(MEKC). MEKC is today considered a major analytical tool in research for the
identification and characterization of pharmaceuticals (Hancu et al. 2013) complementary to CE. MEKC is efficiently combining standard chromatographic principles
with electrophoresis. Usually, suitable surfactants are added to the mobile phase/
buffer above their critical micellar concentrations (CMC). The here-created micelles
undergo electrophoretic migration similar to charged particles and carry, as such,
neutral analytes interacting on the surface of the micelles (Huang et al. 2014). By
adding enantiomeric surfactants, effective chiral selectivity can be easily achieved
(Duan et al. 2012; Silva 2013; Mikuma et al. 2016). By carefully modifying the
buffer composition, significant improvements in sensitivity and selectivity can be
achieved. Mikuma et al. (2016) modified the buffer composition by adding anionic
cyclodextrin (CS) into the micellar buffer solution. By this simple modification, the
sensitivity for the enantiomer-selective separation and detection of the illicit drug
methamphetamine (Meth) could be improved 10,000 times compared to standard
methods. With the here-achieved sensitivity, this method is comparable with HPLC/
UHPLC methods regularly used for forensic investigations. Furthermore, by actively
applying highly sensitive enantiomer-selective methods (as described here) for the
quantitative determination of Meth in human and/or sewage samples, indications for
sources and production pathway can be elucidated and implemented in the forensic
strategies of the respective investigators (Svidrnoch et al. 2014; Tomkova et al.
2017).
One of the earliest applications of surfactants as CPs is reported by Hinze et al.
(1990) who used chiral surfactants as chiral mobile-phase additives, in order to
enhance the resolution of enantiomers. Surfactants have been previously employed
in many different separation science applications, including chromatographic mobile
phases, but reports on chiral mobile-phase additives for the liquid chromatographic
separation of optical isomers have been scarce. Recently, chiral surfactants as CSPs
are getting more and more popular among analytical chemists due to the large
application range and the robustness of these chiral selectors (Fanali 1996; Camilleri
1997; Billiot et al. 1999; El Rassi 2000). Especially, the combination of electrophoresis and micellar electrokinetic chromatography with surfactant CSPs has, thus,
opened many opportunities for enantiomer-selective separation in pharmacology,
medical sciences and product development (Dobashi et al. 2001; Watzig and Gunter
2003; Rizvi et al. 2004; Rizvi and Shamsi 2006; Rizvi et al. 2007; El-Hachemi et al.
2008; Gubitz and Schmid 2008).
For standard HPLC separation, most enantioselective separations were earlier
conducted employing C 18 -based, 5-μm particle-size spherical packing material as
basic substrate. Bile salt surfactants were added including sodium cholate, sodium
deoxycholate and sodium taurocholate. Chiral non-ionic surfactants used comprised
digitonin, dodecyl-β-D-maltoside, octyl-α-D-glucopyranoside and octyl-β-Dglucopyranoside. The usage of bile salts for chromatographic resolution improvement is considered beneficial (Hinze et al. 1990; Billiot et al. 1999; El Rassi 2000).
4.1 Enantiomer-Selective Capillary Electrophoresis
67
identification and characterization of pharmaceuticals (Hancu et al. 2013) complementary to CE. MEKC is efficiently combining standard chromatographic principles
with electrophoresis. Usually, suitable surfactants are added to the mobile phase/
buffer above their critical micellar concentrations (CMC). The here-created micelles
undergo electrophoretic migration similar to charged particles and carry, as such,
neutral analytes interacting on the surface of the micelles (Huang et al. 2014). By
adding enantiomeric surfactants, effective chiral selectivity can be easily achieved
(Duan et al. 2012; Silva 2013; Mikuma et al. 2016). By carefully modifying the
buffer composition, significant improvements in sensitivity and selectivity can be
achieved. Mikuma et al. (2016) modified the buffer composition by adding anionic
cyclodextrin (CS) into the micellar buffer solution. By this simple modification, the
sensitivity for the enantiomer-selective separation and detection of the illicit drug
methamphetamine (Meth) could be improved 10,000 times compared to standard
methods. With the here-achieved sensitivity, this method is comparable with HPLC/
UHPLC methods regularly used for forensic investigations. Furthermore, by actively
applying highly sensitive enantiomer-selective methods (as described here) for the
quantitative determination of Meth in human and/or sewage samples, indications for
sources and production pathway can be elucidated and implemented in the forensic
strategies of the respective investigators (Svidrnoch et al. 2014; Tomkova et al.
2017).
One of the earliest applications of surfactants as CPs is reported by Hinze et al.
(1990) who used chiral surfactants as chiral mobile-phase additives, in order to
enhance the resolution of enantiomers. Surfactants have been previously employed
in many different separation science applications, including chromatographic mobile
phases, but reports on chiral mobile-phase additives for the liquid chromatographic
separation of optical isomers have been scarce. Recently, chiral surfactants as CSPs
are getting more and more popular among analytical chemists due to the large
application range and the robustness of these chiral selectors (Fanali 1996; Camilleri
1997; Billiot et al. 1999; El Rassi 2000). Especially, the combination of electrophoresis and micellar electrokinetic chromatography with surfactant CSPs has, thus,
opened many opportunities for enantiomer-selective separation in pharmacology,
medical sciences and product development (Dobashi et al. 2001; Watzig and Gunter
2003; Rizvi et al. 2004; Rizvi and Shamsi 2006; Rizvi et al. 2007; El-Hachemi et al.
2008; Gubitz and Schmid 2008).
For standard HPLC separation, most enantioselective separations were earlier
conducted employing C 18 -based, 5-μm particle-size spherical packing material as
basic substrate. Bile salt surfactants were added including sodium cholate, sodium
deoxycholate and sodium taurocholate. Chiral non-ionic surfactants used comprised
digitonin, dodecyl-β-D-maltoside, octyl-α-D-glucopyranoside and octyl-β-Dglucopyranoside. The usage of bile salts for chromatographic resolution improvement is considered beneficial (Hinze et al. 1990; Billiot et al. 1999; El Rassi 2000).
4.1 Enantiomer-Selective Capillary Electrophoresis
67