Mass spectrometric methods for monitoring redox processes in electrochemical cells

Abstract

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation–reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

Optimization of reaction parameters for the electrochemical oxidation of lidocaine with a Design of Experiments approach

“…In addition, on-line EC-MS limits the range of parameters, such as solvent, pH and electrolytes, since a compromise between MS and EC conditions must be found. The above-mentioned problems can be avoided by working off-line so that EC parameters can be optimized independent of the MS conditions [15,[27][28][29][30][31][32].…”

Adduct formation of electrochemically generated reactive intermediates with biomolecules

“…By using inductivelycoupled plasma MS (ICP-MS), quantitative data for heteroatomcontaining oxidation products can be obtained, since ICP-MS is an element-specific, matrix-independent method. In recent reviews, the developments and various applications in coupling EC to different ionization methods were summarized with the focus on ionization methods for molecular MS [1,2].…”

Paper-Based Electrochemical Cell Coupled to Mass Spectrometry

“…In ESI, EC processes occur at the solution-capillary interface ('solution' in this context refers to the solvent carrying analytes and reactants through the EC cell and ESI emitter) [24][25][26][27][28][29][30][31], which are influenced by the ESI current, capillary material, and composition of the solution. As a result of this inherent complexity in ESI, a distinct EC cell is desired to enable independent physical, electrical, and chemical control [32][33][34][35]. Two-electrode [34,[36][37][38][39][40][41] and three-electrode [33,35,[42][43][44] EC/ESI-MS configurations have been developed that achieve electrical decoupling through (a) floating the EC cell on the potential induced by the ESI high voltage or (b) electrically isolating the electronic circuits.…”

“…Two-electrode [34,[36][37][38][39][40][41] and three-electrode [33,35,[42][43][44] EC/ESI-MS configurations have been developed that achieve electrical decoupling through (a) floating the EC cell on the potential induced by the ESI high voltage or (b) electrically isolating the electronic circuits. An interesting example is an integrated three-electrode EC/ESI-MS device developed by Cole and coworkers [33,35,42] in which solution interacts with the EC working electrode a few millimeters before the end of the ESI source, leading to shorter response times (t r < 3 s; time between EC conversion and detection at three times the signal-to-noise (S/ N)) compared to typical configurations that have tubing connecting the EC cell and ESI emitter (t r is determined by tubing length, tubing internal diameter, and solution flow rate).…”