Page 62 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 53

Figure 3.2 Valve switching interfaces in (a) heart-cut position and (b) primary column
monitor/secondary column analyse position.

In 1968, Deans (11) introduced a basic principle of pressure switching which has
subsequently been used extensively for heart-cutting, venting and backflushing
within two-dimensional systems. The principle of operation is shown in Figure 3.3 in
a diagram adapted from an earlier review by Bertsch (12). The process indicated
highlights three distinct phases, i.e. that of survey or prefractionation, sample transfer
and backflush of the primary column. The basis of the method is in the diversion of
flows by using pressure balancing at junctions. The flow of carrier gas to each junc-
tion is controlled by solenoid valves, with the magnitude of pressure introduced
being determined by the inlet and outlet pressures of the interacting devices and their
individual flow resistances.
There are three phases in the operation of a Deans switch :

(a) Prefractionation-operation in the SURVEY position indicted in Figure 3.3
results in a balance of pressure such that flow from the primary column is
diverted at the junction between the columns (marked ‘A’) towards Detector 1.
This set of pressures prevents sample entering the second column, but does