PRISMA selectivity optimization

On the basis of Snyder s system for characterization of solvents the PRISMA method for mobile phase optimization has been developed. This system enables the optimization of solvent strength and mobile phase selectivity and also the transfer of the optimized mobile phase to different planar chromatographic techniques, in our case the PLC. 

Procedures used vary from trial-and-error methods to more sophisticated approaches including the window diagram, the simplex method, the PRISMA method, chemometric method, or computer-assisted methods. Many of these procedures were originally developed for HPLC and were apphed to TLC with appropriate changes in methodology. In the majority of the procedures, a set of solvents is selected as components of the mobile phase and one of the mentioned procedures is then used to optimize their relative proportions. Chemometric methods make possible to choose the minimum number of chromatographic systems needed to perform the best separation. 

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... 

The optimization of the solvent strength by varying the selectivity points has to be carried out until at least a beginning separation is obtained. At this point the third part of the PRISMA iK>del can be used to select the appropriate development mode. If an Increase in efficiency is required to improve the overall resolution of the sample then forced-flow linear... 

The mobile phase for a particular separation is usually selected empirically using prior personal experience and literature reports of similar separations as a guide or by use of a systematic mobile-phase optimization scheme, usually the PRISMA model. Typical mobile phases that have been used for separations of many classes of pesticides on sihca gel have been mixtures of hexane-acetone, toluene-acetone, chloroform-diethyl ether, and toluene-methanol, whereas mobile phases for RPTLC analyses on Cig layers are usually methanol-water and acetonitrile-water mixtures. 

Figure 6.15. The Prisma mobile phase optimization model showing the construction of the prism and the selection of selectivity points.

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. 

Stationary phase optimized selectivity liquid chromatography basic possibilities of serially connected columns using the PRISMA principle. /. Chromatogr. A, 1157 (1-2), 122-130. 

Spiegeleer et al., 1987 De Spiegeleer and De Moerloose, 1988), a graphical method (Matyska and Soczewinski, 1993), numerical taxonomy and Information content derived from Shannon s equation (Medic-Saric et al., 1996), and the PRISMA system (Nyiredy et al., 1988, 1989, 1991 Nyiredy and Fater, 1995 Dallenbach-Toelke et al., 1986) (see Section I.D). All of these optimization procedures involve the use of some form of statistical design to select a series of solvents for evaluation or to indicate the best system by comparing the results obtained from an arbitrarily selected group of solvents (Poole and Poole, 1991). 

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 Vario chambers, which can be used in sandwich or tank configurations, allow selection of the optimal vapor-phase conditions and are best suited for working with the PRISMA optimization system (Chapter 6) (Nyiredy, 1992). 

The PRISMA model method was introduced by Nyiredy and co-workers [19] for optimization of the mobile phase in reversed-phase HPLC. It has been effectively used in planar chromatography [20-23]. The PRISMA model is a structured trial and error approach and is a three-dimensional model, correlating the solvent strength and the selectivity of mobile phases. The solvent selection is performed according to Snyder s solvent classification [24], With this optimization model, the most advantageous mobile phase composition may be systematically elaborated, and from one to four solvents can be combined to achieve a suitable separation. 

Nyiredy et al. have developed an optimization model called PRISMA for the optimization of the mobile phase for OPLC (59). PRISMA is a three-dimensional model that correlates the solvent strength and the proportion of eluent constituents, which determine the selectivity of mobile phases applying Snyder s solvent classification (60). 

Nurok (e.g., 26) and Geiss (17) recently summarized the most important possibilities for mobile phase optimization. On the basis of Snyder s system for characterization of solvents (27), a new mobile phase optimization method was developed, the PRISMA system (28) this enables not only optimizaton of solvent strength and mobile phase selectivity (29), but also transfer of the optimized mobile phase between the different planar chromatographic techniques. Multicomponent mobile phases should not be used repeatedly, whereas single-solvent mobile phases can be used repeatedly until they become contaminated (30). 

Selection of the mobile phase depends on the RPC method used the RPC method can also be selected after TLC preassays. We prefer the PRISMA system for TLC optimization (28). 

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. 

In optimizing planar chromatography, peak capacity in 2-D TLC far exceeds HPLC (13). PRISMA has also been very helpful by developing computerized and statistical choices for solvents. Demixing remains a major problem in predicting Rys and the ultimate experimental outcome vs. predicted. Again, 20 chromatograms define experimental variables for optimum Rf. Solvent selectivity (14) has been discussed based on proton donation, acceptance, or dipole interactions (IS). 

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