technical terms
When first introduced, these words and phrases were in bold type. You should now know the meaning of these technical terms. If still in doubt, consult the Glossary at the end of the book:
1 active‐solid column
2 analysis time
3 analyte
4 application engineering
5 atmospheric referencing
6 autosampler
7 baseline
8 calibration factor
9 capillary column
10 carrier gas
11 chromatogram
12 chromatogram signal
13 chromatograph
14 chromatography
15 column
16 column oven
17 column valve
18 component
19 detector
20 elute
21 flame ionization detector
22 flame photometric detector
23 gas chromatograph
24 gas chromatography
25 gas‐liquid chromatography
26 gas‐solid chromatography
27 housekeeping column
28 inert support
29 liquid chromatography
30 liquid loading
31 liquid‐phase column
32 mobile phase
33 molecule
34 open‐tubular column
35 packed column
36 peak
37 peak area
38 peak height
39 PLOT column
40 retention time
41 sample
42 sample conditioning
43 sample injector valve
44 SCOT column
45 separation
46 stationary phase
47 supercritical fluid
48 temperature programming
49 thermal conductivity detector
50 volatile liquid
51 WCOT column
In addition, we introduced several chemical names and you need to know what they are. If you are not familiar with chemical names, refer to the SCI‐FILE: On Chemical Names in Chapter 4. You can also look up individual chemical names in the Glossary.
Note
1 1 There are some exceptions to the principle of instant vaporization that are beyond the scope of this introductory text.
2 Peak shape
“What happens inside a column? What makes the peaks form? Why are peaks that funny shape? Why are some peaks wider than other peaks? These are good questions, and now is the time for answers”.
How columns work
The secret to understanding process gas chromatographs is knowing how the columns separate the components of the sample. PGC training courses often omit this important knowledge, preferring to focus instead on the mechanics and electronics of the instrument itself. It's true that special skills are required to properly set up and maintain the equipment. And you must learn those skills. Yet, even if you gain perfect knowledge of the electromechanical systems, you won't be competent with process gas chromatographs until you clearly understand what the columns are doing.
You'll need to know how a column really separates molecules of one kind from molecules of another kind. It's not sufficient (nor true) to say that some kinds of molecule move faster than others do.
You'll also need to know what determines the shape of a chromatogram peak, particularly its width. So let's look a little closer at some typical peaks.
Looking back at the chromatogram in Figure 1.7, if you examine any individual peak, it is easy to see that even identical molecules don't reach the detector at the same time. Relative to the time of the peak apex, some molecules arrive earlier, and some arrive later. Take a look at the propane peak, for instance: its base width is about 40 s, which means propane molecules start arriving at least 20 seconds before their most frequent and average time (at peak apex) and continue for at least 20 seconds after that, gradually dropping back to zero. This variation in the elution time of identical propane molecules determines the width of the propane peak and its characteristic shape.
At this point, you should be wondering why identical molecules don't spend an identical amount of time in the column. Whatever happens in there, surely identical molecules must experience identical delay and emerge from the column at the same time? No, they don't. Some emerge a little earlier, and some emerge a little later. Any useful explanation of chromatography must account for that inconvenient fact.
Of course, anything that makes a peak wider is a nuisance because it's more difficult to separate wide peaks from each other than to separate narrow peaks. So, as a practical matter, we need to know how to minimize the peak width, and that is one of the most important questions in gas chromatography! The answer will become apparent as you work through the book.
What happens inside the column
Inside the column, sample molecules that are traveling in the carrier gas touch the stationary phase. What happens next depends on what kind of stationary phase is present, a solid or a liquid.
Most PGC columns employ a liquid stationary phase, which works by selectively dissolving the component molecules. Since they are so common, it is reasonable to use liquid‐phase columns as our example for explaining how the chromatographic process works. Therefore, the rest of this chapter will discuss only gas‐liquid interaction.
Columns
