Off-shell behavior of the crossed nucleon pole in pion-nucleus scattering

The contribution of the nucleon-pole piece of the pion-nucleon amplitude to the pion-nucleus optical potential is examined. Straightforward calculations of diagrams which are intermediate between Goldstone and Feynman diagrams produce simple results with well-behaved physical implications. In particular, the direct S-channel nucleon pole contributes to one-body true absorption while the crossed U-channel pole has a novel off-shell behavior and does not contribute to pion true absorption.     

Microcolumn preconcentration and gas chromatography-microwave induced plasma-atomic emission spectrometry (GC-MIP-AES) for mercury speciation in waters

A novel method for the direct determination of mercury species at the ng l^–1 level in natural waters is described. Methyl-, ethyl- and inorganic mercury are preconcentrated on a sulphhydryl cotton microcolumn incorporated in a flow injection system. Retained mercury species are then eluted with hydrochloric acid solution (3 mol/l) and subjected to phenylation before determination by gas chromatography-microwave induced plasma-atomic emission spectrometry. Limits of detection for mercury species are 10 ng l^–1 for methyl- and ethyl-mercury and 16 ng l^–1 for inorganic mercury based on processing 200 ml of sample. Application of the methodology to waters of the Manchester Ship Canal revealed elevated levels of methylmercury and inorganic mercury.     

Rapid sequential determination of inorganic mercury and methylmercury in natural waters by flow injection-cold vapour-atomic fluorescence spectrometry

A novel method for the rapid sequential determination of inorganic mercury and methylmercury in natural waters at the ng/l. level has been developed. Trace enrichment and separation of mercury species are achieved using a microcolumn of sulphydryl cotton which has a relatively high affinity for methylmercury. The limit of detection for methylmercury based on processing of a 0.5-ml sample volume was 6 ng/l. Application to river waters is demonstrated.     

Rapid survey analysis of polymeric materials by laser ablation-inductively coupled plasma spectrometry. An analytical note

An Nd-Yag laser (1064 nm) in combination with a multichannel emission spectrometer was used for rapid survey analysis of polymers and paints. A novel design feature of the ablation chamber was the incorporation of a graphite furnace to effect electrothermal pretreatments of samples. Transient emission signals were studied as a function of laser operating mode, laser duration and flashlamp energy.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Simple oxidative pretreatment for determination of organomercury by cold vapour atomic fluorescence spectrometry

A simple and rapid method has been developed for the direct determination of organomercury in toluene extracts. The oxidative pretreatment, intended for cold vapour-atomic absorption/fluorescence spectrometry, involves the use of bromide/bromate reagent under strong acid conditions. The pretreatment is performed directly in a volumetric flask and avoids the need for back extraction and phase separation. Recoveries for methyl mercury additions to toluene, at the 2 g/1 level, ranged from 100–106%.On leave Xianjiang Environmental Research Institute, Urumqui, Xianjiang Province, Peoples Republic of China     

Voltammetry of Plastocyanin at a Graphite Electrode: Effects of Structure, Charge, and Electrolyte

Comparative voltammetric studies on Anabaena variabilis plastocyanin (positively charged at neutral pH) and spinach and poplar plastocyanins (negatively charged at neutral pH) have been undertaken at an edge-plane graphite electrode as a function of ionic strength, pH, and Mg(2+) concentration at 3 degrees C. The aim was to provide a more detailed understanding of the influence of the electrode-protein (solution) interfacial characteristics, as well as the variation of the formal potential with both the nature of the plastocyanin species and the pH. As might be expected, some of the interfacial properties associated with the positive charge on A. variabilis plastocyanin are the opposite of those observed with the negatively charged plastocyanins. For example, the linear diffusion component of the mass transport process for A. variabilis plastocyanin under the conditions of cyclic voltammetry is decreased and the radial diffusion component is increased by the addition of Mg(2+), whereas the reverse occurs with poplar and spinach plastocyanins. The voltammetrically determined reversible potentials for A. variabilis plastocyanin are considerably less positive than those for spinach and poplar plastocyanins, in agreement with values calculated from chemically based redox studies. Ionic strength effects, as determined by addition of NaClO(4) over the concentration range 0.005-0.20 M, are negligible for all three proteins. The addition of Mg(2+) causes a significant shift in the reversible potential toward more positive values for spinach and poplar plastocyanin but only a small positive shift for A. variabilis plastocyanin. The difference is attributed to a specific binding effect. The addition of Mg(2+) also dramatically alters the pH dependence of the reversible potential, indicating that the equilibrium between the protonated and unprotonated forms of reduced plastocyanin is modified by binding of Mg(2+) to the protein. It is concluded that the effects of biologically relevant redox-inactive cations such as Mg(2+) or Ca(2+) have to be considered carefully in studies of the redox chemistry of metalloproteins.