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The development of Cu-based catalysts for the electrochemical CO2 reduction reaction (eCO2 RR) is of major interest for generating commercially important C2 liquid products such as ethanol. Cu is exclusive among the eCO2 RR metallic catalysts in that it facilitates the formation of a range of highly reduced C2 products, with a reasonable total faradaic efficiency but poor product selectivity. Here, a series of new sulfide-derived copper-cadmium catalysts (SD-Cux Cdy ) was developed. An excellent faradaic efficiency of around 32 % but with a relatively low current density of 0.6 mA cm-2 for ethanol was obtained using the SD-CuCd2 catalyst at the relatively low overpotential of 0.89 V in a CO2 -saturated aqueous 0.10 m KHCO3 solution with an H-cell. The current density increased by an order of magnitude under similar conditions using a flow cell where the mass transport rate for CO2 was greatly enhanced. Ex situ spectroscopic and microscopic, and voltammetric investigations pointed to the role of abundant phase boundaries between CdS and Cu+ /Cu sites in the SD-CuCd2 catalyst in enhancing the selectivity and efficiency of ethanol formation at low potentials.

The development of Cu-based catalysts for the electrochemical CO

We describe formation and detection of transient radical intermediates and 2e − , 2H + oxidation products (OPs) of dopamine (DA) by electrospray ionization (ESI) mass spectrometry (MS) which has not been reported previously. The results confirm formation of DA radicals by proton coupled electron transfer pathway. DA radicals and 2e − , 2H + oxidation products that were identified are of interest because of their reported neurotoxicity. Using different solution compositions and MS detection conditions three different DA radicals were identified that have different stability. DA reactions with reactive oxygen species (ROS) relevant to DA reactivity in vivo are discussed. Detection of DA OPs by electrochemistry/mass spectrometry with a floated on-line electrochemical cell is also illustrated.

Significantly higher than previously reported sensitivity was achieved in the electrochemical determinations of caffeine (CA) and theophylline (TP) at a nanostructured carbon fiber electrode (CFE) sensor by fast scan voltammetry (FSV), where apparent catalysis of the oxidation was observed. For CA the oxidation was reversible and the oxidation potential in aqueous solutions was similar to the potentials reported from cyclic voltammetry (CV) in aprotic solvents, indicating similar oxidation pathways, forming CA radicals. For TP, the reversible oxidation potential was higher, in agreement with TP structure. Formal potentials in aqueous solutions that have been previously unavailable from electrochemistry of CA and TP were thus obtained. In the determinations of CA and TP, sensitivity and linear dynamic range (LDR) in FSV were 1.84 mA cm−2 µM−1 and 0.20–22.0 µmol L−1 and 1.46 mA cm−2 µM−1 and 0.50–30.0 µmol L−1, respectively. FSV determination of CA in coffee sample is demonstrated and the results are in agreement with the results obtained by HPLC-UV.

A new online electrochemistry/liquid sample desorption electrospray ionization mass spectrometry (EC/LS DESI MS) system with a simple electrochemical thin-layer flow-through cell was developed and tested using N,N-dimethyl-p-phenylenediamine (DMPA) as a model probe. Although oxidation of DMPA is observed as a result of ionization of LS in positive ion mode LS DESI, application of voltage to the online electrochemical (EC) cell in EC/LS DESI MS increases yields of oxidation products. An advantage of LS DESI MS is its sensitivity in aqueous electrolyte solutions, which improves efficiency of electrochemical reactions in EC/LS DESI MS. In highly conductive low pH aqueous buffer solutions, oxidation efficiency is close to 100%. EC/ESI MS typically requires mixed aqueous/organic solvents and low electrolyte concentrations for efficient ionization in MS, limiting efficiency of electrochemistry online with MS. Independently, the results verify higher electrochemical oxidation efficiency during positive mode ESI than during LS DESI.

The relationship between one-electron (e−) oxidation processes and the formation of radical cations of endogenous and exogenous compounds in vivo is of considerable interest. This paper reports on the experiments that allow FTICR mass spectrometric (MS) detection of ion signals that are consistent with the formation of radical cations of caffeine (CA) and theophylline (TP) during electrospray ionization (ESI) in ESI FTICR MS and in on-line electrochemistry (EC)/ESI FTICR MS in positive mode. Significantly, the signals of the radicals of CA•+ and TP•+can be enhanced by simple modifications of the operating conditions in ESI MS, facilitating investigations of radical formation and related reactions.

Electrolytic efficiency of battery-operated flow-through thin-layer electrochemical (EC; echem) cell used in EC/ESI-MS was investigated. Two stainless steel capillaries joined by PTFE tubing formed the thin-layer EC cell. Echem reaction efficiency of ferrocyanide, dopamine and hydroquinone oxidation in the EC cell was low and decreased with increasing MeOH volume fraction in the MeOH/H2O carrier solutions with low conductivity. Water oxidation dominated the cell current and migration of ferrocyanide was observed. Flow rate of 50–200 µL/h had a very small effect on EC cell current. Dopamine oxidation current in the EC cell and dopamine ion intensity in EC/ESI-MS showed a direct correlation.

The attributes of large-amplitude Fourier transformed AC voltammetry have been assessed at electrodes constructed from either a single carbon fiber microdisk or a random assembly of carbon fiber microdisks (RAM). Factors that govern the limits of detection for the FcMeOH and uric acid analytes have been established. For the reversible FcMeOH oxidation process, the RAM electrode provides the lower limit of detection through substantial amplification of the peak current magnitude, and hence superior faradaic current-to-noise ratio. In contrast for the overall two-electron oxidation of uric acid, the much faster electron transfer rate achieved at the electrochemically treated single carbon fiber microelectrode provides the lower detection limit, which is again optimal in the sixth harmonic.

Low (15 nM) limits of detection (LOD) of dopamine were obtained at porous 1–3 nm-thick overoxidized polypyrrole (OPPY) membrane-coated microdisk carbon fiber electrodes of small size (r=3.5 µm). The excellent mass transport through the porous OPPY membrane (efficiency as good as solution transport) contributes to the low LOD, together with dopamine preconcentration in the membrane. Structural characterization of the membrane coated electrodes confirms the contribution of membrane nanostructure to the high sensitivity and the low LOD.

The ionization of LS samples in desorption ionization mass spectrometry (LS DESI MS), supplied continuously through a LS interface separated in space from the spray emitter, was investigated in this work. The role of electrochemistry (EC) in the ionization process was addressed. The visual (observation) of the operation of the LS DESI MS system showed a thick spray plume generated by the electrosonic spray ionization (ESSI), forming a liquid cone at the LS interface. When the LS interface was grounded the cone collapsed and the MS ion signal was lost, indicating that the LS was carried to the MS inlet by the spray that emerged from the cone. Ion signals in a new in-line LS DESI MS system, in angled LS DESI MS, and in electrospray ionization (ESI) MS, which produced the most intense ion signals from methanol/water solutions, and in ESSI MS, of dopamine (DA), tyrosine (Tyr) and N,N-dimethyl-p-phenylenediamine (DMPA), were evaluated using methanol/water and aqueous (aq) solutions. In addition, the effect on ion signals of geometric parameters and the LS and the spray solution flow rates was tested in in-line LS DESI MS. Of the methods tested, the analysis of aq LS containing electrolytes was simplest by LS DESI MS. The signal intensity was higher in in-line than in angled LS DESI MS. In online electrochemistry (EC)/LS DESI MS, when 0 V was applied to the EC cell Tyr ion signal was detected only at low pH (2).

Ionically conducting 1–3 nm thick porous films of overoxidized polypyrrole (OPPY) were electrodeposited on nanostructured 7 μm diameter carbon fiber microdisk electrodes. The microdisk electrodes were fabricated from two types of polyacrylonitrile (PAN) carbon fibers, PAN T650 and PAN HCB. The electrodes were nanostructured by electrochemical etching of the microdisk electrode surface. Ultrathin porous polypyrrole (PPY) films were electrodeposited by the electropolymerization of pyrrole (PY) to PPY by a short (10 ms) single potential pulse. During the electropolymerization, the polymer “precipitated” on the nanostructured surface producing ultrathin porous film. OPPY films were fabricated by constant potential overoxidation of PPY. In steady-state voltammetry of ferricyanide, the nanostructured electrodes behave as a random array of microscopic nodules and pores. At potential scan rates of 0.050 V s−1 diffusion fields at the 300–600 nm nodules on the 7 μm diameter microdisk electrode overlap. The surface area of the electroactive nanofeatures decreases after deposition of insulating OPPY. Kinetics of ferricyanide at bare and OPPY-coated nanostructured electrodes reflect the electrode surface area, as predicted by the model for charge transfer at a partially blocked surface. A model reflecting the 58–94% coverage of the nanostructured electrodes by OPPY was developed to address the high permeability of the porous OPPY-coated microdisk electrodes.

Crosslinking of guanine with tyrosine and glutathione is studied by on-line electrochemistry electrospray ionization mass spectrometry (EC ESI MS) in the positive ion mode with a high resolution FT ICR mass spectrometer without the use of an external oxidizer in the sample. Dimeric adducts of guanine with tyrosine ( m / z 331) and ( m / z 333) and a dimer of guanine with glutathione ( m / z 459) are detected in the mass spectra of guanine–tyrosine and guanine–glutathione mixtures in addition to dimers of guanine ( m / z 303) and tyrosine ( m / z 361) and ( m / z 363) and glutathione dimers ( m / z 613) and ( m / z 615). Guanine tetramers ( m / z 627) and tetramers of guanine with 1 or 2 tyrosines, but not with three tyrosines, were also identified in the mass spectra of guanine–tyrosine mixtures. Formation of radicals and other oxidation products during positive ion mode ESI can drive the formation of covalent dimer adducts of guanine with tyrosine and glutathione. When low EC cell voltage is applied in on-line EC ESI MS, changes in ion intensities reflect changes in the oxidation conditions and are more apparent for 2e, 2H + than for the 1e, 1H + oxidation processes. Efficient oxidation during ESI is indicated for analytes with low redox potentials.

2,8-Dihydroxyadenine (2,8-DHA) was identified by high-performance liquid chromatography with ultraviolet detection as a major metabolite in the supernatant of endothelial cells of the pulmonary artery (PAECs) and aorta (AECs), in addition to hypoxanthine, xanthine, uric acid, and uracil. Under normoxic, hypoxic, and hyperoxic conditions, the concentrations of all the identified metabolites change with time, marking the response of endothelial cells to stress, as a result of changes in cellular metabolism. Thus, the metabolites can serve as stress markers, and their concentrations can indicate the type and the level of cell stress. The results verify that PAECs adapt to survive oxidative stress of hyperoxia. However, AECs can adapt to hypoxia only for a short time and do not survive prolonged hypoxia. The role of the polyamine synthesis pathway in the formation of the unsalvaged adenine, as a possible source of 2,8-DHA, is discussed.

A cone-shaped MS inlet and on-line electrochemistry (EC) were used to enhance the ionization efficiency in electrospray ionization mass spectrometry (ESI MS) of purine bases. A pathway of positive ion mode ESI may involve oxidation of purine bases, guanine, adenine, xanthine and hypoxanthine, by 1e−, 1H+ processes. The electrospray process generates dimers of purine bases that are detected in ESI MS as protonated ions, except for xanthine, for which a protonated radical dimer is detected. Thus electrochemical oxidation of purine bases during ESI may generate reactive radicals that can subsequently dimerize. Dimer formation is facilitated in ESI MS when the carrier solution pH is high. The positive ion mode ESI MS ionization is consistent with the reactivity of the bases toward oxidation. Furthermore, the formation of the protonated ions, and Na+ and K+ adducts of the bases, expected in positive ion ESI MS, are observed. In addition, unusual H-bonding of purine bases guanine and xanthine is confirmed by ESI MS. Application of low EC voltage to the on-line EC cell in EC/ESI MS improves the sensitivity and correlates with the decrease of the intensity of the dimers, possibly as a result of their further oxidation.

High signal intensities of glutathione (GSH), [GSH+H]+ (m/z 308), cysteine (CySH), [CySH+H]+ (m/z 122), and homocysteine (hCySH), [hCySH+H]+ (m/z 136), are observed in ESI MS with on-line electrochemistry (EC). Dimers formed by H-bonding, which are not electrochemical products, are detected as [2GSH+H]+ (m/z 615), [2CySH+H]+ (m/z 243) and [2hCySH+H]+ (m/z 271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]+ (m/z 613), [CySSCy+H]+ (m/z 241) and [hCySSCyh+H]+ (m/z 269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.

The sensitivity of detection of uric acid (H2U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pKa1=5.4 of H2U, protonated unoxidized uric acid [H2U+H]+ (m/z 169) was detected together with the protonated uric acid dimer [2H2U+H]+ (m/z 337). The dimer likely forms by 1e− oxidation of urate (HU−) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e−, 2H+ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on-line electrochemical cell voltage is floated on the high voltage of the ES in on-line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pKa2=9.8 of H2U, allantoin (Allnt) (MW 158.04), the final 2e−, 2H+ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]+ (m/z 235) and the [2H2U+H]+ dimer was not detected. In direct ESI MS analysis of 1000-fold diluted urine [NaHU+H]+ (pKsp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH4Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone-shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.

Dopamine [DA]+ (m/z 154), DA dimer [2DA-H]+ (m/z 307) and DA quinone [DAQ]+ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e−, 1H+ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e−, 2H+ oxidation reaction. On-line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone-shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone-cysteine adduct [DAQ+Cys]+ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone-cysteine adduct by 1e− pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.

A miniaturized two-electrode electrochemical (EC) cell was developed and was coupled on-line with an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR MS). Electrochemistry on-line with mass spectrometry, EC/ESI-FTICR MS, of triphenylamine (TPA), which undergoes one-electron oxidation to form a radical cation (TPA•+), demonstrates a significant sensitivity enhancement compared to ESI-FTICR MS. The on-line EC cell configuration with a stainless steel ES needle as the working electrode produces the highest sensitivity in EC/ESI-MS. The results provide evidence that, during the ES ionization, electrolytic reactions occur mainly in the ES tip region, as previously predicted. The results demonstrate that ESI-MS signal suppression by tetrabutylammonium perchlorate electrolyte, which can be a problem, is minimized in EC/ESI-MS. TPA•+ dimer tetraphenylbenzidine (TPB) can be detected by EC/ESI-MS, together with TPA•+, as TPB•+ and TPB2+. The high mass resolving power of...

In the investigation of the anodic oxidation of triphenylamine (TPA) by electrochemistry/particle beam mass spectrometry (EC/PB/MS), the presence of a non-volatile organic electrolyte tetrabutylammonium perchlorate (TBAP) caused a significant MS signal enhancement of the TPA + ion. The magnitude of the signal enhancement was ca. 35-fold and was observed in the presence of 10/1 excess of TBAP. While the mobile phase additives have been frequently used in PB/LC/MS analysis to enhance analyte signal, the use of a non-volatile surface-active organic salt as a mobile phase additive, to enhance the signal of a neutral analyte, has not been reported. The present work demonstrates that in addition to the well-known improved mass transport of the analyte through the PB interface, the presence of TBAP also contributes to the reduced fragmentation of the analyte molecular ion, TPA + , in the EI ion source; this effect also contributes to the observed signal enhancement. This is the first report about the effect of the mobile phase additive on the EI mass spectrum of the analyte. In addition, it is demonstrated that the concentration ratio of TBAP/TPA plays a critical role in the signal enhancement process. Best linearity, sensitivity and reproducibility of the calibration curves are obtained by keeping the TBAP/TPA ratio constant near an optimum value during calibrations.

Hipoxantina, xantina e acido urico sao purinas encontradas em fluidos extracelulares, que podem ser utilizados como marcadores metabolicos, indicadores da idade de carnes e outras aplicacoes. Estudos voltametricos foram desenvolvidos para investigar a possibilidade da determinacao simultânea destes analitos em ultramicroeletrodos de fibra de carbono (diâmetro de 7 mm). Os estudos efetuados mostraram a possibilidade destas determinacoes com coeficientes medios de recuperacao da ordem de 97,6%, ao nivel de mmol L-1.