Retention volume of acidic compounds in ion exclusion chromatography

The separation mechanism of acidic compounds in Ion-Exclusion Chromatography is described assuming that the stationary phase contains immobilised ions (remnants of the' dissociated functional groups) that cannot be transferred to the mobile phase. Other properties of the two chromatographic phases are assumed to be the same. System description is based on computer simulation of column performance by the Craig (finite element) method. The thermodynamic equilibria conditions are applied locally to a small but finite element of the column, corresponding to a theoretical plate, in a recurrent procedure to simulate the passage ofthe sample down the column. Partition of the analysed sample between the stationary and the mobile phases is calculated for every plate. The portion of the mobile phase is then moved to the next plate, the process being continued repeatedly. This procedure makes it possible to study the chromatographic peak evolution with time and also the influence of the parameters characterising the chromatographic column, the mobile phase and the analysed sample, on retention volume and peak shape of the sample. Calculations of retention volume as a function of acidic dissociation constant, sample injection quantity and buffer concentration were performed for pure water and aqueous buffer as the mobile phases. Results are in good agreement with experimental data. It was found that the buffer eliminates peak leading and retention volume dependence on sample concentration. It was also ascertained. that, as in pure water, strong acids are eluted together at the start of the chromatogram and are not retained on the column. Weak acids are also eluted together, at the end of the chromatogram.