Pressure Tuning

The technique of pressure tuning involves variation in the midpoint pressure between coupled columns to alter overall solute selectivity. This is not strictly an MDGC method, and has been given the term multi-chromatography in order to differentiate it from regular MDGC. However, pressure tuning is possible on regular MDGC systems, and some specific multidimensional results can be achieved on multi-chromatography pressure-tuned systems. The unified chromatography procedure, promoted by Bartle, alters the characteristics of the carrier phase during an analysis, for example progressing from GC to SFC conditions by applying a pressure programme to the carrier stream. Thus, one column is used, but different chromatography mechanisms are employed. This variation is again not strictly multidimensional. The pressure-tuning arrangement is shown in Figure 4.

Compounds are immediately presented to the second column as soon as they are eluted from the first column; their motion is not hindered by trapping or any other similar solute-focusing effect. Changing the midpoint pressure alters the relative flows in each column. Flow change by itself does not alter the capacity factor on either column (i.e. k is constant),

Inj Det 1 Pm Det 2

Inj Det 1 Pm Det 2

Column 1 Column 2

Figure 4 The pressure-tuning method involves a coupling between the two columns where additional pressure (Pm, midpoint pressure) can be applied above the natural pressure. p, inlet pressure; inj, injector; Det 1, Det 2, detectors 1 and 2.

Column 1 Column 2

Figure 4 The pressure-tuning method involves a coupling between the two columns where additional pressure (Pm, midpoint pressure) can be applied above the natural pressure. p, inlet pressure; inj, injector; Det 1, Det 2, detectors 1 and 2.

although of course temperature change may affect relative k values. However, the overall k value may change dramatically with midpoint pressure changes. The contribution of each column in determining overall solute capacity on the system is varied, and so relative solute positions may change and best separation conditions may be determined. This procedure is essentially a continuously variable (at least over a given range) phase composition method, simulating column phases of selectable polarity based on the two phases comprising the coupled columns. It is possible to predict the effect of pressure on the overall separation, since individual retention factors on each column can be determined. The effect of carrier flow rate on each individual column's performance should still be considered. Figure 5 represents results which may be obtained, with the unretained peak time giving retention factors on each column. Peak overlap and exchange of relative retention of components are precisely what may be seen experimentally.

For a given column length, the total separation space does not increase in this method.

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