Page 65 - Multidimensional Chromatography

56 Multidimensional Chromatography

capillary columns, (iv) multiple enrichment to enhance sensitivity, (v) reduction in
band broadening, (vi) the addition of auxiliary inlets for the addition of standards,
and (vii) the operation of analytical columns with a different carrier gas to that of the
primary. While some of these potential advantages are perhaps little exploited in
reality, they highlight the flexibility that refocusing at the midpoint can achieve. It is,
of course, this zone compression that lies at the heart of the next generation of
two-dimensional gas chromatographs which have utilised comprehensive chro-
matography.
The practical use of cryogenic traps varies greatly in sophistication. Fully auto-
mated cryogenic traps are available as add-on equipment for capillary GC systems.
Couplings for GC–GC may be readily adapted from such automated preconcentra-
tion devices that were designed initially for headspace analysis. Far simpler interme-
diate refocusing can be achieved by using only a short length of capillary column
dipped into a Dewar flask of liquid nitrogen. The low thermal mass of the capillary
means that on withdrawal from the cryogenic liquid a very rapid release of trapped
analytes occurs, thus introducing a very narrow injection band of analytes to the
secondary column. Care must be taken, however, in ensuring complete retention
of solute at the intermediate stage with the use of thin liquid film coatings aiding this
process. The use of polysiloxane-coated capillaries dipped in liquid nitrogen is par-
ticularly effective since they remain as fluids even at low temperatures. Other com-
monly used phases, such as polyethylene glycol, however, are less effective as they
often undergo phase solidification.
A variation on the method of thermal modulation is the use of a length of capil-
lary column coated with a thick film of stationary phase. At ambient oven tempera-
tures, this results in a retention of semivolatile analytes, which may be subsequently
released to the secondary column once the trap is heated. The rapid cycling time
possible with this methodology has resulted in its common application as the inter-
mediate trap in comprehensive GC.

3.2.5 MULTIPLE OVENS

A very early application of a double-oven system (that is, primary and secondary
columns held at independent temperatures) was reported in 1973 by Fenimore et al.
(13). In this particular application, the need to use a two-oven system was driven by
detector stability considerations, requiring accurate and stable control of the sec-
ondary column temperature. Later systems began to use independent temperature
controls of both ovens to enable more rapid and higher resolution separations.
Refocusing achieved through cooled on-column transfer to a secondary column for
temperature programming introduces a further area for two-dimensional GC method
refinement. The use of multiple ovens held at differing temperatures for primary and
subsequent columns requires a significant additional investment in equipment. It
does, however, offer a number of significant advantages over simple single-column
methods, as follows: