Open tubular columns selectivity optimization

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]. 

Precolumn derivatization is often inadequate for dirty samples. In these cases, application of a postcolumn reaction detection system will often suffice. Deelder et al. (44) and van der Wal (45) have examined different configurations for postcolumn reactors and defined optimal selections on the basis of reaction time and type and effect on resolution and sensitivity. Both studies preferred the packed-bed reactor to the open tubular reactors when conventional column geometries were employed for separation, that is, 4.6 mm i.d. X 15 or 25 cm. 

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