to quickly exit the column system. A laboratory instrument used only a few times each day has plenty of time to recover between sample injections.
Robust column systems and stable devices, all designed to operate for a long time without adjustment. In contrast, the laboratory staff can frequently check and adjust their instruments, as necessary.
Automatic validity checking and automatic calibration as necessary. Most laboratories analyze a quality control sample every day.
Hardened electronic devices to capture and process the detector signal and to schedule timed events.
An analyzer enclosure, shelter, or house to protect the analyzers and workers from the plant environment.
The following paragraphs introduce the basic function of the hardware devices. Later chapters detail their performance and technology.
The oven
Temperature control
The hardware devices used by a gas chromatograph and the separations that occur within its columns are sensitive to temperature change, so a gas chromatograph needs very fine temperature control.
In the first makeshift gas chromatographs the temperature‐controlled enclosure was literally a laboratory oven, and the name stuck; the column compartment of a gas chromatograph is still the column oven.
Early PGCs had a single isothermal oven that housed the sample injection valve, column, and detector; and sometimes the pressure regulator too. The temperature setting was then a compromise that didn't always satisfy the needs of the individual devices. More recent instruments include several temperature‐controlled zones for columns, valves, and detectors, thereby allowing individual temperature settings.
The chromatographic columns are very sensitive to temperature change. A change of column temperature will change the time that a component spends in that column, which might cause an error in analyte detection and measurement. Most columns today reside in a separate column oven often controlled to better than ±0.03 °C.
Temperature programming
A separate column oven may also support temperature programming, a sometimes‐useful technique that gradually increases the temperature of a column during analysis. When temperature programming is employed, the analyzer needs a reproducible cooling system to rapidly lower the column temperature to its original starting point.
Temperature programming is common in laboratory gas chromatographs and allows them to separate a wide range of components, but it's rare in process gas chromatograph due to cost and analysis time issues. This may change with the introduction of less complex methods of heating and cooling, as discussed in Chapter 11.
The sample injection valve
Laboratory and online practice
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.
Plug injection
The injector valve must inject the measured sample volume all at once, in the form of a compact plug. If the injection is slow and the sample starts to mix with the carrier gas, the sample molecules will start to spread out in time even before they reach the column. This would not be good because it's more difficult to separate a wide band of molecules than it is to separate a narrow band. Separation is easier when the injected molecules tightly pack together.
The sample volume is determined by the application. It's crucial to inject the same volume of sample each time because the detector output signal is proportional to the number of molecules it sees. Should the injected volume change, so would the output signal, even if the concentration of the analyte remained the same.
Gas sample injection
The injected volume of a gas sample is typically less than 0.25 mL.
The number of molecules in a fixed gas volume increases with pressure, so it's necessary to maintain constant sample pressure for each injection. Therefore, most PGCs block and bleed the sample line to allow the sample gas to come to atmospheric pressure, a technique known as atmospheric referencing. Chapter 7 discusses some valve systems to achieve this.
But that still leaves the normal variation of atmospheric pressure, which is quite small, as you can see by inspecting any barometer. Discounting stormy weather, the jobsite pressure variation should not be more than about ±2 %.
In practice, atmospheric referencing works well enough for most applications. If greater precision is desired, it's best to measure local barometric pressure and adjust the measurement values to compensate for any variation found.
Some PGCs have a sensor to measure the absolute pressure of the gas sample and use an algorithm to correct for detected changes.
Liquid sample injection
With liquid samples, the main challenge is to avoid gas bubbles in the injected sample as these will cause erratic measurements. To guard against bubbles, keep the pressure of a liquid sample as high as possible, consistent with the pressure rating of the sample injector valve.
A volume of liquid contains about 300 times as many molecules as an equal volume of vapor. Therefore, to inject the same number of molecules, a liquid sample
