Thin-layer Flow-through Cell Research Articles

Electrochemistry/mass spectrometry (EC/MS) for fast generation and identification of novel reactive metabolites of two unsymmetrical bisacridines with anticancer activity

The development of a new drug requires knowledge about its metabolic fate in a living organism, regarding the comprehensive assessment of both drug therapeutic activity and toxicity profiles. Electrochemistry (EC) coupled with mass spectrometry (MS) is an efficient tool for predicting the phase I metabolism of redox-sensitive drugs. In particular, EC/MS represents a clear advantage for the generation of reactive drug transformation products and their direct identification compared to biological matrices. In this work, we focused on the characterization of novel electrochemical products of two representative unsymmetrical bisacridines (C-2028 and C-2045) with demonstrated high anticancer activity. The electrochemical thin-layer flow-through cell μ-PrepCell 2.0 (Antec Scientific) was used here for the effective metabolite electrosynthesis. The electrochemical simulation of C-2028 reductive and C-2045 oxidative metabolism resulted in the generation of new products that were not observed before. The formation of nitroso [M-O+H]+ and azoxy [2M-3O+H]+ species from C-2028, as well as a series of hydroxylated and/or dehydrogenated products, including possible quinones [M-2H+H]+ and [M+O-2H+H]+ from C-2045, was demonstrated. For the latter, a glutathione S-conjugate (m/z 935.3130) was also obtained in measurements supplemented with the excess of reduced glutathione. For the identification of the products of interest, structural confirmation based on MS/MS fragmentation experiments was performed. Novel products of electrochemical conversions of unsymmetrical bisacridines were discussed in the context of their possible biological effect on the human organism.

Merits of online electrochemistry liquid sample desorption electrospray ionization mass spectrometry (EC/LS DESI MS).

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.

Quasi-Continuous Determination of the Apparent Transfer Coefficient of Methanol Oxidation Using a Potential Modulation Technique Under Convection Conditions

The apparent transfer coefficient for methanol oxidation and for the oxidation of the methanol adsorption product was determined using the recently introduced ac voltammetry method [1]. To ensure fast convectional transport and avoid interference with reaction intermediates, the same dual thin-layer flow-through cell, which we typically use for differential electrochemical mass spectrometry, was used. Potential sweep and potential step experiments have been performed at polycrystalline platinum, superimposing a small sinusoidal voltage to the normal dc voltage, and the components of the ac current have been recorded. The potential dependence of the electrochemical reaction rate, described by the apparent transfer coefficient (α′), has been recorded quasi-continuously as a function of potential or time, and the corresponding Tafel slopes have been calculated. In control experiments, using adsorbed CO, values previously determined using the H cell was reproduced. This demonstrates that the method is applicable to the thin-layer cell despite of the high electrolyte resistance. Contrary to the case of the oxidation of adsorbed CO, where the transfer coefficient varies from about 1.5 at low potentials to 0.5 at high potentials, due to a change of the rate-determining step, the apparent transfer coefficient for the oxidation of methanol adsorption product is around 0.5 at all potentials, suggesting that at all potentials, the first reaction step, the adsorption of OH, is the rate-determining step (rds) and not in equilibrium. A typical α′ value of 0.5 (Tafel slope of 118 mV dec−1) has been calculated at the onset potential during the potentiodynamic and potentiostatic oxidation of bulk methanol at polycrystalline platinum demonstrating that the rds is the first charge transfer with loss of one electron.

Formation of methylformate during methanol oxidation revisited: The mechanism

One of the key questions for the performance of the direct methanol fuel cell is the role of intermediates and the poisoning of the surface. Here, we demonstrate that methylformate, which electrochemically is less reactive than methanol itself, is not formed in a reaction between formic acid and methanol in solution, contrary to what is usually assumed. This reaction is far too slow as shown here in a simple kinetic measurement ( τ = 1.5 × 10 5 s). Rather, methylformate originates from a direct oxidation reaction at the surface. A corresponding reaction scheme is presented. The effect of various parameters such as convection, methanol concentration and potential on the current efficiency for CO 2 and methylformate and the extent of poisoning is evaluated using a special potential step program in combination with a dual thin-layer flow-through cell which is connected directly to a mass spectrometer by a hydrophobic Teflon (PTFE) membrane. In agreement with previous results, the current efficiency with respect to CO 2 is independent of flow rate; also the extent of CO poisoning does not depend on convection. Both confirm the parallel path mechanism. Under the present conditions, the formation of CO 2 is solely due to the path via adsorbed CO. The current efficiency for CO 2 increases with potential and decreases with methanol concentration, whereas that for methylformate shows the opposite behavior.

Miniaturization of flow-through generation cells for electrochemical hydride generation in AAS

Abstract The construction and optimization of five new types of miniaturized flow-through electrolytic cells with lead cathode and platinum anode for electrochemical hydride generation in atomic absorption spectrometry (HG-QFAAS) were achieved during this research study. The ion-exchange membrane was not part of these cells and only one carrying electrolyte for both electrode chambers was used. Hydride generation efficiency achieved was either comparable or higher than the one recorded for the classic thin-layer generation cell. The inner volume of the cathode chamber was reduced to a quarter of the classic thin-layer flow-through cell. Compared to the commonly used thin-layer flow-through cell, higher sensitivity (7.32×103 dm3 µg−1) and better limit of detection (0.32 µg dm−3) were obtained for selenium determination using two of these new generators.

DEMS Study on Methanol Oxidation at Poly- and Monocrystalline Platinum Electrodes: The Effect of Anion, Temperature, Surface Structure, Ru Adatom, and Potential

Methanol oxidation on Ru-unmodified and modified polycrystalline platinum, Pt(111), and Pt(332) was studied by on-line differential electrochemical mass spectrometry (DEMS) in combination with the dual thin-layer flow-through cell. The effects of anion, temperature, surface structure of Pt electrodes, Ru adatom, and potential on methanol oxidation were discussed. Methanol oxidation on smooth polycrystalline platinum, Pt(111), and Pt(332) electrodes proceeds via the parallel pathway mechanism, and mainly forms the soluble intermediates, i.e., formaldehyde and formic acid. Anion, temperature, and surface structure of Pt electrodes markedly influence the apparent rate of methanol oxidation; however, they do not significantly alter the current efficiency of CO2, and therefore do not change the mechanism of methanol oxidation. On pure platinum electrodes current efficiency of CO2 for methanol oxidation increases from ca. 20% to 32% with the increase of potential from 0.6 to 0.8V, accompanied by the increasing ...

Electrochemical generation of mercury cold vapor and its in-situ trapping in gold-covered graphite tube atomizers

The combination of more efficient flow-through electrochemical mercury cold vapor generation with its in-situ trapping in a graphite tube atomizer is described. This coupled technique has been optimized to attain the maximum sensitivity for Hg determination and to minimize the limits of detection and determination. A laboratory constructed thin-layer flow-through cell with a platinum cathode served as the cold vapor generator. Various cathode arrangements with different active surface areas were tested. Automated sampling equipment for the graphite atomizer with an untreated fused silica capillary was used for the introduction of the mercury vapor. The inner surface of the graphite tube was covered with a gold foil placed against the sampling hole. The results attained for the electrochemical mercury cold vapor generation (an absolute limit of detection of 80 pg; peak absorbance, 3 σ criterion) were compared with the traditional vapor generation using NaBH 4 as the reducing agent (an absolute limit of detection of 124 pg; peak absorbance, 3 σ criterion). The repeatability at the 5 ng ml − 1 level was better than 4.1% (RSD) for electrochemical mercury vapor generation and better than 5.6% for the chemical cold vapor generation. The proposed method was applied to the determination the of Hg contents in a certified reference material and in spiked river water samples.

Methanol Oxidation on a Carbon-Supported Pt Fuel Cell CatalystA Kinetic and Mechanistic Study by Differential Electrochemical Mass Spectrometry

Methanol oxidation on a supported Pt fuel cell catalyst was investigated by on-line differential electrochemical mass spectrometry (DEMS) at continuous electrolyte flow and defined catalyst utilization, employing a thin-film electrode setup and a thin-layer flow-through cell. The active surface of the Pt/Vulcan (E-TEK) high surface area catalyst was characterized quantitatively by Hupd and preadsorbed CO monolayer stripping. Methanol stripping DEMS experiments, oxidizing the adsorbed dehydrogenation products formed upon methanol adsorption at potentials in the hydrogen adsorption region, show that the coverage of these products and hence the methanol uptake depend on the electrode potential, in contrast to the potential-independent COad coverage. The dehydrogenation products cannot be displaced by Hupd. The number of close to two electrons used per oxidation of one adsorbed dehydrogenation product identifies this as COad species. Further methanol dehydrogenation is hindered when the CO adlayer reaches a d...

Determination of [formula omitted]-Cysteine in Amino Acid Mixture and Human Urine by Flow-Injection Analysis with a Biamperometric Detector

Based on the electrocatalytic oxidation of cysteine at pretreated platinum electrode and the flow-injection biamperometry for irreversible couple, a novel electrochemical detector is proposed for the selective determination of cysteine in amino acid mixtures and human urine samples. A thin-layer flowthrough cell was used to achieve large electrode surface area to volume ratio. Two identical pretreated platinum electrodes were mounted in the cell with an applied potential difference of 10 mV. By coupling two independent and irreversible electrode processes, namely, the oxidation of cysteine and the reduction of platinum oxide, the biamperometric detection scheme has been established. The resulting current is linear to cysteine over the range 4 × 10−7–4 × 10−5 M with the detection limit 1 × 10−7 M (15 pmol). The selectivity of the detector is tested by 55 foreign species including 26 ions, 11 amino acids, 6 vitamins, and 12 other compounds possibly found in urine. The detector performs well as a routine assay, showing high efficiency (180 samples/h) and good reproductivity shown by a RSD of 0.6% for eight repeated determinations of 2 × 10−6 M cysteine. The urine samples are detected directly without the need of pretreatment or adding other reagents.

The kinetic [formula omitted] isotope effect in electroless copper plating. A DEMS study

DEMS studies in electroless copper plating solution were carried out using a thin-layer flow-through cell within the electrode potential range − 1.0 V to 0.7 V ( SHE). The effect of H D substitution in both formaldahyde and water on the rate of anodic oxidation of formaldehyde to formate was investigated by on-line mass spectrometry in electroless copper plating solutions and in Cu(H)-free solutions under controlled electrode potential as well as under open-circuit conditions. A negligible H D isotope effect was found in electroless copper plating solution for H 2O replacement by D 2O within the potential range − 0.7 to −0.4 V and under open-circuit conditions. The dependence of the k H k D value for H D substitution in formaldehyde on the electrode potential was enhanced in electroless plating solution at low ( ca. −0.6 V) anodic potentials. The rupture of the CH bond of formaldehyde is concluded to be the rate-limiting step in anodic oxidation of formaldehyde on copper and in electroless copper plating.

Chemiluminescent immunoenzyme biosensor with a thin-layer flow-through cell. Application for study of a real-time bimolecular antigen-antibody interaction

A simple flow enzyme system for real-time continuous monitoring of interaction of biological molecules has been developed. It relies upon a thin-layer flow-through cell placed directly into the measuring compartment of the luminometer. One ligand (antibody) is immobilized on the inner surfaces of the flow cuvette, and a second ligand (antigen) labeled with a peroxidase molecule moves through the flow cell. The quantity of the complex on the surface of the cell may be monitored by measurement intensity of chemiluminescence in the reaction of peroxidase label with substrates ( p-iodophenol, luminol and hydrogen peroxide). In such a way one can detect in a real-time regime the kinetics of association (or dissociation) of the complex labeled ligand-receptor on the surface of the cuvette. Due to the small thickness of the flow cell the diffusion limitations of interaction for two kinds of biomolecules (soluble and immobilized) are negligible, so the resulting intensity of chemiluminescent signal reflects the kinetics of interaction between soluble and immobilized components. The system may be successfully used for molecular recognition studies, analyzing the kinetics of bimolecular interaction and for concentration determination.

Differential electrochemical mass spectrometry study of the electroless copper plating process using a thin-layer flow-through cell

Differential electrochemical mass spectrometry (DEMS) using a thin-layer flow-through electrochemical interface was used for simultaneous on-line studies of the dependence of mass spectra and Faradaic current on the electrode potential during electrocatalytic reduction of copper(II) ions by formaldehyde. The feasibility of the thin-layer flow-through DEMS cell for investigating electrode processes involving metal phase formation was demonstrated. The method permits to obtain the instantaneous values of partial reactions of the catalytic process.

Amperometric detection at a carbon-fibre array ring/glassy-carbon disk electrode in a wall-jet cell

A wall-jet cell incorporating a carbon fibre array ring/glassy-carbon disk electrode has been constructed, and characterized by the cyclic voltammetry and flow-injection techniques. The ring (composed of several microdisks) and glassy-carbon disk electrode, can be used separately for different purposes, e.g., detection in solution without a supporting electrolyte, collection/shielding detection with dual-electrode and voltammetric/amperometric detection with series dual-electrode. The electrode shows better collection and shielding effects than the usual ring—disk electrode in quiescent solution and the series dual-electrode in a thin-layer flow-through cell. The detection limit at the ring electrode is comparable with that at a conventional-size electrode, and has been used in the mobile phase without a supporting electrolyte, proving to be a promising detector for normal-phase liquid chromatography.