Linear flow velocity optimum

In liquid chromatography, the diffusion rates are slower than that in gas chromatography, and the values of DM and D are very small therefore, the minimum H value is obtained at a low flow rate, as shown by curve E in Figure 5.6. The value of H increases slowly at higher flow rates in liquid chromatography. An experimental result is shown in Figure 5.7. The HETP was minimal at a certain flow rate, and the measured optimum value was less than 10 pm for this column. The optimum flow rate was about 0.9 ml min - corresponding to a linear flow velocity of about 55 mm min -. ... 

An optimum of flow profile has recently been achieved for capillary electrophoresis [76], when the mobile phase migration is done by electroosmosis. It is the situation that has been utilised for electrochromatography. For planar chromatography, the optimum of the linear flow velocity is approximated when the convex shape of a forced-flow profile chiefly counterbalances the concave profile of the advancing meniscus, it is possible to reach optimal efficiency as a function of linear flow velocity [67]. This is demonstrated in Fig. 10.6. At the optimum of efficiency, the microflow profile is nearly linear as the convex and concave forms of laminar flow and the concave form of the advancing meniscus counterbalance each other (Fig. 10.7). 

Even if a medium dispersion FIA system is required, it is economical to use narrow channels. The sample and reagent economy is improved when narrow channels are used because, for the same linear flow velocity, the pumping rate (2 in a tube of radius R is only one-fourth of that required for a tube of radius 2R. The optimum internal diameters of mbes coimecting the injection port and the detector are 0.5 to 0.8 mm. The narrower channels are more subject to clogging. 

How high should the value of the mobile phase linear flow velocity, u, be for chromatographing phenetol (Dm = 0.49 x lO m s ) and a hypothetical macromolecnle (Dm = 6.2 x 10 " m s ) with an optimum reduced flow-rate of Vopt = 3 The particle diameter dp of the stationary phase is 5 pm. 

Three factors were studied regarding the resolution using fully off-line OPLC (50). Use of the optimal linear flow velocity in relation to HETP produces the highest resolution. The relationship between resolution and distance of development is approximately linear, and the resolution increases with increasing front distance. The layer thickness shows an optimum value (80-160 pm) in terms of resolution. 

Fig. 10.7. Optimum of flow velocity reflects the linear shape of the microflow profile.

Differentiation of Eq. 16.29 shows that the optimum mobile phase velocity, which is a fimction of the displacer retention factor and concentration, is minimum for Kd = 1, which also corresponds to the optimum displacer conditions for iiiinimiun SLT (see previous section) [11]. This combination of results is obviously imattractive, as it does not correspond to what would have been required for a high production rate. The optimum flow velocity is almost always much lower in displacement chromatography than the optimum velocity imder linear... 

The measurement and control of carrier gas flow is essential for both column efficiency and for qualitative analysis. Column efficiency depends on the proper linear gas velocity which can be easily determined by changing the flow rate untU the maximum plate number is achieved. Typical optimum values are 75 to 90 mL/min for 1/4" outside diameter (o.d.) packed columns 25 mL/min for 1/8" o.d. packed columns and 0.75 mL/min for a 0.25 fim i.d. open tubular column. These values are merely guidelines the optimum value for a given column should be determined experimentally. 

You should generate your own van Deemter plot for the analysis temperature you intend to use and determine the optimum linear velocity best for your analysis. Most people set their linear velocity at the initial oven temperature. As the temperature rises during a temperature-programmed run, the optimum linear velocity will change. For many separations, you do not need to consider optimization, as it is not critical to the separation and the change in optimum flow velocity is small. 

The cavities can carry catalyst spheres, cylinders or rings with diameters of about 2-8 mm. The very narrow slit between the catalyst body and reactor wall leads to linear gas velocities of 0.2-3.2m/s in the cavities, which is fully comparable with linear gas velocities in industrial reactors. The high gas flow in the connecting channels leads to optimum mixing in the following cavity. Four modules are charged per reactor of this seven-tuhe parallel unit. ... 

An important parameter when considering GC resolution of the sample components is the carrier gas linear velocity (flow rate, F), which can be determined by injecting 5-50 /A of argon or butane and measuring the time from injection to detection by the mass spectrometer. An optimum linear velocity using helium as a carrier gas is approximately 30 cm/sec and... 

The plate height, and thus the total number of theoretical or effective plates, depends on the average linear carrier gas velocity (van Deemter relationship) and, for a particular carrier gas, the efficiency will maximize at a particular flow rate. Only at the optimum carrier gas flow rate are n, N, and HETP Independent of the column length. The efficiency will also depend on the column diameter (see section 1.7.1) where typical values for n, N, and HETP for different column types can also be found. Values for n, N, and HETP are reasonably independent of temperature but may vary with the substance used for their determination, particularly if the test substance and statioKary phase are not compatible. 

Resolution in forced-flow development is not restricted by the same limitations that apply to capillary flow controlled systems. The maximum resolution achieved usually corresponds to the optimum mobile phase velocity and R, increases approximately linearly with the solven)t migration distance (48). Thus there is... 

Figure 2.2—Optimum linear velocity and viscosity of carrier gas. The optimal mean linear velocities of the various carrier gases are dependent on the diameter of the column. The use of hydrogen as a carrier gas allows a faster separation than the use of helium while giving some flexibility in terms of the flow rate (which can be calculated or measured). This is why the temperature program mode is used. The significant increase in viscosity with temperature can be seen for gases. In addition, the sensitivity of detection depends on the type of carrier gas used.

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