Investigation of electrostatic ion chromatography for the separation of inorganic ions

The new technique of ‘electrostatic ion chromatography’ (ion chromatography using a zwittenomc stationary phase) has been applied to the separation of ions using pure water as an eluent, without the addition of any inorganic buffers or organic modifiers. The nature of the separation, le cationic or anionic, is dependent upon the nature of the zwittenomc stationary phase. In the work presented here, the zwittenomc surfactant Zwittergent 3-14 was used to functionalise an octadecylsihca stationary phase with its subsequent application to the separation of common inorganic anions, using direct conductivity detection and/or UV absorption detection. Ion redistnbution, a major drawback of electrostatic ion chromatography, has been eliminated by development of an off-line sample pre-treatment procedure, employing a cation exchange resin in the sodium form. This resulted in the quantitative exchange of analyte cations to sodium ions. When the method was applied to water samples, such as tap water, mineral water or river water, the vanous mono- and divalent cations present in the water samples were quantitatively exchanged, resulting in single ion - pairs for all analyte anions and hence a simpler separation was achieved. The beneficial effect this had upon peak retention, resolution and efficiency is descnbed and explained. Another advantage of electrostatic ion chromatography is that as water can be used as the eluent, the entire system can be set up in such a way that the eluent can be constantly recycled. This negates the need for re-coating of the column due to ‘column bleeding’, and hence improves the problem of poor reproducibility, inherent in any method based on dynamic coating Work here employed the use of post-detector cation and anion exchange columns in the acid and hydroxide forms to remove sample anions and cations from the eluent stream, allowing complete recycling of the water eluent. In addition, since Zwittergent 3-14 passed undetected and unretamed through the complete chromatographic system, small amounts could be added to the water eluent. When this eluent was recycled, the resulting system provided superior separations and much improved reproducibility for all anions tested With the addition of a UV absorption detector, the method was successfully applied to the determination of chloride, sulphate and nitrate in drinking and river waters. The developed method proved extremely sensitive Detection limits were found to be in the ppb range for the common inorganic anions such as sulphate, chloride, nitrite, nitrate and iodide When the method was compared to standard ion exchange chromatography, the agreement was excellent. The concentration of nitrate in a sample of mineral water was found to be 0 18 mM by electrostatic ion chromatography and also by ion exchange chromatography, while the level of chloride in the same sample was found to be 1 17 mM by ion exchange chromatography and 1 18 mM by electrostatic ion chromatography. The method was also suited to the analysis of complex matrix samples such as seawater where the high concentration of chloride salts would normally cause problems for ion chromatographic techniques. Using electrostatic ion chromatography, iodide was successfully separated from chlonde in samples of iodised sodium chloride table salt. The trace iodide in the sample was easily quantified using direct UV detection. Application of the developed method was shown with the determination of iodide at a concentration of 77 jig/L in a sample solution containing 20 g/L of sodium chlonde