Effect of temperature on the chromatographic retention of ionizable compounds. I. Methanol-water mobile phases.
The retention mechanism of acids and bases in reversed-phase liquid chromatography (RPLC) has been experimentally studied by examining the temperature dependence of retention, with emphasis on the role of the buffer ionization equilibria in the retention and selectivity. Retention factors of several ionizable compounds in a typical octadecylsilica column and using buffers dissolved in 50% (w/w) methanol as eluents at three temperatures in the range of 25-50 degrees C were measured. Two pairs of buffer solutions were prepared by a close adjusting of their pH at 25 degrees C; differences in their ionization enthalpies determined a different degree of ionization when temperature was raised and, as a consequence, a different shift in the eluent pH. Predictive equations of retention that take into account the temperature effect on both the transfer and the ionization processes are proposed. This study demonstrates the significant role that the selected buffer would have in retention and selectivity in RPLC at temperatures higher than 25 degrees C, particularly for co-eluted solutes.
Castells
Cecilia B
CB
División Química Analítica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata y Cidepint, 47 y 115, 1900 La Plata, Argentina. castells@isis.unlp.edu.ar
Gagliardi
Leonardo G
LG
Ràfols
Clara
C
Rosés
Martí
M
Bosch
Elisabeth
E
D000143:Acids; D000465:Algorithms; D002021:Buffers; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D000432:Methanol / Q000737:chemistry*; D010756:Phosphoric Acids / Q000737:chemistry; D000077489:Piperazine; D010879:Piperazines / Q000737:chemistry; D011199:Potentiometry; D012997:Solvents; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D000143:Acids; D002021:Buffers; D007202:Indicators and Reagents; D010756:Phosphoric Acids; D010879:Piperazines; D012997:Solvents; D014867:Water; D000077489:Piperazine; C030242:phosphoric acid; D000432:Methanol