Wall Coated Open Tubular Column

A wall coated open tubular column, as the name suggests, consists of a tube in which the wall is coated with a material acting as a stationary phase.

In general the tube itself is a capillary tube with a narrow inner diameter, less than 1 mm, but of very long length measuring up to tens of meters. The tubes are so narrow that they are easily coiled up and suspended in an oven for temperature control.

The coating is usually a film of a polymer that uniformally wets the inside of the column. A variety of functional groups may be present in such a polymer so that specific polarity and selectivity is provided. The film is thermostable, within reasonable temperatures, so that a WCOT can work over a range of tempertures. The polymer is also nonextractable meaning that the column can be flushed with pure solvents to remove contaminants.

The thickness of the coating allows one to optimize columns for separation of very volatile (thick films, 3- 5 mm) or high molecular weight compounds (thin films , < 1 mm) and achieve separations within a reasonable analysis time. The usual thickness of the film is 1-2 mm.

Advantages

The advantages of open tubular columns over packed columns are:

  1. faster analysis (high flow of carrier possible)
  2. shorter retention times
  3. more inert
  4. longer life
  5. less bleed of coating material
  6. higher efficiencies
  7. greater reproducibility

Animation

Things to note in the animation of the separation are:

  1. Although it is not shown, at any point in time the carrier gas is moving through the column.
  2. At any point in time more molecules of the more retained compound are in the stationary phase, and stay there for a longer period of time. These are the result of differences in the kinetics and the thermodynamics of the partioning of the material between the stationary and mobile phase.
  3. The compound that stays on the column for a longer time has more broadening of the plug of material (also shown by the peak above the column).
  4. When the peak broadens the total integral remains the same (we are not loosing any material) but the concentation and therefor the intensity of the peak decreases and the signal to noise ratio in the detector is negatively affected.
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