The Separation and Purification of Compounds for Isotopic Analysis

An effective and rapid method of separation of acidic products (such as carboxylic acid or oxyacids) is by precipitation as the silver or barium 

Volatile products can be separated under mild conditions by sweeping out with an inert gas such as nitrogen. This has been used by Hamilton and Westheimer (1959) to recover rapidly exchanging acetone from aqueous solution. For involatile products Hamilton and Westheimer (1959) also froze the aqueous solution and removed the solvent by lyophilization to dryness. The product to be analyzed is then separated by conventional chemical or physical methods. 

In order to insure accurate and reproducible isotopic analyses, the compounds to be analyzed must be rigorously purified. The usual methods of purification of organic and inorganic materials are used, bearing in mind that often only very small quantities are available. Thus microtechniques (Cheronis, 1954) are usually employed for distillation or even recrystallization. In recrystallization as in extraction, hydroxylic solvents should not be used in order to reduce the possibility of exchange, and to reduce isotopic contamination in analysis. 

One of the best methods of purification is sublimation in a high vacuum, although it is. not often feasible to separate mixtures of compounds in this manner. 

The various forms of chromatography could be of considerable use in the separation, purification, or identification of products for analysis. Absorption (column) chromatography has not been much used since the isotopic exchange between alumina or silica and the oxygen in the compounds absorbed has not yet been investigated. Anion ion-exchangers have not been used either, since the extent of exchange of the anions on the column with water is not known. 

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