HERFD XAS/ATR-FTIR batch reactor cell

An autoclave reactor was modified to perform simultaneously high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy measurements without altering the reactor design. This operando cell allows one to follow changes in the electronic and geometric structure of the catalyst with HERFD XAS and relate them to the simultaneously detected activity and reaction species with ATR-FTIR formed during the reaction in the liquid phase. The capability of the cell is demonstrated by two studies. The first shows the reduction of Au/CeO(2) precursors in different solvents. The second shows that mainly Au(0) is present in the hydrogenation of nitrobenzene over Au/CeO(2).     

New luminescence measurement facilities in retrospective dosimetry

This paper gives a review of recent developments in luminescence measurement facilities on the Risø TL/OSL reader including radio-luminescence (RL), exo-electron and violet stimulation attachments, and a method for characterising and if necessary correcting for beta irradiation source non-uniformity.We first describe improvements to the existing RL option to allow near infra-red detection (NIR) during irradiation by the built-in ⁹⁰Sr/⁹⁰Y beta source. The RL optical signal is collected by a liquid light guide through an F34-901 interference filter and detection is based on a dedicated thermoelectrically cooled NIR sensitive PMT (detection window peak at 855 nm, FWHM 27 nm). Software and electronics have been modified to allow standard TL and OSL measurements in the same sequence as RL measurements. Together with a new bleaching source based on a high-power UV LED (395 nm; 700 mW/cm²), this facility has been used to measure natural doses in feldspar using the decaying NIR RL signal.Secondly, we present a method for mapping radiation field of the built-in ⁹⁰Sr/⁹⁰Y β-source and estimating grain-location specific dose-rates. This is important for the accuracy of single grain results, when radiation field is spatially non-uniform across the sample area. We document the effect of this correction method and further investigate on the effect of lifting the source to achieve a better dose-rate uniformity.Finally we summarise two recently-developed novel facilities to help investigate (i) the time scales involved in OSL processes (time-resolved exo-electron detection) and (ii) extending the age range (violet stimulated signals from deep quartz OSL traps).     

Square-free perfect graphs

We prove that square-free perfect graphs are bipartite graphs or line graphs of bipartite graphs or have a 2-join or a star cutset.     

Universally signable graphs

In a graph, a chordless cycle of length greater than three is called a hole. Let be a {0, 1} vector whose entries are in one-to-one correspondence with the holes of a graphG. We characterize graphs for which, for all choices of the vector , we can pick a subsetF of the edge set ofG such that |F H| H (mod 2), for all holesH ofG and |F T| 1 for all trianglesT ofG. We call these graphsuniversally signable. The subsetF of edges is said to be labelledodd. All other edges are said to be labelledeven. Clearly graphs with no holes (triangulated graphs) are universally signable with a labelling of odd on all edges, for all choices of the vector . We give a decomposition theorem which leads to a good characterization of graphs that are universally signable. This is a generalization of a theorem due to Hajnal and Surányi [3] for triangulated graphs.This work was supported in part by NSF grants DMI-9424348 DMS-9509581 and ONR grant N00014-89-J-1063. Ajai Kapoor was also supported by a grant from Gruppo Nazionale Delle Ricerche-CNR. We also acknowledge the support of Laboratoire ARTEMIS, Université Joseph Fourier, Grenoble.     

Enrichment of rare-earth elements (REE) and Gd-DTPA in surface water samples by means of countercurrent chromatography (CCC)

An analytical scheme was developed for the determination of rare-earth elements and gadolinium diethylenetriaminepentaacetate (Gd-DTPA) in river water by ICP-MS. Since the concentration of Gd-DTPA and the rare-earth elements in river water is often lower than the limits of detection in quadrupole ICP-MS applying pneumatic nebulization, a preconcentration procedure is essential.In this work, the capabilities of countercurrent chromatography (CCC) for preconcentration of REE were investigated. For preconcentration ethylhexylphosphates as stationary phase had been used. Acidified aqueous samples (pH 2) and river water spiked with REE and Gd-DTPA were under study. The procedure was compared with solid phase extraction (SPE) using the same samples and ethylhexylphosphates as stationary phase. The recovery of the light and middle REE was found to be at about 100%, which was found to be more efficient than applying SPE. In contrast, the recovery rates for Yb and Lu were low (and poorly reproducible) using CCC (57% and 73%, respectively) while significant better results were obtained with SPE (89% and 84%, respectively).The recoveries of Gd applying Gd-DTPA were 80 ± 4% in the investigation of river water samples.     

Library screening for synthetic agonists and antagonists of a Pseudomonas aeruginosa autoinducer

The autoinducer (AI) that initiates the quorum sensing (QS) signaling cascade in Pseudomonas aeruginosa is an acyl-homoserine lactone (acyl-HSL). We initiated a study of the requirements for binding of the AI to its protein effector LasR by synthesizing a library of analogs with the HSL moiety replaced with different amines and alcohols. We tested each compound for both agonist and antagonist activity using a QS-controlled reporter gene assay and found several new agonists and antagonists. A representative antagonist was further tested for its ability to inhibit virulence factors. This data progresses our understanding of the LasR-AI interaction toward the rational design of therapeutic inhibitors of QS.     

Reactions of Bis(p-methoxyphenyl)trisulfane and Its Oxides with Dimethyldioxirane and (Trifluoromethyl)methyldioxirane

The reactions of dimethyldioxirane and (trifluoromethyl)methyldioxirane with bis(p-methoxyphenyl)trisulfane, its 1-oxide, its 2-oxide, and its 1,1-dioxide derivatives have been investigated. The reactions were followed by careful monitoring of the methoxy region of the (1)H NMR spectra and where possible by doping with authentic samples of the products. The decomposition of labile intermediates and products was investigated. A new mechanism for the rearrangement of a trisulfane 1,3-dioxide to a trisulfane 1,1-dioxide is proposed.