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At least 1,315 records · Page 73Linked to original sources
PubMed2002

Direct transesterification of gases by "dry" immobilized lipase.

Several different reactor configurations, including single pass, continuous recycle, and batch reactor modes, were used to investigate the effects of temperature and water activity, or relative humidity, on lipase-catalyzed, gas-phase transesterifications. Temperature and relative humidity were controlled both inside reactors and throughout the course of the reaction to account for and optimize their effects. Results indicated that, at low relative humidity, reaction rates increased with temperature up to 60 degrees C. However, when relative humidity was increased, a similar increase in temperature resulted in the loss of nearly all enzyme activity. These results are consistent with the idea that enzymes without free water are more thermally stable. Furthermore, at constant ambient temperatures, production increased dramatically with an increase in relative humidity, confirming the idea that an increase in water activity increases catalytic activity. A mass balance performed on reactors at higher relative humidity revealed that hydrolysis (rather than transesterification) of the ester substrate could significantly decrease product yields.

Animals↗
PubMed1988

Mass spectrometric investigation of dissociative ionization of toxic gases by electrons at 20-1000 eV.

Measurements of the dissociative ionization cross-sections of CO, CO2, NO and N2O molecules for electrons from 20 up to 1000 eV were performed. Total collection of ions was achieved by using a specially constructed cycloidal mass spectrometer, an ion source without slit and a collector with large area. In that open ion source the effusion molecular beam formed by a capillary was crossed by the electron beam. Owing to such parameters of the mass spectrometer the mass discrimination effects were reduced. At the ionization of CO molecules, the ions CO+, C+, O+ and CO2+ were registered; at CO2, the ions CO2+, CO+, O+, C+ and CO2(2)+; at NO, the ions NO+, N+, O+ and NO2+; at N2O, the ions N2O+, NO+, N2+, O+, (N+,N2(2)+) and N2O2+. The relative values of the partial cross-sections were normalized to the absolute total ionization cross-sections obtained by Rapp and Englander-Golden.

Carbon Dioxide↗
PubMed2004

Molecular complexes of organometallic molecules with noble gases: the rotational spectrum of dimethylsilane-argon.

The rotational spectrum of the molecular complex dimethylsilane-argon was investigated by free-jet absorption millimeter-wave and molecular-beam Fourier transform spectroscopy. The absolute energy minimum corresponds to a conformation in which the argon atom lies in the plane of symmetry of dimethylsilane, perpendicular to the C-Si-C plane. The distance of Ar atom is tilted 14 degrees away from the Si atom. The zero-point dissociation energy was estimated from the centrifugal distortion constant D(J) to be 2.2 kJ mol(-1). Small splitting, due to tunneling of the Ar atom and internal rotation of the two methyl groups, was observed, measured, and used to determine the potential energy surface for these motions.

Argon↗
PubMed1998

Low frequency NMR polarimeter for hyperpolarized gases.

An inexpensive and self-contained apparatus for pulsed NMR at 30-250 kHz is described. The intended application is monitoring of the spin polarization of rare gas nuclei in a laser-polarizing apparatus in fields of order 30 G. In addition, the device provides a convenient method for following the polarization decay during storage and transport. Some of the features are a flexible pulse generator, splitting of transmitter RF cycles by the RF gate, a Q switch, and a wide range of receiver gains.

Equipment Design↗
PubMed2000

Simulated navigation based on observed gradients of atmospheric trace gases (Models on pigeon homing, part 3).

An earlier developed model, simulating pigeon homing based on fictitious gradients of atmospheric odours, was applied to actually observed spatial distributions of volatile hydrocarbons. The model calculations demonstrate that sufficient information on a bird's current position with respect to home can be derived from the ratios among three or more chemical compounds which gradually vary over distances of several hundreds of kilometres, differently in different directions. Flight directions computed by model birds from such observed ratios are roughly but not perfectly homeward-oriented from most positions within the investigated radius of 200 km around home. Performances of model birds are at least as good as those of real pigeons in the field. According to calculations using atmospheric data collected under different wind directions, the birds might, but possibly need not, take the current weather conditions into account when evaluating olfactory signals. It is necessary, however, that the birds acquire, during their long-term stay at the home site, some knowledge of the directions of relevant gradients. Homing experiments with pigeons as well as measurements of atmospheric trace substances are consistent with the hypothesis that this knowledge is gained by correlating wind directions with specific changes of ratios among a number of compounds. This assumed process requires further elucidation.

Animals↗