Page 337 - Tandem Techniques

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After passing through the extraction coil, the segmented flow enters a phase separator, similar to that
described by Johnson et al. [21], and the separated solvent then flows through a UV absorption
detector. The eluent leaving the UV detector then passes directly into a spray jet assembly, similar to
that manufactured by Lab Connections Inc., which has already been described. Nitrogen is passed
through a heater to the spray jet assembly, to aid in the nebulization process. In order to confirm that the
extraction system did not contribute significantly to peak dispersion, a separation was monitored with a
UV detector placed before and after the solvent extraction process. The chromatograms obtained are
shown in Figure 8.24. It is seen from Figure 8.24, that very little peak dispersion takes place in the
extraction tube with segmented flow, and that the resolution obtained from the chromatographic column
is not significantly denigrated. Due to the finite volume of the extraction tube, however, there is a
significant retention delay manifested between the two chromatograms. Nevertheless, as all the peaks
are displaced by the same amount, this displacement has no effect on resolution. The delay amounted to
about 3.5 min, but this will vary with both the flow rate and volume of the extraction system. In general
the delay (At)will be given by,

Where (VE) is the volume of the extraction system,

and (Q) is the flow rate.

An example of the use of the system for the separation and identification of a series of phenylureas is
shown in Figure 8.25.

It is seen that an excellent separation was obtained, and as the chromatogram was monitored after the
extraction process, the results again confirm that the integrity of the separation is not impaired by the
extraction procedure. The spectrum shown below was obtained as a result of an injection of 150 µl of a
solution, containing 180 ng/ml of acenaphthenequinone, and thus represents a total mass of about 27 ng.
It is clear that the system has a very useful sensitivity and can handle aqueous mobile phases very
easily. However, the solvent extraction technique does