The temperature dependence of the inelastic neutron-scattering intensity from vitreous silica has been studied between 50 and 300 K down to frequencies of 150 GHz. Above 500 GHz one finds essentially harmonic behavior. Low-frequency anharmonic behavior can be described by a relaxational model using parameters determined from ultrasonic measurements. The dynamic structure factor shows that relaxation involves coupled rotational jumps of SiC>4 tetrahedra, with a jump width of the atoms smaller than 0.8 A. These results support the hypothesis of a common origin of low-temperature glass anomalies.PACS numbers: 63.50. +X, 62.80.+f, 78.30.Ly Although low-frequency excitations in glasses have been studied extensively over the last few years 1 "" 3 their nature remains a puzzle. In addition to sound waves, common to both glasses and crystals, glasses appear to contain three different kinds of "excess" excitations. First, infrared, 4 Raman, 5 thermal, 1,3 and inelasticneutron 3 studies show a density of harmonic modes in the range 200 GHz to 1 THz over and above the soundwave (Debye) contribution. Both heat-capacity and quantitative inelastic-neutron measurements 3 in vitreous silica, for example, give a density of states up to 8 times larger than the Debye value calculated from the measured sound velocities (up to 500 GHz). The neutron experiment also identifies the local motion involved in these excess modes as harmonic libration of coupled SiC>4 tetrahedra.Second, both Raman 5,6 and, less clearly, infrared 4 measurements below room temperature show a relaxational or quasielastic contribution to the excitation spectrum, reminiscent of states seen in acoustic studies 1,2 of thermally activated relaxation processes in a range of glasses. However, attempts to link optical and acoustic measurements by the description of relaxation with a model based on symmetric double-well potentials 5,7 have not been successful in matching both the temperature and frequency dependences of the quasielastic contributions.The third group of additional excitations comprises the tunneling or two-level states 1,8 studied in a wide range of glasses between 10 mK and 1 K. Results of thermal, acoustic, and electrical measurements have been successfully interpreted and related by theory although in most cases the microscopic nature of tunneling states is as yet unknown.The lack of any microscopic picture of low-frequency excitations has meant that no convincing synthesis of these three classes of phenomena has been possible, and each has usually been studied separately. In this Letter we report a study of quasielastic neutron scattering in vitreous silica which allows us to develop a picture of the microscopic motion involved in relaxation similar to that carried through previously 3 for excess harmonic modes. Analysis of the results using a distribution of asymmetric double-well potentials allows a consistent interpretation of all relaxation phenomena.The measurements were done on two cold-neutron time-of-flight spectrometers, the IN6 at the In...
This study aimed to prepare a molecularly imprinted monolithic extraction column (MIMC) inside a micropipette tip by situ polymerization with roxithromycin as the dummy template. The polymers possessed excellent adsorption capacity and class-specificity to multiple macrolide drugs. MIMC was directly connected to a syringe for template removal and for the optimization of extraction conditions without any other post-treatment of polymers. A liquid chromatography-tandem mass spectrometric method was developed for the selective microextraction and determination of macrolide antibiotics in animal muscles based on MIMC. High recoveries of 76.1–92.8% for six macrolides were obtained with relative standard deviations less than 10.4%. MIMC exhibited better retention ability and durability when compared with the traditional C18 and HLB cartridges. The proposed method shows a great potential for the analysis of macrolide drugs at the trace level in animal foodstuffs.
Before being subjected to the aging process, raw tobacco leaves (TLs) must be threshed and redried. We propose that threshing and redrying affect the bacterial communities that inhabit the TL surface, thereby influencing the aging process. However, these effects remain unclear. In this study, Illumina sequencing was applied to analyze the bacterial communities on both raw and redried TLs. Shannon's diversity value decreased from 3.38 to 2.52 after the threshing and redrying processes, indicating a large reduction in TL bacterial diversity. The bacterial communities also largely differed between raw TLs and redried TLs. On unaged raw TLs, Proteobacteria was the most dominant phylum (56.15%), followed by Firmicutes (38.99%). In contrast, on unaged redried TLs, Firmicutes (76.49%) was the most dominant phylum, followed by Proteobacteria (21.30%). Thus, the dominant genus Proteobacteria, which includes Sphingomonas, Stenotrophomonas, and Pantoea, decreased after the threshing and redrying processes, while the dominant genus Firmicutes, which includes Bacillus and Lactococcus, increased. Changes in the bacterial communities between raw and redried TLs were also noted after 1 year of aging. The relative abundance of dominant Proteobacteria taxa on raw TLs decreased from 56.15 to 16.92%, while the relative abundance of Firmicutes taxa increased from 38.99 to 79.10%. However, small changes were observed on redried TLs after 1 year of aging, with a slight decrease in Proteobacteria (21.30 to 17.64%) and a small increase in Firmicutes (76.49 to 79.10%). Based on these results, Firmicutes taxa may have a higher tolerance for extreme environments (such as high temperature or low moisture) than Proteobacteria bacteria. This study is the first report to examine the effects of threshing and redrying on bacterial communities that inhabit TLs.
The disposition and metabolism of nicotine in the brain is an important determinant of its exposure. We have developed a novel method for the dynamic determination of nicotine and its metabolites in rat brain and blood by simultaneous microdialysis sampling, stable-isotope labeling, and ultra high-performance liquid chromatography−high-resolution mass spectrometry (UHPLC−HRMS) assaying. Microdialysis probes were inserted into both the right striatum and jugular vein of Sprague−Dawley rats. The collections of dialystes after nicotine intraperitoneal injection were analyzed by the optimized UHPLC−HRMS. Nicotine-pyridyl-d 4 was used as a metabolic tracer, and several stably labeled isotopes were applied to calibrate the in vivo recoveries of retrodialysis. The quadrupole-Orbitrap HRMS provided reliable characterization of the nicotine derivatives with less than 3.5 ppm mass measurement accuracy. Good precision and accuracy were obtained for different analytes within the predefined limits of acceptability and the range of the standard curve. Nicotine and its 11 metabolites were identified in most microdialysis samples from the blood and brain tissue samples. Besides cotinine as the main metabolic product of nicotine, trans-3′-hydroxy-cotinine, nicotine-N-oxide, and norcotinine were proven to be the second most abundant metabolites. The other seven nicotine products, including 4-oxo-4-(3-pyridyl)-butanoic acid, 4-hydroxy-4-(3-pyridyl)-butanoic acid, cotinine-N-oxide, nicotine-N-glucuronide, cotinine-N-glucuronide, and trans-3′-hydroxy-cotinine-O-glucuronide, which have not been determined previously in animal brain, were present in minor amounts. The pharmacokinetic profile of nicotine metabolites indicated that the metabolic characteristic of nicotine in the brain was relatively different from that in the blood. The present work would provide comprehensive evidence for clarifying the differences between nicotine metabolism in the brain and peripheral system.
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