Structural modifications of dicationic acetylcholinesterase reactivators studied under ion‐pairing mechanism in reversed‐phase liquid chromatography

Abstract

A study focused on the chromatographic behavior of several acetylcholinesterase reactivators under ion-pairing mechanism is reported. Among these reactivators, dicationic oximes and carbamoyl-based pyridinium congeners were studied, which form ion pairs with alkylsulfonate anions. This mechanism was studied for some major experimental parameters, such as the chain length of the ion-pairing agent added to the aqueous phase, its concentration, temperature, and nature of the organic modifier from mobile phase. Retention data showed one or two possibilities of forming ion pairs and the tautomerism of the studied reactivators, for different pH values of the aqueous component. Double sigmoid shapes were obtained for the studied compounds for the dependence between retention factor and pH, indicating the possibility of one or two tautomeric equilibria: at pH close to 7 these compounds are not stable as dicationic species and they participate in the retention process as nitroso forms, which are not able to form ion pairs with alkylsulfonates. The dependences of the retention factor on the organic modifier content from mobile phase were linear. Two complementary theoretical models were used to explain the functional dependences for the retention data on the experimental parameters.

Variability of temperature dependences of the retention of strongly polar compounds under ZIC‐HILIC liquid chromatographic mechanism

“…, ΔH 0 is the standard enthalpy for the transfer of the analyte from mobile phase to the stationary phase, ΔS 0 is standard entropy for the same partition process, R is the universal gas constant (8.314 J K −1 mol −1 ), T is the absolute column temperature (namely t + 273.15, t being column temperature in Celsius degree) and ϕ represents the phase volume ratio (the volume of stationary phase to that of the mobile phase). According to this equation the dependence between ln k and 1/T should be linear, but in several circumstances a deviation from this linearity has been observed and explained by various factors that are either column dependent, or due to the complex processes occuring in the mobile phase, or due to the solute molecular characteristics , such as: ‐the phase ratio is not always constant over the temperature interval applied to the chromatographic column used in the retention study ; ‐the existence of secondary equilibria involving analytes, such as, for example, their structural modifications by temperature modification, which may influence their interactions with stationary phase or mobile phase components ; ‐the non‐linear variation of acidic properties of analytes (pK a ) with temperature variation . …”