Nanometer Calibration Particles: What is Available and What is Needed?

Information on available polystyrene calibration spheres is presented regarding the particle diameter, uncertainty in the size, and the width of the size distribution for particles in a size range between 20 and 100nm. The use of differential mobility analysis for measuring the single primary calibration standard in this size range, 100nm NIST Standard Reference Material®1963, is described along with the key factors in the uncertainty assessment. The issues of differences between international standards and traceability to the NIST Standard are presented. The lack of suitable polystyrene spheres in the 20–40nm size range will be discussed in terms of measurement uncertainty and width of the size distributions. Results on characterizing a new class of molecular particles known as dendrimers will be described and the possibilities of using these as size calibration standards for the size range from 3 to 15nm will be discussed.     

Use of cyclodextrin‐modified gold nanoparticles for enantioseparations of drugs and amino acids based on pseudostationary phase‐capillary electrochromatography

The application of chemical‐modified gold nanoparticles (GNPs) as chiral selector for the enantioseparation based on pseudostationary phase‐CEC (PSP‐CEC) is presented. GNPs modified by thiolated β‐CD were characterized by NMR and FT‐IR. The nanoparticle size was determined to be of 9.5 nm (+2.5 nm) by Transmission Electron Microscopy (TEM) and UV spectra. Four pairs of dinitrophenyl‐labeled amino acid enantiomers (DL‐Val, Leu, Glu and Asp) and three pairs of drug enantiomers (RS‐chlorpheniramine, zopiclone and carvedilol) were analyzed by using modified GNPs as the chiral selector in PSP‐CEC. Good theoretical plate number (up to 2.4×10⁵ per meter) and separation resolution (up to 4.7) were obtained even with low concentration of modified GNPs (0.8–1.4 mg/mL). The corresponding concentration of β‐CD in the buffer was only 0.30?0.53 mM, which was much lower than the optimum concentration of 15 mM if pure β‐CD was used as chiral selector. Our results showed that thiolated β‐CD modified GNPs have more sufficient interaction with the analytes, resulting in significant enhancement of enantioseparation. The study shed light on potential usage of chemical modified GNPs as chiral selector for enantioseparation based on PSP‐CEC.     

Thermal degradation and flame retardance in copolymers of methyl methacrylate with diethyl(methacryloyloxymethyl)phosphonate

Methyl methacrylate (MMA) has been free radically copolymerized, both in bulk and in solution, with diethyl(methacryloyloxymethyl)phosphonate (DEMMP), to give polymers which are significantly flame retarded when compared with PMMA, as indicated by the results of limiting oxygen index (LOI) measurements, UL 94 tests, and the results of cone calorimetric experiments. The physical and mechanical properties of the copolymers are similar to those of PMMA, except that the bulk copolymers are slightly crosslinked, and are better than those of PMMA flame retarded to a similar extent by some phosphate and phosphonate additives. Examination of the some of the gaseous products of pyrolysis and combustion, and of chars produced on burning, show that flame retardation occurs in the copolymers by both a condensed-phase and a vapour-phase mechanism. The condensed-phase mechanism is shown to involve generation of phosphorus acid species followed by reaction of these with MMA units giving rise to methacrylic acid units. The methacrylic acid units subsequently form anhydride links, which probably impede depolymerization of the remaining MMA sequences, resulting in evolution of less MMA (the major fuel when MMA-based polymers burn). By undergoing decarboxylation, leading to interchain cyclisation and, eventually, to aromaticisation, the anhydride units are probably also the principal precursors to char.     

Bivariate distributions characterized by one family of conditionals and conditional percentile or mode functions

It is well known that full knowledge of all conditional distributions will typically serve to completely characterize a bivariate distribution. Partial knowledge will often suffice. For example, knowledge of the conditional distribution of X given Y and the conditional mean of Y given X is often adequate to determine the joint distribution of X and Y. In this paper, we investigate the extent to which a conditional percentile function or a conditional mode function (of Y given X), together with knowledge of the conditional distribution of X given Y will determine the joint distribution. Finally, using this methodology a new characterization of the classical bivariate normal distribution is given.     

Thermodynamics of hypoelasticity

A thermodynamic framework for hypoelasticity is constructed based on a modified Gibbs Function which depends on the stress deviator and specific volume. In the hypoelastic equations considered here, the stress deviator is obtained from a rate equation involving the corotational derivative and coefficients which depend on the invariants of the stress deviator as well as specific volume. Non-negative entropy production is enforced, leading to a non-dissipative condition in the sense that entropy is produced only by heat transfer. The resulting conditions lead to relations among the coefficients in the rate equation. These relations underdetermine the coefficients, so that there is leeway in specifying these coefficients. An example of a set of rate equations which satisfy the thermodynamics is presented.     

Base-catalyzed tritium exchange between tritium-containing heavy water and deuterated chloroform–kinetics and equilibrium

We develop a kinetic model for the exchange of tritium and deuteriumbetween tritium-containing heavy water and deuterated chloroform in a two-phase liquid system, catalyzed by hydroxide ion (sodium hydroxide). For the exchange reaction yielding tritiated chloroform andheavy water, the measured equilibrium constant is 0.518 at 25–45°C, independent of temperature. The rate constant (for the controlling step, deuteron abstraction from chloroform) is 0.22 L/mol s at 25°C, 0.72 at 35°C, and 2.0 at 45°C, corresponding to an activation energy of 87 kJ/mol. Comparison is made with literature data, and effects of mass transfer on the apparent rate constant are discussed.     

Array measurements of the surface pressure beneath a forced axi-symmetric separation bubble

An array of microphones is used to study the space–time characteristics of the wall-pressure field beneath a forced separation bubble downstream of an axi-symmetric backward-facing step. To excite the flow, an externally driven Helmholtz resonator is employed. A unique aspect of the present study is the utilization of an amplitude-modulated forcing scheme in order to avoid contamination of the measured hydrodynamic pressure fluctuations by acoustic radiation from the forcing device. The results lead to the hypothesis that the optimal forcing frequency is achieved when the forced disturbance originates near the center of the unforced separation bubble in the limit of very low levels of forcing. Moreover, a frequency–wavenumber spectrum analysis highlights the possibility for achieving separation control while minimizing potential acoustic radiation due to coupling between the forced disturbance and resonant modes of the underlying surface.