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

A study of electrospray ionization emitters with differing geometries with respect to flow rate and electrospray voltage

The performance of several electrospray ionization emitters with different orifice inside diameters (i.d.s), geometries, and materials are compared. The sample solution is delivered by pressure driven flow, and the electrospray ionization voltage and flow rate are varied systematically for each emitter investigated, while the signal intensity of a standard is measured. The emitters investigated include a series of emitters with a tapered outside diameters (o.d.) and unaltered i.d.s, a series of emitters with tapered o.d.s and i.d.s, an emitter with a monolithic frit and a tapered o.d., and an emitter fabricated from polypropylene. The results show that for the externally etched emitters, signal was nearly independent of i.d. and better ion utilization was achieved at lower flow rates. Furthermore, emitters with a 50 μm i.d. and an etched o.d. produced about 1.5 times more signal than etched emitters with smaller i.d.s and about 3.5 times more signal than emitters with tapered inner and outer dimensions. Additionally, the work presented here has important implications for applications in which maximizing signal intensity and reducing frictional resistance to flow are necessary. Overall, the work provides an initial assessment of the critical parameters that contribute to maximizing the signal for electrospray ionization sources interfaced with pressure driven flows.

Silanization of inner surfaces of nanoelectrospray ionization emitters for reduced analyte adsorption

During the course of nanoelectrospray ionization (nanoESI) of substance P, an unusual type of signal reduction was observed with flow rates <10 nL/min. This reduction in signal appears to be induced by the adsorption of positively charged analytes onto negatively charged free silanol groups on the inner surface of emitters; analytes with higher pI values (such as substance P) exhibit greater tendency for adsorption. Support for this hypothesis is demonstrated by the decrease in signal reduction in the presence of concentrated salts or for emitters whose internal silanols have been covalently silanized. Emitters treated with hexamethyldisilazane or 3-aminopropyltriethoxysilane showed higher analyte signals for substance P than untreated emitters, suggesting a reduction of analyte adsorption onto the inner walls of silanized emitters. The efficacy of reduced peptide adsorption was demonstrated for emitters silanized with 3-aminopropyltriethoxysilane using a simple peptide mixture as well as a more complex peptide mixture (a tryptic digest of horse hemoglobin).

Coupling CE with atmospheric pressure photoionization MS for pharmaceutical basic compounds: Optimization of operating parameters

The use of CE coupled with MS (CE-MS) has evolved as a useful tool to analyze charged species in small sample volumes. Because of its sensitivity, versatility and ease of implementation, the ESI interface is currently the method of choice to hyphenate CE to MS. An alternative can be the atmospheric pressure photoionization (APPI) source, however, numerous parameters must be optimized for its coupling to CE. After evaluation of the sheath liquid composition and the CE capillary outlet position, an experimental design methodology was assessed for optimizing other ionization source parameters, such as sheath liquid flow rate, drying gas flow rate and temperature, nebulizing gas pressure, vaporizer temperature, and capillary voltage. For this purpose, a fractional factorial design (FFD) was selected as a screening procedure to identify factors which significantly influence sensitivity and efficiency. A face-centered central composite design (CCD) was then used to predict and optimize sensitivity, taking into account the most relevant variables. Sensitivity was finally evaluated with the optimized conditions and height-to-noise ratios (H/N) around 10 were achieved for an injection of 200 ng/mL of each analyte.

Resin-packed nanoelectrospray in combination with video and mass spectrometry for the direct and real-time molecular analysis of mast cells

A nano-electrospray ionization (nanoESI) emitter for analysis of a biological solution was developed by packing a nanoESI needle with two types of resins for desalting and preconcentration of target molecules. Determination of secreted histamine and serotonin molecules in cell culture buffers was demonstrated using 5-methyltryptamine as internal standard. The results showed good linearity of target signals in the concentration range from 0.25 to 50.0 ng/mL of histamine or serotonin. These molecules were monitored to be secreted by A23187 (calcium ionophore) stimulant in rat peritoneal mast cells. Using a combination of a video-microscope and a mass spectrometer, we could visualize exocytotic moments and analyze secreted molecules by mass spectrometry simultaneously. Time-dependent release of histamine and serotonin from activated mast cells showed that significant production of these molecules occurred and reached a maximal level at 15 min for serotonin and at 30 min for histamine, respectively. These results showed that this method allows the direct and timely analysis of secreted molecules in biological responses.

Analytical performance characteristics of nanoelectrospray emitters as a function of conductive coating

As miniaturization of electrospray continues to become more prevalent in the mass spectrometry arsenal, numerous types of conductive coatings have been developed with miniaturized electrospray emitters. Different conductive coatings have different properties that may lead to differences in analytical performance. This paper investigates and compares the analytical properties of a series of applied conductive coatings for low-flow electrospray ionization developed in this laboratory vs. commercially-available types. Evaporated graphite is thoroughly compared with commercially available polyaniline (PANI) coated emitters and metal coated emitters. Each set of emitters was investigated to determine various performance characteristics, including susceptibility to electrical discharge in both positive and negative ionization modes, as well as emitter reproducibility and generation of a standard curve to determine each emitter coating's limit of detection and limit of quantitation. Furthermore, evaporated graphite and polyaniline coated fused silica capillaries were investigated to determine which coating is more stable over long-term analyses and during electrical discharge.

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.

A soft on-column metal coating procedure for robust sheathless electrospray emitters used in capillary electrophoresis-mass spectrometry

An on-column metal coating procedure was developed for sheathless electrospray emitters, based on Justus von Liebig's electroless silver mirror reaction followed by electrochemical deposition of gold onto the silver layer. The coating procedure is straightforward, mild, inexpensive, and can be performed with standard laboratory equipment. A long-term (600 h) stability investigation of the conductive coating was carried out by continuous electrospray in the positive electrospray mode, and no degradation in performance was found. The simplicity of the coating procedure and the robustness of the spray tips makes the spray tips highly suitable to couple delicate wall-coated or monolithic capillary columns to mass spectrometry. Peptide mixtures were separated by capillary electrophoresis and injected into either a Hadamard-transform time-of-flight mass analyzer or a commercial quadrupole mass analyzer using the described sheathless electrospray emitters. The performance was judged to be excellent.

Comparison of sheathless and sheath-flow electrospray interfaces for the capillary electrophoresis-electrospray ionization-mass spectrometry analysis of peptides

Capillary electrophoresis coupled to mass spectrometry via an electrospray interface provides a powerful system for separation and characterization of a high number of biomolecules. The present paper describes a home-made sheathless interface and compares it with a commercial sheath-flow interface, using a separation method based on a peptide hormone mixture of therapeutic interest. In a previous work, we optimized the parameters involved in a sheath-flow interface and obtained good results in sensitivity and reproducibility. The sheathless interface is performed with a graphite-coated electrospray ionisation (ESI) tip attached to the separation capillary. We demonstrate that electrolyte composition is the main parameter affecting signal sensitivity and separation resolution. The effect of the nature and concentration of the organic solvent added to the separation electrolyte is carefully studied. Furthermore, a general comparison of both interfaces is made in terms of separation, reproducibility, and sensitivity obtained under the optimized conditions described. Advantages and disadvantages of both coupling setups have been evaluated.

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.

Sheathless electrospray from polymer microchips

In this study, sheathless electrospray from polymer microchips with conducting layer on the emitter tip is described for the first time. The electrospray emitter tips were fabricated directly from the end of the microchips that were made of polycarbonate or poly(methyl methacrylate) with injection molding. A variety of tip shapes and conducting coatings were evaluated using an electrospray time-of-flight mass spectrometer run in the sheathless mode. Stable electrospray was obtained both from hand-polished and machine-milled three-dimensional tips coated with either polymer-embedded gold particles or graphite particles as the conducting layer. Sputtered gold, on the other hand, suffered from poor stability mainly due to bad adhesion to the polymer tip. The durability of the different coatings was confirmed with electrochemical experiments under simulated electrospray conditions. The relative standard deviations of the response received from the ion current of the MS analysis were in the range of 3.5-12%. The detection limit for a standard mixture containing five neuropeptides was lower than 0.5 fmol. The low detection limit makes the emitter tips highly attractive for the analysis of low-abundance biological species.

A Glassy Carbon Microfluidic Device for Electrospray Mass Spectrometry

Due to the broad impact of microfabrication technology on chemistry and biology, new methods to pattern and etch a variety of materials are being explored in a number of laboratories. Here, we report the design, fabrication, and operation of a glassy carbon (GC) microchip interfaced to a nanoelectrospray ionization source and a quadrupole mass spectrometer. The method involves standard photolithographic pattern transfer to a photoresist layer and anodization of the exposed GC substrate in basic electrolyte to produce a series of channels with well-defined wall structure. The performance of the microchip was evaluated with standard polymer and peptide samples.

Graphite-Coated Nanoelectrospray Emitter for Mass Spectrometry

A new, more rapid method for coating nanoelectrospray emitters with graphite is to use a vacuum deposition chamber and a graphite carbon electrode. This method allows for mass production of nanoelectrospray emitters in a short period of time. The emitters are laser-pulled borosilicate glass micropipets and have tapers of around 4 microm i.d. The conductive coating applied to the emitter is only 20-30 nm thick, allowing for optical transparency with the borosilicate emitters. The conductive coating is stable for a number of hours at the high voltages used for nanoelectrospray ionization and is durable in both positive and negative ion modes-even during electrical discharge. This stability will make it possible to couple these emitters with online separations such as capillary liquid chromatography or capillary electrophoresis.

Phosphoprotein isotope-coded affinity tags: application to the enrichment and identification of low-abundance phosphoproteins

The use of a phosphoprotein isotope-coded affinity tag (PhIAT), which employs differential isotopic labeling and biotinylation, has been shown capable of enriching and identifying mixtures of low-abundance phosphopeptides. A denatured solution of beta-casein was labeled using the PhIAT method, and after proteolytic digestion, the labeled peptides were isolated using immobilized avidin. The recovered peptides were separated by capillary reversed-phase liquid chromatography and identified by tandem mass spectrometry. PhIAT-labeled peptides corresponding to known O-phosphorylated peptides from beta-casein were identified along with the phosphorylated peptides from alphas1-casein and alphas2-casein, known low-level (<5%) contaminants of commercially available beta-casein. All of the casein-phosphorylated residues identified by the present PhIAT approach correspond to previously documented sites of phosphorylation. The results illustrate the efficacy of the PhIAT-labeling strategy to not only enrich mixtures for phosphopeptides but also, more importantly, permit the detection and identification of low-level phosphopeptides. In addition, the differences in the phosphorylation state could be determined between phosphopeptides in comparative samples by stoichiometric conversion using the light and heavy isotopic versions of the PhIAT reagents. Overall, our results exemplify the application of the PhIAT approach and demonstrate its utility for proteome-wide phosphoprotein identification and quantitation.

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.