Packings selectivity differences

FIGURE 15 Selectivity differences between different packings. Plotted is the standard deviation of graphs of the logarithm of the retention factors for the different columns. Courtesy of Clayton McNeff, Zirchrom Separations. 

Coupled columns packed with different stationary phases can be used to optimize the analysis time (71, 75). In this approach the different columns are connected in a series or in parallel. liie sample mixture is first fractioned on a relatively short column. Subsequently the fractions of the partially separated mixture are separated on other columns containing the same or other stationary phases in order to obtain the individual components. Columns differing in length (number of theoretical plates), adsorptive strength or phase ratio (magnitude of specific surface area), and selectivity (nature of the stationary phase) can be employed, whereas, the eluent composition remains unchanged. Identification of the individual sample components via coupled column technique requires a careful optimization of each column and precise control of each switching step. 

Various commercially-packed columns and packing materials are available and have been used for the separation and analysis of mixtures of sulphur gases (Bremner and Banwart, 1974 De Souza et al., 1975 Supelco, 1977). The most commonly-used columnpacking materials for sulphur-gas analysis are Chromosil 310, acetone-washed Porapak-QS and three types of Carbopak B. Selection must be based on the analytical data desired. Each of these packings separates different components and has advantages and disadvantages. 

YMC Inc. has simplified the process of scaling up an analytical separation for preparative isolation by developing matched R D column sets. Each R D column set contains an analytical methods development column and a preparative isolation column packed from the same lot of packing material. This provides assurance that any separation developed on the analytical column will scale-up directly on the matched preparative column without further method modification. The use of these column sets eliminates potential selectivity differences caused by different types of silica and different particle and pore size packings by providing matched columns. 

The nature of the packing material differs depending on the analytes to be separated, so it is very important to have a good selection of materials and elution solvents. A general scheme of optimizing steps is shown in Figure 2.11. These steps depend on the namre of the solution. 

Micropacked columns are available with most of the bonded-phase packings used in HPLC. Porous and non-porous silica particles are optionally functionalized with covalently ound silanes or other strongly adsorbed materials. ALkyl-bonded silicas produce separations, generally based on solute volatility, but with the potential for selectivity differences based on interaction with silanol groups. Underivatized silica is popular for petroleum separations of aliphatic and aromatic hydrocarbons. Silver-ion-containing silica columns are selective for olefin separations. Huoroalkyl-bonded... 

Three HiGee units were installed by Dow Chemicals in the US for stripping hypochlorous acid, used as the reactive chemical in a process. The rotating packed-bed used for stripping was selected after other alternatives such as a spray distillation tower were rejected for capital cost reasons. The packing selected was a woven wire screen, with the gas flowing counter-current to the liquid. Various packing surface areas were tested, with 2000-3000 m /m area densities, but these exhibited no performance differences so the lower value was used. 

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