Page 89 - Multidimensional Chromatography

Orthogonal GC–GC 81

two dimensions to give two important results, and benefits, which arise from the
modulation process. These are as follows:
(i) the zone to be passed (or more correctly, pulsed) from 1D to 2D must be com-
pressed in space;
(ii) the compressed zone must be delivered to 2D very rapidly, and as a sharp
pulse;
(iii) 2D must be capable of giving fast GC results, achieved by a combination or all
of the following, i.e. a short column, thin film thickness, narrow id column
(giving high carrier linear velocity) and higher temperature (if a two-oven sys-
tem is used);
(iv) the peaks produced at the detector will be increased in peak height response
due to the above process;
(v) all of the first column solute is transferred to the second column.
It is tempting to draw the analogy between GC GC and 2D planar chromatogra-
phy. On a TLC plate, the original single spot may be developed along one edge of
the plate, which is then rotated 90° and placed in a second eluting solvent system to
give rise to a different separation mechanism for separating compounds that were
unresolved in the first step. The final plate could have solute spots distributed any-
where over the plate space that had been ‘developed’ by eluent. In a similar sense,
the GC GC experiment, if properly designed, could theoretically have peaks dis-
tributed over a space corresponding to the full range of possible distribution con-
stants available to the mixture components on the columns used in the experiment. In
TLC, however, zone visualization is conducted on the final plate (e.g. by densitome-
try), whereas in the GC case we must have a single detector recording the effluent
from the second column. The GC GC experiment also has all of the normal
method advantages of the GC technique–sensitive analysis, hyphenation with mass
spectrometry should be possible, readily automated methods, etc.
The ability of a GC column to theoretically separate a multitude of components is
normally defined by the capacity of the column. Component boiling point will be an
initial property that determines relative component retention. Superimposed on this
primary consideration is then the phase selectivity, which allows solutes of similar
boiling point or volatility to be differentiated. In GC GC, capacity is now defined
in terms of the separation space available (11). As shown below, this space is an area
determined by (a) the time of the modulation period (defined further below), which
corresponds to an elution property on the second column, and (b) the elution time on
the first column. In the normal experiment, the fast elution on the second column is
conducted almost instantaneously, so will be essentially carried out under isothermal
conditions, although the oven is temperature programmed. Thus, compounds will
have an approximately constant peak width in the first dimension, but their widths in
the second dimension will depend on how long they take to elute on the second col-
umn (isothermal conditions mean that later-eluting peaks on 2D are broader). In
addition, peaks will have a variance (distribution) in each dimension depending on