Criteria of Identity and Purity in Chromatographic Separations

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Abstract
One of the fundamental objectives of both experimentalists and theoreticians is to establish chromatography as a primary criterion of identity and purity. In part, meeting this objective consists of the experimental aspect of measuring and cataloging the behavior of all possible compounds and classes, but equally important it consists of describing the theoretical basis for system definitions. These system definitions include the discrimination ability or chromatographic “mesh size,” the density or incidence of retention values in the test chromatogram, and the quality of measurements used to compare reference and experimental material. The latter is composed both of the instrument detection capabilities and column efficiencies, and of the experimenter's choice of peak-measurement techniques; the greatest difference in these is between the integral (flow) and incremental (fraction) methods. The uses, advantages, and disadvantages of each depend in a critical manner upon the limiting efficiencies and resolution of which the instrument is capable, but the quality of the measurements can always be improved by the use of the incremental technique. In particular, the incremental method permits the distinction between two nonidentical species to be made without achieving partial peak separation or even in the absence of a perceptible discontinuity in the peak envelope. A theoretical basis for classifications of systems is described. This is based upon the ability to determine the basic parameters characterizing a chromatographic peak and is illustrated by reference to synthetic chromatograms generated by digital computation and by reference to representative column-performance figures in actual separations.