Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 m of the CO₂ laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 s and a long pulse of 250 s. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO₂ radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm³. Histological examination revealed a minimal zone of 10–15 m of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 m is lower than at 10.3 and 10.6 m. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO₂ radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm³.     

Identification of ochratoxin A in food samples by chemical derivatization and gas chromatography-mass spectrometry.

The contamination of foods with ochratoxin A can be determined very sensitively by high-performance liquid chromatography (HPLC) with fluorescence detection. A novel procedure is described to confirm OA-positive results quantitatively down to the HPLC detection limit of 0.1 ppb. For this, ochratoxin A in the sample extract is converted into its O-methylochratoxin A methyl ester derivative, which is identified subsequently by gas chromatography-mass spectrometry negative-ion chemical ionization and multiple ion detection modes using the hexadeuterated O-methyl-d3-ochratoxin A methyl-d3 ester derivative as internal standard for quantification. In the analysis of more than 60 contaminated samples, the procedure was found to be very accurate.     

Liquid chromatographic separation of the thyroid hormones

The liquid Chromatographic separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine with a nonpolar stationary phase was studied as a function of pH, temperature, organic content of the mobile phase, and ionic strength using aqueous phosphate—acetonitrile, aqueous phosphate—methanol, and aqueous phosphate— n-propanol mobile phase systems. It was demonstrated that the quality of the thyroid hormone separations, as determined by normalized peak capacity values, was unchanged with temperature, remained relatively constant with increasing ionic strength, and was affected to the greatest extent by changes in pH and organic modifier content of the mobile phase. Chromatographic behavior of the compounds studied as a function of these variables was found to be consistent with existing Chromatographic theory and/or empirical observations. Recommended conditions are aqueous phosphate—methanol mobile phase, pH 2–5 (aqueous portion), and high temperature (60–70°C).     

Collisional depolarization of NO2 fluorescence as studied by Hanle effect measurements

We report on collisional depolarization of NO₂ fluorescence with use of Hanle effect (zero magnetic field level crossing) experiments. Single fine structure levels of NO₂ in several regions of the visible absorption spectrum predominantly near 593 nm and 514 nm are prepared by selective optical excitation and the depolarization of the fluorescence light versus a magnetic field is investigated. We find that the Hanle signal is in general a superposition of two Lorentzian shaped signals, each with a characteristic dependence on light intensity and NO₂ pressure. For NO₂ pressures >1 µ bar the collisional depolarization follows simple Stern-Volmer kinetics. However, an unusual pressure dependence is observed at NO₂ pressures <1 µ bar. In the same pressure range (<1 µ bar) we see also an unexpected resonance with significantly different properties as the Hanle signal.     

Activated alpha-alkyl-alpha-arylacetic acid enantiomers for stereoselective thin-layer chromatographic and high-performance liquid chromatographic determination of chiral amines

The separation of racemic benoxaprofen into the two benoxaprofen enantiomers by preparative high-performance liquid chromatography and the application of the activated enantiomers as derivatization reagents for the simultaneous stereoselective determination of chiral amines in biological material is described. Activated (+)- and (-)-benoxaprofen are both shown to be very sensitive and stable chiral fluorescence markers, applicable to thin-layer chromatography as well as to high-performance liquid chromatography.     

Consequences of the ‘Pedersen papers’ on crown type chemistry at Würzburg and Bonn Universities: From heteroaromatic crowns and podands to large molecular and crystalline cavities including multisite receptors,cascade molecules,chromoionophores, siderophores,surfactant-type,and extreme ligands

We thank the editors of this issue for the opportunity to present the historic development of crown chemistry at the Universities of Wurzburg and Bonn in memory of C. J. Pedersen, the originator of the crown ethers. His legacy of science has tremendously stimulated research at both universities.This paper is dedicated to the memory of the late Dr C. J. Pedersen.