Computerized pneumatics facilitate setup and operation of both capillary and packed columns and offer chromatographers improved
performance over manually operated pneumatics in terms of retention time stability and split or splitless quantitation. The
first part of this series (1) introduced the basics of computerized pneumatics, showed how a gas chromatography (GC) system
controls flow or pressure, and discussed the fundamentals of operation as well as some pitfalls that can arise when changing
the pneumatic configuration.
The capabilities of computerized pneumatic control extend beyond the basic capillary-column modes of total flow control for
inlet splitters and pressure-drop control for columns. As discussed in the first part of this series, given the correct column
dimensions and carrier gas type, computerized pneumatic systems will calculate capillary column flows and velocities that
correspond to specified pressure drops and oven temperatures, and then report the resulting flow and velocity to the operator.
Going further, given a specific flow rate or average carrier gas linear velocity, the pneumatic system also can calculate
and set the pressure drop required to produce the desired flow or velocity.
This is particularly convenient for columns attached to split–splitless inlet systems, which control the column pressure drop
directly but not the column flow rate itself. The pressure drop that is required for a particular flow depends upon the oven
temperature as well as the column dimensions and carrier gas, so the operator must specify the temperature at which the desired
flow is to be achieved: this is nearly always the same as the initial or operating oven temperature. When the GC system receives
a flow or velocity setpoint, the corresponding pressure is set automatically by the pneumatic system.
So far so good, but what happens when the column temperature increases during oven temperature programming? The column flow
rate depends upon the oven temperature, as well as many other factors, and so chromatographers expect the flow to change during
temperature programming. The physics of flow through an open tube are well understood, though, and computerized pneumatic
systems incorporate a mathematical model that accurately predicts the interrelationships of pressure, flow, and linear velocity.
Pressure Programming
Figure 1
The dependency of capillary column flow and velocity on the column pressure drop is determined by the column dimensions and
the carrier-gas viscosity. As temperatures increase, so does the viscosity, which causes the flow and velocity to decrease
at higher temperatures. Figure 1 shows the measured relationships of temperature and gas viscosity for three common GC carrier
gases. Computerized pneumatic systems use the functions of temperature represented by the lines plotted in Figure 1, which
are fitted to the measured viscosity data, to calculate viscosity dynamically as the oven temperature changes. At an inlet
pressure below 30 psig, the column flow rate and outlet velocity are roughly proportional to the viscosity, to within about
10%. Figure 1 gives a good idea of how large the viscosity effect can be when a temperature program spans a large temperature
range. Going from 50 °C to 250 °C, for example, causes helium viscosity to increase by around 40%. The other carrier gases
undergo viscosity changes of a similar magnitude.
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