Design and performance of a low-flow capillary electrophoresis-electrospray-mass spectrometry interface using an emitter with dual beveled edge

A low-flow electrophoresis-mass spectrometry (CE-MS) interface has been developed for interfacing capillary zone electrophoresis (CZE) with electrospray- ionization-mass spectrometry (ESI-MS). The interface consists of two parallel capillary columns (a separation column and a makeup column), and an emitter with a dual beveled edge. While maintaining a relatively low optimum flow rate, the dual-beveled-edge ESI emitter allows the use of a tip with larger orifice. Therefore, this interface is less prone to column blocking in comparison with a flat tip. Primarily attributed to low sample dilution and smaller initial droplet, the interface showed better sensitivity than a conventional sheath liquid interface. Furthermore, the interface was found to be more resistant to the presence of nonvolatile salts. By using 40 mM borate and 20 mM alpha-cyclodextrin (alpha-CD) as the running buffer, four major forms of gangliosides were detected by CE-MS.     

Two capillary approach for a multifunctional nanoflow sheath liquid interface for capillary electrophoresis-mass spectrometry

CE hyphenated to ESI-MS (CE-ESI-MS) is a well-established technique to analyze charged analytes in complex samples. Although various interfaces for CE-MS coupling are commercially available, the development of alternatives which combine sensitivity, simplicity, and robustness remains a topic of research. In this work, a nanoflow sheath liquid CE-MS interface with two movable capillaries inside a glass emitter is described. The setup enables a separation mode and a conditioning mode to guide the separation capillary effluent either into the electrospray or to the waste, respectively. This enables to exclude parts of the analysis from MS detection and unwanted matrix components reaching the mass spectrometer, comparable to divert valves in LC-MS coupling. Also, this function improves the overall robustness of the system by reduction of particles blocking the emitter. Preconditioning with electrospray interfering substances and even the application of coating materials for every analysis is enabled, even while the separation capillary is built into the interface with running electrospray. The functionality is demonstrated by analyses of heavy matrix bioreactor samples. Overall, this innovation offers a more convenient installation of the interface, improved handling with an extended lifetime of the emitter tips and additional functions compared to previous approaches, while keeping the higher sensitivity of nanoflow CE-MS-coupling.     

Microelectrospray interface with coaxial sheath flow for high-resolution capillary electrophoresis/mass spectrometry separation

We have fabricated a coaxial sheath liquid flow microelectrospray ionization (microESI) interface for capillary electrophoresis coupled with mass spectrometry (CE/MS). The ESI interface, which features a reduced probe diameter (130 microm i.d. x 174 microm o.d.) with a nebulizer-free format, can relatively easily electrospray a large amount of make-up sheath liquid (5-10 microL/min) over the long term (more than 80 runs) with a high degree of stability. The interface also provides higher separation qualities and improved detection sensitivities compared with a conventional ion spray (IS) interface.     

Sheathless reverse-polarity capillary electrophoresis-electrospray-mass spectrometry for analysis of underivatized glycoconjugates

We report on the development of a novel methodology to extend the limits of capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) general applicability. A sheathless on-line CE-ESI-MS setup was optimized on standard monosaccharide mixture to operate in reverse polarity and negative ion mode for MS detection without pressure to assist the sample migration, coating of the capillaries, and/or sample derivatization. This approach was further applied for screening of a complex glycopeptide mixture obtained from the urine of a patient diagnosed with N-acetylhexosaminidase deficiency, known as Schindler's disease. The potential of this methodology in terms of high sensitivity, separation efficiency, resolution, and reproducibility is demonstrated. In combination with the high quality of MS data, a new, significantly improved insight into the sample heterogeneity is possible.     

A beveled tip sheath liquid interface for capillary electrophoresis-electrospray ionization-mass spectrometry

A simple and durable sheath liquid interface for capillary zone electrophoresis-electrospray ionization-mass spectrometry (CZE-ESI-MS) has been developed. This interface utilized a beveled tip emitter and was found to be more sensitive than the conventional sheath liquid interface. The use of a beveled tip reduces the optimal flow rate and therefore decreases sample dilution. The interface utilized a 380 microm inner diameter and 400 microm outer diameter beveled tapered tip. Because of the large inner diameter and outer diameter of the tip, the interface is robust and can be easily implemented. The performance of this interface for CZE-ESI-MS and micelle electrokinetic capillary electrophoresis-electrospray-mass spectrometry, as demonstrated by the analysis of synthetic drugs and triazine mixtures, was significantly better than results obtained using a conventional sheath liquid interface.     

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.     

Quantitative analysis of six pesticides in fruits by capillary electrophoresis-electrospray-mass spectrometry

A method to identify and quantify six pesticide residues - dinoseb, pirimicarb, procymidone, pyrifenox, pyrimethanil, and thiabendazole - in peaches and nectarines using capillary electrophoresis-electrospray ionization-quadrupole ion trap-tandem mass spectrometry (CE-ESI-MS/MS) is described. Separation was carried out using a buffer of 0.3 M ammonium acetate at pH 4 with 10% methanol. Pesticide residues present in peach and nectarine samples were preconcentrated by solid-phase extraction using C(18), eluted with CH(2)Cl(2), concentrated to dryness, and redissolved in buffer to obtain lower detection limits. The recoveries of the analytes ranged from 58 to 99% and the relative standard deviations were 9 to 19%. Under optimized CE-MS/MS conditions the minimum detectable levels for the six pesticides in spiked peach samples were between 0.01 mg/kg for pirimicarb and 0.05 mg/kg for procymidone with pressure injection of 50 mbar for 5 s (5 nL) at a signal-to-noise ratio of 3, which constitutes a severalfold increase in sensitivity compared to CE-MS, using a single quadrupole, and to conventional CE-UV. The potential of the method was demonstrated by analyzing different samples taken from regional agricultural cooperatives. The pesticides most often detected were thiabendazole and procymidone.     

Use of a fritless dual tapered column and a low flow interface for capillary electrochromatography-mass spectrometry

To avoid problems associated with the use of sintered frits to retain packing material, tapered columns were investigated for use with capillary electrochromatography-mass spectrometry (CEC-MS) analysis. Taking the advantage that negatively charged stationary phase particles have a net velocity directed towards the buffer reservoir (inlet) over a wide range in pH, a fritless CEC column with a single taper tip was prepared for CEC-MS analysis. During CEC-MS analysis, the tapered end was immersed in the buffer reservoir and the unmodified end was pointed toward the ionization source. For better sensitivity, this single tapered CEC column was coupled to ESI/MS using a low flow sheath liquid interface. With this setup, occasional blockage of the ESI sprayer by stationary phase particles was observed. In addition, significant dead volume was observed because the unmodified tip could not be inserted into the very end of the sprayer of the low flow sheath liquid interface. To circumvent these problems, a dual tapered CEC column was prepared. This fritless dual tapered column CEC-MS approach alleviated the problems of frit, sprayer blockage and extensive dead volume.     

Analysis of triazines by capillary electrochromatography/electrospray ionization-mass spectrometry using a low-flow sheath liquid interface

CEC-MS has been used for the analysis of eight-triazine herbicides. It showed significantly better S/N ratio than reversed EOF CE-MS and MEKC-MS, due to the lack of a surfactant in the separation buffer. By optimizing the pH, the organic content of the running buffer, and the separation potential, optimal separation was achieved within 18 min using a running buffer of pH 7.0, containing 70% v/v ACN, and an applied voltage of 17 kV. Gradient CEC showed superior separation when compared with isocratic elution. The combination of a tapered CEC column and a low-flow interface confers several advantages including better sensitivity, low dead volume, and independent control of the conditions used for CEC separation and ESI analysis.     

A sheath flow gating interface for the on-line coupling of solid-phase extraction with capillary electrophoresis

A sheath flow gating interface (SFGI) is presented for the on-line coupling of solid-phase extraction (SPE) with capillary electrophoresis (CE). The design, construction and operation of the SFGI are described in detail. After operating conditions were investigated and selected, the SFGI was evaluated on a SPE–CE–UV setup using hydroxylated poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith as the absorbent and using three phenols as the test analytes. The preconcentration factors obtained with the SPE–CE–UV system and the SPE–UV part are 530 and 550, respectively. The plate numbers obtained using the SPE–CE–UV system are slightly better than or comparable to those with the CE–UV part. The precisions (RSDs) of 100 consecutive injections are 2.43%, 3.86%, and 4.25% for peak height, peak area and migration time, respectively. The measured recoveries for the river water samples spiked at three different levels are in the range of 93.6–102.8% with the interday RSD values ranging from 2.0 to 4.5% (n = 3). These data collectively demonstrate that the SFGI has the ability to exactly and reproducibly transfer nanoliters of fractions from SPE onto CE with no degradation of the efficiencies of SPE and CE, suggesting a great potential to be routinely used for the coupling of SPE, microcolumn LC or FIA with CE.     

Microfabricated liquid junction hybrid capillary electrophoresis‐mass spectrometry interface for fully automated operation

One of the challenging instrumental aspects in coupling an automated CE instrument with ESI mass spectrometry (CE‐MS) is finding the balance between the stability, reproducibility and sensitivity of the analysis and compatibility with the standard CE instrumentation. Here, we present a development of a new liquid junction based electrospray interface for automated CE‐MS, with a focus on the technical design followed by computer modeling of transport conditions as well as characterization of basic performance of the interface. This hybrid arrangement designed as a microfabricated unit attachable to the automated CE instrument allows using of a wide range of separation capillaries with respect to their diameter, length or internal coating (e.g., for suppressed electroosmotic flow). Different compositions of the ESI liquid and background electrolyte solutions can be used if needed. The microfabricated part, prepared by laser machining from polyimide, includes a self‐aligning liquid junction, a short transport channel, and a pointed sprayer for the electrospray ionization. This microfabricated part is positioned in a plastic connection block securing the separation capillary and flushing ports. Transport conditions were modelled using computer simulation and the real life performance of the interface was compared to that of a commercial sheath liquid interface. The basic performance of the interface was demonstrated by separations of peptides, proteins, and oligosaccharides.     

Development of a simple vent-free interface for capillary gas chromatography-mass spectrometry

A novel and simple interface for capillary gas chromatography-mass spectrometry (GC-MS) was developed using a piece of deactivated stainless-steel tubing. This interface eliminated the need to vent the MS ion source when changing columns. Various chromatographic performance indicators, such as inertness, and thermal and chemical stability, were confirmed to be unaffected by using this interface at an elevated temperature of around 300°C. The new interface should facilitate the characterization of polymeric materials using analytical pyrolysis techniques in which frequent switching is required in the measuring mode, such as evolved gas analysis-MS and flash pyrolysis-GC-MS.     

A critical evaluation of high performance liquid chromatography-electrospray ionisation-mass spectrometry and capillary electrophoresis-electrospray-mass spectrometry for the detection and determination of small molecules of significance in clinical and forensic science

A critical review of applications for the period 2000-2003, taken from the Web of Knowledge database, of the techniques high performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) and capillary electrophoresis-electrospray ionisation-mass spectrometry (CE-ESI-MS) to the detection and determination of small molecules of significance in clinical and forensic science is presented. The molecules of mass less than 500 Da are chosen according to selected structural classes in which they give ESI signals primarily as [M+H](+) ions although other ions such as [M-H](-), [M+Na](+) and [M+NH(4)](+) are also reported. The structural classes are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, 1,4-benzodiazepines, carbohydrates, benzimidazoles, other heterocycles, sulphonylureas, anthracyclines, sulphonamides, penicillins, cephalosporins, tetracyclines, nitrocatechols, steroids, flavonoids, oxazaphosphorines, cannabinols, and miscellaneous molecules. Details are given on the fragmentations, where available, that these ionic species exhibit in-source and in ion-trap, triple quadrupole and time-of-flight mass spectrometers. The review then gives a critical evaluation of these recent HPLC-ESI-MS and CE-ESI-MS analytical methods for the detection and determination of small molecules of clinical and forensic significance. Analytical information on, for example, sample concentration techniques, HPLC and CE separation conditions, recoveries from biological media and limits of detection (LODs) are provided.     

Glass nebulizer interface for capillary electrophoresis-Fourier transform infrared spectrometry

Despite the continuing development of SPME (solid-phase microextraction) fibre coatings, their selection presents some difficulties for analysts in choosing the appropriate fibre for a certain application. There are two distinct types of SPME coatings available commercially. The most widely used are poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA). Supelco has developed new mixed phases consisting of porous polymer particles, either poly(divinylbenzene) (DVB) or Carboxen suspended in a matrix of PDMS or Carbowax for extracting analytes via adsorption. In addition to the nature of the extracting phase, the thickness of the polymeric film must be taken into account and, surprisingly, the construction of the fibres when apparently they bear the same coating, as it is the case of the three PDMS-DVB fibres available. Other fibre structure properties not well explored were identified and must be taken into consideration. To elucidate their extraction efficiency, three PDMS-DVB fibres, namely 60 microm for HPLC use, 65 microm for GC use and 65 microm StableFlex for GC use, were compared with regard to the extraction of 36 compounds included in four pesticide groups. The first was particularly suited for the extraction of organophosphorus pesticides and triazines whereas the StableFlex exhibited advantages in the analysis of organochlorine pesticides and pyrethroids. An explanation for the extraction differences is suggested based on the different structure of the fibres. Detection limits in the range of 1-10 ng/l for organochlorine pesticides, 1-30 ng/l for organophosphorus pesticides, 8-50 ng/l for triazines and 10-20 ng/l for pyrethroids were attained in a method using the 60 microm PDMS-DVB fibre. The fibre maintains its performance at well above 100 extractions with between-day precision below 10%.     

Modified liquid junction interface for nonaqueous capillary electrophoresis-mass spectrometry

Electrospray ionization (ESI) is the most widely used ionization method in on-line coupling of capillary electrophoresis-mass spectrometry (CE-MS). The conventional coaxial sheath flow electrospray interface is currently being replaced by the more sensitive nanoelectrospray technique. The usual limitation of nanoelectrospray CE-MS interface has been its short lifetime caused by deterioration of the metal coating on the CE capillary terminus. This article describes an easy way to construct a more durable and sensitive nanospray interface for nonaqueous CE-MS. In this approach a very thin glass spray capillary (ca. 30 microm outer diameter) is partly inserted inside the CE capillary, the junction being surrounded by the electrolyte medium, which is in contact with the platinum electrode. The interface was tested with five pharmaceuticals: methadone, pentazocine, levorphanol, dihydrocodeine, and morphine. Detection limits ranged from 12 to 540 fmol. Separation efficiency and reproducibility were also studied. The CE current was found to be stable and the migration times were highly reproducible. All the CE separations were carried out in a nonaqueous background electrolyte solution.     

Twenty years of interface development for capillary electrophoresis-electrospray ionization-mass spectrometry

Capillary electrophoresis-electrospray ionization-mass spectrometry has the potential to become a preferred tool for the analysis of biological mixtures and other complex samples. The development of improved interfaces in the past twenty years has been critical in demonstrating the feasibility of this technique. However, a compromise still exists between interfaces that give optimal performance and those that are practical for commercial applications. The first section of this review focuses on the technological advances in CE-ESI-MS as they relate to the key interface features for both sheath-flow and sheathless systems: delivery of the sheath liquid, shaping of the emitter tip, formation of electrical contact, and practicality in terms of ease of use and lifetime. In the second section, we review the fundamental processes that affect interface performance. Because of the complex natures of both capillary electrophoresis and electrospray ionization, flow rate, arrangement of the electrical circuit, electrochemistry, tip geometry and location of electrical contact must all be carefully managed in the design of a successful interface.     

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.     

Simple interface for capillary electrophoresis-inductively coupled plasma atomic emission spectrometry

A simple interface has been developed to couple capillary electrophoresis (CE) to inductively coupled plasma atomic emission spectrometry (ICP-AES) for metal speciation. A concentric glass nebulizer with elongated tip is used as the CE-ICP interface. The CE capillary is the central tube of the nebulizer. A platinum wire is wrapped across the exit end of the CE capillary to provide electrical connection to the CE power supply. No sheath flow of buffer solution is needed. A simple cooling system has also been developed. A peristaltic pump circulates water through a plastic tube that encloses the section of the CE capillary between the CE instrument and the ICP spectrometer. Characteristics of the CE-ICP interface, e.g., elution time, nebulization and transport efficiency and peak broadening, versus carrier gas flow-rate have been studied. Comparisons to a previous design with the Pt electrode inserted into the end of the CE capillary are made where appropriate. The reproducibility (RSD) in ICP emission intensity of the system is <4%. Detection limits of Cr and Cu are approximately 5 ng/ml.     

Influence of capillary dimensions on the performance of a coaxial capillary electrophoresis-electrospray mass spectrometry interface

The dimensions of the capillaries used to construct a typical coaxial capillary electrophoresis-mass spectrometry (CE-MS) interface, i.e. the inner diameter, the outer diameter and the wall thickness, have been shown to affect the performance of the CE-MS system. The influence of these parameters has been investigated in both MS and MS-MS modes. The initial results indicate that by reducing all the sheath capillaries' dimensions and the CE capillary outer diameter, better operation and increased sensitivity can be achieved. The capillary arrangement which gives optimum sensitivity and stable operation has been suggested.     

An automated, sheathless capillary electrophoresis-mass spectrometry platform for discovery of biomarkers in human serum

A capillary electrophoresis-mass spectrometry (CE-MS) method has been developed to perform routine, automated analysis of low-molecular-weight peptides in human serum. The method incorporates transient isotachophoresis for in-line preconcentration and a sheathless electrospray interface. To evaluate the performance of the method and demonstrate the utility of the approach, an experiment was designed in which peptides were added to sera from individuals at each of two different concentrations, artificially creating two groups of samples. The CE-MS data from the serum samples were divided into separate training and test sets. A pattern-recognition/feature-selection algorithm based on support vector machines was used to select the mass-to-charge (m/z) values from the training set data that distinguished the two groups of samples from each other. The added peptides were identified correctly as the distinguishing features, and pattern recognition based on these peptides was used to assign each sample in the independent test set to its respective group. A twofold difference in peptide concentration could be detected with statistical significance (p-value < 0.0001). The accuracy of the assignment was 95%, demonstrating the utility of this technique for the discovery of patterns of biomarkers in serum.