Ion pair chromatography of carboxylic and sulfonic acids with trialkylamines : An investigation of the relationship between the adsorption isotherm of the IPR and the retention factor of the analyte

Sammanfattning: Ion pair chromatography has found many uses within analytical chemistry and understanding of the chromatographic process with an ion-pairing reagent have been studied since the 1970s. The adsorption of the Ion-pairing reagent to the stationary phase is understood to be important and many studies have been dedicated to looking at the adsorption isotherms of the earlier quaternary alkylammonium ions. Trialkylamines have been found to have some advantages over quaternary alkylammonium ions and are almost exclusively used in the analysis of oligonucleotides. Despite this there is a lack of studies looking at the adsorption isotherms of these molecules. A method for determining the adsorption isotherm of triethylamine and tributylamine was recently published by Haseeb et al. (2023) were the adsorption isotherm of tributylamine displayed and unusual behaviour. This raised the question of how the adsorption isotherm of the ion-pairing reagent relates to the retention of the analytes, which is the topic of this project. The retention factor of a set of carboxylic and sulfonic acids were determined at increasing concentration of triethylamine and tributylamine. The retention data was used to assess the validity of the electrostatic retention model by Ståhlberg and the Extended thermodynamic approach by Cecchi, as both these models can be used to relate the adsorption to the retention factor. Results from triethylamine were harder to interpret and tributylamine subsequently became the main focus of the project. The results from tributylamine showed a clear division between carboxylic and sulfonic acids in terms of selectivity and determined surface potential. This could not be explained by the electrostatic retention model and led to the adoption of the Cecchi retention model. The adsorption isotherm of tributylamine was determined by two methods and used to fit the Cecchi model to the retention data of three analytes. This could potentially be used to interpret the adsorption isotherms influence on the retention factor, although the attained relationship should be questioned due to uncertainty in the determined adsorption isotherms.