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Chromatographic scince series - Cazes J.

Cazes J. Chromatographic scince series - Marcel Dekker, 1996. - 1098 p.
ISBN 0-8247-9454-0
Download (direct link): сhromatography1996.pdf
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Part I.
Part II.
Part III.
SELECTION OF THE BASIC PARAMETERS
Selection of the stationary phase [normal or bonded phase]
Selection of the vapor phase
Selection of the suitable solvents
[According to the Snyder classification of solvent properties as proton acceptors, proton donors & dipole interactions]
OPTIMIZATION OF MOBILE PHASE
SELECTION OF THE FINAL PARAMETERS
Transfer of the optimized analytical TLC mobile phase
Selection of the development mode
[circular, linear, anticircular or the different types of multiple developments]
Selection of other operating parameters [flow rate; particle size; sample loading; relative humadtty, temperature]
Figure 4 PRISMA optimization system for planar chromatographic separation.
830
Szepesi and Nyiredy
also extended this approach to continuous development. The application of a sequential simplex method was also reported (50,51).
A structural approach was suggested by Geiss (52) that assumes selectivity and solvent strength are independent variables. For this optimization procedure a Vario KS chamber was used, with three strong solvents. All three solvents (methyl-f-butyl ether, acetonitrile, and methanol) were diluted with a suitable amount of a weaker fourth solvent (e.g., 1,2-dichloro-ethane) to obtain a series of solutions spanning the solvent strength (e) range from 0.0-0.70 in increments of 0.05e. In the next step, the appropriate solvent strength must be determined. Once this has been identified, fine-tuning is accomplished by blending solvent mixtures of this strength but of different selectivity. However many elegant separations have been achieved in this manner, this method reduces the number of solvents available for optimization.
Based on Snyders 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(Ps)2 + (Ps) + c.
The vertical correlation at constant selectivity points between various solvent strengths was also described by ST = d In hRf+ e. Because the vertical correlation can be linearized, measurements on
3 solvent strengths levels are needed to calculate the hRf values in all selectivity points in the spatial design. These correlations are also relevant when modifiers are used in constant amounts, using various substance classes of naturally-occurring compounds. With these correlations of the h/fy values and the selectivity of the mobile phase, the chromatographic behavior of substances to be separated can be predicted at all selectivity points within the PRISMA model in saturated chromatographic chambers. The separation quality of predicted chromatograms is assessed by the chromatographic response function (CRF). This optimal composition is found by a simple mathematical procedure, which maximizes the CRF in dependence upon the mobile phase combination. Twelve measurements are necessary for a local optimum, and 15 for the global one. To increase the accuracy, six measurements at three different solvent strength levels (18 experiments) are proposed (60).
Strategies for optimizing the solvent systems for planar chromatography, including the two-dimensional TLC separations, have been summarized by Geiss (52) and Nurok (61,62).
E. Mobile Phase Transfer
The possibilities of transferring mobile phases between the various chromatographic methods (40) are summarized in Figure 5. The thick lines indicate where direct transfers are possible, the thin lines where transfers are possible but where in general the selectivity will change. The dashed lines indicate the direct transfer possibilities for fully on-line separation processes. With the characterization of the different saturation grades of chromatographic chambers (40), excellent mobile phase transfer between analytical and preparative planar chromatographic methods and analytical HPLC can be achieved.
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