Gas chromatographic detection of organometallic compounds by reactive-flow photometry

The response of the reactive flow detector (RFD) toward organometallic compounds of several transition metals has been explored, and several of its strong elemental responses have been characterized in detail. The RFDs minimum detectable flow (measured in picograms of metal per second at S/N_p-t-p=2) is 0.1 for ruthenium, 3 for chromium, 10 for manganese, 5 for nickel, 15 for iron, and 2 for osmium. Typically, the linearity of response spans four orders of magnitude, with atomic selectivity of metal versus carbon ranging from 2 to 3 orders. Response quenching by co-eluting hydrocarbons is not observed. As a demonstration experiment, the common analysis of methylcyclopentadienyl manganese tricarbonyl (MMT) in gasoline is carried out on an RFD system modified for dual-channel operation. The results show that dual-channel operation of the RFD can increase the native elemental selectivity of manganese over carbon by a factor of 100, in accordance with earlier results obtained on a special version of the flame photometric detector (FPD). Significant differences exist between the optical spectra of carbon flame species resulting from various combustion modes carried out in the RFD capillary, and also between conventional FPD and typical RFD spectra. Consideration of these spectral differences suggests that the virtual absence of in the reactive flow may be the primary reason why analyte response quenching by hydrocarbons, while prominent in the FPD, is not observed in the RFD.     

Global existence for degenerate parabolic equations with a non-local forcing

We establish local existence and comparison for a model problem which incorporates the effects of non-linear diffusion, convection and reaction. The reaction term to be considered contains a non-local dependence, and we show that local solutions can be obtained via monotone limits of solutions to appropriately regularized problems. Utilizing this construction, it is further shown that, under conditions of either ‘weak reaction’ or ‘sufficiently small’ initial mass, solutions exist for all time. Finally, we provide an alternative analysis of global existence and investigate blow up in finite time for the case of power law diffusion and convection. These results show the extent to which the assumption of weak reaction may be relaxed and still obtain global existence. © 1997 by B. G. Teubner Stuttgart–John Wiley & Sons Ltd.     

Improved isotopic abundance measurements for high resolution fourier transform ion cyclotron resonance mass spectra via time-domain data extraction

The simultaneous high resolution and accurate mass measurements possible with Fourier transform ion cyclotron resonance mass spectrometry coupled with the gentle ionization of electrospray hold attractions for protein, peptide, and oligonucleotide characterization, including multistage-mass spectrometry measurements for assignment of fragment masses and greater confidence in structural measurements. The detection of cyclotron motion over extended periods of time (in some cases for several minutes) allows higher resolution and mass accuracy. Generally, signal duration has been considered to be limited primarily by background pressure, with ion-neutral collisions leading to the reduction and dephasing of cyclotron motion, causing signal loss. However, recent theoretical work has shown that the ion cloud stability that is a prerequisite for high performance measurements is highly dependent on the electric field generated by the ion cloud, thus giving rise to a minimum number of charges or ions required for extended time-domain signals. The effects of ion population on ion cloud stability and signal duration, and the subsequent effects on resolution and measured isotopic abundances are reported. Individual time-domain signals for bovine insulin isotopic peaks were extracted to allow a comparison of the damping rates for each of these ion clouds and the measured time-domain amplitude maxima are shown to provide a better match with the theoretically predicted isotopic abundances for insulin. These results show that different damping rates of ions of very similar mass, but different ion cloud population sizes, can have dramatic effects on the observed isotopic patterns. Additionally, more accurate, high resolution spectra can be produced by correcting for the effects of the different damping rates that are observed for different ion population sizes.     

A high transmission hemispherical energy analyzer for ion spectrometry

A hemispherical energy analyzer was constructed by using a novel approach to control the fringing electrostatic field. It provides several properties useful in ion spectrometers: namely, rather simple fabrication and compact size, high transmission efficiency at moderate resolution, and the capability to adjust resolution by changing the intersphere potentials. A computer program was developed to evaluate ion trajectories through the hemispherical analyzer. Data obtained from the trajectories were used to predict the characteristics of the analyzer. Experiments performed to determine the kinetic energy dependence of the absolute transmission and the resolution functions are in accord with theoretical calculations.     

Nutrient cycling responses to fire frequency in the Kruger National Park (South Africa) as indicated by Stable Isotope analysis

Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N 2 fixation by legumes balances the N losses caused by fires. In this study, the N 2 fixation activity of woody legumes was estimated by analyzing foliar i 15 N and proportional basal area of N 2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N 2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N 2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N 2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil i 15 N and fire frequency found in this study indicates that the effects of fires on ecosystem i 15 N are unpredictable. Similar soil i 15 N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases i 15 N) and N 2 fixation other than that associated with woody legumes (which lowers i 15 N) on isotopic signatures.     

SONET-Framed Optical Packet Networking in the Context of Generalized Multiprotocol Label Switching

This article discusses the evolution of Synchronous Optical Network (SONET) towards support for shared packet media, particularly in light of new developments in the control plane, such as Generalized Multiprotocol Label Switching (GMPLS) and the (UNI) specification. Whereas the current SONET network is largely static and circuitcentered, and typically supports DS-1 and DS-3-level granularities, the emerging packet-centric, GMPLS-controlled optical network will be significantly more dynamic, and characterized by concatenated or virtually concatenated payloads such as STS-3c/12c/48c/192c, and, possibly, STS-1-24v (i.e., 'STS-24c'). In this environment, it will be shown that the differences between such packet-centric SONET networks and 'IP over Optical' network architectures begin to disappear.     

Polyethylene Sandwich Technique for Far Infrared Powder Samples

Recent research on the far infrared absorption of the free radical “tanane” (nitroxyde tetramethyl-2, 2, 6, 6,-piperidine oxyle, C₉H₁₈NO) in connection with its ferroelectric-ferroelastic transition at 14° C^{1, 2, 3} has shown that conventional sample preparation techniques are unsuitable. A new method which offers certain advantages has been developed and is described here since it has potential applicability to other powder samples and can be readily adapted for measurements at higher frequencies.     

Cartesian Dynamics of Simple Molecules VIII planar quadratomics (D3h symmetry)

The vibrations of planar A₃B type molecules with D_3h symmetry are analysed, by treating these molecules as a special case of A₂BC types with C_2v symmetry. Analytical expressions for the four normal mode frequencies are derived in terms of four force constants, and optimized values of the latter for the boron trihalides and sulphur trioxide are obtained and briefly discussed. Frequencies of various isotopic species are also calculated.     

Cartesian Dynamics of Simple Molecules III Non-Linear Triatomics (C2v Symmetry)

A simple spring model for the molecular vibrations of non-linear triatomic molecules with C_2v symmetry is described in terms of Cartesian co-ordinates. Analytical expressions for the stretching and bending mode frequencies are obtained and compared with previous derivations. Optimized values of force constants are calculated for models involving two or three adjustable parameters. The validity of the model is demonstrated by the satisfactory agreement between calculated and observed frequencies of isotopic species. The model provides a simple explanation of the near degeneracy of the symmetric and asymmetric stretching frequencies of H₂S and H₂Se.