Membrane permeation continuous‐flow isotope ratio mass spectrometry for on‐line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous‐flow isotope ratio mass spectrometry for on‐line δ¹³C measurements. The experimental setup of membrane permeation‐gas chromatography/combustion/isotope ratio mass spectrometry (MP‐GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady‐state conditions and performs on‐line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO₂ demonstrate that the whole process does not fractionate the C isotopes of CO₂. Moreover, the δ¹³C values of CO₂ permeated on‐line give the same isotopic results as off‐line static dual‐inlet IRMS δ¹³C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on‐line δ¹³C determination. The feed‐formaldehyde δ¹³C value was pre‐determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on‐line yields a ¹³C enrichment relative to the feed δ¹³C value, the isotopic fractionation being 1.0026 ± 0.0003. The δ¹³C values have been normalized using an adapted two‐point isotopic calibration for δ¹³C values ranging from ?42 to ?10‰. The MP‐GC/C/IRMS system allows the δ¹³C determination of formaldehyde without chemical derivatization or additional analytical imprecision. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthetic studies in the intramolecular carbocyclization of N-acyloxyiminium ions. Stereoelectronic and steric implications of nucleophilic alkene, alkyne, and allene tethers

N-Acyloxyiminium ions generated from 4-substituted l-pyroglutamic esters with 4-(3-butenyl), 4-(3-butynyl), 4-(3-cinnamylmethyl), and 4-allenic tethers undergo rapid Lewis acid mediated carbocyclization to give stereodefined azacyclic compounds depending on the nature of the nucleophilic tether. In general, reactions of alkenes and alkynes with terminal alkyl or aryl substituents, as well as allenes, proceed through transient vinylic carbocations that are attacked internally by the N-Boc group to give tricyclic dihydrooxazinones. Diastereotopic bis-4-(3-butenyl) and 4-(3-butynyl) tethers undergo stereochemically controlled attack favoring an antiperiplanar rather than synclinal approach to give enantiopure 6-halo octahydroindole-2-carboxylic acids and 6-halo hexahydroindole-2-carboxylic acids as their methyl esters, respectively. The aza bicyclic and tricyclic compounds are excellent scaffolds for diversification.     

Universal multifractal properties of circle maps from the point of view of critical phenomena I. Phenomenology

The strange attractor for maps of the circle at criticality has been shown to be characterized by a remarkable infinite set of exponents. This characterization by an infinite set of exponents has become known as the multifractal approach. The present paper reformulates the multifractal properties of the strange attractor in a way more akin to critical phenomena. This new approach allows one to study the universal properties of both the critical point and of its vicinity within the same framework, and it allows universal properties to be extracted from experimental data in a straightforward manner. Obtaining Feigenbaum's scaling function from the experimental data is, by contrast, much more difficult. In addition to the infinite set of exponents, universal amplitude ratios here appear naturally. To study the crossover region near criticality, a correlation time, which plays a role analogous to the correlation length in critical phenomena, is introduced. This new approach is based on the introduction of a joint probability distribution for the positive integer moments of the closest-return distances. This joint probability distribution is physically motivated by the large fluctuations of the multifractal moments with respect to the choice of origin. The joint probability distribution has scaling properties analogous to those of the free energy close to a critical point.     

Beryllium-7 as a tracer of stratospheric ozone: A case study

A new method is presented for the determination of Au and Pt in biological materials based on neutron activation analysis with radiochemical separation of gold. Separation of gold by electrolytic deposition on a niobium cathode ascertains the highest radiochemical purity without any interference from calcium or other major elements. With¹⁹⁹Au as indicator for platinum the gold content of the sample not only strongly affects the limit of detection, but also causes interference by double neutron capture. Replicate analyses of BCR Certified Reference Materials No. 184, 185 and 186 were carried out.Presented at the MTAA-8 Conference, September 16–20, 1991, Vienna, Austria.     

Computing resonance energies, widths, and wave functions using a Lanczos method in real arithmetic

We introduce new ideas for calculating resonance energies and widths. It is shown that a non-Hermitian-Lanczos approach can be used to compute eigenvalues of H+W, where H is the Hamiltonian and W is a complex absorbing potential (CAP), without evaluating complex matrix-vector products. This is done by exploiting the link between a CAP-modified Hamiltonian matrix and a real but nonsymmetric matrix U suggested by Mandelshtam and Neumaier [J. Theor. Comput. Chem. 1, 1 (2002)] and using a coupled-two-term Lanczos procedure. We use approximate resonance eigenvectors obtained from the non-Hermitian-Lanczos algorithm and a very good CAP to obtain very accurate energies and widths without solving eigenvalue problems for many values of the CAP strength parameter and searching for cusps. The method is applied to the resonances of HCO. We compare properties of the method with those of established approaches.