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71.
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On-column solvent exchange, using many of the principles of solid-phase extraction, has been implemented to significantly reduce evaporation cycle time following reverse-phase preparative HPLC. Additional benefits, such as a reduced potential for salt formation, thermal decomposition, and residual solvent, are also described. Fractions obtained from preparative separations, typically in a large volume of acetonitrile:water, are injected into the preparative HPLC and then eluted in acetonitrile, creating a new fraction in a volatile organic solvent. Minimal modification to the instrument was required, and unattended operation is possible. Acetonitrile evaporation is achieved within 3 h, compared with 17 h for aqueous-based fractions; lower temperatures can be used during the evaporation step; mobile-phase additives, likely to form salts with the target compound if concentrated in the fraction, are removed before evaporation; sample recovery and purity are unaffected. 相似文献
73.
Thomas Hilker Forrest G. Hall Nicholas C. Coops Yujie Wang Zoran Nesic T. Andrew Black Natascha Kljun Laura Chasmer 《Remote sensing of environment》2010,114(12):2863-3435
Eddy covariance (EC) measurements have greatly advanced our knowledge of carbon exchange in terrestrial ecosystems. However, appropriate techniques are required to upscale these spatially discrete findings globally. Satellite remote sensing provides unique opportunities in this respect, but remote sensing of the photosynthetic light-use efficiency (ε), one of the key components of Gross Primary Production, is challenging. Some progress has been made in recent years using the photochemical reflectance index, a narrow waveband index centered at 531 and 570 nm. The high sensitivity of this index to various extraneous effects such as canopy structure, and the view observer geometry has so far prevented its use at landscape and global scales. One critical aspect of upscaling PRI is the development of generic algorithms to account for structural differences in vegetation. Building on previous work, this study compares the differences in the PRI: ? relationship between a coastal Douglas-fir forest located on Vancouver Island, British Columbia, and a mature Aspen stand located in central Saskatchewan, Canada. Using continuous, tower-based observations acquired from an automated multi-angular spectro-radiometer (AMSPEC II) installed at each site, we demonstrate that PRI can be used to measure ? throughout the vegetation season at the DF-49 stand (r2 = 0.91, p < 0.00) as well as the deciduous site (r2 = 0.88, p < 0.00). It is further shown that this PRI signal can be also observed from space at both sites using daily observations from the Moderate Resolution Imaging Spectro-radiometer (MODIS) and a multi-angular implementation of atmospheric correction (MAIAC) (r2 = 0.54 DF-49; r2 = 0.63 SOA; p < 0.00). By implementing a simple hillshade model derived from airborne light detection and ranging (LiDAR) to approximate canopy shadow fractions (αs), it is further demonstrated that the differences observed in the relationship between PRI and ε at DF-49 and SOA can be attributed largely to differences in αs. The findings of this study suggest that algorithms used to separate physiological from extraneous effects in PRI reflectance may be more broadly applicable and portable across these two climatically and structurally different biome types, when the differences in canopy structure are known. 相似文献
74.
In numerical weather prediction (NWP) data assimilation (DA) methods are used to combine available observations with numerical model estimates. This is done by minimising measures of error on both observations and model estimates with more weight given to data that can be more trusted. For any DA method an estimate of the initial forecast error covariance matrix is required. For convective scale data assimilation, however, the properties of the error covariances are not well understood.An effective way to investigate covariance properties in the presence of convection is to use an ensemble-based method for which an estimate of the error covariance is readily available at each time step. In this work, we investigate the performance of the ensemble square root filter (EnSRF) in the presence of cloud growth applied to an idealised 1D convective column model of the atmosphere. We show that the EnSRF performs well in capturing cloud growth, but the ensemble does not cope well with discontinuities introduced into the system by parameterised rain. The state estimates lose accuracy, and more importantly the ensemble is unable to capture the spread (variance) of the estimates correctly. We also find, counter-intuitively, that by reducing the spatial frequency of observations and/or the accuracy of the observations, the ensemble is able to capture the states and their variability successfully across all regimes. 相似文献
75.
Gaussian elimination and LU factoring have been greatly studied from the algorithmic point of view, but much less from the
point view of the best output format. In this paper, we give new output formats for fraction free LU factoring and for QR
factoring. The formats and the algorithms used to obtain them are valid for any matrix system in which the entries are taken
from an integral domain, not just for integer matrix systems. After discussing the new output format of LU factoring, the
complexity analysis for the fraction free algorithm and fraction free output is given. Our new output format contains smaller
entries than previously suggested forms, and it avoids the gcd computations required by some other partially fraction free
computations. As applications of our fraction free algorithm and format, we demonstrate how to construct a fraction free QR
factorization and how to solve linear systems within a given domain. 相似文献
76.
为了明确Cr的质量分数高低对埋地用输油输气管线钢性能的影响,设计了4种Cr的质量分数的X70管线钢,研究了不同Cr的质量分数下管线钢组织及其力学性能,并采用高温高压反应釜进行了实验钢的CO2腐蚀试验.结果表明:钢中加入0.1%~0.8%的Cr后,其组织均由针状铁素体和准多边形铁素体构成,Cr元素均呈现明显的沿晶界分布状态;随着Cr的质量分数的增加,钢板强度随之升高,晶界中Cr分布密度随之增大,Cr(OH)3在腐蚀产物膜中的富集量增加,促使钢板的平均腐蚀速率降低;同时由于Cr(OH)3可以有效阻碍阴离子穿透腐蚀产物膜,因而大大减少了Cl-的催化作用导致的点蚀,使得钢板点蚀速率明显降低. 相似文献
77.
78.
在工程技术和科学研究的许多领域,傅里叶积分变换极为重要,但逆傅里叶积分变换手工计算比较困难,限制了傅里叶积分变换的应用范围。研究发现,逆傅里叶积分变换可以变换成沿复平面虚轴上的无穷区间主值积分,由此,导出一个逆傅里叶积分变换的计算公式,可用来快速完成逆傅里叶积分变换计算。 相似文献
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80.