Extra Column Dispersion - Low Dispersion Connecting Tubes > Serpentine Tubes > Page 33
The serpentine tube is protected by an outer sheath which also provides some mechanical strength. In a coiled tube, the flow of fluid is continually deflected in the same direction, but in the serpentine tube the reversal of the flow at each bend also induces some turbulence, which strongly augments the increase in diffusivity. As a result low dispersion is produced at relatively low solvent velocities.
Curves for a serpentine tube having the dimensions given in figure 12 relating the variance per unit length of the tube (H) against flow rate are shown in figure 13. The flow rate is employed as the independent variable as an alternative to the more usual linear velocity because, in practice, the flow rate is defined by the column with which the low dispersion tubing is to be used. In fact, the column flow rate is independently defined by the chromatographic characteristics of the column. The curve obtained for the serpentine tube is similar to that for the coiled tube, but the maximum value of (H) occurs at a significantly lower flow rate with the serpentine tube. It is seen that once the flow rate exceeds about 1.5 ml/min., the dispersion is small and remains more or less constant over a wide range of flow rate range that embodies those usually employed in LC separations.
Ref (P) J. Chromatogr. 268(1978)681
Figure 13. Graph of Variance against Flow Rate for Coiled and Serpentine Tubes