Microcalorimeter Detectors and Low Voltage SEM Microanalysis

The development of the microcalorimeter energy-dispersive X-ray spectrometer (µ-cal EDS) offers a significant advancement in X-ray microanalysis, especially for electron beam instruments. The benefits are especially pronounced for low voltage (5kV) X-ray microanalysis in the field emission scanning electron microscope (FE-SEM) where the high energy resolution of the µ-cal EDS minimizes the peak overlaps among the myriad of K, L, M and N lines in the 0–5keV energy range. The availability of L- and M-shell X-ray lines for microanalysis somewhat offsets the absence of X-ray lines traditionally used above 5keV energy. The benefits and challenges of the µ-cal EDS will be discussed, including P/B ratio for characteristic X-rays, collection angle, count rate capability and the impact of polycapillary X-ray optics on microanalysis.     

Efficiency for unretained solutes in packed column supercritical fluid chromatography II. Experimental results for elution of methane using large pressure drops

At near-critical temperatures and pressures, experimental results for elution of methane with neat carbon dioxide on a 150 mm x 2.0 mm I.D. column packed with 5 microm porous silica with a bonded octylsilica stationary phase show much greater efficiency losses than predicted by theory if isothermal conditions are assumed. Experiments with insulated, air- and water-thermostatted columns demonstrate that significant axial and radial temperature gradients are produced by Joule-Thomson cooling of the mobile phase, and that radial temperature gradients can be a major cause of band spreading at low temperatures and pressures. The use of thermal insulation on the column can greatly improve efficiency under these conditions.     

Allylation of erythromycin derivatives: introduction of allyl substituents into highly hindered alcohols

Functionalized erythromycin 9-oxime derivatives are 6-O-allylated under mild conditions using substituted allyl tert-butyl carbonates under palladium(0) catalysis. This allylation works well where traditional ether-forming protocols function poorly. Allyl tert-butyl carbonates provide higher yields in this reaction than lesser substituted carbonates such as ethyl or isopropyl. Aryl-substituted allyl carbonates or carbamates may be employed as well and, when used, produce trans-olefinic products.     

The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Tracer diffusion and electrical conductivity in sodium-cesium silicate glasses

The tracer diffusion coefficients of ²²Na and ¹³⁷Cs, and the electrical conductivity have been measured in the (Na, Cs)₂O:3SiO₂ glasses as a function of temperature and Cs/Na ratio. Complex impedance analysis was used for the conductivity measurements. The Haven ratio at 396.5°C increases from 0.3–0.4 in single-alkali glasses to 0.8 for the mixed-alkali compositions. The results are explained in terms of a single-jump mechanism; interactions between alkali ions and non-bridging oxygen ions, and between different alkali ions, produce the observed correlation effects.