a lot of information to absorb, and it's all important for understanding how columns separate components. Before we look into what causes separation, you might want to read it again.
The SCI‐FILE that follows is optional reading, but is not difficult to understand. It gives a more technical account of solubility, so you may find it interesting.
SCI-FILE: On Solubility
Solubility
The main text uses the concept of “solubility” because it's intuitive and easy to understand. The solubility of a component depends on how easily its molecules can interact with the molecules of the stationary phase.
In that text, we make the simplifying assumption that “75 % solubility” means 75 % of the molecules dissolve in the liquid phase and 25 % remain in the gas phase.
While convenient and applicable to a given column, this representation fails to account for the volume of each phase inside the column, an important variable. Instead of using the number of molecules, the formal expressions of solubility use the concentration of the solute in each phase, thus incorporating the volume of each phase.
Partition
In the gas chromatography literature, the process and outcome of equilibrium is often called partition. The partition coefficient is the ratio of solute concentrations in each phase.
Chemists now prefer to reserve partition for liquid‐liquid systems. For gas‐liquid systems, the equivalent term is distribution, and the ratio of the solute concentrations in the gas and liquid phases is called the distribution constant.
Distribution
The distribution of molecules between gas and liquid phases in a chromatograph column is a special case of Henry's Law: the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas in contact with that liquid.
The distribution constant (Kc) 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
