Ultra thick film open tubular traps with an increased inner diameter.

In this paper, the concept of open tubular traps, coated with a very thick film of polydimethyldisiloxane for enrichment of trace volatile components has been further explored. From theoretical calculations as well as practical experiments it is demonstrated that it can be advantageous to increase the inner diameter of such traps. For a given sampling flow rate and phase ratio, the plate number of the traps is not dependent on the inner diameter, provided that the linear flow velocity remains sufficiently high to offset the effect of axial diffusion. It is shown that this is due to the basic fact that for a given sampling flow rate, the average linear flow velocity in the trap is inversely proportional to the square of the inner diameter of the trap. However, in contrast to chromatographic separations, the linear flow velocity is not important. Under conditions of a constant phase ratio, an increased inner diameter also increases the amount of sorbent in the trap, which is a key parameter for obtaining high breakthrough volumes. Open tubular traps with an expanded inner diameter have very low pressure drop characteristics, which provides the possibility to construct new, simplified sampling systems.