Mass spectrometry of sterols. Electron ionization induced fragmentation of C-4-alkylated cholesterols.
The electron impact ionization of C-4-alkylated cholest-5-en-3beta-hydroxysterols has been investigated. The mass spectra of the C-4-alkylated cholesterols contain a number of ions in the high mass region for which analogous ions are not found in the spectrum of cholesterol. Detailed studies of the composition and origin of these ions have been made by high resolution mass spectrometry and analysis of metastable ions. In addition, a large number of isotopically (deuterium and 18O) substituted C-4-alkylated analogues have been prepared to assist in the interpretation of the spectra. The combined results indicate the occurrence of a number of very complex and unusual electron ionization induced fragmentations. Most notable of the findings reported herein concerns the demonstration of the formation of an ion involving loss of the elements of ring A with an intramolecular shift of the oxygen and hydrogen atoms of the hydroxyl function to the charge-retaining species.
Mass spectrometry in structural and stereochemical problems-XCVII
Abstract Aliphatic aldoxime and ketoxime spectra are characterized by substantial McLafferty rearrangement peaks, the site specificity of the accompanying hydrogen rearrangements having been demonstrated by deuterium labeling. In contrast to dipropyl ketone, the McLafferty rearangement of its oxime is accompanied to a small extent by methyl migration. Another diagnostic, though less prevalent process of such oximes is fission of the γ-bond. Alicyclic ketoximes, such as cyclopentanone and cyclohexanone axime, yield more complex spectra with many hydrocarbon ions. A characteristic heteroatom fission is the loss of an oxygen atom from the molecular ion—a reaction which is not observed in aliphatic oximes but does occur in aromatic ones such as benzophenone oxime. The previously claimed occurrence of an electron impact-induced Beckmann rearrangement in benzophenone oxime could not be substantiated. In terms of fragmentation-directing ability, the oxime function does not surpass the carbonyl group as shown by the mass spectra of the oximes of 2-, 3- and 11-keto steroids. Through the aid of several deuterated analogs plausible structures and reaction paths could be presented for several characteristic ions.
The effect of collimation on image quality and radiation safety in digital radiography of small animals
Collimating the primary x-ray beam to the specific area of anatomic interest has traditionally been strongly recommended to produce high quality radiographic images, even though some small animal patients can fit their entire torso on a single radiographic casette or detector. However, modern digital radiography may allow for whole-body projections for these patients at similar quality to specific collimated projections. Lateral and ventrodorsal projections of 31 small dogs were obtained to evaluate the effect of collimation on the quality of the resulting digital radiograph. Each projection was performed using both a collimated and whole-body technique. Scatter radiation was detected during each exposure as well to determine the effect of collimation on radiation safety for veterinary personnel. The resulting images were cropped to portray only the thorax, and were evaluated by five radiologists blinded to the procedure used for each image. Visual grading characteristics analysis was performed. An overall trend towards higher image quality was found in the collimated approach compared to the whole-body, but in most cases this change was not significant. Overall image quality for the ventrodorsal images was found to be significantly better in the collimated approach compared to the whole-body. Evaluation of image histograms suggests that the application of digital processing and the look-up table differs between them, and this may account for a large portion of any quality difference. Over twice as much scatter radiation was detected during the whole-body images as during the collimated images. Overall image quality was rated as adequate to high for almost all images with both approaches. Adequate collimation remains recommended to provide the highest quality images with the lowest exposure to veterinary personnel.