Micellar separation methods

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

Electromigration methods compose a family of analytical separation methods based on differences in the mobilities of charged analytes in the electric field. In this chapter, we discuss mainly such electromigration methods that are performed in thin capillaries with inner diameter (i.d.) <0.1 mm. These methods are commonly known as capillary electrophoretic methods where the most important modes are capillary zone electrophoresis (CZE), micellar electrokinetic capillary chromatography (MEKC), capillary gel electrophoresis (CGE), and capillary electrochromatography (CEC). 

Micellar electrokinetic chromatography has been proven to be a highly efficient separation method for neutral analytes (63,67,68), neutral and charged compounds (69-72), and ionized compounds (67,73) including PTH-amino acids (18). 

Although not used in any of the overall methods found, Fourier transform-infrared spectroscopy for detection after GC can supplement MS to verify the presence of DNOC in samples (Budzinski et al. 1992 Gurka et al. 1991 Schneider et al. 1991). Alternative separation methods have also been shown to be applicable to nitrophenols, including DNOC, but have not yet become routine. These methods include supercritical fluid chromatography (Ong et al. 1992 Pospisil et al. 1992), capillary zone electrophoresis (Chao and Whang 1994), and micellar electrokinetic chromatography (Ong et al. 1991). 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

Other separation methods Sorbitol and their paraben derivatives have been separated by micellar electrokinetic chromatography by using 25 mmol 1 phosphate buffer, pH 7.0, containing lOOmmoll ... 

However, additives are normally combined to complement and promote their activity as a result, it is necessary to develop analytical methods for the determination of additive mixtures. Although some spectroscopic and chemical methods are used, it is preferable to use separation methods for this purpose. Most analytical methods used to determine food additives are based on chromatographic techniques, although several recent papers have demonstrated the usefulness of electrophoresis for the analysis of food colors, sweeteners, antioxidants, and/or preservatives. The separation of food colors has received most attention, with a number of articles published on both capillary zone electrophoresis and micellar electrokinetic chromatography. 

A micellar isolation method using an analytical ultracentrifuge and so avoiding the inherent errors associated with the use of membranes in the dialysis and ultrafiltration techniques, has been proposed by Park and Rippie [70]. In principle the solubilized systems are centrifuged under selected conditions such that about 40 % separation of micelles is achieved. By assuming that the apparent partition coefficient is independent of surfactant concentration, equations are presented whereby the distribution of the solubilizate can be calculated from analysis of the upper and lower portions of the contents of the centrifuge tube. 

Recently a new method was developed for the complete liquid chromatographic separation and diode array detection of standard mixtures of the 14 most frequently used synthetic colorants. Protocols for RP-HPLC - " and IP-HPLC techniques have been extensively described and the techniques were compared with micellar electrokinetic capillary chromatography, - which has been shown to be suitable for the analysis of synthetic colorants. 

Ki is Ki just below the collector s critical micelle concentration, Cs . Kj is Ki at some higher collector concentration, C. E is the relative effectiveness, in adsorbing colligend, of surface collector versus micellar collector. Generally, > 1. F, is the surface excess of collector. More about each K is available [Eemlich, Adsubble Methods, in Li (ed.), Beeent Developments in Separation Seienee, vol. 1, CRC Press, Cleveland, 1972, pp. 113-127 Jashnani and Eemlich, Ind. Eng. Chem. Proeess Des. Dev., 12, 312 (1973)]. 

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

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