Separation selectivity length

Separation selectivity was demonstrated by extraction of salicylic acid from pH 2 solutions in the presence of a 60-fold excess of ethanol and sixfold excesses of barbituric acid and caffeine. No measurable interference was observed. Experiments showed that the principal selectivity is in the extraction rather than the back extraction step. This finding indicated that the polar and/or ionic nature of these interferences prevents retention by polymer. Preconcentration of analyte was examined by means of extraction from a flowing stream and back extraction into a minimum volume. The extractor tube length was 4.3 m, and the sample was 10 mL of 1 mM oxine with an extraction time of 2.0 min. The back extractant was 80 pL of 0.2 M NaOH. A sevenfold increase in concentration was observed. Enhanced preconcentration can be expected with smaller tubing diameter-to-length ratios, larger sample volumes, and repetitive use of back extractant. 

The resolution achievable by a system depends upon the number of theoretical plates generated by that system and upon the selectivity of the system. The number of theoretical plates can be varied widely with separation path length, flow conditions, field strength, etc., and is thus highly variable. However, selectivity 1s a more Intrinsic property of the fractionation method and serves as a basis of comparison of the different systems. 

The problems described here are noticed more often when the sample contains so-called irregular components, discussed later. On the other hand, in isocratic systems a larger dead volume (= volume between the autosampler and the detector - without the column) leads only to broader peaks and thus to a deterioration of the resolution. Analog, with isocratic separations a longer column affects the retention time, peak shape, and resolution. Unlike gradient separations, the length of the column can change neither the selectivity nor the elution order ... 

The carrier gas flow rate affects the analysis in the same way that temperature does (see above). The higher the flow rate the faster the analysis, but the lower the separation between analytes. Selecting the flow rate is therefore the same compromise between the level of separation and length of analysis as selecting the column temperature. 

Approximately 0.75 mL of sample is introduced into a special g ass adsorption column packed with activated silica gel. A small layer of the silica gel contains a mixture of fluorescent dyes. When all the sample has been adsorbed on the gel, alcohol is added to desorb the sample down the column. The hydrocarbons are separated according to their adsorption affinities into aromatics, olefins, and saturates. The fluorescent dyes are also separated selectively, with the hydrocarbon types, and make the boundaries of the aromatic, olefin, and saturate zones visible under ultraviolet light. The volume percentage of each hydrocarbon type is calculated from the length of each zone in the column. 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

Various support media may be employed in electrophoretic techniques. Separation on agarose, acrylamide, and paper is influenced not only by electrophoretic mobiUty, but also by sieving of the samples through the polymer mesh. The finer the weave of selected matrix, the slower a molecule travels. Therefore, molecular weight or molecular length, as well as charge, can influence the rate of migration. 

Thus, the length of a settling chamber is inversely proportional to the square of the particle diameter. For example, if it is desirable to separate out particles that are two times smaller than the selected size, then the length of the chamber must be increased by a factor of four. The equation may also be used to determine the smallest particle diameter that can be removed by a chamber of specified dimensions. The following example problem illustrates some of these design principles. 

---