Page 64 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 55

provide the carrier gas for the second column. This is supplied as excess pres-
sure at the junction A.
(b) Sample transfer-The second pressure configuration results in both columns
being coupled in a sequential manner. A minor portion of the primary eluent is
split at junction A to go to Detector 1, with the majority passing directly on to
the secondary column.
(c) Analysis of fraction-once the sample transfer is complete, the third pressure
configuration is adopted. The carrier gas flow through the secondary column is
maintained by excess pressure at junction A, supplied from regulator B.
Concurrently, the primary column is backflushed by also using the pressure
supplied from regulator B.

Following the backflush of the primary column and separation of the analytes on
the second column, the system can then be returned to its original prefractionation
position, ready for the next sample injection.
The system can be made more sophisticated through the addition of fused silica
restrictors acting as bleeds, thus preventing back diffusion of analytes towards the
solenoid valves. Additional pre-concentration stages can also be provided between
the primary and secondary columns to reduce the peak widths introduced to the sec-
ondary column.
Overall, the technical complexity of the Deans switch system is considerably
greater than that of a mechanical switching valve and it is accepted that reliability
and ease of use is reduced as the system complexity increases. For many compound
types, however, the completely non-intrusive nature of the Deans method offers suf-
ficient advantages to justify its application. However, the use of modern electronic
pressure and flow controls integrated into the overall computer control of the chro-
matographic system does now make the operation of Deans switches significantly
easier or more reliable than has been reported in its earlier applications.

3.2.4 INTERMEDIATE TRAPPING

Whilst the most simple form of two-column coupling results in a switch at the mid-
point between two columns in a single oven, large numbers of more complex adapta-
tions do exist. Perhaps the most significant additional modification that can be made
is refocusing of analytes prior to the secondary separation. As highlighted earlier, the
peak widths from the primary column separation fundamentally limit the resolving
power of the second column, with refocusing being the key method in reducing this
dispersion effect. Refocusing can be enabled by a number of means, including ther-
mal focusing by using a cryogenic trap, cooled typically with CO 2 or liquid nitrogen,
a thermally modulated retaining column, or reconcentration at the secondary column
head prior to temperature programming. W. Bertsch highlighted a large number of
refocusing advantages, which included (i) knowing the exact starting point of the
second dimension, (ii) addition of several heart-cuts to form a composite prior to
injection, (iii) the ability to connect high-flow packed columns with low-flow narrow