Direct analysis in real time (DART) is an open atmospheric pressure soft ionization method which was originally developed for immediate qualitative analysis of a wide range of compounds without sample preparation. In its classical form, DART finds applications in forensic science, pharmaceutical industry, food industry, materials analysis, environmental analysis etc. We designed a novel, customized inlet to make the DART - time-of-flight mass spectrometer (DART-TOF-MS) suitable for analysis of liquids in real time. When combined with an electrochemical flow reactor, the inlet-modified DART-TOF-MS is able to characterize the liquid products of electrochemical reactions in real time with excellent temporal and potential resolution. We operate a JMS-T100LP AccuTOF LC-plus 4G mass spectrometer (JEOL), equipped with an IonSense DART-SVP ion source. The time-of-flight mass analyzer has a resolving power of ≥ 10,000 measured at nominal m/z=609 according to FWHM definition.
Proton-transfer-reaction mass spectrometry was originally designed for the quantitative analysis of volatile organic compounds (VOCs) in the atmosphere or breath, with fast response times and low detection limits. We modified the inlet of a Proton-transfer-reaction - time-of-flight mass spectrometer (PTR-TOF-MS) to enable the characterization of liquids in real time and thereby the analysis of liquid products of electrochemical reactions (e.g. alcohols, aldehydes, ketones, organic acids, etc.) when the mass spectrometer is coupled to an electrochemical flow cell. We use an IONICON PTR-TOF-MS 6000 X2, which is mainly operated with H3O+, but allows the use of other primary ions, such as O2+, NO+, Kr+ or Xe+, when the detection compounds is otherwise very challenging.
Electron ionization quadrupole mass spectrometers (EI-QMS) are used to analyze gases in real time. Similar to classical MIMS or DEMS approaches, we extract gases dissolved in the electrolyte, using a hydrophobic PTFE membrane. The m/z resolution offered by our quadrupole mass analyzers is sufficient to distinguish gas products formed during most of the electrochemical reactions. The gas analyzers are essential components of EC-RTMS. We operate three EI-QMS instruments: (i) a Pfeiffer Omnistar GSD 320 (for mass range m/z: 1-100), (ii) a Hiden QGA (for mass range m/z: 1-100), and (iii) an Extrel MAX300-LG (for mass range m/z: 1-250).
Gas chromatography (GC)
Gaseous products formed during steady-state electrolysis experiments are analyzed with online gas chromatography. The gas outlet of the electrochemical reactor is connected with the sample loop of the GC and the instrument is automated to perform sample injections periodically. We use a PerkinElmer Clarus 580 chromatograph equipped with two detectors in series, a thermal conductivity detector (TCD) and a flame ionization detector (FID) with a methanizer. The instrument is set up in the ARNL3878 configuration, modified model 4016, which enables the separation and detection of H2, CO, CO2, CH4, C2H4, N2 and O2 in a temperature-controlled program that lasts 13 minutes.
Gas chromatography mass spectrometry (GC-MS)
GC-MS is used for offline quantitative analysis of volatile organic compounds (alcohols, aldehydes, ketones, etc.) produced after steady-state electrolysis experiments. We operate a PerkinElmer Clarus 580 gas chromatograph, coupled to a Clarus SQ 8 T mass spectrometer with electron ionization (70 eV) and a quadrupole mass analyzer. To enable the analysis of solutions that contain dissolved salts as electrolytes, a thermostated PerkinElmer TurboMatrix 40 headspace autosampler is used, so the sample is heated in a crimped vial and after the establishment of the gas-liquid equilibrium, the autosampler withdraws an aliquot from the vapor. The instrument is additionally equipped with a flame ionization detector (FID), which can be used for the detection of compounds when the analysis with mass spectrometry is not necessary.
High performance liquid chromatography (HPLC)
High performance liquid chromatography (HPLC) is used for the offline separation and quantification of liquid reaction products from steady-state electrochemical measurements. We use an Agilent 1260 Infinity II, equipped with an autosampler, a column oven, a fraction collector, and two different detectors: a UV-Vis diode array detector (DAD) for light absorbing species and a refractive index detector (RID), which is sensitive to all species as long as their refractive index differs from that of the eluent. The HPLC can be operated with different eluents and columns for the separation and analysis of a broad variety of compounds.
Ion Chromatography (IC)
Ion chromatography is used for offline quantitative analysis of anionic and cationic species (e.g. carboxylate ions, ammonium, etc) produced after steady-state electrolysis experiments. We use a ThermoScientific dual-channel Dionex ICS-5000+DC, equipped with electrochemical and chemical detectors, electrolytic eluent generator and carbonate removal device. The two channels can be operated independently.
We use a Gilson GX-271 liquid handler for fully automated sample withdrawal and preparation, during steady-state electrolysis. The liquid handler is programmed to withdraw electrolyte aliquots of pre-defined volume from continuous flow electrolyzers at certain times, dispense the sample on appropriate vials, as well as to make dilutions with water of another solution of choice in case derivatization is needed, so the samples are ready for analysis without any user interference. The liquid handler has been customized to be compatible with vials for analyses with various methods, such as GC-MS, ion chromatography, HPLC and ICP/MS.
We use several Gamry Reference600 potentiostats, able to make measurements with all classical potentiostatic, potentiodynamic and galvanostatic methods, electrochemical impedance spectroscopy etc. Each potentiostat, together with other components of the respective electrochemical station, is controlled and synchronized via home-made LabVIEW-based software, which enables the creation of worklists for automated execution of experiments.
We use various home-made electrochemical cells for different applications. For real-time investigations, we use electrochemical flow cells in which a capillary is positioned close to the electrode for continuous electrolyte withdrawal with high collection efficiency. The scanning flow cell (SFC) is a sophisticated electrochemical flow micro-cell which allows spatially-resolved investigations on selected locations of a sample. The spatial resolution offered by the SFC is appropriate to investigate material libraries, i.e. inhomogeneous samples with gradient material composition along one axis (binary alloys) or along two axes (ternary alloys). By scanning different sample locations, information for different electrode compositions can be obtained, using one sample only. For fundamental investigations, we use three-electrode electrochemical cells which allow investigations with the rotating (ring-) disk electrode and bead-type single-crystal electrodes. For long-term electrolysis, we use a three-electrode, two compartment electrolyzer which can be coupled to online gas chromatography and can operate with continuous flow of gases or of the electrolyte. The electrolyzer is compatible with a wide range of electrodes (metal foils, nanostructured catalysts, sputtered films) and has been designed to ensure high surface-to-volume ratio, decreased electrolyte resistance, elimination of contaminations from the counter electrode or cell components etc.
Rotating (ring-) disc electrodes
The rotating (ring-) disc electrode method is ideal for electrochemical measurements under well-defined mass transport conditions. For rotation control, we use PINE MSR rotating electrodes, compatible with rotating disk and rotating ring-disk electrodes. Using custom-made electrode holders or shafts, measurements are possible in the hanging meniscus configuration, as well as with single-crystal electrodes, flat samples, high-surface-area catalysts etc.