Minimizing analyte electrolysis in an electrospray emitter

The electrospray (ES) ion source is a controlled-current electrolytic flow cell. Electrolytic reactions in the ES emitter capillary are continually ongoing to sustain the production of charged droplets and ultimately gas-phase ions from this device. Under certain circumstances, the analytes under study may be directly involved in these electrolytic processes. It is demonstrated that a simple means to minimize analyte electrolysis is to exchange the normal metal emitter capillary of commercial ES sources with one made of fused silica. This change is shown to provide an ES mass spectrometric system of similar performance in terms of gas-phase ion signal generated for non-electroactive analytes and also assures minimal oxidation of electroactive analytes even at low (2.0 microl x min(-1)) solution flow-rates and high (millimolar) solution electrolyte concentrations.     

Efficient analyte oxidation in an electrospray ion source using a porous flow-through electrode emitter

This article describes the components, operation, and use of a porous flow-through electrode emitter in an electrospray ion source. This emitter electrode geometry provided enhanced mass transport to the electrode surface to exploit the inherent electrochemistry of the electrospray process for efficient analyte oxidation at flow rates up to 800 microL/min. An upstream current loop in the electrospray source circuit, formed by a grounded contact to solution upstream of the emitter electrode, was utilized to increase the magnitude of the total current at the emitter electrode to overcome current limits to efficient oxidation. The resistance in this upstream current loop was altered to control the current and "dial-in" the extent of analyte oxidation, and thus, the abundance and nature of the oxidized analyte ions observed in the mass spectrum. The oxidation of reserpine to form a variety of products by multiple electron transfer reactions and oxidation of the ferroceneboronate derivative of pinacol to form the ES active radical cation were used to study and to illustrate the performance of this new emitter electrode design. Flow injection, continuous infusion, and on-line HPLC experiments were performed.     

Ionization and transmission efficiency in an electrospray ionization—mass spectrometry interface

The ionization and transmission efficiencies of an electrospray ionization (ESI) interface were investigated to advance the understanding of how these factors affect mass spectrometry (MS) sensitivity. In addition, the effects of the ES emitter distance to the inlet, solution flow rate, and inlet temperature were characterized. Quantitative measurements of ES current loss throughout the ESI interface were accomplished by electrically isolating the front surface of the interface from the inner wall of the heated inlet capillary, enabling losses on the two surfaces to be distinguished. In addition, the ES current lost to the front surface of the ESI interface was spatially profiled with a linear array of 340-microm-diameter electrodes placed adjacent to the inlet capillary entrance. Current transmitted as gas-phase ions was differentiated from charged droplets and solvent clusters by measuring sensitivity with a single quadrupole mass spectrometer. The study revealed a large sampling efficiency into the inlet capillary (>90% at an emitter distance of 1 mm), a global rather than a local gas dynamic effect on the shape of the ES plume resulting from the gas flow conductance limit of the inlet capillary, a large (>80%) loss of analyte ions after transmission through the inlet arising from incomplete desolvation at a solution flow rate of 1.0 microL/min, and a decrease in analyte ions peak intensity at lower temperatures, despite a large increase in ES current transmission efficiency.     

Observation of gas-phase molecular dications formed from neutral organics in solution via the controlled-current electrolytic process inherent to electrospray

This article reports the first electrospray (ES) mass spectrometry observation of molecular dications that were formed in solution by sequential one-electron oxidation of the neutral molecules [viz., nickel(II) and cobalt(II) octaethylporphyrin] via the controlled-current electrolytic (CCE) process inherent to electrospray. Dication formation was found to require (1) the addition of electrolyte to the sample solution, which increased the magnitude of the ES current and, therefore, increased the extent of analyte electrolysis in the ES capillary, (2) a relatively low solution flow rate, which increased the electrolysis time (i.e., the time the analyte remained in the capillary), thereby providing more time for the analytes to diffuse to the metal-solution interface and react, and (3) the use of a platinum ES capillary, which, because it is difficult to oxidize, increased the proportion of the faradaic current that might be provided by electrolysis of solution species compared to that proportion available when the typical stainless steel capillary is used. These interpretations of the data are made on the basis of the known characteristics of the CCE process inherent to ES, supplementary data obtained from direct solution-phase observation of the metalloporphyrin redox products formed within the different metal ES capillaries by means of a novel ES ion source, and off-line cyclic voltammetry studies of the metalloporphyrins performed by using platinum and stainless steel working electrodes.     

Polarography��past, present, and future

Polarography was first developed as an automated method of voltage-controlled electrolysis with dropping mercury electrode. The spontaneously renewed pure electrode surface provided reproducible electrochemical results which enabled scientists to work out adequate theory and rich analytical applications. The original method was then instrumentally modified in various ways. Later, hanging mercury drop was added as an alternative indicator electrode??in this way, polarography turned formally into voltammetry with mercury drop electrodes. Beside, in potential-controlled electrolysis, the mercury drop electrodes have been also used in current-controlled electrolysis (chronopotentiometry)??there, it has provided new experimental effects. Polarography has thus gradually covered a wide field of electrolytic methods based on the use of mercury electrodes, in which it continues developing.     

LiCl-KCl-LaCl3熔盐体系中La(III)的电化学行为

在773 K条件下,研究了La（III）在LiCl-KCl熔盐中W和Ni电极上的电化学行为。La（III）还原反应是一步三电子转移的准可逆反应;通过在Ni电极上直接电沉积La的方法可以获得La-Ni金属间化合物;恒电位电解可以获得含三种金属间化合物（LaNi₅、La₇Ni₁₆和La₂Ni₃）的La-Ni合金层,并且通过X射线衍射仪（XRD）和扫描电镜-能谱分析仪（SEM-EDS）确定物相并表征结构。采用开路计时电位法估算了LaNi₅金属间化合物的标准生成吉布斯自由能。揭示了恒电位电解方法是制备La-Ni镀层合金以及提取熔盐中La的有效方法。     

Redox buffering in an electrospray ion source using a copper capillary emitter.

An electrospray ion source used in electrospray mass spectrometry is a two-electrode, controlled-current electrochemical flow cell. Electrochemical reactions at the emitter electrode (oxidation and reduction in positive and negative ion modes respectively) provide the excess charge necessary for the quasi-continuous production of charged droplets and ultimately gas-phase ions with this device. We demonstrate here that a copper capillary emitter, in place of the more commonly used stainless-steel capillary emitter, can be utilized as a redox buffer in positive ion mode. Anodic corrosion of the copper capillary during normal operation liberates copper ions to solution and in so doing maintains the interfacial potential at this electrode near the equilibrium potential for the copper corrosion process [E degrees = 0.34 V versus standard hydrogen electrode (SHE)]. Fixing the interfacial potential at the emitter electrode provides control over the electrochemical reactions that take place at this electrode. It is shown that the oxidation of N-phenyl-1,4-phenylenediamine to N-phenyl-1,4-phenylenediimine (E(p/2) = 0.48 V versus SHE) can be completely avoided using the copper emitter, whereas this analyte is completely oxidized with a stainless-steel capillary emitter under the same conditions. Moreover, using N-phenyl-1,4-phenylenediimine, we demonstrate that reduction reactions can occur at the copper emitter electrode in positive ion mode. Emitter corrosion, in addition to redox buffering, provides a convenient means to introduce metal ions into solution for analytical use in electrospray mass spectrometry.     

Flow visualisation: a voltammetric approach

A novel approach to the visualisation and quantification of mass transfer in liquid flow is presented. The technique utilises an array of microstrip electrodes sited within a duct through which the electrolyte solution is pumped. The electrolyte solution contains a reagent that may be oxidised and in the process undergo a colour change. The electrolysis products are then swept through the cell and the pathway imaged via a digital video. Results are presented for an experimental geometry where a rectangular obstruction is deliberately introduced into the duct. Transport rates within the cell are restricted such that Stokes flow conditions are maintained throughout and the streamlines generated from the electrolysis used to map the flow profiles through the devices. The merits of the approach are discussed and the potential of numerical modelling to provide quantitative analysis are highlighted.     

Minimizing analyte electrolysis in electrospray ionization mass spectrometry using a redox buffer coated emitter electrode

An emitter electrode with an electroactive poly(pyrrole) (PPy) polymer film coating was constructed for use in electrospray ionization mass spectrometry (ESI‐MS). The PPy film acted as a surface‐attached redox buffer limiting the interfacial potential of the emitter electrode. While extensive oxidation of selected analytes (reserpine and amodiaquine) was observed in positive ion mode ESI using a bare metal (gold) emitter electrode, the oxidation was suppressed for these same analytes when using the PPy‐coated electrode. A semi‐quantitative relationship between the rate of oxidation observed and the interfacial potential of the emitter electrode was shown. The redox buffer capacity, and therefore the lifetime of the redox buffering effect, correlated with the oxidation potential of the analyte and with the magnitude of the film charge capacity. Online reduction of the PPy polymer layer using negative ion mode ESI between analyte injections was shown to successfully restore the redox buffering capacity of the polymer film to its initial state. Published in 2010 by John Wiley & Sons, Ltd.     

Application of the superposition principle to the study of a charge transfer reaction in cyclic chronopotentiometry. Part II

The general analytical expressions corresponding to the response obtained for a charge transfer process in cyclic chronopotentiometry are presented. The different geometries considered for the mass transport operator are planar, tubular and spherical. In the case of spherical electrodes (such as the dropping mercury electrode and the static mercury dropping electrode), we have analyzed the following two cases: solution soluble product and electrode soluble product or amalgamation. The solutions deduced here are independent of the method used in solving the differential equations system since we have applied the superposition principle for which we have only used the properties of the linear operators.     

Dielectrophoretic cell trapping for improved surface plasmon resonance imaging sensing

The performance of conventional surface plasmon resonance (SPR) biosensors can be limited by the diffusion of the target analyte to the sensor surface. This work presents an SPR biosensor that incorporates an active mass‐transport mechanism based on dielectrophoresis and electroosmotic flow to enhance analyte transport to the sensor surface and reduce the time required for detection. Both these phenomena rely on the generation of AC electric fields that can be tailored by shaping the electrodes that also serve as the SPR sensing areas. Numerical simulations of electric field distribution and microparticle trajectories were performed to choose an optimal electrode design. The proposed design improves on previous work combining SPR with DEP by using face‐to‐face electrodes, rather than a planar interdigitated design. Two different top‐bottom electrode designs were experimentally tested to concentrate firstly latex beads and secondly biological cells onto the SPR sensing area. SPR measurements were then performed by varying the target concentrations. The electrohydrodynamic flow enabled efficient concentration of small objects (3 μm beads, yeasts) onto the SPR sensing area, which resulted in an order of magnitude increased SPR response. Negative dielectrophoresis was also used to concentrate HEK293 cells onto the metal electrodes surrounded by insulating areas, where the SPR response was improved by one order of magnitude.     

Expanded use of a battery-powered two-electrode emitter cell for electrospray mass spectrometry

A battery-powered, controlled-current, two-electrode electrochemical cell containing a porous flow-through working electrode with high surface area and multiple auxiliary electrodes with small total surface area was incorporated into the electrospray emitter circuit to control the electrochemical reactions of analytes in the electrospray emitter. This cell system provided the ability to control the extent of analyte oxidation in positive ion mode in the electrospray emitter by simply setting the magnitude and polarity of the current at the working electrode. In addition, this cell provided the ability to effectively reduce analytes in positive ion mode and oxidize analytes in negative ion mode. The small size, economics, and ease of use of such a battery-powered controlled-current emitter cell was demonstrated by powering a single resistor and switch circuit with a small-size, 3 V watch battery, all of which might be incorporated on the emitter cell.     

A glass capillary ultramicroelectrode with an electrokinetic sampling ability.

A glass capillary ultramicroelectrode (tip diameter approximately 1.2 microm) having an electrokinetic sampling ability is described. It is composed of a pulled glass capillary filled with an inner solution and three internal electrodes (Pt working and counter electrodes and an Ag/AgCl reference electrode). The voltammetric response of the capillary electrode is based on electrokinetic transport of analyte ions from the sample solution into the inner solution across the conical tip. It was found that the electrophoretic migration of analytes at the conical tip is faster than electroosmotic flow, enabling electrokinetic transport of analyte ions into the inner solution of the electrode. By using [Fe(CN)6]4- and (ferrocenylmethyl)trimethylammonium (FcTMA+) ions as model analytes, differential pulse voltammetric responses of the capillary electrode were investigated in terms of tip diameter of the capillary, sampling voltage, sampling time, detection limit and selectivity. The magnitude of the response depends on the size and charge of analyte ions. With a capillary electrode having a approximately 1.2-microm tip diameter, which minimizes non-selective diffusional entry of analytes, the response after 1 h sampling at +1.7 V is linearly related to [Fe(CN)6]4- concentration in the range of 0.50-5.0 mM with the detection limit of 30 microM. Application of a potential of the same sign as that of the analyte ion forces the analyte to move out from the electrode to the solution, enabling reuse of the same capillary electrode. The charge-selective detection of analytes with the capillary electrode is demonstrated for [Fe(CN)6]4- in the presence of FcTMA+.     

Hittorf法测定离子迁移数实验的改进

以不同浓度的CuSO4溶液为电解质,Pt电极和Cu电极分别为电解电极,通过观察通电前后电解质溶液阳极区和阴极区颜色的变化,定性分析离子的电迁移现象.通过电极上产生/溶解Cu的变化量以及CuSO4溶液浓度的变化,计算了不同浓度CuSO4溶液中离子的迁移数,并对浓度等因素对迁移数的影响规律进行了探讨.本文对传统离子迁移数的实验进行了改进,旨在加深学生对离子电迁移现象的认识及对迁移数概念的理解.     

Electrochemical processes in a wire-in-a-capillary bulk-loaded, nano-electrospray emitter

Experiments are described that illustrate solvent oxidation, emitter electrode corrosion, and analyte oxidation in positive ion mode nano-electrospray mass spectrometry using a wire-in-a-capillary, bulk-loaded nano-electrospray emitter geometry. Time-lapsed color photography of pH and metal specific indicator solutions within operating nano-electrospray emitters, as well as temporal changes in the ions observed in the nano-electrospray mass spectra, are used to probe these reactions, judge their magnitude, and study the time dependent changes in solution composition and gas-phase ion signal brought about as a result of these electrochemical reactions. The significance of these observations for analytical applications of nano-electrospray mass spectrometry are discussed.     

Determination of sulfite in beer samples using an amperometric fill and flow channel biosensor employing sulfite oxidase

A simple method is described to determine sulfite in beer samples using a fill and flow channel biosensor. A droplet of sample is placed into the inlet of a rectangular flow cell and begins to flow through the channel by capillarity. The flow is maintained and controlled by a porous outlet plug of defined porosity. In a rectangular flow cell, the sample solution flows through three consecutive zones: over a predictor electrode, an enzyme layer and a detector electrode. Together these three zones enable the differentiation between current due to sulfite and current due to other electroactive species in the sample. The predictor electrode is located upstream, and on the opposite channel wall to the enzyme layer and detector electrode, and is poised at the same potential (+0.65 V versus Ag/AgCl) as the detector electrode. On this electrode, the current contribution from all species in the sample solution that are oxidized at that potential is determined. The enzyme layer contains sulfite oxidase, which, in the process of oxidizing sulfite, produces hydrogen peroxide, which itself is reduced by excess sulfite. The current at the downstream detector electrode is therefore different from that at the predictor electrode as a result of the enzyme reaction and the difference of the currents, corrected for the dimensions of the electrodes, is proportional to the concentration of sulfite. The method enables a straightforward correction of the interfering current at the detector electrode and a determination of the analyte concentration. The effect of interferences from ascorbic acid, ethanol, sorbic acid and tartaric acid in the detection of sulfite is efficiently removed. The concentration of sulfite in a sample of beer measured by the biosensor is equivalent to that measured using a reference method based on the AOAC-recommended Monier-Williams method.     

Dynamics of copper electrodeposition onto fibrous carbon electrodes

Distribution of copper electrodeposited from a sulfuric acid solution onto fibrous carbon electrodes, copper deposition rate, and current efficiency by the metal were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The variation of the electrode thickness on which copper ions discharge at the limiting diffusion current at various solution flow rates and the electrode thickness on which the whole amount of oxygen dissolved in the electrolyte is reduced were calculated in relation to the solution flow rate and geometric current density. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters from the beginning of the process till ??clogging?? of a part of the electrode by the metal were determined.     

Diffusion mass transfer in a closed electrolytic microcell. Numerical simulation for the Cu/CuSO4(H2SO4)/Cu system

This paper reports the results of mathematical simulation of electrolysis in a closed electrolytic microcell in a Cu/CuSO₄(H₂SO₄)/Cu system, in which a two-stage electrode reaction, forming a relatively stable intermediate, and chemical reactions between the electrolyte components take place. The dependence of the current that flows through the cell on the electrolysis time and voltage on the cell electrodes was studied by numerical methods. The steady-state profiles of the concentrations of copper-containing species were calculated. The diffusion mass transfer rate in the cell was evaluated. The results were compared with the calculated data for the cell with a recessed electrode configuration with the indicator electrode lying at the bottom of a hole in the insulator. The mass transfer rate in the closed microcell was higher than on the recessed electrode.     

Monolayers of photosystem II on gold electrodes with enhanced sensor response—effect of porosity and protein layer arrangement

Mass transport of the bulk of the analyte to the electrode and through the bioactive layer can be significantly improved by use of the nanoelectrode array and defined arrangement of protein film. This phenomenon has been studied by (i) atomic-force microscopy, (ii) electrochemical measurements of PSII activity, and (iii) digital simulations for an oriented monolayer of histidine-tagged photosystem II (PSII) immobilized on nitrilotriacetic acid (NTA)-modified gold electrodes. The output signal of the electrochemical biosensor is controlled by (i) mass transport from the bioactive layer to electrode and (ii) mass transport between the bulk of the analyte and the electrode. Mass transport through the bioactive layer was electrochemically studied for PSII self-assembled on gold screen-printed electrodes. A densely packed monolayer of PSII has a significant shielding effect toward the diffusion of redox mediator duroquinone (DQ). Mass transport to the planar electrode surface was improved by co-immobilization of bovine-serum albumin (BSA) as spacer biomolecule in the monolayer of PSII. Correlation between the electrochemical properties and surface arrangement of the resulting protein films was clearly observable and confirmed the improved mass-transport properties of structured enzyme monolayers. On the basis of this observation, the application of a bottom-up approach for improvement of electrode performance was proposed and digitally simulated for an infinite array of electrodes ranging in diameter from 50 nm to 5 m. The nanoelectrode array, with the optimum time window selected for measurements, enables enhancement of mass transport between the bulk of the analyte and the macroelectrode by a factor of up to 50 in comparison with classical planar electrodes. Use of a time window enables minimization of crosstalk between individual electrodes in the array. The measurements require methods which suppress the double-layer capacity.     

Current measurements within the electrospray emitter

A movable disc-like wire probe electrode placed inside the electrospray (ES) capillary was used to measure currents flowing within the ES device for the first time. Currents were measured between the wire probe and the ES capillary. Current maps revealing measured current versus wire probe position were generated for a variety of solution conditions in the positive and negative ion modes and are compared to potential maps. The electrospray device was found to subsist on highly stable total currents; this current regulator aspect of the ES device showed remarkable resiliency regardless of the proportion of current produced at the wire probe electrode versus the ES capillary. However, kinks observed in the current and potential maps are attributed to adsorbed air participating in electrochemical reactions, and turbulence in solution flow in the region of the Taylor cone. From differential electrospray emitter potential (DEEP) maps, current maps, and cyclic voltammetry experiments performed at different wire probe locations, evidence is provided for separate regimes of current flow in the bulk solution and in the thin "skin" of highly conductive electrolyte constituting the outer surface (air interface) of the Taylor cone. Current maps reveal that current is drawn more evenly along the length of the ES capillary when solutions are highly conductive, in agreement with previous results for DEEP maps. In less conductive solutions, the area close to the capillary exit contributes more heavily to current production. Evidence that contaminant participation in electrochemical processes occurring within the electrospray device can be largely responsible for production of the excess charge in ES droplets is also provided. These investigations complement previous DEEP mapping studies to further elucidate the details of the electrochemical processes occurring within the electrospray device.