Enhanced performance membrane preconcentration-capillary electrophoresis-mass spectrometry (mPC-CE-MS) in conjunction with transient isotachophoresis for analysis of peptide mixtures

Mobility-based selective on-line preconcentration of proteins in capillary electrophoresis by controlling electroosmotic flow

A simple method to perform selective on-line preconcentration of protein samples in capillary electrophoresis (CE) is described. The selectivity, based on protein electrophoretic mobility, was achieved by controlling electroosmotic flow (EOF). A short section of dialysis hollow fiber, serving as a porous joint, was connected between two lengths of fused silica capillary. High voltage was applied separately to each capillary, and the EOF in the system was controlled independently of the local electric field intensity by controlling the total voltage drop. An equation relating the EOF with the total voltage drop was derived and evaluated experimentally. On-line preconcentration of both positively charged and negatively charged model proteins was demonstrated without using discontinuous background electrolytes, and protein analytes were concentrated by approximately 60-200-fold under various conditions. For positively charged proteins, positive voltages of the same magnitude were applied at the free ends of the connected capillaries while the porous joint was grounded. This provided a zero EOF in the system and a non-zero local electric field in each capillary to drive the positively charged analytes to the porous joint. CE separation was then initiated by switching the polarity of the high voltage over the second capillary. For negatively charged proteins, the procedure was the same except negative voltages were applied at the free ends of the capillaries. Mobility-based selective on-line preconcentration was also demonstrated with two negatively charged proteins, i.e. beta-lactoglobulin B and myoglobin. In this case, negative voltages of different values were applied at the free ends of the capillaries with different values, which provided a non-zero EOF in the system. The direction of EOF was the same as that of the electrophoretic migration velocities of the protein analytes in the first capillary and opposite in the second capillary. By controlling the EOF, beta-lactoglobulin B, which has a higher mobility, could be concentrated over 150-fold with a 15 min injection while myoglobin, which has a lower mobility, was eliminated from the system.

Coupling continuous separation techniques to capillary electrophoresis

One of the weak points of capillary electrophoresis is the need to implement rigorously sample pretreatment because its great impact on the quality of the qualitative and quantitative results provided. One of the approaches to solve this problem is through the symbiosis of automatic continuous flow systems (CFSs) and capillary electrophoresis (CE). In this review a systematic approach to CFS-CE coupling is presented and discussed. The design of the corresponding interface depends on three factors, namely: (a) the characteristics of the CFS involved which can be non-chromatographic and chromatographic; (b) the type of CE equipment: laboratory-made or commercially available; and (c) the type of connection which can be in-line (on-capillary), on-line or mixed off/on-line. These are the basic criteria to qualify the hyphenation of CFS (solid-phase extraction, dialysis, gas diffusion, evaporation, direct leaching) with CE described so far and applied to determine a variety of analytes in many different types of samples. A critical discussion allows one to demonstrate that this symbiosis is an important topic in research and development, besides separation and detection, to consolidate CE as a routine analytical tool.

On-line preconcentration methods for capillary electrophoresis

The limits of detection (LOD) for capillary electrophoresis (CE) are constrained by the dimensions of the capillary. For example, the small volume of the capillary limits the total volume of sample that can be injected into the capillary. In addition, the reduced pathlength hinders common optical detection methods such as UV detection. Many different techniques have been developed to improve the LOD for CE. In general these techniques are designed to compress analyte bands within the capillary, thereby increasing the volume of sample that can be injected without loss of CE efficiency. This on-line sample preconcentration, generally referred to as stacking, is based on either the manipulation of differences in the electrophoretic mobility of analytes at the boundary of two buffers with differing resistivities or the partitioning of analytes into a stationary or pseudostationary phase. This article will discuss a number of different techniques, including field-amplified sample stacking, large-volume sample stacking, pH-mediated sample stacking, on-column isotachophoresis, chromatographic preconcentration, sample stacking for micellar electrokinetic chromatography, and sweeping.

Liquid-phase microextraction and capillary electrophoresis of acidic drugs

Vial liquid-phase microextraction (LPME) combined with capillary electrophoresis (CE) was evaluated for the determination of the acidic drugs ibuprofen, naproxen, and ketoprofen present in water samples and in human urine. The 2.5 mL samples containing the drugs were filled into conventional vials and subsequently acidified by 250 microL of 1-10 M HCl. Porous hollow fibers of polypropylene containing 25 microL of an aqueous solution of 0.01-0.1 M NaOH (acceptor solution) and with dihexyl ether immobilized in the pores of the wall were placed into each of the samples. The acidic drugs were extracted from the acidified sample solutions into the dihexyl ether phase, in the pores of the hollow fiber, and further into the alkaline acceptor solution forced by high partition coefficients. The drugs were extracted almost quantitatively (75-100% extraction efficiency) from the 2.5 mL samples and into the 25 microL acceptor solutions, providing 75-100 times preconcentration. The acceptor solutions were collected for automated CE analysis, which enabled the drugs to be detected down to the 1 ng/mL level.

Characterization of a solid-phase extraction device for discontinuous on-line preconcentration in capillary electrophoresis-based peptide mapping

Peptide mapping by capillary electrophoresis (CE) with UV detection is problematic for the characterization of proteins that can only be obtained at low micromolar concentrations. Dilution of peptide fragments during digestion of the protein can further reduce the detection sensitivity in peptide mapping to the point where analysis at sub-micromolar concentrations is not possible. A remedy to this problem is preconcentration (sample enrichment) of the proteolytic digest by solid-phase extraction (SPE). To minimize non-specific adsorptive losses during sample handling, on-line SPE-CE is preferred. However, packed-inlet SPE-CE is not always feasible due to either instrument or sample limitations. We describe here a simple method of preconcentration by discontinuous on-line SPE-CE, specifically applied to peptide mapping in low-pH separation buffer after protein digestion in a solid-phase enzyme microreactor. The SPE-CE system does not require application of a low pressure during electrophoretic separation to overcome reversed electroosmotic flow because the preconcentrator device is disconnected from the separation capillary before the electric field is applied. Up to a 500-fold preconcentration factor can be achieved with this device, which can be reused for many samples. Parameters such as the volume of desorption solution, the adsorption/desorption (chromatographic) process, reproducibility of packing the SPE preconcentrator and effects of sample concentration on the peptide map are investigated.

Capillary electrophoresis as a versatile tool for the bioanalysis of drugs--a review

This review article presents an overview of current research on the use of capillary electrophoretic techniques for the analysis of drugs in biological matrices. The principles of capillary electrophoresis and its various separation and detection modes are briefly discussed. Sample pretreatment methods which have been used for clean-up and concentration are discussed. Finally, an extensive overview of bioanalytical applications is presented. The bioanalyses of more than 200 drugs have been summarised, including the applied sample pretreatment methods and the achieved detection limits.

Liquid-liquid-liquid microextraction for sample preparation of biological fluids prior to capillary electrophoresis

Methamphetamine as a model compound was extracted from 2.5-mL aqueous samples adjusted to pH 13 (donor solution) through a thin phase of 1-octanol inside the pores of a polypropylene hollow fiber and finally into a 25-microL acidic acceptor solution inside the hollow fiber. Following this liquid-liquid-liquid microextraction (LLLME), the acceptor solutions were analyzed by capillary zone electrophoresis (CE). Extractions were performed in simple disposable devices each consisting of a conventional 4-mL sample vial, two needles for introduction and collection of the acceptor solution, and a 8-cm piece of a porous polypropylene hollow fiber. From 5 to 20 different samples were extracted in parallel for 45 min, providing a high sample capacity. Methamphetamine was preconcentrated by a factor of 75 from aqueous standard solutions, human urine, and human plasma utilizing 10(-1) M HCl as the acceptor phase and 10(-1) M NaOH in the donor solution. In addition to preconcentration, LLLME also served as a technique for sample cleanup since large molecules, acidic compounds, and neutral components were not extracted into the acceptor phase. Utilizing diphenhydramine hydrochloride as internal standard, repetitive extractions varied less than 5.2% RSD (n = 6), while the calibration curve for methamphetamine was linear within the range 20 ng/microL to 10 micrograms/mL (r = 0.9983). The detection limit of methamphetamine utilizing LLLME/CE was 5 ng/mL (S/N = 3) in both human urine and plasma.

Identification of proteins in complexes by solid-phase microextraction/multistep elution/capillary electrophoresis/tandem mass spectrometry

A method to directly identify proteins in complex mixtures by solid-phase microextraction (micro-SPE)/multistep elution/capillary electrophoresis (CE)/tandem mass spectrometry (MS/MS) is described. A sheathless liquid-metal junction interface is used to interface CE and electrospray ionization MS/MS. A subfemtomole detection limit is achieved for protein identification through database searching using MS/MS data. The SPE serves as a semiseparation dimension using an organic-phase step-elution gradient in combination with the second separation dimension for increased resolving power of complex peptide mixtures. This approach improves the concentration detection limit for CE and allows more proteins in complex mixtures to be identified. A 75-protein complex from yeast ribosome is analyzed using this method and 80-90% of the proteins in the complex can be identified by searching the database using the MS/MS data from a complete analysis. This multidimensional CE/MS/MS methodology provides an alternative to multidimensional liquid chromatography/MS/MS for direct identification of small amounts of protein in mixtures.

Combining solid-phase preconcentration, capillary electrophoresis and off-line matrix-assisted laser desorption/ionization mass spectrometry: intracerebral metabolic processing of peptide E in vivo

The in vivo metabolism of peptide E was studied in the anesthetized rat using a combination of microdialysis sampling, solid-phase preconcentration capillary electrophoresis and imaging matrix-assisted laser desorption/ionization mass spectrometry (MALDI/MS). The metabolic profile of peptides identified by MALDI/MS showed that the primary enzymatic activity for degradation of peptide E was due to carboxypeptidases and, to a lesser extent, aminopeptidases and some trypsin-like endopeptidases. Over 75 metabolic fragments were detected from the action of these enzymes in vivo.

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.

Comparison of protein mixtures in aqueous humor by membrane preconcentration - capillary electrophoresis - mass spectrometry

The significance of proteomic research is coupled with the recent exponential growth of these investigations. Currently, the most popular techniques used for these studies include the coupling of 1- and 2-dimensional electrophoresis with mass spectrometric analysis of the extracted and digested proteins. However, detection limits of gel staining methods have led to a search for complimentary techniques that afford the detection of lower concentrations of biologically relevant proteins. In the present studies, we have evaluated the applicability of on-line capillary electrophoresis - mass spectrometry (CE-MS) for this application. Specifically, we used membrane preconcentration-CE-MS (mPC-CE-MS) to analyze 13 samples of human aqueous humor (AH) from patients with various ocular pathologies (cataract, cataract plus glaucoma, and cataract plus pseudoexfoliation syndrome). This approach enabled rapid analysis of a relatively large volume (1 microL of each specimen, and a protein map for each was created. Measured average molecular weights (Mr) were used to tentatively identify proteins after search of the SWISS-PROT database using TagIdent from ExPaSy. Among those proteins tentatively identified are beta-2 microglobulin (Mr 11731.2), apolipoprotein A1 (Mr 28078.6) and serum albumin (Mr 66400). Proteins with Mr of 4349 (unidentified), 11731.2 (beta-2 microglobulin), 13400-14100 (immunoglobulin fragments), 28078.2 (apolipoprotein A1) and approximately 68000 (serum albumin) were observed in the majority of specimens. Generally no significant differences were noted in the protein composition of aqueous humor samples from different pathologies. However, the absence of an Mr 13345 protein and its oxidized form (Mr 13361) in samples from patients with pseudoexfoliation syndrome was noted. Occasionally the alpha-and beta-chains of hemoglobin, a contaminant in aqueous humor introduced during sampling, were also detected. We conclude from these studies that mPC-CE-MS is an attractive complimentary technique for proteome research, as this approach enables direct mapping and characterization of low concentrations of proteins that are present in complex physiologically derived fluids.

Enhanced sensitivity for sequence determination of major histocompatibility complex class I peptides by membrane preconcentration-capillary electrophoresis-microspray-tandem mass spectrometry

Sequence analysis of antigenic major histocompatibility complex (MHC) class I peptides requires minimizing sample loss and enhancing mass spectrometric sensitivity. In order to facilitate such analyses, we have coupled on-line membrane preconcentration-capillary electrophoresis (mPC-CE) with microspray mass spectrometry (mPC-CE-microMS) and tandem mass spectrometry (mPC-CE-microMS/MS). Specifically, cell lysate from approximately 10(9) EG-7 mouse tumor cells was immunoprecipitated and the released MHC class I peptides were subjected to reverse-phase HPLC. An HPLC fraction containing antigenic peptide(s) shown to induce T-cell stimulation was subjected to mPC-CE-microMS. Approximately 10 microL (from 100 microL) of the fraction was pressure-injected and concentrated on a styrenedivinylbenzene (SDB) impregnated membrane. The peptides were eluted from the membrane with approximately 100 nL of 80% methanol, sandwiched between a leading stacking buffer (LSB, also serving as CE separation medium) of approximately 110 nL of 0.1% acetic acid in 10% methanol, and a trailing stacking buffer (TSB) of approximately 110 nL of 0.1% NH4OH. On application of the CE voltage the peptides are subjected to moving boundary transient isotachophoresis and focused. The peptides were separated in a Polybrene-coated capillary with application of -20 kV in reverse polarity mode and subsequently sprayed via an emitter coupled to the CE capillary by a liquid junction containing a platinum wire. An ion at m/z 482.3 was detected and subjected to mPC-CE-microMS/MS and determined to be SIINFEKL, a peptide (OVA) known to be antigenic in the mouse model system. Sensitivity enhancement over conventional mPC-CE-MS and MS/MS was approximately 100-fold.

Protein analysis by membrane preconcentration-capillary electrophoresis: systematic evaluation of parameters affecting preconcentration and separation

Fast and efficient analysis of proteins in physiological fluids is of great interest to researchers and clinicians alike. Capillary electrophoresis (CE) has proven to be a potentially valuable tool for the separation of proteins in specimens. However, a generally acknowledged drawback of this technique is the limited sample volumes which can be loaded onto the CE capillary which results in a poor concentration limit of detection. In addition, matrix components in samples may also interfere with separation and detection of analytes. Membrane preconcentration-CE (mPC-CE) has proved to be effective in overcoming these problems. In this report, we describe the systematic evaluation of parameters affecting on-line preconcentration/clean-up and separation of protein mixtures by mPC-CE. Method development was carried out with a standard mixture of proteins (lysozyme, myoglobin, carbonic anhydrase, and human serum albumin). First, using MALDI-TOF-MS, membrane materials with cation-exchange (R-SO3H) or hydrophobic (C2, C8, C18, SDB) characteristics were evaluated for their potential to retain proteins in mPC cartridges. Hydrophobic membranes were found most suitable for this application. Next, all mPC-CE analysis of protein samples were performed in polybrene coated capillaries and parameters affecting sample loading, washing and elution, such as the composition and volume of the elution solvent were investigated. Furthermore, to achieve optimal mPC-CE performance for the separation of protein mixtures parameters affecting postelution focusing and electrophoresis, including the composition of the background electrolyte and a trailing stacking buffer were varied. Optimal conditions for mPC-CE analysis of proteins using a C2 impregnated membrane preconcentration (mPC) cartridge were achieved with a background electrolyte of 5% acetic acid and 2 mM ammonium acetate, 60 nl of 80% acetonitrile in H2O as an elution solvent, and 60 nl of 0.5% ammonium hydroxide as a trailing stacking buffer. The developed method was used successfully to separate proteins in aqueous humor, which contains numerous proteins in a complex matrix of salts.

Recent advances in capillary electrophoresis/electrospray/mass spectrometry

Successful on-line interfacing of capillary electrophoresis (CE) with electrospray (ES) mass spectrometry (MS) has progressed substantially in recent years. Of particular note also is the development which has occurred in combining the more advanced capillary-based electromigration separation techniques, such as capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF), capillary isotachophoresis (CIT), micellar electrokinetic chromatography (MEKC) and capillary electrochromatography (CEC), with ES/MS. The union of these electromigration schemes with MS detection provides a useful and sensitive analytical tool for the separation, quantitation and identification of biological, therapeutic, environmental and other important classes of chemical analytes. By making optimal use of the characteristics inherent with these separation mechanisms, greatly enhanced MS performance may be obtained. The following review summarizes the significant issues and challenges involved with CE/ES/MS analysis as well as results which have recently been obtained.

Capillary zone electrophoresis of proteins

This review article with 237 references is focused on capillary zone electrophoresis (CZE) of proteins. It includes discussion of modeling electrophoretic migration of proteins, sample pretreatment before the analysis, methods reducing the sorptions of proteins on the capillary wall, and techniques for increasing selectivity by using electrolyte additives including the sieving matrices. Significant progress in detection techniques, namely in laser-induced fluorescence and mass spectrometry, is emphasized. Modifications of CZE using specific interactions, such as affinity capillary electrophoresis or capillary immunoelectrophoresis, are debated as well as combination of CZE with other separation methods such as high performance liquid chromatography (HPLC). A number of practical applications of CZE of proteins are described.

New approaches in clinical chemistry: on-line analyte concentration and microreaction capillary electrophoresis for the determination of drugs, metabolic intermediates, and biopolymers in biological fluids

The use of capillary electrophoresis (CE) for clinically relevant assays is attractive since it often presents many advantages over contemporary methods. The small-diameter tubing that holds the separation medium has led to the development of multicapillary instruments, and simultaneous sample analysis. Furthermore, CE is compatible with a wide range of detectors, including UV-Vis, fluorescence, laser-induced fluorescence, electrochemistry, mass spectrometry, radiometric, and more recently nuclear magnetic resonance, and laser-induced circular dichroism systems. Selection of an appropriate detector can yield highly specific analyte detection with good mass sensitivity. Another attractive feature of CE is the low consumption of sample and reagents. However, it is paradoxical that this advantage also leads to severe limitation, namely poor concentration sensitivity. Often high analyte concentrations are required in order to have injection of sufficient material for detection. In this regard, a series of devices that are broadly termed 'analyte concentrators' have been developed for analyte preconcentration on-line with the CE capillary. These devices have been used primarily for non-specific analyte preconcentration using packing material of the C18 type. Alternatively, the use of very specific antibody-containing cartridges and enzyme-immobilized microreactors have been demonstrated. In the current report, we review the likely impact of the technology of capillary electrophoresis and the role of the CE analyte concentrator-microreactor on the analysis of biomolecules, present on complex matrices, in a clinical laboratory. Specific examples of the direct analysis of physiologically-derived fluids and microdialysates are presented, and a personal view of the future of CE in the clinical environment is given.

Rapid loading of large sample volumes, analyte cleanup, and modified moving boundary transient isotachophoresis conditions for membrane preconcentration-capillary electrophoresis in small diameter capillaries

Using a removable membrane preconcentration (mPC) cartridge, large sample volumes can be loaded prior to final assembly of the mPC capillary electrophoresis (CE) capillary. For narrow-bore (< or = 25 microns ID) uncoated mPC-CE capillaries, applied to peptide analysis, efficient moving boundary transient isotachphoresis (tITP) conditions at the onset of electrophoresis are described. The enhancement of mPC-CE-mass spectrometry (MS) technology afforded by rapid sample loading and modified moving boundary tITP conditions are demonstrated by analysis of major histocompatibility complex (MHC) class I peptides that were derived from a Kb precipitation of mouse EL-4 cells. Furthermore, we demonstrate the structural characterization of these immunologically significant molecules by mPC-CE-tandem mass spectrometry (mPC-CE-MS/MS).

Membrane preconcentration-capillary electrophoresis-mass spectrometry in the analysis of biologically derived metabolites and biopolymers

On-line capillary electrophoresis-mass spectrometry (CE-MS) is finding increased use in the analysis of a wide variety of chemically diverse complex mixtures. It is characterized by minimal sample loss and enhanced separation efficiencies as compared to conventional techniques such as high-performance liquid chromatography-mass spectrometry (HPLC-MS). However, the major limitation of both CE and CE-MS is the limited sample loading capacity of conventional CE capillaries. Typical loading volumes are approximately 1-100 nL, which afford optimal CE performance, and this is in stark contrast to the 1-100 microL commonly injected onto capillary HPLC columns. The limited loading of CE leads to relatively poor concentration limits of detection. In this work a unique method for analyte preconcentration with CE is described. A cartridge containing an impregnated membrane is installed at the inlet of the CE capillary, and we term this approach membrane preconcentration-CE (mPC-CE), and in conjunction with mass spectrometry, mPC-CE-MS. It allows both on-line sample concentration and cleanup and concentration limits of detection of fg/mL are possible using this approach. The analysis of in vivo derived metabolites, peptides, and proteins is described. This demonstrates the wide applicability of the technology in the analysis of any class of compounds, ranging in molecular weight from 100 to 70,000 Da with loading capacities of approximately 1- > 100 microL sample volumes.

Strategy for isolating and sequencing biologically derived MHC class I peptides

The presentation of MHC class I peptides at cell surfaces and the subsequent cytolytic T-lymphocyte response are critical components of the mammalian immune response. However, the identification and sequencing of such peptides present a considerable analytical challenge since > 10,000 peptides at 10(-15)-10(-18) M concentrations are often present in the mixture. We describe a two-dimensional chromatography approach in conjunction with tandem mass spectrometry to sequence and identify such peptides. After immunoaffinity concentration, and subsequent acetic acid release of MHC class I peptides from MHC protein complex, the peptides are subjected to reversed phase HPLC, where they are separated based on their hydrophilic-hydrophobic character. These coarse fractions are then loaded onto a specially designed membrane preconcentration-capillary electrophoresis cartridge (mPC-CE) and subsequently subjected to on-line mPC-CE-MS analysis. The second dimension of chromatography by CE separation affords resolution of peptides based on their charge/mass (to a first approximation) ratio. Ultimately peptides are sequenced using mPC-CE-tandem mass spectrometry (mPC-CE-MS-MS). We describe the strategy for sequencing < 60 femtomoles of a peptide obtained from 3.10(9) Kb-derived EL-4 cells.

Application of capillary electrophoresis and related techniques to drug metabolism studies

The use of capillary electrophoresis (CE) for the separation of small organic molecules such as pharmaceutical agents and drug/xenobiotic metabolites has become increasingly popular. This has arisen, at least in part, from the complimentary mode of separation afforded by CE when compared to the more mature technique of HPLC. Other qualities of CE include relative ease of method of development, rapid analysis, and low solvent consumption. The recent introduction of a variety of detector systems (including UV diode array, laser-induced fluorescence, conductivity) and the demonstrated coupling of CE to MS have also aided acceptance of this technology. In the present report, we review the role of CE coupled to various detector systems including a mass spectrometer for the characterization of both in vitro and in vivo derived drug metabolite mixtures. Attributes of CE for this application are demonstrated by discussion of metabolism studies of the neuroleptic agent haloperidol. Various aspects of the development and use of CE and CE-MS for the characterization of haloperidol metabolites, including criteria for selection of parameters such as pH, ionic strength, extent of organic modification, and the use of nonaqueous capillary zone electrophoresis are discussed. We also consider potential limitations of CE and CE-MS for drug metabolism research and describe the introduction of membrane preconcentration-CE (mPC-CE) and mPC-CE-MS as a solution that overcomes the rather poor concentration limits of detection of CE methods without compromising the resolution of analytes or separation efficiency of this technique.