Correlation and Prediction of the k′ Values for Mobile Phase Optimization in HPLC

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Abstract
The tripartite "PRISMA" optimization model is summarized, which includes all possible solvent combinations between 1-5 solvents. The solvent composition is characterized by the solvent strength (ST) and the selectivity points (PS). The results show that a correlation between the selectivity points for equilibrated column systems at a constant solvent strength (horizontal function) can be described by the function k'' = 1 (Ps)2 + b(PS) + c. In all cases in 12, 8, and 4 selectivity points, the k'' values were measured along the edges of the triangle through two basic selectivity points (181-118, 118-811 or 811-181). The function obtained from the k'' values measured at 12 or 8 points correlated with a high significance with the function obtained from only 4 points. The vertical correlation at constant selectivity points between various solvent strengths can be described by ST=a In(k'') + b. Because the vertical correlation can be linearized, measurements on 3 solvent strengths levels are needed to calculate the k'' 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 of differing polarity, both correlations were shown to be valid. From the presented correlations of the k'' values and the selectivity points, the chromatographic behaviour of substances to be separated can be predicted at all selectivity points within the "PRISMA" model for isocratic separation. Based on these relationships a mobile phase optimization strategy is suggested.