Page 61 - Multidimensional Chromatography

52 Multidimensional Chromatography

course, an injection period of 10 s (which is essentially what is occurring between the
primary and secondary columns) would be generally considered excessive. In prac-
tice, a number of mechanisms are used to enable refocusing and zone compression at
the point of transfer, ranging from cryogenic and chemical micro-traps, to phase ratio
refocusing at cooler oven temperatures, when the secondary column is held at temper-
atures independent of the first. An additional consideration of this simple two-dimen-
sional system, beyond just that of peak dispersion, highlights that the sample will be in
contact with a metallic or polymer surface of the valve, and that a significant pressure
drop may exist with two columns in line. In addition, consideration must be made to
match the flow rates, and sweep dead flows in the system along with precise and
instantaneous transfer from the primary to the secondary column. The switching pro-
cess itself must also not upset flows or general system or detector stability.
Solutions do exist to these problems. The engineering of mechanical valves is
now significantly advanced and very low dead volume units which minimize band
broadening are commercially available. Similarly, the inertness of the rotor material
used to make the gas-tight seal between the metal body channels has been vastly
improved. The extent to which the problems of band dispersion and inertness have
been overcome can perhaps be assessed from some anecdotal reports of two-dimen-
sional GC performed by using modern valves. A study in 1985 compared the Deans
switch and mechanical valve performance for a range of reactive species, although
the results obtained were largely inconclusive (7). More recent work by Mills and
Guise (8) reported the successful analysis of free acids and anhydrides in a valve-
based two-dimensional system with few problems. However contradictory evidence
is available for some compounds such as aldehydes and alcohols, e.g. acetal and
acetaldehyde were analysed successfully by Adam (9) whilst furaneol gave signifi-
cant problems (10). It is clear, therefore that individual methodologies must be care-
fully examined when considering the use of mechanical valve interfaces. Given,
however, the various deactivation techniques, such as silanization, which are now
used for the production of stainless steel columns, future developments in deactiva-
tion, and high-temperature stability make this simple route worthy of consideration.
Figure 3.2 shows the typical valve switching arrangements used for two-dimensional
gas chromatography in both the heart-cut and analyse/monitor positions. In the anal-
yse position, flow from the primary column passes into the two-position switching
valve and is diverted to the first detector. This allows the progress of the primary sep-
aration to be actively monitored. At the moment of transfer, the valve positions are
switched and flow exiting the first column is passed directly to the secondary
column. On completion of the heart-cut, the valve is returned to its original position.
The secondary separation is monitored on the second detector.

3.2.3 DEANS SWITCHING

The non-intrusive manipulation of carrier gas effluent between two columns clearly
has significant advantages in two-dimensional GC. In addition, a pressure-driven
switch between the columns introduces no extra band broadening to an eluting peak.