Pre‐equilibrium Approximation for Analysis of Chemical Kinetics in Chromatography: Application to Reversed‐Phase On‐column Kinetics of Decomposition of Murexide

Abstract

AbstractA kinetic process can sometimes be observed during a chromatographic separation. The chemical reaction is coupled with the rapid distribution of the reacting compounds between the mobile and stationary phases. The use of the pre‐equilibrium approximation enables the derivation of the rate law for the chemical reaction(s) taking place in the mobile and/or stationary phase. A simple procedure is proposed for writing down this rate equation. It can be applied to any on‐column reaction, whether reversible or irreversible. For first‐ or pseudo‐first‐order kinetics, integration of the rate equation gives an exponential function of concentration with respect to time. To demonstrate the application of the proposed method, the kinetics of the decomposition of murexide in a reversed‐phase C8 column was studied. Murexide decomposes through a fast reversible protonation process to give purpuric acid, which then decomposes irreversibly. The stopped‐flow HPLC technique was used to obtain the entire kinetic profile for the on‐column reaction. Additionally, the kinetics was measured off‐column using the same mobile phase. The values of the kinetic parameters obtained from the batch experiments enabled the determination of all kinetic parameters of the on‐column reaction using non‐linear least squares fitting of the derived rate equation.