Analysis and characterization of aluminum chlorohydrate oligocations by capillary electrophoresis

Aluminum chlorohydrates (ACH) are the active ingredients used in most antiperspirant products. ACH is a water soluble aluminum complex which contains several oligomeric polycations of aluminum with degrees of polymerization up to Al₁₃ or Al₃₀. The characterization and quantification of ACH oligo-cations remain a challenging issue of primary interest for developing structure/antiperspirant activity correlations, and for controlling the ACH ingredients. In this work, highly repeatable capillary electrophoresis (CE) separation of A_l3⁺, Al₁₃ and Al₃₀ oligomers contained in ACH samples was obtained at pH 4.8, owing to a careful choice of the background electrolyte counter-ion and chromophore, capillary I.D. and capillary coating. This is the first reported separation of Al₁₃ and Al₃₀ oligomers in conditions that are compatible with the aluminum speciation in ACH solution or in conditions of antiperspirant application/formulation. Al₁₃ and Al₃₀ effective charge numbers were also determined from the sensitivity of detection in indirect UV detection mode. The relative mass proportion of Al₁₃ compared to Al₁₃+Al₃₀ could be determined in different aluminum chlorohydrate samples. Due to its simplicity, repeatability/reproducibility, minimal sample preparation and mild analytical conditions, CE appears to be a promising analytical separation technique for the characterization of ACH materials and for the study of structure/antiperspirant activity correlations.