MEEKC Systems

In the last time, MEEKC has become a powerful tool in the analytical laboratory, specially in the determination of a wide range of compounds present in different biological and pharmacentical samples [42,43]. 

The convenience of working with miCToemnlsions is based on their properties thermodynamic stability, optical transparency, and easy preparation that make possible to quantitate drugs of different hydrophobicity, both nentral and ionic compounds in the same run. 

Moreover, MEEKC has been shown to be a highly apphcable system for the analysis of complex mixtures such as mirlticomponent formirlations, related substances of active principles and excipients in birlk drugs, chiral analysis, natural products, and bioanalytical separations. 

Commonly, mictoemulsions used in MEEKC are based O/W systems. A carefully selection of the componaits of a microemulsion should be planned in the development of a MEEKC system. 

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SDS, octane, borate, butan-l-ol MEEKC system with direct detection Neutral and charged solutes 52... 

The log P for 10 reference compounds are given in Table 19.2 together with the measured log k in three MEEKC systems and the Equation 19.10 for each system is included. 

MEEKC systems has been studied for different compounds. Typical microemulsions with SDS (MEEKC-SDS) and MEEKC based on PC and bile salts like sodium cholate (MEEKC-PCSC) and deoxycholate (MEEKC-PCSDC) have been studied. 

The selected compounds studied were the corticoids, betamethasone, and derivatives. Corticoids are a group of drugs with therapeutic action as antiinflammatory, antialergic, and antirheumatic properties. Betamethasone and synthetic derivatives have similar chemical structures (Figure 19.4). Esterification in position 17 and/or 21 leads to compounds with different hydrophobicity and they have been used in different pharmaceutical administrations, log kf for betamethasone and derivatives have been determined in those MEEKC systems and the Equation 19.10 applied for calculation of log P. The measured mobilities and retention factors of the analytes in different MEEKC systems are shown in Table 19.3. 

The log P calculated in the three MEEKC systems (Table 19.4) agree for bethametasone and bethametasone acetate, both for experimental and software values. In the case of bethametasone phosphate, which is in anionic form, the software does not calculate the log and the log P agree in less grade than the others. For bethametasone valerate and dipropionate, the MEEKC systems with... 

Mobilities and Retention Factors for Betamethasone and Derivatives for MEEKC Systems... 

FIGURE 19.5 Electropherograms obtained by MEEKC systems. (1) Betamethasone 21-phosphate, (2) Betamethasone, (3) Betamethasone 21-acetate, (4) Betamethasone 17—valerate, (5) Betamethasone 17-21-dipropionate. (From Lucangioli S. E., Carducci C. N., Sciosda S. L, Carlucci A., Bregni C., Kenndler E., Electrophosesis, 24,984,2003. With permission.)... 

The mobilities of the BA were calculated in the MEEKC system and in free solution (Equation 19.6). Methanol and dodecaphenone were the EOF and microdroplets marker, respectively (Table 19.5). 

Figure 19.8 shows the correlation between log k versus 1/CMC for free BAs. The retention of BAs in MEEKC system seems to reflect their role in living systems as detergents, and probably might serve as a model for estimation of detergency of hydrophobic compounds [38]. 

Another double-chain, anionic, and hydrophobic surfactant is sodium bis(2-ethylhexyl) sulfosuccinate (AOT) used in the development of a novel microemulsion in a MEEKC system for the separation of estrogens with different hydrophobicity in the same run [45]. 

Octane, heptane, hexane, and octanol are the oils used in MEEKC [2]. They show similar selectivity and retention of the analytes. However, octane is the mostly used oil in a MEEKC system. Another oil is isopropyl myristate, employed as cosurfactant with PC and salts of BAs [7,22]. 

The MEEKC technique is effective for the separation of complex mixtures. A typical example is the separation of proteins (33). This separation was carried out using a microemulsion system consisting of SDS-heptane-butane-... 

Separation of antibiotics and cephalosporins can be achieved successfully by CZE because most of them are ionic species. As an alternative to CZE, antibiotics and cephalosporins have been separated by MEEKC. The separation of cephalosporins in different systems (micelles, mixed micelles, and microemulsions) was investigated. The best separation was achieved in microemulsions (Fig. 4). Figure 4 shows that cephalosporins have better affinity to ME in the ME systems than in the MC systems. The affinity of cephalosporins in the ME systems decreases with decrease in the migration time. The MEEKC was also particularly suitable for neutral cephalosporins that could not be separated by CZE or MEKC (14) (see Fig. 5). The method provided good reproducibility and rapid separation with high efficiency. 

Please note that I have not covered the whole of MECC. One notable omission involves the use of surfactants that generate in-situ charge [28]. Another related omission is microemulsion electrokinetic capillary chromatography (MEEKC) [29]. I have not had the opportunity or the need to try these systems. 

The microemulsion system to estimate the CMC of free, glycine, and taurine derivative is MEEKC-SDS. This system employs SDS as surfactant, butanol as cosurfactant, octane as oil and an aqueous buffer at pH 7.50. At that pH, the BAs are in the anionic form. 

It must be noticed that resolution of compounds with different and high hydrophobicity using SDS as surfactant in MEEKC sometimes may be unsuccessful [44], Double-chain structure surfactants such as phosphatydUchoUne achieved a better selectivity compared to the traditional MEEKC-SDS system [7,22]. 

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