EOF modifier

Separation is performed using free-zone electrophoresis, where the capillary is filled with a separating buffer at a defined pH and molarity. This buffer is also called a BGE. During separation, the polarity is set to cathodic or anodic mode, also called normal and reverse mode, depending on the charge of the molecule cation or anion. For anions, the capillary is usually dynamically coated with an electroosmotic flow (EOF) modifier to reverse the EOF and separate the analytes in the co-electroosmotic mode. 

The low electroosmotic flow (EOF) of the PMMA chip material facilitated the rapid switching between analyses of explosive-related cations and anions using the same microchannel and run buffer (and without an EOF modifier) [29], This led to a rapid (<1 min) measurement of seven explosive-related cations and anions down to the low micromolar level. The presence of an 18-crown-6 ether modifier in the run buffer allowed separation of the peaks of the co-migrating ammonium and potassium ions. 

Various EOF modifiers were compared [23]. The size of the alkyl ammonium ion affects not only the EOF, but also determines the optimum concentration range of the surfactant which decreases with increasing ion size. When using high-molecular-mass additives, e.g. HDM, a good efficiency can be obtained even at very low surfactant concentrations (0.0001%). This is important for two reasons first,... 

Fig. 3. Trace determination of some inorganic and organic anions in pure water, after an electrophoretic enrichment at 5 kV for 45 s with an addition of 75 p,M octanesulfonate to the sample [42]. The electrolyte 10 mM sodium chromate and 0.5 mM OFM-BT (a surfactant used as the EOF modifier), adjusted to pH 8 with sulfuric acid 15 kV 60 cmX75 fjim I.D. capillary, distance to detector, 52 cm UV photometric detection at 254 nm. Anions (concentrations in mg/1) 1, chloride (3.5) 2, sulfate (4.8) 3, nitrate (6.2) 4, oxalate (5) 5, fluoride (1.9) 6, formate (5) 7, phosphate (3.2) 8, acetate (5) 9, propionate (5).

Fig. 4. Analysis of an anion standard solution by IC (a) and CE (b) [48]. IC conditions aVydac 302IC4.6 column, a flow-rate of 2.5 ml/min, an injection volume of 25 xl, an isophthalic acid mobile phase, UV detection at 280 nm. CE conditions an electrolyte of potassium dichromate, sodium tetraborate, boric acid and the DETA (diethylenetriamine) EOF modifier, pH 7.8 65 cmX75 xm I.D. capillary 20 kV indirect UV detection at 280 nm. Anions 1, chloride 2, nitrite 3, chlorate 4, nitrate 5, sulfate 6, thiocyanate 7, perchlorate 8, bromide.

Sorbic acid has also been shown to be a suitable probe for the analysis of the alkylphosphonic acids and their monoester derivatives (14). A run buffer consisting of 5 mM sorbic acid and 0.1 mM decamethonium bromide, adjusted to pH 6, was used to separate a mixture of eight alkyl alkylphospho-nates. The use of the EOF modifier decamethonium bromide reduced the EOF at pH 6 to enhance the resolution and gave complete separation of the alkylphosphonic acids and their monoester derivatives (e.g. MPA and IMPA) in less than 15 minutes, as shown in Figure 3. 

To this aim, Hargadon and McCord used CZE (65 cm x 75 mm i.d. capillary) in borate buffer (2 miW borate, 40 miW boric acid) containing 1 mJW diethylenetriamine as EOF modifier, at the final pH of 7.8. The applied potential was 20 kV, with reversed polarity. The addition to the running buffer of a dichromate chromophore (1.8 mM) permitted the use of indirect UV detection at 280 nm. 

Phthalate, myristyltrimethylammonium bromide type EOF modifier Chromate,... 

UV absorbing complexes Cyanide Inorganic cations Involves the formation of metal-ligand complexes The resultant negatively charged complexes require an appropriate electrolyte and an EOF modifier... 

Besides the parameters discussed above, some other factors can also be used to optimize chiral resolution by CE. These parameters include the reversal of polarity, the volume of sample injected, the use of EOF modifiers and pre-derivatization of the chiral pollutants with a suitable reagent. In the normal CE machine, the anode (+) and cathode (—) are always at the inlet and outlet ends, respectively. In this modality, the EOF always tends to travel towards the cathode (detector). On the other hand, in the reverse mode, the direction of the EOF is away from the detector, and hence only negatively charged diastereomeric complexes with an electrophoretic mobility greater than the EOF will pass the detector. This format is typically used with capillaries that are coated with substances that reverse the net charge of the... 

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

A wide choice of cationic surfactants such as CTAB (cetyltrimethylammonium bromide), CTAH (cetyltrimethylammonium hydroxide), TTAB (tetradecyltrimethylammonium bromide), TTAOH (tetradecyltrimethylammonium hydroxide), MTAB (myristyltrimethylammo-nium bromide), OFM (OFM Anion-BT, Waters, Milford, MA, USA), HDB (hexadimethrine bromide), and many others may be used to reverse the EOF. CTAH and TTAOH should be preferred to CTAB and TTAB to avoid interference from bromate contamination. The capillary coating is performed just by rinsing with the BGE containing this flow modifier or even with an additional rinse step with a solution containing this flow modifier. 

From Eq. (13.3), it is clear that neutral molecules will have a net velocity. In normal electrophoresis, cations will migrate faster than neutrals, and neutrals will migrate faster than anions. Anions are electrophoretically migrating in a direction opposite to EOF. Separations of neutral molecules, such as organic explosives, can only be achieved by using buffer additives, such as micelles, ionic cyclodextrins, and bile salts. The interaction of neutral analytes with these ionic buffer additives results in a modified mobility that enables separation. 

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