Problems and Restrictions in the Use of Oxygen

Mass spectrometers can only analyze a gas or a volatile liquid and usually require about 1 ml. at NTP for each analysis. Organic and inorganic compounds must therefore be converted to a suitable gas (usually carbon dioxide) for analysis. The methods of conversion are discussed in Section V, B, 4. Even by the most efficient methods fairly large quantities of material are required to produce enough gas and most analyses should be done at least in duplicate. Therefore, biochemical reactions must be run on a fairly large scale in order to isolate sufficiently large quantities of the products to be analyzed. This means an over-all increase in costs since all stable isotopes, with the exception of deuterium, are relatively expensive. 

Attempts are now being made to find methods of reducing the quantities required for isotopic analysis, particularly by the newer methods suggested. 

Apart from the difficulties of conversion of many compounds to carbon dioxide, or any other suitable gas for mass spectrometry, nearly double the quantity eventually used for analysis is often required in order to handle and purify such compounds effectively. 

Compounds for isotopic analysis must be rigorously purified, since any oxygen containing impurities will give spurious results. The two most frequent impurities are water, which can usually be removed by prolonged vacuum drying, and carbonate (particularly in phosphates and other salts), which can only be removed by chemical methods, such as treatment with dilute acid. Other impurities can be removed by the usual methods of purification summarized in Section IV. 

Finally, work with the stable isotopes of oxygen and of hydrogen (unlike and N ) is made more difficult by the ever-present problem of isotopic exchange between the solvent, water, and the reactants, products, or intermediates of the reaction. 

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