Abstract: The ideal model of liquid chromatography in gradient elution was solved by using the characteristic approach on the assumption of a linear solvent strength model. By considering the influence of the dwelling time of the chromatographic system on retention time, the retention time formulae suited for any combination of isocratic, linear and stepwise gradients were obtained. It was found that the values of the retention time obtained from these formulae were in well accordance with those computed by using the finite-difference method, which confirms the validity of the formulae. Due to the simplicity and broad scope of applications, these formulae can be easily applied in practice.

Key words: formulae, isocratic elution, linear gradient elution, retention time, stepwise gradient elution , liquid chromatography