PubMed · 3340174
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J B Pethica. 1988-01-28. Scanning tunnelling microscopes. Atomic-scale engineering.. https://doi.org/10.1038/331301a0
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Detection limits of poly(ethylene glycol) were examined in the mass range 2000-6000 Da. Using an aerospray sample deposition technique, highly uniform sample surfaces were produced. This method allows signal averaging of spectra from up to 400 shots on the same sample spot. It is found that, as the material available for desorption is decreased, the overall average sample consumption per shot is decreased. Experimentally determined detection limits of 40 and 280 fmol (based on the average molecular masses of 2000 and 6000) were found for PEG 2000 and PEG 6000, respectively. The sample spectra show oligomer distributions in agreement with their higher concentration counterparts. However, at the lowest signal-to-noise levels, oligomers at the extremes of the distribution are no longer detected, making the polymer distribution appear to be narrower in mass range.
Polycrystalline silver catalysts are used extensively for the partial oxidation of methanol to formaldehyde, which is then primarily incorporated in the synthesis process for adhesives and resins. In order to maximize formaldehyde production it is essential to gain a comprehensive understanding of the complex microstructural changes which occur in the catalyst during reaction conditions. However, conventional electron microscopic techniques are incapable of imaging catalysts at high temperatures and in the presence of a gaseous atmosphere. Therefore, an environmental scanning electron microscope (ESEM) has been used to image polycrystalline silver catalysts during simulated industrial conditions. The most dramatic effect of heating various catalysts to 700 degrees C in the ESEM chamber was the formation of "pinholes" in the silver surface. These "pinholes" occur at specific temperatures and are inherently associated with the catalytic process, resulting from near-surface explosions caused by subsurface hydroxyl recombination. Of particular interest was the nature and location of the holes, which preferentially occur in the vicinity of surface defects such as platelets and edge structures. To the best of our knowledge, this study represents the first time that the progress of a catalytic reaction has been observed under in situ conditions by scanning electron microscopy.
The objects of this investigation were (i) to prepare and characterize a new anhydrous theophylline phase that is metastable under ambient conditions, and (ii) to prepare model tablet formulations containing either this metastable anhydrate (I*) or stable anhydrous theophylline (I), store them under different relative humidity (RH) conditions, and compare their dissolution behavior. I* was prepared by dehydration of theophylline monohydrate (II). Variable temperature X-ray powder diffractometry of II revealed the following series of transitions: II-->I*-->I. The metastable anhydrate, I*, which has not yet been reported in the literature, appears to be related monotropically to I. It was characterized by ambient and variable temperature X-ray powder diffractometry, Karl Fischer titrimetry, and thermoanalytical techniques (differential scanning calorimetry and thermogravimetric analysis). Tablet formulations containing either I* or I were prepared and stored at 33 and 52% RH (room temperature). The solid state of the drug was monitored by X-ray powder diffractometry and the tablets were subjected to the USP dissolution test. In tablets, I* completely converted to I in < or = 10 days when stored at either 33 or 52% RH. Scanning electron microscopy provided direct visual evidence of recrystallization. This recrystallization was accompanied by a decrease in the dissolution rate of the stored formulations that was so pronounced in the formulations stored at 52% RH that they failed the USP dissolution test. The in situ solid-state transition appears to be responsible for the decrease in dissolution rate observed following storage. Stored tablets containing I showed neither a phase transition nor an alteration in their dissolution behavior.