MEKC analysis

Pietta, P., Mauri, P., and Bauer, R. 1998a. MEKC analysis of different Echinacea species. Planta Med. 64, 649 -652. 

CE and CE-ESI-MS were exploited for the analysis of lorazepam (3-hydroxy-1,4-benzodiazepine) and its metabolites in urine [157]. This drug is commonly used for the treatment of anxiety and as a sedative and hypnotic agent. The authors found that 75% of administered lorazepam is excreted in urine as its 3-0-glucuronide. Interestingly, since glucuronidation occurs at the chiral center of the molecule, two diastereoisomers can be formed, which was confirmed by MEKC analysis of urine extracts and also in vitro, via incubation of the drug with human liver microsomes and 5 -diphospho-glucuronic acid as coenzyme. The evidence of stereoselectivity of lorazepam... 

If the molecule of interest is neutral or weakly charged then the sample can be analyzed directly without removing the protein or any treatment using MEKC. Analysis by MEKC can tolerate proteins since the surfactants in the MEKC solubilize them. 

Despite the high sensitivity of amperometric detection, its use in environmental applications has not been exploited accordingly. Nevertheless, a few examples include the MEKC analysis of herbicides in tap water and the CZE and MEKC analysis of aromatic amines in contaminated water samples. ... 

Cationic sm-factants such as tetradecyltrimethyl-ammonium bromide (TTAB), cetyltrimethylammoni-um bromide (CTAB), and cetyltrimethylammonium chloride (CTAC) have also been useful for MEKC analysis. Most cationic sm-factants have an alkyltri-methylammonium group, and their counterions are halides. The addition of cationic surfactants to the backgroimd electrolytes (BGE) caused the reversal of electroosmotic flow (EOF) owing to a positively charged capillary wall on account of the adsorption of cationic sm-factants. As a result of the reversed EOF, the polarity of the electrodes has to be reversed in order to detect the analytes. 

Gotti R, Furlanetto S, Lanteri S, Ohno S, Ragaini A, Cavrini V (2009) Diffaentiation of green tea samples by chiral CD-MEKC analysis of catechins content. ElectrophOTesis 30 2922-2930. doi 10.1002/elps.200800795... 

Figure 11.27 MEKC analysis of urine sample (a) and extracted components by the Wulff-type boronate-functionalized monolithic column (b). Peak identity 5, c)4idine 12, guanosine other, unknown. (Reproduced from ref. 12 with permission. Copyright 2011, Royal Society of Chemistry.)...

Electropherograms of a mixture of phenolic constituents in Echinacea root extracts separated by MEKC analysis. From [44]. 

Pietta P, Mauri P, Bauer R (1998) MEKC Analysis of Different Echinacea Species. Plan-ta Med in press... 

This experiment describes a quantitative analysis for caffeine, theobromine, and theophylline in tea, pain killers, and cocoa. Separations are accomplished by MEKC using a pH 8.25 borate-phosphate buffer with added SDS. A UV detector set to 214 nm is used to record the electropherogram. An internal standard of phenobarbital is included for quantitative work. 

CE was recently used for anthocyanin analysis because of its excellent resolution. This technique has different modes capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), micellar electrokinetic chromatography (MEKC), capillary electrochromatography (CEC), capillary isoelectric focusing (CIEE), and capillary isotachophoresis (CITP)."° CZE is the most popular method for anthocyanin... 

Desiderio, C., Marra, C., and Fanali, S., Quantitative analysis of synthetic dyes in lipstick by MEKC, Electrophoresis, 19, 1478, 1998. 

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. 

There is a recent trend towards simultaneous CE separations of several classes of food additives. This has so far been applied to soft drinks and preserved fruits, but could also be used for other food products. An MEKC method was published (Lin et al., 2000) for simultaneous separation of intense sweeteners (dulcin, aspartame, saccharin and acesulfame K) and some preservatives (sorbic and benzoic acids, sodium dehydroacetate, methyl-, ethyl-, propyl- and isopropyl- p-hydroxybenzoates) in preserved fruits. Ion pair extraction and SPE cleanup were used prior to CE analysis. The average recovery of these various additives was 90% with good within-laboratory reproducibility of results. Another procedure was described by Frazier et al. (2000b) for separation of intense sweeteners, preservatives and colours as well as caffeine and caramel in soft drinks. Using the MEKC mode, separation was obtained in 15 min. The aqueous phase was 20 mM carbonate buffer at pH 9.5 and the micellar phase was 62 mM sodium dodecyl sulphate. A diode array detector was used for quantification in the range 190-600 nm, and limits of quantification of 0.01 mg/1 per analyte were reported. The authors observed that their procedure requires further validation for quantitative analysis. 

The advantages of CE and MEKC (small sample size, high separation efficacy and speed) have been exploited in the analysis of flavonoids too. The results obtained in the analysis of wines have been reviewed earlier [211]. 

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