separations involve a fluid material moving across the surface of a stationary material.
In the formal terminology of chromatography, the moving material is the mobile phase, and the immobile material is the stationary phase.
The mobile phase may be a gas or a liquid, from which we derive the terms:
gas chromatography, in which the mobile phase is a gas.
liquid chromatography, in which the mobile phase is a liquid.
A few applications have used a supercritical fluid as the mobile phase.
This book is about the analytical use of gas chromatography for the online measurement of industrial processes. We won't be discussing liquid chromatography.
In gas chromatography, the mobile phase is always a gas, and it's common to call it the carrier gas.
The carrier gas flows through a long narrow tube called a chromatographic column, which contains the stationary phase. The stationary phase may be an adsorbent solid or a non‐volatile liquid. More about that later.
The gas chromatograph
The basic instrument
A gas chromatograph is an analytical instrument that uses the techniques of gas chromatography to measure the concentration of selected chemical compounds in a small sample containing a mixture of compounds.
In a gas chromatograph, the mobile phase is a gas carefully selected for the application. It's usually hydrogen, helium, or nitrogen, but any gas will do the job, as long as is doesn't react with the sample components or the column materials. The gas should not contain oxygen or water vapor, as these substances might damage the columns.
The pressure of the carrier gas is closely controlled, after which it flows continuously through the column.
The analyzed fluid can be a gas or a volatile liquid. A special valve injects a small volume of that fluid into the flowing carrier gas. A liquid sample usually vaporizes instantly upon injection, so it's all vapor by the time it reaches the column.1
Note that when a gas chromatograph accepts a liquid sample, it doesn't become a liquid chromatograph. A liquid chromatograph is an entirely different instrument that employs a liquid mobile phase and separates components in the liquid phase. Liquid chromatographs are rarely employed as industrial online analyzers and are not considered here.
After injection, the carrier gas carries the gas or vapor sample into the column, where it contacts the stationary phase. It's the contact with the stationary phase that accomplishes the desired separation.
It's common to use the word component for a chemical substance or a group of chemical substances that are present in the sample. The gas chromatograph may not measure every component, but each component measured is an analyte.
A gas chromatograph can separate and measure one, several, or all the components in a gas or liquid sample.
After separation, the carrier gas carries the components into a detector that provides a measurable signal to the data‐processing circuits.
When actuated, the sample injection valve transfers a minute aliquot of the sample fluid into the flowing carrier gas. Later chapters provide full details of the many varieties of sample injector valve used in process gas chromatographs.
Figure 1.2 Basic Gas Chromatograph.
Figure 1.2 introduces the essential hardware devices found in any gas chromatograph:
A column oven with one or more controlled temperature zones.
A carrier gas supply and pressure control system.
A sample injector to inject a repeatable volume of sample into the flowing carrier gas.
One or more separating columns.
One or more detectors.
All gas chromatographs have these basic functions, yet we see a large variation in their design and fabrication.
The process instrument
The early development of the gas chromatograph was unusual. After the invention of the technique in 1952, the oil and chemical companies soon recognized its potential for process control, and those industrial companies did much of the original development work. The contribution of the instrument manufacturers came later.
Consequently, gas chromatographs intended for process monitoring and control evolved differently from those intended for laboratory use. Although both types of instrument use the same core technology, their sphere of application is quite different.
For example, a process gas chromatograph performing a two‐minute analysis receives 720 samples per day. The laboratory chromatograph might only receive three.
Thus, the design specifications for a gas chromatograph installed in an industrial processing plant are quite different than for a gas chromatograph sitting on a laboratory bench. The main reasons for these differences are:
The process instrument operates in a potentially hot, cold, dusty, wet, windy, corrosive, or hazardous environment.
The process instrument operates continuously twenty‐four hours per day, seven days per week.
The process instrument must operate reliably with almost no human intervention – perhaps only one calibration check each month.
The process instrument can focus on measuring just a few of the components in a sample – the ones needed for process control.
The process instrument suffers from a fanatical quest to reduce analysis time, so its measurements are valid for process control.
For all the above reasons, a process chromatograph (PGC) may include devices not shown in Figure 1.2. Later chapters will further discuss those devices. To whet your appetite, expect to see:
Devices external to the instrument to condition the incoming process sample to make it compatible with the chromatograph; i.e. a sample conditioning system.
Multiple columns with special valves to switch analyte molecules from one column to another, thus maximizing the rate that separated components arrive at the detector. This is an additional complexity rarely found in laboratory instruments.
Housekeeping columns that allow strongly‐retained
