John V. Hinshaw

First, let us clear up some terminology. Gas chromatography (GC) instrument manufacturers refer to computerized pneumatic systems in a variety of ways, including electronic pneumatics control or electronic pressure control (EPC), programmable pneumatic control (PPC), advanced flow control (AFC), digital pressure and flow control (DPFC), detector gas flow control (DGFC), and electronic flow control (EFC). Each of the manufacturers' variations has some unique modes and capabilities; many have advanced to a second or third generation since they were first offered. It is beyond the scope of this article to enumerate all of the possibilities. The reader is encouraged to explore companies' literature and documentation to learn specific details. Here, we will focus on the general functionality and utility of computerized pneumatics with the aim of describing the major operating principles and modes common to many of these systems.

Development

The literature from that time discusses the relative merits of various positive or negative, linear, exponential, or hyperbolic pressure and flow profiles applied over the course of a GC separation on both packed as well as open-tubular columns. The mechanical pneumatic controllers of the time were crude by today's standards, but they contained the necessary elements for automated pneumatic control: a valve or other controller and the mechanical means to change its operation during a GC analysis. Later controllers began to supplant mechanical controls and added feedback using simple operational amplifiers and pressure or flow sensors.

As GC liquid-phase maximum temperatures increased — permitting higher and higher oven temperature program limits — efforts in pressure–flow programming essentially ceased. From the mid 1970s on, little work was done in this area. Most analysts operated their open-tubular columns under isobaric (constant pressure) conditions using mechanical pressure regulators and their packed columns under isoheric (constant flow) conditions using mechanical constant mass-flow controllers. During this period electronic constant-flow controllers crossed over from the semiconductor industry and were produced specifically for GC requirements, but they were relatively expensive compared with simpler mechanical devices.

Computer control: In the1980s, microprocessors migrated into GC instruments as instrument designers established digital control of temperatures and other parameters. Computerized digital pressure–flow control was a natural extension of these capabilities. In early implementations, the microprocessor controlled only the pneumatic setpoints. The operator entered a flow rate, for example, and the microprocessor sent the setpoint to the controller digitally. The actual control of flow occurred in the pneumatic device itself using analog circuitry. The controller reported the measured flow rate back to the microprocessor through another digital channel, and the microprocessor reported the measured flows to the instrument display. However, these systems did not have extensive pressure or flow programming capability.