Retention coupled columns

One potential problem associated with column coupling in reversed phase is relatively high back-pressure ( 2600 psi at 1 mL miir ). This will place a limit on the flow rate, which in turn limits the further reduction of analysis time. Also, compared to the new polar organic mode, the retention in reversed phase on coupled columns is deviated more from the average retention on the individual stationary phases. 

Similar to the new polar organic mode, the retention of analytes in normal phase is not difficult to predict. For all the compounds, the average of the retention on individual columns is fairly close to the retention on the coupled columns. The selectivity of most compounds on coupled columns is an average of the selectivities of individual columns (Fig. 2-9). However, it was found that the elution order for some compounds was reversed on ristocetin A and teieoplanin or vancomycin. As a result. 

Gray, M.J., Dennis, G.R., Slonecker, P.J., Shalhker, R.A. (2005). Utilising retention correlation for the separation of oligostyrenes by coupled-column liquid chromatography. J. Chromatogr. A 1073, 3. 

Coupled column (multidimensional) systems in chromatography have also been developed to improve resolution. The coupled column procedure, as noted in Section 6.4, requires two or more columns of different kinds having different retention mechanisms. 

Coupled-column separations or multidimensional chromatography can be considered as a sample preparation form, as one column is used to derive fractions for the second column. It provides a two dimensional separation in which sample substances are distributed over a retention plane formed by the operation of two independent columns. This type of two dimensional based separation method is more powerful than a single dimensional based one. A retention plane has more peak capacity than a retention line and so can accommodate much more complex mixtures. Component identification is more reliable because each substance has two identifying retention measures rather than one. These type of combinations offer high selectivity and high sensitivity, and could be used with less expensive and more robust detectors (e.g., flame ionization). ... 

Series coupling of columns containing the same stationary phase is used to enhance efficiency and with different stationary phases to fine tune selectivity [8]. Series coupling of packed columns became popular after it was demonstrated that the column pressure drop did not limit the total column efficiency to the extent that had been predicted (section 7.4.2). Serial coupling of 2 or 3 standard columns is practical for routine applications and provides a total plate count in excess of 50,000. There is no theory for selectivity optimization for coupled packed columns but suitable conditions often can be estimated from separations on the individual columns. Effective selectivity changes require that the coupled columns have different retention properties. A number of practical examples for the separation of polymer additives, polycyclic aromatic hydrocarbons, phytic acid impurities and enatiomers have been described [137,195,196]. Series coupling of open tubular columns with different stationary phases is less common, but changes in selectivity are predictable, at least when the pressure drop is low [197]. 

The actual determination of which factors are most Important In determining the accuracy of calculated retention will require further Investigation. The fact remains, however, that the developed approach and corresponding computer programs can predict retention under complicated experimental conditions with acceptable accuracy. The program as written can accommodate up to 6 coupled columns, each with their own particular k vs mobile phase composition function, over any desired gradient function. This represents then a nearly universal approach to the calculation of retention for solutes under all possible elution conditions. 

In column coupling, columns of the same type or of different types are connected in series. This technique is standmd practice in size-exclusion chromatography, but is only rarely used in retention chromatography. Another common application of column coupling is the use of guard columns. 

Pasti, L. Dondi, F. van Hulst, M. Schoenmakers, R Martin, M. Felinger, A. Experimental validation of the stochastic theory of size exclusion chromatography Retention on single and coupled columns. Chromatographia 2003,57 (Suppl ), S171-S186. 

The retention factor of a solute in a coupled column series, can, for equal column temperatures... 

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