Reversal of elution sequence and selectivity resulting from the use of an ionic liquid as a mobile phase modifier

For the binary mixture of N-CBZ-L-amino acid+L-amino acid, the effects of using the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), as a mobile phase modifier on the elution sequence and the selectivity were investigated. The aqueous solution of 80% methanol by volume at pH 3.0 was employed as a mobile phase and a reversed-phase C18 media as a stationary phase. The use of ionic liquid as a mobile phase modifier caused the reversal of elution sequence for the aforementioned binary mixture when the ionic liquid concentration was increased beyond a certain value (or critical ionic liquid concentration). Such effect of ionic liquid gave rise to the occurrence of a minimum in the selectivity between the mixture components when the ionic liquid concentration reached the critical one. Below the critical ionic liquid concentration, the increase of the ionic liquid concentration reduced the selectivity. Beyond the critical ionic concentration, the selectivity was enhanced as the ionic liquid concentration increased. These results indicate the importance of determining a proper ionic liquid concentration if the ionic liquid is to be chosen as a mobile phase modifier.