Mobile phase velocity chromatographic

By definition, the e]q>erlmentally determined average mobile phase velocity Is equal to the ratio of the column length to the retention time of an unretalned solute. The value obtained will depend on the ability of the unretalned solute to probe the pore volume. In liquid chromatography, a value for the Interstitial velocity can be obtained by using an unretalned solute that Is excluded from the pore volume for the measurement (section 4.4.4). The Interstitial velocity Is probably more fundamentally significant than the chromatographic velocity in liquid chromatography (39). 

Under chromatographic conditions, the flow profile Is usually laminar and therefore the mobile phase velocity can be described by Darcy s law... 

For an understanding of band broadening in chromatographic systems, the linear velocity of the mobile phase is more important than the column volumetric flow rate. The mobile phase velocity and flow rate in an open tubular column are simply related by... 

One advantage of small bore columns is that they are operated at much lower mobile phase flow rates than the 4.6 mm columns, so there is a large reduction in solvent consumption and hence operating costs of the chromatograph. The efficiency of hplc columns does not depend on their diameter but it does depend on the velocity of the mobile phase in the column, so microbore columns are operated at velocities corresponding to the flow rates used with larger columns. If / (cm3 min-1) is the flow rate in a column with diameter d cm, and the mobile phase velocity is v cm min-1, / and v are related by ... 

In HPLC the following variables may be programmed, that is, changed in the course of a chromatographic run (a) mobile phase velocity (flow or ptessure programming) (h) column temperature (temperature program-... 

This makes it possible to tune solvent properties to optimize chromatographic separations. Because of the lower viscosity and higher diffusivity of supercritical fluids compared to common solvents, a higher mobile phase velocity can be used in the column, leading to a higher process throughput than that of liquid chromatography. 

FLOW. The rate at which zones migrate down the column is dependent upon equilibrium conditions and mobile phase velocity on the other hand, how the zone broadens depends upon flow conditions in the column, longitudinal diffusion, and the rate of mass transfer. Since there are various types of columns used in gas chromatography, namely, open tubular columns, support coated open tubular columns, packed capillary columns, and analytical packed columns, we should look at the conditions of flow in a gas chromatographic column. Our discussion of flow will be restricted to Newtonian fluids, that is, those in which the viscosity remains constant at a given temperature. 

The effects of mass transfer are different in the stationary and mobile phases. The resistance to mass transfer in the mobile phase varies with the reciprocals of mobile phase velocity and the diffusivity of the species. The resistance to mass transfer inside the stationary phase varies with the reciprocal of diffusivity and is proportional to the radius of the adsorbent granules attached to the chromatography plate, or the structural complexity of the internal pores in chromatographic paper. For both types of mass-transfer resistance, band stretching is proportional in each direction, as measured from the geometrical spot center, and increases in magnitude the greater the resistance. 

As an example, qualitative reproduction of the experimental concentration profiles shown in Figs. 3 and 4 is given in Fig. 5. The Eq. 11 constants of the adsorption isotherm, the mobile phase velocity, and effective diffusion coefficients were chosen to reproduce the shapes of the lengthwise cross sections of the chromatographic bands obtained in the experimental densitograms. 

Mobile phase velocity. Chromatographers define the cross-section average velocity of the mobile phase as the ratio, u = L/fo, of the column length and the hold-up time or retention of a nonretained compoimd. This time should be corrected for the transit time through the extracolumn volumes between injection valve and column inlet and between column inlet and detector. Chemical engineers tend to prefer the superficial velocity, or ratio of the flow rate and the column cross-section area, wq = Fv/S. Obviously, we have ... 

Chromatographers have always used a velocity that is straightforward to calculate from chromatographic data, the linear mobile phase velocity or chromatographic velocity... 

Finally, the validity of the chromatographic methods for the determination of isotherms is based on the assumption that phase equilibrium is reached rapidly during the experiment. The rate constant for phase equilibration must be large enough for the experimental results to be independent of the mobile phase velocity. When carrying out FA, FACP, or ECP measurements on proteins that tend to equilibrate slowly, it is advisable to check the influence of the flow velocity on the isotherms (Figures 3.15 and 3.44 [38]). 

This noninvasive method could allow the differentiation between the various packing materials used in chromatography, a correlation between the chromatographic properties of these materials that are controlled by the mass transfer kinetics e.g., the coliunn efficiency) and the internal tortuosity and pore coimectivity of their particles. It could also provide an original, accurate, and independent method of determination of the mass transfer resistances, especially at high mobile phase velocities, and of the dependence of these properties on the internal and external porosities, on the average pore size and on the parameters of the pore size distributions. It could be possible to determine local fluctuations of the coliunn external porosity, of its external tortuosity, of the mobile phase velocity, of the axial and transverse dispersion coefficients, and of the parameters of the mass transfer kinetics discussed in the present work. Further studies along these lines are certainly warranted. 

Morbidelli et al. [41] discussed a numerical procedure for the calculation of numerical solutions of the GRM model in the case of an isothermal, fixed-bed chromatographic column with a multicomponent isotherm. These authors considered two different models for the inter- and intra-particle mass transfers. These models can either take into account the internal porosity of the particles or neglect it. They include the effects of axial dispersion, the inter- and intra-particle mass transfer resistances, and a variable linear mobile phase velocity. A generalized multicomponent isotherm, initially proposed by Fritz and Schliider [34] was also used ... 

The accurate determination of the competitive equilibrium isotherms of the feed components of importance in the chromatographic system selected, and the measurement of the other parameters of importance (column efficiency as a function of the mobile phase velocity, viscosity of feed solutions in the mobile phase). 

Peclet number, Pe A dimensionless number used frequently by chemical engineers. Pe = (ux)/Dm, where x is a characteristic length, u the mobile phase velocity, and Dfn the molecular diffusivity. In chromatography, there are two Peclet numbers, defined by respect to the column length and to the particle size. The former must be large (i.e., 100 or more) for the simple models of chromatography to be valid. The second is known by chromatographers as the reduced mobile phase velocity. 

Velocity There are three different definitions of the mobile phase velocity, the chromatographic, the interfacial, and the superficial velocity They are all defined as the ratio of the mobile phase flow rate to an estimate of the column volume and differ by the estimate of the column volume used. The chromatographic velocity uses the total volume in the column that is available to the mobile phase the superficial velocity, the geometrical column tube volume and the intersticial velocity the external or extra-particle volume. See Chapter 2, Section 2.3.3. 

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