(3.3)
Inserting the data from Figure 3.4 gives:
A plate number of 576 is very low and would indicate a very inefficient column, since most columns generate about 2000 plates per meter. Of course, this peak is intentionally drawn wide so the measurements are clearly seen. A real peak would be much narrower than this: a more typical peak might have one third of the width and nine times the plate number.
The alternative equation using the peak width at half height (w0.5) is:
(3.4)
Chromatographers calculate plate number as a way to evaluate the performance of a column. Any change of operating parameters that increases plate number automatically improves the separating power of a column.
Another performance indicator is the plate height (H), which is the length of a column required to generate one plate. It's usually reported in mm. Knowing the column length (L), you can easily calculate the plate height:
(3.5)
Plate height gives a measure of column performance that is independent of the column length and is the primary variable used for optimizing performance.
Knowledge Gained
It's obviously true to say that peaks come out of the column at different times.
It's not true to say that molecules move at different speeds inside the column.
For all injected molecules, there are only two speeds possible inside the column; stop or go.
When molecules are in the liquid phase, they cannot move along the column.
When molecules are in the gas phase, they move at full carrier gas speed.
The “air peak” doesn't dissolve in the liquid phase so it travels at the same speed as the carrier gas.
It's impossible for any peak to travel faster than the carrier gas.
No peak from the same injection can appear on the chromatogram earlier than the air peak.
To get through the column, all injected molecules must travel for the same time as the air peak does.
If the column had no liquid in it, all the peaks would elute together with the air peak.
Any additional retention time beyond the air peak time is the time a peak stopped in the liquid phase.
Separation is not caused by motion, it's due to peaks stopping for different times in the liquid.
The time that a peak stops in the liquid phase is directly proportional to its solubility in that liquid.
The spacing of peaks on the chromatogram is different than their spacing inside the column.
Peaks that migrate slowly along the column come out very much later on the chromatogram.
Retention time is the sum of time traveling in the gas phase and time stopped in the liquid phase.
Holdup time is the time in the gas phase; adjusted retention time is the time in the liquid phase.
Column operating performance can be evaluated from chromatogram measurements.
Chromatogram measurements may be made in seconds or millimeters.
You can measure the holdup time, peak retention time, and peak width on the chromatogram.
When measuring base width, triangulate the peak and extend the baseline under the peak.
Alternatively, measure the peak width at half the peak height.
Plate number is calculated from measurements of retention time and peak width.
Any change in operating conditions that increases plate number increases column separating power.
Did you get it?
Self‐assessment quiz: SAQ 03
These questions relate to gas‐liquid chromatography:
1 Q1. At what speed do component molecules move along the column when they are in the gas phase?
2 Q2. At what speed do component molecules move along the column when they are in the liquid phase?
3 Q3. What causes the separation between components?
4 Q4. What information may be derived from the retention time of an unretained peak, such as air?
5 Q5. If all the liquid phase was washed out of the column, where would all the peaks be on the chromatogram?
6 Q6. What causes the additional retention time beyond the holdup time?
7 Q7. What is the purpose of triangulating a peak?Check your SAQ answers with those given at the end of the book.
Student evaluation test: SET 03
Your instructor will provide answers to these questions.
All of the questions relate to gas‐liquid chromatography.
1 S1. Which one of the statements below best defines the retention time of a component peak? Select one:It's the average time that the component molecules spend in the column.It's the average time that the component molecules spend in the gas phase.It's the average time that the component molecules spend in the liquid phase.It's the average time that the component molecules spend in the detector.
2 S2. Which one of the statements below best explains the separation of component peaks on the chromatogram? Select one:The molecules of some components move at different speeds than others do.The molecules of some components spend less time moving than others do.The molecules of some components spend more time moving than others do.The molecules of some components spend more time not moving than others do.
3 S3. Which one of the statements below best explains the significance of the air peak? Select one:The air peak marks the time when the component molecules start to dissolve in the liquid phase.The air peak indicates the time that every component molecule spends in the gas phase.The air peak indicates the time that every component molecule spends in the liquid phase.Any peaks traveling faster than the air peak cannot be separated on that liquid phase.
