Determination of inorganic anions in biological fluids with direct sample injection by electrostatic ion chromatography using zwitterionic micelles in both stationary and mobile phases

Abstract

A new ion chromatographic (IC) system, which uses zwitterionic (e.g., Zwittergent 3-14) micelles as both stationary and mobile phases, highly useful for the analysis of inorganic anions in biological samples, was developed. The zwitterionic micellar stationary phase (which is obtained by immobilizing the zwitterionic surfactant on surfaces of the reversed-phase ODS) showed high ability to confine the elution bands of the large amount of SO₄²⁻ and Cl⁻ to narrow zones. As a result, a base-line separation of NO₂⁻, Br⁻ and NO₃⁻ from SO₄²⁻ and Cl⁻ is always achieved. The zwitterionic micellar mobile phase, (which is obtained by dissolving the zwitterionic surfactant with a suppressive electrolytic solution, e.g., aqueous NaHCO₃ solution), on the other hand, showed high ability for rapid elution of proteins. The separation column is therefore always being cleaned up even after the protein-containing sample is directly injected. The zwitterionic micelles are also insensitive to conductivity detection, therefore either the suppressed or the non-suppressed conductivity detection method is applicable for detection of the analyte ions. Urine and serum were chosen as the model real samples and were analysed with direct sample injection; the results of successful determination of a number of inorganic anions (SO₄²⁻, Cl⁻, NO₂⁻, Br⁻ and NO₃⁻) in both samples have demonstrated the usefulness of this new IC system.