Micellar electrokinetic chromatography: Methodological and instrumental advances focused on...

Review

Micellar electrokinetic chromatography:Methodological and instrumental advancesfocused on practical aspects

This article reviews the latest methodological and instrumental improvements for

enhancing sensitivity and resolution in MEKC-based determinations. On-line sample

concentration (stacking, sweeping and combination of two protocols) and other on-

capillary approaches such as in-capillary derivatization and the coupling of flow-injection

systems with MEKC are the most relevant methodological approaches discussed for

improving sensitivity. At the same time, changes in the BGE through the use of organic

modifiers, ionic liquids, cations, CDs, non-ionic and zwitterionic surfactants, mixed

micelles, vesicles and carbon nanostructures as well as 2-D CE and chemometric tech-

niques for enhancing resolution are also considered in detail. Two instrumental

approaches such as MS and LIF are also discussed, covering the proposals for over-

coming the difficulties arising from the direct coupling of MEKC with MS and the

excellent detection sensitivity achieved by LIF using the typical argon-ion laser and recent

growth in the use of diode lasers as excitation sources. Some thoughts on potential future

directions are also expressed.

Keywords:

LIF / MEKC / MS / Stacking / Sweeping DOI 10.1002/elps.200800414

1 Introduction

Many recently published studies highlight how MEKC

systems have become viable and popular ways to analyze

samples containing a broad range of compounds. That this

is truly a reality at the present time is reflected in the

bibliography covered in this review (2006 and 2007) where

the number of routine MEKC methods considerably

outweighs the number based on methodological and

instrumental advances (which, in general, also focuses on

more practical determinations than those reported some

years ago). This situation clearly reflects just how readily this

analytical technique is being accepted by the scientific

community. Consequently, the assertion that MEKC is a

useful alternative to the liquid chromatographic technique is

becoming a more tangible reality with each passing day. As

in the previous review [1], some current relevant aspects to

do with MEKC, such as the use of polymeric pseudo-

stationary phases (PSPs), its implementation on microchip

devices and advances in MEEKC (at present, a consolidated

extension of MEKC), have been omitted in this review as

they are now the subject of dedicated reviews in this issue.

On the other hand, some papers related to theoretical

aspects of MEKC have also been published in this period,

the most relevant being those focusd on studies on the

selectivity of single, mixed and modified PSPs [2], on the

origin of peak asymmetry and isotherm non-linearity [3], on

the analysis of substances to be used as internal standards

[4] and on migration models for acidic solutes [5].

The methodological and instrumental approaches covered

by this review are focused on expanding MEKC ability to

improve sensitivity by providing LODs in the sub-mg/L region

and resolving more and more complex mixtures of neutral and

charged analytes in real samples. The use of sample stacking

and sweeping techniques in routine analysis (the most widely

used methods for on-line sample concentration in MEKC) in

order to enhance sensitivity has grown considerably in the

period covered by this review. At present, these techniques can

Manuel Silva

Department of AnalyticalChemistry, Rabanales Campus,University of Cordoba, Cordoba,Spain

Received June 30, 2008Revised August 27, 2008Accepted August 27, 2008

Abbreviations: ANNs, artificial neural networks; APFOA,

ammonium perfluorooctanoate; CD-MEKC, CD-modifiedMEKC; DTAF, 5-(4,6-dichloro-s-triazin-2-ylamino)fluorescein; EKSI, electrokinetic stacking injection; FASI,

field-amplified sample injection; FQ, 3-(2-furoyl)quinoline-2-carboxaldehyde; MCA, methyl chloroacetate; MPE,

multiphoton excitation fluorescence; NBD-F, 4-fluoro-7-nitro-2,1,3-benzoxadiozole; NDA, naphthalene-2,3-dicarboxaldehyde; PEO, poly(ethylene oxide); PF, partialfilling; PSP, pseudo-stationary phase; RMM, reversedmigrating micelles; SAMF, 6-oxy-(N-succinimidyl acetate)-9-(20-methoxy-carbonyl) fluorescein; SC, sodium cholateSC-CNT, surfactant-coated carbon nanotube; SWNT, single-walled nanotube

Correspondence: Professor Manuel Silva, Department of Analy-tical Chemistry, Marie-Curie Building (Annex), RabanalesCampus, University of Cordoba, E-14071 Cordoba, SpainE-mail: [email protected]: 134-957-218614

& 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com

Electrophoresis 2009, 30, 50–6450

really be considered useful alternatives to classical off-line pre-

concentration procedures. Regarding approaches for improv-

ing resolution, few contributions have been reported with

respect to the proposal for alternative PSPs to SDS, except for

polymeric ones (the study of which, as stated above, is outside

the scope of this review). Thus, SDS has become the most

effective pseudostationary phase (PSP), as reflected in the great

number of papers published over the last 2 years; in many

studies, conventional or modified with an organic solvent, SDS

micelles provide the required selectivity for analysis, although

in others, SDS micelles are coupled with other separation

equilibriums in the aqueous phase to enhance MEKC

separation, such as CDs and carbon nanostructures among

others, as well as those provided by other micellar systems

(mixed micelles) to enhance MEKC separation. The use of

non-aqueous MEKC, ionic liquids as alternatives to organic

modifiers, mixed micelles instead of conventional SDS and

coated capillaries has not totally succeeded in fulfilling the

expectations hoped for in the last review. On the other hand,

2-D CE and chemometric techniques have grown extensively

as tools for improving resolution in MEKC, and therefore they

have been included in the present review. Regarding detection

techniques, new contributions involving MS and LIF detection

are discussed here as in the last review. Despite improvements

in MS detection, especially those based on the use of polymeric

surfactants, this subject is still pending in MEKC, as stated by

Klampfl and Buchberger [6] in a recent article: ‘‘Whereas in

some fields CZE–MS may be accepted as a routine technique

in the near future, coupling of MEKC or even MEEKC with

MS is still in its very early stages’’. On the other hand, the use

of LIF detection in MEKC has grown dramatically over the

period covered by this review, the most relevant research topics

being: the consolidation of ‘‘classical’’ fluorescent probes with

an argon-ion laser as the excitation source, the development of

new contributions based on LEDs and the novel proposal for

fluorescence detection by using continuous wave-based

multiphoton excitation. Worthy of special note in this context

is a new contribution on the use of thermal lens microscopic

detection in MEKC. In an interesting study, Kitagawa et al. [7]

describe how this detection system in MEKC is improved

through the use of an interface chip to achieve highly sensi-

tivity detection and high reproducibility for the determination

of non-fluorescent and neutral analytes. Furthermore, nearly

million-fold sensitivity enhancement can be achieved by

coupling this detection system with sweeping as an on-line

sample concentration procedure.

2 Approaches for improving sensitivity

2.1 On-line sample concentration methods

On-line sample concentration methods are one of the

simplest ways for sample enrichment in MEKC, since the

concentration step is performed within the same capillary

used for analysis. Sample stacking based on the manipula-

tion of analyte migration velocity and sweeping on its

partitioning into a moving PSP are by far the most widely

used methods. Different trends were observed in the topics

dealt with during the period covered by this review with

respect to those included in the last one, namely: (i) few

studies have been carried out to expand the scope of

application of sweeping techniques by modifying the PSP,

resulting in SDS micelles being the most commonly

sweeping carrier used in many analytical applications; (ii)

a considerable growth in the use of sample stacking

methods; (iii) a lack of development in the expected

potential of other on-line sample concentration approaches

such as those based on ‘‘selective exhaustive injection-

sweeping’’ and ‘‘dynamic pH junction-sweeping’’, probably

due to their low reproducibility, which often prevents their

use as quantitative tools for the ultra-trace analysis of

analytes in complex matrices and (iv) some new proposals

have also been reported, mainly focused on increasing the

injected sample volume. These trends lead to the conclusion

that these techniques now offer enough reliability to be

considered useful alternatives to off-line concentration ones

to increase sensitivity in MEKC-based determinations. In

this context, the combination of both on- and off-line sample

concentration approaches is another significant trend to

achieve higher enhancement factors; SPE and solid-phase

microextraction are the most frequent off-line sample

concentration techniques, while the use of other current

alternatives such as stir bar sorptive extraction and

liquid–liquid semimicroextraction has also been reported.

2.1.1. Sample stacking

Sample stacking in MEKC is an on-line sample concentra-

tion technique based on differences between electric field

strength in the sample zone and that of BGE. The focusing

effect occurs at the boundary between the high electric field

sample zone and the low electric field BGE zone. It is a

result of a rapid change in micelle migration velocity when

passing from one region to another. There have been

remarkable reports on the use of the different stacking

modes and new alternatives in MEKC over the period

covered in this review. Ordinarily SDS is used as PSP and

the most frequent approaches are field-enhanced sample

injection [8–10], stacking with reversed migrating micelles

(RMM) [11–14] and field-enhanced sample injection

with RMM [14, 15]. The ensuing analytical methods

have been used to determine a great variety of analytes,

namely aristolochic acids in Chinese medicine preparations

[8], PAHs in airborne particulates [9], melatonin and

related indoleamines in rat pineal glands [10], melatonin

and its precursors/metabolites in human serum [11],

paraben preservatives in cosmetic products [12], pesticides

in fruits and vegetables [13], non-steroid anti-inflammatory

drugs in mineral water [14] and saponins in Panax

notoginseng [15]. Between ca. 100- and 200-fold enhance-

ment in detection sensitivity can be achieved compared with

that obtained in the simple MEKC method; which can be

increased to 300-fold by using the method proposed by

Electrophoresis 2009, 30, 50–64 CE and CEC 51


Zhuang et al. [12] in which large volumes of sample can be

successfully injected.

In this context it is noteworthy to point out two inter-

esting stacking approaches for the analysis of large-volume

samples. Chiu and Chang [16] reported a stacking method

in the presence of poly(ethylene oxide) (PEO) for neutral or

anionic solutes, such as amino acids after their derivatiza-

tion with naphthalene-2,3-dicarboxaldehyde (NDA). In

addition to SDS, PEO is essential for the stacking and

separation of large-volume (e.g. 0.53 mL) NDA–amino acid

derivatives, which allows sensitivity enhancements to be

achieved in the 50–800-fold range. The basic principles

behind this approach are illustrated in Fig. 1. Initially, the

capillary is filled with Tris-borate buffer containing SDS in

order to reduce PEO adsorption and thus generate a high

and repeatable EOF (Fig. 1A). A mixture of NDA–amino

acid derivatives is hydrodynamically injected from the

anodic end to the capillary for a length of time, e.g. 240 s,

(Fig. 1B). When high voltage is applied, the PEO solution

enters the capillary from the anodic end. SDS micelles

migrating from the zone of Tris-borate buffer interact with

and thus sweep the negatively charged NDA–amino acid

derivatives, demonstrating less electrophoretic mobility than

that of SDS micelles in the sample zone. Both SDS micelles

and NDA–amino acid derivatives migrate against EOF

(Fig. 1C). When anionic NDA–amino acid derivatives and

the aggregates between SDS micelles and NDA–amino acid

derivatives migrate into the PEO zone, they are stacked in a

narrow band as a result of the decrease in electric field and

increase in viscosity (Fig. 1D). Finally, aggregates between

SDS micelles and NDA–amino acid derivatives are separated

according to the mechanisms of MEKC. Zeng and

co-workers [17] proposed the so-called pressure-assisted

field-amplified sample injection (FASI) with RMM for the

on-line concentration of neutral analytes in MEKC with a

low-pH BGE. Briefly, the stacking mechanism of pressure-

assisted FASI with RMM is as follows: in step 1, after the

capillary is initially conditioned with the micellar BGE, a

water plug was hydrodynamically injected by pressure. In

step 2, the injection end was switched into the sample vial

that contained the analytes, which were prepared in low-

conductivity matrices. At the same time, the outlet end was

still in the micellar BGE vial. Analyte focusing was started

by applying a negative voltage in conjunction with a pres-

sure that generates a hydrodynamic flow in the direction

opposite to the EOF, which prevents the water plug from

moving toward the inlet of the capillary too fast. Thereafter,

micelles and neutral analytes solubilized in it were injected

electrokinetically into the capillary for a period of time much

longer than usual for FASI (e.g. 8 min). Under these

conditions, 1000–3000 stacking enhancement factors in

terms of peak area can be observed for the determination of

trace steroids. The method also provides about seven times

more peak height enhancement for the analytes when

compared with those obtained by the sweeping technique.

Other ‘‘classical’’ stacking approaches have been used

for concentrating analytes from different samples. Rodrı-

guez-Delgado and co-workers [18] reported several meth-

odologies based on the reversed-electrode polarity stacking

mode for pesticide analysis in water and wine [19–21]

samples. To increase concentration factors, off-line

concentration strategies were combined with the reversed-

electrode polarity stacking mode; thus, SPE was useful for

multiresidue analysis of pesticides in water samples,

providing LODs at ng/L levels, whereas solid-phase micro-

extraction was recommended for the determination of these

compounds in wine samples, resulting in less sensitivity,

only just reaching LODs at mg/L levels. The combination of

so-called ‘‘ACN stacking’’ and ACN deproteinization with

salting-out extraction is proposed by Li and Huie [22] as a

new sample pre-treatment approach for biological samples

for additional enhancement in concentration detection

sensitivity. The approach was useful for the determination

of hydrophobic porphyrins with clinical significance at

ng/mL in urine samples. In contrast to other methods

already commented on, sodium cholate (SC) was used as the

PSP. This micellar system has also been used by Collins and

co-workers [23] for the on-line concentration of nitroaro-

matic explosives in seawater by using the high-salt stacking

mode. Although longer injection times result in a loss of

resolution due to limited interaction between the explosives

and the SC micelles, LODs below 100 mg/L can be obtained

successfully. At this point, it is worth mentioning the

interesting results also reported by Collins and co-workers

[24] to understand the role of stacked micelles in the sample

concentration. The paper is a fine study on the differences

between the mechanisms of sweeping and high-salt stack-

ing as on-line sample concentration techniques.

Detection

+

+_

_

SiO µEOF µEP PEO solution TB buffer

NDA-amino acid derivatives SDS micelle

Detection

A

B

C

D

+

+_

_

SiO µEOF µEP PEO solution TB buffer

NDA-amino acid derivatives SDS micelle

Figure 1. Evolution of stacking and separation of NDA–aminoacid derivatives by CE-LED induced fluorescence in the presenceof EOF and PEO solutions. Reprinted from [16] with permission.

Electrophoresis 2009, 30, 50–6452 M. Silva


Finally, there is a noteworthy new on-line sample

concentration approach based on the combination of large

volume sample stacking and the dynamic pH junction

technique for the determination of weak acidic compounds

[25]. Figure 2 shows the scheme of the method. The capil-

lary is filled with the sample solution in sodium borate pH

9.5 while sodium phosphate pH 2.5 with SDS is in the inlet

and the outlet vials (Fig. 2A). Application of a negative

electric field causes the co-electroosmotic migration of

analytes and pre-concentration due to the pH boundaries.

Analytes are immobilized in the junction of phosphate and

borate due to changes in their dissociation. This leads to a

situation where these compounds have mobility close to

zero at pH 2.5 and are stopped on reaching the phosphate

zone. Analytes are also pre-concentrated at the second pH

boundary between the inlet vial and the capillary. They

migrate with the EOF to the boundary where they are

protonated and get stacked (Fig. 2B). These two pre-

concentrated zones are joined during migration (Fig. 2C)

and then the common MEKC proceeds (Fig. 2D). The

method was tested to determine nanomolar concentration

levels of sorbic and benzoic acids and compared favorably

with another on-line sample concentration technique,

isotachophoresis, providing 10–70 times lower LODs.

2.1.2 Sweeping

Sweeping is another effective and convenient way to do on-

line sample concentration in MEKC. It is based on the

accumulation and isolation of analytes, injected by micelles

in a large sample volume, to concentrate them into a narrow

zone and enhance detection sensitivity. From an experi-

mental point of view, sweeping occurs when the sample

matrix is prepared in a buffer solution with conductivity that

is similar to or higher than BGE and without PSP. The basic

condition for sweeping is for the separation buffer to

contain a surfactant at a concentration above its CMC, while

the sample solution is free of the surfactant. When charged

micelles in BGE penetrate the sample zone during the

application of voltage, the picking or accumulating of

analytes occurs due to the partitioning or interaction of

analytes with PSP. As stated above, little work has been

conducted to improve the phenomenon of sweeping; thus,

the contributions have mainly been focused on the analytical

use of this on-line concentration technique. In most cases

the protocol for sweeping neutral and anionic analytes uses

anionic micelles like SDS and suppressed EOF in a low pH

BGE provided by phosphoric acid or phosphate buffer (pH

2–3) [26–32]. Under suppressed EOF conditions, other

anionic surfactants, di(2-ethylhexyl) sulfosuccinate and

Brij-S, and cationic ones, CTAB, octyltrimethylammonium

bromide and tetradecyltrimethylammonium bromide, have

also been used as alternatives for SDS, but with lower

enhancement factors [33]. In general, up to 100-fold

improvement in concentration sensitivity can be achieved

in the determination of different analytes, mainly in

samples of pharmaceutical or clinical interest such as

strychnine and brucine in Chinese medicinal preparations

[26, 27], phenethylamine designer drugs [28] and ketamine

and norketamine [29] in urine samples and carbamazepine

in tablet and human serum [32]. In some cases, LODs were

also enhanced by combining on-line sweeping with an off-

line concentration approach such as liquid-phase microex-

traction [27] or by using a more sensitive detection

technique such as LED-LIF detection [28, 30]. The sweeping

protocol has also been applied under basic conditions (BGE

with borate buffer at pH over 8.5–9.5), but only two

approaches have been reported. The first one, concerning

the determination of all-trans- and 13-cis- retinoic acids in

rabbit serum, was developed in order to overcome the poor

resolution obtained under suppressed EOF conditions

caused by strong interaction of the analytes with the PSP

[34]. Although under basic conditions the resolution was

increased, only ca. 19-fold improvements in sensitivity were

achieved for retinoic acids. The second one deals with a

sweeping technique using CTAB to improve sensitivity

detection in flunitrazepam and its major metabolites in

urine samples [35]. In this approach the focusing effect is as

follows: after a negative voltage was applied from inlet

(cathode), the EOF, under the influence of the cationic

CTAB surfactant, moved toward the outlet (anode). Because

the velocity of the EOF was higher than that of the CTAB

micelle, the analytes stacked at the boundary by the CTAB

micelle and moved toward the anode. Using this sweeping

protocol, sensitivity enhancement for each compound was

within the range of 110–200-fold.

_ +

_

_ +

_

detector

concentrated analytes

separated analytes

sodium phosphatepH 2.5 with SDS

sodium borate pH 9.5with analyte

+

+

_ +

_

_ +

_

detector

concentrated analytes

separated analytes

sodium phosphatepH 2.5 with SDS

sodium borate pH 9.5with analyte

A

B

C

D+

+

Figure 2. (A–D) The scheme of proposed large volume samplestacking and the dynamic pH junction technique pre-concentra-tion method. The dots represent micelles. Reprinted from [25]with permission.



2.1.3 Combination of stacking and sweeping

protocols

The so-called ‘‘selective exhaustive injection-sweeping’’

techniques based on the combination of stacking and

sweeping protocols are the most sensitive approaches for

on-line sample concentration; up to a million-fold sensitivity

increase for cationic analytes (cation-selective exhaustive

injection-sweeping) has been reported by Quirino and

Terabe [36]. Some contributions to these techniques have

been reported in the period covered by this review [37–41],

but fewer than expected despite the high values of

improvement factors it can involve. This behavior can be

ascribed to the poor reported run-to-run reproducibility, in

many cases over 10% for low concentrations. In an

interesting study, Chen and co-workers [37], after a

systematic optimization of the process, concluded that there

are five key factors that must be controlled to achieve highly

reproducible results, namely, the conductivity of the sample

solution, the conductivities of high- and medium-conductiv-

ity buffers, the fraction of the column filled with high-

conductivity buffer, electrokinetic injection time and the

surfactant concentration. The authors recommended prepar-

ing the sample in a solution with moderately low conductiv-

ity in order to increase the reproducibility of both the

injection process and the whole analytical procedure. Illicit

amphetamines [38, 39] and other abuse drugs [40, 41] in

urine [38–40] and hair [41] samples can be detected at a few

ng/mL or even at pg/mL levels, thanks to the higher

enrichment factors that can be obtained, which ranged from

1000- to 6000-fold. Anion-selective exhaustive injection-

sweeping is a more recent approach than cation-selective

exhaustive injection-sweeping although its principle is

basically the same. Few contributions in this respect have

been reported in this on-line sample concentration technique

for negatively charged analytes during the period covered by

this review [42], and there has even been a straightforward

and easy approach reported by Heineman and co-workers

[43] as an alternative to the anion-selective exhaustive

injection-sweeping scheme, which does not require polarity

changes during stacking and sweeping steps. The authors

developed a new electrokinetic stacking injection (EKSI)

scheme to concentrate anionic analytes by sweeping with

cationic surfactants in a dynamically coated capillary. Figure

3 shows a schematic diagram of the stacking and sweeping

processes. Under optimal experimental conditions, the

injected sample plug length for analytes under 20.1 kV for

60 min was estimated as ca. 800 cm, the effective capillary

length corresponding to ca. 25-fold. Compared with tradi-

tional pressure injection, the EKSI scheme resulted in an

increased detection factor of ca. 4.5� 103.

2.1.4 Other on-capillary approaches

In addition to the on-line sample concentration methods

reported above, other on-capillary approaches with different

aims have also been proposed in the period covered by this

review, e.g. to achieve full automation and simplification of

the analytical procedure with a further improvement in

reproducibility by using so-called ‘‘in-capillary derivatiza-

tion’’ [44–47] or by the combination of flow injection with

MEKC [48, 49], and to increase sensitivity by coupling stir

bar sorptive extraction [50] or liquid–liquid semimicroex-

traction with MEKC [51].

Despite its attractive features, little work has been

reported on the use of in-capillary derivatization in MEKC in

recent years. On the other hand, a great deal has been

devoted to studying enzymatic reactions through electro-

phoretically mediated microanalysis [44]. Only three refer-

ences to in-capillary derivatization have been reported so far

in the context of MEKC analysis; the first one evaluated the

potential of 4-fluoro-7-nitro-2,1,3-benzoxadiozole (NBD-F) as

an in-capillary derivatization reagent for the analysis of

organophosphorus pesticides [45]. A mixed in-capillary

mode combining zone-passing mixing and at-inlet reaction

was proposed, in which plugs of sample and NBD-F solu-

tions were introduced successively to the anodic end of the

capillary. The sample and reagent zones were electro-

kinetically mixed by applying a potential of 5 kV for 30 s, and

the reaction was allowed to stand for 7.5 min in the absence

of potential. Although the LODs achieved for the determi-

nation of pesticides were comparable with those obtained

+

+

+_

+_

+_

_

Steadyinjection

Vial exchange

Sweeping

Zone migration

Separation

_

Startingsituationof EKSI

+_

lSB, max

lL

Veo1Veo2

Veo1

lSB, max

lSB

Véo2 Véo1

lSB, max

Véo2 Véo1Véo1

Véo1 Véo2

Véo1

Detectionpoint

+

+

+__

+_ +__

+__

__

Steady stateinjection

exchange

Sweeping

Zone migration

Separation

__

A

B

C

D

E

F

Startingsituationof EKSI

+__

lSB, max

lL

Veo1Veo2

Veo1

lSB, max

lSB

Véo2 Véo1

lSB, max

Véo2 Véo1Véo1

Véo1 Véo2

Véo1

Detectionpoint

Figure 3. Evolution of the EKSI and sweeping. The dense-dotpart represents sample solution in phosphate buffer. The blankpart refers to the separation buffer containing dodecyltrimethyl-ammonium bromide (DTAB), phosphate and ACN. The sparse-dot denotes the phosphate sample buffer. I and L are theeffective and total capillary lengths, respectively. ISB and ISB,max

are the sample buffer plug length and its maximum, respec-tively. Veo1 (Veo1) and Veo2 (Veo2) are the local EOF velocities.Reprinted from [43] with permission.



from conventional pre-capillary derivatization, the sensitivity

could be improved remarkably by its coupling with an on-

line sample concentration technique. The other two contri-

butions rely on the in-capillary derivatization of enantiomers

of chiral amino compounds [46] and amino acid mixtures

[47] with o-phthalaldehyde and the chiral reagent N-acetyl-L-

cysteine as derivatizing reagents using DAD and LIF

detection, respectively. As with in-capillary derivatization

methods, few works have been devoted to flow injection-

MEKC approaches. Two contributions from Chen and

co-workers [48, 49], one related to the separation and

determination of alpinetin and cardamonin in Alpinia

katsumadai Hayata [48] and the other to the determination

of berberine, palmatine and jatrorrhizine in herbal medicine

[49], are the only ones that have been reported. The main

features of these methods were automation, rapidity (typical

sample throughput ranges from 10 to 24 h�1) and even high

sensitivity because low LODs can be achieved by using an

additional on-line sample concentration step, as in the

second approach, in which 64–86-fold improvement in the

detection sensitivity was obtained by using head-column

FASI and sweeping.

Finally, an interesting contribution of Wang et al. [51]

about the use of methyl chloroacetate (MCA) as an extrac-

tion solvent for coupling liquid–liquid semimicroextraction

with MEKC is worthy of note: it is based on its on-capillary

decomposition for the separation of neutral compounds

with concentration enhancement. This solvent overcomes a

significant drawback involved in the on-line coupling liqui-

d–liquid extraction with MEKC, which is the poor electrical

conductivity of the solvents that are not able to conduct a

high enough current along the capillary. Thus, after

microextraction of analytes into MCA, the organic phase was

directly subjected to separation by MEKC, where MCA

underwent in-capillary decomposition into methanol and

chloroacetic acid. Due to the features of MCA, a plug of

catalyst (e.g. NaOH) was not required as in the method

reported by Zhan et al. [52] based on the use of ethyl acetate

as the extraction solvent. Concentration enhancement for

alkylphenones such as acetophenone, butyrophenone and

valerophenone was between several multiples of 10 to more

than 100 times.

3 Approaches for improving resolution

The approaches described in this section for improving

resolution in MEKC have been classified into two main

topics: (i) incorporation of additives into the aqueous phase

to modify the distribution constant between micelles and

the surrounding medium or to alter its apparent distribution

constant by using coupled equilibriums and (ii) modifica-

tion of the micellar phase by using other surfactants as an

alternative to SDS as well as mixed micelles. This section

also considers the use of coated capillaries, 2-D CE and

chemometric techniques in MEKC as approaches for

improving resolution.

3.1 Incorporation of additives into the aqueous

phase

Organic modifiers are a widely known option for enhancing

MEKC separations because they alter the micellar structure

(solute partition coefficients) and expand the migration

window by reducing the EOF. Up to now, ACN has been the

most commonly found organic modifier used in MEKC; of

the methods reported based on the use of organic modifiers

for improving resolution in MEKC, over 75% use ACN as an

organic modifier [53–58], and to a lesser extent, methanol

[59–62], ethanol [63, 64] and mixtures [65]. Urea [66], ionic

liquids [67, 68] and cations [69] are the most significant

alternatives for organic modifiers in MEKC reported during

the period covered by this review. Urea was used as an

alternative to methanol to reduce the migration window

without detriment to the resolution of eight adrenocortico-

tropic steroid hormones with SDS micelles and acidic BGE

[66]. As stated by the authors, urea forms hydrogen bonds

with SDS micelles and weakens the hydrophobic interac-

tions of the separated components with the surfactant,

which results in an increase in separation efficiency. Few

contributions have been reported in the last 2 years on the

use of ionic liquids in MEKC as an alternative to organic

modifiers, despite their recent introduction in MEKC by

Warner and co-workers in 2003 [70]. These authors reported

a comparison of ionic liquids and organic solvents as

additives for chiral separations in MEKC using polysodium

oleyl-L-leucylvalinate as molecular micelle and 1-alkyl-3-

methylimidazolium tetrafluoroborate as ionic liquids, where

the alkyl group was ethyl, butyl, hexyl or octyl [68].

Equivalent chiral resolution and selectivity can be achieved

when using low contents of ionic liquids as an alternative to

high volumes of organic solvents; however, ionic liquids

provide faster separations without current breakdowns.

Another way to modify the EOF to improve resolution is

reported by Qu and co-workers. [69], based on the addition

of divalent and monovalent cations to the BGE as additives,

which is used to enhance the separation of amino acids as

p-acetamidobenzenesulfonyl fluoride derivatives. A compro-

mise solution can be derived from this study, because the

resolution cannot be improved easily using the buffer cation

that provides the fastest decrease for the EOF. Thus,

although the reduction of the EOF followed the order

Mg2+4K+4Na+4Li+, the authors propose Na+ as the best

additive to reach a compromise among migration time,

resolution and peak shape. Therefore, 20 mM Na+ as

electrolyte was added to the BGE consisting of 20 mM

borate at pH 9.3 containing 140 mM SDS.

The addition of CDs as additives to the aqueous phase is

another effective option for increasing resolution in MEKC,

especially in enantiomeric separations. The so-called

CD-modified MEKC (CD-MEKC) method is widely used to

achieve chiral separations [71], which is based on the

combining of chiral CDs with achiral micelles, such as SDS.

However, chiral separations in MEKC have been recently

drawn toward using chiral polymeric surfactants (also called



molecular micelles); the description and applications of

molecular micelles is beyond the scope of this review, which

is now focused on the use of CD-MEKC and other reported

chiral alternatives. As in the previous review, b-CDs are by

far the most widely used additives for improving resolution

in CD-MEKC; SDS is commonly used as a micellar system

[9, 28, 72–76], but other micellar systems such as bile salts

[77] and mixed micelles [78] have also been employed. In

addition to b-CDs, derivatives such as hydroxypropyl-b-CD

[79–82] and 6-O-a-D-glucosyl-b-CD [83], g-CD [84, 85],

hydroxypropyl-g-CD [31] and the currently reported native

d-CD [86] have also been used as chiral selectors in MEKC to

increase enantioselectivity. Pharmaceutical [28, 74–77, 79,

84, 85] and food [31, 72, 80, 81, 83] fields are the main areas

of application of the methods reported on. In this context,

two relevant contributions are noteworthy. The first one is

devoted to the study on the competition mechanism

between b-CD and SDS reported by Liu and co-workers [74],

which led the authors to conclude that CD-MEKC improves

selectivity in hydrophobic compounds, because for hydro-

philic analytes, which have weaker hydrophobic interactions

with SDS, this approach is sometimes not efficient because

the competition between SDS and the aqueous PSP is too

strong and will lead to a negative effect on separation. The

second one introduces native d-CD as a chiral selector in

CD-MEKC [86] comparing it with conventional BGE addi-

tives a-, b- and g-CDs. Experimental results showed that,

depending on the structure of the analytes, the resolving

property of d-CD is similar to or quite different from g-CD.

Although CD-MEKC is the most widely used approach for

chiral analysis in MEKC, some interesting alternatives have

been proposed, one of which reports a new way to find novel

types of chiral derivatizing reagents for amine compounds

[87]. The method is based on the synthesis of isothiocyanate

derivatives of chiral amine reagent, like dehy-

droabietylamine, through reaction with Et3N, CS2 and

POCl3 in a dried ethyl ether medium.

3.2 Use of a different micellar phase

The choice of surfactant seems to be the most important

variable in optimizing the resolution in MEKC; out of the

numerous surfactants that have been evaluated, SDS is by

far the most widely used. However, SDS-MEKC does not

provide suitable resolution in all cases and therefore other

micellar phases from monomeric (e.g. bile salts, cationic and

non-ionic surfactants, mixed micelles, etc.) to polymeric

surfactants have been used to extend the application of

MEKC to more complex matrices. As in other sections, the

description of contributions based on the use of polymeric

surfactants is beyond the scope of this review.

Lithium dodecyl sulfate [88] and sodium octane sulfo-

nate [89] are two salient examples of the use of monomeric

surfactants as PSPs with a similar structure to SDS. When

using lithium dodecyl sulfate, selectivity can be improved

because Li+ ions will bind the surface of the micelles less

tightly than Na+ ions and therefore the repulsions with

sulfate groups are more important and a more ‘‘open’’

micelle with a higher mass transfer is obtained. In conse-

quence, the residence time of analytes in the hydrophobic

core of micelle becomes more important and narrower peak

widths can be obtained [88]. On the other hand, surfactants

with a carbon number lower than that of SDS, such as

sodium octane sulfonate, can be a useful alternative to the

use of SDS micelles in the presence of organic modifiers to

decrease the migration window without affecting separation

efficiency [89]. The number of contributions/year using

other conventional PSPs such as bile salts, cationic and non-

ionic surfactants, and mixed micelles is practically the same

as in the last review. In general, the methods reported have

been developed in order to determine analytes of clinical

and pharmaceutical interest in different samples, and only

some of them are worth pointing out. Regarding bile salts,

SC micelles have been used for the separation of bioactive

licorice compounds [77] (alkyltio)acridines [90] amino acids

after derivatization with phanquinone [91] four streoisomers

of palonosetron hydrochloride [92], whereas sodium deox-

ycholate was a useful PSP for the separation of nitrofuran

antibiotics [93] and a mixture of herbs commonly present in

herbal Chinese formulas [94]. Mixed micelles of SC with

SDS [95] or another bile salt like taurine [96] have also been

reported for the separation of ten flavonoid aglycons

belonging to four different classes, and for the determina-

tion of shikimate in raw plant extracts, respectively. As is

well known, relatively few cationic surfactants have been

used in MEKC, and consequently few contributions have

been reported in the period covered by this review [97, 98].

In an interesting paper, Palmer and co-workers [98] char-

acterize the solute distribution between water and self-

assemblies formed from ionic liquid type mono-chain

cationic surfactants containing a cyclic pyrrolidinium head

group. The kind of PSPs formed by these N-alkyl-N-

methylpyrrolidinium bromides interacts more strongly with

polar compounds and less strongly with compounds having

non-bonding or p-electrons and are more cohesive

compared with the well-studied CTAB. Regarding non-ionic

surfactants, polyoxyethylene sorbitan monolaurate (Tween

20) [49, 99], polyethylene glycol dodecyl ether (Brij-35) [85,

100] and mixed micelles of Brij-35 with SC [78] and 3-(N,N-

dimethylhexadecylammonium) propanesulfonate [101, 102]

are the useful PSPs reported for the separation of compo-

nents in herbal medicines [99], evaluation of the enantio-

meric purity of an oxazolidinone drug candidate [85],

determination of phosphoamino acids [100], profiling of

amine metabolites in human biofluids [78] and peptide

mapping [101] by using acidic [49, 85, 101, 102] or basic

[78, 99, 100] BGEs.

Between monomeric surfactants and polymeric ones

with multiple hydrophobic chains and ionic head groups are

the dimeric surfactants, which are made up of two amphi-

philic moieties connected at, or close to, the head groups by

a spacer. In an interesting paper, van Biesen and Bottaro

[103] describe the synthesis of dimeric anionic surfactants



with a flexible hydrophobic spacer (2, 4, 6, 8, 10 and 12

methylene groups) close to the two head groups and the

influence of these spacers on the selectivity. No significant

differences were observed in the selectivity for each surfac-

tant, which means that the cohesiveness of the micelles and

their interface with the aqueous bulk phase is very similar,

regardless of the length of the hydrophobic spacer.

Compared with SDS, these dimeric surfactants do seem to

be better at separating more hydrophobic solutes at much

lower concentrations because of their lower CMCs. This

could be an advantage for applications where a low current

is required to minimize Joule heating or for direct coupling

of MEKC with MS. One other contribution of this kind of

surfactants is that of using di(2-ethylhexyl) sulfosuccinate as

the PSP for the separation of synthetic phenolic antioxidants

in edible oils [104].

Liposomes are self-assembled vesicles commonly

consisting of phospholipid bilayers enclosing an aqueous

solution, with other lipid aggregates found as well. When

using liposomes in MEKC, the lipophilic phase is suspen-

ded in the aqueous mobile phase (or vice versa) in the form

of micelles, microemulsions or other particles forming a

PSP [105–107]. These particles are normally electrically

charged and move thus with their own electrophoretic

velocity under the influence of the electric field applied.

Kenndler and co-workers [108] reviewed in detail the work

on liposomes in the context of electrically driven separation

methods in the capillary format; and concretely one topic is

related to the use of liposomes as PSP in MEKC. These

vesicular structures obtained by mixing cationic and anionic

surfactants at a concrete mass ratio, phospholipids or lipids

as well have rarely been used in MEKC before now for the

separation of hydrophobic compounds. Recently, Jiang

et al. [109] have demonstrated that the use of liposomes as

PSP in MEKC is a good and dynamic method to study

of the interaction between cell membranes and important

biomolecules.

The recent introduction of carbon nanostructures as

PSPs in MEKC is also worth commenting on [110–112]. The

use of nanoparticles as PSP in CE is a growing area of

interest because they have the advantage of introducing a

novel interaction phase for every analysis, as can be stated in

the current and excellent review by Nilsson and Nilsson

[113]. These authors discuss the use of micelle polymers,

dendrimers, silica nanoparticles, molecularly imprinted

polymer particles, polymer nanoparticles and gold nano-

particles; however, carbon nanostructures (carbon nano-

tubes and fullerenes) are not considered in this review.

Valcarcel and co-workers [110] have recently developed a

new MEKC procedure based on the use of surfactant-coated

carbon nanotubes (SC-CNTs) as PSPs, in which single-

walled nanotubes (SWNTs) are dispersed into a micellar

media by sonication to obtain SC-CNTs. The approach

allows the use of CNTs as PSPs (avoiding their insolubility

in water) and makes use of their advantages, namely, both

the high surface area and the capacity to adsorb organic

analytes; thus, additional interactions are introduced into

the micellar medium with the subsequent improvement in

the resolution. Figure 4 shows the significant effect of SC-

CNTs on the separation of a mixture of chlorophenols by

MEKC using SDS micelles. As can be seen, overlapped

peaks can be resolved by adding SC-CNTs to the BGE

without an appreciable increase in the migration window.

Other interesting potential features of SC-CNTs as PSPs in

MEKC are their use as an analytical tool in sequential

injection to improve sensitivity and resolution and as a

strategy for isomeric separation. Regarding the latter,

SWNTs and multiwalled nanotubes (MWNTs) have been

evaluated as chiral selectors for direct enantiomeric separa-

tion of ephedrines [111]. MWNTs demonstrate a better

capability to resolve enantiomeric mixtures by using partial

filling (PF) of the capillary with concentrated SC-MWNTs,

and the approach is a promising way of using carbon

nanostructures in MEKC to develop new direct enantio-

meric methods that can be a rapid and simple alternative to

A

B

Figure 4. Electropherograms of chlorophenols obtained in(A) absence and (B) presence of SC-SWNTs (3.2 mg/L).The BGE was 25 mM sodium hydrogenphosphate, 15 mMsodium tetraborate, 100 mM SDS and 6% ethanol at pH 7.95.Other operating conditions: capillary 30 cm�75 mm id, 15 kV,201C. Peaks: (1) 2,6-dichlorophenol; (2) 3,5-dichlorophenol;(3) 2,3-dichlorophenol; (4) 2,5-dichlorophenol, (5) 3,4-dichloro-phenol; (6) 2,3,5-trichlorophenol; (7) 2,4,5-trichlorophenol;(8) 2,3,6-trichlorophenol; (9) pentachlorophenol. Reprinted from[110] with permission.



those based on the use of CDs. Surfactant-coated fullerenes

C60 have also been proposed as novel PSPs in MEKC to

improve the separation of different aromatic compounds,

such as b-lactams antibiotics, non-steroidal anti-inflamma-

tory drugs and amphenicols [112]. Compared with other

carbon nanostructures such as SWNTs and MWNTs, full-

erenes C60 have a similar potential as PSPs to improve

resolution, but for some analytes they can provide better

results; hence, additional work is required to assess the real

potential of carbon nanostructures in MEKC as PSPs.

3.3 Use of other approaches

At present, the potential of coated capillaries, 2-D CE and

chemometric techniques in MEKC as tools for improving

resolution is quite different because, as stated above, coated

capillaries have not lived up to the expectations raised in the

last review, whereas the use of 2-D CE has grown widely as

well as that of chemometric techniques during the period

covered by this review.

The most relevant contributions on coated capillaries in

MEKC reported in the last 2 years are the characterization

of the SDS-induced EOF in MEKC by using the polyelec-

trolyte multilayer coating approach [114] and a current

review of the application of physically adsorbed polymer

coatings in CE [115]. The first report constitutes a new

contribution of Pranaityte and Padarauskas [114] on the use

of bilayer coatings whose capillaries are coated with

poly(diallyldimethylammonium chloride) followed by the

physical adsorption of SDS in a second layer under MEKC

conditions. The concentration of SDS is a critical parameter

in this process: at approximately or just below the condi-

tional CMC value, a variable but highly reproducible cath-

odal EOF can be achieved, whereas at greater concentrations

the EOF is independent of the pH. The paper also revises in

detail the effect of organic solvents, urea, b-CD and non-

ionic surfactants on EOF behavior, and the results can be

helpful for separation optimization in MEKC using dyna-

mically coated capillaries. In addition, Blomberg and co-

workers [116] report on the first approach for the separation

and characterization of anacardic acids by MEKC-MS using

polydimethylacrylamide-coated capillaries. The best resolu-

tion of the analytes (nine different isomers were identified)

requires the use of a complex BGE consisting of 10 mM

phosphate buffer pH 6.5 with the addition of 1 M urea,

20% ACN, 20 mM SDS, 10 mM of b-CD and 1 mM of

heptakis-6-sulfo-b-CD.

The use of 2-D separation systems is an emerging area

in CE for achieving high-resolution capabilities, especially

for protein analysis in clinical samples. Most of the papers

on this topic in the period covered by this review are

contributed by the Dovichi group [117–122], who are an

important reference in this field. Two important contribu-

tions can be highlighted based on modifications of the first-

generation 2-D CE instrument developed by the group in

2004, both focused on decreasing analysis time. In the first,

narrow inner diameter capillaries are used to facilitate the

application of higher electric fields than in the original 2-D

CE instrument in order to reduce separation time from ca.3–5 to 1 h [117]. In this 2-D CE system, the first capillary

employs capillary sieving electrophoresis using a replaceable

sieving matrix, and the fractions are periodically transferred

across an interface into a second-dimensional capillary

where components are further resolved by MEKC. In addi-

tion to high resolution, the 2-D CE system also provides

high sensitivity because analytes are detected by LIF using

3-(2-furoyl)quinoline-2-carboxaldehyde (FQ) as the labeling

fluorogenic reagent. Analysis of Barrett’s esophagus tissues

[117], characterization of single MCF-7 breast cancer cells

[118], separation of the subcellular fractioning of isolated

components from mouse adrenal gland cells (AtT-20) [119]

and characterization of protein expression from a single

mouse embryo [120] are interesting applications of this

approach. The second modification of the original 2-D CE

design is the substitution of the one pair of capillaries to

analyze a single sample for a multiplexed system that allows

separation of five samples in parallel [121]. Therefore,

samples are injected into the five first-dimension capillaries,

capillary sieving electrophoresis fractions are transferred

across an interface to five second-dimension capillaries

(MEKC) and finally FQ-labeling analytes are detected by LIF

in a five-capillary sheath-flow cuvette. The 2-D CE system

has been successfully used for the separation of proteins and

biogenic amines from homogenates prepared from lung

cancer (A549) and mouse AtT-20 cell lines [121, 122].

The design of the capillary–capillary interface is a

significant feature for developing a 2-D CE instrument with

an optimum transfer of the analytes from the first to the

second dimension. Recently, Sahlin [123] has developed a

novel low dead-volume capillary–capillary interface that

allows straightforward filling of the systems with different

solutions in the two different capillaries. The design of this

interface allows two channels to be tangentially in contact

with each other and connected through a small opening at

the contact area. Since the channels do not cross each other

in the same plane, the capillaries can easily be filled with

different solutions. This home-made 2-D CE system is easy

to implement in the laboratory because commercial fused-

silica capillaries can be used and the interface can be easily

made up reproducibly with simple equipment without

microfabrication technology. The approach has great

potential for use in different types of CE applications, such

as CZE at two different pH values and CZE followed by

MEKC. Finally, another noteworthy and relevant contribu-

tion is a simple methodology for converting a commercial

CE-MS instrument into an integrated 2-D CE system [124],

which only requires the construction of a modified electrode

and a two-way electrical switch. The modified electrode

(constructed from a commercial electrode) allows the

simultaneous introduction of two capillaries and the two-

way electrical switch is used to apply the voltage difference

to the first or second capillary but must be compatible with

the application of high voltage. The first-dimension capillary



operates as a typical CE instrument with UV/visible detec-

tion; then fractions leaving the first dimension are auto-

matically collected and introduced into the second

dimension, for analysis on a CE-MS apparatus. The

approach proposed, which operates in a highly automatic

manner, is flexible and allows various combinations of CE

modes to be implemented, such as the screening of anti-

biotic families (nitroimidazoles and tetracyclines) in the first

CZE dimension and the subsequent resolution of each

family (nitroimidazoles by MEKC and tetracyclines by CZE)

in the second dimension.

The use of chemometric techniques is another current

approach for improving MEKC resolution. Thus, optimiza-

tion of separation parameters for the baseline resolution and

short migration times has been the subject of studies

reported by several authors using different approaches in

the period covered by this review [125–129]. Statistical

experimental [125] and factorial [126, 127] designs, multi-

linear regression [128] and genetic algorithms [129] are the

chemometric tools used to search for the optimum condi-

tions in MEKC separation. In general, resolution, peak

symmetry and analysis time are established as responses in

the approaches tested, which provide a good optimization of

the MEKC buffer system. The selection of the appropriate

chemometric technique is closely related to the complexity

of the mixture under study, and therefore the use of

powerful chemometric tools is not always the best choice.

However, these powerful chemometric techniques are

needed to improve resolution based on the quantification of

highly overlapping CE peaks, which is a growing area of

interest in the last years. Therefore, Li and co-workers [130]

reported the application of three different kinds of artificial

neural networks (ANNs) such as radial basis function,

generalized regression and linear neural networks in order

to resolve overlapped peaks. ANN inputs are selected by

applying principal component analysis to the 2-D array of

data generated by MEKC with DAD detection. The results

proved that the proposed ANN approach based on principal

component analysis input selection is suitable for the

quantification of the four components at their overlapped

MEKC peaks.

4 Detection techniques

4.1 MS

As the hyphenation of MEKC and MS has been a

challenging topic for several years, an array of means has

been researched to accomplish this coupling. As stated in

the previous review [1], additional work is still required to

reach a similar potential to that shown by this detection

technique in CZE, where robust CZE-MS methods are

currently available, especially in pharmaceutical analysis.

Two interesting overview articles have been published on

the period covered by this review [6, 131], mainly focused on

recent developments in interfacing and soft ionization

techniques, atmospheric-pressure photoionization versusthe typical ESI, as approaches to circumvent compatibility

problems in MEKC-MS. However, the contributions

reported in the period covered by this review are largely

focused on the use of polymeric surfactants with ESI-MS

detection to afford practical MEKC-MS determinations. This

trend can be ascribed to the fact that the use of these PSPs

with ESI interface is to date a more useful and practical

choice because the atmospheric-pressure photoionization

interface, although seeming to overcome many of the

deficiencies of MEKC-MS, does not provide the needed

sensitivity for the analysis of real samples [6].

As stated above, polymeric surfactants have been the

most frequent choice for developing practical MEKC-MS

determinations in the period covered by this review,

although the PF approach with conventional SDS micelles

has also been proposed along with the use of BGEs

containing volatile surfactants and buffers to afford the

direct coupling of MEKC to MS. The PF approach was

employed for the determination of nitroimidazoles using

ammonium acetate as the electrolyte [124] as a second

dimension of a 2-D CE system in which nitroimidazoles and

tetracyclines are separated in the first dimension by CZE at

pH 8.5. The method was really optimized with the aim of

being an application in the proposed integrated 2-D CE

system described in Section 3.3, instead of a sensitive

MEKC-MS practical methodology. Also the potential of the

hyphenation of MEKC with ESI-MS for the impurity

profiling of drugs using galantamine and ipratropium as

test samples has been evaluated using anionic SDS micelles

although in a BGE containing sodium phosphate and ACN

[132]. The authors have demonstrated that despite ionization

suppression by non-volatile buffer and surfactant, MS

detection of impurities present around 0.1% level can be

possible, demonstrating the great potential of the MEKC-

MS/MS system for the detection and structure elucidation

of minor impurities in drug substances. As in the previous

review, little work has been done toward finding suitable

volatile BGEs for the direct coupling of MEKC to MS. Biesen

and Bottaro [133] have tested the potential of ammonium

perfluorooctanoate (APFOA) for the analysis of N-methyl-

carbamate pesticides by MEKC-ESI-MS. This volatile

surfactant can be introduced into an MS without the adverse

effects of less volatile surfactants such as SDS, and its

residue can be easily removed from the ion-source after use,

as has recently been reported by Petersson et al. [134]. The

proposed MEKC system using a BGE consisting of 50 mM

APFOA at pH 9.0/isopropanol 98:2 was sensitive enough to

afford LODs between 0.01 and 0.08 mg/L in the SIM mode,

and even lower if an SPE pre-concentration step is used.

APFOA seems to be an effective alternative to SDS in

MEKC-ESI-MS because more cumbersome techniques such

as PF and reverse migrating micelles can be avoided.

Another strategy for the direct coupling of MEKC to

ESI-MS is based on the use of polymeric surfactants,

Professor Shamsi’s research group being a significant

reference in this field. In the period covered by this review,



several contributions have been reported by the Shamsi

group, such as the determination of ephedrine alkaloids

using polysodium N-undecenoxycarbonyl-L-leucinate as

chiral polymeric surfactant [135, 136], bezodiazepines and

benzoxazocine with poly(sodium N-undecenoxy carbonyl-L,L-

leucyl-valinate) [137] and warfarin enantiomers using poly-

sodium N-undecenoyl-L,L-leucyl-valinate [138]. The sensitiv-

ity and enantioselectivity provided by these methodologies

are in general better than those achieved through UV

detection. The spray chamber parameters as well as sheath

liquid conditions are found to significantly influence the MS

S/N of the analytes, whereas enantioselectivity is closely

related to the selected polymeric surfactant. Recently, this

research group has synthesized three amino acid-derived

(L-leucinol, L-isoleucinol and L-valinol) sulfated chiral

surfactants and evaluated their potential in MEKC and

MEKC-MS [139]. Among the three polymeric sulfated

surfactants studied, polysodium N-undecenoyl-L-isoleucine

sulfate with two chiral centers on the polymer head group

provided an overall higher enantioresolution in acidic

medium for acid and/or basic analytes, such as pseudoe-

phedrine, b-blockers, phenoxypropionic acid, benzoin deri-

vatives, benzodiazepinones, etc. Regarding sensitivity, lower

LODs were also obtained at acidic pH 2.0 (325 ng/mL),

approximately 16 times better than at pH 8.0 (5.2 mg/mL).

This work opens new perspectives in MEKC-MS using

polymeric surfactants, concretely anionic chiral polymeric

surfactants, due to the superiority of chiral separation and

sensitive MS detection at low pH over conventional high-pH

chiral separation and detection.

4.2 LIF spectroscopy

MEKC with LIF detection is currently a powerful analytical

tool routinely used for the sensitive monitoring of fluorescent

analytes, either native fluorescent or dye-labeled compounds.

LIF detection is consolidated in MEKC analysis as shown in

the large number of applications that have been reported over

the period covered by this review, mainly involving the

determination of analytes showing no native fluorescence.

Despite this fact, the contributions reported on MEKC-LIF are

mainly focused on the consolidation of ‘‘classical’’ excitation

sources like the argon-ion laser and evaluation of the use of

diode lasers as alternative sources, instead of on proposals for

new labeling schemes. This is because the existing probes

provide adequate sensitivity for the determination of a great

number of compounds in many samples; the new contribu-

tions try to achieve better derivatization chemistry, especially

by decreasing the derivatization time and increasing the

stability of the derivatives.

The argon-ion laser emitting at 488 nm is the most

frequent excitation source employed in LIF detection.

Although this excitation source has been used for the

sensitive determination of native fluorescent compounds

such as doxorubicin and doxorubicinol by CD-MEKC [84],

the majority of the MEKC-LIF methods reported in the

period covered by this review are focused on the determi-

nation of amine compounds after their derivatization with

two popular amino-reactive labels: fluorescein- and nitro-

benzo-2-oxa-1,3-diazol-based reagents. Among fluorescein

analogs, classical FITC [72, 73], 5-(4,6-dichloro-s-triazin-2-

ylamino) fluorescein (DTAF) [140] and 6-oxy-(N-succinimi-

dyl acetate)-9-(20-methoxy-carbonyl) fluorescein (SAMF) [58,

100, 141] have been used for the determination of amino

acids, biogenic amines and catecholamines in foods [58, 72]

and pharmaceutical [140] and biological samples [100, 141].

SAMF is the best alternative due to its better labeling

chemistry: derivatization was carried out at room tempera-

ture for ca. 10 min whereas FITC needs hours to complete

the derivatization and DTAF 30 min at 451C. Moreover,

FITC forms several by-products and DTAF and derivatives

are relatively unstable in water. Regarding resolution, a BGE

consisting of SDS [58, 141] or Brij-35 [100] and borate buffer

at an alkaline pH provides efficient baseline separation of

labeled analytes and also from the interfering peaks of label

excess. NBD-F [45, 78, 142] and 4-chloro-7-nitro-2,1,3-

benzoxadiazole [143, 144] are the amino-reactive labels with

the nitrobenzo-2-oxa-1,3-diazol moiety that are employed

most frequently for MEKC-LIF determination of amine

compounds using the argon-ion laser as excitation source.

These reagents, and especially NBD-F, provide improved

label chemistry with respect to fluorescein analogs, except

for SAMF. The proposal of the 4-chloro-7-nitro-2,1,3-

benzoxadiazole as an alternative to NBD-F is mainly based

on the high cost of the latter for practical applications. The

improved labeling chemistry afforded by NBD-F has fores-

ted its implementation in the in-capillary format for the

determination of organophosphorus pesticides in spiked

river water samples [45]. Profiling of amine metabolites in

human biofluids [78] and determination of spermine

synthase activity [142], epinephrine and dopamine in tradi-

tional Chinese medicines [143] and catecholamines and

amino acids in human urine samples [144] are the MEKC-

LIF applications reported using these labels in the period

covered by this review. As with fluorescein derivatives, good

separations can easily be achieved between derivatives and

from the excess of the label using a simple BGE containing

SDS and borate buffer (organic modifiers are required in

some cases) with significant sensitivity: LODs at ng/mL

level are generally reported for the analytes. Another inter-

esting contribution to LIF detection in MEKC is that

reported by the Dovichi group for 2-D CE systems [117–122].

As stated in Section 3.3, MEKC analysis is carried out in the

second dimension and LIF is used for the detection of

compounds of biological and clinical interest after their

labeling with FQ. In the built in-house instrument devel-

oped by the Dovichi group, the fluorescence of the labeled

analytes is detected by using a sheath-flow cuvette, and

argon-ion [119] and solid-state diode [117, 118, 122] lasers

were used as excitation sources.

The use of diode lasers as light sources for LIF has

noticeably increased in the last 2 years as an alternative to

the argon-ion laser because comparable results in terms of



sensitivity can be achieved with these relatively lower cost

and longer lifetime excitation sources. The increase in

research based on the use of LEDs as excitation sources in

LIF has also contributed to this growth; although LEDs have

recently been extended to the UV region, built in-house

equipment must still be used to date. As with MEKC-LIF

methods using the argon-ion laser as the excitation source,

the reported methods are useful for the determination of

amine compounds after their labeling; as a variety of diode

lasers can be used due to their lower cost, from UV to near

infrared spectrum, different labels have been proposed as a

result. Sulfoindocyanine succinimidyl ester (Cy5-NHS) [57,

145] and NDA [88, 146] are the labels whose luminescent

properties match well with the 635 and 415 nm laser

modules, respectively, built-in commercial equipment.

These reagents have a good labeling chemistry, derivatiza-

tion time between 10 and 30 min at room temperature and

provide good sensitivity, although in some cases an SPE pre-

concentration step is necessary, such as in the determina-

tion of b-lactam antibiotics [145] and glyphosate [146] in

environmental waters, in which LODs at ng/L level can be

achieved.

The contributions toward the use of LED-induced

fluorescence detection in MEKC in the period covered by

this review are essentially those reported by Li et al. [28, 30,

147]. Thus, blue (476 nm) [28], violet (410 nm) [30] and UV

(380 nm) [147] LEDs have been used for the determination

of phenethylamine designer drugs with FITC, dopamine

and norepinephrine with NDA, and tryptophan with

phenylglyoxal, respectively. In all cases, on-line sample

concentration techniques were used to improve sensitivity.

Another interesting contribution in this field is that reported

by Aspinwall and co-workers [47] based on the utilization of

a high power UV (365 nm) LED in MEKC-LIF. In addition to

its high emission intensity, also noteworthy is its excitation

wavelength, which is particularly useful for the fluorescence

detection of common small-molecule fluorogenic labels.

This compact and inexpensive UV-LED allows the deter-

mination of native fluorescent PAHs and glutamic and

aspartic acids, proteins and peptides after their derivatiza-

tion with o-phthalaldehyde with LODs at nM levels and

without the need for on-line sample concentration.

Finally it is noteworthy to point out the introduction by

Chen et al. [148, 149] of a novel luminescence detection

method in MEKC based on continuous wave-based multi-

photon excitation fluorescence (MPE) detection using cheap

diode laser instead of high cost femto-second pulse laser.

Two major benefits can be reported on the use of MPE in

MEKC: (ii) an ultra-low detection background that allows

LODs about zeptomole level to be readily achieved and (ii)

capability of simultaneous multicolor excitation in a broad

range; hence, using only one excitation line it is possible to

detect complex components labeled with different fluor-

escent tags. Although the end-column MPE configuration

exhibits better detectability, due to less light scattering,

the LODs achieved are not as good as those provided by

the fento-second pulse laser; however, when compared with

the classical argon-ion laser excitation source, better mass

detectability and higher separation selectivity were achieved,

although the sensitivity in concentration was not so

encouraging.

5 Conclusions and prospects

From the advances reported in the different topics covered

by this review on approaches for increasing sensitivity and

resolution in MEKC-based determinations, it can be inferred

that further improvements are still needed, especially in

sensitivity, because of the extremely rapid and extensive

developments in resolution in recent years. Reasonably,

initial improvements in MEKC were focused on increasing

resolution and thus research areas such as MEEKC and

polymeric PSPs in MEKC have developed to such an extent

so far that they warrant separate discussion and thus are not

included in this review. The ‘‘classical’’ BGE containing SDS

micelles with the incorporation of different additives is a

useful and consolidated choice to afford the resolution of

complex mixtures of charged or neutral analytes. The recent

introduction of carbon nanostructures as PSPs is an

interesting topic that requires further studies to assess their

real potential. One of the biggest drawbacks of MEKC so far

is problems associated with its direct connection with MS.

The advances in this field have not progressed as expected in

the period covered by this review, and from the reported

contributions it can be inferred that the use of polymeric

surfactants seems to be the alternative with the greatest

potential for avoiding the problems associated with this

direct coupling, although the design of new approaches for

interfacing MEKC to MS is another topic of great relevance.

The consolidation of on-line sample concentration techni-

ques in routine MEKC analysis, the noticeable growth in LIF

detection using the ‘‘classical’’ argon-ion laser excitation

source also for routine analysis and the improvements in

diode lasers including LEDs as excitation sources are the

most significant approaches for improving sensitivity in

MEKC. With advances in laser technology, lower cost diode

excitation sources will be integrated in commercial instru-

ments providing excellent detection sensitivity, especially

through the use of LEDs.

The author gratefully acknowledges funding by SpainsDepartment of Research of the Ministry of Education andScience under Project CTQ2007-63962 and by FEDER.

The author has declared no conflict of interest.

6 References

[1] Silva, M., Electrophoresis 2007, 28, 174–192.

[2] Fuguet, E., Rafols, C., Bosch, E., Abraham, M. H.,Roses, M., Electrophoresis 2006, 27, 1900–1914.



[3] Williamson, Y., Davis, J. M., Electrophoresis 2006, 27,

572–583.

[4] Hasemann, P., ter Balk, M., Waetzig, H., Electrophor-

esis 2007, 28, 1798–1804.

[5] Tellez, A., Fuguet, E., Roses, M., J. Chromatogr. A 2007,

1139, 143–151.

[6] Klampfl, C. W., Buchberger, W., Anal. Bioanal. Chem.

2007, 388, 533–536.

[7] Kitagawa, F., Tsuneka, T., Akimoto, Y., Sueyoshi, K.

et al., J. Chromatogr. A 2006, 1106, 36–42.

[8] Shi, S., Li, W., Liao, Y., Cai, Z., Liu, H., J. Chromatogr. A

2007, 1167, 120–124.

[9] Song, G. Q., Peng, Z. L., Lin, J. M., J. Sep. Sci. 2006, 29,

2065–2071.

[10] Wu, X., Wu, W., Zhang, L., Xie, Z. et al., Electrophoresis

2006, 27, 4230–4239.

[11] Musijowski, J., Pobozy, E., Trojanowicz, M., J. Chro-

matogr. A 2006, 1104, 337–345.

[12] He, S., Zhao, Y., Zhu, Z., Liu, H. et al., Talanta 2006, 69,

166–171.

[13] Juan-Garcıa, A., Font, G., Pico, Y., J. Chromatogr. A

2007, 1153, 104–113.

[14] Macia, A., Borrull, F., Calull, M., Aguilar, C., J. Chro-

matogr. A 2006, 1117, 234–245.

[15] Wang, S., Ye, S., Cheng, Y., J. Chromatogr. A 2006,

1109, 279–284.

[16] Chiu, T. C., Chang, H. T., J. Chromatogr. A 2007, 1146,

118–124.

[17] Fang, H., Yang, F., Sun, J., Zeng, Z., Xu, Y., Electro-

phoresis 2007, 28, 3697–3704.

[18] Ravelo-Perez, L. M., Hernandez-Borges, J., Cifuentes, A.,

Rodrıguez-Delgado, M. A., Electrophoresis 2007,28,1805–1814.

[19] Ravelo-Perez, L. M., Hernandez-Borges, J., Borges-

Miquel, T. M., Rodrıguez-Delgado, M. A., Electrophor-esis 2007, 28, 4072–4081.

[20] Molina-Mayo, C., Hernandez-Borges, J., Borges-

Miquel, T. M., Rodrıguez-Delgado, M. A., J. Chroma-togr. A 2007, 1150, 348–355.

[21] Ravelo-Perez, L. M., Hernandez-Borges, J., Borges-

Miquel, T. M., Rodrıguez-Delgado, M. A., J. Sep. Sci.2007, 30, 3240–3246.

[22] Li, Q., Huie, C. W., Electrophoresis 2006, 27,

4219–4229.

[23] Giordano, B. C., Copper, C. L., Collins, G. E., Electro-

phoresis 2006, 27, 778–786.

[24] Giordano, B. C., Newman, C. D., Federowicz, P. M.,

Collins, G. E., Burgi, D. S., Anal. Chem. 2007, 79,6287–6294.

[25] Horakova, J., Petr, J., Maier, V., Znaleziona, J. et al.,

J. Chromatogr. A 2007, 1155, 193–198.

[26] Wang, C., Han, D., Wang, Z., Zang, X., Wu, Q., Anal.

Chim. Acta 2006, 572, 190–196.

[27] Wang, C., Li, C., Zang, X., Han, D. et al., J. Chromatogr.

A 2007, 1143, 270–275.

[28] Tsai, C. C., Liu, J. T., Shu, Y. R., Chan, P. H., Lin, C. H.,

J. Chromatogr. A 2006, 1101, 319–323.

[29] Jen, H. P., Tsai, Y. C., Su, H. L., Hsieh, Y. Z., J. Chro-matogr. A 2006, 1111, 159–165.

[30] Chang, Y. S., Shih, C. M., Li, Y. C., Lin, C. H., Anal. Sci.2006, 22, 557–561.

[31] Ibrahim, W. A. W., Hermawan, D., Sanagi, M. M.,J. Chromatogr. A 2007, 1170, 107–113.

[32] Zang, X., Anal. Lett. 2006, 39, 1927–1939.

[33] Shen, H. J., Lin, C. H., Electrophoresis 2006, 27,1255–1262.

[34] Zhao, Y., Kong, Y., Wang, B., Wu, Y., Wu, H., J. Chro-matogr. A 2007, 1146, 131–135.

[35] Huang, C. W., Jen, H. P., Wang, R. D., Hsieh, Y. Z.,J. Chromatogr. A 2006, 1110, 240–244.

[36] Quirino, J. P., Terabe, S., Anal. Chem. 2000, 72,1023–1030.

[37] Fang, N., Meng, P., Zhang, H., Sun, Y., Chen, D. D. Y.,Analyst 2007, 132, 127–134.

[38] Cheng, H. L., Jong, Y. J., Li, J. H., Ko, W. K., Wu, S. M.,Electrophoresis 2006, 27, 4711–4716.

[39] Meng, P., Fang, N., Wang, M., Liu, H., Chen, D. D. Y.,Electrophoresis 2006, 27, 3210–3217.

[40] Lin, Y. H., Li, J. H., Ko, W. K., Wu, S. M., J. Chromatogr.A 2006, 1130, 281–286.

[41] Lin, Y. H., Lee, M. R., Lee, R. J., Ko, W. K., Wu, S. M.,J. Chromatogr. A 2007, 1145, 234–240.

[42] Zhu, J., Yu, K., Chen, X., Hu, Z., J. Chromatogr. A 2007,1166, 191–200.

[43] Gong, M., Wehmeyer, K. R., Limbach, P. A., Heineman,W. R., Anal. Chem. 2006, 78, 6035–6042.

[44] Zhang, J., Hoogmartens, J., van Schepdael, A., Elec-trophoresis 2006, 27, 35–43.

[45] Zhou, L., Luo, Z., Wang, S., Hui, Y. et al., J. Chroma-togr. A 2007, 1149, 377–384.

[46] Cheng, J., Kang, J., Electrophoresis 2006, 27,865–871.

[47] Hapuarachchi, S., Janaway, G. A., Aspinwall, C. A.,Electrophoresis 2006, 27, 4052–4059.

[48] Liu, L., Chen, X., Hu, Z., Talanta 2007, 71, 155–159.

[49] Liu, L., Chen, X., Hu, Z., Microchim. Acta 2007, 159,125–131.

[50] Do Rosario, P. M. A., Nogueira, J. M. F., Electrophor-esis 2006, 27, 4694–4702.

[51] Wang, T., Tang, J., Wan, W., Zhao, S., J. Chromatogr.A 2007, 1147, 105–110.

[52] Zhan, W., Wang, T., Li, S. F. Y., Electrophoresis 2000,21, 573–578.

[53] Zhao, S., Wang, H., Zhang, R., Tang, L., Liu, Y. M.,Electrophoresis 2006, 27, 3428–3433.

[54] Maguregui, M. I., Alonso, R. M., Barandiaran, M.,Jimenez, R. M., Garcıa, N., J. Chromatogr. A 2007,1154, 429–436.

[55] Wang, S., Cheng, Y., J. Pharm. Biomed. Anal. 2006, 40,1137–1142.

[56] Baskan, S., Daut-Oezdemir, A., Guenaydin, K., Erim, F. B.,Talanta 2007, 71, 747–750.

[57] Serrano, J. M., Silva, M., Electrophoresis 2006, 27,4703–4710.



[58] Cao, L., Wang, H., Ma, M., Zhang, H., Electrophoresis2006, 27, 827–836.

[59] Yu, K., Wang, Y. W., Cheng, Y. Y., Chromatographia2006, 63, 389–393.

[60] Injac, R., Kac, J., Mlinaric, A., Karljikovic-Rajic, K.,J. Sep. Sci. 2006, 29, 1288–1293.

[61] Hancu, G., Gaspar, A., Gyeresi, A., J. Biochem.Biophys. Meth. 2007, 69, 251–259.

[62] Cornelius, M. G., Schmeiser, H. H., Electrophoresis2007, 28, 3901–3907.

[63] Zhang, Y., Chen, J., Ma, X. M., Shi, Y. P., J. Pharm.Biomed. Anal. 2007, 45, 742–746.

[64] Tavares, M. F. M., Electrophoresis 2007, 28, 3722–3730.

[65] Gong, S., Liu, F., Li, W., Gao, F. et al., J. Chromatogr. A2006, 1121, 274–279.

[66] Kartsova, L. A., Bessonova, E. A., J. Anal. Chem. 2007,62, 68–75.

[67] Chen, X., Qi, S., Curr. Anal. Chem. 2006, 2,411–419.

[68] Mwongela, S. M., Siminialayi, N., Fletcher, K. A.,Warner, I. M., J. Sep. Sci. 2007, 30, 1334–1342.

[69] Qu, Q., Tang, X., Mangelings, D., Wang, C. et al.,J. Chromatogr. B 2007, 853, 31–37.

[70] Mwongela, S. M., Numan, A., Gill, N. L., Agbaria, R. A.,Warner, I. M., Anal. Chem. 2003, 75, 6089–6096.

[71] Ha, P. T. T., Hoogmartens, J., van Schepdael, A.,J. Pharm. Biomed. Anal. 2006, 41, 1–11.

[72] Carlavilla, D., Moreno-Arribas, M. V., Fanali, S.,Cifuentes, A., Electrophoresis 2006, 27, 2551–2557.

[73] Herrero, M., Ibanez, E., Fanali, S., Cifuentes, A., Elec-trophoresis 2007, 28, 2701–2709.

[74] Li, W., Chen, Z., Liao, Y., Liu, H., Electrophoresis 2006,27, 837–841.

[75] Kartsova, L. A., Strel’nikova, E. G., J. Anal. Chem. 2007,62, 716–720.

[76] Zhao, Y., Luo, X., Ming, Y., Chen, L., Li, Y., J. Pharm.Biomed. Anal. 2006, 41, 124–128.

[77] Cai, J., Li, P., J. Liq. Chromatogr. Relat. Technol. 2007,30, 2805–2817.

[78] Tseng, H. M., Li, Y., Barrett, D. A., Anal. Bioanal. Chem.2007, 388, 433–439.

[79] Zhang, Y., Chen, J., Zhao, L., Shi, Y. P., Biomed.Chromatogr. 2007, 21, 987–992.

[80] Kodama, S., Yamamoto, A., Sato, A., Suzuki, K. et al.,J. Agric. Food Chem. 2007, 55, 6547–6552.

[81] Gotti, R., Furlanetto, S., Pinzauti, S., Cavrini, V.,J. Chromatogr. A 2006, 1112, 345–352.

[82] Baldacci, A., Thormann, W., J. Sep. Sci. 2006, 29,153–163.

[83] Kodama, S., Ito, Y., Nagase, H., Yamashita, T. et al.,J. Health Sci. 2007, 53, 491–495.

[84] Eder, A. R., Chen, J. S., Arriaga, E. A., Electrophoresis2006, 27, 3263–3270.

[85] Antczak, A., Ramstad, T., Johnson, R., Chromato-graphia 2006, 64, 57–64.

[86] Wistuba, D., Bogdanski, A., Larsen, K. L., Schurig, V.,Electrophoresis 2006, 27, 4359–4363.

[87] Wang, H., Zhang, R., Zhao, S., Tang, L., Pan, Y., Anal.Chim. Acta 2006, 560, 64–68.

[88] Siri, N., Lacroix, M., Garrigues, J. C., Poinsot, V.,Couderc, F., Electrophoresis 2006, 27, 4446–4455.

[89] Lin, S. J., Chen, Y. R., Su, Y. H., Tseng, H. I., Chen, S. H.,J. Chromatogr. B 2006, 830, 306–313.

[90] Nejmanova, J., Coufal, P., Bosakova, Z., Suchankova, J.,Tesarova, E., J. Sep. Sci. 2007, 30, 2123–2129.

[91] Gotti, R., Gioia, M. G., Gatti, R., Cavrini, V., J. Sep. Sci.2006, 29, 1259–1267.

[92] Tian, K., Chen, H., Tang, J., Chen, X., Hu, Z., J. Chro-matogr. A 2006, 1132, 333–336.

[93] Wickramanayake, P. U., Tran, T. C., Hughes, J. G.,Macka, M. et al., Electrophoresis 2006, 27, 4069–4077.

[94] Chang, L. C., Sun, S. W., J. Pharm. Biomed. Anal. 2006,40, 62–67.

[95] Herrero-Martınez, J. M., Oumada, F. Z., Roses, M.,Bosch, E., Rafols, C., J. Sep. Sci. 2007, 30,2493–2500.

[96] Petersen, I. L., Andersen, K. E., Sorensen, J. C.,Sorensen, H., J. Chromatogr. A 2006, 1130, 253–258.

[97] Trenerry, V. C., Caridi, D., Elkins, A., Donkor, O., Jones, R.,Food Chem. 2006, 98, 179–187.

[98] Schnee, V. P., Baker, G. A., Rauk, E., Palmer, C. P.,Electrophoresis 2006, 27, 4141–4148.

[99] Wang, X. K., He, Y., Z., Qian, L. L., Talanta 2007, 74, 1–6.

[100] Cao, L. W., Wang, H., Zhang, H. S., Chromatographia2006, 63, 571–578.

[101] Huang, J., Kang, J., J. Chromatogr. B 2007, 846,364–367.

[102] Huang, J., Kang, J., J. Chromatogr. A 2007, 1175,294–296.

[103] van Biesen, G., Bottaro, C. S., J. Chromatogr. A 2007,1157, 437–445.

[104] Delgado-Zamarreno, M. M., Gonzalez-Maza, I.,Sanchez-Perez, A., Carabias Martınez, R., Food Chem.2007, 100, 1722–1727.

[105] Pascoe, R. J., Foley, J. P., Electrophoresis 2003, 24,4227–4240.

[106] Wiedmer, S. K., Jussila, M. S., Riekkola, M. L., TrAC2004, 23, 562–582.

[107] Owen, R. L., Strasters, J. K., Breyer, E. D., Electro-phoresis 2005, 26, 735–751.

[108] Bilek, G., Kremser, L., Blaas, D., Kenndler, E., J. Chro-matogr. B 2006, 841, 38–51.

[109] Jiang, J., Geng, L., Qu, F., Luo, A. et al., J. Chromatogr.Sci. 2007, 45, 587–592.

[110] Suarez, B., Simonet, B. M., Cardenas, S., Valcarcel, M.,Electrophoresis 2007, 28, 1714–1722.

[111] Moliner-Martınez, Y., Cardenas, S., Valcarcel, M.,Electrophoresis 2007, 28, 2573–2579.

[112] Moliner-Martınez, Y., Cardenas, S., Valcarcel, M.,J. Chromatogr. A 2007, 1167, 210–216.

[113] Nilsson, C., Nilsson, S., Electrophoresis 2006, 27,76–83.

[114] Pranaityte, B., Padarauskas, A., Electrophoresis 2006,27, 1915–1921.



[115] Robb, C. S., J. Liq. Chromatogr. Relat. Technol. 2007,30, 729–759.

[116] Cesla, P., Blomberg, L., Hamberg, M., Jandera, P.,J. Chromatogr. A 2006, 1115, 253–259.

[117] Kraly, J. R., Jones, M. R., Gomez, D. G., Dickerson, J. A.et al., Anal. Chem. 2006, 78, 5977–5986.

[118] Harwood, M. M., Bleecker, J. V., Rabinovitch, P. S.,Dovichi, N. J., Electrophoresis 2007, 28, 932–937.

[119] Abul-Fazal, M., Palmer, V. R., Dovichi, N. J., J. Chro-matogr. A 2006, 1130, 182–189.

[120] Harwood, M. M., Christians, E. S., Abul-Fazal, M.,Dovichi, N. J., J. Chromatogr. A 2006, 1130, 190–194.

[121] Zhu, C., He, X., Kraly, J. R., Jones, M. R. et al., Anal.Chem. 2007, 79, 765–768.

[122] Chen, X., Abul-Fazal, M., Dovichi, N. J., Talanta 2007,71, 1981–1985.

[123] Sahlin, E., J. Chromatogr. A 2007, 1154, 454–459.

[124] Santos, B., Simonet, B. M., Rıos, A., Valcarcel, M.,Electrophoresis 2007, 28, 1345–1351.

[125] Lin, Y. H., Yang, Y. H., Wu, S. M., J. Pharm. Biomed.Anal. 2007, 44, 279–282.

[126] Micke, G. A., Fujiya, N. M., Tonin, F. G., Costa, A. C. O.,Tavares, M. F. M., J. Pharm. Biomed. Anal. 2006, 41,1625–1632.

[127] Nemutlu, E., Celebier, M., Uyar, B., Altinoez, S.,J. Chromatogr. B 2007, 854, 35–42.

[128] Williams, A. A., Fakayode, S. O., Huang, X., Warner,I. M., Electrophoresis 2006, 27, 4127–4140.

[129] Yu, K., Lin, Z., Cheng, Y., Anal. Chim. Acta 2006, 562,66–72.

[130] Zhang, Y., Li, H., Hou, A., Havel, J., Chemometr. Intell.Lab. Syst. 2006, 82, 165–175.

[131] Somsen, G. W., Mol, R., de Jong, G. J., Anal. Bioanal.Chem. 2006, 384, 31–33.

[132] Mol, R., Kragt, E., Jimidar, I., de Jong, G. J., Somsen,G. W., J. Chromatogr. B 2006, 843, 283–288.

[133] van Biesen, G., Bottaro, C. S., Electrophoresis 2006, 27,4456–4468.

[134] Petersson, P., Jornten-Karlsson, M., Stalebro, M.,Electrophoresis 2003, 24, 999–1007.

[135] Hou, J., Zheng, J., Rizvi, S. A. A., Shamsi, S. A., Elec-trophoresis 2007, 28, 1352–1363.

[136] Hou, J., Zheng, J., Shamsi, S. A., Electrophoresis 2007,28, 1426–1434.

[137] Hou, J., Rizvi, S. A. A., Zheng, J., Shamsi, S. A., Elec-trophoresis 2006, 27, 1263–1275.

[138] Hou, J., Zheng, J., Shamsi, S. A., J. Chromatogr. A2007, 1159, 208–216.

[139] Rizvi, S. A. A., Zheng, J., Apkarian, R. P., Dublin, S. N.,Shamsi, S. A., Anal. Chem. 2007, 79, 879–898.

[140] Wang, W., Li, C., Li, Y., Hu, Z., Chen, X., J. Chromatogr.A 2006, 1102, 273–279.

[141] Cao, L. W., Biomed. Chromatogr. 2007, 21, 708–715.

[142] Sano, M., Nishino, I., J. Chromatogr. B 2007, 845,80–83.

[143] Dong, Y., Chen, H., Chen, Y., Hui, Y. et al., J. Sep. Sci.2006, 29, 2049–2055.

[144] Zhou, L., Wang, S., Tian, K., Dong, Y., Hu, Z., J. Sep.Sci. 2007, 30, 110–117.

[145] Serrano, J. M., Silva, M., Electrophoresis 2007, 28,3242–3249.

[146] Jiang, J., Lucy, C. A., Talanta 2007, 72, 113–118.

[147] Chang, Y. S., Shih, C. M., Lin, C. H., Anal. Sci. 2006, 22,235–240.

[148] Chen, S., Liu, B. F., Fu, L., Xiong, T. et al., J. Chromatogr.A 2006, 1109, 160–166.

[149] Chen, S., Xu, Y., Xu, F., Feng, X. et al., J. Chromatogr.A 2007, 162, 149–153.



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