Carmen García-Ruiz María Luisa Marina Departamento de Química Analítica, Facultad de Química, Universidad de Alcalá, Alcalá de Henares, Madrid, Spain Received June 8, 2005 Revised September 1, 2005 Accepted September 1, 2005 Review Sensitive chiral analysis by capillary electrophoresis In this review, an updated view of the different strategies used up to now to enhance the sensitivity of detection in chiral analysis by CE will be provided to the readers. With this aim, it will include a brief description of the fundamentals and most of the recent applications performed in sensitive chiral analysis by CE using offline and online sam- ple treatment techniques (SPE, liquid–liquid extraction, microdialysis, etc.), on-column preconcentration techniques based on electrophoretic principles (ITP, stacking, and sweeping), and alternative detection systems (spectroscopic, spectrometric, and electrochemical) to the widely used UV-Vis absorption detection. Keywords: Detection sensitivity / Isotachophoresis / Offline and online sample treat- ment techniques / Review / Sensitive chiral analysis / Stacking DOI 10.1002/elps.200500418 1 Introduction For some applications in the field of chiral analysis by CE it is necessary to achieve a high sensitivity of detection. As examples, it can be mentioned the determination of the enantiomeric purity of a drug, the determination of chiral compounds in biological samples, the analysis of environ- mental samples containing low levels of chiral pesticides or pollutants, and the analysis of food samples. Thus, the analysis of the stereochemical purity of compounds is of critical importance in chiral drug synthesis and develop- ment as well as for quality control of drug substances. In analytical chemistry the enantiomeric impurity (ei) is usually quantified as the percentage of one enantiomer (i.e., the S form) in the mixture (R and S forms), Eq. (1). ei ¼ S= R þ S ð Þ ½  100 (1) Since nowadays, a high percentage of chiral drugs is commercialized as pure enantiomers, the determination of their purity is essential. The ICH guidelines on impu- rities (Topics Q3A and B) (International Conference on Harmonisation, http://www.ich.org; checked on March 2005) can be applied to study the enantiomeric impurities as impurities for achiral drugs. They define certain thresholds for the content of impurities above which they should be identified and/or quantified. These thresholds have recently been revised (February 2002 and 2003) establishing that for drug substances where the max- imum daily dose (MDD) is 2 g/day or below, impurities must be reported if they are present above 0.05%, iden- tified if above 0.10%, and qualified if above 0.15%. Obviously, to accomplish these requirements the sensi- tivity of detection has to be taken into consideration [1–3]. On the other hand, the determination of chiral com- pounds in biological samples (plasma, urine, cere- brospinal fluid, tissues, cells, etc.) is one of the most interesting applications of chiral CE. The analysis of these small mass/volume samples requires appropriate selec- tivity (usually a sample treatment is used to avoid matrix interferences) and sensitivity [4]. In addition, chiral analy- sis of environmental samples by CE is a clear challenge nowadays. The main problem is the high sensitivity required for the detection of herbicides, fungicides, and organic persistent pollutants in environmental samples such as water, soil, and slug [5]. In fact, the levels of pol- lutants in environmental samples are below ppbs levels, that is, molar concentrations lower than 3610 29 M for chiral pollutants with a molecular weight of ,300 g/mol. Finally, the determination of the enantiomeric purity of food components or the analysis of chiral compounds in food samples, which among others, enables obtaining important information on adulterations and food proces- Correspondence: Professor Dr. María Luisa Marina, Departamento de Química Analítica, Facultad de Química, Universidad de Alcalá, Ctra. Madrid-Barcelona Km. 33.600, E-28871 Alcalá de Henares, Madrid Spain E-mail: mluisa.marina@uah.es Fax: 134-91-8854-971 Abbreviations: CM-â-CD, carboxymethylated-b-CD; DM-â-CD, heptakis(2,6-di-O-methyl)-b-CD; FT-IR, Fourier transform infrared; HP-â-CD, 2-hydroxypropyl-b-CD; LLE, liquid–liquid extraction; LPME, liquid-phase microextraction; NACE, nonaqueous CE; QA- â-CD, 2-hydroxypropyltrimethylammonium-b-CD; SC, sodium cho- late; SLE, solid–liquid extraction Electrophoresis 2006, 27, 195–212 195  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com