Tube diameter, effect, small-volume

Figure 4.13 shows the influence of dead volume and connection design on the peak distortion and dead time of the analytical HPLC unit. Three runs were performed with the same flow rate and sample size (injection time 2 s). The straight black line describes the elution profile of a tracer in an optimal system with small dead volume and optimal connections. In the next example one of the tube connections was insufficient because, within the connector, the tube endings did not touch and a small dead volume is formed. The dashed line shows the new elution profile. A poor connection results in higher back mixing and, thus, peak distortion. This effect should be kept in mind for systems with many connections and small tube diameters where bad coimections can have a larger impact. 

It is critically important to understand this last point. There are two tubing volumes that can dramatically affect the appearance of your separation the one coming from the injector to the column and from the column to the detector flow cell. It is important to keep this volume as small as possible. The smaller the column diameter and the smaller the packing material diameter, the more effect these tubing volumes will have on the separation s appearance (peak sharpness). 

With the measurements subject to fluctuations of 20 or 30%, no accurate description of the profile is possible. All that can be said is that with moderate ratios of tube to particle diameter, the maximum velocity is about twice the minimum, and that when the particles are relatively small, the profile is relatively flat near the axis. It is fairly well established that the ratio of the velocity at a given radial position to the average velocity is independent of the average velocity over a wide range. Another observation that is not so easy to understand is that the velocity reaches a maximum one or two particle diameters from the wall. Since the wall does not contribute any more than the packing to the surface per unit volume in the region within one-half particle diameter from the wall, there is no obvious reason for the velocity to drop off farther than some small fraction of a particle diameter from the wall. In any case, all the variations that affect heat transfer close to the wall can be lumped together and accounted for by an effective heat-transfer coefficient. Material transport close to the wall is not very important, because the diffusion barrier at the wall makes the radial variation of concentration small. 

The high electric fields which can be used effectively because of the high surface-area-to-volume ratio offered by the small-diameter capillary tube, permitting an efficient heat dissipation... 

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