David M. Wayne

Durability of PEFCs at high humidity conditions

This work addresses issues of long-term durability of hydrogen-air polymer electrolyte fuel cells (PEFCs). The chromium in a Pt 3 Cr binary alloy catalyst has been found to migrate from cathode to anode during the course of life testing when operating within the oversaturated, or high-humidity, gas feed regime (one or both inlet feeds with a dew point equal to or higher than cell operating temperature) above 1 A/cm 2 current density. Other major factors such as membrane degradation, dissolution of catalyst-layer recast ionomer, catalyst oxidation, and catalyst agglomeration/growth have been identified as simultaneous, gradual processes that can lead to long-term PEFC failure. In situ cyclic voltammetry measurement of electrochemically active catalyst surface area shows a continuous decrease, revealing that catalyst agglomeration and/or growth may be a major cause of membrane electrode assembly degradation during middle-term life tests (i.e., operation times up to about 2000 h) under high-humidity conditions. Membrane and/or recast ionomer degradation was confirmed by the presence of fluoride and sulfate anions in the cathode outlet water. Scanning and transmission electron microscopy observation of a tested MEA suggest the loss of carbon-supported catalyst clusters and possible dissolution of recast Nafion ionomer.

Analysis of Water Ice and Water Ice/Soil Mixtures Using Laser-Induced Breakdown Spectroscopy: Application to Mars Polar Exploration

Recently, laser-induced breakdown spectroscopy (LIBS) has been developed for the elemental analysis of geological samples for application to space exploration. There is also interest in using the technique for the analysis of water ice and ice/dust mixtures located at the Mars polar regions. The application is a compact instrument for a lander or rover to the Martian poles to interrogate stratified layers of ice and dusts that contain a record of past geologic history, believed to date back several million years. Here we present results of a study of the use of LIBS for the analysis of water ice and ice/dust mixtures in situ and at short stand-off distances (<6.5 m) using experimental parameters appropriate for a compact instrument. Characteristics of LIBS spectra of water ice, ice/soil mixtures, element detection limits, and the ability to ablate through ice samples to monitor subsurface dust deposits are discussed.

Direct determination of trace noble metals (palladium, platinum and rhodium) in automobile catalysts by glow discharge mass spectrometry

Most automobile catalysts consist of a CeO2- and Al2O3-rich washcoat deposited on a porous, honeycombed cordierite support block. The active ingredients in the automobile catalyst are several noble metals (usually Pd, Rh and Pt) contained in the washcoat. Rapid, precise and accurate quantification of noble metals in automobile catalysts is desirable for noble metal tracking and accountability. Determination of Pd, Rh and Pt in the catalysts by ID–ICP-MS is possible only after extensive treatment with highly corrosive acids in sealed quartz tubes at high temperatures. By contrast, GDMS permits the direct determination of noble metals in the catalysts without prior dissolution. Although the auto catalyst powders are non-conducting, they may be combined with a metallic binder and analysed. Aluminium powder was chosen as the binder material for its superior gettering abilities, and because the Al mass spectrum in the vicinity of the Pd, Rh and Pt peaks was clear of interferences. Results from the GDMS analysis of noble metals in NIST Standard Reference Material 2557 Used Auto Catalyst (monolith) are precise and accurate to <10% RSD for Pd, Rh and Pt. The current generation of auto catalyst materials also contain significant amounts (≈0.3% by mass) of ZrO2, which causes isobaric (ZrO)+ interference on m/z 106, 107, 108, 110 and 112. A simple mathematical peak-stripping routine was devised to eliminate the (ZrO)+ interference.

Glow discharge source interfacing to mass analyzers: theoretical and practical considerations.

The fundamental requirements for the optimum mechanical interface between a glow discharge ion source and a mass spectrometer are described in this paper. Specifically, the properties of a typical glow discharge ion source are compared and contrasted to those of a typical inductively coupled plasma ion source. The critical parameters and theoretical considerations of glow discharge and inductively coupled plasma ion source interfaces are reviewed, and the results of experiments using both quadrupole and time-of-flight mass spectrometers with a glow discharge source are presented. The experimental results clarify several important problems in the glow discharge ion sampling process. Our findings indicate that a shock wave structure does not occur in the supersonic expansion of the glow discharge ion source. Ions of different masses have similar initial kinetic energies in the glow discharge; thus, the angle of the skimmer cone is not a critical parameter for efficient ion beam extraction. Another consquence is that space charge effects in glow discharge ion sources repel heavy ions farther off axis than light ions. Thus, there are distinct and fundamental differences between glow discharge and inductively coupled plasma ion sources which are relevant to both ion sampling and ion extraction processes.

The thermal ionization cavity (TIC) source: elucidation of possible mechanisms for enhanced ionization efficiency

We discuss the ionization efficiency performance of a thermal ionization cavity source which has been adapted for use in an orthogonal acceleration linear thermal ionization cavity time-of-flight mass spectrometer (TIC-TOFMS). Our results indicate that the maximum ionization efficiency of thorium, a notoriously difficult element to ionize thermally, in the present TIC-TOFMS system is about 1–3%. This represents a 10–30× enhancement of the typical Th sample utilization for conventional, Re filament-based, thermal ionization mass spectrometry sources. Thorium samples were small (0.075–25 ng), therefore, analytical blanks for Th in the tungsten TIC were also measured and subtracted from the total Th ion signal arriving at the detector. Prominent peaks that correspond to multiply-charged (2 + and 3 + ) tungsten ions are always evident in high-temperature (>2400 ◦ C) spectra. Furthermore, TIC-TOFMS spectra also contained peaks corresponding to elements with ionization potentials in excess of 8 eV, such as beryllium. The presence of multiply-charged tungsten peaks, and beryllium peaks, are indicative of significant ionization via non-thermal processes, such as electron impact, in or near the TIC. (Int J Mass Spectrom 216 (2002) 41–57)

Characterization of a Pulsed Glow Discharge Laser Ablation System Using Optical Emission

Investigations involving laser-based sampling of copper into an auxiliary pulsed glow discharge for ionization and excitation are presented. The interaction of the ablated copper with the auxiliary glow discharge was studied by monitoring the copper atom emission signal at 368.744 nm. Results demonstrate the ability to time ablation appropriately to access specific temporal regions of the pulsed plasma. More specifically, laser-ablated material was introduced into the glow discharge negative glow during the afterpeak. Ionization and excitation was accomplished by collisions with a metastable argon population produced by the glow discharge (Penning ionization) followed by recombination to yield excited-state Cu atoms. The work presented investigates parameters that affect the atomic emission signal intensity of the ablated material, including cathode-to-target distance, discharge gas pressure, and relative timing of discharge and ablation. Results demonstrate that decreasing the glow discharge working gas pressure increases the transport efficiency of laser-ablated material into the negative glow. These investigations are part of an ongoing series of studies on sample introduction schemes that utilize different ionization and excitation mechanisms found in pulsed glow discharge plasmas.

Characterization of an improved thermal ionization cavity source for mass spectrometry

A new thermal ionization source for use with a quadrupole mass spectrometer has been designed and characterized. The new source provides significant advantages over the previously reported prototype source and traditional filament-type thermal ionization sources. The operational interface between the source and the quadrupole mass spectrometer has been redesigned. A vacuum interlock, a translational stage, and an adjustable insertion probe are added to improve the source performance. With these modifications, the source is easier to operate while maximizing sample throughput. In this work, the performance of the newly developed source is examined. The ionization efficiencies are measured with a quadrupole mass spectrometer. The efficiency values obtained with this system are comparable to those obtained from a large scale isotope separator. The relationships among the ionization potential, vapor pressure, and measured ionization efficiency results are discussed. The crucible lifetime has been quantitatively estimated by measuring the crucible sputtering rate. Diagnostic studies of the new source show that the crucible position is a crucial parameter for sensitivity and performance. Stability tests demonstrate that the source can be run several weeks at a fixed emission current without significant degradation.

Dating fluid flow in shear zones: Rb-Sr and U-Pb studies of syntectonic veins in the Néouvielle Massif, Pyrenees

Abstract Rb-Sr and Pb-Pb data are reported from separated minerals (amphibole, albite, calcite, epidote), altered wall rocks, and fluid inclusions trapped in quartz, from veins associated with retrogressive shear zones cutting the Néouvielle granodiorite massif in the central French Pyrenees. Alteration in the shear zones was caused by movement of hypersaline brines during deformation. Shear zones with similar kinematics occur in granodiorites and high grade gneisses throughout the northern Axial Zone of the Pyrenees, and have been attributed variously to both the Variscan and Alpine orogenies. Rb-Sr data on a syntectonic, polymineralic vein and its altered wall rock support an Alpine age for deformation and fluid flow, giving an isochron of 47 ± 8 Ma, or 48 ± 2 Ma if albite is excluded. Another vein gives a similar age for altered wall rock and albite, but chlorite and quartz-hosted fluid inclusions are out of equilibrium. Pb data plot as quasi-linear arrays on ratio plots, with no age significance. The usefulness of this latter technique is restricted by the slow rate of uranium decay in these young rocks, but in any case Pb isotopic ratios do not appear to have homogenized during vein formation.

A linear time-of-flight mass analyzer for thermal ionization cavity mass spectrometry ☆

Abstract In this paper we report the basic design characteristics, typical operating parameters, and isotope ratio performance of an orthogonal acceleration linear thermal ionization cavity time-of-flight mass spectrometer (TIC-TOFMS). The present system is capable of mass resolution of 750–850 (FWHM) over a wide range of masses, and can generate and analyze multi-element spectra from sub-μg samples (solids and solution residues) in

Selective removal of matrix ion peaks in plasma source time-of-flight mass spectrometry: ion deflection and detector gating

A significant problem in plasma source time-of-flight mass spectrometry (TOFMS) is the saturation of detectors due to plasma gas and matrix ions. This paper outlines two experimental procedures to minimize the detector response to large ion currents in a glow discharge time-of-flight mass spectrometer. One novel solution is a deflection device that includes two parallel plates for ion ejection and two sleeve plates (after the deflection plates) to reduce ion and neutral scattering. Another approach is to gate the detector, by lowering the voltage on the microchannel plates, when an intense ion pack is expected. Both approaches are efficient in eliminating the plasma gas peaks and matrix ions in plasma source time-of-flight mass spectrometry. The matrix ion intensity can be reduced by several orders of magnitude, with minimal influence on the analyte ions. The detector gating technique requires a high peak current pulse with a flexible rise time. However, this approach will cause an electrical ringing to be superimposed on the analytical signal. Therefore, ion deflection is the method of choice for plasma TOFMS.