Features of gas chromatographic analysis of thermally unstable compounds

Confirming the stability of analytes during gas chromatographic (GC) analysis is an important criterion, especially for previously uncharacterized compounds. However, the variations of absolute peak areas at different injector temperatures usually do not allow us to reveal the thermal instability of analytes during GC analysis. Such variations may be caused by peak area known discrimination typical for using capillary columns, especially at low split injection. The thermal instability can be revealed only when using relative peak areas. The dependences of the relative peak areas of unstable analytes on the injector temperature (descending), as well as similar dependences for products of their decomposition (ascending), are characterized by the existence of two limits. Low-temperature limits correspond to the initial quantities of unstable analytes, and high-temperature limits, to their complete transformations. Such dependences can be approximated with an equation of logistic regression (synonymous: sigmoid or Boltzmann approximation). The relationships and features of gas chromatographic analysis of thermally unstable compounds are considered with products of free-radical isopropylbenzene (cumene) chlorination and with solutions of ethyl diazoacetate in different solvents as examples. The major cumene chlorination product, (1-chloro-1-methylethyl)benzene, undergoes dehydrochlorination at injector temperatures above 200 °С to form a single product, α-methylstyrene. The analysis of the ethyl diazoacetate solutions is accompanied by the formation of ethyl alkoxyacetates, products of the insertion of intermediate ethoxycarbonylcarbene into OH bonds of alcohols, if they are used as solvents. Comparing the temperatures of half-conversion of the initial ester, T(50 %), and half-formation of the products shows that they are equal to each other. This confirms both the process mechanism and the correctness of the data approximation.