Gas Chromatography - GC Columns > Coating the Supports > Page 23
The macroporous polymer beads are used as supports as well as adsorbents. They exhibit significant adsorption as the support itself acts as a stationary phase and makes a substantial contribution to retention. However, with normal sample loads, the adsorption isotherm is linear and so the eluted peaks are symmetrical. Only stationary phases that do not affect the polymer in any way can be used with such beads, which is a distinct disadvantage. They also have relatively poor temperature stability.
Coating the Supports
It is important to have an accurate measure of the amount of stationary phase that has been placed on a support to ensure retention time reproducibility and qualitative accuracy. The reproducibility of the coating procedure may have particular significance when the analytical results are to be used for forensic purposes. The material can be coated by the direct addition of the stationary phase to the support, by the filtration method or by the slurry method. The slurry method of coating is the one that is recommended.
Coating by direct addition would appear to be the ideal quantitative method of preparing the column packings. A weighed amount of stationary phase is added directly to a known mass of support contained in a glass flask. The material is well mixed by rotating the flask for several hours, but even with extensive mixing, the stationary phase being is still irregularly distributed throughout the packing. As a result, the efficiency of the column slowly increases with use, as the stationary phase distributes itself more evenly throughout the packing. It may take several weeks of use for the column to give a constant maximum efficiency.
The filtration method gives a packing with the stationary well distributed over the support but the loading can not be accurately calculated. A known mass of stationary phase is dissolved in sufficient solvent to provide excess liquid when mixed with a weighed amount of the support. The mixture is filtered under vacuum and the volume of the filtrate measured. From the volume of filtrate, the amount of solvent remaining on the support can be calculated and hence this stationary phase loading can be accessed. The bed is then sucked dry, the solvent evaporated and the coated support packed into the column. The amount of stationary phase on the support is not determined accurately by this method due to solvent losses by evaporation.