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.     

Formation conditions for InAs/GaAs quantum dot arrays by droplet epitaxy under MOVPE conditions

The first stage of formation of InAs/GaAs quantum-dot heterostructures by droplet epitaxy is investigated. Factors influencing the geometrical size and density of arrays of indium nanodrops deposited by trimethylindium pyrolysis on the GaAs(100) substrate are analyzed, and the possibility of using these factors in the process of metal-organic vapor phase epitaxy (MOVPE) are studied. To refine the temperature dependence of the In evaporation rate, a computational experiment taking into account real MOVPE conditions is conducted. An ultimate change in the composition of In droplets contacting the substrate at a high temperature is estimated, and the thickness and composition of crystallizing In-Ga-As solid solution are calculated. It is shown that the size and density of the droplet array to a great extent depend on the crystallochemical structure of the substrate surface and deposition conditions.     

Tunable laser diodes for the 1250-1650 nm spectral range

A series of tunable laser diodes was developed with emission wavelength in spectral range 1250-1650 nm. This paper describes the configuration and basic characteristics at temperatures from 5 to 80 degrees C.     

Catastrophic degradation of pulsed lasers based on AlGaAs/InGaAs/GaAs heterostructures with electron-beam pumping

Catastrophic degradation of pulsed lasers based on InGaAs/AlGaAs/GaAs structures with different design of the active domain with transverse pumping by the electron beam at T = 300 K is studied. In lasers based on structures with a InGaAs single quantum well and with seven quantum wells, the maximal values of pulsed power are 70–90 and 10–20 W, respectively.     

Syntheses and reactions of some fluorinated dienes,cyclobutenes and furans

There are many reports¹ of the pyrolysis of fluorinated organic compounds, including the defluorination of cyclic fluorocarbons over iron to give aromatic compounds. Extending this technique we have investigated the flow pyrolysis of some readily accessible unsaturated fluorocarbons, such as I, II, and III, and found these to be synthetically

useful routes to fluorinated dienes, cyclobutenes, and furans. Pyrolyses were carried out using a nitrogen flow over platinum, iron or caesium fluoride heated at 430–700°. The various products can all be rationalized in terms of intermediate allylic radicals, and the solid substrate influences which allylic radicals are formed.We are also investigating the chemistry of those now accessible compounds, such as IV, V, and VI, and some of the preliminary results are described.

For example the fluoride ion induced dimerisation of IV gave two major products VII and VIII via a particular interesting mechanism.

     

ChemInform Abstract: The Ground-State Potential Energy Surface of Diazene

has been studied by using a complete active space MCSCF wave function generated by distributing eight electrons among eight orbitals.