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Vacancy Chromatography Vacancy chromatography is a unique method of chromatographic development that can provide both negative and positive peaks in the chromatogram. It is not in common use, despite it having certain characteristics that make it especially applicable to process monitoring. If a mobile phase that contains a solute at a given concentration is continually passed through a column until equilibrium is achieved, then the concentration of solute in the column eluent will be the

Despite the possibilities, vacancy chromatography is still rarely used in analytical laboratories although there are a number of applications where it might be useful. Zhukhovitski recommended its use for quality control with a particularly interesting application. Consider a pharmaceutical product that contains a number of ingredients that must be kept in specified proportions. To monitor the drug for quality control purposes, a mobile phase is prepared that contains the components of the product, in the proportions

the approximate binomial form of the elution curve in their procedure. Scott et al. (21) provided some experimental evidence supporting equation (65). The mixture contained uracil, hypoxanthine, guanine and cytosine, each present in the mobile phase at a concentration of 14 mg/l. The column employed was 1m long, 1.5 mm I.D., packed with a pellicular cation exchange resin and operated at a flow rate of 0.3 ml/min. Courtesy of the Journal of Analytical Chemistry (ref. 21) Figure 20. Vacancy Chromatograms for the Separation of a Four Component Mixture   The mobile phase was a 0.14 M potassium phosphate buffer solution adjusted to pH 4.0. Figure 20 reveals that the positions of the peaks are accurately predicted by the theory; the peak heights differ because the relative responses of the detector to the different bases were not taken into account in calculating the theoretical curves. The sample with excess concentration of solutes over that in the mobile phase,

injection volume is increased, so the retention volume of the peak also increases. The retention volume of the small negative peak produced by the smallest charge will be the same as that for a sample where Xi>X0 and the same as that for a solute chromatographed in the normal way with the column carrying pure mobile phase only. The significant dispersion that occurs with larger charges is clearly demonstrated.   Courtesy of th Journal of  Chromatography (ref. 21) Figure 19.Vacancy Elution Curves from Different Injection Volumes on a Column of 500 Theoretical Plates This theoretical treatment assumes that the presence of a relatively low concentration of solute in the mobile phase does not influence the retentive characteristics of the stationary phase. The concentration of solute in the eluted peak does not fall to zero until the sample volume is in excess of 100 plate volumes and, at this sample volume, the peak width has become about five times the standard

Vacancy chromatography can be considered theoretically using the equations derived from the plate theory for the elution of positive concentration profiles. If a mobile phase contains a solute at a concentration (Xo), then, after equilibrium, the eluent from the column will also contain the solute at a concentration (Xo). Let a sample containing the same solute at a concentration (Xi) be injected onto the column under where either (Xo<Xi) or (Xo>Xi),but (XoXi). Such an injection will