Retention Behavior of Purine-Like Compounds on Titania and the Effect of Organic Modifiers Content in the Mobile Phase

Abstract

The retention behavior of purine and purine-like compounds on titania was investigated in order to elucidate the strong retention of purine on titania. It was verified that, as expected, the strong retention stemmed from the formation of a chelate ring using the two nitrogen atoms N3 and N9 with titania. Purine has two possible tautometric forms because of proton tautomerism between N7 and N9. Hence, the lone pair of either of the two nitrogen atoms could be located immediately in an sp² orbital. Therefore, it was found that the lone pairs of N3 and N9 must be located in an sp² orbital, and the location is essential to forming the chelate with titania. Moreover, the effect of the presence of methanol and acetonitrile in the mobile phase on retention was also investigated. The retention strength increased with their content, but the tendency of this increase was different for the two organic solvents. It was concluded that the difference stems from the differing degrees of strength of the ligands, which includes the analytes and the solvents, as a result of competing with each other. Water and methanol acted as ligands strong enough to compete with the analytes, but acetonitrile did not, probably because it lacks hydroxy groups, which have a strong affinity for titania.