Page 123 - Tandem Techniques

Page 105

Chapter 3
Interface Conduits

All tandem interfaces contain conduits, through which the column eluent from the separation system
(the chromatograph) passes, in order to enter the identifying device (the spectrometer). The interface
can range in complexity from a short length of small diameter tubing to an elaborate device that
concentrates the solute in the mobile phase, prior to it entering the spectrometer. However, irrespective
of the complexity of the interface, it must be designed in such a manner that it causes little, or
preferably no, loss in chromatographic resolution, and delivers the necessary amount of solute to the
spectrometer. The solute bands must not be allowed to spread during passage through the interface, and
cause closely eluted peaks to merge into one another, before being analyzed in the spectrometer. The
most likely and certain sources of peak dispersion in any interface will be the connecting tubes.
Furthermore, as lengths of open tubing are commonly included as conduits in most interfaces, the
dispersion that takes place in open tubes will be considered in some detail.

Dispersion in Open Tubes

The magnitude of the dispersion that takes place in open tubes depends strongly on the nature of the
fluid passing through it. It follows that dispersion in GC connecting tubes will differ considerably from
those in LC, and consequently each system will be considered separately.

Gas Chromatography Columns and Connecting Tubes

The dispersion in simple open tubes results from the parabolic velocity profile that exists across the
radii of such tubes, and is depicted in Figure 3.1. The length of the arrows represent the relative velocity
of the mobile