Process Gas Chromatographs. Tony Waters

(K_c) is defined for a specified solute A as the equilibrium concentration of A in the stationary phase divided by the equilibrium concentration of A in the mobile phase:

      SCI‐FILE: (2.1)

      Where the square brackets are a standard chemical shorthand for [the concentration of].

      Limitations

      The theory assumes that the distribution constant will remain constant. It is assumed, for example, that the distribution of propane molecules at equilibrium is constant regardless of the number of propane molecules present, and that the distribution is unaffected by the presence of other solute molecules in the stationary phase.

While generally true at the low concentrations of solute normally reached in a liquid phase as a peak passes through, either of these assumptions may fail at higher concentrations.

      The first assumption is that the distribution is constant for varying amounts of solute. At low concentrations of solute in the liquid phase, the distribution constant is indeed constant for most solute‐solvent pairs. But high concentrations of solute may exceed the linear range of Henry's Law, causing a large peak to become wider and distorted in shape.

      The second assumption is that solute molecules are at such low concentration in the liquid phase that they do not interact with each other; i.e., the presence of one kind of molecule dissolved in the liquid phase will not affect the solubility of another kind of molecule.

      This independence of solubility is generally found to be true in practice. A rapidly moving peak will pass right through a slower one, usually with no effect on the retention time of either of them. Even so, you should be aware that a very large peak can saturate the liquid phase and displace a low‐concentration peak from solution, thereby reducing the distribution constant and the retention time of the smaller peak.

      Knowledge Gained

       It's not sufficient to say that some kinds of molecule travel faster in the column than others do.

       Identical molecules don't spend the same time in the column; some elute earlier than others do.

       Variation in the retention time of identical molecules is the root cause of the peak width and shape.

       Wide peaks are more difficult to separate (resolve) than narrow peaks are.

       Solid columns work by a different mechanism than liquid columns, but the end result is similar.

       Gases dissolve in liquids and rapidly reach dynamic equilibrium.

       Component solubility is affected by temperature, pressure, and the kind of liquid; nothing else.

       Component solubility in a liquid phase decreases with temperature increase or pressure decrease.

       At equilibrium, component molecules enter the liquid phase at the same rate as they are escaping.

       Although chromatography is continuous, it can be modeled as a succession of discrete equilibria.

       A succession of equilibria forms a symmetrical peak shape which narrows with more equilibria.

       Ideally, peaks would have perfect symmetry, but real peaks are often somewhat asymmetric.

       For every injected molecule there are only two speeds along the column; stop or go.

       All component molecules in the gas phase must move along the column at full carrier gas speed.

       The molecules that by random chance are stuck in the liquid phase cannot move with the carrier gas.

       Even identical molecules suffer random time in the liquid, which is the main cause of peak width.

       Narrower peaks are taller, and more easily separated from adjacent peaks.

       Plate number is the effective number of equilibria and is estimated from the chromatogram.

       Peaks become narrower as the plate number increases, making separation easier.

       Peaks become taller as the plate number increases, thereby increasing signal‐to‐noise ratio.

       The plate number affects only the width of a peak, not its position on the chromatogram.

       The peak apex is at the elution time for an average molecule, so is the best measure of retention time.

Did you get it?

      Self‐assessment quiz: SAQ 02

      1 Q1. The way a column works depends on whether the stationary phase is a solid or a liquid. What is the mechanism involved when the stationary phase is liquid?

      2 Q2. What is the mechanism involved when the stationary phase is solid?

      3 Q3. What are the four variables that determine the solubility of a gas in a liquid?

      4 Q4. When a gas and a liquid are in equilibrium with each other, what is happening?

      5 Q5. Why do peaks get wider as they travel through a column?

      6 Q6. Predict the effect of increased column temperature. If all else were constant, what effect would it have on the peaks?

      7 Q7. Predict the effect of increased column pressure. If all else were constant, what effect would it have on the peaks?Check your SAQ answers with those given at the end of the book.

Student evaluation test: SET 02

      Your instructor will provide answers to these questions.

      1 S1. What are the two main mechanisms at work in the different kinds of gas chromatographic column?Select the two correct statements:Columns containing a solid stationary phase work by selectively adsorbing components of the sample.Columns containing a solid stationary phase work by selectively dissolving components of the sample.Columns containing a liquid stationary phase work by selectively adsorbing components of the sample.Columns containing a liquid stationary phase work by selectively dissolving components of the sample.

      2 S2. Consider the identical molecules within a certain component peak, say, propane. Which of the statements listed here are false?Select the two false statements:If all variables are held constant, identical molecules take the same time to pass through a column.When identical molecules travel through a column, some randomly stay in the liquid phase longer than others do.When identical molecules travel through a column, some randomly stay in the gas phase longer than others do.The width of a peak is due to identical molecules randomly taking different times to travel through a column.

      3 S3. What is the effect of temperature and pressure on the solubility of sample gases in the liquid phase?Select the two correct statements:The solubility of a gas in a liquid increases when the temperature is increased.The solubility of a gas in a liquid decreases when the temperature is increased.The solubility of a gas in a liquid increases when the pressure is increased.The solubility of a gas in a liquid increases when the pressure is decreased.

      4 S4. When applied to the discontinuous model of chromatography presented in the text, which two of these statements are true?Select the two true statements:When the carrier gas moves, all the sample molecules move with it.Each time the carrier gas moves, the same number of sample molecules move with it.Each time an equilibrium forms, it contains the same percentage of the original sample molecules.Each time an equilibrium forms, the same percentage of the available molecules is dissolved in the liquid phase.

      5 S5. Consider a gas‐liquid equilibrium. Which of the statements listed here is true?Select the one true statement:Whenever a gas is in equilibrium with a liquid, 50 % of the gas molecules are dissolved in the liquid.At equilibrium, the liquid phase cannot dissolve any of the sample