Manipulation of separation selectivity for alkali metals and ammonium in ion-exchange capillary electrochromatography using a suspension of cation exchange particles in the electrolyte as a pseudostationary phase

The viability of using ion-exchange particles as a pseudostationary phase i n capillary electrochromatography for the separation of monovalent inorgani c cations has been investigated. Using sulfonated polymeric particles (aver age diameter 225 nm) as the pseudostationary phase, the separation selectiv ity for alkali metals and ammonium was examined under a range of background electrolyte compositions and employing indirect absorbance detection. Addi tion of ion-exchange particles to the background electrolyte resulted in a reduction in the observed electrophoretic mobility of the analytes due to t he establishment of ion-exchange interaction with the pseudostationary phas e, with the decrease in mobilities following the ion-exchange interaction o rder for these analytes with a sulfonated stationary phase. Increasing the concentration of the particles resulted in a uniform reduction in the elect rophoretic mobility of the analytes, similar to that observed in micellar e lectrokinetic chromatography. Conversely, increasing the concentration of t he cationic indirect detection probe (which also acted as an ion-exchange c ompeting cation) resulted in a decrease in the ion-exchange interaction wit h the particles and a reduction of the relative ion-exchange contribution t o the overall separation mechanism. Plots of log[retention factor] versus l og[electrolyte concentration] were linear, as is the case for ion-exchange chromatography, but the observed slopes were greater than predicted from io n-exchange theory. Indirect absorbance detection was found to give poor sen sitivity due to light scattering effects caused by the particles of pseudos tationary phase.