Page 35 - Multidimensional Chromatography

Coupled HPLC with HRGC 25

Figure 2.6 Gas chromatogram of a 10 ml test sample containing C 14 –C 26 alkanes in
n-hexane (about 1 ppb each): the carrier gas (H 2 ) inlet pressure was 2.5 bar for a 22 m 0.32
mm id separation column coupled with a 2 m 0.32 mm id uncoated precolumn (no vapour
exit). Reprinted from Journal of High Resolution Chromatography, 9, K. Grob et al.,
‘Concurrent solvent evaporation for on-line coupled HPLC–HRGC’, pp. 95–101, 1986, with
permission from Wiley-VCH.

The main drawback of this technique is represented by the loss of volatile com-
pounds. A solution to this problem is represented by the so-called co-solvent trap-
ping technique used during concurrent eluent evaporation. In practice, a small
amount of a higher boiling co-solvent is added to the main solvent at such a concen-
tration that some co-solvent is left behind as a liquid, while some is co-evaporated
with the main solvent. The co-solvent remaining forms a barrier of liquid film, which
evaporates from the rear to the front, as in the retention gap technique. Volatile com-
ponents are retained by the solvent trapping effect, and released when the evapora-
tion of the co-solvent is completed. Although this technique shows some similarities
to the retention gap technique, the main difference is represented by the interface
used: the latter technique uses an on-column interface, while concurrent eluent evap-
oration is designed for a loop-type interface.
While partially concurrent eluent evaporation is easier to use, and is preferred for
the transfer of normal phase solvents, concurrent eluent evaporation with co-solvent
trapping is the technique of choice for transfer of water-containing solvents, because
wettability is not required.
Figure 2.7 (11) shows a gas chromatogram obtained by co-solvent trapping and
concurrent eluent evaporation after injecting 500 l of diluted gasoline. The main
solvent was n-pentane with 5% of n-heptane as co-solvent. It is noteworthy that
without the co-solvent, higher-boiling compounds could be lost.

2.3 VAPORIZATION WITH HOT INJECTORS

Together with the techniques described above, other techniques using hot injectors
for the transfer of large-volumes in capillary gas chromatography have been devel-
oped. Transfer of large-volume solvents in a programmed temperature vaporizing