Application to impurity analysis

The use of directed chemical reactions opens up a number of new opportunities in trace impurity analysis, such as the determination of compounds whose peaks are overlapped completely or partly by the zone of the main component or elimination of the superposition of the peaks of impurity components in their incomplete separation. Table 5.8 shows the application of the subtraction method to impurity analysis. 

The use of this method makes possible the solution of the two principal problems in impurity analysis first, to demask the impurities covered by the zone of the main component, and second, to improve the sensitivity of their determination by using super-selective chemical methods. Simultaneously, the subtraction methods can be used for solving such complex problems as the identification of impurities and the quantitative analysis of impurities of different types, which form a common unresolved task. The methods for solving the last two problems were discussed in Section 5.2. 

Let us now consider the demasking and concentration of impurities in more detail. If the natures of the main component and the impurities are different, it is possible to absorb the main component by choosing an appropriate selective reagent and to detect impurities that were previously masked by the zone of the main component. Selective absorption of the main component has been used for determining non-olefln impurities in ethylene [111] and impurities in toluene [119], butadiene-1,3 [127] and acetic acid [128]. The method of frontal chemical concentration was used for the intermediate concentration of heavy impurities in ethylene and propylene [129]. 

Main substance Separation of impurities and main substance, which forms involatile compound with reagent 11,119 

Impurities Concentration of impurities by using chemical absorbers, which form low-volatility compounds with impurity after concentration (sometimes after regeneration) gas chromatographic impurity analysis is carried out 123-124 

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