Photon channelling in foams

Experiments by Gittings, Bandyopadhyay and Durian (Europhys. Lett. 65, 414 (2004)) demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first derive a central relation in the work of Gitting et al. without any free parameters. It links the photon's path-length fraction f in the liquid phase to the liquid fraction ɛ. We then construct two-dimensional Voronoi foams, replace the cell edges by channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling only occurs in a finite range of ɛ.     

High precision delta(17)O isotope measurements of oxygen from silicates and other oxides: method and applications

The use of infrared laser-assisted fluorination to release oxygen from milligram quantities of silicates or other oxide mineral grains is a well-established technique. However, relatively few studies have reported the optimisation of this procedure for oxygen-17 isotope measurements. We describe here details of an analytical system using infrared (10 μm) laser-assisted fluorination, in conjunction with a dual inlet mass spectrometer of high resolving power ( approximately 250) to provide (17)O and (18)O oxygen isotope measurements from 0.5-2 mg of silicates or other oxide mineral grains. Respective precisions (1) of typically 0.08 and 0.04 per thousand are obtained for the complete analytical procedure. Departures from the mass-dependent oxygen isotope fractionation line are quantified by Delta(17)O; our precision (1) of such measurements on individual samples is shown to be +/-0.024 per thousand. In turn, this permits the offset between parallel, mass-dependent fractionation lines to be characterised to substantially greater precision than has been possible hitherto. Application of this system to investigate the (17)O versus (18)O relationship for numerous terrestrial whole-rock and mineral samples, of diverse geological origins and age, indicates that the complete data set may be described by a single, mass-dependent fractionation line of slope 0.5244+/- 0.00038 (standard error). Copyright 1999 John Wiley & Sons, Ltd.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.     

A study of some parameters relevant to the response of harwell PMMA dosimeters to gamma and electron irradiation

The effects on the radiation response of Harwell polymethylmethacrylate (PMMA) dosimeters of dose-rate, radiation type, temperature during irradiation and post-irradiation storage, and post-irradiation stability, are of importance to the operators of commercial irradiation facilities.

This paper describes recent studies of the effects of some of these parameters on the radiation response of Harwell Red 4034, Amber 3042, and Gammachrome YR Perspex dosimeters, and provides data on batch to batch variation and shelf-life.     

Electroanalytical exploitation of quinone-thiol interactions: application to the selective determination of cysteine

Square wave voltammetry was applied to the detection of cysteine through the use of an indirect assay that exploits the reaction of the thiol with a quinone indicator. Voltammetric discrimination between unreacted quinone and the corresponding quinone-cysteine adduct is possible with clear resolution of the latter peak providing a linear response from 5 to 47 microM. The selectivity of the approach was assessed with no interference from cystine, lysine, paracetamol or 4-aminophenol. The response recorded in the presence of a massive excess of ascorbic acid was also investigated and the integrity of the approach confirmed. The effects of other sulfhydryl thiols, homocysteine and glutathione, were also assessed and found to present no appreciable change in the voltammetric profile. The practical utility of the approach was investigated through examining the response to cysteine in urine.     

Evaluation of phenolic assays for the detection of nitrite

A series of generic nitrite assay systems based on the single step nitrosation of phenol derivatives are presented. The chemical reactivity offered by the C-nitroso compounds provides an opportunity to pursue a number of analytical strategies of which three spectroscopic (UV/Vis) and two electrochemical options (linear sweep voltammetry/differential pulse voltammetry) were evaluated. The capacity for multiple detection options from a single analyte species without significant sample manipulation is a major advantage with each assay system providing sub ppm detection limits with linear ranges up to milli-molar concentrations of nitrite. The influence of common interferents such as nitrate, ascorbate and paracetamol was investigated. The applicability of the assay procedures to the analysis of authentic biological samples (saliva and urine samples) was assessed with the analytical accuracy independently corroborated with a standard Griess protocol. In addition, a brief comparison with alternative nitrite detection strategies is also presented.     

A clinical assessment of direct electrochemical urate measurements

The response characteristics of various carbon substrates towards the direct oxidative measurement of urate and other key purine biomarkers have been compared. A novel carbon fibre laminate assembly has been proposed as an alternative substrate for the preparation of disposable sensing strips. The fabrication method is generic and readily transferable to a number of sensor applications. Its performance in the determination of urate within biofluids (serum and plasma) has been critically assessed. An inter-laboratory pilot study demonstrated the bioanalytical efficacy of the approach with the responses validated through comparison with the standard colorimetric (uricase/peroxidase) assay.     

Electrochemical tagging of urate: developing new redox probes

The electro-oxidation of uric acid in the presence of nucleophilic species is shown to produce conjugates that can aid our understanding of antioxidant interactions and also provide opportunities for advancing electroanalytical detection strategies involving purine species.     

A disposable sensor for point of care wound pH monitoring

The development of a disposable pH sensor for the point of care monitoring of wound pH is described. The system exploits the pH dependence of an endogenous biomarker (urate) to yield an unambiguous signal. The sensor responses have been characterised and the responses validated in whole blood.