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Process Gas Chromatographs. Tony WatersЧитать онлайн книгу.

Process Gas Chromatographs - Tony Waters


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how a gas can interact with a liquid.

      Gases dissolve less at higher temperature. Heating the water expels the dissolved gas, visible as a cluster of small bubbles. Schematic illustration of gases dissolve in liquids. Gases dissolve less at higher temperature. Heating the water expels the dissolved gas, visible as a cluster of small bubbles.

      A moment's thought should reveal the answer. The gas can't be steam because steam bubbles can't exist under water unless the water is boiling. The gas can't be hydrogen because water doesn't decompose at this low temperature. No, the gas must be coming from something dissolved in the water, most likely oxygen and nitrogen from the air.

       Gases dissolve in liquids.

       Gases dissolve less in hot liquids than they do in cold liquids.

      Gases dissolve less at lower pressure. Popping the cork lets the pressure drop, releasing lots of bubbles. Schematic illustration of different gas. Gases dissolve less at lower pressure. Popping the cork lets the pressure drop, releasing lots of bubbles.

      Again, where did the gas come from? There is only one place that it could have been hiding; it was dissolved in the liquid. This confirms our theory that gases dissolve in liquids, but far more was dissolved this time; the solubility of carbon dioxide in water is greater than the solubility of air in water.

      This second example demonstrates two additional principles:

       Gases dissolve less at low pressure than they do at high pressure.

       Some gases dissolve in a given liquid more than other gases do.

      A final example (not illustrated) is what happens when you heat cooking oil in a pan. No bubbles appear. Apparently, the oil doesn't dissolve any air − and it won't dissolve much carbon dioxide either!

      This last example gives a fifth principle:

       Some liquids dissolve more of a gas than other liquids do.

      These five principles are all the science you will need to truly understand what happens in a column. They remind you that a gas can dissolve in a liquid and that the amount of gas that can dissolve depends on only four simple variables; the temperature and pressure, the type of gas, and the type of liquid.

      That's it. Nothing else affects the solubility of a gas in a liquid.

      The four variables are very easy to understand, yet they are the hidden foundation of all gas chromatography. Let's see how that can be …

      In most process gas chromatographs, three of the four variables are closely controlled and do not vary:

       The column temperature is held constant.

       The carrier gas pressure is held constant.

       The liquid phase is predetermined and doesn't change.

      The fourth and most important variable is due to the different gases in the sample. And this is the real cause of chromatographic separation:

       Different gases have different solubility in the liquid phase.

      This is the process of separation. It's all about solubility.

      Chemists call the liquid phase a solvent and each dissolved component a solute. But no real chemistry is involved. If a chemical reaction occurred, it might destroy some of the molecules that we are trying to measure and likely would cause a gradual and irreversible deterioration of the column itself.

      Before moving on, a quick reminder. The discussion in this chapter focuses on the most common kind of column; one that has a liquid stationary phase. As noted earlier, another kind of column uses a solid stationary phase. The solute molecules can't dissolve in a solid, but they can and do adhere to its solid surface, and the final outcome is much the same.

      Troubleshooting tips

      The household examples used above may provide some valuable help with troubleshooting and are worth remembering:

       When a column works at higher temperature, gas solubility is reduced, and all the peaks come out earlier on the chromatogram, thereby reducing their separation. For an easy way to remember this, recall the heated water!

       When a column works at higher pressure, gas solubility is increased and all the peaks come out later on the chromatogram, thereby increasing their separation. For an easy way to remember this, recall the bubbly champagne!

      These troubleshooting tips assume that the carrier gas flow rate is held constant. Later chapters discuss the effect of other variables.

      The rest of this chapter explains how solubility causes the classic peak shape. The following chapter examines how a difference in solubility will cause the peaks to become separated from each other.

      Forming an equilibrium

Schematic illustration of forming an equilibrium. (a) Consider helium gas in contact with a liquid, then add propane. (b) The initial rate of entry into liquid is fast, with zero rate of escape. (c) Then, the rate of entry drops as the rate of escape increases. (d) Equilibrium occurs when the rate of entry equals the rate of escape.
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