Experimental characterization of elution profiles in gas chromatography using central statistical moments : Study of the relationship between these moments and mass transfer kinetics in the column

Abstract

A high-precision gas chromatograph under computer control, with digital data acquisition of the chromatographic signal, was used to measure the central statistical moments of chromatographic zones. It is shown that for fairly symmetrical peaks, the moments are determined with a better accuracy, even at a low signal-to-noise ratio, from a least-squares fit of the Gram-Charlier series on the experimental elution profiles. The Gram-Charlier series is a good model for representing the peak elution profile provided that the skew coefficient is small. The second central moment derived from the Gram-Charlier model was studied as a function of the carrier gas velocity, using the usual HETP concept. The third and the fourth central moments were found to follow similar relationships to the carrier gas velocity. For strongly tailing peaks, it is shown that the moments are not sufficient for characterizing the peak shape. This tailing has a kinetic origin and results from slow mass transfer in the stationary phase. The time constant of the exponential decay adjusted on the peak tail gives a good indication of the desorption time of the molecule from the most active sites of the packing.