Page 32 - Multidimensional Chromatography

22 Multidimensional Chromatography

Transfer of an LC fraction of 300 l volume occurred by the conventional retention
gap technique. In fact, Figure 2.3(b) shows the GC chromatogram obtained after the
transfer of the LC fraction.
An additional technique, called ‘partially concurrent eluent evaporation,’has been
introduced in order to overcome some drawbacks of the retention gap technique. In
this case, a large amount of solvent is evaporated during introduction (concurrently),
yet still producing a zone flooded by the eluent, thus providing solvent trapping (5).
This technique allows the use of shorter retention gaps or larger transfer volumes. In
theory, an early vapour exit (6) should be placed between the uncoated precolumn
and the main column, but in practice a short section of the main column (7) is placed
between the precolumn and the vapor exit, so rendering the closure of the vent less
critical.
In particular, Figure 2.4(a) (7) shows the LC chromatogram of fat extracted from
an irradiated chicken. Irradiation of foods containing fat produces radiolysis frag-
ments of triglycerides, such as acids, propanediol esters, alkenes, aldehydes and
methyl esters. The alkane/alkene fraction transferred to the gas chromatograph cor-
respond to the first 200 l eluted after the dead volume. The second fraction of inter-
est was that of the aldehydes, eluted after a few minutes. The two fractions (Figure
2.4(b)), both of about 200 l, were transferred by partially concurrent eluent evapo-
ration by using a precolumn of 12 m 0.50 mm id, with a 3 m 0.32 mm id fused
silica section of the separation column serving as a retaining precolumn before the
solvent vapour exit.

2.2.2 LOOP-TYPE INTERFACES (CONCURRENT
ELUENT EVAPORATION)

The retention gap techniques, essential for the analysis of very volatile components,
are often replaced by concurrent eluent evaporation techniques, due to their simplic-
ity and the possibility of transfering very large amount of solvent. In this case, the
solvents are introduced into an uncoated inlet at temperatures at or above the solvent
boiling point.
In this way, the liquid can be transferred at a speed corresponding to the evapora-
tion speed. The fraction to be analysed is contained in a loop (see Figure 2.5), con-
nected to a switching valve. By opening the valve, the sample in the loop is driven by
the carrier gas into the GC unit (8), instead of the LC pump. An early vapour exit is
usually placed after a few metres of the deactivated precolumn (9) and a short piece
(3–4 m) of the main column (retaining precolumn). This valve is opened during sol-
vent evaporation in order to reduce the amount of solvent that would reach the detec-
tor, and at the same time, to increase the solvent evaporation rate (6).
When the sample solvent evaporates at the front end of the liquid, volatile com-
pounds co-evaporate with the solvent and start moving through the main column. In
this way, volatile components can be lost through the early vapour exit or, if venting
is delayed, the most volatile compounds reach the detector even before the end of