Column geometry

Macroscopic flow structure in the heterogeneous flow regime (Chen et al., 

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For this purpose, two gradients with a different tc are necessary (20 and 60 min are acceptable for a conventional LC column geometry) [9]. This methodology was... 

Flow rate. Its value must be selected according to the column geometry and back-pressure Umitations of the system. For 4.6 mm internal diameter columns (5 pm), 1 mLmin is adapted for a conventional length, while for short columns packed with smaller particles the flow rate can be increased. 

The demand for analytical laboratories to increase sample throughput provided the impetus for HPLC column manufacturers to introduce new stationary phases and a range of column geometries to meet requirements for speed, high sensitivity, and reduced sample availability. 

Jacobson, S. C., R. Hergenroder, L. B. Koutny, R. J. Warmack, and J. M. Ramsey, Effects of injection schemes and column geometry on the performance of microchip electrophoresis devices. Anal. Chem., 66, 1107-1113 (1994). 

Since 1992, a vast variety of rigid organic monolithic stationary phases with different chanistry, functionality, and column geometry has been reported for HPLC as well as CEC applications, as summarized by the number of excellent reviews [25-32,213], The development and enhancanent of monolithic stationary phases is stiU a rapidly growing area of research with scientific and industrial interest. 

In a column packed with a swollen gel, two solvent phases may be distinguished, one within the gel (the stationary phase), and the other outside (the mobile phase). The volume of solvent in the gel is known as the internal volume, designated V4, and the volume of solvent outside the gel particles is the void volume of the column, designated V0. A solute will distribute itself between the two phases to an extent measured by the distribution coefficient Kd, a constant determined by the nature of the solute, the solvent, and the gel, but independent of column geometry.27 The volume of solvent required for eluting the solute from the column in maximum concentration is called the elution volume and is designated Ve it is equal to the sum of the void volume and the volume of the stationary phase available to the solute, given by Kd V. Hence,... 

When one is deciding what column geometry is optimal for trace analysis with unlimited sample volume, two additional points should be evaluated. First, to what extent does the analysis require accurate and reproducible injections Strict performance specifications may eliminate microbore columns from consideration. The accuracy and reproducibility of injection systems that deliver 0.1-, 0.2-, and 0.5-/xL samples have not been adequately characterized. Second, if the analyte of interest requires postcolumn derivatization, construction of a postcolumn reaction system that is compatible with the exceedingly small band volumes characteristic of microbore columns may be extremely difficult, but not impossible. Apffel et al. (28) developed and evaluated both packed-bed and open tubular postcolumn reactors for use with 1-mm i.d. analytical columns. Catecholamines were postcolumn derivatized with o-phthal-aldehyde and detected spectrofluorometrically. The 5-/zm particle... 

Precolumn derivatization is often inadequate for dirty samples. In these cases, application of a postcolumn reaction detection system will often suffice. Deelder et al. (44) and van der Wal (45) have examined different configurations for postcolumn reactors and defined optimal selections on the basis of reaction time and type and effect on resolution and sensitivity. Both studies preferred the packed-bed reactor to the open tubular reactors when conventional column geometries were employed for separation, that is, 4.6 mm i.d. X 15 or 25 cm. 

Both models, (a) and (b), can formally be described by means of Fick s second law with a suitable common time boundary and the corresponding space boundaries, as shown in Fig. 16. Since the diffusion of the P-mer from the sol into the gel can be assumed as one-dimensional according to the column geometry and, in addition, the laterally diffusing macromolecules have all the same probability to reach the gel layer from the position c(0) of the sol at t = 0 (spontaneous diffusion of the P-mer from the sol into the gel), the second Fick s law... 

The amount of resin to pack in a column, column geometry, flow rates, pressure, column hardware, and wetted materials of construction should all be evaluated in development. Chromatography columns must be properly packed prior to validating the purification process. From a business perspective there should be some criteria other than purification of the product by which the quality of the packed column can be assessed prior to applying the feedstream, which by this time in the process is quite expensive. Height equivalent to a theoretical plate (HETP) and asymmetry determinations can be used to evaluate the quality of column packing, but may have limited value for some types of packed columns... 

In methods involving a gradient elution, the column geometry has more of an influence and it is preferable to have a length to diameter ratio of at least 5. 

The scope of the following chapter will be an attempt to identify system parameters limiting the efficiency of fluidized bed adsorption and to define operating parameters, e.g. adsorbent design, flow rate, column geometry, and... 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

One of the most important decisions that is left to the analyst when operating a liquid chromatograph is the choice of detector sensitivity. In some instruments the output from the sensor is monitored continuously over its entire dynamic range and so sensitivity is not an optional experimental parameter. Nevertheless, in this case, the sample size determines the concentration range over which the eluted solutes are monitored and thus an optimum sample size must be chosen. The detector should never be operated at its maximum sensitivity unless such conditions are enjoined by limited sample size or column geometry. Provided that there is adequate sample available, and the sample concentration when eluted is within the linear dynamic range of the detector, the maximum sample size that the column can tolerate should be used. This ensures that the detector noise is always minimal... 

The two derivations quoted above rely on the boundary condition that the gas flow through the annulus near the top of the bed is sufficient to fluidize the solids, and they therefore are valid only for beds of maximum spoutable depth. The derived values of APa/APmf thus represent the upper limit which would be approached with increasing bed depth for a pven system. This is borne out by the experimental results plotted in Fig. 11, which cover different materials as well as column geometries. The maximum APa/APmf ratios attained are seen to be in remarkably good agreement with the predicted values of 0.64-0.75. 

In Isothermal conditions a constant inlet gas pressure would automatically maintain a constant volumetric and linear flow rate. When the temperature of the column is programmed while the inlet gas pressure is kept constant, the flow rate will change considerably, becoming lower because of the modifications of the column geometry, expansion of the gas, and modifications of the gas viscosity, which increases with (T) . Besides isobaric operation, which is common, constant flow at programmed temperature capability is also available for certain instruments. 

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