Local Observation of Field Polarity Dependent Flux Pinning by Magnetic Dipoles

A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.     

Determining a 'safe' high-mass limit in matrix-assisted laser desorption/ionisation time-of-flight mass spectra of coal derived materials with reference to instrument noise

Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd.     

The use of wood veneer in decomposition experiments

Lignified cellulose (sycamore wood veneer) has been included in a range of simple biological soil tests that are being assessed as field tests to demonstrate the decomposing abilities of soils and the effect of management practices. The tests measure the weight loss of individual nitrogen and cellulose substrates and also plant litter (Eriophorum leaves). The veneer pieces are 6 × 2 cm and the weight loss is the measure of decomposition. All the experimental results are examined in relation to the growth of one or more test plants and to the chemical and physical characteristics of the soils. The tests have been made in a range of 76 soils representing eight major soil types ranging from brown earths to peat, and were collected from sites throughout Britain. The soils were brought back to the research station for testing under common conditions and all tests were made in separate pots. The tests have also been made in a range of five soils pre-stored for 6 months at five moisture levels from 20% to more than 100% of field capacity. The results for the veneer test from all experiments will be presented in relation to the chemical and physical characteristics of the soils and the potential of the soils for plant growth. The loss of veneer sometimes relates to the other soil tests. The use of veneer allows an integrated assessment of the environmental conditions for decomposition over a longer period of time than for other tests and may be used in future field experiments.     

Construction of multiporphyrin arrays via selective cross-metathesis

Selective cross-metathesis of type I and type II meso-functionalized porphyrin olefins afforded alkenyl-coupled dimeric and trimeric porphyrin systems in good yield with excellent E/Z selectivity. The synthetic utility of the method is demonstrated through the preparation of mixed metalated (M = 2H, Zn) porphyrin dimer and trimer. [reaction: see text]