Anion-exchange chromatography on short reversed-phase columns modified with amphoteric (N-dodecyl-N,N-dimethylammonio)alcanoates

Short reversed-phase columns (50 mm × 4.6 mm Gemini C18) were dynamically coated with carboxybetaines of the general structure, C12H25N+(CH3)2(CH2)nCOOH, namely (N-dodecyl-N,N-dimethylammonio)undecanoate, DDMAU (n = 10) and (N-dodecyl-N,N-dimethylammonio)butyrate, DDMAB (n = 3), and investigated for the separation of inorganic anions in ion chromatography. The role of the ionic strength of coating surfactant solutions on their adsorption and resultant column capacity was studied. The retention of inorganic anions was investigated with different eluents at various concentrations and pH. Interestingly, no retention for anions was found with pure water as the eluent, but the addition of small amounts of electrolytes, up to 0.1 mM, caused a sharp increase in the retention of analytes. The effect of increasing anion retention with an increase in eluent cation charge was also observed. Based on this effect a new cation charge gradient concept was proposed and applied to the separation of a standard mixture of anions.