Page 334 - Tandem Techniques

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They produced a segmented flow, by mixing the aqueous eluent from a reversed phase column with
chloroform. The extraction solvent was then separated from the segmented flow by means of a
'hydrophobic' (dispersive) membrane. A diagram of their apparatus is shown in Figure 8.22. There are
two pumps, one provides the solvent for the chromatographic development and the other the extraction
solvent, which can be either chloroform or carbon tetrachloride. After passage through the column, the
two streams are mixed at a T junction and form a segmented flow as both solvents are virtually
immiscible. The segmented flow passes through an extraction coil, which allows sufficient time for the
extraction to take place, and then to a separator. A diagram of the separator is shown in the lower
portion of Figure 8.22.

The separator is made of stainless steel, and the volume on either side of the membrane is about 16 µl,
the membrane itself having pores about 0.2 µm in diameter. The membrane is sealed against the steel
flanges by compression. The amount of solvent that is passed through the membrane is controlled by
adjusting the differential pressure across the membrane. This device could obviously cause serious peak
dispersion and, in the form described, would be unsuitable for use with high-efficiency or small-bore
columns. It was found that samples containing 300 µg of material were necessary to produce a
satisfactory spectrum, which indicated a relatively poor sensitivity.

Hellgeth and Taylor [22] developed the segmented flow interface further, improving both the method of
producing the segmented flow, and the phase extractor. The segmented flow generator was constructed
from a 1/16. in Swagelok T union. The union was drilled out 1/16 in. I.D. to allow the 1/16 in. tubes to
be inserted, so that the ends of the tubes were only 0.45 mm apart. The aqueous solvent and the organic
extraction solvent passed into the mixing T through tubes 0.020 in. I.D. The segmented flow left the
mixing T through a length of Teflon tube, 75 cm long and 0.8 mm I.D., which also acted as the
extraction conduit. The membrane separator consisted of two stainless steel plates with grooves in each
surface, and a triple-layer membrane of Gore-Tex sheet. The basic system is diagramatically
represented in Figure 8.23