Prisma model selectivity points

Horizontal and vertical correlations of hR values of nonpolar compounds and the selectivity points at different levels of the solvent strength using samrated TLC systems were given by Nyiredy et al. [18,67] applying the PRISMA model ... 

Pelander et al. [71] studied the retardation behavior of cyanobacterial hepato-toxins in the irregular part of the PRISMA model for TLC at 16 selectivity points. The mobile phase combination and the area of the triangular plane were selected in the preassay. The retardation of all the toxins followed the relation for ftRp. The cyanobacterial hepatotoxins behaved predictably in the selected systems in the irregular part of the PRISMA model. 

Figure 7.14 The PRISMA Bobile phase optiaization model showing M the construction of the prism and the selection of selectivity points (Reproduced with permission from ref. 170. Copyright Marcel Dekker, Inc.) ...

Optimization of the solvent strength by varying the selectivity points is carried out until the required separation is obtained. If no adequate separation is obtained then a different layer or additional solvents must be selected and the new system optimized by the previous procedure. Nearly adequate separations can be improved in the third part of the Prisma model by selecting a different development mode. If an increase in efficiency is required to improve the overall separation then forced flow methods should be used. If the separation problem exists in the upper Rp range then anticircular development may be the best choice, if in the lower Rp range, then circular development is favored. 

Figure 3.41 Basic selectivity points of the PRISMA model.

Figure 3 PRISMA model after Nyiredy et al. The corners of the triangular base (1, 2, 3) represent solvents of different selecti-vities and the height at each corner (4, 5, 6) is a measure of the P value of the individual solvents. Stronger solvents are reduced by mixing them with hexane (P = 0) to arrive at the strength of the weakest solvent (7, 8). The selectivity of the mobile phase system is varied by changing the proportions of the adjusted corner solvents (6, 7, 8), beginning with the center point (9=3, 3, 3) and then moving to points close to the corners (10 = 3, 1, 1 11 =1, 3, 1 12 = 1, 1, 3).

The PRISMA model has three parts an irregular frustum, a regular middle part, and a platform (Figure 6), The three top comers of the model represent the selected three individual solvents which can be diluted with hexane. The solvent strength is represented by the height of the prism (Sj, Sjb,Sjc). points along the edges stand for combination of two solvents, points on the sides for combination of three, and the point in the interior of the prism for mixtures of four solvents. 

Based on Snyder s solvent characterization (25), a new mobile phase optimization method, the PRISMA system (Figure 4) has been developed by Nyiredy et al. (53-58). The system consists of three parts In the first part, the basic parameters, such as the stationary phase, vapor phase and the individual solvents are selected by TLC. In the second part, the optimal combination of these selected solvents is selected by means of the PRISMA model. The third part of the system includes selection of the appropriate FFPC technique (OPLC or RPC) and HPTLC plates, selection of the development mode, and finally application of the optimized mobile phase in the various analytical and preparative chromatographic techniques. This system provides guidelines for method development in planar chromatography. The basic system for an automatic mobile phase optimization procedure, the correlation between the selectivity points for saturated TLC systems at a constant solvent strength (horizontal function), was described (59) by the function hRf= a(Pj) + (Fj) + c. 

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