Compensating for variation in 1JCH coupling constants in HSQC spectra acquired on small organic molecules

The HSQC sequence provides a sensitive way of determining the ¹³C chemical shift of protonated carbons. It uses INEPT elements for magnetization transfer, which can only be optimized for one value of ¹J_CH, but small organic molecules contain a wide range of ¹J_CH values. One popular method of compensating for ¹J_CH variation is to incorporate adiabatic pulses into the INEPT elements. This article shows that this method fails for a significant subset of functional groups. It also shows that the effects of this failure can be reduced by avoiding refocusing delays and by using a J‐compensated excitation element. Copyright © 2010 John Wiley & Sons, Ltd.     

Einen elektrischen Widerstands-Tiegel-Ofen aus Magnesia

Ohne Zusammenfassung     

Polymerization of aromatic nuclei. XXV. ESR and chemical studies on the radical cation nature of poly(p-phenylene)

The radical cation nature of poly(p-phenylene) (PPP) was examined by electron spin resonance (ESR) and chemical means. ESR studies revealed a radical concentration of 1.0 × 10²¹ spins/g for the crude polymer. Workup with aqueous acid decreased the value to 1.5 × 10¹⁸ spins/g. Reactions of the polymer with certain nucleophiles followed the half-regeneration mechanism, whereas with others, electron transfer mainly occurred. The origin of halogen in the polymer was found to arise from reaction of the radical cation with the oxidant, and not with halide during workup. Oxidation of PPP with various species increased the concentration of radical cations.     

Studies of potassium ferrite,K1+xFe11O17. II. Mössbauer effect and spin flopping

The Mössbauer spectrum of the iron analog of K-β-alumina, K_1+xFe₁₁O₁₇, where x = 0.35 to 0.86, has been obtained. Four magnetically split Fe³⁺ components can be resolved. At 295°K the magnetic flux densities at the nuclei are, for the 2a octahedral site 51.8 T, for the 12k octahedral site 43.8 T, and for the two tetrahedral sites, for which an assignment is suggested, 49.5 and 38.7 T. The random distribution of the excess potassium ions resulted in line broadening of the 12k octahedral, and bridging tetrahedral, spectra.Spin flopping in the antiferromagnetic lattice at 4.2°K in an applied magnetic flux density of 2.5 T was inferred from the absence of any effect of the applied field on the spectrum. The consequently low values of the magnetocrystalline anisotropy constants are compared and contrasted with those of the similarly structured ferrimagnetic hexagonal ferrites, and the possible role of Fe²⁺ in reducing the anisotropy is discussed.     

Negative ion mass spectrometry: The generation of high concentrations of low energy molecular negative ions at high source pressures

The effect of the addition of argon and other gases upon the intensities of negative ion species formed in an electron impact source has been investigated. The negative ion current generated for a series of aromatic compounds has been investigated as a function both of sample and argon pressure in the ion source of a ZAB-2F mass spectrometer. For all compounds studied, a striking enhancement of molecular negative ion current occurred on increasing either the presure of the sample or of argor. The results are consistent with thermalization of the 50 eV electrons by collisions with neutral molecules in the high pressure ion source and collisional stabilization of the negative ions initially formed. Analytical applications of the technique are discussed.     

79Br NMR spectroscopy as a practical tool for kinetic analysis

⁷⁹Br NMR spectroscopy has been used to monitor a series of reactions in which the bromide ion is produced, including the Menschutkin reaction of pyridine with a range of substituted benzyl bromides and a Heck coupling process. In cases where the process could also be monitored using ¹H NMR spectroscopy, the kinetic analyses using heteronuclear magnetic resonance spectroscopy were shown to be completely consistent. Both the utility of the process in following reactions which may be difficult to analyse using other techniques and the practical limitations associated with solvent choice are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Magnetic Resonance Microscopy of Heterogeneity in Polymer Electrolyte Membranes

Magnetic resonance microscopy of Nafion 117 is used to measure the spatial variation of solvent dynamics in protonated (acid form) membranes immersed in solvent. Spatial resolution allows determination of membrane material property heterogeneity via T ₂ and diffusion variations to be easily studied in the presence of strong free solvent signal. Variation of T ₂ and diffusion as a function of solvent methanol–water mole fraction is measured. Spatially averaged T ₂ and diffusion data are in general agreement with previous bulk nuclear magnetic resonance studies; however, significant disparities from sample to sample at fixed solvent concentration are present in the spatially averaged data. The variation is determined to be due to changes in solvent mobility within the membrane over scales of the order of 10 mm, indicating macroscale heterogeneity of the solvent saturated membrane morphology. Authors' address: Sarah L. Codd, Department of Mechanical and Industrial Engineering, Montana State University, Bozeman, MT 59717-3800, USA     

Methodologies for detection of hemoglobin-based oxygen carriers

Blood substitutes based on hemoglobin or hemoglobin-based oxygen carriers (HBOCs) are oxygen-carrying therapeutic agents developed for use in operations and emergencies in place of donated blood. Increased oxygen-carrying capacity through the use of blood substitutes could help elite athletes to lengthen endurance capacity and improve their performance. As blood substitutes become more readily available, it is essential that a qualitative detection method for their abuse in sport is available. Ideally, such a method would be simple and inexpensive. This study investigates methods that could be used as screening procedures to easily detect HBOCs in plasma and develops tests that can unequivocally confirm their presence. The investigation into the screening method indicates that the direct visual screening of plasma discoloration is the most appropriate with detection limits of less than 1% HBOC in plasma. Two methods are shown to confirm the presence of exogenous hemoglobin in plasma samples, size-exclusion chromatography with photodiode array detection and high-performance liquid chromatography analysis of enzymatic digests with detection by electrospray mass spectrometry. This work emphasizes the need for cooperation between drug developers and sports testing laboratories to ensure that methods for the detection of putative doping agents are available prior to product release.     

Classification of brain tumours using short echo time 1H MR spectra

The purpose was to objectively compare the application of several techniques and the use of several input features for brain tumour classification using Magnetic Resonance Spectroscopy (MRS). Short echo time 1H MRS signals from patients with glioblastomas (n = 87), meningiomas (n = 57), metastases (n = 39), and astrocytomas grade II (n = 22) were provided by six centres in the European Union funded INTERPRET project. Linear discriminant analysis, least squares support vector machines (LS-SVM) with a linear kernel and LS-SVM with radial basis function kernel were applied and evaluated over 100 stratified random splittings of the dataset into training and test sets. The area under the receiver operating characteristic curve (AUC) was used to measure the performance of binary classifiers, while the percentage of correct classifications was used to evaluate the multiclass classifiers. The influence of several factors on the classification performance has been tested: L2- vs. water normalization, magnitude vs. real spectra and baseline correction. The effect of input feature reduction was also investigated by using only the selected frequency regions containing the most discriminatory information, and peak integrated values. Using L2-normalized complete spectra the automated binary classifiers reached a mean test AUC of more than 0.95, except for glioblastomas vs. metastases. Similar results were obtained for all classification techniques and input features except for water normalized spectra, where classification performance was lower. This indicates that data acquisition and processing can be simplified for classification purposes, excluding the need for separate water signal acquisition, baseline correction or phasing.