On-line Sulfur Isotope Analysis of Organic Material by Direct Combustion: Preliminary Results and Potential Applications

Abstract

Sulfur isotopes have received little attention in ecology studies because plant and animal materials typically have low sulfur concentrations (< 1 wt.%) necessitating labor-intensive chemical extraction prior to analysis. To address the potential of direct combustion of organic material in an elemental analyzer coupled with a mass spectrometer, we compared results obtained by direct combustion to results obtained by sulfur extraction with Eschka's mixture. Direct combustion of peat and animal tissue gave reproducibility of better than 0.5‰ and on average, values are 0.8‰ higher than values obtained by Eschka extraction. Successful direct combustion of organic material appears to be a function of sample matrix and sulfur concentration. Initial results indicate that direct combustion provides fast, reliable results with minimal preparation. Pilot studies underway include defining bear diets and examining fluctuations between freshwater and brackish water in coastal environments.     

A Laboratory,Computer and Calculus Based Course in Mathematics

The paper describes the designing and testing of a laboratory, computer and calculus based course in mathematics. The laboratory is central to the course and stimulates in the student the need and desire to know more about mathematics. Further, it enables mathematics to be taught in a real world context. Computers are used to take the drudgery out of the mathematics and make it possible to attack real scientific and technical problems. This new approach to calculus is less formal and depends to a smaller extent upon prior mathematical training so that it appeals to a much wider audience. The proposed course, with its emphasis on laboratory measurements, is ideally suited to the exploration of numerical methods and their application to the calculation of derivatives, definite integrals and the solution of differential equations.     

Direct observation of the phenomenology of a solid thermal explosion using time-resolved proton radiography

We present a new phenomenology for burn propagation inside a thermal explosion based on dynamic radiography. Radiographic images were obtained of an aluminum cased solid cylindrical sample of a plastic bonded formulation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. The phenomenology observed is ignition followed by cracking in the solid accompanied by the propagation of a radially symmetric front of increasing proton transmission. This is followed by a further increase in transmission through the sample, ending after approximately 100 micros. We show that these processes are consistent with the propagation of a convective burn front followed by consumption of the remaining solid by conductive particle burning.     

Chemical reaction fronts in ordered and disordered cellular flows with opposing winds

We present experiments on the motion of chemical fronts in ordered and disordered vortex flows with imposed uniform winds. Fronts in a chain of alternating vortices are found to freeze (pin to the separatrix) for a wide range of opposing winds that grows nonlinearly with the characteristic strength of the underlying vorticity. Experiments in spatially disordered flows demonstrate that freezing of fronts is common to cellular flows; furthermore, it is not dependent on boundary conditions. We therefore anticipate similar pinning in a wide range of 2D cellular flows and front-producing systems.     

Linear covering codes and error-correcting codes for limited-magnitude errors

The concepts of a linear covering code and a covering set for the limited-magnitude-error channel are introduced. A number of covering-set constructions, as well as some bounds, are given. In particular, optimal constructions are given for some cases involving small-magnitude errors. A problem of Stein is partially solved for these cases. Optimal packing sets and the corresponding error-correcting codes are also considered for some small-magnitude errors.     

Tracking controlled chaos: Theoretical foundations and applications

Tracking controlled states over a large range of accessible parameters is a process which allows for the experimental continuation of unstable states in both chaotic and non-chaotic parameter regions of interest. In algorithmic form, tracking allows experimentalists to examine many of the unstable states responsible for much of the observed nonlinear dynamic phenomena. Here we present a theoretical foundation for tracking controlled states from both dynamical systems as well as control theoretic viewpoints. The theory is constructive and shows explicitly how to track a curve of unstable states as a parameter is changed. Applications of the theory to various forms of control currently used in dynamical system experiments are discussed. Examples from both numerical and physical experiments are given to illustrate the wide range of tracking applications. (c) 1997 American Institute of Physics.     

Spiroacetal biosynthesis in insects from Diptera to Hymenoptera: the Giant Ichneumon wasp Megarhyssa nortoni nortoni Cresson

The volatile components of the mandibular gland secretion generated by the Giant Ichneumon parasitoid wasp Megarhyssa nortoni nortoni Cresson are mainly spiroacetals and methyl ketones, and all have an odd number of carbon atoms. A biosynthetic scheme rationalizing the formation of these diverse components is presented. This scheme is based on the results of incorporation studies using (2)H-labeled precursors and [(18)O]dioxygen. The key steps are postulated to be decarboxylation of beta-ketoacid equivalents, beta-oxidation (chain shortening), and monooxygenase-mediated hydroxylation leading to a putative ketodiol that cyclizes to spiroacetals. The generality of the role of monooxygenases in spiroacetal formation in insects is considered, and overall, a cohesive, internally consistent theory of spiroacetal generation by insects is presented, against which future hypotheses will have to be compared.     

Got Sulfate? Luring Axons This Way and That

Glycosaminoglycans are sulfated in complex and changing patterns that affect neural development. These sugars mediate interactions between macromolecules, and their biological contributions are of high interest. In this issue of Chemistry & Biology, Shipp and Hsieh-Wilson describe microarrays to probe these complex modifications.