Retention behavior of cationic aluminium chelate with o,o'-dihydroxyazobenzene in HPLC using the C18-bonded silica stationary phase.

The retention behavior of cationic aluminium chelate with o,o'-dihydroxyazobenzene (DHAB or H2L), [AIL](+), in high-performance liquid chromatography (HPLC) was investigated using the C-18-bonded silica stationary phase. In this study, it was found that when the pH of the mobile phase was 5.5, a significant tailing of the alminium chelate was observed, whereas when the pH was 2, a marked improvement of the elution was attained. These results were interpreted in terms of a silanol masking effect: when an acidic mobile phase is employed, protonation of the silanol groups on the surface is maintained, while eliminating the strong interaction between the cationic chelate and the stationary phase. The addition of sodium chloride in the eluent can also be used to eliminate peak tailing of the cationic chelate. Based on these results, the underlying immobilization mechanism of the aluminium chelate on the C-18-bonded silica thin layer is explained by an electrostatic interaction between the cationic aluminium chelate and deprotonated silanol groups on the solid surface. The example in this study should provide valuable information to improve the separation quality of cationic solutes in HPLC.