Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

Review

Ewa Dabek-ZlotorzynskaRocio Aranda-RodriguezKatherine Keppel-Jones

Analysis andAir Quality Division,Environmental TechnologyCentre, Environment Canada,Ottawa, ON, Canada

Recent advances in capillary electrophoresis andcapillary electrochromatography of pollutants

An overview of major developments in capillary electrophoresis and capillary electro-chromatography systems in the environmental field is presented, covering relevantpublications between the second half of 1999 and early 2001. Contributions are re-viewed in relation to developments in detection, sample preparation/preconcentration,precision and applications. Many interesting examples are shown and the influence ofimportant parameters on the performance of developed methods is discussed.

Keywords: Capillary electrophoresis / Capillary electrochromatography / Pollutants / ReviewEL 4679

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 42622 Advances in detection . . . . . . . . . . . . . . . . . . 42632.1 Indirect UV absorption detection . . . . . . . . . 42632.2 Fluorescence detection . . . . . . . . . . . . . . . . . 42632.3 Electrochemical detection . . . . . . . . . . . . . . 42632.4 MS and ICP-MS detection . . . . . . . . . . . . . . 42643 Sample preparation and preconcentration . 42653.1 SPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42663.2 FIA-CE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42663.3 Sample stacking and sweeping . . . . . . . . . . 42674 Separation on microchips . . . . . . . . . . . . . . . 42675 CEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42686 Reproducibility and quantification . . . . . . . . 42707 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 42717.1 Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42717.2 PAHs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42717.3 Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42727.4 Carbonyls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42727.5 Explosives . . . . . . . . . . . . . . . . . . . . . . . . . . . 42727.6 Sulfonates and surfactants . . . . . . . . . . . . . . 4273

7.7 Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42737.8 Humic acids . . . . . . . . . . . . . . . . . . . . . . . . . . 42737.9 Inorganic and small organic ions . . . . . . . . . 42747.10 Element speciation . . . . . . . . . . . . . . . . . . . . 42767.11 Miscellaneous applications . . . . . . . . . . . . . 42778 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 42779 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 4278

1 Introduction

The ever-present need for new fast and efficient methodswithin the environmental sector fuels research into betterand more selective and sensitive analytical procedures.Thus, the interest continues in capillary electrophoresis(CE), an analytical separation technique that has enor-mous potential for environmental analysis especially forpolar and ionic analytes. This review represents the thirdin a series in this journal on the application of CE for deter-mination of environmental pollutants. Covering the periodbetween the second half of 1999 and early 2001, thisreview focuses on significant developments of pollutantanalysis using CE and capillary electrochromatography(CEC). In this way, it is a continuation of the previousreview on organic pollutants by Sovocool et al. [1] pub-lished in this journal, but in addition covers major devel-opments for inorganic pollutants. Because of other re-views that appear in this issue, coverage of pesticidesand herbicides has been excluded.

This paper reviews current research in the field through alook at the major approaches and achievements. Thus,the latest breakthroughs and improvements in detectiontechniques, in sample preparation and preconcentrationin addition to the latest uses of electromigration techni-ques for fabricated microchip analysis and the applicationof these methods to the detection of a variety of environ-

Correspondence: Dr. Ewa Dabek-Zlotorzynska, Analysis and AirQuality Division, Environmental Technology Centre, EnvironmentCanada, Ottawa, ON, CanadaE-mail: [email protected]: +613-990-8568

Abbreviations: AES, atomic emission spectrometry; CSEI,cation-selective exhaustive injection; DAD, diode-array detector;ECD, electrochemical detection; FASI, field-amplified stackinginjection; FIA, flow injection analysis; HIBA, hydroxyisobutyricacid; ICP, inductively coupled plasma; IDLIF, indirect laser-induced fluorescence; LIDF, laser-induced dispersed fluores-cence; NCAC, nitrogen-containing aromatic compounds; PAH,polyaromatic hydrocarbon; SFE, supercritical fluid extraction;TNT, trinitrotoluene; TTAB, tetradecyltrimethylammonium bro-mide

4262 Electrophoresis 2001, 22, 4262–4280

ª WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 0173-0835/01/1911–4262 $17.50+.50/0

mental pollutants are described. It is hoped that thisarticle will provide a flavor for the current directions inwhich CE is evolving in the environmental field, assessthe areas where CE has matured, and perhaps offerinsight into some of the latent, untapped promise of CE.

2 Advances in detection

2.1 Indirect UV absorption detection

CE with indirect UV detection is still the most popular andwidely utilized detection method for the analysis of a vari-ety of UV-transparent species like metal ions, simpleanions, and small organic acids. Recent principles andstrategies influencing optimal performance of indirect UVdetection were discussed in detail by Macka et al. [2, 3].These include factors which affect detection sensitivityand peak shape, procedures for buffering the electrolyte,and approaches to the design and optimization of thecomposition of background electrolytes (BGEs). NewBGE compositions and new applications using indirectUV detection for inorganic anions and organic acids havebeen reviewed in detail elsewhere [4, 5]. Current majorapplications will be discussed later in this review.

To date, the majority of applications of indirect UV detec-tion have been reported in conjunction with capillary zoneelectrophoresis (CZE) separation. Recently, Boyce et al.[6] used indirect photometric detection for a CEC systemin which an anion-exchange stationary phase (in the formof aminated latex particles) was coated onto the wall of afused-silica capillary. This study focused on the choice ofthe type and concentration of the absorbing coion (probe)added to the BGE and the role of the species in manipu-lating the ion-exchange contributions to the separation,with a view to controlling the selectivity of the separation.It was reported that when variation of the separationselectivity – from predominantly electrophoretic to pre-dominantly ion-exchange in nature – was desired, thiswas best achieved by varying the type of probe ratherthan its concentration. For example, the nitrate probeprovided predominantly electrophoretic separations withgood peak shapes and high efficiencies.

2.2 Fluorescence detection

Laser-induced dispersed fluorescence (LIDF) detectionusing a liquid-nitrogen cooled charge-coupled device de-tector in combination with CEC has been shown to bea very powerful tool for the quick identification of poly-aromatic hydrocarbons (PAHs) and nitrogen-containingaromatic compounds (NCACs) in complex environmentalsamples [7]. This paper also demonstrated the utility of

using dispersed fluorescence in the deconvolution ofcoeluting peaks in CEC. Limits of detection (LODs)ranged from 0.5 to 9.6�10 M for selected PAHs and0.9–3.7�10 M for selected NCACs. CE interfaced withfluorescence line narrowing (FLN) and non-line narrowing(FNLN) spectroscopy has been reported as a novelapproach for the separation and on-line spectral struc-tural identification of diastereomeric dibenzo[a,l]pyrenediol epoxide-derived deoxyadenosine (–dA) adducts [8].It was shown that low-temperature CE-FNLN/FLN meth-odology not only allows for on-line identification viavibrationally resolved 4.2 K fluorescence spectra, butalso provides conformational information on the –dAadducts.

A few recent reports have utilized indirect LIF detectionfor samples which inherently have no fluorophore [9–12].Bailey and Wallenborg [11] demonstrated the first exam-ple of indirect laser-induced fluorescence (IDLIF) detec-tion in packed channel CEC. Laser excitation was pro-vided by a near-infrared laser diode, which can be usedin a compact, inexpensive, low power consumption sys-tem. IDLIF detection was performed using an epifluores-cence system with excitation provided by a 635 nm diodelaser and micromolar concentrations of the dye Cy-5 asthe visualizing agent. To achieve a stable fluorescencebackground in CEC using a charged visualization agent,it was necessary to use a nonporous stationary phase.The developed system was used to separate nitro-aromatic and nitramine explosives.

2.3 Electrochemical detection

Using electrochemical detection (ECD) methods in CE isalso widespread in the literature. New approaches as wellas new applications of ECD of environmental pollutantshave been proposed in the last two years [13–25]. Mostof the work has been performed in the amperometricmode, although some have applied conductivity [21] andpotentiometric methods [23–25]. Investigations into theoptimal configuration for coupling CE with ECD continuedduring the review period. An improved fabrication methodfor a decoupler for on-column amperometric detectionin CE was recently described by Zhang et al. [17]. Thedecoupler was fabricated by combining laser focusingetching and HF etching which provided a more durablejoint with higher strength, shorter stabilization time, higherelectric conductivity efficiency, lower noise and less ofa decrease in separation efficiency. Application of thedeveloped on-column amperometric detection using acarbon-fiber microelectrode and practical small electro-chemical detection cell was demonstrated for the detec-tion of para-substituted phenols.

Electrophoresis 2001, 22, 4262–4280 CE and CEC of pollutants 4263

CE

and

CE

C

Hilmi and co-workers [19, 20] developed a simple end-column amperometric detector in conjunction withelectrokinetic CE for analysis of trinitrotoluene (TNT) andother explosive compounds. Using an off-column cellconsisting of a Ag/AgCl reference electrode, a platinumcounter electrode, and a gold, silver, or silver-plated goldworking electrode resulted in elimination of the require-ment for a capillary coupler, and electrochemical detec-tion was achieved by maintaining the working electrodeat –700 m V. This detection system offered a 10-fold detec-tion improvement over UV measurement for most of theexplosives tested in this study [19].

Electrochemical cells can be miniaturized fairly easily, andthe detection technique is quite sensitive, if the analyte iscapable of electrochemical oxidation or reduction underconditions used by CE instrumentation. Hilmi and Luong[20] described the coupling of a microelectrochemicaldetection system with CE chips. As part of an electro-chemical detection system, the working gold electrodewas inserted into a specially designed detection cell,with its sensing area positioned just outside the separa-tion channel outlet to provide high sensitivity detectionwith negligible interferences from the separation electricfield. The design is in contrast to disposable CE chipsequipped with integrated thin-film or sputtered workingelectrodes that require replacement of the entire analyti-cal system in the case of electrode passivation or severeelectrode fouling.

In a recently published study, Wilke et al. [21] describedthe development of amperometric detection of ions inCE by ion transfer across a liquid-liquid microinterface.The applicability of this detection scheme for nonredoxions was demonstrated by means of the cations cholineand acetylcholine and by some alkyl and aryl sulfate andsulfonate anions. A new electrode composed of carbonsol-gel composite material has been developed andapplied for amperometric detection of phenols and othercompounds in CE [16]. The performance of these sol-gelbased electrodes revealed some desirable properties,namely stability, renewability, and versatility in differentCE modes. Various buffer systems containing cationic oranionic surfactants were found to be suitable for ECD withthe sol-gel derived electrodes and had no deleteriouseffects on the electrode surface.

Poels and Nagels [23, 24] reported the possibility of usingconducting polymers and oligomers as electrode materialin potentiometric detection for CE. The electrode materi-als were successfully utilized for the determination ofcarboxylic acids and inorganic anions under co-electro-osmotic conditions. A new potentiometric analytical tech-nique termed pulsed potentiometric detection (PPD) hasbeen developed and introduced as an end-capillary de-

tection technique in CE [25]. In contrast to normal poten-tiometry where the potential is measured in a steadystate, PPD involves the application of one or more inde-pendent pre-pulses to the detection electrode prior to themeasurement period. The preliminary utilization of PPD incombination with the use of inert metal microelectrodesfor the detection of common anions in end-capillarydetection with CE was also reported.

2.4 MS and ICP-MS detection

CE-MS, CE-MS-MS, and CEC-MS continue to garner agreat deal of attention because of their universality, speci-ficity, and sensitivity. In particular, the introduction ofelectrospray ionization (ESI) has heralded tremendousprogress in the use of CE-MS for the determinationof various compounds. Although MS is generally moresensitive than UV detection, the current CE-MS inter-faces employ make-up liquids to increase the flow upto 10 �L/min, which dilutes the CE efflux. This results inmuch lower concentration sensitivity than LC-MS orGC-MS, or even CE-UV. Attempts are ongoing toimprove the interfaces between CE and MS to increasesensitivity.

In the last two years, several research groups have de-scribed the development of CE-MS methods to analyzeenvironmental pollutants [26–30]. Poiger et al. [27]coupled CE with negative ion electrospray MS for sensi-tive and selective identification and characterization ofnegatively charged metallized dyes. The dyes were sepa-rated using a 5 mM ammonium acetate buffer (pH 9) con-taining 40% acetonitrile. Despite the excellent sensitivityof CE-MS for negatively charged dyes, identification ofthese compounds in environmental samples required arigorous sample concentration procedure [28]. In anotherstudy on CE-MS [31], the conditions for coupling CEwith MS using an ion-trap analyzer for the separation of18 positional isomers of chlorophenols were optimized(Fig. 1). Diethylmalonic acid (5 mM) at pH 7.25 as BGEand isopropanol-250 mM dimethylamine (80:20 v/v) as asheath liquid were used. Debusschere et al. [29] usedCE-ESI-MS to facilitate the identification of mineral andorganometallic compounds of arsenic in a speciationstudy. Optimization of the coupling conditions (geometryof concentric interface, composition and flow rate ofthe sheath liquid, electronebulization and detection con-ditions) was described. The results showed that the geo-metry of the concentric interface and the positioning ofthe outlet of the separation capillary have a crucial effecton stability and sensitivity. Comparison with detectionlimits reported for CE-inductively coupled plasma-MS(CE-ICP-MS) showed that coupling CE with ICP-MSyields superior sensitivity; however, the advantage of

4264 E. Dabek-Zlotorzynska et al. Electrophoresis 2001, 22, 4262–4280

Figure 1. CE-MS separation of a standard solution of18 chlorophenols. Peak identification: 1, 2-chlorophenol(2CP); 2, 3CP; 3, 4CP; 4, 2,3-dichlorophenol (23DCP);5, 24DCP; 6, 25DCP; 7, 26DCP; 8, 34DCP; 9, 35DCP;10, 2,3,4-trichlorophenol (234TCP); 11, 235TCP;12, 236TCP; 13, 245TCP; 14, 246TCP; 15, 345TCP;16, 2,3,4,6-tetrachlorophenol (2346TeCP); 17, 2356TeCP;18, pentachlorophenol (PCP). CE conditions: BGE asin text; capillary separation length, 100 cm; temperature,25�C; voltage, 20 kV; hydrodynamic injection, 4 s. MSconditions: ESI ion source, ion-trap detector; capil-lary temperature, 175�C; electrospray capillary potential,–3.5 kV; electrospray current, 12 �A; data scanning fromm/z 100–300. Reprinted from [31], with permission.

ESI-MS over ICP-MS is that the ESI ionization processdoes not alter the structure of all the arsenic species thatare detected as simply charged molecular species.

Another promising instrumental setup especially for ele-ment speciation is the combination of the high separationefficiency and speed achievable by CE with the sensitiveelement-specific capabilities of ICP-MS and atomic emis-sion spectrometry (ICP-AES). The wide-ranging oppor-tunities made possible by coupling CE with ICP-MS orICP-AES ensure continued interest in exploring theseavenues [32–40].

The chief obstacle to be overcome in CE-ICP like anyhyphenated technique is the development of the inter-face, in this case one which can maintain the high voltagerequired for the CE experiments and is capable of accom-

modating the CE solution flow-rate of �L/min or lower.In almost all CE-ICP experiments described to date, aconducting makeup buffer flowing concentrically aroundthe CE capillary exit is used to complete the necessaryground connection as well as to supplement the low EOFin order to achieve stable nebulizer operation [32–36]. Interms of injection, the performance of two different sam-ple injection systems for the separation of mercury spe-cies was reported by Tu et al. [35]. The results of theseexperiments showed that both cross-flow and microcon-centric nebulizers (MCN) with appropriate spray cham-bers were suitable for this purpose. A standard cross-flow nebulizer with a Scott spray chamber offered ashorter analysis time, better resolution and ruggedness.By comparison, a MCN-100 with a cyclone spray cham-ber provided higher sensitivity and it reduced the detec-tion limits to the low �g/L level. These detection limits areadequate for the analysis of biological materials with mar-ine origin without the need for derivatization or extraction,so that potential error involved in these steps can beavoided. In another study, Chen et al. [40] demonstrateda simple interface by employing a cross-flow nebulizercoupled with either CEC or CE for the on-line detectionof different species of chromium, arsenic and selenium.To accommodate this electrophoretic separation, an aux-iliary capillary was used with nitric acid (0.05 M) as make-up liquid. Concentration detection limits for these metalions were in the low �g/L range.

Tangen and Lund [37] developed an interface with mini-mum dead volume. The interface was based on a com-mercial microconcentric nebulizer, but the standard capil-lary was replaced by one with 100 �m ID. A plate numberof 3.6�106 plates/m was achieved, which is an orderof magnitude better than results previously reported forCE-ICP-MS. Another option for the setup of CE-ICP is anelectrical connection between the outlet of the CE capil-lary and the CE power supply via a platinum wire [38, 39].This approach ensures stable electrical contact betweenthe CE electrode and electrolyte. Furthermore, using arelatively small nebulizer orifice, a sheath flow is notneeded in this arrangement for stable nebulization andCE operation.

3 Sample preparation and preconcentration

Various sample extraction, cleanup and preconcentrationprocedures have been utilized for CE analysis of pollutantspresent at low concentration levels and/or in complex mix-tures. Among these techniques, solid-phase extraction(SPE), liquid-liquid extraction (LLE), supercritical fluidextraction (SFE) and flow injection analysis (FIA) havebeen the most attractive. Several reviews dealing with thistopic were published during this two-year period [41–44].


3.1 SPE

SPE constitutes the most widely used preconcentration/cleanup technique prior to CE analysis. Detailed informa-tion on SPE sample pretreatment used in the determina-tion of various pollutants prior to CE analysis can be foundin a recent review by Martínez et al. [43]. In the majority ofwork reported, sampling, pretreatment and introductioninto the capillary have been performed using off-line tech-nology. For instance, Morales and Cela [45] utilized solid-phase polymeric functionalized cartridges (Oasis) forthe extraction of priority phenols from volumes of up to500 mL of wastewater at neutral pH with a concentrationfactor of 500:1. Employing the electrolyte as eluent, theextract could be directly injected into the CE systemwithout any dilution or solvent change. In another study[28], anionic dyes were extracted from wastewater usingion-pair reverse C18 SPE cartridges prior to their CE-MSanalysis. Thus, preconcentration and desalting of hydro-lysis products of reactive dyes were achieved. Withoutdesalting, only very poor separation was obtained withthese samples.

Since miniaturization of sample preparation has been ageneral trend in the last decade, many research groupshave focused their attention on on-line sample prepara-tion prior to CE analysis. Some studies have shown thepotential of on-line SPE in CE [46, 47]. In one example, asmall amount of sorbent is placed on-line as a microcar-tridge at the inlet end of the capillary to be used in on-lineSPE-CE. More recently, Breadmore et al. [48, 49] reporteda novel on-capillary SPE-CE method using ion-exchangeopen-tubular CEC, with elution being accomplished bythe use of a transient isotachophoretic gradient with twoelectrolytes. A short section of capillary (8.5 cm, in whichan anion-exchange stationary phase was adsorbed to thecapillary wall) was used to on-line preconcentrate anions,and the remainder of the capillary was used as a conven-tional fused-silica surface. Analytes were adsorbed ontothe stationary phase in the weakly eluting electrolyte,but were efficiently desorbed in the stronger elutingelectrolyte. Peak compression was demonstrated for I–,for which detection limits were reduced by a factor of2400 relative to conventional injection techniques.

3.2 FIA-CE

The FIA-CE hyphenation technique, first described byKuban et al. [50], can easily be used in conjunction withappropriate on-line sample preparation techniques. Flowsystems are among the most powerful tools for imple-menting pretreatment and conditioning the samples in anautomatic flow [51]. As a result, there is much ongoingresearch into exploring and developing various FIA-CE

assemblies for this purpose [52–56]. Mardones and co-workers [52] developed an integrated FIA-CE methodfor the automatic determination of chlorophenols inhuman urine. The FIA system allowed the continuous pre-concentration and cleanup of chlorophenols, while the CEsystem afforded highly sensitive separations over broadconcentration ranges. The reported detection limits werebetween 4 and 12 �g/L for all chlorophenols exceptpentachlorophenol and 4-chlorophenol, which could notbe determined by this method. Another study [53] re-ported a new approach to automate analyses by couplinga supercritical fluid extractor on-line to CE equipment,both commercially available. Extracted analytes werecollected in a trap following depressurization in the SFEand were transferred to the CE equipment across theinterface. The key elements of the experimental assemblyare a laboratory-made programmable arm and the auto-sampler of the CE equipment, both of which are con-trolled by a built-in microprocessor using an appropriateinterface and customized software. This combined sys-tem was successfully used to determine cresols andchlorophenols in liquid samples (river water and humanurine).

The recently developed valveless FIA-CE system hasbeen shown by Kuban and Karlberg [55] to be an efficienttool for on-line monitoring purposes. The analytical per-formance of the system was comparable to that of aconventional FIA-CE system with an injection valve, andthe system was assessed for on-line monitoring of themajor anions present in kraft pulping liquors. Sampleswere taken every 6 min, fully automatically; in addition,oxidation rates of liquor samples can be monitored andthe reaction mechanisms evaluated. An interesting appli-cation of the FIA-CE concept was reported recently bySirén et al. [56]. This study described on-line monitoringof soluble inorganic and organic ions in water at differentstages of the pulp and paper process with real time ana-lyses by two on-line CE methods. A reconstructed com-mercial CE system was connected to a papermakingmachine via a combined sampling and sample pretreat-ment instrument which filtered and diluted the samplesbefore on-line determination by CE. On-line determina-tion of anions and cations in pulp and paper mills wastested in three periods during 1 year; over 2000 on-linesamples were analyzed, with over 9000 anion and cationdeterminations. The successful design, construction andevolution of a three-dimensional CE system has beendescribed by Hanna et al. [57]. This novel 3-D CE systemfacilitates on-line sample cleanup and preconcentrationvia capillary isotachophoresis in addition to providingthe means whereby multimode electrophoresis may beachieved with four independent detection capabilities.The authors concluded that the utilization of such a sys-


tem should be of particular importance for researchersinvolved in trace analyses in clinical, environmental andpharmaceutical fields.

3.3 Sample stacking and sweeping

Over the last two years, a great deal of attention has beenpaid to concentration sensitivity enhancement and im-proved detection systems. Without modification of theinstrument, improvement of the concentration sensitivityis accomplished either by analyte enrichment during asample preparation step as discussed above or by ex-tended volume injection followed by analyte focusing dur-ing the CE analysis. The latter concept involves samplestacking. Strategies to improve the sensitivity in CE in-cluding sample stacking have been covered in severalrecently published reviews [58–63]. Considerable workhas been published regarding the theory of the effect oflow sample conductivity zones on the electroosmoticflow (EOF), on peak broadening, and on the stackingboundary, as reviewed recently [64].

Several sample stacking variations have been developedfor the analysis of anionic and cationic species [29, 32,65–71]. For instance, Liu and Lee [67] reported an en-hancement of detection up to 1700-fold for multiple spe-cies of lead, mercury and selenium with field-amplifiedstacking injection (FASI), and sub-�g/L detection limitswere obtained. More recently, Timerbaev et al. [70] evalu-ated on-capillary approaches for the improvement ofdetection sensitivity for UV-absorbing anions in samplesof high salt content such as seawater. The approachestested involved on-capillary preconcentration techniquescommon in the practice of CE, such as FASI and isotacho-phoretic sample stacking. Of these two, transient isota-chophoresis (ITP) stacking has not so far proven to be sui-table for the analysis of high-salt samples. For example, ina preliminary study by Fukushi et al. [72], nitrite and nitratein seawater preconcentrated using transient isotacho-phoresis yielded only a 2-fold improvement in detectionlimits over a system with no transient FTP. Artificial sea-water was used as the BGE and different concentrationsof chlorate served as the terminating ion for the twoanalytes. In spite of the improvement being small, thismethod shows promise and may yet prove useful atlow environmental levels. In contrast, more encouragingresults have already been obtained with FASI sample intro-duction [70]. The up to 10-fold advancements in detectionlimits make this approach attractive for the determinationof trace anions in loaded samples.

The use of sample stacking in electrokinetic chromato-graphy, nonaqueous CE, and on-chip electrophoresishas also appeared in the literature [58, 73–75]. Morales

and Cela [75] recently reported a significant sensitivityenhancement when FASI in nonaqueous media wasperformed for phenolic compounds included in prioritypollutant lists of the European Union and the U.S.Environmental Protection Agency. Otsuka et al. [74]reported an application study using stacking with reversemigrating micelles and a water plug for the analysis ofseveral dioxins and related compounds by cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC). By applying this technique, the detection limitswere successfully decreased to around 0.1 mg/L, roughlya 200-fold improvement in detection sensitivity in com-parison to normal CD-MEKC without any preconcentra-tion procedures.

Sweeping is another approach which, in theory, providesfor an almost unlimited improvement in concentrationdetection sensitivity for analytes that have high affinitiesfor the pseudostationary phase: 5000-fold improvementshave been demonstrated experimentally [76]. Moreover,sweeping is independent of the EOF and works forcharged and uncharged solutes, making it a widelyapplicable technique. Some studies reported the use ofsweeping for on-line concentration of various pollutants[73, 77–79]. The combination of sample stacking withelectrokinetic injection and sweeping for the on-line con-centration of cations (cation-selective exhaustive injec-tion-sweeping, CSEI-sweeping) was recently reported[78]. An increase in peak height of more than 100 000-foldwas obtained for naphthylamine and laudanosine byCSEI-sweeping and MEKC.

4 Separation on microchips

In recent years, microscale separation using micro-machined CE has received considerable attention be-cause these microchip-based separations are expectedto become a highly powerful tool for future clinical analy-sis, environmental screening and industrial monitoringof toxic substances. The initial results confirm the highapplication potential of this technology. Several studieshave reported the development and use of CE micro-systems for the analysis of pollutants [10, 12, 13, 20].Wallenborg and Bailey [12] reported the use of micro-fabricated glass chips as a quick screening tool for thepresence of explosive compounds after MEKC sepa-ration and indirect LIF detection. Figure 2 shows theseparation of nine nitroaromatic compounds in ex-tracts from spiked soil samples. As can be seen, thepresence of 1 �g/g of nitroaromatics in soil could readilybe separated and detected using microchip MEKC-indirect LIF.


Figure 2. MEKC-IDLIF analysis of extracts from explo-sive-spiked soil samples using microchip: (a) soil blank,(b) soil containing 1 �g/L of each analyte, (c) soil contain-ing 5 �g/L of each analyte. Peak identification: 1, trinitro-benzene (TNB); 2, dinitrobenzene (DNB); 3, nitrobenzene(NB); 4, TNT; 5, tetryl; 6, 2,4-dinitrotoluene (DNT);7, 2,6-DNT; 8, 2-,4- and 4-nitrotoluene (NT); 9, 2,4-amino-2,6-DNT. Conditions: MEKC buffer, 50 nM borate,pH 8.5, 50 mM SDS, 5 �M Cy7; voltage, 4 kV; separationdistance, 65 mm. Reprinted from [12], with permission.

Hilmi and Luong [20] developed a CE chip with ampero-metric detection, which was used to determine explo-sives in soil. Electrophoretic separation of TNT and fournitroaromatic explosives in 4 min with detection limitsof 100–200 �g/L demonstrated the good resolution aswell as sensitivity of the CE microchip. Another study byWang et al. [13] described a miniaturized analytical sys-tem for separating and detecting toxic phenolic com-pounds, based on the coupling of a micromachined CEchip with a thick-film amperometric detector. The inte-grated microsystem offers a rapid (4 min) simultaneousdetermination of seven priority chlorophenolic pollutantsdown to the 1–2�10–6 M level. Applicability to river watersamples was also demonstrated. A micromachined CEsystem has been fabricated on a glass device for separa-tion and indirect fluorescence detection of phenols [10].Using this device, two phenols – 2,4-dichlorophenol andpentachlorophenol – were separated within 12 s com-pared to nearly 19 min on a conventional CE system usingdirect UV detection. Although poorer detection limitswere obtained with the glass device and indirect fluores-cence detection, these results provide a foundation forthe development of a miniaturized chemical analysissystem for the on-line analysis of phenols in water.

5 CEC

Interest in CEC continues to grow at a rapid pace asrefinements in the technique are reported [80–83]. Themotivation for this interest is the high separation efficiencywhich is comparable to that obtained by gas chromato-graphy and CE. Other inherent advantages of CECinclude high peak capacity, a wide range of applicability,short analysis times, and high, tunable selectivity. Be-cause of these advantages, there is currently a tremen-dous surge of research in CEC among separation scien-tists. In the last few years, column technology and instru-mentation for CEC have made large steps forward [82,83]. Extremely high efficiencies were obtained with non-porous octadecyl silica gel and wide-pore packing mate-rial [11]. Many alternatives to capillaries packed with thestationary phase have been developed, including open-tubular columns [84, 85] and monoliths [86–88]. Althoughboth ionic and neutral compounds can be separated byCEC in theory, most of the reported CEC developmentshave focused on the analysis of neutral compounds usingreversed-phase (RP) stationary phases. The separationselectivity is therefore similar to that encountered in RP-HPLC. Interest in the behavior of charged analytes in CECis growing and a number of publications have reportedthe determination of such solutes, generally using RPstationary phases [80]. However, in addition to the RPchromatographic supports, polar stationary phases suchas ion-exchangers [89–93] and mixed-mode phases [94,95] have been tested for CEC separations of chargedanalytes.

Hilder et al. [92] demonstrated the feasibility of pressur-ized anion-exchange CEC for the separation of inorganicanions with unique separation selectivity, and showedthat polymeric stationary phases are suitable for this pur-pose. In addition, the use of an artificial neural networkto determine optimum separation conditions was shownto be particularly useful in this case where the system hasa mixed-mode separation mechanism, together with amixture of voltage-induced and pressure-induced flow.Another CEC study investigated the viability of addingion-exchange particles to the background electrolyte asa pseudostationary phase for the separation of mono-valent inorganic cations [96]. These ion-exchange parti-cles reduced the observed electrophoretic mobility of theanalytes through their interaction with the pseudostation-ary phase, following the same interaction order as with asulfonated stationary phase. Indirect absorbance detec-tion was found to give poor sensitivity due to light scatter-ing effects caused by the particles of the pseudostation-ary phase. Separation of acidic compounds like aromaticacids was reported using ion-exchange CEC with stronganion-exchange packing as the stationary phase [89].


Because these acids were only partially ionized, theirretention on the strong anion-exchange packing was rela-tively weak, and both chromatographic and electrophore-tic processes contributed to separation. Recently, Chenet al. [40] utilized a bonded phase capillary column con-taining macrocyclic polyamine functional groups for theCEC separation of arsenic, chromium and selenium spe-cies with on-line detection by ICP-MS. The separationefficiency of the bonded phase was compared with abare fused-silica system and the resolution results weregenerally similar, although the bonded phase yieldedhigher theoretical plates and appeared to be better suitedfor complex matrices. Concentration detection limits forthese metals were in the low �g/L range.

Widespread acceptance of CEC within the chromato-graphic community can only be realized by demonstrat-ing the robustness of CEC in the analysis of real-worldsamples. However, the majority of the CEC work that hasbeen presented to date reports on the separation of stan-dards, mostly PAHs, which are used in CEC for columnand system evaluation. At present, this novel techniqueis not yet mature or rugged enough to be implemented inroutine analysis/quality control work, and development inall aspects of the technique is still needed. Nevertheless,three important environmental applications (PAHs [7], car-bonyls [97], phenols [98]) have been reported using CECin the period covered by this review. Dabek-Zlotorzynskaand Lai [97] showed the highly efficient separation of car-bonyls in 2,4-dinitrophenylhydrazine-cartridge sampledambient air and automobile exhaust (Fig. 3), and foundCEC to give far better resolution than LC. Under optimalisocratic conditions (60% acetonitrile-4% tetrahydro-furan-5 mM Tris, pH 8), a 3 �m porous C18-bonded silicacapillary column yielded a separation profile similar tothose reported for gradient HPLC, with significant im-provements in efficiency (to 150 000–250 000 theoreticalplates/m) and analysis time (by a factor of 4). A retentiontime precision better than 0.2% (RSD) from run to run and1% from day to day was reported. The limits of detectionfor individual carbonyl hydrazones ranged between 0.1and 0.5 mg/L.

The next study demonstrated the use of CEC with LIDFfor the quick identification of PAHs and NCACs in soilextracts [7]. Figure 4 shows the CEC separation andLIDF spectra for a creosote-contaminated soil extractobtained by Soxhlet extraction with dichloromethane.This sensitive LIDF detection technique provided informa-tion on compound identity and, when coupled with thehigh separation efficiency of the CEC technique, proveduseful in the analysis of complex real samples as well asstandards. Finally, Fung and Long [98] reported a newanalytical procedure developed to couple SFE with CEC

Figure 3. CEC separation of carbonyl hydrazones in(a) ambient air and (b) automobile exhaust. Detectionwavelength, � = 360 nm (—) and 300 nm (��). Conditions:column, 75 �m�27/20 cm packed with 3 �m C18-bondedporous silica particles; mobile phase, 60% acetonitrile-4% tetrahydrofuran-5 mM Tris-HCl (pH 8); voltage, 20 kV;temperature, 35�C; injection, 10 kV, 10 s for air, 20 s forexhaust. Reprinted from [97], with permission.

(SFE-CEC) to extract and determine phenols in soil.Baseline separation was achieved for the ten selectedphenols under optimized CEC conditions at 20 kV andusing a mobile phase of acetonitrile-4 mM Tris (35:65 v/v)in a 45 cm (25 cm packed with 3 �m octadecyl silica(ODS) stationary phase)�75 �m ID fused-silica capillarycolumn. Using SFE with a 10-fold preconcentration factor,all alkyl-substituted phenols in soil could be determinedwith detection limits ranging from 0.0032 to 0.014 mg/kgand working range from 0.019 to 2.72 mg/kg. The SFE-CEC procedure developed was successfully applied todetermine phenols extracted from soil samples contami-nated with medical disinfectant.


Figure 4. LIDF spectra and CEC separation of acreosote-contaminated soil extract. Peak identification:(a), acridine or degradation product; 1, phenanthrene;2, fluoranthene; 3, pyrene; 4, benz[a]anthracene; 5, chry-sene; 6, benzo[b]fluoranthene; 7, benzo[k]fluoranthene;8, benzo[ghi]perylene. Spectra obtained using second-order dispersed fluorescence collection and mono-chromator slit width of 500 �m. Inset shows the chro-matogram obtained by integrating the entire spectralregion. CEC conditions: column packed length, 21 cm;voltage, 20 kV; mobile phase, 85% ACN and 15% 5 mM

Tris; 10 s, 15 kV injection. Reprinted from [7], with permis-sion.

6 Reproducibility and quantification

While CE is growing in importance as an efficient, fast,versatile and economical technique in the environmentalfield, there still exist major challenges that limit the techni-que’s practical acceptance by analytical chemists. To ourbest knowledge, very few works which used CE in routine

environmental monitoring have been published so far[99–101]. Since existing criteria for quantification andaccuracy have generally been set by regulatory bodiesfor application to HPLC and GC methods, their useful-ness in validating CE methods may be limited which mayexplain in part this lack of routine CE use [102]. Hence, inrecent years, CE publications are demonstrating thatmore emphasis is being placed not only on separationproblems but also on aspects of method validation. Inaddition to instrumental improvements in the control/stabilization of the power supply and temperature in thedetection and injection devices, more research andeffort have focused on some CE-specific factors, suchas electrolyte stability, instrument transfer, data handlingprocedures, and system suitability and ruggedness test-ing [99, 103–106].

For routine analysis it is essential to keep migration timesconstant in order to allow automatic peak identificationby means of commercial data analysis software. Auto-matic peak identification and quantification are only pos-sible if the relative standard deviation of migration timesis less than 0.5% [104]. Several recently publishedreviews and studies described the improvement ofmigration time precision by using buffered backgroundelectrolytes [2–5] and capillary conditioning [99, 103,104]. It was found that the precision of migration timesstrongly depends on appropriate pretreatment of theinner capillary surface before and during the analysis.Hence, the selection of the proper rinse steps is animportant optimization parameter in CE method devel-opment. Use of the constant-current separation modein the first part of the run was also reported to improvereliability of the analysis in samples of wide-ranging con-centration [99].

Another approach recently reported for quantitative andqualitative precision improvements in CE is the trans-formation of the total time x-scale of electrophoretic datainto the corresponding effective mobility scale (�-scale)[105, 106]. The conversion leads to a better interpretationof the obtained electropherograms in terms of separationprocesses, and enables better direct comparison ofelectropherograms and easier “peak tracking” when try-ing to identify single components from complex matrices,especially when UV-visible signatures of the componentsare also available [106]. The authors found that, besidesthe qualitative possibilities, a quantitative improvementwas achieved in the �-scale with significantly better peakarea reproducibility which equates to better precision inquantitative analysis than with the primary time-scaleintegration. However, electrophoretic-based data proces-sing CE software is needed to be able to handle directlythe electrophoretic data.


7 Applications

New and innovative applications of CE continue toappear in the environmental field. However, the role ofCE in the analysis of various pollutants can be justifiedonly by examining applications toward real samples.Methods must be successfully transferred into workingenvironments for use by non-CE experts before thepower of CE can be realized. Although gradual, therehave been increasing indications that this transfer oftechnology is beginning to occur.

7.1 Phenols

CE remains an important area of investigation for the ana-lysis of phenols. Because of their acid-base characteris-tics, research effort has focused on buffer compositionand pH optimization. Reports include the determinationof priority pollutant phenols with common aqueous buf-fers [17, 31, 107], the use of buffer additives for improvingtheir resolution [15, 108], MEKC [16, 52, 109], CEC [98],and the use of nonaqueous buffers [14, 45, 75,110]. Jáur-egui and co-workers [107] developed a CE-UV methodto separate 16 chlorophenols with a buffer compositionof 30 mM diethylmalonic acid at pH 7.23. Migration timeprecision ranged from 3 to 8%. In order to couple CE toMS, the concentration of carrier electrolyte was loweredto 5 mM to avoid discharge effects in the source and tominimize the Joule heating effect [31]. The LOD rangedbetween 0.3 and 2 mg/L using the CE-UV diode arraydetector (DAD) and between 0.5 and 10 mg/L in CE-MS. The use of a newly synthesized cationic calixarene,p-(quaternary ammonium) calix[4]arene, as a modifier ina phosphate buffer for CE-EC detection was reportedfor the separation of benzenediols and aminophenols,with this setup attaining LODs from 0.7 to 4 �M (0.09–0.5 �g/L) [15].

Effective separation of phenols has also been achievedby MEKC. Preliminary studies showed the simultaneousseparation of bisphenols and alkylphenols using MEKCwith sodium dodecyl sulfate (SDS) in 5% acetonitrile[109]. Alternatively, MEKC with amperometric detectionusing sol-gel carbon composite electrodes efficientlyseparated four phenols in 35 min. Negatively chargedelectrodes minimized the adsorption of SDS onto theelectrodes, decreasing the limit of detection to 1.5 �M

[16]. Reduction in analysis time and improvement inseparation have been accomplished by the use of nona-queous buffers such as methanol and/or acetonitrile.Because of acetonitrile’s low viscosity, moderate polarityand good UV transparency, it is the solvent of choice.Luong and co-workers [14] described a nonaqueous CE-EC method using a buffer consisting of acetonitrile, acetic

acid, sodium acetate, tetrabutylammonium perchlorateand triacetyl-�-CD, which gave 3-8-fold better LODsfor 11 chlorophenols than those obtained by aqueousbuffers. With an acetic acid-tetrabutylammonium hydrox-ide-acetonitrile buffer, Miller [110] obtained separationof 11 priority pollutants (chloro- and nitrophenols) within5 min and 19 chlorophenols in 6 min.

Strategies for the determination of phenols at low con-centrations have recently been addressed. These haveincluded preconcentration steps such as SPE [45, 52]and SFE [53, 98] prior to CE, and the use of nonaqueousCE-FASI [75]. Spiked river and urine samples were ana-lyzed using SFE-CE-DAD detection, and good recoverywas found in both matrices at concentrations of 0.076–0.14 mg/L and 0.35–1.0 mg/L, respectively [53]. Recentdevelopments in microscale analysis such as microchipshave found their application in the analysis of phenols[10, 13]. Separation of four phenols in spiked river watersamples was achieved within 2 min using CE-microchipand amperometric detection [13]. Novel approachesutilizing alternate buffer compositions in MEKC for theanalysis of polychlorinated biphenyls (PCBs) have beenreported [111, 112]. Edwards and Shamsi [112] usedpolysodium undecyl sulfate (poly-SUS) as an alternativeto the commonly used CDs. The buffer consisted ofborate, acetonitrile and poly-SUS at pH 9.2, and the sys-tem achieved separation of nine congeners within 11 min.In another study, rapid enantiomeric separation of chiralPCBs was performed using an anionic CD (carboxy-methylated �-CD) as pseudostationary phase and a2-morpholinoethanesulfonic acid buffer at pH 6.5 [111].The addition of neutral CDs (�-CD and permethylated�-CD) facilitated the separation of five PCBs in less than12 min.

7.2 PAHs

Although the CE separation of neutral and highly hydro-phobic PAHs is not straightforward, research using capil-lary electrophoretic techniques continues. Due to theirlack of charge, this class of important environmentalpollutants cannot be separated in a free solution by CE.This obstacle is overcome by using electrokinetic chro-matography (EKC) through the use of charged bufferadditives such as micelles or polymeric surfactants aspseudostationary phases [8, 9, 113–116] and CEC (seeSection 5). CD-modified MEKC was applied to the sepa-ration of benzo[a]pyrene (BaP) and 12 positional isomersof monohydroxybenzo[a]pyrenes [114]. A full separationof BaP and 12 monohydroxy BaPs was performed using20 mM phosphate-5 mM borate buffer (pH 8.5) containing20 mM �-CD and 50 mM SDS with an effective voltage of12 kV at 20�C. A novel approach utilizing fullerenes as


micelle additives in MEKC for the separation of PAHswas recently reported by Treubig and Brown [113]. Thepreliminary results showed promise in improving exist-ing MEKC methods for various compounds includingPAHs.

Polymeric surfactants have been proposed as alterna-tives to normal micellar systems [115, 116]. Fujimoto etal. [115] examined polymers of sodium 11-acrylamidoun-decanoate with a very high molecular mass as a pseudo-stationary phase in MEKC. It was reported that, foraromatic compounds such as PAHs, the polymer affordsa different selectivity from SDS and high stability in thepresence of an organic modifier. Additionally, the use ofthis polymer is advantageous over conventional micellesfor on-line coupling of MEKC with ESI-MS because thehigh molecular mass of the polymer is well beyond themass range of the spectrometer.

Another study [116] reported that the combined use ofa poly(sodium 10-undecenyl sulfate) with various typesof neutral CDs was successful in CD-EKC separation ofmonomethylbenz[a]anthracene (MBA) isomers. Baselineresolution of 10 of the 12 isomers, except for 9-MBA and2-MBA, was achieved with �-CD at pH 9.75. Differentresolution and selectivity were found with other CDs.Zhan et al. [117] outlined a strategy to couple liquid-liquidsemimicroextraction (LLsME) with CE based on a newlyintroduced on-column decomposable sample matrix,ethyl acetate. Alkylphenones of C8-C12 with concentra-tions of about 10 �g/L could be determined using the pro-posed methods. The method developed appears to havegreat potential for the analysis of environmental pollu-tants, especially those found in water samples.

7.3 Amines

Aromatic amines such as aniline and its derivatives con-stitute an important class of environmental water pollu-tants. Anilines are used in the manufacturing of rubberand plastics, dyes, agrochemicals and pharmaceuticals.Using CE, Li and Fritz [118] achieved the separation of tenanilines using either ethanesulfonic acid or protonatedtriethylamine at pH 3.45 as the electrolyte and isopropa-nol as an organic modifier, obtaining a reproducibilitybetter than 1%. An alternate method was proposed byAsthana et al. [18], which involved derivatization of theamines with fluorescamine and separation by MEKC-fluorescence detection. This method performed as wellas CZE with EC detection for the same set of com-pounds, yielding the same LOD (1 �g/L) and reproduci-bility of 1–3%. Application of this method was demon-strated via analysis of aromatic amines in surface watersamples collected in regions with textile and leather

industries. Benzenediamines and aminophenols, whichare present in dye intermediates in hair dye production,were separated using a phosphate buffer at pH 5 by CZE[119]. Using a combination of field-enhanced sampleinjection in the CZE format and sweeping in the MEKCformat (CSEI-sweeping), 104 to 105-fold improvement indetector response was achieved for the seven positivelychargeable anilines studied, which were separated in25 min [77].

7.4 Carbonyls

Aldehydes and ketones are of environmental relevance,since they are ubiquitous gases in the atmosphere. Ana-lyses of carbonyl compounds include two approaches:the formation of anionic species with bisulfite [120] andthe formation of neutral species with 2,4-dinitrophenyl-hydrazine [97, 120, 121]. In the case of bisulfite adducts,CE with indirect detection showed good sensitivity withdetection limits of the order of 10–40 �g/L [120]. Twoapproaches for the analysis of neutral hydrazine deriva-tives were reported. The first consisted of MEKC usinga borate buffer containing SDS and �-CD by whichLODs between 0.5 and 2 mg/L were obtained [120]. Analternate buffer for MEKC was tetraborate decahydratewith SDS at pH 9.2 by which separation of 13 carbonylcompounds and LODs of 0.1–0.52 mg/L were reported[121]. The second approach in the analysis of hydrazinederivatives was the use of CEC resulting in similar detec-tion limits (0.1–0.5 mg/L) but faster separation (10 min)[97].

7.5 Explosives

The need for fast and sensitive methods to screen largenumbers of environmental samples for the presence ofexplosives ensures continuing interest in the explora-tion of CE for their determination. Soil and groundwatersamples were analyzed by CE-EC detection using a poly-imide-coated fused-silica capillary with a borate bufferat pH 8.7 containing SDS [19]. It was found that the pre-sence of acetonitrile interfered with detection when usinga gold electrode. This problem was solved by using aplatinum or silver on gold electrode. Nitroaromatic andnitramine explosives were separated using nonporousCEC and MEKC systems coupled to indirect LIF [11].The LODs for nitroaromatics ranged from 1–10 mg/L andwere limited by high running currents which affected thebackground, though this was more of an issue in MEKC.Nitramines, however, could only be detected at muchhigher concentrations in both systems, probably due totheir low fluorescence quenching efficiency.


Fast analysis of TNT biotransformation products in an-aerobic sludge cultures was performed by SDS-MEKCwith UV detection [122]. Although this method does notidentify TNT metabolites, it allows their fast monitoringwhen complemented by LC-MS results. The method hada linear range from 0.5–25 mg/L and yielded �5% peakarea reproducibility. Recent developments in microchiptechnology have produced a reusable, sensitive and low-cost analytical device which has been used in the analysisof explosives [12, 20]. Hilmi and Luong [20] demonstratedits use in groundwater and soil samples by CE-microchipwith EC detection. Wallenborg and Bailey [12] describeda CE-MEKC microchip with indirect LIF detection usingthe dye Cy7, by which system ten peaks were resolvedfrom a mixture of 14 explosives in 1 min.

7.6 Sulfonates and surfactants

Aromatic anionic surfactants such as naphthalene andbenzene sulfonates have been separated by MEKC withdirect UV detection [123–125]. The use of buffer modifierssuch as CTAB and hexadimethrin bromide [123], Brij 35in isopropanol [124, 125] and Brij 58 [126] has beendescribed. In contrast, linear anionic surfactants such asSDS have been indirectly detected using p-hydroxyben-zoic acid in the buffer [127] and ECD [21]. Wilke and co-workers [21] described the use of ion-transfer ampero-metry using a borate buffer at pH 10 (see Section 2.3).The estimated LODs in preliminary results were in therange of 0.3 mg/L, and analysis time was 8 min. It wasshown that ion-transfer detection was also compatiblewith �-CD as a modifier.

The main challenges in the separation of cationic sur-factants arise from their ability to sorb onto the capillarywall and to form micelles at low concentration. Theseeffects can be avoided by the addition of CDs and organicmodifiers. Good separation of cationic surfactants suchas alkylbenzyldimethyl ammonium compounds (C10-C18)above their critical micellar concentration was achievedusing phosphate buffer containing �- or �-CD and 40%acetonitrile or methanol [128]. Homologues of the cationicsurfactants benzalkonium chloride and cetylpyridiniumchloride were quickly resolved and determined in a boratebuffer with the addition of bile salts and a high concentra-tion of organic solvent [129].

7.7 Dyes

Dyes comprise an extensive group of compounds with awide range of polarities. Despite their widespread useand the associated potential hazards of dyes, dye impu-

rities, and metabolites to human and environment health,little is known about their fate in sewage treatment or inthe aquatic environment. As a result, the developmentof new methods for the analysis of dyes and their degra-dation products is of paramount importance. CE hasbecome an alternative to HPLC for the analysis of dyessuch as bis-monochlorotriazinyl sulfonated dyes [130]and sulfonated azo dyes [131]. However, more importantis the identification of their degradation products, forwhich CE-MS is the most reliable technique [27, 28, 30].Takeda and co-workers [30] determined the degradationproducts of Orange II after oxidation with a solid catalystby CE-MS and CE-UV/DAD. Poiger and co-workers [27]applied CE-MS to the analysis of dyes containing Cu(II),Cr(III), and Co(III) with LODs in selected ion monitoringmode at the low mg/L level. Better detection limits (23–42 �g/L) were achieved through the use of SPE prior toanalysis [28].

7.8 Humic acids

Humic substances (HS) contain functional groups suchas phenolic, carboxylic and carbonyl groups. The analysisof HS involves their fractionation by ultrafiltration prior toCE in order to obtain information on molecular mass dis-tribution. Because of their large molecular size and poly-electrolytic character, HS electropherograms normallyshow a broad peak commonly termed a humic “hump”.To date, CE could be used only as a fingerprinting techni-que to visually compare the changes in electrophero-grams of complex HS mixtures obtained from differentsources or during degradation processes. This humichump was observed in fog and interstitial aerosol sam-ples analyzed by CE, and it was concluded that thesesamples contained humic-like substances, which wasalso confirmed by size-exclusion chromatography [132].Schmitt-Kopplin and co-workers [133] proposed conver-sion of the time scale to effective electrophoretic mobilitywhich allows calculation of statistically defined “descrip-tors” that can be directly compared instead of just ob-serving changes in CZE fingerprints.

De Nobili and co-workers [134] described a procedure inwhich the capillary was filled with long-chain polyethyleneglycols above its entanglement threshold, creating whatis known as a physical gel. Under these conditions, themobility of HS decreased with an inverse linear relation-ship to mean Mr, making possible the determination ofthe molecular size distribution of HS. Another obstacleto HS analysis is the considerable sorption of some HSfractions on the fused-silica capillary wall. The use of asapphire epoxy-coated capillary and modification of theborate buffer with the addition of �-, �-CDs or their


Figure 5. CE separation ofinorganic anions and organicacids from fine airborne particu-late matter extract using (a) 30 spressure injection and (b) 10 s/10 kV electrokinetic injection.Peak identification: 2, chloride;3, sulfate; 4, nitrate; 5, oxalate;7, malonate; 8, fumarate; 9,formate; 10, malate; 11, succi-nate; 12, glutarate; 14, metha-nesulfonate; 15, adipate; 17, pyr-uvate; 18, suberate; 19, glyco-late; 20, acetate; 21, azelate;22, glyoxylate; 24, phosphate;25, lactate; I.S. = internal stan-dard (pentanesulfonate); * =unidentified peaks. Conditions:capillary, 75 �m�57/50 cm; in-direct UV detection at 214 nm;BGE, 4 mM NDC, 14.4 mM Bis-Tris, 0.2 mM TTAB, pH 6.2; runmode, constant current 6.6 �Afor 7 min, then constant voltageat –15 kV. Reprinted from [99],with permission.

mixtures yielded stable and reproducible electrophero-grams that confirmed the presence of more than 30 peaksin most of the humic acids under examination [135].

7.9 Inorganic and small organic ions

There has been active research into the separation of in-organic and small organic ions, with several reviews beingpublished in the last two years demonstrating successfulenvironmental CE applications [5, 136–141]. Some recentapplications are summarized in Table 1 [69, 99, 142–152].Determination of perchlorate in groundwater [143]; in-organic anions at trace levels in snow [144]; silicate withother nutrients in river water [145]; cationic and anionicspecies in cleanroom wipers [147], environmental watersamples [148], and size-classified ice crystals [150]; in-organic anions in microbial fermentation [146]; trace

metal cations in rain water [69]; and low molecular mass(LMW) mono- and dicarboxylic acids in airborne and vehi-cle-emitted samples [99, 100, 142] has been successfullyperformed and validated with the results obtained byother methods or using laboratory-made reference sam-ples. For example, Dabek-Zlotorzynska et al. [99] report-ed a reliable CE method with indirect UV detection forthe determination of a large number of airborne andvehicle-emitted LMW organic acids. The method useda buffered BGE containing 4 mM 2,6-naphthalenedicar-boxylic acid adjusted to pH 6.2 with Bis-Tris (14.4 mM)and 0.2 mM tetradecyltrimethylammonium bromide (TTAB)as an EOF modifier. Reliability of the analysis wasimproved by using the buffered BGE, proper rinse steps,and constant current mode. The method is now in routineuse for monitoring low-molecular-weight carboxylic acidin atmospheric aerosol and vehicle emission samples(Fig. 5). Fung and Tung [152] employed a newly developed


Table 1. Applications of CE analysis for the separation of inorganic and small organic ions

Analyte Samplematrix

Electrolyte Injection Detectionmode

LODs Ref.

Metal cations Rainwater 30 mM hydroxylamine hydrochloride,0.1 mM 1,10-phenanthroline,1% methanol, 15 mM ammonium chloride,0.08 M urea, pH 3.7

40 s/3kV Direct UV,265 nm

0.1–0.5 �g/L [69]

Organic acids Atmosphericaerosolsand vehicleexhaust

4 mM naphthalenedicarboxylate,0.2 mM TTAB, 14.4 mM Bis-Tris,pH 6.2

10 s/10 kV (a),10 S/70 mbar (b)

Indirect UV,214 nm

3–10 �g/L (a),50–180 �g/L (b)

[5, 99]

Inorganic anions/cations,organic acids

Vehicularemission

Organic anions: 7.5 mM 3,5-dinitrobenzoate,0.115 mM CTAB, pH 5; inorganic anions:7.5 mM chromate, 0.115 mM CTAB,pH 8.5; cations: 5 mM imidazole,5 mM HIBA, 1 mM bicyclohexane18-crown-6 ether, pH 4.5

1 s/5 in. Hg Indirect UV,anions254 nm,cations214 nm

[142]

Perchlorate Groundwater 6 mM sodium chromate, 0.04 mM H2SO4,1.5% OFM-Anion BT

30 s/10 cm Indirect UV,254 nm

400 �g/L [143]

Inorganic anions Snow 0.01 mM p-sulfonated calix(4)arene,4 mM KBr, pH 3.1

50 s/10 kV (a)and 15 shydro-dynamic (b)

Indirect UV,204 nm

1.0–10.2 �g/L (b) [144]

Silicate, nitrite,nitrate,phosphate

River water 5 mM sodium chromate, 0.2 mM TTAB,pH 11

30 s/10 cm Indirect UV,254 nm

0.6–1.3 mM [145]

Inorganic anions Microbialfermentationsamples

10 mM sodium chromate, 0.5 mM TTAB,pH 9

5 s/50 mbar Indirect UV,254 nm

1–2.5 mg/L [146]

Inorganic anionsand cations

Cleanroomwipers

Anions: 4.7 mM sodium chromate,4.0 mM Waters OFM-OH, 0.1 mM Cagluconate, 10 mM CHES buffer, pH 9.1;cations: 5.0 mM methylbenzyl amine,6.5 mM HIBA, 2.0 mM 18-crown-6 ether,pH 4.5

Hydrostatic,�10 nL

Indirect UV,anions254 nm,cations184 nm

Anions 100 �g/L;cations 50 �g/L

[147]

Inorganic anionsand cations

Environmentalwater

Anions: 2.25 mM pyromellitate, 6.5 mM NaOH,0.75 mM hexamethonium hydroxide,1.6 mM triethanolamine, pH 7.7;cations: 9 mM pyridine, 12 mM glycolicacid, 5 mM 18-crown-6 ether, pH 3.6

0.5 psi,anions 50 s,cations 10 s

Indirect UV,254 nm

Anions 10–40 �g/L;cations20–80 �g/L

[148]

Inorganic anions/cations andorganic acids

Atmosphericaerosols andwet, bulk anddry deposition

Anions: 7.5 mM salicylate, 15 mM Tris,0.4 mM dodecyltrimethylammoniumhydroxide, 1.05 mM Ca+2, 0.6 mM Ba+2,pH 8.1; cations: 4 mM ECOL, 4 mM

18-crown-6 ether, pH 4.2

30–60 srange/1.6 psi

Indirect UV,anions230 nm,cations220 nm

Bulk/wet dep.8.5–58.5 �g/dm3;dry dep.13.3–92.0 �g/m2/day; aerosol0.42–2.87 ng/m3

[149]

Inorganic anions/cations andorganic acids

Ice crystals Anions: 8.5 mM salicylate, 21 mM Tris,0.001% hexadimethrin bromide,2% methanol, 2 mM NaOH, pH 8.25;cations: 5 mM 4-methylaminophenolsulfate,4 mM 18-crown-6 ether, 5 mM HIBA,2.5 mM triethanolamine, pH 4.2

10 cm; anions60 s, cations45 s

Indirect UV,anions 232nm, cations220 nm

Anions 0.3–0.8 �M,cations0.4–0.9 �M

[150]

Organic acids Water (snowas example)

5 mM Tris, 2 mM trimellitate, 0.2 mM TTAB,0.5 mM Ca+2, pH 8.5 using NaOH

45 s/–5 kV (a),90 s/10 cm (b)

Indirect UV,254 nm

1–20 �g/L (a),0.1–1.0 mg/L (b)

[151]

Metal cations Particulatematter in air

30 mM hydroxylamine hydrochloride, 0.1 mM

1,10-phenanthroline, 1% methanol, pH 3.725 s/8 cm Direct UV,

265 nm0.5–3 �g/L [152]


method for the simultaneous determination of water/acid-leachable metal ions in respirable, fine, and coarse parti-culate matter in air. The method utilizes a system consist-ing of 1,10-phenanthroline for direct UV detection of Zn,Cu, Fe and Cd cations. High iron and lower copper andzinc concentrations were found in respirable suspendedparticles sampled in Hong Kong.

Tam et al. [149] described a simple protocol for collectionof bulk dry deposition on a daily basis, and for size-selected aerosol sampling. The soluble components ofthese samples were analyzed by CE, which has beenshown to provide the sample throughput, accuracy andprecision required for routine analysis in the �M-mM

ranges with very low sample consumption. The reportedscheme enabled the simultaneous analyses of inorganicanions and organic acids, as well as major cations.Representative results from samples collected in HongKong were included.

New methods for the simultaneous determination of in-organic anions and cations [153] and alkali, alkalineearth and transition metal ions [154] have been reportedrecently. Haumann et al. [153] investigated the principle ofboth-side injection using two different BGE systems. Thefirst system consisted of imidazole as the cation compo-nent and thiocyanate as the anion component. To resolveammonium and potassium, 18-crown-6 ether was addedto the electrolyte; citric acid was used to improve separa-tion of the alkaline earth ions. Using this system, efficientsimultaneous separation of inorganic cation and anionswas obtained. However, if real samples contained cationswith low electrophoretic mobility, these appeared to inter-fere with the anion determination. A second system uti-lized a stronger EOF to avoid overlapping of cation andanion signals. The BGE consisted of dimethyldiphenyl-phosphonium iodide as the cationic UV absorbent, trime-sic acid as the anionic UV probe, and hydroxyisobutyricacid (HIBA) and 18-crown-6 as additives. Thiosulfate asthe most mobile anion was well separated from the EOFsignal, so it was not possible for any cationic componentof the sample to disrupt the determination of anions. Thedevelopment and optimization described for these sys-tems is more extensive than electrolyte systems for sepa-rate cation or anion analysis. The applicability of themethod was shown by investigation of reproducibilityand linearity, and by the analysis of drinking water. An-other study demonstrated the successful determinationof 16 metal ions with 4-aminopyridine as UV-absorbingprobe and HIBA as complexing reagent (pH 4.5) [154].The potential of the method for quantitative and qualita-tive analysis of metal ions in river water was presented.Verma et al. [155] developed an improved CE methodwith indirect UV detection for quantifying rare-earth ele-

ments (REEs) in synthetic geochemical standards. Theseparation of the REE total group (lanthanum to lutetium)within 2 min was obtained using a BGE consisting of10 mM UVCat-1 and 4 mM HIBA (pH 4.4 with acetic acid)at 15�C. Under these conditions, an excellent separationof europium and gadolinium peaks, which usually hadoverlapping problems in earlier applications, was ob-tained.

The popularity of on-column UV-absorbing chelatingreagents for metal ion analysis continues [156–159].Vanadium(V), niobium(V) and tantalum(V) were separatedas ternary mixed-ligand complexes by CE using 4-(2-pyr-idylazo)resorcinol (PAR) [156]. Another work describedthe use of 4-(2-thiazolylazo)resorcinol (TAR) to separateuranium(VI), cobalt(II), cadmium(II), nickel(II), titanium(IV)and copper(II) metal ions [157]. Efficient separation ofnine transition metals with N, N’-bis(hydroxybenzyl)eth-ylenediamine-N, N’-diacetic acid using ion electrokineticchromatography was also reported [158]. Separation anddirect UV detection of lanthanides complexed with cup-ferron by CE was investigated by Öztekin and Erim [159].The resolution of partially complexed positively chargedcupferron complexes was improved by using HIBA asbuffer and competing ligand. An on-column separationof 14 lanthanides was achieved in only 7 min using0.1 mM cupferron and 15 mM HIBA at pH 4.9. Thereported detection limits ranged between 0.24 and0.47 mg/L.

7.10 Element speciation

Recent reviews [160, 161] summarize the present statusand background of element speciation by CE. Extensivedetails of developments in the application of CE to simul-taneous separation and determination of different chemi-cal forms of an inorganic element, with particular empha-sis placed on metal speciation analysis, can be found inthe first review [160]. The methods’ strengths and currentlimitations with regard to chemical speciation studies arealso critically discussed. In the second excellent review,Timerbaev [161] focuses on the ways to overcome thelimitations of CE in the context of solving practical specia-tion problems, rather than dwelling on the techniques’strengths and developments. Recommendations on howto produce reliable speciation measurements by CE aregiven. More recently, new methods and applicationshave been proposed for nitrogen [162–164], sulfur [165,166] and various metal speciation analyses [29, 35, 36,66, 167–169]. Some examples will be described below.

Ultrarapid analysis of nitrate and nitrite using CE wasrecently published by Melanson and Lucy [162]. With thismethod, nitrate and nitrite are separated in just over 10 s.


Direct UV detection at 214 nm was employed and offeredsub-�M detection limits using electrokinetic injection.Total time (pre-rinse, injection, separation) was less than1 min, making this method ideal for high-throughputanalysis. Padarauskas et al. [163, 164] developed a CEmethod for the simultaneous determination of nitrate,nitrite and ammonium based on electrokinetic injectionfrom both ends of the capillary, using both direct andindirect UV detection. High precision could be obtained iftwo oppositely charged internal standards were used.The proposed system was applied to the speciation ofinorganic nitrogen in rainwater [164]. In an effort to over-come the analytical challenges posed by the high-saltconditions of seawater, nitrate and nitrite have beenanalyzed using transient ITP with artificial seawater asthe BGE, but so far this method is optimized separatelyfor the two analytes and the detection limits are not yetsufficient for low environmental levels [72].

CE with indirect UV detection was developed for rapidspeciation of sulfite, sulfate, thiosulfate and peroxodi-sulfate [165]. By adding 5% propanol as a stabilizer toboth the working electrolyte and the sample solution,good stability for sulfite and separation of the sulfuranions within 4 min were obtained. Another study [166]evaluated the performance of CE with indirect UV detec-tion in the simultaneous analysis of several sulfur speciesleached from oxidized pyrite. The developed method wassuccessfully applied to the separation and quantificationof seven sulfur species in aqueous solution. Calibrationcurves were linear for all the studied species, includingthe unstable sulfite and sulfide species, for which the cali-bration curves were deduced from those of their hydro-lysis/oxidation products.

CE can be utilized as a powerful new tool for kinetic inves-tigation of metal complexes if the exchange kinetics ofthe species are sufficiently slow [167]. The kinetic studyof Cr(III) chlorocomplexes showed that the solution ob-tained by dissolving Cr(III) chloride cannot be regardedas a pure, one-component system but as a mixture ofthree Cr(III) species, the concentrations of which changewith time depending on the pH of the solution. For thesereasons most environmental and biological samples con-taining Cr(III) should be regarded as a mixture of differentCr(III) species with relatively long lifetimes. The determi-nation of Cr(III) hydrolytic polymerization products usinga CE-ICP-MS method was described in another study[36]. The reported results indicated that the developedmethod could be used to separate and detect polymericCr(III) species. In general, the data suggested that therelative mobility follows the order trimer � dimer � mono-mer. Other recently developed methods and applicationsusing CE-ICP-MS, CE-ICP-AES and CE-MS techniquesin metal speciation studies are discussed in Section 2.

7.11 Miscellaneous applications

Ethylenediaminetetraacetic acid (EDTA) and nitrilotriace-tic acid (NTA) form stable complexes with metals; as aresult, they can modify metal transport and distributionin the environment and uptake by plants. Free chelatesand their metal complexes have been separated by CEusing UV detection [170, 171]. The development of suchmethodology has allowed for speciation in nutrient mediawith LODs of 2–50 �M and reproducibility better than 5%for corrected peak area [171]. Amnesic and paralyticshellfish poisoning, which have recently become seriousproblems for human and fish health, are caused by phyto-planktonic toxins which are incorporated into the marinefood chain, making their analytical determination of pri-mary importance. Piñeiro and co-workers [172] describedan efficient separation of these toxins by CE, whichinvolved sample homogenization and a series of clean-up steps prior to analysis by CE-UV detection. The flota-tion of metals from crude ore was in the past oftenaccomplished using alkyl xanthates and phosphonateswhich could then leach into river and groundwater. Thesereagents have been determined in tin ore tailings by aCE-UV method. Improvement in the sensitivity up to4–8-fold was achieved by pressure-assisted FASI, andup to 10-fold by stacking injection (which consisted offilling a third of the capillary, followed by negative voltagefor 85 s to eliminate aqueous solution). The latter injec-tion gave LODs from 10 to 49 �g/L [71].

8 Conclusions

Recent advances in CE and CEC separation, detectionand sample preparation methods applied for the determi-nation of a variety of pollutants have been described. Thereviewed literature has illustrated the wide applicationrange of CE, which indicates the continuing interest inCE and CEC in the environmental field. From the abovediscussion, it is clear that CE techniques have developedto the stage where they are having a significant impact onCE environmental usage, especially for the determinationof small ions. In addition, both techniques but especiallyCE are now maturing from a phenomenon of interest toacademics to a technique of potential practical interestto the analytical chemist.

During the period covered by this review, progress hasbeen made in sample preparation/preconcentration anddetection although much work still deals with the separa-tion of model analytes. In terms of detection, the inter-facing of CE to MS, ICP-MS and ECD was explored indepth by several groups. Instrumental advances haveled to increased sensitivity and therefore a growing num-ber of applications. Important efforts have been made


during the reviewed period to increase the robustness ofthe developed CE methods. However, for much wideruse, especially for the trace species levels in real sam-ples, it is still desirable for the sensitivity and precision inquantitative analysis to be improved in order to becomecomparable with other mature separation techniques.Thus, a real breakthrough in broader acceptance of CEtechniques in environmental routine usage will only bepossible with additional improvements in detection sensi-tivity and/or in on-line sample treatment including sampleenrichment steps prior to CE. Finally, the recent develop-ments of chip-based CE systems have demonstrated thefeasibility of portable CE systems, which should becomean important field of interest in the near future.

Received June 13, 2001

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Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

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