The Measurement of Solute Diffusivity and Molecular Weight

Analytical information taken from a chromatogram has almost exclusively involved either retention data (retention times, capacity factors, etc.) for peak identification or peak heights and peak areas for quantitative assessment. The width of the peak has been rarely used for analytical purposes, except occasionally to obtain approximate values for peak areas. Nevertheless, as seen from the Rate Theory, the peak width is inversely proportional to the solute diffusivity which, in turn, is a function of the solute molecular weight. It follows that for high molecular weight materials, particularly those that cannot be volatalized in the ionization source of a mass spectrometer, peak width measurement offers an approximate source of molecular weight data for very intractable solutes. 

The major problem in this procedure is not in obtaining an accurate value for the solute diffusivity from peak width measurements (which is relatively straightforward) but in identifying the best relationship between diffusivity and molecular weight to employ in the subsequent data processing. 

Unfortunately, there are many expressions in the literature that give molecular weight as a function of diffusivity, and the most appropriate expression must be identified in order to permit a reasonably accurate value for the molecular weight to be calculated. Thus, the diffusivities of a large number of solutes of known molecular weight need to be measured in a solvent that is commonly used in the liquid chromatography, so that a practical relationship between diffusivity and molecular weight can be identified. 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

Katz and Scott [1] measured the diffusivity of 69 different solutes having molecular weights ranging from 78 to 446. The technique they employed was to measure the dispersion of a given solute band during passage through an open tube. 

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