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To produce a chromatographic separation, the instrument needs a small sample of the gas or volatile liquid for analysis. The introduction of this sample into the carrier gas stream is known as sample injection. After injection, the carrier gas carries the sample into the column. As used here, a volatile liquid is one that will rapidly and completely vaporize at the injector temperature.

It used to be a standard laboratory practice to inject samples manually, using a glass syringe, but this routine procedure is now automatic. In the laboratory, an autosampler accepts an array of small vials containing the liquids for analysis. Then, according to a preloaded time program, it pulls a sample from each vial in turn and injects it into the chromatograph.

In contrast, an online gas chromatograph needs to periodically extract a minute sample from a continuously flowing process fluid and inject that sample into the carrier gas flow. To do this, most PGCs use a mechanical sample injector valve having a pneumatic actuator powered by an air signal from the chromatograph control unit. A few use electric power.

Figure 1.3 shows a typical valve configuration for injecting gas samples. For clarity, the diagram shows a rotary valve, but there are several other types of valve in use, including slide valves, diaphragm valves, and plunger valves. Chapter 8 details the function, design, and usage of these valves.

PGCs use several types of valve. As an example, this sketch shows a rotary valve. The rotor turns 60° to inject a sample.

Figure 1.3 Typical Gas Sample Injector Valve.