Chiral pollutants resolution

As in the case of chromatography, a chiral selector is also required in CE for enantiomeric resolution. Generally, suitable chiral compounds are used in the background electrolyte (BGE) as additives and hence are called chiral selectors or chiral BGE additives. There are only a few publications available that deal with the chiral resolution on a capillary coated with the chiral selector in CE. The analysis of the chiral pollutants discussed in this chapter is restricted only to using chiral selectors in the BGE. The most commonly used chiral BGE additives are cyclo-dextrins, macrocyclic glycopeptide antibiotics, proteins, crown ethers, ligand exchangers, and alkaloids.A list of these chiral BGE additives is presented in Table 1. 

Normally, the chiral pollutants in the environment occur at low concentrations and therefore a sensitive detection method is essential and is required in chiral CE. The most commonly used detectors in the chiral CE are UV, electrochemical, fluorescence, and mass spectrometry. Mostly, the detection of the chiral resolution of drugs and pharmaceutical in CE has been achieved by a UV mode and therefore the detection of the chiral pollutants may be achieved by the same method. The selection of the UV wavelength depends on the type of buffer, chiral selector, and the nature of the environmental pollutants. The concentration and sensitivity of UV detection are restricted insofar as the capillary diameter limits the optical path length. It has been observed that some pollutants, especially organochloro pesticides, are... 

Analysis of the chiral pollutants at trace levels is a very important and demanding field. In recent years, capillary electrophoresis has been gaining importance in the direction of chiral analysis of various racemates. A search of the literature cited herein indicates a few reports on the chiral resolution of environmental pollutants by CE. It has not achieved a respectable place in the routine chiral analysis of these pollutants due to its poor reproducibility and to the limitations of detection. Therefore many scientists have suggested various modifications to make CE a method of choice. To achieve good reproducibility. 

CE has been used for the analysis of chiral pollutants, e.g., pesticides, polynuclear aromatic hydrocarbons, amines, carbonyl compounds, surfactants, dyes, and other toxic compounds. Moreover, CE has also been utilized to separate the structural isomers of various toxic pollutants such as phenols, polyaromatic hydrocarbons, and so on. Sarac, Chankvetadze, and Blaschke " resolved the enantiomers of 2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl)propanoic acid using CD as the BGE additive. The CDs used were native, neutral, and ionic in nature with phosphate buffer as BGE. Welseloh, Wolf, and Konig investigated the CE method for the separation of biphenyls, using a phosphate buffer as BGE with CD as the chiral additive. Miura et al., used CE for the chiral resolution of seven phenoxy acid herbicides using methylated CDs as the BGE additives. Furthermore, the same group resolved 2-(4-chlorophenoxy) propionic acid (MCPP), 2-(2,4-dichlorophenoxy) propionic acid (DCPP), (2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chlorophenoxy) propionic acid (2,4-CPPA), [(2,4,5-... 

Today, chromatographic modalities are used frequently for the analysis of chiral pollutants. The wide application of HPLC is due to the development of various chiral stationary phases and excellent reproducibility. However, HPLC suffers from certain drawbacks, as the chiral selectors are fixed on the stationary phase and, hence, no variation in the concentrations of the chiral selectors can be carried out. Moreover, a large amount of the costly solvent is consumed to establish the chiral resolution procedure. Additionally, the poor efficiency in HPLC is due to the profile of the laminar flow, mass transfer term, and possible additional interactions of enantiomers with the residual silanol groups of the stationary phase. GC also suffers from certain drawbacks as discussed in the Introduction. ... 

Chiral resolution in GC is carried out using chiral stationary phases (CSPs) that is, GC columns containing chiral compounds. Many chiral compounds are used for the preparation of chiral stationary phases. The most commonly used chiral compounds are cyclodextrins (CD) and their derivatives. The merit of CD derivatives for chiral resolution in GC is the great spectmm of resolvable classes of compounds. The modified CDs are capable of resolving chiral pollutants over a high range of GC temperatures. 

The separation of the isomers of some environmental pollutants (chlor-danes, toxaphenes etc.) is a challenging job, while the chiral resolution of these pollutants is extremely difficult. A multidimensional gas chromatographic (MDcGC) approach for such types of chiral resolution of environmental pollutants has been proposed as the best choice. The MDcGC technique involves the use of two chiral columns of different polarities in series, each with a separate temperature control. The remarkable advantage of this technique, in both a qualitative and a quantitative sense, is a consequence of the fact that a valveless pneumatic system, which involves a live T-piece, allows a preselected small fraction to be cut from the eluate of the first column and transferred quantitatively and reproducibly to the second column. This technique may be used for the complete separation of chiral pollutants, with increased sensitivity and selectivity. Examples of this technique for the chiral resolution of environmental pollutants can be found in several publications [93-96, 122, 123]. 

The chiral resolution of environmental pollutants by CE depends on the formation of diastereomeric complexes and, therefore, the stmctures and sizes of the chiral pollutants are responsible for their enantiomeric resolution. To study this aspect, phenoxy acid herbicides (see Table 9.4) may be considered as the best class of chiral pollutant. Mechref and El Rassi [40] studied these herbicides using cyclodextrins as chiral selectors. It has been reported that the chiral resolution of these herbicides was in the order 2-PPA > 2,2-CPPA > 2,3-CPPA. 2-PPA has no chlorine atom on the phenyl ring, while 2,2-CPPA and 2,3-CPPA have chlorine atoms in the ortho- and meta- positions, respectively. Therefore, it may be concluded that the chlorine atom creates some sort of hindrance in the formation of diastereomeric complexes. Furthermore, it may be observed that the ortho- position creates a greater strain in comparison to the meta- position in the formation of diastereoisomeric complexes, and hence the above-mentioned order of resolution is observed. Briefly, the steric effect due to... 

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... 

Various approaches to chiral resolution have been developed for the analysis of pharmaceuticals and drugs but, unfortunately, few reports and monographs are available on the chiral separation of pollutants. Therefore, we have set out to write this book, which deals with the distribution, toxicities and art of analysis of chiral pollutants by gas chromatography and liquid chromatography that is, by high performance liquid chromatography (HPLC), sub- and supercritical fluid chromatography (SFC),... 

Ali, L Gupta, V.K. Aboul-Enein, H. Y, Chiral resolution of racemic environmental pollutants by capillary electrophoresis Crit. Rev. Anal. Chem. 2008, 38, 132-146. 

Only a few reports are available in the literature dealing with the limits of the detection for the chiral resolution of environmental pollutants by CE, indicating mg/L to fig/L as the limits of the detection. Tsunoi et al. " carried out an extensive study on the determination of the limits of the detection for the chiral resolution of herbicides. The authors used a 230-nm wavelength for the detection and the minimum limit of the detection achieved was 4.7 X 10 M for 2,4-dichhophenoxy acetic acid. On the other hand, Mechref and El Rassi reported better detection limits, for herbicides, in the derivatized mode, in comparison to the underivatized mode. For example, the limit of the detection was enhanced by almost 1 order of magnitude from 1x10 " M (10 pmol) to 3 x 10 M (0.36 pmol). In the same study, the authors reported 2.5 X 10 M and 1 x 10 M as the limits of detection for the herbicides by fluorescence and laser-induced fluorescence detectors, respectively. 

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