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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 employing a solid stationary phase are used to separate simple gases like hydrogen, oxygen, nitrogen, or methane. Gas‐solid columns work by selectively adsorbing the sample molecules. This is a different mechanism, but it has the same effect; the column retains one kind of molecule longer than it retains another kind of molecule.

      How gas and liquid interact

      In a gas‐liquid column, the component molecules touch a stationary liquid phase and obviously interact with it in a way that causes separation. So we need to

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