共查询到19条相似文献,搜索用时 375 毫秒
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考虑到毛细管色谱分析过程中径向浓度梯度对传质过程的影响,建立了相应的色谱质量平衡模型,模仿Giddings的假设,将模型适当化简,并通过数学变换的方法得到了流出曲线的一级矩及二、三级中心矩表达式。结果表明,溶质的径向扩散不仅对柱效、峰展宽、峰形特征等有影响,而且对溶质的保留时间也有一定的影响。 相似文献
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色谱流出峰的拖尾现象普遍存在于制备和大型色谱分离过程中,它直接影响分离的产率和回收率.本文提出采用色谱流出峰形的不对称偏差度来表征色谱流出峰的拖尾程度;并基于液相制备色谱分离过程FAD-SMT模型及吸附速率理论,通过计算机模拟,定量分析了吸附剂性能和操作参数对色谱流出峰形不对称性的影响。结果表明:不仅是吸附剂的热力学和动力学性能(包括吸附相平衡关系、液固两相间的传质阻力);而且柱的设计和吸附剂的装填状况(包括轴向扩散系数),以及色谱分离的操作条件(进料时间、浓度和流速等)都直接影响色谱流出峰形的不对称性。随着吸附相平衡等温线的非线性程度增大,或者总传质系数的减小,色谱流出峰形的不对称偏差度明显增大;吸附剂吸附容量的减小也将引起色谱流出峰形的不对称偏差度的增加;色谱流出峰形的不对称偏差度与进科体积、浓度和流体线速,以及轴向扩散系数的增大成正比。 相似文献
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本文在C、Horvath建立的液相色谱柱内质量平衡的基础上考虑到柱外效应的影响,建立起整个色谱系统的质量平衡模型、并求得了色谱系统的一阶距,二、三阶中心距的表达式。同时,通过指数修正的高斯模型中描述色谱峰展宽的参数σ和拖尾因子τ与二、三阶中心距的关系,详细地考察了色谱参数D_m,k_d,d_p和检测池体积对我们以所报导的c、 与保留值之间线性关系的影响。结果表明在一般高效柱液相色谱系统中,σ、τ与保留值间线性关系是成立的,但在K′值小于0.5时与线性关系有一定的偏差;柱外效应对τ的影响比对σ的影响大得多。 相似文献
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基于溶质在径向色谱柱内输运的质量平衡方程, 在线性分配条件下, 得到了描述分离柱效和流出曲线形状各参数的理论表达式, 也对柱效和流出曲线对称性的变化趋势加以系统讨论. 结果表明: 径向色谱中, 柱效与体积流速之间的关系与轴向色谱中柱效与流动相线速度的关系在趋势上相同; 在较高流速下运行时, 径向色谱仍可以得到高柱效. 随着溶质容量因子、进样时间的增加, 柱效单调降低. 柱直径和柱长对柱效的影响存在交叉, 设计半径较大而长度较短的色谱柱将更有利于提高分离柱效. 径向色谱适宜于大分子样品的稳定分离方法建立, 也预示其对于蛋白、DNA等样品的制备分离具有明显优势. 相似文献
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The effects of molecular collisions between the mobile phase and the solute in gas–solid chromatography 下载免费PDF全文
In chromatographic processes, molecular collisions between the mobile phase and the solute result in the transfer of kinetic energy. Based on these interactions, the relationship between the gauge pressure of the carrier gas at the column inlet and the partition frequency of the solute is derived; consequently, the relationship between the column temperature and partition frequency can be obtained. These relationships have been experimentally validated. The change in the peak shape described herein has been successfully explained using this relationship: the partition frequency was calculated from the theoretical plate number of a tailing peak. We propose a new mechanism for peak tailing using plate theory, which states that as the number of plates increases, the symmetry of the peak increases. 相似文献
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In a previous paper a model function was tested in order to approximate the peak shape obtained on non-polar column by injecting different compounds. The simulation of the symmetrical or non-symmetrical shape of gas chromatographic peaks was satisfactory. In this paper, the influence of the amount of injected substance was investigated at different values of inlet pressure and carrier gas velocity, in order to evaluate the relative contribution to the total peak area and shape of the symmetrical distribution due to partition phenomena and of the non-symmetrical and tailing distribution due to adsorption-desorption kinetics. The effect of the molecular mass and of the chain length of compounds belonging to the homologous series of 1-alcohols and n-alkanes on the adsorption phenomena was evaluated. 相似文献
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In analogy to the focusing effective in capillary GC, performed with temperature programming but also with sectional cooling of the column inlet as in multidimensional capillary GC, peak focusing can easily be attained in SFC by adjustment of the mobile phase pressure as well as the column temperature. This may be of practical use in connection with sampling techniques giving poor, i.e. broad and unsymmetrical, peak shapes. Such disturbances may occur, for example, in time controlled valve sampling over longer switching times. Generally, all other negative influences on peak shape can be suppressed or compensated by trapping within the column inlet. Special trapping devices and “retention gaps” may also be coupled to the column inlet in order to create narrow starting plug widths. Positive pressure (density) and negative temperature programs give rise to peak compression besides the increase of peak capacity of the separation. 相似文献
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An HETP equation for the capillary column is developed that takes into account the dependence of gaseous diffusion on pressure, the compressibility of the mobile phase, together with the unique relationship between mobile phase velocity, and the resistance to mass transfer in the stationary phase. The equation is used to develop a procedure for column optimization and expressions are derived that allow the optimum column radius and optimum column length to be calculated for a given fixed inlet pressure. It is shown that fast, simple separations are optimally achieved using relatively short small diameter columns. Conversely, optimum performance for the separation of complex mixtures requiring higher efficiencies requires the use of long columns with relatively large diameters. 相似文献
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Philip L. Wylie Kenneth J. Klein Michael Q. Thompson Bruce W Hermann 《Journal of separation science》1992,15(11):763-768
A split/splitless capillary injection port has been developed for electronic pressure programming (EPP) in gas chromatography. The inlet may be operated in several modes: constant pressure, constant flow, vacuum compensation (for gas chromatography–mass spectrometry (GC-MS)), pressure-programmed, or a combination mode enabling a pressure program to be followed by constant flow. A pressure-programming technique has been tried which uses high pressure (high column flow rate) at the time of injection followed by reduction in inlet pressure to a value required for normal chromatography. Sample is swept rapidly from the inlet and into the column, reducing contact with the hot, active inlet surfaces which cause sample decomposition. The decomposition of endrin and 4,4′-DDT, two labile pesticides, can be substantially reduced using this technique and modest improvements were also observed with the carbamate pesticide carbaryl. 相似文献
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The temperatures of the metal wall along a chromatographic column (longitudinal temperature gradients) and of the liquid phase across the outlet section of the column (radial temperature gradients) were measured at different flow rates with the same chromatographic column (250 mm x 4.6 mm). The column was packed with 5 microm C18-bonded silica particles. The measurements were carried out with surface and immersion thermocouples (all junction Type T, +/-0.1 K) that measure the local temperature. The column was either left in a still-air bath (ambient temperature, T(ext) = 295-296 K) or insulated in a packing foam to avoid air convection around its surface. The temperature profiles were measured at several values of the inlet pressure (approximately = 100, 200, 300 and 350 bar) and with two mobile phases, pure methanol and a 2.5:97.5 (v/v, %) methanol:water solution. The experimental results show that the longitudinal temperature gradients never exceeded 8 K for a pressure drop of 350 bars. In the presence of the insulating foam, the longitudinal temperature gradients become quasi-linear and the column temperature increases by +1 and +3 K with a water-rich (heat conductivity approximately = 0.6 W/m/K) and pure methanol (heat conductivity approximately = 0.2 W/m/K), respectively. The radial temperature gradients are maximum with methanol (+1.5 K at 290 bar inlet pressure) and minimum with water (+0.8 K at 290 bar), as predicted by the solution of the heat transfer balance in a chromatographic column. The profile remains parabolic all along the column. Combining the results of these measurements (determination of the boundary conditions on the wall, at column inlet and at column outlet) with calculations using a realistic model of heat dispersion in a porous medium, the temperature inside the column could be assessed for any radial and axial position. 相似文献
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A procedure previously described for the prediction of the plate height of capillary columns operated at different inlet pressure of the carrier gas and at various column temperatures by using few retention data measured under isobaric conditions was modified and improved in order to permit the prediction of the retention times and of the peak widths at various heights. It is therefore possible to calculate the ratio, delta, between the peak width at different heights and the peak width at half height, whose value is used to predict the resolution at different height of two closely eluting peaks. It was found that the delta values do not depend on temperature and inlet pressure and are a characteristic of the used column; they can therefore be used in order to calculate the resolution in any temperature and inlet pressure condition. The method was used to predict the retention time, the peak width and the resolution of polar and non-polar compounds (alkanes, alkenes, chloroalkanes, alcohols, ketones) on capillary columns of different length and polarity by using as the starting data retention and width values measured in three isobaric runs only. 相似文献
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Fifteen liquid chromatographic experiments were investigated using a whole‐column detection (WCD) system and a conventional post‐column UV/Vis detector. The peak widths obtained from chromatograms were found dependent on the retention factor; the larger the retention factor was the greater the peak width. However, the on‐column spatial peak widths were dependent on the locations where they were measured in the column. The peak widths monitored at 17 cm from the column inlet were found essentially the same no matter what their retention factors were. In addition, a linear relationship was found between the chromatographic peak width and the reciprocal of the average linear rate of the solute migration. The peak widths on chromatograms did not reflect how they appeared in the column; instead, the widths were determined by the solute speed passing the detector. 相似文献
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Krzysztof Kaczmarski Fabrice Gritti Joanna Kostka Georges Guiochon 《Journal of chromatography. A》2009,1216(38):6575-6586
Advanced instruments for liquid chromatography enables the operation of columns packed with sub-2 μm particles at the very high inlet pressures, up to 1000 bar, that are necessary to achieve the high column efficiency and the short analysis times that can be provided by the use of these columns. However, operating rather short columns at high mobile phase velocities, under high pressure gradients causes the production of a large amount of heat due to the viscous friction of the eluent percolating through the column bed. The evacuation of this heat causes the formation of significant axial and radial temperature gradients. Due to these thermal gradients, the retention factors of analytes and the mobile phase velocity are no longer constant throughout the column. The consequence of this heat production is a loss of column efficiency. We previously developed a model combining the heat and mass balance of the column, the equations of flow through porous media, and a linear isotherm model of the analyte. This model was solved and validated for conventional columns operated under moderate pressures. We report here on the results obtained when this model is applied to columns packed with very fine particles, operated under very high pressures. These results prove that our model accounts well for all the experimental results. The same column that elutes symmetrical, nearly Gaussian peaks at low flow rates, under relatively low pressure drops, provides strongly deformed, unsymmetrical peaks when operated at high flow rates, under high pressures, and under different thermal environments. The loss in column efficiency is particularly important when the column wall is kept at constant temperature, by immersing the column in a water bath. 相似文献