Incremental gradient elution

The response index for a series of compounds of different chemical types ranged from 0.96 to 1.04. The response of the detector was found to be proportional to the carbon content of the solutes tested, which would be expected. 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... 

The Separation of Blood Liquids Employing Incremental Gradient Elution and Monitored by the Modified Moving Wire Detector... 

Figure 19, Chromatogram of blood lipids employing incremental gradient elution.

Gradient elution in AMD starts with the most polar eluent and is varied toward decreasing polarity. Figure la shows a typical universal elution gradient, made up of the three solvents methanol, dichloromethane, and hexane. Figure lb illustrates the increasing duration of the development cycles. Time increments are chosen to obtain uniform increases of the running distance of 3 mm/step. 

The backward-forward finite difference scheme is identical to the Craig model if we choose the time and space increments such that = H. The Craig model has been used by many authors, including Eble et ah [45], Czok and Guiochon [49,50], and El Fallah and Guiochon [55]. This model affords a good numerical solution of the gradient elution problem, which is very difficult to solve numerically with the forward-backward finite difference scheme [55,56]. 

Step Gradient A method of gradient elution in which the composition of the mobile phase is changed by incremental steps. 

Other methods for solvent fractionation are by precipitation method where a ratio of solvent and nonsolvent is incrementally adjusted from solvent-rich to nonsolvent-rich. In this technique, the higher molecular weight fractions will be precipitated first. A reverse technique is solvent gradient elution where a liquid mixture of increasing solvent power is used to remove the lowest molecular weight materials first (10). 

Polyzonal TLC with a multicomponent mobile phase represents the simplest technique for stepwise gradient elution. A continuous gradient can be realized only if the eluent contains a great number of different components with very small increments in solvent strength. However, since each solvent has a different elution effect, such a mixture can seldom be realized. For a general discussion of polyzonal TLC, see the review by Niederwieser and Honegger (6). 

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