Coupling of Liquid Adsorption Chromatography and SEC

From the theoretical point of view, a more feasible way of analyzing copolymers is the prefractionation through HPLC in the first dimension and subsequent analysis of the fractions by SEC [89,90]. HPLC was found to be rather insensitive towards molar mass effects and yielded very uniform fractions with respect to chemical composition. 

The major disadvantage of all early investigations on chromatographic crossfractionation was related to the fact that both separation modes were connected to each other either off-line or in a stop-flow mode. Regardless of the separation order, SEC vs. HPLC or HPLC vs. SEC, in the first separation step fractions were collected, isolated, and then subjected to the second separation step. This procedure, of course, is very time-consuming and the reliability of the results at least to a certain extent depends on the skills of the operator. 

A fully automated two-dimensional (2D) chromatographic system was developed by Kilz et al. [91-93]. It consists of two chromatographs, one which separates by chemical composition or functionality and an SEC instrument for subsequent separation by size. Via a storage loop system, fractions from the first 

The most sophisticated way of coupling the two chromatographs is a fraction transfer system comprising one eight-port injection valve (see Fig. 15). With two storage loops, it is possible to collect fractions continuously without losses. When starting the separation, loop 1 is in the LOAD position, whereas loop 2 is in the INJECT position. Each of the loops has a volume of 100-200 pi. When the first fraction leaves the HPLC system, it enters loop 1 and fills it. When loop 1 is completely filled with the fraction, the injection valves automatically switch to the opposite positions, i.e. loop 1 is then connected to the SEC system in the 

An important feature for such an automated system is the proper coordination of the flow rates of the HPLC and the SEC systems. Since fractions are continuously collected from HPLC and subjected to SEC, the collection time of one fraction must exactly equal the analysis time in the SEC mode. Depending on the number and size of the SEC columns, about 7-15 min are required for one SEC analysis. The flow rate in HPLC must be such that one storage loop is filled exactly within this time. In practice, typical flow rates are 20-40 pl/min and 1.5-2.5 ml/min for the first and second dimensions, respectively. 

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