Phase Optimization by TLC Following the PRISMA Model

5 Mobile-Phase Optimization by TLC Following the PRISMA Model 

Before starting extensive experiments, a procedure recommended by Kaiser and Oelrich (1981) to rule out adsorbents by fast experiments should be employed. Each elution experiment takes about 20 s. For this purpose, samples are applied on a 50 X 50 mm TLC plate at nine points, which are exactly 10 mm apart. Five microlitres of methanol are dravm into a microcapillary with a platinum-iridium point. By applying the point of the filled capillary on one of the sample points on the plate, methanol is introduced onto the plate. A miniature radial chromatogram of about 7 mm diameter is produced. If the sample components remain at the point of application, the use of this adsorbent type is ruled out for HPLC usage. To make sure, the procedure is repeated with 5 [xl of acetonitrile and tetrahydrofuran, respectively. If the products still remain at the point of application, the situation will not be changed by using any other mobile phase that is suitable for preparative chromatography work. 

However, if the entire sample migrated to the outer ring with methanol, the test is repeated with n-heptane. If the entire sample still migrates to the outer ring, then the used adsorbent is again ruled out as stationary phase. If, with w-heptane, the entire sample or part of it remains at the starting point, the unlimited possibilities of combining solvents can be exploited to further optimize the method (Kaiser and Oelrich, 1981). 

The PRISMA model developed by Nyiredy and coworkers (Nyiredy et al., 1985 Dallenbach-Tolke et al., 1986 Nyiredy and Fater, 1995 Nyiredy, 2002) for use in Over Pressured Layer Chromatography is a three-dimensional model that correlates solvent strength and the selectivity of different mobile phases. Silica is used as the stationary phase and solvent selection is performed according to Snyder s solvent classification (Table 3.27). 

The PRISMA model is a structured trial-and-error method that covers solvent combinations for the separation of compounds from low to high polarity. Initial experiments are done with neat solvents, covering the eight groups of the Snyder solvent classification triangle. 

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