Capillary anions

Figure 8.137 Separation of nucleoside di- and triphosphates together with NAD and NADP by capillary anion-exchange chromatography coupled with high-resolution mass spectrometry. Separator column lonPac AS19 column dimensions 250 mm x 0.4 mm i.d. eluent KOH (EG) gradient 8-40 mmol/L linearly from 0 to 15 min, then to 80 mmol/L in 10 min, to...

The direction of electroosmotic flow and, therefore, the order of elution in CZE can be reversed. This is accomplished by adding an alkylammonium salt to the buffer solution. As shown in Figure 12.45, the positively charged end of the alkylammonium ion binds to the negatively charged silanate ions on the capillary s walls. The alkylammonium ion s tail is hydrophobic and associates with the tail of another alkylammonium ion. The result is a layer of positive charges to which anions in the buffer solution are attracted. The migration of these solvated anions toward... 

Capillary zone electrophoresis also can be accomplished without an electroosmotic flow by coating the capillary s walls with a nonionic reagent. In the absence of electroosmotic flow only cations migrate from the anode to the cathode. Anions elute into the source reservoir while neutral species remain stationary. 

Capillary zone electrophoresis provides effective separations of any charged species, including inorganic anions and cations, organic acids and amines, and large biomolecules such as proteins. For example, CZE has been used to separate a mixture of 36 inorganic and organic ions in less than 3 minutes.Neutral species, of course, cannot be separated. 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

There are several methods of buffer and capillary surface modification used to prevent electrostatic interactions. Two modes have been examined mn buffer with pH >10 in the uncoated capillary and anionic polymer coating capillary, developed in lAI. 

In trne thermospray, charging of the droplets is dne to the presence of a bnffer in the mobile phase. Both positively and negatively charged droplets are formed dne to the statistical flnctnation in anion and cation density occnrring when the liqnid stream is disrnpted. As with the interfaces previonsly described, involatile bnffers are not recommended as blocking of the capillary is more likely to occnr if temperatnre control is not carefnlly monitored and for this reason ammoninm acetate is often nsed. 

Galli, V., Garcia, A., Saavedra, L., and Barbas, C. (2003). Capillary electrophoresis for short-chain organic acids and inorganic anions in different samples. Electrophoresis 24, 1951-1981. 

Figure 10 Capillary ion analysis of 30 anions 1 = thiosulfate, 2 = bromide, 3 = chloride, 4 = sulfate, 5 = nitrite, 6 = nitrate, 7 = molybdate, 8 = azide, 9 = tungstate, 10 = monofluorophosphate, 11 = chlorate, 12 = citrate, 13 = fluoride, 14 = formate, 15 = phosphate, 16 = phosphite, 17 = chlorite, 18 = galactarate, 19 = carbonate, 20 = acetate, 21 = ethanesulphonate, 22 = propionate, 23 = propanesulphonate, 24 = butyrate, 25 = butanesulphonate, 26 = valerate, 27 = benzoate, 28 = D-glutamate, 29 = pentane-sulphonate and 30 = D-gluconate. Experimental conditions fused silica capillary, 60 cm (Ld 52 cm) x 50 p i.d., voltage 30 kV, indirect UV detection at 254 nm, 5 mM chromate, 0.5 mM NICE-Pak OFM Anion-BT, adjusted to pH 8.0, with 100 mM NaOH. (From Jones, W. R. and Jandik, R, /. Chromatogr., 546, 445,1991. With permission.)...

Friedl, W., Reijenga, J. C., and Kenndler, E., Ionic strength and charge number correction for mobilities of multivalent organic anions in capillary electrophoresis, /. Chromatogr. A, 709, 163, 1995. 

Gassner, B., Friedl, W., and Kenndler, E., Wall adsorption of small anions in capillary zone electrophoresis induced by cationic trace constituents of the buffer, /. Ckromatogr., 680, 25, 1994. 

Jandik, P. and Jones, W. R., Optimization of detection sensitivity in the capillary electrophoresis of inorganic anion, /. Chromatogr., 546, 431, 1991. 

Heegard, N. H. H. and Robey, R A., Use of capillary zone electrophoresis to evaluate the binding of anionic carbohydrates to synthetic peptides derived from human serum amyloid P component, Anal. Chem., 64, 2479, 1992. 

He Y. and Lee H.K., Large-volume sample stacking in acidic buffer for analysis of small organic and inorganic anions by capillary electrophoresis, Anal. Chem. 71, 995, 1999. 

Moffatt, F., Cooper, P.A., and Jessop, K.M., Capillary electrochromatography. Abnormally high efficiencies for neutral-anionic compounds under reversed-phase conditions, Anal. Chem. 71, 1119, 1999. 

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