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Cromatography Handbook of HPLC - Rizzi A.

Rizzi A. Cromatography Handbook of HPLC - John Wiley & Sons, 2005. - 14 p.
Download (direct link): chromatographyhandbook2005.pdf
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IX. CAPILLARY ELECTROPHORESIS
Since its inception, capillary electrophoresis (CE) has been used primarily for biochemical applications because of its high efficiencies and mild separation conditions. The high efficiencies of CE holds significant potential for the separation of inorganic ions, as evidenced recently by the separation of the isotopes of chloride [209]. However, it is other characteristics of CE that make it attractive for inorganic analyses. These advantages include reduced reagent consumption, inexpensive columns, and quick and easy switching between separation conditions. More significant than all of these is the simple fact that CE separations use a totally different separation process from ion chromatography.
The separation selectivity of CE is based on the ionic mobilities of the sample ions. Therefore, the order of elution of ions is significantly altered from that traditionally seen in IC. For example, fluoride is a difficult ion to quantify in complex matrices by IC because it is weakly retained and so is often obscured by the weakly retained carboxylic acids and the water dip. The ionic mobility of fluoride, however, is significantly different from that of the carboxylic acids, and so in CE fluoride is well resolved from these interferences. Thus, the two techniques can be viewed as complementary, for an analysis that is difficult on one may well be straightforward using the other technique [210].
Analysis of Ions and Inorganic Species 823
Several reviews of CE for the analysis of inorganic and other small ions are available [210-214].
A. Anions
In typical capillary zone electrophoresis operating conditions, anions migrate toward the pos-itive electrode, but are overwhelmed by the electroosmotic flow, such that the anions, as well as neutrals and cations, are detected at the negative end of the capillary. Although this ap-proach is recommended for large, low-mobility ions, such as proteins and peptides, it is not appropriate for the high-mobility inorganic anions. The inorganic anions migrate more rapidly than the electroosmotic flow and thus would not be detected using the typical capillary zone electrophoresis conditions. Thus, in CE of inorganic and other highly mobile anions, the sample is injected at the cathode and detection occurs at the amode, a reversal of the typical configuration. Cationic surfactants, such as cetyltrimethylammonium bromide (—“ņ¬), tetra-decyltrimethylammonium (TTAB), diethylenetriamine (DETA), and hexamethylammonium bromide are added to the electrolyte to reverse the electroosmotic flow and thus accelerates the overall analysis.
Under these conditions the earliest detected anions are small inorganic mono- and divalent anions, with those inorganic species that possess larger Stokes radii eluting later. The migration rates of ions can be predicted using the ionic equivalent conductances of the ions [215]. The selectivity and resolutions achieved for inorganic ions is affected by the concentration of the background electrolyte, the pH, the concentration of the electroosmotic flow modifier, and the mobility of the primary electrolyte component [215,216]. If the electrolyte mobility differs significantly from that of the analyte, peak distortion will be observed [217]. Thus, the optimal electrolyte should match the mobility of the analytes of interest: chromate and pyromellitic [216] acid for most common inorganic anions that possess high electrophoretic mobilities; /7-hydroxybenzoate for moderate mobility anions; and sorbate for slow-migrating anions (i.e., those eluting after bicarbonate) [218].
Detection is typically accomplished using indirect UV absorption, wherein the decrease in absorbance observed when an analyte ion displaces a chromophoric ion in the electrolyte is monitored. Thus, strongly absorbing buffers, such as chromate. are most commonly used as the electrolyte. Methods for the determination of sub-ppm to ppm quantities of fluoride, chloride, bromide, nitrite, phosphate, and sulfate in drinking water, wastewater, and reagent water [219] and for 0.5- to 50-ppb quantities of the same anions in high-purity deionized water and nuclear reactor moderator water [220] have been submitted to regulatory agencies for approval. For specific applications of CE for inorganic ion analysis, the reader is referred to the review articles [210-214] and to the annual proceedings of the International Ion Chromatography Symposium [9-15].
B. Cations
Inorganic and low molecular weight cations are separated by capillary electrophoresis using the typical electrophoretic configurationóinjection at the anode and detection at the cathode. By using untreated capillaries, the cation migration is in the same direction as the electroos-motic flow and rapid high-efficiency separations result. Thus, the relative migration rates for the cations can be deduced from the associated equivalent conductances [221]. Many metal cations possess similar ionic mobilities, making their separation difficult. In such cases modification of the mobilities through weak complexation of the metal with a complexing agent, such as a-hydroxyisobutyric acid (HIBA), can be used to achieve resolution [222]. The migration rate of amines can be adjusted by altering the electrolyte pH [221].
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