Analytical Sciences

Evaluation of an ODS Column Modified with Zwitterionic/Nonionic Mixed Surfactants and Its Application to Direct Injection Determination of Inorganic Anions

Takuya HASEGAWA,* Tomonari UMEMURA,* Akira KOIDE,* Koichi CHIBA,* Yuji UEKI,** Kin-ichi TSUNODA,** and Hiroki HARAGUCHI*

*Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
**Department of Chemistry, Faculty of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan

An octadecylsilica (ODS) column modified with zwitterionic/nonionic mixed surfactants was evaluated for the direct injection determination of inorganic anions in biological fluids by ion chromatography. A zwitterionic surfactant (sulfobetaine-type) and a nonionic surfactant (polyoxyethylene-type) were used for a stationary-phase modification. When aqueous electrolyte solutions with concentrations of sub-mM to several mM were used as a mobile phase, the zwitterionic surfactant coated on the ODS surface exhibited unique separation selectivity for ionic species, while the nonionic surfactant coated on the ODS might have formed a hydrophilic network over the ODS surface and restricted matrix proteins from adsorbing on the stationary phase. Consequently, the mixed surfactant-modified column system allowed an efficient ion chromatographic separation of inorganic anions as well as a size-exclusive removal of column-fouling proteins. This separation system was applied to the direct injection determination of UV-absorbing anions in human saliva. The detection limits for nitrite, nitrate, iodide and thiocyanate were 3.1, 2.7, 4.5 and 25 µM, respectively, with UV detection at 210 nm (injection volume; 20 µl), and their relative standard deviations for 5 replicate measurements of saliva samples spiked with 100 µM each of those anions were 1.4, 0.9, 2.2 and 5.5%, respectively.

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