Computerized pneumatics facilitate set-up 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 quantification. The first part of this series introduced the basics of computerized pneumatics,¹ 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 can also 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.

Figure 2

Until computerized pneumatics was widely available, nearly all split–splitless inlets operated at a constant inlet pressure drop. This meant that the column flow would decrease as the temperature went up during temperature programming, because the viscosity was increasing as well. Figure 2 illustrates the effect on corrected theoretical carrier gas flow and average linear velocity of changing the oven temperature while holding the pressure drop constant, for a 30 m × 0.530 mm wide-bore column with 4.1 psig of helium carrier gas and with the column outlet at 1 atm. The column outlet flow-rate at a column temperature of 50 °C is 6.5 mL/min, and it decreases to 4.3 mL/min at 250 °C, resulting in a 34% loss. Figure 2 gives flow-rates corrected from the column temperature to a standard temperature (usually 25 °C), which amounts to a larger 57% drop in flow as would be measured with a flow meter or as is displayed on the gas chromatograph. Across the same temperature range, the average linear velocity decreases from 40 cm/s at 50 °C to 28 cm/s at 250 °C, a 30% loss.