Process Gas Chromatographs. Tony Waters
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phase.At equilibrium, the rate of molecules dissolving in the liquid has become zero.At equilibrium, the rate of molecules dissolving in the liquid is exactly equal to the rate of molecules leaving the liquid phase.
6 S6. For a 10,000‐plate peak having a retention time of 500 s, what is the average time for the column to generate one equilibrium?Your answer must be an integer number of milliseconds.
7 S7. In the example given in Figure 2.9, why is the propane peak exactly halfway along the column when the carrier gas gets all the way through?Select all the correct answers and none of the incorrect explanations:Because, in the example, the propane solubility was adjusted to 50 %.Because, when in the gas phase, the average speed of a propane molecule is half the speed of the carrier gas.Because, on average, the propane molecules stop in the liquid phase for half the time, so they only travel in the gas phase for half the time.Because, in the example, the plate number of the column was N = 5.
8 S8. What change in operating variables might influence the height of a peak on the chromatogram?Select all the correct answers and none of the incorrect answers:A change in the number of molecules of that component present in the injected sample.A change in the width of the peak.A change in the plate number of the peak.A change in the volume of sample injected.
9 S9. This question requires you to make deductions from what you have learned so far.For a particular sample gas, what properties of the gas‐liquid equilibrium are necessary to produce a symmetrical peak?Select all the correct answers and none of the incorrect answers:The proportion of molecules in the gas and liquid phases must remain constant when the component concentration changes.The equilibrium must form very rapidly so more equilibria occur while the peak is in the column.The overall time the molecules spend in the gas phase must be equal to the time they spend in the liquid phase.The solubility of the sample gas must not change with its concentration.
References
1
Figures
1 2.1 Gases Dissolve in Liquids
2 2.2 A Different Gas
3 2.3 Forming an Equilibrium
4 2.4 The Carrier Gas Moves
5 2.5 The Second Equilibrium
6 2.6 The Third Equilibrium
7 2.7 The Fourth Equilibrium
8 2.8 The Fifth Equilibrium
9 2.9 Effect of Having More Equilibria
Equation
| 2.1 |
|
Distribution constant |
Symbols
| [A]M | Equilibrium concentration of substance A in mobile phase |
| [A]S | Equilibrium concentration of substance A in stationary phase |
| H | Plate height |
| K c | Distribution constant |
| N | Plate number |
New 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 distribution
2 distribution constant
3 dynamic equilibrium
4 Gaussian
5 Henry's law
6 partition
7 plate height
8 plate number
9 signal noise
10 solubility
11 solute
12 solvent
3 Separation
“It's a paradox worth repeating. The different kinds of molecules that are injected into a column all travel along the column at the same speed, yet emerge from the column at different times, thus becoming separated from each other. Separation is what chromatographs do. You certainly need to know how”.
How peaks get separated
Looking back at Figure 2.9, why is the propane peak exactly halfway along the column? Think about it.
Taking the question one stage further: What would have to happen for another peak to be at a different location in the column, separated from the propane peak?
Simple explanations of gas chromatography often say that different peaks move at different speeds and come out of the column at different times. While this is empirically true, it's a circular argument and explains nothing. It's plainly true that if the peaks come out of the column at different times they must be traveling at different average speeds, but it's illogical to then conclude that the peaks are separated because they move at different speeds. It doesn't explain anything.
A more realistic explanation
We need a better explanation. Let's start from the obvious truism that when the sample molecules are in the column, they must always be in the gas phase or the liquid phase. There's nowhere else for them to be.
And we know that the liquid phase doesn't move.
Therefore:
When the sample molecules are dissolved in the liquid phase, they are held in place and are not moving along the column.
When the sample molecules are in the gas phase, they move along the column at the same speed as the carrier gas.
Note that we refer to movement along the column. Molecules are always moving randomly in every direction, and this contributes to peak broadening, but random motion contributes nothing to positive movement along the column.
There are only two speeds along a chromatograph column; stop or go!
This is true for every peak. Every injected molecule must spend enough time in the gas phase to transit through the column. Therefore, each molecule travels at the same speed and spends the same time traveling as the other molecules do. It's not true to say that different molecules travel at different speeds.
Separation is not caused by motion at all. It's caused by stopping; the time that different molecules stay motionless in the liquid phase. The less soluble molecules don't stop for long and move quickly through the column, while the more soluble ones hang around in the liquid, slowing them down. It's that simple.
Notwithstanding the stop‐go mechanism, one might successfully argue that the peaks really are travelling at different average speeds through the column. Yes, of course, this is the overall result. But talking about the average speed of a peak obscures what is really happening in the column.