Process Gas Chromatographs. Tony Waters
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Part One PGC fundamentals
1975 Beckman Model 6800 Air Quality Chromatograph.
Figure 1.1 A Classic Process Gas Chromatograph.
Source: Beckman Historical Collection, Box 58, Folder 28. Science History Institute, Philadelphia. https://digital.sciencehistory.org/works/474299142 Reproduced with permission of Rosemount, Inc.
“We cannot teach people anything; we can only help them discover it within themselves.”
Attributed to Galileo Galilei 1564–1642
Why study this?
Part One introduces the art and science of gas chromatography (GC) as applied to the industrial process instrument.
These four chapters explain how a GC column works, why the compounds in the injected sample form the characteristic peak shape, how one peak becomes separate from another peak, and how we can predict the position and shape of peaks on a chromatogram from known patterns of peak timing and width.
The text presents this information in an easy‐to‐read and mostly non‐mathematical manner. Yet it shuns simplistic analogies of what happens inside a GC column because they tend to mislead rather than to inform. Instead, it offers a challenging insight into real chromatographic behavior.
The knowledge gained here is a necessary preparation for understanding the function of the hardware devices and software techniques introduced in later chapters of the book. For those who aspire to be proficient in the application or troubleshooting of process gas chromatographs, mastery of these concepts is not optional.
1 An introduction
“Books on gas chromatography, of which there are many, usually start by reviewing the historical development of the science, so we won't do that here. Instead, we'll start by understanding the basic technique: what a chromatograph does and how it does it. To read the fascinating history of chromatographic science, see the beautiful book by Ettre (2008)”.
Chromatographic separation
Let's start by looking briefly at the various forms of chromatography.
Chromatography by itself is not a complete analytical technique. It's just a way to separate one kind of molecule from another kind of molecule. Of course, for those reading this book, the reason for separating those molecules is to measure them alone, without interference from other molecules. This is the analytical use of chromatography.
While analytical measurement is the main use of chromatography, it is not the only one. Some laboratory‐scale and industrial‐scale processes use a chromatographic separation to isolate extremely pure batches of valuable chemicals. This usage is known as preparative chromatography, and it works with much larger quantities of material than analytical chromatography does. This textbook focuses on analysis, so it doesn't further discuss the preparative use of chromatography.
When used as part of an analytical technique, chromatography is a very effective way to separate the measured compounds from each other and from all the other chemical compounds present in the analyzed material. After all desired compounds have been isolated, another device measures each one independently.
Keep in mind, then, that chromatographic analysis is always a two‐stage process: first separation, then measurement.
There are many ways to produce a chromatographic separation, and they involve all possible combinations of gases, liquids, and solids. While quite different in practice, these various forms of chromatography share some common features. All practical chromatographic 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.