Review Theron J. Pappas Melissa Gayton-Ely Lisa A. Holland Department of Chemistry, West Virginia University, Morgantown, WV, USA Recent advances in micellar electrokinetic chromatography This review contains nearly 200 reference citations, and covers advances in electro- kinetic capillary chromatography based on micelles, including stabilized micelle com- plexes, polymeric and mixed micelles from 2003–2004. Detection strategies, analyte determinations, and applications in micellar electrokinetic capillary chromatography (MEKC) are discussed. Information regarding methods of analyte concentration, analyte specific analyses, and nonstandard micelles has been summarized in tabular form to provide a means of rapid access to information pertinent to the reader. Keywords: Micellar / Electrokinetic capillary chromatography / Review DOI 10.1002/elps.200410191 Contents 1 Introduction ........................... 719 2 Surfactants and additives ................ 720 3 Detection ............................. 721 3.1 Absorbance ........................... 721 3.2 Fluorescence .......................... 721 3.3 Electrochemical ........................ 721 3.4 Mass spectrometry ..................... 721 3.5 Other methods of detection .............. 722 4 Applications ........................... 722 4.1 Pharmaceuticals ....................... 722 4.2 Natural sources ........................ 722 4.3 Environmental analysis .................. 723 4.4 Forensic analysis ....................... 723 4.5 General product analysis ................ 723 5 Future directions ....................... 723 6 References ............................ 725 7 Addendum ............................ 728 1 Introduction Since the first report of capillary electrophoresis modified by the addition of micelles in the background electrolyte [1], micellar electrokinetic capillary chromatography (MEKC) has been defined, redefined, furiously “opti- mized”, and expanded to a host of applications. MEKC, a hybrid of electrophoresis and chromatography, incorpo- rates the ability of certain surfactants to spontaneously form aggregates (micelles). Advantages of MEKC include high efficiency, fast analyses, and a powerful flexibility in rapidly tuning or changing the running buffer composition and subsequently the selectivity of the separation. Many recent tutorials and reviews have been published in the area of MEKC [2–8], or as a means of advancement in fields which have incorporated MEKC applications [9–14]. In this journal, a review covering MEKC advances and progress was published in 2002 [2]. The purpose of this review is to continue the thorough discussion of advances and progress of Molina and Silva [2] and to summarize recent applications of the method. This review covers additives, methods of detection, and applications in MEKC from 2003–2004. Information regarding con- centration methods and applications has been summa- rized in tabular form to provide a means of rapid access to information pertinent to an individual reader. This review is strictly focused on the use of micelles, including stabilized micelle complexes, polymeric, and mixed micelles. As a further note, the related field of micellar emulsion electro- kinetic chromatography has been thoroughly reviewed recently by others [15, 16] and the diversity in characteri- zation and recent advances are not covered here. Correspondence: Dr. Lisa A. Holland, Chemistry Department, West Virginia University, 217 Clark Hall, P.O. Box 6045, Morgan- town, WV 26506, USA E-mail: [email protected]Fax: 1304-293-4904 Abbreviations: CHAPSO, 3-[(3-cholamidopropyl)-dimethylam- monio]-2-hydroxy-1-propanesulfonate; DM-â-CD, dimethyl-b- CD; FITC, fluorescein isothiocyanate; HP-â-CD, hydroxypropyl- b-CD; MAGF , micelle affinity gradient focusing; Me-â-CD, methyl-b-CD; PFOA, perfluorooctanoic acid; TM-â-CD, heptakis (2,3,6-tri-O-methyl)-b-cyclodextrin Electrophoresis 2005, 26, 719–734 719 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim CE and CEC
Transcript
Review
Theron J. PappasMelissa Gayton-ElyLisa A. Holland
Department of Chemistry,West Virginia University,Morgantown, WV, USA
Recent advances in micellar electrokineticchromatography
This review contains nearly 200 reference citations, and covers advances in electro-kinetic capillary chromatography based on micelles, including stabilized micelle com-plexes, polymeric and mixed micelles from 2003–2004. Detection strategies, analytedeterminations, and applications in micellar electrokinetic capillary chromatography(MEKC) are discussed. Information regarding methods of analyte concentration,analyte specific analyses, and nonstandard micelles has been summarized in tabularform to provide a means of rapid access to information pertinent to the reader.
Since the first report of capillary electrophoresis modifiedby the addition of micelles in the background electrolyte[1], micellar electrokinetic capillary chromatography(MEKC) has been defined, redefined, furiously “opti-mized”, and expanded to a host of applications. MEKC, ahybrid of electrophoresis and chromatography, incorpo-rates the ability of certain surfactants to spontaneouslyform aggregates (micelles). Advantages of MEKC includehigh efficiency, fast analyses, and a powerful flexibility inrapidly tuning or changing the running buffer compositionand subsequently the selectivity of the separation.
Many recent tutorials and reviews have been published inthe area of MEKC [2–8], or as a means of advancement infields which have incorporated MEKC applications [9–14].In this journal, a review covering MEKC advances andprogress was published in 2002 [2]. The purpose of thisreview is to continue the thorough discussion of advancesand progress of Molina and Silva [2] and to summarizerecent applications of the method. This review coversadditives, methods of detection, and applications inMEKC from 2003–2004. Information regarding con-centration methods and applications has been summa-rized in tabular form to provide a means of rapid access toinformation pertinent to an individual reader. This review isstrictly focused on the use of micelles, including stabilizedmicelle complexes, polymeric, and mixed micelles. As afurther note, the related field of micellar emulsion electro-kinetic chromatography has been thoroughly reviewedrecently by others [15, 16] and the diversity in characteri-zation and recent advances are not covered here.
Correspondence: Dr. Lisa A. Holland, Chemistry Department,West Virginia University, 217 Clark Hall, P.O. Box 6045, Morgan-town, WV 26506, USAE-mail: [email protected]: 1304-293-4904
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2 Surfactants and additives
Micelles are formed above a critical micelle concentration(CMC) when it is thermodynamically more favorable for asurfactant molecule to accept an orientation in which theirnonpolar tails (or regions) and polar groups respectivelyalign. At the CMCa specificnumberof surfactant molecules,the aggregation number (AN), align in this ordered orienta-tion and form an aggregate structure. By far, the mostreported surfactant is the anionic sodium dodecyl sulfate(SDS). Sodium deoxycholate (DOC) and sodium cholate,which are bile salts, were also reported in 2003–2004 [17–20]. Cationic surfactants used in MEKC including CTABcontinue to be reported [21] as do zwitterionic surfactants:CHAPS [22, 23] and 3-[(3-cholamidopropyl)-dimethyl-ammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) [23].Alternative surfactants or additives continue to be explored.See Tables 1 and 3 (Addendum) for a collection of thesesurfactants used in 2003–2004. For example, per-fluorooctanoic acid (PFOA) is particularly useful for MEKC-MS [24]. Mixed micelles and micelle complexes are anotherinterestingoption inMEKC.Theirusehas led tomorediversepseudostationary phases [17, 25–35]. Polyelectrolyte-sur-factant complexes (Fig. 1a) provide additional stability tomicelles in the BGE, which results in improved reproducibil-ity, and can be utilized at low concentrations [36, 37].Bilayered micelles, recently introduced to MEKC (Fig. 1b)are an exciting alternative to bilayered aggregates, such asvesicles and liposomes [26, 27]. They are highly stable andprovide excellent interaction for peptides with bilayerinduced secondary structure [27].
Polymer micelles (high-molecular-weight micelles)(Fig. 1c) are a further variation on MEKC. Since 2003, theliterature regarding polymer micelles has focused on theircharacterization [38–44] and applicability [4, 36, 43, 45–51]. Polymer micelles have notably different character-istics than conventional micelles. Polymer micelles do nothave a CMC and can be used below the CMC of theirmonomer counterparts because they are held together bycovalent bonds. This allows the use of lower-ionic-strength solutions, which diminishes Joule heating andincreases reproducibility. The inherent stability in polymermicelles allows them to be used in a wider range of bufferconditions, especially those with increased amounts oforganic modifiers. This greater stability also reduces thepossibility of unwanted competitive interactions betweensolutes and partial micelles or individual surfactant mole-cules that may have degraded. The stability, consistentsize, and high molecular weights of polymer micellesmake them more compatible with MEKC-MS coupling[52]. Polymer micelles can be screened for micelles of aparticular size or molecular weight, which allows for morereproducible conditions. Polymer micelles are alsodiverse (see Table 1, Addendum). Chiral and achiral poly-
Figure 1. (A) Depiction of a water-soluble polyelectrolytemolecule consisting of polyacrylic acid and alkyl-trimethylammonium salts, used as an alternative pseu-dostationary phase for the MEKC separation of variousphenols. Reproduced with permission from [37]. (B)Depiction of a bilayered micelle (bicelle) resultingfrom 1,2-dihexanoyl-sn-glycero-phosphocholine (DHPC)(edges) and 1,2-dimyristoyl-sn-glycero-phosphocholine(DMPC) bilayer. These membrane-like molecules wereused for the separation of peptides, proteins andb-blockers. Bicelles in EKC show promise as a new wayto study membrane-based interactions. Reproduced withpermission from [26]. (C) Generic structure of a polymersurfactant, based on an amino acid functional group.Both the L- and D-isomers of these surfactants in thepresence of various cyclodextrins were used in theseparation of three binaphthyl derivatives. Reproducedwith permission from [40].
mers can be combined to form copolymers at variousratios for more selective separations of enantiomericcompounds [38, 41, 43, 53].
Further flexibility in MEKC in regard to the separationmechanisms can be attained by including additional addi-tives. Cyclodextrins (CDs) are frequently used in conjunc-tion with micelles. During 2003–2004 there were manyreports of MEKC combined with CDs: a-CD [54, 55], b-CD[18, 19, 40, 54–64], methyl-b-CD (Me-b-CD) [65], dimethyl-b-CD (DM-b-CD) [66], heptakis(2,3,6-tri-O-methyl)-b-CD(TM-b-CD) [56], hydroxypropyl-b-CD (HP-b-CD) [64, 66,
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67], mono-3-O-phenylcarbamoyl-b-CD [59], sulfated b-CD [62, 65, 68], and g-CD [40, 54, 55, 69, 70]. Organic sol-vent additives can alter analyte selectivity, which may bebeneficial in separating highly hydrophobic components.Reported solvent additives have included methanol [20,29, 31, 32, 61, 71–77], ethanol [78–80], propyl alcohol [60,81–90], acetonitrile [31, 32, 46, 64, 79, 91–102], or tetra-hydrofuran [103, 104]. Ionic liquids have also been used asa solvent additive [105]. There are a few examples of otheradditives for additional separation mechanisms, 18-crown-6 [106], or additives that interact with the capillarysurface [79, 90]. For a general picture of how these addi-tives are used in tandem, see Table 3 (Addendum).
3 Detection
3.1 Absorbance
The most reported method of detection for MEKC is ab-sorbance detection in the UV region. UV absorbancedetection is applicable to most molecules, provides a goodlinear range and is easily interfaced with conventionalfused-silica capillary. The drawback, of course, is thedetection limit. Physical modification of the detection pathlength via the use of a bubble cell has been implemented inthe time frame of this review [79, 107, 108]. Generally, theuse of this larger path-length detection cell improves thedetection limit approximately 3-fold. Off-line sample pre-concentration has been employed to alleviate this issuethrough the use of solid-phase extraction [60, 64, 81, 83,109–115], solid-phase microextraction [85], liquid extrac-tion [60, 64, 81], and poly(dimethylsiloxane) (PDMS) facili-tated sorptive extraction-liquid desorption [107]. On-capil-lary preconcentration continues to be applied throughstacking, which relies on conductivity difference, andsweeping, which is based on analyte concentration withinmicelles that sweep a micelle free injection zone. From2003–2004, stacking [93, 96, 116–118] and sweeping [31,32, 34, 56, 63, 71, 72, 91, 119]havebeenutilized fora host ofapplications for improvement in detection limit (Table 2,Addendum). Also during the time frame of this reviewmicelle affinity gradient focusing (MAGF), a focusing meth-od based on temperature-dependent viscosity differencescombined with micelle-sweeping was reported for conven-tional capillary electrophoresis [78]. This communicationdemonstrated a 27-fold concentration increase for a 30sinjection of anthracene [78]. Preconcentration techniquesare summarized in Table 2 (Addendum).
3.2 Fluorescence
Fluorescence detection, also coupled to MEKC, pro-vides better detection limits than absorbance-baseddetection due to fundamental differences between these
techniques. Fluorescence detection typically requiresderivatization with a fluorescent moiety. Examplesinclude coupling with BODIPY [74, 75, 90], 7-fluor-4-nitrobenzo-2-oxa-1,3-diazole (NBD-F) [73, 90], 2,3-dicarboxaldehyde (NDA) [61, 67, 120], 3-(2-furoyl)quino-line 2-carboxaldehyde (FQ) [17, 100, 121], 4-chloro-7-nitrobenzo[1,2,5]-oxadiazol (NBD) [122] 5-TAMRA [89],and FITC [123]. Applications based on native fluorescentare possible [31, 124, 125]. In one instance, detection ofnatively fluorescent lysergic acid diethylamide (LSD) wasperformed at a low temperature to verify the analytepeak was LSD via fluorescence scanning [32]. This wasaccomplished by stopping the MEKC separation whenanalyte reached the detection window and exposing thecapillary to liquid nitrogen in a specially designeddetection cell coupled to CE [71]. Excitation light is gen-erally provided by a laser, such as an argon ion [61, 67,73–75, 89, 100, 121–123, 125] or He/Cd [67, 124], al-though Xe lamps [31, 71] and light-emitting diode (LED)sources [120] have been reported. During the time frameof this review there have been several reports of MEKC-fluorescence and the detection limits from laser sourceshave been at the submicromolar [122] and sub-nanomolar [100, 123] levels. When combined with cati-on-selective exhaustive sweeping, an LED light sourcehas provided detection limits as low as subnanomolar[120].
3.3 Electrochemical
Electrochemical detection coupled to MEKC also gen-erally provides better detection limits than absorbancedetection. Amperometric detection has been applied tobiogenic amines in rice spirits [126], pesticides [127], andhomogenized fruit fly (heads or bodies) [128]. For thesereports, detection limits were submicromolar [126, 127]. Ithas also been operated in a redox-cycling mode using aninterdigitated array [129]. Finally, an electrochemical arraycombined with channel electrophoresis was used to ana-lyze catecholamines [130].
3.4 Mass spectrometry
Mass spectrometry provides structural information andis helpful in further verifying the identity of an analytepeak. Surfactant commonly used in MEKC, such asSDS, is generally incompatible with the MS instru-mentation. This has been obviated through partial filling-MEKC. In this method, the detection end of theseparation capillary is filled with running buffer contain-ing no micelle, while the injection end of the capillarycontains a plug of micelle modified running buffer. Theseparation is halted before the plug containing micelles
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is introduced into the MS. The approach requires someoptimization, but has been successfully implementedduring the time frame of this review [29, 98, 131]. Alter-native surfactants have been introduced, which arecompatible with MS. The fluorinated surfactant PFOA isvolatile, has a low CMC (reportedly 12 mM), and hasbeen applied for MEKC-MS analysis at concentrationsas high as 100 mM [24]. Polymerized micelles have noCMC value and significantly higher molecular masses.These additives have been successfully coupled withMS [52]. Triaxial interfaces, which couple surfactant freesheath flow with MEKC running buffer prior to introduc-tion on the MS, continue to be in use [132]. A modifiedlow-flow sheath interface was introduced during thetime frame of this review which allowed coupling oflower concentration SDS (25 mM) with ESI-MS [133].
3.5 Other methods of detection
Other spectroscopic methods of detection have beencoupled with MEKC in 2003 and 2004. Fourier transforminfrared (FTIR) detection can provide structural informa-tion which is particularly useful in identifying unknownanalyte peaks in a nondestructive manner. The method,previously interfaced with CE [134] is based on the use ofa 10067 mm channel in a CaF2 substrate interfaced with afused-silica capillary [135]. SDS micelles reportedly didnot interfere with detection and the system provideddetection limits in the mM range [135].
Applications of MEKC-chemiluminescence remain in theliterature. Chemiluminescence facilitated by derivatiza-tion with N-(4-amino-butyl)-N-ethylisoluminal (ABEI) wasreported during the time frame of this review [136]. Themethod was based on post column reaction followingMEKC and provided detection limits as low as 35 nM forbiogenic amines [136]. Electrochemiluminescence facili-tated by the end-column electrochemical oxidation ofRu(bpy)3
21 which forms the active agent inducing chemi-luminescence (by its reaction with the analyte of interest)was also reported [137].
Advances in the use of near field thermal lens detectioncoupled to MEKC continue. The method relies on meas-urement of nonradiative relaxation (heat) induced by alight source (pump beam), which is facilitated with a sec-ond laser (probe beam) [138–140]. The application report-ed during the time frame of this review incorporated afrequency doubled argon ion laser (120 mW, 257 nm) forthe detection of etoposide, which is natively fluorescent(Ex ,257 nm). The method provided detection limits5-fold lower than that obtained with UV absorbance(214 nm).
4 Applications
MEKC has been tailored for a number of applications. Themethodology of these analyses has been fine tuned toaddress the application at hand, for example, in terms ofsample preparation or the running buffer composition.Physicochemical parameters continue to be addressedby MEKC [41]. Constituent analysis in the areas of phar-maceuticals, natural products, forensics, and environ-mental samples from 2003–2004 demonstrate the cap-ability of MEKC to handle diverse analytical criteria. Asummary of analyte specific MEKC analyses from 2003–2004 is listed in Table 3 (Addendum). This table is limitedto reports developed for, or with the intention of, a partic-ular application. Running buffer recipes, detection, anddetection limits are outlined in more than 140 references.Most analyses are accomplished using SDS as the sur-factant, and UV-visible absorbance as the primary modeof detection.
4.1 Pharmaceuticals
During the time frame of this review, a majority of theapplications have centered on physiological interaction,including drug analyses, natural products studies andbiomarker assay. Drug determinations have beenaccomplished in plasma [117, 138, 141, 142], urine [60,68, 81], serum [79, 117, 143–145], cerebral spinal fluid[141], and cell lines [125]. Impurity (and purity) analysis ofdrugs [18, 59], preparations [146], and tablets [102, 147–149] have also been demonstrated with MEKC. Otherreports have demonstrated means of determining me-tabolites [60, 68, 81, 117, 125], degradation products [92]and constituent analysis [65, 94, 150, 151]. Drug enantio-mers are a pervading topic in pharmaceutical research,and MEKC separations of drug enantiomers have beendemonstrated during the time frame of this review [48, 59,60, 62, 65, 144].
Although, not strictly related to pharmaceutical analyses,the determination of biomarkers of stress [119, 152], dis-ease [64, 74, 75, 99, 115, 153], or the progress of clinicaltreatment [55, 154] in biological fluids have been accom-plished. In one study, 81 clinical cases were investigatedby MEKC to determine the validity of using DNA methyl-ation with prognostic outcome for patients suffering fromchronic lymphocytic leukemia [74].
4.2 Natural sources
Ethnobotanicals and efficacious compounds from naturalsources have been addressed by MEKC. This includesthe analysis of plant materials in the form of medicinalpreparations [35, 82], herbs [155], fruit [95, 114, 156], fruit
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Figure 2. Separation of 17 plant secondary metabolitestandards: coumarin, verbenone, camphor, eucalyptol,carvone, a-terpineol, linalool, jasmone, bergapten, roseoxide, geraniol, t-anethole, citronellal, citronellol, p-cym-ene, limonene, caryophyllene, and nerol, with 20 mM tet-raborate buffer, 20 mM SDS, 20 kV, and at pH 9.4. Theoptimal conditions for this separation were calculatedbased on partition coefficients and solute-micelle asso-ciation constants. Reprinted from [158], with permission.
constituents [30, 33, 91], root [77], root bark [84], bark[157], leaves [157], raw plants [88, 96, 103], essential oil[158], and tea [80, 108, 159, 160]. Prior to introduction onthe MEKC system, the sample generally undergoes asimple alcohol extraction [35, 77, 82, 84, 91, 95, 155–157], although some samples required different proce-dures [30, 33, 96, 103]. The determined constituents areprimarily phenolic [30, 33, 35, 77, 80, 84, 95, 96, 108, 156,157], and UV-visible absorbance detection has been usedfor quantification. The capability of MEKC to handle thediverse compounds in natural sources is demonstrated inFig. 2.
4.3 Environmental analysis
Herbicides and pesticides and their metabolites havebeen separated using MEKC. With sample pre-concentration, such as solid-phase extraction, manymethods can be used for determinations at ppb or lowerlevels [83, 85, 109–111]. Triazine herbicides were addres-sed most during the time frame of this review [85, 109,111, 127, 161]. One report outlined methodology for adiverse set of estrogens and phenols [97]. The workdemonstrates the potential of MEKC to handle the diversechemicals either suspected or known to be involved inendocrine disruption. Much work has been designed forwater analysis [83, 85, 97, 109–111, 127, 136, 162]. Anal-ysis of pesticides in air collected from a greenhouse wasreported [112]. In another report, aldehydes in indoor air
were determined by passing air through a solution of3-methyl-2-benzothiazoline hydrazone (MBTH) for 2.5 h.The aldehydes reacted with the MBTH to form azinesdetected by UV-visible absorbance detection [163]. Pes-ticides in carrot [76] were also reported.
4.4 Forensic analysis
Illicit drugs and impurities have been addressed. LSD hasbeen determined by MEKC in tablets [32] and its half-lifein blood following administration to mice has also beenreported [31]. An interesting report outlines an MEKCmethod designed for the identification of impurities,adulterants, and diluents in heroin samples [66]. Gun-powder residue [164] and munitions components [165]have also been reported by MEKC, although in the lattercase, MEKC was deemed less reliable than HPLC.
4.5 General product analysis
Finally, in regard to product analysis, a host of samplesunrelated to drugs, natural products, and forensics appli-cations have been addressed. Beer, wine, and spiritswere subject to MEKC analysis [71, 107, 126, 166, 167].These and other beverages were analyzed for aminoacids [126, 166], phenols [71, 166], hop acid [107], car-bohydrates [167], and even vitamin content [166, 168].Several components of vanilla extract from a variety ofsources were determined using MEKC, and the resultswere verified using HPLC [28]. Preservatives in cos-metics, pharmaceutical products and food have beenreported [113, 169], and even components of X-raydeveloping solutions [170] have been separated andquantified by MEKC.
5 Future directions
Based on the current reports it is clear that MEKC has anevident role in pharmaceutical analyses. The innovativemodifications and expanded repertoire of applicationsthat occurred during 2003 and 2004 also point to con-tinued developments in the future. A few current andanticipated topics require further mention. To begin with,the exciting and innovative field of multidimensionalseparations has harnessed the flexibility, speed, and effi-ciency of MEKC. Capillary sieving electrophoresis hasbeen coupled to MEKC-fluorescence resulting in aseparation system with an outstanding ability to handlelow volumes. The system has a good two-dimensionalpeak capacity (,375), which defines the hypotheticalability of a separation technique to separate singlet com-ponents (peaks) within the separation space (or time). Theresults of these attributes can be visualized in Fig. 3,
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Figure 3. Two-dimensional separation of single MC3T3-E1 cell (A–C), and a single MC3T3-E1 cell (D)transfected with the transcription regulator TWIST. The first dimension of this separation was performed bycapillary sieving electrophoresis and the second dimension was performed by MEKC, extracting informa-tion about molecular weights and retention times, respectively. This technology shows an alternative to2-D gel electrophoresis as a protein fingerprinting technique. Reprinted from [121], with permission.
which is comprised of four separate two-dimensional plotsof four separate single mammalian cells. Electro-pherograms A–C are protein fingerprints of “normal cells”,while electropherogram D is of a cell transfected with ahuman transcription regulator. In 3.5 h this system pro-vides researchers with the ability to individually screen fordifferences in single cells [121]. A two-dimensionalseparation system in which the second dimension wasoperated under different modes of CE (including MEKC)demonstrated potential for metabolomics [171].
Two-dimensional MEKC-CE on a microchip (see Fig. 4)provided high peak capacities (,4200) and short total runtimes (15 min). This system was used to map tryptic pep-tides from bovine serum albumin and demonstrated rapidfingerprinting of human versus bovine hemoglobin [89].Chip-based MEKC has been demonstrated in channel-for-mat [130] coupled to electrochemical (EC) detection.Another chip system incorporated MEKC-fluorescence andcould be operated in a high-throughput mode [90]. Thismethod was methodically designed as a high-throughputscreening assay for phosphoinositide-specific phospholi-pase C and phospholipase A2 lipid hydrolysis [90]. Otherexamples of MEKC based assays, although not chip based,point to continued advances in affinity screening [21, 73].
These exciting applications are accompanied by evolu-tion in the development of MEKC. The transfer to chip-based systems is becoming more common. New detec-
Figure 4. Microfluidic chip for two-dimensional separa-tion of protein digests. The first dimension of this 15 minseparation was performed by MEKC and the second di-mension was performed by CE. The turns in the chip wereconstructed to be asymmetrically cut to minimize bandbroadening. Reprinted from [89], with permission.
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tion methods, and critical improvements in interfacingcommon detection methods, for example MS, continue toappear in the literature. These innovations coupled withthe new and expanding list of “micelles” makes theextension into unexplored applications certain, asunsolved or unaddressed research challenges becomecandidates for harnessing the ever expanding attributesof MEKC.
This material is based upon work supported by theNational Science Foundation under Grant No. 0307245.
Received September 14, 2004
6 References
[1] Terabe, S., Otsuka, K., Ichikawa, K., Tsuchiya, A., Ando, T.,Anal. Chem. 1984, 56, 111–113.
[2] Molina, M., Silva, M., Electrophoresis 2002, 23, 3907–3921.[3] Terabe, S., Anal. Chem. 2004, 77, 240A–246A.[4] Pirogov, A. V., Shpigun, O. A., Electrophoresis 2003, 24,
2099–2105.[5] Peric, I., Kenndler, E., Electrophoresis 2003, 24, 2924–2934.[6] Fritz, J. S., Electrophoresis 2003, 24, 1530–1536.[7] Urbánek, M., Krivánková, L., Bocek, P., Electrophoresis
2003, 24, 466–485.[8] Welsch, T., Michalke, D., J. Chromatogr. A 2003, 1000, 935–
951.[9] Makino, K., Itoh, Y., Teshima, D., Oishi, R., Electrophoresis
2004, 25, 1488–1495.[10] Trojanowicz, M., Pobozy, E., Gübitz, G., J. Sep. Sci. 2003,
26, 983–995.[11] Boyce, M. C., Haddad, P. R., Electrophoresis 2003, 24,
2013–2022.[12] Dabek-Zlotorzynska, E., Chen, H., Ding, L., Electrophoresis
2003, 24, 4128–4149.[13] Poole, S. K., Poole, C. F., J. Chromatogr. B 2003, 797, 3–19.[14] Davankov, V. A., J. Chromatogr. A 2003, 1000, 891–915.[15] Hansen, S. H., Electrophoresis 2003, 24, 3900–3907.[16] Klampfl, C. W., Electrophoresis 2003, 24, 1537–1543.[17] Liu, X., Ma, L., Lu, Y.-T., Anal. Chim. Acta 2004, 512, 297–
304.[18] Deubner, R., Holzgrabe, U., J. Pharm. Biomed. Anal. 2004,
Flavonoids FESI-RMM Extract of Epimedium 16–79 ng/mL [96]brevicornum Maxim SE ,6–50
Drug and metabolite FASS Plasma 0.01 mg/mL [117]Alkaloids a) Sweeping Herb extracts, mouse a) 0.2–0.7 mg/mL [91]
b) CSEI-sweeping sera b) 3–32 ng/mLDrug a) Sweeping Mouse blood a) 16 ng/mL [31]
b) CSEI-sweeping b) 58 pg/mLDrug Sweeping Tablets SE ,200 [32]Phenolic compound LLE Wine 5 ppb [71](stilbene) SweepingSteroids Sweeping Aqueous standards 1.1–7.2 nM [171]
Bacterial cellsSteroid biomarker a) Sweeping a) Mouse sera a) 3 ng/mL [119]
b) CSEI-sweeping b) Human urine b) 5 ng/mLSteroid biomarker Sweeping Mouse plasma 5 ng/mL [72]Biogenic amines a) Sweeping Aqueous standards a) 2 nM, 30 nM [120]
b) CSEI-sweeping b) 0.2 nM
Chlorophenols a) Sweeping Aqueous standards a) 0.1–0.25 mg/mL [56]Chlorophenoxy acids SE 3–7
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Table 3. Continued
Surfactant additive Other running buffercomponents
Detection LOD Ref.
CarbohydratesSDS Borate (pH 8) A N.R. [182]SDS Phosphate (pH 6) A 1.5 ng [167]SDS Cholate, borate (pH 9.7) 1
1-propanolA 10 mM [88]
ExplosivesSDS Borate (pH 9.24) A N.R. [164]SDS Borate (pH 9.2) A N.R. [165]SDS Acetate, SBE-b-CD (pH 5) A 0.2 mg/mL [179]
BiomarkersSDS Borate (pH 10) A N.R. [183]SDS Borate (pH 9) A 5 mMa) [154]
OtherSDS Borate, (pH 9) 1 isopropanol A 3.9, 5.9 mg/mL [82]SDS Borate (pH 9.3) A 1–4 mg/mL [184]SDS Phosphate/borate, g-CD (pH 6.6) A 0.16–0.24 mg/L [70]SDS Phosphate, borate 1 alcohol A 2.1, 0.9 mg/mL [155]SDS Borate, MBTH (pH 9.3) A 0.54–11 ng/mL [163]SDS Ammonium acetate (pH 6) A N.R [131]
MS N.R.SDS Borate, Tris, EDTA, a-,b- or
g-CD (pH 8.3)A N.R. [54]
SDS Phosphate-borate (pH 6.6) A N.R. [113]SDS Borate 1 tetrahydrofuran A N.R. [104]SDS, 18-crown-6 Borate (pH 9.5) A 0.1, 0.9 mM [106]DOC Tris (pH 8.5) 1 1-propanol A N.R. [90]
DrugsSDS Tris (pH 8, 9) A 0.2, 0.3 mg/mL [141]SDS Borate, phosphate (pH 9.2) 1
isopropanolA N.R. [81]
SDS Phosphate-borate (pH 7.5) A 00.1–0.4%b) [146]SDS Boric acid /borate (pH 8) A 0.5 mg/mL [143]DOC Borate, b-CD (pH 10) A N.R. [18]SDS Borate (pH 10) A 0.5 mg/mL [144]SDS Phosphate- borate (pH 9) 1
acetonitrileA 0.4–0.5 mg/mL [94]
SDS Borate (pH 7.5) 1 acetonitrile A 0.33 mg/mL [176]SDS Phosphate/borate (pH 8.2) A 0.03–0.22 mg/mL [150]SDS Borate (pH 10.5) A 50 ng/mL [116]SDS Borate (pH 9.5) A 0.5 mg/mL [145]SDS Borate, urea, b-CD, mono-3-O-
phenylcarbamoyl-b-CD (pH 9.5)A 20 mg/mL [59]
SDS Phosphate, borate (pH 9) A 0.2–0.8 mg/mL [142]SDS Phosphate-borate (pH 6.5) A N.R. [66]SDS Boric acid, phosphoric acid,
