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]. 

FIGURE 4.11 The PRISMA model. (Adapted from Siouffi, A.-M. and Abbou, M., Optimization of the mobile phase, in Planar Chromatography, A Retrospective View for the Third Millennium, Nyiredy, Sz., Ed., Springer Scientific, Budapest, 2001, chap. 3. With permission.)... 

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 ... 

Investigation of the chromatographic system using the PRISMA model... 

Morita et al. [69] optimized the mobile phase composition using the PRISMA model for rapid and economic determination of synthetic red pigments in cosmetics and medicines. The PRISMA model has been effective in combination with a super modihed simplex method for fadhtating optimization of the mobile phase in high performance thin layer chromatography (HPTLC). 

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. 

Pelander et al. [81] developed a computer program for optimization of the mobile phase composition in TLC. They used the desirability function technique combined with the PRISMA model to enhance the quahty of TLC separation. They apphed the statistical models for prediction of retardation and band broadening at different mobile phase compositions they obtained using the PRISMA method the optimum mobile phase mixtures and a good separation for cyanobacterial hepatotoxins on a normal phase TLC plate and for phenolic compound on reversed-phase layers. 

For practical purposes, it is most suited to combine the analyst s practice with computer assistance, in this context the PRISMA model being very efficient. 

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... 

A good correlation between experimental retention times and calculated selectivities and molecular connectivities was found using the PRISMA model for seventeen dansylated biogenic amines present in foodstuffs and animal fodder251. 

The separation of synthetic red pigments has been optimized for HPTLC separation. The structures of the pigments are listed in Table 3.1. Separations were carried out on silica HPTLC plates in presaturated chambers. Three initial mobile-phase systems were applied for the optimization A = n-butanol-formic acid (100+1) B = ethyl acetate C = THF-water (9+1). The optimal ratios of mobile phases were 5.0 A, 5.0 B and 9.0 for the prisma model and 5.0 A, 7.2 B and 10.3 C for the simplex model. The parameters of equations describing the linear and nonlinear dependence of the retention on the composition of the mobile phase are compiled in Table 3.2. It was concluded from the results that both the prisma model and the simplex method are suitable for the optimization of the separation of these red pigments. Multivariate regression analysis indicated that the components of the mobile phase interact with each other [79],... 

HPLC analysis with three-component mobile phase optimization through PRISMA model. 

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 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. 

Mobile Phase Optimization by TLC Following the PRISMA Model... 

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. 

Fig. 4.16 C raphic representation of the PRISMA model (a) complete model (b) regular part of the...

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

Nyiredy, S., Fater, Z. Automatic mobile phase optimization, using the PRISMA model for the TLC separation of apolar compounds, Jpc-/. Planar Chromatography-Mod. TLC, 1995, 8,... 

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. 

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