Under conditions of constant mass-flow the volume flow measured at atmospheric temperature and pressure at the end of the column will also constant. As a consequence, providing the appropriate pressure correction is applied (as derived by Martin) accurate and repeatable, absolute retention data can be obtained. In addition, ancillary gas supplies that are required by specific detectors (e.g. hydrogen and oxygen or air for the flame ionization detector) are also fitted with mass flow controllers to facilitate detector stability. As an example, the operation of the Porter mass-flow controller is described here. A diagram of this mass-flow controller that demonstrates its mode of action is shown in figure 55. As the temperature of the gas will be proportional to the product of the mass of gas flowing per unit time and its specific heat, the differential temperature will also be proportional to the mass flow rate. The voltage generated from the differential sensor signal is compared to that of a set voltage (set to represent the desired mass-flow) and the difference is used to generate a signal that actuates the valve controlling the gas-flow. It follows, that, a closed loop control system is formed that maintains the mass flow rate set by the reference voltage. These devices can be made extremely compact, are very reliable and affords accurate control of the carrier gas flow rate irrespective of gas viscosity changes due to temperature programming. A further advantage is that the device can also be controlled by the chromatograph computer. Under computer control, the mass flow controller also allows the flow rate to be changed, or stopped, during injection, or during a separation, if so desired.

Courtesy of Porter Instrumentation Company Inc.

Figure 55. The Mass Flow Controller

A further advantage of the mass-flow control system is that it can also be used for flow programming. Flow programming is analogous technique to that of temperature programming and is used to accelerate the late eluting peaks through the column and, thus, reduces the analysis time. Flow programming might be the alternative technique to choose if the late eluting solutes were thermally labile. Mass flow control by the computer is also used in special sampling and development procedures.