The precise estimation of deuterium afforded by modem isotope mass spectrometers suggests the possibility of employing deuterated precursors in a 

Carbon-13. The increasing availability of C-labelled substrates has provided a marked impetus in the use of these tracers in clinical studies. Most recent interest centres round the use of C02 breath tests for the in vivo estimation of the rate of oxidation of specific C-labelled substrates. Some clinical implications of these tests will be discussed later. Mention should be made of the available methods for respiratory COj collection. They include the collection of whole breath in evacuated glass tubes prior to CO2 separation [92], the direct cryogenic trapping of exhaled CO2 in liquid nitrogen [93] and the precipitation of CO2 as carbonate in sodium hydroxide [94]. The latter method is suspect in that it results in isotopic fractionation which may not be reproducible. 

The estimation of the content in carbonate, resulting from C02 trapping, involves the reaction with 100% phosphoric acid under rigorously controlled conditions originally described over 25 years ago [96]. 

Finally, organic samples requiring analysis of isotopic content, may be combusted in a stream of oxygen to provide the carbon dioxide for isotopic analysis. Several combustion systems have been used that differ only in minor components designed to remove contaminants such as sulphur or the halogens [97,98]. 

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