Journal of research of the National Bureau of Standards (1977)最新文献

This short paper describes several experimental approaches for dealing with chemical reactions or decomposition which can occur when making thermophysical property measurements at high temperature and high pressure. The associated equipment was designed and built to allow thermophysical property data to be cast in a more realistic perspective by taking explicit account of chemical changes which may occur during an experiment. As an example of these methods, the measurements on the methanol system are discussed in detail.

The use of linear interpolation and simplified iteration procedures are shown to introduce inaccuracies to the rectangular grid method of characteristics, particularly when applied to subcritical flows. Comparisons of experimental and computational results are presented illustrating the use of Everett and Newton-Gregory interpolation, in addition to a more complex iteration procedure, to substantially improve the method's ability to maintain both steady uniform flows under subcritical conditions, and retain wave steepness during propagation along the drainage pipe. The results presented will be directly applicable to the building drainage network model previously developed at Brunei University with the support of NBS CBT grant aid.

Biotechnological processing of inorganic, heavy elements has only begun to emerge as we start to understand microbial strategies and mechanisms of heavy element transformations. Chemical speciation of key, diagnostic intermediates and products of bioprocessing in gas, liquid, and cellular phases, and on surfaces, is required to understand and optimize important reactions. Recent discoveries of microorganisms in metal-enriched thermal environments, and further investigations into production of exocellular metal transforming metabolites, offer exciting prospects for development of new technologies for strategic and precious materials recovery and processing.

Apparatus and procedures used for high-precision microcalorimetric measurements are described. The calorimeter is of the heat-conduction type and utilizes semi-conductor thermoelectric modules. The bicompartmental reaction vessel is made of high-density polyethylene and holds about 0.5 mL of solution in each compartment. Imprecision of heat measurement is 0.2 percent when measuring 300 mJ of heat produced by a rapid chemical reaction. Three microcalorimeters are operated simultaneously using a microcomputer and a data acquisition system. Thermochemical and kinetic applications are described. The acquisition of data from an isoperibol solution calorimeter is also described.

In this short note, we describe the design and construction of several modifications of miniature mercury contact switches for use in laboratory temperature control applications. Commercial contact switches, or contact thermometers as they are commonly called, are limited in their application because of their large size. The units which we present here are much more compact and are thus suitable for a wider range of applications. The limitations of the miniature contact switches in their present configurations are also discussed.

This article summarizes two studies made in preparation for standards development, by differential scanning calorimetry, for instruments such as scanning calorimeters, differential thermal analyzers, differential mechanical analyzers, and related thermal analysis devices. The first was an extensive study of the variability of differential scanning calorimeters when used for determining transition temperatures and enthalpies. The second was an evaluation of calibration procedures recommended by the American Society of Testing and Materials. These studies are described in detail in National Bureau of Standards Special Publication 260-99.

In order to adequately ensure the protection of human health and the environment from the thousands of presently suspected hazardous substances and the new compounds added to those by new industrial processes, sophisticated approaches to hazard assessment and monitoring are being established. Environmental specimen banking (ESB) is necessary, useful, and important for environmental monitoring currently, and in the future for monitoring the past. ESB has already proved a good lool for recording inorganic and/or organic pollution trends over the years. Moreover, ESB offers the possibilities and potentials for retrospective analysis of authentic samples from the past by improved future analytical procedures, including the detection of presently unnoticed environmental chemicals of biological interest. Among the specimens representing the environment, specimens of human origin play a key role. The selection criteria for human specimens include ethical and legal considerations together with the appropriate scientific approaches and epidemiological criteria. Technical considerations for sampling, preparation, transportation, and storage of the specimens include the selection and development of specific materials and implements, cold storage, and clean room technology in order not to compromise the original composition of the sample.

The problems regarding storage and pre-neutron activation analysis treatment for the elements aluminum, calcium, vanadium, selenium, copper, iodine, zinc, manganese, and magnesium in a urine matrix are reviewed. The type of collection and storage procedure and pre-neutron activation analysis treatment of mine depend on the specific trace element; that is, its inherent physical and chemical properties. Specifically polyethylene in teflon containers are the most suitable for general determinations. Whether any preservative is added would depend upon the stability of the trace element and its tendency for surface adsorption. Preferably preservatives should contain no radioactivatable elements for maximum efficacy. Freeze drying or packing urine shipments under dry ice needs to be explored on an individual basis. Each pre- or post-neutron activation analysis treatment is specific and optimized for the trace element analyzed.

Choosing the right kind of samples from human subjects for trace element studies poses many difficult problems. First of all, due to practical considerations, specimens with clinical relevance are restricted to a few such as whole blood, hair, nail, urine, and faeces. Although autopsies provide access to collect various organs, their usefulness is restricted to monitoring type of activities and not for clinical diagnosis. Besides these basic differences one is also confronted with procuring "valid" samples for analysis. Validity refers to both analytical and biological aspects and the material collected should satisfy both the demands to make the specimen meaningful. In practice this is not a simple task because a number of presampling factors need to be taken into account. Significant situations among these are the biological variations, post mortem changes, intrinsic errors resulting from internal contaminations, etc. The impact of these factors alters the status of the sample and calls for adequate discription of the specimen. In the absence of a well defined sample protocol accurate characterization of the material will not be possible and renders the analytical effort worthless. Solutions to these problems should be sought at interdisciplinary level and effective team work is mandatory to make any meaningful progress in our endeavours to answer public health questions.