Comprehensive Multidimensional Chromatography

Comprehensive multidimensional chromatography is closest to a true continuous multidimensional column method since it subjects every emerging peak in the first dimension to second-dimension separation in almost continual fashion. Figure 3C is a representation of this approach. The term comprehensive chromatography is attributed to Bushey and Jorgen-son, who demonstrated the advantages of the technique for coupled HPLC dimensions. Two major

Figure 5 Pressure tuning allows retention factors to be determined on each separate column section provided tRs of compounds (solid lines) and tM (dotted line) are known on each dimension. After the midpoint, various solutes may swap position, possibly leading to better overall separation. The effect of the pressure tuning can be predicted by moving the right-hand vertical line along the horizontal axis.

Figure 5 Pressure tuning allows retention factors to be determined on each separate column section provided tRs of compounds (solid lines) and tM (dotted line) are known on each dimension. After the midpoint, various solutes may swap position, possibly leading to better overall separation. The effect of the pressure tuning can be predicted by moving the right-hand vertical line along the horizontal axis.

variations depend on whether all the column flow or only a portion from the first column is transferred to the second column. These require different technical implementation of column coupling.

The second column elution time must be very short with respect to that on the first dimension - less than the frequency at which the first column effluent is sampled. This ensures that each second-dimension elution is completed before the subsequent band is introduced into the second column. Result presentation is best if the second-dimension analysis is rapid with respect to bandwidths on the first column, for example, a second-dimension analysis time about one-fifth of the peak width time on the first column. Given this requirement, first column performance leading to broad peaks may be required. The two columns chosen should ensure orthogonality. Retention of compounds in dimensions one and two can be defined as 1iR and 2tR respectively.

The final chromatogram will be a two-dimensional array of retentions, with a third dimension of peak height, leading to a contour plot chromatogram. Data presentation protocols and concepts such as retention indices, quantitative analysis considerations and relationships between peak position and phase polarities are only just being explored. Much further work is needed to evaluate these systems fully. Figure 6 demonstrates how the three-dimensional data are presented in terms of a contour plot. The peak contour comprises a series of slices in the second dimension which is reconstructed as a peak with dispersion in both dimensions. The original first-dimension separation is shown.

In the comprehensive gas chromatography (C(GC)2) method, peak compression by a means of a focusing step between the two dimensions may be advantageous. This allows a very narrow band to be introduced to column 2 and allows the best peak capacity to be achieved on this column. Ideally, overlapping components in dimension 1 will be resolved on dimension 2. Peak compression of 20-50 times have been demonstrated, and this immediately translates into significant peak sensitivity enhancement with C(GC)2.

In HPLC, the collected fraction of effluent would typically be analysed on a conventional column, so it would have a typical retention time in minutes.

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