Topics - Serpentine

Serpentine Serpentine is the term given to the shape of a particular type of low-dispersion connecting tubing. The velocity profile of fluid flow through a cylindrical tube is parabolic which (as Golay has shown) causes significant dispersion (band spreading) as the solute passes down the tube. The dispersion or band spreading is inversely proportional to the diffusivity of the solute in the mobile phase and, as the diffusivity of a solute in a gas is relatively large, dispersion in GC connecting tubes is not usually significant. In liquid chromatography, however, the diffusivity may be four orders smaller than in a gas and, thus, dispersion can become very serious. Connecting tubes are an essential feature of most chromatographs and so in liquid chromatography, tubes providing minimum dispersion are necessary to maintain the integrity of the separation. To reduce dispersion, convective mixing must be introduced into the connecting tube which is achieved by making it serpentine in shape. As the mobile phase continually changes direction as it passes from one semicircular loop to the next “swirls’ are produced that provide convective mixing and dramatically increases the effective solute diffusivity. As a result dispersion in serpentine connecting tubes is almost negligible. The energy required to produce the convective mixing is provided by a greater pressure drop across the serpentine connecting tube. The serpentine tube reduces dispersion to a greater extent than simple coiling as the direction of flow is not merely continually changed but is actually reversed at each serpentine bend. Diffusion and thermal conductivity are very similar types of transfer processes and are described by very similar mathematical functions. The serpentine tube is also an extremely efficient heat exchanger.