Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.     

Self-assembled monolayer initiated electropolymerization: a route to thin-film materials with enhanced photovoltaic performance

Continuing progress in the field of organic polymer photovoltaic (PV) devices requires the development of new materials with better charge-transport efficiency. To improve this parameter, we have investigated surface-attached bilayer polymer PV thin films prepared starting from a covalently attached monolayer of an electroactive initiator using sequential electropolymerization of dithiophene and its derivatives. These systems were found to show significantly increased photocurrent generation quantum yields as compared to systems made through conventional approaches. In addition, the described PV thin films possess remarkable mechanical, air, and photostability. These properties likely arise from the more uniform and better ordered bulk layer morphologies as well as tighter covalently bonded contacts at the interfacial junctions, contributing to improved charge transport. While more studies on the fundamental reasons behind the discovered phenomenon are currently underway, this information can be readily applied to build more efficient organic polymer photovoltaics.     

Ion transport in Ca2+, Sr2+, Ba2+, and Pb2+ beta″ aluminas

Single crystals of Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺ beta″ alumina were prepared from sodium beta″ alumina by ion exchange. The ionic conductivities of Ca²⁺, Sr²⁺, and Ba²⁺ beta″ alumina are comparable, about 3 × 10^?2 (ohm-cm)^?1 at 300°C. Surprisingly, Pb²⁺ beta″ alumina is much more conductive, 1.5 × 10^?1 (ohm-cm)^?1 at 300°C and 4.6 × 10^?3 (ohm-cm)^?1 at 40°C. Its conductivity approaches that of sodium beta″ alumina at temperatures below 25°C. The diffusion coefficient for Sr²⁺ in beta″ alumina at 600°C was estimated from radiotracer experiments. It is consistent with that expected from conductivity measurements.     

How does cold constrain life cycles of terrestrial plants and animals?

Short summer seasons with low thermal energy input characterise polar ecosystems. Climatic variables and physical isolation of terrestrial habitats act as selective filters which must be passed to allow colonisation, establishment and survival in these extreme environments. Life history studies of the terrestrial biota of such ecosystems give little evidence of adaptive responses to low temperatures having evolved in situ, even though behavioural, ecophysiological and biochemical features allowing tolerance of the likely extremes are well-developed. Observed life history strategies are often consistent with the general predictions of adversity (A) or stress (S) selection. However, biota successful in these extreme environments may be better-grouped by the lack of particular life history features, rather than common possession.