Direct determination of bromide, nitrate, and iodide in saline matrixes using electrostatic ion chromatography with an electrolyte as eluent

A new ion chromatographic method that is applicable to the direct determina tion of W-absorbing inorganic anions in saline matrixes is described. An oc tadecylsilica column modified with a zwitterionic surfactant (3-(N,N-dimeth ylmyristylammonio)propanesulfonate) is used as the stationary phase, and an electrolytic solution is used as the eluent. Under these conditions, the m atrix species (such as chloride and sulfate) are only retained weakly and s how little or no interference. It is proposed that a binary electrical doub le layer (EDL) is established by retention of the eluent cations on the neg atively charged (sulfonate) functional groups of the zwitterionic surfactan t (forming a cation-EDL) and by retention of eluent anions on the positivel y charged (quaternary ammonium) functional groups of the zwitterionic surfa ctant (forming an anion-EDL). Sample anions are able to distribute into the cation-EDL and to form ion pairs with the EDL cations, while at the same t ime experiencing repulsion from the anion-EDL. Anions are therefore eluted in order of increased propensity to form ion pairs. The method has been app lied to the determination of bromide, nitrate, and iodide in artificial sea water, giving detection limits of 0.75 ppb for bromide, 0.52 ppb for nitrat e, and 0.8 ppb for iodide using UV absorbance detection at 210 nm and relat ive standard deviations of <1.2%. Real seawater samples have also been anal yzed successfully.