Volume 78, Issue 10, October 2007
Index of content:
78(2007); http://dx.doi.org/10.1063/1.2785156View Description Hide Description
A dual-sample ultrasonicdroplet ejector array is developed for use as a soft-ionization ion source for multiplexed mass spectrometry (MS). Such a multiplexed ion source aims to reduce MS analysis time for multiple analyte streams, as well as allow for the synchronized ejection of the sample(s) and an internal standard for quantitative results and mass calibration. Multiplexing is achieved at the device level by division of the fluid reservoir and separating the active electrodes of the piezoelectric transducer for isolated application of ultrasonicwave energy to each domain. The transducer is mechanically shaped to further reduce the acoustical crosstalk between the domains. Device design is performed using finite-element analysis simulations and supported by experimental characterization. Isolated ejection of diameter water droplets from individual domains in the micromachined droplet ejector array at around 1 MHz frequency is demonstrated by experiments. The proof-of-concept demonstration using a dual-sample device also shows potential for multiplexing with larger numbers of analytes.
Quantitative analysis of transient surface reactions on planar catalyst with time-resolved time-of-flight mass spectrometry78(2007); http://dx.doi.org/10.1063/1.2791950View Description Hide Description
A procedure for the quantitative analysis of transient surfacecatalyticreactions in millisecond time resolution has been studied constructing a specially designed apparatus employing (1) pulsed-gas valves for the injection of reactant molecules onto catalysts and (2) a time-of-flight mass spectrometer (TOF-MS) to detect every reaction product simultaneously. For a better understanding of the catalytic activity and selectivity for products quantitatively, a procedure for measuring an amount of reactant molecules injected onto catalyst surface and calibrating the intensity of mass signal were proposed and implemented. We tested the applicability of this procedure for the quantitative analysis of products of reaction on catalysts (a planar catalyst: substrates inserted into a micro-tube-reactor with SiC balls). Although the surface area of the planar catalyst was very small, the mass signal intensities of the reaction products were found to be sufficient for the above procedure. We measured the fragmentation patterns and the inherent sensitivity factors in the TOF-MS using the mixture of the internal standard gas Ar and the N-containing gases. The relative sensitivity factors for , , NO, and and the relative intensities of fragment peaks to the molecular ion peak of and were estimated. The procedure constructed here has enabled us to analyze the transient consecutive secondary catalyticreactions as well as primary reactions based on the formation rate of product molecules per millisecond instead of the mass signal intensities of the reaction products.
78(2007); http://dx.doi.org/10.1063/1.2791948View Description Hide Description
We describe an improved temporal analysis of products (TAP) reactor design whose main new features in comparison to the recent TAP-2 design of Gleaves et al. [Appl. Catal. A160, 55 (1997)] are the use of a turbomolecular pump, piezoelectrically driven pulse valves, and a newly designed, differentially pumped gate valve. The gate valve allows fast and simple changes between high pressure operation, in which in situ catalyst treatment can be performed, and the analytic mode with a direct line-of-sight connection to the analysischamber and the mass spectrometer. The heating system and pulse valves are located outside the vacuum chamber, resulting in a system that is easy to operate and modify. The high stability and reproducibility of the pulse intensity allows for direct, quantitative evaluation of single-pulse and multipulse experiments. The performance of the system is demonstrated using the CO oxidation over a catalyst as test reaction.
An advanced molecule-surface scattering instrument for study of vibrational energy transfer in gas-solid collisions78(2007); http://dx.doi.org/10.1063/1.2796149View Description Hide Description
We describe an advanced and highly sensitive instrument for quantum state-resolved molecule-surface energy transfer studies under ultrahigh vacuum (UHV) conditions. The apparatus includes a beamsourcechamber, two differential pumpingchambers, and a UHV chamber for surface preparation, surface characterization, and molecular beamscattering. Pulsed and collimated supersonic molecular beams are generated by expanding target molecule mixtures through a home-built pulsed nozzle, and excited quantum state-selected molecules were prepared via tunable, narrow-band laser overtone pumping. Detection systems have been designed to measure specific vibrational-rotational state, time-of-flight, angular and velocity distributions of molecular beams coming to and scattered off the surface. Facilities are provided to clean and characterize the surface under UHV conditions. Initial experiments on the scattering of from Au(111) show many advantages of this new instrument for fundamental studies of the energy transfer at the gas-surface interface.
78(2007); http://dx.doi.org/10.1063/1.2785028View Description Hide Description
The increasing use of micromechanical cantilevers in sensing applications causes a need for reliable readout techniques of micromechanical cantilever sensor (MCS)bending. Current optical beam deflection techniques suffer from drawbacks such as artifacts due to changes in the refraction index upon exchange of media. Here, an adaptation of the Fabry-Perot interferometer is presented that allows simultaneous determination of MCSbending and changes in the refraction index of media. Calibration of the instrument with liquids of known refraction index provides an avenue to direct measurement of bending with nanometer precision. Versatile construction of flow cells in combination with alignment features for substrate chips allows simultaneous measurement of two MCS situated either on the same, or on two different support chips. The performance of the instrument is demonstrate in several sensing applications, including adsorption experiments of alkanethioles on MCSgold surfaces, and measurement of humidity changes in air.