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use. Rapid Separations Employing Gradient Elution Rapid LC separations are relatively easy to accomplish with isocratic development assuming low dispersion instrumentation with a fast response is available. However, if a sample mixture contains components that extend over a wide polarity or molecular weight range then gradient elution development will be necessary and fast gradients are almost impossible to form with conventional LC solvent programmers. As a consequence, for high speed gradient separations, a unique procedure must be used in conjunction with specially designed apparatus. The solution to the problem of fast gradient generation is to employ a preformed gradient a concept that was first introduced by Snyder and Saunders (13) as long ago as 1969. A diagram of a gradient preformer is shown in figure 27. In the particular apparatus that was used to provide the fast analyses that are described below the required gradient was formed in a column 25 cm long, 4.6 mm

nbsp; Figure 44  The Separation of Blood Liquids Employing Incremental Gradient Elution and Monitored by the Modified Moving Wire Detector However, due to the limited number of compounds that were tested this relationship should be assumed only with caution. A chromatogram of blood lipids obtained by incremental gradient elution and monitored by the modified detector is shown in figure 44. As incremental gradient elution involves a program of 12 solvents ranging from hydrocarbons, chlorinated hydrocarbons, nitro-paraffins, esters, ketones and alcohols. This separation illustrates the versatility that is provided by this detector for solvent selection. Van Dijk (37), attempted to improve the sensitivity of the detector by using a spray procedure for coating the wire. The column eluent entered an

in a relatively short length of packed tube or a simple wall-coated open tube. The rest of the apparatus is merely there to support this relatively trivial, but critical device. The oven also will contain a temperature sensor and if necessary an appropriate temperature programmer. As the mobile phase is a gas, there are virtually no interactions between the sample components and the mobile phase and thus the elution time can not be controlled by techniques such as solvent programming or gradient elution. The counterpart to gradient elution in gas chromatography is temperature programming. The column temperature is raised continuously during development to elute the more retained peaks in a reasonable time. It is a similar technique to flow programming but decreases the retention exponentially with temperature as opposed to linearly with flow rate. The temperature was originally programmed in a linear manner using electro-mechanical devices but modern temperature

Although the actual elution took only 22 seconds, due to the time required to form the gradient and regenerate the column the total gradient cycle was 5-6 minutes. To fully utilize the speed of the system a number of gradient storage columns would be necessary, that could be operated in parallel, if the gradient analysis was to be repeated continuously. The quantitative repeatability of the system was tested with 8 replicate analyses of the mixture. The results obtained are shown in Table 6. It is seen that despite the complexity of the analytical procedure, and the need for

and (Z) will continually increase. Such a system is extremely difficult to treat theoretically particularly as the boundaries are not as sharp as those depicted in Figure 3. In fact, the overall effect is as though the separation was carried out sequentially on three separate sections of a plate, each section having a different stationary phase and mobile phase. In each section, the separation will then be achieved by elution development, but the overall effect will be a form of gradient elution. The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the same as that between the liquid solvent and the surface.

Low Dispersion Gradient Elution Apparatus Gradient elution with high efficiency low dispersion columns, particularly small bore columns that operate at very low flow rates, present another instrumental problem for the designer of the modern chromatograph. Not only must the gradients be formed accurately and precisely, they must be also formed at very low flow rates and sometimes the total volume employed for the analysis will be less than 1 ml.  The apparatus can be basic and accommodate only to solvents