Sensitive high-resolution analysis of biological molecules by capillary zone electrophoresis coupled with reflecting time-of-flight mass spectrometry

Capillary electrophoresis–mass spectrometry for the analysis of intact proteins

Developments in the fields of protein chemistry, proteomics and biotechnology have increased the demand for suitable analytical techniques for the analysis of intact proteins. In 1989, capillary electrophoresis (CE) was combined with mass spectrometry (MS) for the first time and its potential usefulness for the analysis of intact (i.e. non-digested) proteins was shown. This article provides an overview of the applications of CE-MS within the field of intact protein analysis. The principles of the applied CE modes and ionization techniques used for CE-MS of intact proteins are shortly described. It is shown that separations are predominantly carried out by capillary zone electrophoresis and capillary isoelectric focusing, whereas electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most popular ionization techniques used for interfacing. The combination of CE with inductively coupled plasma (ICP) MS for the analysis of metalloproteins is also discussed. The various CE-MS combinations are systematically outlined and tables provide extensive overviews of the applications of each technique for intact protein analysis. Selected examples are given to illustrate the usefulness of the CE-MS techniques. Examples include protein isoform assignment, single cell analysis, metalloprotein characterization, proteomics and biomarker screening. Finally, chip-based electrophoresis combined with MS is shortly treated and some of its applications are described. It is concluded that CE-MS represents a powerful tool for the analysis of intact proteins yielding unique separations and information.

Advances in the analysis of proteins and peptides by capillary electrophoresis with matrix-assisted laser desorption/ionization and electrospray-mass spectrometry detection

High throughput, outstanding certainty in peptide/protein identification, exceptional resolution, and quantitative information are essential pillars in proteome research. Capillary electrophoresis (CE) coupled to mass spectrometry (MS) has proven to meet these requirements. Soft ionization techniques, such as matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), have paved the way for the story of success of CE-MS in the analysis of biomolecules and both approaches are subject of discussion in this article. Meanwhile, CE-MS is far away from representing a homogeneous field. Therefore the review will cover a vast area including the coupling of different modes of CE (capillary zone electrophoresis, capillary isoelectric foscusing, capillary electrochromatography, micellar electrokinetic chromatography, nonaqueous capillary electrophoresis) to MS as well as on-line preconcentration techniques (transient capillary isotachophoresis, solid-phase extraction, membrane preconcentration) applied to compensate for restricted detection sensitivity. Special attention is given to improvements in interfacing, namely addressing nanospray and coaxial sheath liquid design. Peptide mapping, collision-induced dissociation with subsequent tandem MS, and amendments in mass accuracy of instruments improve information validity gained from MS data. With 2-D on-line coupling of liquid chromatography (LC) and CE a further topic will be discussed. A special section is dedicated to recent attempts in establishing CE-ESI-MS in proteomics, in the clinical and diagnostic field, and in the food sector.

Design, optimisation, and evaluation of a sheath flow interface for automated capillary electrophoresis-electrospray-mass spectrometry

A sheath-flow capillary electrophoresis-mass spectrometry (CE-MS) system utilizing a fully integrated large-bore stainless-steel emitter electrode tapered at the end for micro-ionspray operation has been developed and evaluated. A separation capillary with an outer diameter of up to 360 microm was inserted into the electrode thus forming a void volume of less than 15 nL between the capillary end and the electrospray ionisation (ESI) tip. The sheath liquid, usually methanol-water (80:20) with 0.1% formic acid for positive ion mode or methanol for negative ion mode, was delivered at 0.5-1.0 microL/min. Unlike previously reported CE-MS interfaces, the CE-MS probe was incorporated directly onto an Applied Biosystems/MDS SCIEX orthogonal-spray Turbo "V" ion source for ease of use and automatic operation. This integration enables fast and facile coupling and replacement of the separation capillary without interrupting the ion source configuration, and the sheath liquid supply. The reusable electrospray electrode was precisely fabricated and aligned with the length of the nebulizing gas tube for improved reproducibility. Automation was achieved through software control of both CE and tandem MS (MS/MS) for unattended batch sample analysis. The system was evaluated for attomole- to low femtomole-level profiling of model peptides and protein mixtures, bisphosphates, as well as antiviral nucleosidic drugs in cellular extracts.

Analysis of melamine resins by capillary zone electrophoresis with electrospray ionization-mass spectrometric detection

A method for the determination of the major components of (methoxymethyl)melamine resins, with quantitative analysis of unreacted melamine by capillary zone electrophoresis (CZE) using electrospray ionization-mass spectrometry (ESI-MS) is presented. Using a low background electrolyte (BGE) pH, components are separated according to their charge/ionic radius ratio with a distinctly different separation selectivity compared to the HPLC methods commonly employed in melamine-resin analysis. The use of a time-of-flight mass spectrometer (TOF-MS) was concluded to be necessary, as the complex samples studied required maximum sensitivity and resolution, which is clearly superior for TOF-MS detectors over their quadrupole counterparts. A standard curve of free melamine was determined with an R(2) = 0.999 over a concentration range of an order of magnitude. This method offers the unique separation selectivity of CZE as well as a quicker analysis time, especially for dimers compared to the HPLC methods used to date.

A sheath-flow nanospray interface for capillary electrophoresis/mass spectrometry

We have developed a novel sheath-flow interface for low-flow electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis/electrospray mass spectrometry (CE/ESI-MS). The interface is composed of two capillaries. One is a tapered fused-silica ESI emitter suitable for microliter and nanoliter flow rate electrospray and the other is a tail-end gold-coated CE separation column that is inserted into the emitter. A sheath liquid is supplied between the column and the emitter capillaries. The gold coating and the sheath liquid are used as the conducting media for ESI and the CE circuit. This novel design was initially evaluated by an infusion ESI-MS analysis of the most common antiretroviral dideoxynucleosides, followed by CE/MS coupling analysis of several antidepressant drugs. With infusion studies, the effects of the sheath liquid and the sample flow rates on detection sensitivity and signal stability were investigated. For an emitter with an internal diameter of 30 microm, the optimum flow rates for the sheath and the sample were 200 and 300 nL/min, respectively. The main improvement of this approach in comparison with conventional sheath liquid approaches using an ionspray interface is the gain in sensitivity. Sensitivities were three times better for dideoxynucleosides analyzed by infusion and 12 times higher for antidepressant drugs analyzed by CE/MS with this interface compared with ionspray. The emitter is durable, disposable, and simple to fabricate.

Online CE−MALDI-TOF MS Using a Rotating Ball Interface

We report on the construction and performance of a rotating ball interface for online coupling of capillary electrophoresis (CE) to matrix-assisted laser desorption ionization (MALDI) mass spectrometry with a time-of-flight (TOF) mass analyzer. The interface is based on a rotating stainless steel ball that transports samples from atmospheric pressure to the high vacuum of the mass spectrometer for desorption and ionization. The sample is deposited directly from a 50-microm-i.d. separation capillary onto the 19-mm ball that is rotating at 0.03 to 0.3 rpm. The sample is mixed online with matrix flowing from a separate 50-microm-i.d. capillary. The sample deposit dries before it is rotated past a polymer gasket and into the laser ionization region. Cleaning of the interface is accomplished using solvent-saturated felt, which cleans the ball surface after it rotates out of the ionization chamber. On-line CE-MALDI is demonstrated, and the performance is evaluated with the analysis of a mixture of three peptides: [Lsy8] vasopressin, substance P, and neurotensin. The rotating ball interface to MALDI-TOF MS demonstrated mass detection limit in the high femtomole range. The interface has negligible memory effect and shows no significant electrophoretic peak broadening when operated under optimized conditions.

Nanospray Mass Spectrometry with Indirect Conductive Graphite Coating

An easy and cost-effective method to manufacture a robust conductive graphite coating for nanospray mass spectrometry (nESI-MS) and capillary electrophoresis (CE)-nESI-MS is described. The method involves graphite coating of a tube sleeve, into which the nESI emitter is inserted and connected to a transfer capillary, instead of coating the actual emitter. The coating, made of graphite from a pencil and epoxy glue, was stable over long periods of use (>80 h) and showed excellent resistance toward various solvents. Stable electrospray was achieved in the investigated flow range (150-900 nL x min(-)(1)), and salbutamol, diphenhydramine, and nortriptyline (M(w): 239-263 g x mol(-)(1)) were detected in the nanomole per liter range during continuous pumping. CE-nESI-MS analysis gave excellent signal-to-noise ratios for 100-fmol injections. The technique allows simple exchange of the nESI emitter to suit a specific flow rate, and it minimizes risk of corona discharge.

Coupling of a large-size capillary column with an electrospray mass spectrometer. A reliable and sensitive sheath flow capillary electrophoresis-mass spectrometry interface

The concept of interfacing a large-size column for capillary electrophoresis (CE) to electrospray ionization mass spectrometry (ESI-MS) for robust and automatic CE-MS operation is reported. Both standard ionspray interface and microionspray interface have been modified to operate in a sheath flow pattern to overcome the common stability problem in CE-MS coupling. To make the interface sensitive, a step-down stainless steel tube with smaller inner diameter and tapered tip was incorporated onto a larger tube embracing the CE column via cold soldering. The devices were evaluated for quantitative analysis of nucleotides at femtomole level and stable analytical performance in peptide profiling.

Field-enhanced sample injection for high-sensitivity analysis of peptides and proteins in capillary electrophoresis–mass spectrometry

Field-enhanced sample injection (FESI) was used to improve the concentration sensitivity of a capillary electrophoresis (CE)-mass spectrometry (MS) system with sheath flow configuration. Using some bioactive peptides, more than 3000-fold improvement in signal was obtained, permitting analysis in the low nM (fmol/microl) levels. The system was further evaluated for analysis of complex peptide mixtures by using low concentration tryptic digests of standard proteins. Rapid identification of the original protein was obtained by database searching using the observed molecular masses of the peptides, and by comparison of actual MS-MS spectra of selected peptides with the predicted fragmentation patterns.

A colloidal graphite-coated emitter for sheathless capillary electrophoresis/nanoelectrospray ionization mass spectrometry

A colloidal graphite-coated emitter is introduced for sheathless capillary electrophoresis/nanoelectrospray ionization time-of-flight mass spectrometry (CE/ESI-TOFMS). The conductive coating can be produced by brushing the capillary tip to construct a fine layer of 2-propanol-based colloidal graphite. The fabrication involves a single step and requires less than 2 min. Full cure properties develop in approximately 2 h at room temperature and then the tip is ready for use. The coated capillary tip is applied as a sheathless electrospray emitter. The emitter has proven to bear stable electrospray and excellent performance for 50 microm i.d. x 360 microm o.d. and 20 microm i.d. x 360 microm o.d. capillaries within the flow rate of 80-500 nL/min; continuous electrospray can last for over 200 h in positive mode. Baseline separation and structure elucidation of two clinically interesting basic drugs, risperidone and 9-hydroxyrisperidone, are achieved by coupling pressure-assisted CE to ESI-TOFMS using the described sheathless electrospray emitter with a bare fused-silica capillary at pH 6.7. It is found that the signal intensity of m/z in sheathless CE/ESI-TOFMS at pH 6.7 is approximately 50 times higher than that at pH 9.0 for the two analytes, although the electroosmotic flow (EOF) at pH 9.0 provides sufficient flow rate (approximately 150 nL/min) to maintain electrospray.

A new sheathless electrospray interface for coupling of capillary electrophoresis to ion-trap mass spectrometry

A simple laboratory-made sheathless electrospray interface for coupling of capillary electrophoresis to ion-trap mass spectrometry (CE/MS) was developed. The interface was machined in-house and it was designed to be freely interchangeable with the commercially available ionization sources for the mass spectrometer. Sharpened fused-silica capillaries were coated with nickel by a simple electrodeless plating procedure and were used as all-in-one columns/emitters. The electrodeless plating produced a 2-5- micro m thick smooth nickel layer that lasted for more than 8 h of continuous electrospraying. The performance of the CE/MS interface was examined by using four cationic imipramine derivatives as test substances. Relative detection limits were calculated on the basis of the extracted ion electrophorograms and were in the range 6-130 nmol/L, corresponding to absolute detection limits in the range of 20-400 amol. The system was applied for analysis of impurities in an impure imipramine N-oxide preparation, and two of the impurities could be identified on the basis of online-MS(MS) spectra recorded in scan-dependent mode.

Evaluations of the stability of sheathless electrospray ionization mass spectrometry emitters using electrochemical techniques

The processes that cause the failure of sheathless electrospray ionization (ESI) emitters, based on different kinds of gold coatings on fused-silica capillaries, are described and explained. The methods chosen for this study include electrochemical methods, ICPMS analysis of the electrolytes used, SEM studies, and electrospray experiments. Generally, the failure occurs by loss of the conductive coating. It is shown that emitters with sputter-coated gold lose their coatings because of mechanical stress caused by the gas evolution accompanying water oxidation or reduction. Emitters with gold coatings on top of adhesion layers of chromium and nickel alloy withstand this mechanical stress and have excellent durability when operating as cathodes. When operating as anodes, the adhesion layer is electrochemically dissolved through the gold film, and the gold film then flakes off. It is shown that the conductive coating behaves as a cathode even in the positive electrospray mode when the magnitude of a superimposed reductive electrophoretic current exceeds that of the oxidative electrospray current. Fairy-dust coatings developed in our laboratory (see Barnidge, D. R.; etal.Anal. Chem. 1999, 71, 4115-4118,) bygluing gold dust onto the emitter, are unaffected by the mechanical stress due to gas evolution. When oxidized, the fairy-dust coatings show an increased surface roughness and decreased conductivities due to the formation of gold oxide. The resistance of this oxide layer is however negligible in comparison with that of the gas phase in ESI. Furthermore, since no flaking and only negligible electrochemical etching of gold was found, practically unlimited emitter lifetimes may be achieved with fairy-dust coatings.

Robust and cost-effective capillary electrophoresis-mass spectrometry interfaces suitable for combination with on-line analyte preconcentration

This paper describes several successful cost-effective attempts to couple capillary electrophoresis (CE) and mass spectrometry (MS) without make-up flow or nebulizing gas. An in-depth analysis of several interfaces using conductive spray tips was performed as well as an easy-to-prepare T-junction with direct electrode contact, the latter being the most robust interface. No coating is necessary and the spray voltage is applied through a gold wire positioned at the gap between the separation and spray capillaries. The T-junction interface is made by puncturing a small piece of transparent rubber. The on-line preconcentration CE-MS system allows immunoassay sensitivity, as is demonstrated by a calibration plot in the picomolar range for angiotensin II and gonadorelin. It also shows good reproducibility and has the ability of excellent automation. The secure electrical contact gives a constant spray quality, even with 100% aqueous separation buffers. The described setup has a wide applicability as is demonstrated by the analysis of larger peptides, such as insulin and cytochrome c. Detailed information is given on critical factors in the preparation of the described interfaces.

A simple and robust conductive graphite coating for sheathless electrospray emitters used in capillary electrophoresis/mass spectrometry

A graphite-polyimide mixture was used as a conductive coating for sheathless electrospray emitters. The coating procedure described is simple and inexpensive compared to previously described methods. An investigation of the stability of the conductive coating carried out by electrochemical methods revealed good performances during oxidative stress. In addition, no decrease in emitter performance was seen during continuous electrospray in the positive electrospray mode for two weeks. Fast capillary electrophoresis with attomole sensitivity demonstrated the excellent performance of the described sheathless interface when used in conjunction with an orthogonal time-of-flight mass spectrometer. The overall simplicity, stability and low cost of this type of sheathless emitter make the described approach highly suitable for any on-column coupling of low flow rate separation techniques to a mass spectrometer.

Quantitative analysis of pharmaceutically active peptides using on-capillary analyte preconcentration transient isotachophoresis

An on-capillary adsorptive phase in combination with capillary electrophoresis (CE), frequently referred to as preconcentration CE, for quantitative analysis of low peptide concentrations was developed. The capillary containing the on-line analyte preconcentrator can be constructed within 5 min from commercially available extraction disks. These disks contain poly(styrenedivinylbenzene) adsorbent particles incorporated in a matrix of inert Teflon, creating a mechanically stable sorbent. Therefore, no frits are needed in the capillary to hold the stationary phase in place. Several parameters, such as the required minimal elution volume, required elution strength, sample application speed or ionic strength, and the capacity were investigated and special interest was given to the quantitative properties of the method. Instead of nL injections, volumes up to a least 25 microL are possible, yielding improvements in detection limits of 3-4 orders of magnitude. The observed limit of detection for both model peptides was 20 pg, corresponding to a 20 microL injection of a 1 ng/mL solution of both model peptides. Using low-wavelength UV detection, reproducibility and linearity in the low nanogram range were satisfactory. No influence of matrix salt concentrations was observed, extending the use of CE to all kinds of samples.

Design of a sheathless capillary eletrophoresis-mass spectrometry probe for operation with a Z-spray ionization source

The construction of a sheathless interface for capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS), for operation with a Z-Spray source on a Micromass Quattro-LC triple quadrupole mass spectrometer is described. Designing the interface involved machining a probe compatible with the setup already in place on the mass spectrometer, i.e., MegaFlow-Z ESI. The probe was made of Lexan with the same dimensions as the ESI probe supplied with the instrument. The electrical connection at the electrospray end of the CE capillary was made possible by gold-coating (sheathless CE-ESI-MS). The probe design as well as the electrical and power supply requirements are described in detail. Experiments were performed using this interface, and CE separations of mixtures containing pmole and sub-pmole amounts of peptides were monitored by on-line MS. For a standard peptide mixture (10(-4) M), separation efficiency was typically characterized by N > 10(4) theoretical plates with S/N > 400. Using the same experimental setup, it was also possible to conduct on-line CE-ESI-tandem MS (MS/MS) experiments on the same peptide mixture, and to determine the sequence of the peptides. MS/MS scan functions for different precursor ions were used either alternately or sequentially and the results from both methods were compared. The possibility of peptide mass mapping was explored, and CE-ESI-MS results were obtained for the digestion products of equine myoglobin. Separation efficiencies and S/N values were similar to those obtained for standard peptides. A complete map of the digestion products was obtained.

On-column conductive coating for thermolabile columns used in capillary zone electrophoresis sheathless electrospray ionisation mass spectrometry

A simple and very mild approach for the application of a conductive layer for sheathless electrospray has been developed. A modified 'fairy dust' method is employed in which 2 microm gold particles are applied by a thin layer of silicone on shaped tips. This novel approach comprises fabrications at room temperature, under atmospheric pressure, and involves no etching, extensive cleaning or otherwise harsh conditions. With this approach, sheathless electrospray emitters have been fabricated from fused silica capillaries with chemically pre-modified inner walls and from heat-sensitive polypropylene hollow fibres. Long term stability for more than two weeks of continuous spraying has been achieved. Capillary zone electrophoresis/time-of-flight mass spectrometry demonstrates attomole sensitivity and no detectable band broadening. A comparison with a chromium-gold coated emitter in terms of chemical noise is made with continuous infusion experiments, showing no significant increase in background from the polymer involved.

Characterization of multipole storage assisted dissociation: implications for electrospray ionization mass spectrometry characterization of biomolecules

Ions accumulated in an rf-only multipole for extended periods of time prior to mass analysis can experience a significant degree of fragmentation and produce mass spectra which do not reflect the true nature of the intact analyte(s). This phenomenon, termed multipole storage assisted dissociation (MSAD), places constraints on the maximum number of ions which can be accumulated in the multipole storage device as a result of its finite space charge limit. This phenomenon can be exploited to produce dissociation spectra that are dominated by fragment ions providing important sequence/structure information. In this work we further explore MSAD and characterize parameters including accumulation time, source pressure, and the electrostatic configuration of the multipole storage device, which mediate the phenomenon. Operating parameters are identified that can either enhance or eliminate the phenomenon.

Electrospray ionization mass spectrometry and on-line capillary zone electrophoresis - mass spectrometry for the characterization of citrate synthase

The enzyme citrate synthase from E. coli is a protein with a molecular weight (Mr) of 47 885 Da (wild type). This enzyme has been studied extensively, and its amino acid sequence has been characterized. This model protein has been used in this work for development and validation of methods involving capillary electrophoresis (CE) and electrospray ionization mass spectrometry (ESI-MS). The Mr determinations were conducted using sample infusion ESI-MS, and the tryptic digestion products of wild-type citrate synthase were characterized by on-line CE-ESI-MS coupled with a sheathless interface. On-line experiments were conducted on two different mass spectrometers, a Quattro-LC triple quadrupole instrument equipped with a Z-SprayTM source (Micromass), and a reflecting time-of-flight (TOF) mass spectrometer built in-house in the Time-of-Flight Laboratory, Department of Physics, University of Manitoba. This is the first article to be written on the interfacing of a Z-SprayTM source with CE. Unmodified fused silica capillaries gold-coated sheathless interfaces were used. The on-line CE separations yielded theoretical plate numbers greater than 104 on average. Selected ion electrophorograms (SIE) of the tryptic peptides recorded on the Quattro-LC displayed S/N ratios ranging from ca. 14 to 120 on raw data. These SIE enabled identification of each peptide. The use of reflecting time-of-flight mass spectrometry (TOFMS) afforded mass resolution values of ca. 6000 (m/deltamFWHM), which enabled isotopic resolution of the peptide components. CE-ESI-MS and CE-ESI-TOFMS experiments enabled the generation of a complete tryptic map of citrate synthase.Key words: capillary electrophoresis, electrospray ionization, mass spectrometry, citrate synthase, tryptic digestion, triple quadrupole analyzer, time-of-flight analyzer.

Comparison between transient isotachophoretic capillary zone electrophoresis and reversed-phase liquid chromatography for the determination of peptides in plasma

Low levels of peptide drugs in human plasma can be determined employing off-line solid-phase extraction, followed by capillary zone electrophoresis with UV detection. A bioanalytical procedure is presented, using gonadorelin and angiotensin II in human plasma as model compounds. The solid-phase extraction method, based on a weak cation exchange mechanism, is able to remove interfering endogenous components from the plasma sample, extract the model peptides quantitatively, and give a possibility of concentrating the sample at the same time. Transient isotachophoretic conditions were kept to increase the sample loadability by about two orders of magnitude. Up to about 70% of the capillary was filled with the reconstituted extract, whereafter the peptides were selectively concentrated during the first 15 min. Subsequently, the concentrated sample zones were separated under capillary zone electrophoresis conditions, showing the technique's high resolution. For the model cationic peptides (gonadorelin, angiotensin II) good linearity and reproducibility was observed in the 20-100 ng/mL concentration range. A more extensive washing procedure permits quantitation of gonadorelin at the 5 ng/mL level. In comparison with a liquid chromatography analysis, superior mass sensitivity and separation are obtained with the transient isotachophoretic capillary zone electrophoresis method. Moreover, in this case equivalent sensitivity is achieved when it is directly compared with a liquid chromatography method with UV detection, keeping in mind that 60 times more sample is needed for the latter method. A further gain in sensitivity can be obtained when the analysis is combined with native fluorescence detection, as is demonstrated by combining liquid chromatography separation with fluorescence detection.

Monitoring impurities in biopharmaceuticals produced by recombinant technology

The unique nature of recombinant technology and the biotherapeutic production process means that regulatory agencies around the world not only require extensive characterization of the product, but they have also provided 'guidelines' to control and monitor product- and process-derived impurities and contaminants. Not only might these impurities and contaminants have a profound effect on product quality and efficacy, but they might also introduce unwanted and unknown side effects, even in trace amounts. The authors present a comprehensive understanding of the nature of possible product- and process-related impurities, and also describe current and future methodologies to control and monitor these impurities.

Capillary electrophoresis/tandem mass spectrometry for analysis of proteins from two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis

Capillary electrophoresis/time-of-flight mass spectrometry(CE/TOFMS) has been used for analysis of in-gel digests of protein spots excised from two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (2-D SDS-PAGE). An off-line purification and preconcentration procedure with a Zip Tip is used before CE/TOFMS analysis which allows for detection of protein spots with <1 picomole of material from 2-D gels. The off-line procedure provides sufficient purification for analysis while maintaining the quality of the CE separation. Using this procedure, several proteins from Coomassie Blue and zinc negatively stained gels are identified by the peptide maps generated and database searching. CE/TOF tandem mass spectrometry is used for the confirmation of database searching results and structural analysis of peptides that do not match the expected peptide maps obtained from the database in order to identify structural modifications. Several modifications were pinpointed and identified by this method.

Analyte identification in capillary electrophoretic separation techniques

A review on applications of on-line hyphenation in capillary electrophoresis and capillary electrochromatography for the identification of migrating analytes is presented. There is an urgent need for unambiguous analyte identification by combining spectral information and observed migration times, because the parameters influencing the migration times and separation efficiencies in these separation techniques are not easily controlled, especially when real samples containing unknown interferences have to be analyzed. The spectrometric techniques covered here are ultraviolet and visible radiation (UV/Vis) absorption, fluorescence including fluorescence line-narrowing spectroscopy, Raman spectroscopy, nuclear magnetic resonance and mass spectrometry. Attention is essentially confined to literature reports in which the extra information provided by the detector is really used for identification purposes, especially in real-life samples, while the interfacing as such and analyte detectabilities in standard solutions are only briefly discussed. This article covers an extensive fraction of the literature published on this topic until the beginning of 1998.