Differentiation between dexamethasone and betamethasone in a mixture using multiple mass spectrometry

Direct gas chromatographic methods to analyse phenylurea pesticides are discouraged by the thermal instability of these compounds, that in conventional hot splitless inlet systems leads to extensive and irreproducible formation of isocyanates and amines. However a careful control of the operating conditions, like the inlet temperature, the pressure and the presence of suitable chemical additives (as acetic acid, low-molecular-mass amines, organic anhydrides) can either: (i) minimise the thermal decomposition enabling the direct GC-MS analysis of phenylureas, or (ii) lead to reproducible conversion to isocyanates. Experimental design was employed to study the effect of the experimental variables on the thermal transformation of phenylurea pesticides in splitless inlet system. Two strategies were alternatively optimised: (i) the minimisation of degradation reactions to increase the signal of phenylureas; (ii) the maximisation of the degradation to isocyanates that are in turn determined. The maximal yields in isocyanate were obtained with high inlet temperatures, low carrier flows in the injection phase and the presence of acetic anhydride. By contrast, the use of relatively low inlet temperatures, high carrier flows during the injection and the presence of an amine maximise the response of the parent compounds.     

Absorptiometric determination of microgram amounts of uranium with Rhodamine B

Uranium (VI) can be extracted as a complex with rhodamine B into a benzeneetherhexone solvent from a benzoate buffered solution. Optimum conditions for the colour development are defined, leading to a molar absorptivity of 102700 mmol^-1 cm² at 555 nm, the highest yet reported for a uranium complex. The determination of uranium in the range 0.02–3 μg/ml in nitrate samples is described. The relative standard deviation ranges from 20 to 0.6%. The interference of several ions is investigated.     

Analysis of ion binding on humic substances and the determination of intrinsic affinity distributions

Humic substances are characterized by a variable electric potential and by a variety of binding sites leading to chemical heterogeneity. Binding of ions to these substances is influenced by both factors. A methodology based on acid—base titrations at several salt levels is presented that allows for the assessment of an appropriate electrostatic double-layer model and the intrinsic proton affinity distribution. The double-layer model is used for the conversion of pH to pH_S for each data point, where H_S is the proton concentration in the diffuse layer near the binding site. It is shown that with an appropriate double-layer model the proton binding curves at different salt levels converge into one “master curve” when plotted as a function of pH_S. The intrinsic proton affinity distribution can then be derived from the “master curve” using the LOGA method. A rigorous analysis of metal binding to humic substances is complex and in practice is not feasible. Under two different (simplifying) assumptions, namely fully coupled and uncoupled binding, it is shown how intrinsic metal ion affinity distributions can be obtained. Model calculations show that apparent metal ion affinity distributions do not resemble the intrinsic metal ion affinity distribution.     

Study of a glassy carbon electrode in amperometric detection using d.c., normal and differential pulse techniques

The results of a comparative study on d.c., normal pulse and differential pulse techniques applied to anodic amperometric detection at a glassy carbon electrode in a voltammetric flow-through cell are presented. The important aspects examined are response time, linearity, limit of detection and selectivity. It is shown that the d.c. mode is the most favourable as long as no adsorption of oxidation products takes place. If strong adsorption occurs, normal pulse detection is recommended, although the limit of detection is somewhat larger.     

Low-temperature infrared and raman studies: XI. The vibrational behaviour of ammonium perchlorate- its phase changes and the rotational freedom of its ions

An infrared, Raman and X-ray diffraction study on NH₄CIO₄. was conducted in the temperature ranges 300 to 17 °K, 300 to 130 °K and 300 to 120 °K respectively. The infrared studies also included the deuterated salt. The spectra are discussed and vibrational assignments made. Changes in the infrared spectra, as well as intensity measurements of the vibrational bands indicate anomalous behaviour in the region 100–110 °K as well as 17–50 °K. Possible reasons for these changes are proposed. The rotational behaviour of the ammonium ion upon cooling and hydrogen bonding are discussed.     

Determination of limonin in grapefruit juice and other citrus juices by high-performance liquid chromatography

A method has been developed for the quantitation of the bitter component limonin in grapefruit juice and other citrus juices. The sample clean-up consisted of centrifugation, filtration and a selective, rapid and reproducible purification with a C2 solid-phase extraction column. The limonin concentration was determined by high-performance liquid chromatography on a C18 column with UV detection at 210 nm. A linear response was obtained from 0.0 to 45 ppm limonin. The minimum detectable amount was 2 ng. The minimum concentration which was detected without concentration with good precision was 0.1 ppm. The method was also used for the determination of limonin in different types of oranges, including navel oranges, mandarins, lemons, limes, pomelos and uglis.     

Ab initio studies of structural features not easily amenable to experiment. 17. The molecular structures of some strained cyclic hydrocarbons and estimates of their strain energies

The molecular structures of tricyclohexane, norbornane, quadricyclane, and cubane were completely refined by standard ab initio force relaxation on the 4-21G level. The results can be used as a basis to interpret some contradictory experimental reports found in the literature and to establish some hitherto unobserved structural trends involving the C? C and C? H bond distances of the cases studied. Group Delta;E values for C? (H)₃(C), C? (H)₂(C)₂, and C? (H)(C)₃ (Benson's notation) derived from the total energies of the completely relaxed 4-21G geometries of a number of unstrained hydrocarbons are also listed. The values are used to estimate the strain energies of the systems studied and of the optimized 4-21G geometries of cyclopropane, cyclobutane, cyclohexane, and bicyclo (2.1.0)pentane. Cooperative effects in the strain energies are discussed.