Extra-thermodynamic relationships in chromatography-study of the relationship between the slopes and intercepts of plots of ln k′ vs. mobile phase composition in reversed-phase chromatography

The chemical validity of the frequently reported linear relationship between the slopes (S) and intercepts (ln k′_w) of plots of ln k′ for various solutes against mobile phase volume fraction organic modifier in reversed-phase liquid chromatography (RPLC) has been investigated. We have shown that under certain conditions, the linear S-ln k′_w relationship for a chemically variegated set of solutes, even though it is mathematically quite real, is an artifact that results from statistical considerations and does not reflect chemical reality. However, S-ln k′_w correlations are chemically meaningful when the set of test solutes is restricted to a homologous series. Based on conventional extra-thermodynamic reasoning, such correlations can only be due to chemistry under two circumstances; i.e., when only a single retention-governing solute property varies within the set of solutes studied, or when various retention governing processes have the same compensation composition (φ{symbol}_iso). The existence of a real correlation is also shown to be at odds with the concepts of linear salvation energy relationships and other models of retention that describe retention in RPLC as the result of a variety of independent intermolecular interactions. Finally it is shown that if linearity between S and ln k′_w were universal, the well-known phenomena of changes in elution sequence with the volume fraction of organic modifier in the mobile phase would not be possible.