Solvents Prisma model

The PRISMA model is a system for the optimization of two- to five-eomponent mobile phases, developed by Nyiredy et al. to simplify the optimization proeess in different planar and column chromatographic systems [66]. This model for the seleetion of solvents and optimization of the mobile phase was developed first for TEC and high-performanee liquid ehromatography (HPLC) [38,67]. 

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 ... 

An important difference between the statistical mixture design techniques popular in HPLC and the PRISMA model is that the former yields a computed optimum solvent composition id>ile the latter relies on a structured trial and error approach, which is readily adaptable to TLC. Solvent changes and re-equilibration in HPLC can be quite time consuming, so that it becomes attractive to ainimize the number of experiments, while for TLC, experiments can be performed in parallel and time constraints are less significant. Changes in solvent strength are also more rapidly adjusted empirically within the PRISMA model when theoretical considerations are found inadequate or require modification due to differences in the experimental approach. 

The PRISMA model was developed by Nyiredy for solvent optimization in TLC and HPLC [142,168-171]. The PRISMA model consists of three parts the selection of the chromatographic system, optimization of the selected mobile phases, and the selection of the development method. Since silica is the most widely used stationary phase in TLC, the optimization procedure always starts with this phase, although the method is equally applicable to all chemically bonded phases in the normal or reversed-phase mode. For the selection of suitable solvents the first experiments are carried out on TLC plates in unsaturated... 

Solvent system optimisation can be done on the basis of trial and error according to the literature data or the intuition and experience of the chromatographer 57. The mobile phase optimisation procedure is based on Snyder s solvent characterisation 58 and is called the PRISMA system 157). which uses a three-step optimisation procedure. The proper stationary phase and the possible individual solvents are chosen, and their combination is. selected by means of the PRISMA model, while this combination is adapted to the selected technique (e.g.. FF-TLC. saturated immersion mode, etc.). 

The PRISMA model developed by Nyiredy and co-workers (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 gel is used as the stationary phase and solvent selection is performed according to Snyder s solvent classification (Tab. 4.7). 

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. 

The whole strategy of solvent optimization via the PRISMA model includes the following steps ... 

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. 

The optimization of the stationary-phase combination can be approached by following the PRISMA-Model (Section 3.2.4.5). Instead of using different solvents, in this case the solvent is fixed and the different selectivities are obtained by using different surface modifications on the stationary phases. 

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 system for mobile-phase optimization is a more elaborate, structured trial-and-error version of the normal-phase and reversed-phase procedures described above. The PRISMA system, which is the most widely used of the systematic optimization methods, involves selection of the stationary phase, individual solvents, and vapor phase optimal combination of the solvents by means of the PRISMA model and selection of the appropriate development mode. With silica gel, 10 mobile phases representing the Snyder (1978) selectiv-... 

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. 

A chemometric approach where the /ty-values of forty-seven flavonoids in seven TLC systems were studied using principal component and cluster analyses, has made it possible to choose the minimum number of chromatographic systems needed to perform the best separation (20). Another method (the PRISMA model) based on Snyder s solvent selectivity triangle has been described to aid mobile phase optimization (21). This model is reported to give good separation of flavonol glycosides from Betula spp. (1). When tested in our laboratory no improvements were obtained in comparison with established systems (22) such as the solvent ethyl acetate-formic acid-acetic acid-water (100 11 11 27) on silica support, which can be used for separation of a wide range of flavonoids. 

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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