Real Time Characterization of Catalysis by On-Line Electrochemistry/Mass Spectrometry. Investigation of Quinone Electrocatalysis of Amine Oxidation

Integrating brucine with carbon nanotubes toward electrochemical sensing of hydroxylamine

Based on the intrinsic electrochemical features of brucine integrated with carbon nanotubes (brucine/SWNTs), dimeric quinoid brucine was electrochemically generated by electroactivation of a brucine/SWNTs-modified GC electrode and used as a novel electrocatalyst for efficient electro-oxidation of hydroxylamine (HA). The electrocatalytic activity was investigated with cyclic voltammetry in the range pH 2.0 to pH 11.0, and the best electrocatalytic performance of the electrocatalyst was obtained at pH 10.0. By taking advantage of the electrocatalytic activity of the dimeric quinoid brucine toward HA, we have developed an electrochemical sensor for HA measurements based on a brucine/SWNTS-modified GC electrode using amperometry with the applied potential of + 0.1 V (vs. Ag/AgCl). Under the optimized conditions, the current response toward HA concentration shows a linear relationship in the dynamic ranges of 0.1-10 μM and 10-1000 μM with a detection limit of 0.021 μM based on the 3σ criterion. The sensor was used to assay HA in pharmaceuticals including hydroxyurea tablets and pralidoxime iodide injections with satisfactory results. The spike-and-recovery for samples of tap water (n = 9) and lake water (n = 9) was within 97.17-100.16%. Graphical abstract Schematic illustration of electrochemical sensing of hydroxylamine (HA) enabled by integrating brucine with single-walled carbon nanotube (brucine/SWNTs) based on electro-activation of brucine/SWNTs-modified GC electrode.

Perspective on electrospray ionization and its relation to electrochemistry

The phenomenon of electrospraying of liquids is presented from the perspective of the electrochemistry involved. Basics of current and liquid flow in the capillary and spray tip are discussed, followed by specifics of charging and discharging of the sprayed liquid surface. Fundamental theories and numerical modeling relating electrospray current to solution and spray parameters are described and then compared with our own experimentally obtained data. The method of mapping potentials and currents inside the electrospray capillary by using an inserted electrically-isolated small wire probe electrode is discussed in detail with illustrations from new and published data. Based on these experimentally obtained results, a new mathematical model is derived. The introduced "nonlinear resistor electrospray capillary model" divides the electrospray capillary into small sections, adds their contributions, and then, by transition to infinitely small section thickness, produces analytical formulas that relate current and potential maps to other properties of the electrospraying liquid: primarily conductivity and current density. The presentation of the model is undertaken from an elementary standpoint, and it offers the possibility to obtain quantitative information regarding operating parameters from typical analytical systems subjected to electrospray. The model stresses simplicity and ease of use; examples applying experimental data are shown and some predictions of the model are also presented. The developed nonlinear resistor electrospray capillary model is intended to provide a new quantitative basis for improving the understanding of electrochemical transformations occurring in the electrospray emitter. A supplemental material section gives full derivation of the model and discusses other consequences.

Electrochemical/electrospray mass spectrometric studies of I- and SCN- at gold and platinum electrodes: direct detection of (SCN)3-

Results on the electrochemistry of I- and SCN- at gold and platinum electrodes using an electrochemical cell coupled to an electrospray mass spectrometer are reported. We demonstrate that our apparatus is capable of these very challenging electrochemical/electrospray experiments and that B(C6H5)4- is a suitable internal standard for negative-ion studies in acetonitrile. With I- at a platinum electrode, we observe well-behaved oxidation to I3-. Experiments on I- at gold electrodes are more complex, showing AuI2- as well as I3-. The AuI2- mass spectrometric ion intensity varies in a complex way throughout the applied electrochemical voltage range studied; we propose that this variation involves the adsorption of I- on the gold electrode surface. In experiments on SCN- from (C4H9)4NSCN at gold electrodes, we observe Au(SCN)2-. Finally, at platinum electrodes, we directly observe (SCN)3-, a species analogous to I3- and (CN)3- that has been previously postulated but unverified. This important finding was confirmed by the isotope pattern and demonstrates the stability of the anion.

Product analysis of caffeic acid oxidation by on-line electrochemistry/electrospray ionization mass spectrometry

On-line electrochemistry/electrospray ionization mass spectrometry (EC/ESI-MS) was developed using a microflow electrolytic cell. This technique was applied to electrochemical oxidation of caffeic acid (CAF) which is known to be a highly antioxidative agent. Effects of electrolytic potentials on ion intensities of product ions and on electrolytic currents were examined at different pHs. Dimer products were detected at electrolytic potentials of E = 0.7 V (vs. Ag/AgCl) and trimer products at 1.0 V at pH 9. Dimer products were distinguished from hydrogen-bonded complexes by MS/MS experiments. Hydrogen/deuterium exchange experiments determined the number of hydroxyl and carboxyl groups in the Dimers formed by electrolysis. The mechanism of oxidative polymerization of CAF is discussed with speculation as to the structure of the dimer product.

A Setup for the Coupling of a Thin-Layer Electrochemical Flow Cell to Electrospray Mass Spectrometry

A novel setup for the coupling of a commercially available thin-layer cell to electrospray mass spectrometry (ESI-MS) which allows the electrochemical reactions at the counter electrode to be straightforwardly separated from the flow into the mass spectrometer has been developed. In this way, interferences from reaction products formed at the counter electrode can be minimized. This reduces the risk of changes in the mass spectra as a result of electrochemical reactions in the solution. The described setup also enables the working electrode to be positioned close to the electrospray (ESI) emitter without the need for a grounding point or a long transfer line between the electrochemical cell and the electrospray emitter. By decoupling the electrochemical reactions in the flow cell and those in the electrospray emitter, improved facilities for studies of electrochemical reactions are obtained through a better control of the potential of the working electrode. The setup has been used to study the oxidation of a drug (Olsalazine), which previously has been found to involve chemical follow-up reactions. It is also demonstrated that uncharged thiols can be detected in ESI-MS after spontaneous adsorption on a gold working electrode, followed by oxidative desorption to yield sulfinates or sulfonates. This adsorption and potential-controlled desorption has been used for the preconcentration of micromolar concentrations of 1-hexanethiol as well as for desalting of solutions containing micromolar concentrations of thiols. The results indicate that the present on-line coupling of an electrochemical cell to ESI-MS provides promising possibilities for sample preconcentration, matrix exchange (including desalting), and ionization of neutral compounds, such as thiols.

On-line investigation of the generation of nonaqueous intermediate radical cations by electrochemistry/mass spectrometry

Anodic oxidation of triphenylamine (TPA) in acetonitrile was investigated by electrochemistry (EC) combined online with mass spectrometry (MS) through a particle beam (PB) interface, EC/PB/MS. Electrooxidation of TPA generates a TPA*+ radical cation (m/z 245) which dimerizes to tetraphenylbenzidine (TPB, MW 488). TPB is readily oxidized to TPB*+ (m/z 488) and TPB2+ (m/z 244) at the oxidation potential of TPA. In EC/PB/MS, direct monitoring of the oxidation of TPB to TPB*+ radical cation as a function of the electrode potential was achieved via selective ion monitoring of the ion peak at m/z 488. By using the relative intensity ratio of ions at m/z 244 (TPB2+) to 245 (TPA*+), the formation of TPB2+ as a function of the electrode potential was also monitored. EC/PB/MS showed a maximum rate of formation of TPB*+ at +1.2 Vvs Pd, while TPB2+ is generated at a maximum rate at +1.6 V vs Pd. The effect of spectral interference from the electron impact ionization of TPA, on EC/PB/MS results, is also discussed. Finally, a significant signal enhancement is observed in the presence of tetrabutylammonium perchlorate (TBAP) and is reported for the first time. Compatibility of coupling of EC with MS via PB interface for EC/MS studies in nonaqueous solvents is demonstrated. The observation of significant signal enhancement in the presence of TBAP may facilitate other applications of LC/MS.