Open tubular column retention mechanism

There are surprisingly few studies of the retention mechanism for open tubular columns but the theory presented for packed columns should be equally applicable. For normal film thicknesses open tubular columns have a large surface area/volume ratio and the contribution of interfacial adsorption to retention should be significant for those solutes that exhibit adsorption tendencies. Interfacial adsorption has been shown to affect the reproducibility of retention for columns prepared with nonpolar phases of different film thicknesses [322-324]. The poor reproducibility of retention index values for columns prepared from polar phases was demonstrated to be c(ue to interfacial... 

Although the exact mechanism is not very clear, the following factors may contribute to the modifier effect on chromatographic retention. Polar modifiers may cover the active sites of the stationary phase (deactivation) so that solute retention is reduced. This can be explained by the differences in retention change between packed and open-tubular columns when small amounts of modifiers were used. Open-tubular columns normally do not show the drastic changes in retention or efficiency upon the addition of small amounts (<2%) of modifier as most packed columns do. These less drastic differences were caused by the differences in the degree of deactivation of the packed column stationary phase as compared with the open-tubular-column stationary phase. An open-tubular column has fewer active sites present and, thus, fewer active sites are present for the modifier to deactivate. 

In this chapter the general applications of adsorption layer open tubular (ALOT) columns will be discussed. As the basic separation on adsorbents is realized by a gas-adsorption mechanism, the ALOT columns are generally used for relatively volatile compounds. Depending on the type of the adsorbent and its physical characteristics each adsorbent has its own specific retention behavior. 

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