Howard D. Dewald

Online Coupling of Electrochemical Reactions with Liquid Sample Desorption Electrospray Ionization-Mass Spectrometry

The combination of electrochemistry (EC) and mass spectrometry (MS) is a powerful analytical tool to study redox reactions. This work reports the online coupling of a thin-layer electrochemical flow cell with liquid sample desorption electrospray ionization mass spectrometry (DESI-MS) and its applications in investigating various electrochemical reactions of biological molecules such as oxidative formation and reductive cleavage of disulfide bonds and online derivatization of peptides/proteins. As a result of the direct sampling nature of DESI, several useful features of such a coupling have been found, including simple instrumentation, fast response time (e.g., 3.6 s in the case of dopamine oxidation), freedom to choose a favorable ionization mode of DESI or traditional electrolysis solvent systems, and the absence of background signal possibly resulting from ionization when the cell is off (e.g., in the case of dopamine oxidation). More importantly, with the use of this new coupling apparatus, three disulfide bonds of insulin were fully cleaved by electrolytic reduction and both the A and B chains of the protein were successfully detected online by DESI-MS. In addition, online tagging of free cysteine residues of peptides/proteins employing electrogenerated dopamine o-quinone can be performed. These revealed characteristics of the coupling along with examined electrochemical reactions suggest that EC/DESI-MS has good potential in bioanalysis.

Online mass spectrometric analysis of proteins/peptides following electrolytic cleavage of disulfide bonds.

The disulfide bond bridge is an important post-translational modification for proteins. This study presents a structural analysis of biologically active peptides and proteins containing disulfide bonds using electrochemistry (EC) online combined with desorption electrospray ionization mass spectrometry (DESI-MS), in which the sample undergoes electrolytic disulfide cleavage in an electrochemical flow cell followed by MS detection. Using this EC/DESI-MS method, the disulfide-containing peptides can be quickly identified from enzymatic digestion mixtures, simply based on the abrupt decrease in their relative ion abundances after electrolysis. Peptide mass mapping and tandem MS analysis of the ions of the resulting free peptide chains can possibly establish the disulfide linkage pattern and sequence the precursor peptides. In this regard, the method provides much more chemical information than previous analogous electrochemical analyses. In addition, derivatization of thiols by selective selenamide reagents is useful for easy recognition of reduced peptide ions and the number of their free thiols. Furthermore, electrolytic reduction of proteins (e.g., α-lactalbumin) leads to increased charges on the detected protein ions, revealing the role of disulfide bonds on maintaining protein conformation. This electrochemical mass spectrometric method is fast (completed in few minutes) and does not need chemical reductants, potentially having valuable applications in proteomics research.

Electrochemistry-Assisted Top-Down Characterization of Disulfide-Containing Proteins

Covalent disulfide bond linkage in a protein represents an important challenge for mass spectrometry (MS)-based top-down protein structure analysis as it reduces the backbone cleavage efficiency for MS/MS dissociation. This study presents a strategy for solving this critical issue via integrating electrochemistry (EC) online with a top-down MS approach. In this approach, proteins undergo electrolytic reduction in an electrochemical cell to break disulfide bonds and then undergo online ionization into gaseous ions for analysis by electron-capture dissociation (ECD) and collision-induced dissociation (CID). The electrochemical reduction of proteins allows one to remove disulfide bond constraints and also leads to increased charge numbers of the resulting protein ions. As a result, sequence coverage was significantly enhanced, as exemplified by β-lactoglobulin A (24 vs 75 backbone cleavages before and after electrolytic reduction, respectively) and lysozyme (5 vs 66 backbone cleavages before and after electrolytic reduction, respectively). This methodology is fast and does not need chemical reductants, which would have an important impact in high-throughput proteomics research.

Anodic stripping voltammetry of heavy metals with a flow injection system

The design and operation of an anodic stripping voltammetric system based on the flow injection technique are described. A flow cell with a wall-jet glassy carbon disk electrode and a 500-μl sample volume are employed. The system allows trace metals at the μg l-1 level to be quantified simultaneusly at a rate of ten samples per hour. A low differential pulse background current allows a detection limit for lead of about 3 × 10-9 M (0.3 ng) with a 3-min deposition time. Deposition is done in the presence of oxygen in the sample solution, but the carrier solution is oxygen-free.

Potential scanning voltammetric detection for flow injection systems

Abstract A flow cell with a stationary carbon disk electrode coupled with rapid differential pulse voltammetry is used. The voltammograms are recorded at a scan rate of 2 V min −1 during passage of the 200-μl sample plug through the detector. This detection mode allows several electroactive species to be quantified simultaneously at an injection rate of 20 samples per hour. Acetaminophen, dopamine, caffeic acid, chlorpromazine, and copper, lead, and cadmium ions are used as test species.

Study of electrochemical reactions using nanospray desorption electrospray ionization mass spectrometry.

The combination of electrochemistry (EC) and mass spectrometry (MS) is a powerful analytical tool for studying mechanisms of redox reactions, identification of products and intermediates, and online derivatization/recognition of analytes. This work reports a new coupling interface for EC/MS by employing nanospray desorption electrospray ionization, a recently developed ambient ionization method. We demonstrate online coupling of nanospray desorption electrospray ionization MS with a traditional electrochemical flow cell, in which the electrolyzed solution emanating from the cell is ionized by nanospray desorption electrospray ionization for MS analysis. Furthermore, we show first coupling of nanospray desorption electrospray ionization MS with an interdigitated array (IDA) electrode enabling chemical analysis of electrolyzed samples directly from electrode surfaces. Because of its inherent sensitivity, nanospray desorption electrospray ionization enables chemical analysis of small volumes and concentrations of sample solution. Specifically, good-quality signal of dopamine and its oxidized form, dopamine o-quinone, was obtained using 10 μL of 1 μM solution of dopamine on the IDA. Oxidation of dopamine, reduction of benzodiazepines, and electrochemical derivatization of thiol groups were used to demonstrate the performance of the technique. Our results show the potential of nanospray desorption electrospray ionization as a novel interface for electrochemical mass spectrometry research.

Electrochemical Stability of Copper in Lithium‐Ion Battery Electrolytes

The electrochemical stability of copper substrate was studied in three different lithium-ion battery electrolytes. Cyclic voltammetry was used to study the oxidation-reduction behavior of copper in these electrolyte solutions. The reduction of electrolyte and its effect on the oxidation of copper was also studied. Bulk electrolysis was used to quantitatively study the dissolution of copper in dry electrolytes and in electrolytes doped with impurities of H 2 O or HF. The stability of copper was closely related to the composition of the electrolytes. Impurities dramatically increased the oxidation tendency of copper.

Differential pulse anodic stripping voltammetry of lead and antimony in gunshot residues

Abstract Differential pulse anodic stripping voltammetry (DPASV) using a glassy carbon/mercury film electrode (GC/MFE) has been applied for characterization and quantitative detection of gunshot residues (GSR). This technique allows for simultaneous detection of Pb and Sb from GSR hand swabs that is simple, fast, and non-destructive. The use of a KNO 3 /hydrazine sulfate matrix, instead of HCl, reduces possible interferences and allows for a small sample size to be used in analysis but yielding greater sensitivity.

Theoretical and experimental aspects of the response of stripping voltammetry in flow injection systems

Abstract The coupling of stripping voltammetry with flow injection systems offers significant advantages over conventional stripping methods. Equations solved for conventional stripping voltammetry, with steady-state deposition current, are not applicable to flow injection systems. Theoretical equations for the peak current under flow injection conditions are derived. As the total charge passed during the deposition step, ∫i dt, is independent of the shape of the sample plug, the stripping peak current is independent of the degree of dispersion. As a result, precise control of the dispersion or deposition period may not be required. The peak current is predicted to be directly proportional to the sample volume. Experimental results are incorporated to support the theoretical conclusions. The effects of experimental variables such as flow rate, length of tubing, deposition period, or sample volume are presented using cadmium ion as test species.

Periodic and Chaotic Current Oscillations at a Copper Electrode in an Acetate Electrolyte

Periodic and chaotic current oscillations are observed during electrodissolution of copper in a pH 3.5 sodium acetate/ acetic acid buffer under potentiostatic conditions using a rotating copper disk electrode. Periodic or chaotic oscillations are observed depending on the applied potential and electrode rotation rate. The oscillations arise after the formation and dissolution of an acetate salt film precursor to oxide passivation. The nature and composition of the surface films were examined using scanning electron microscopy and x‐ray powder diffraction data. Nonlinear dynamic analysis methods have been used to show the existence of deterministic chaos. Time series data of the chaotic oscillations were used to generate a return map that appears to be one‐dimensional. The chaotic attractor was reconstructed in a three‐dimensional state space using the method of time delays and the largest Lyapunov exponent was calculated and was positive for the chaotic oscillations indicating a sensitive dependence on initial conditions characteristic of deterministic chaos.