erkncyln asked: "hot to increase resolution"?
cjwitt comment:
There are so many factors such as type of phase and support, temperature, column ID and film thickness, carrier flow, column length, etc... that go into the equation when looking at manipulating resolution.
Yes, it is messy, but not as messy as it might appear. Several parameters affect the resolution in a predictable way, and that helps a lot.
All following comments are intended for the capillary columns. It is also assumed that the temperature program is always translated to accommodate the changes described below. You can do the translation manually (ask me how) or use GC Capillary Column Method Translation Software, available free of charge at
www.chem.agilent.com/cag/main.html#mxlator
1. Carrier gas flow rate.
Using optimal flow rate always helps. Choose the flow rates this way:
a) for 0.1mm column: 1mL/min for hydrogen and 0.8mL/min for helium
b) for other column ID (internal diameter), use flow rate proportional to the ID.
Example: helium flow rate in 0.25mm column should be 2mL/min.
WARNING! DO NOT (I repeat: DO NOT) rely on recommended average linear velocities (in cm/sec). If you do not know what I am talking about, don't bother to learn. It is not relevant to the topic of gaining the resolution. For those who are familiar with the widely recommended average velocities, I would like to remind that, for many applications, the recommended average velocities are way off the mark (please check my earlier postings).
Conclusion. Always use optimal gas flow rate (in mL/min) and have one less parameter to worry about.
*************************
2. Carrier gas type has no effect on potential resolution in the sense that
resolutions of ALL peak pairs that can be obtained with (for example) hydrogen at optimal flow can be reproduced with helium or nitrogen at their optimal flows. The only difference is that, with helium, the analysis would run 25 to 70 % longer than with hydrogen, and with nitrogen, it would run three to four times longer than with hydrogen.
Conclusion. Use optimal flow rate and don’t worry about the flow any more when trying to increase the resolution.
*************************
3. Column dimensions – length and internal diameter (ID). Their effect on the resolution is straightforward: potential resolution is proportional to sqrt(length/ID) as long as
a) the ratio Flow/ID for a given carrier gas does not change when ID changes
b) stationary phase type and phase ratio does not change (the phase ratio is proportional to the ratio ID/FilmThickness)
In fact, increasing the length/ID-ratio is practically THE ONLY predictable way of raising the resolution.
Example:
Double the column length, or replace 25m-0.25mm column with 20m-0.1mm column and the resolution of EACH peak pair will increase by 41%. Guaranteed! Try these or similar changes and see it for yourself.
Conclusion.
Effects of four things on the resolution are clear. They are:
carrier gas type
flow rate
column length
column ID
*************************
Additional comments to items #1-#3.
If it is necessary to improve resolutions of only a few peaks, several additional techiques can be used (see below). However, the additional techniques can barely increase the total number of resolved peaks because the techniques typically increase the resolutions of some peak pairs while reducing the resolutions of others. So, if the goal is to resolve more peaks then items #1 and #3 are about all that one can do.
The following techniques might work for improving the resolutions of a few targeted pairs.
4. Column temperature (isothermal analysis).
Effect of the column temperature on the resolution is difficult to predict. The temperature change might improve resolutions of some peaks while reducing the resolutions of others.
Try to increase the column temperature by 30ºC.
a) I the resolution does not change then changing the temperature is not the way to go. Most likely, any temperature change would be ineffective.
b) If the resolutions increase, you might be on the right path. Try further temperature increase. And don't forget about the flow optimization and column dimensions (items #1 and #3)
c) If the temperature increase reduced the resolutions then lowering the temperature would most likely improve the resolution.
*************************
5. Column heating rate (temperature-programmed analysis) is, in many ways, similar to column temperature in isothermal analyses. The effect of the heating rate on the resolution is difficult to predict. It might improve resolutions of some peaks while reducing the resolutions of others.
Make sure that the heating rate is close to optimal (10ºC per hold-up time) and then double the rate
a) I the resolutions of the pairs of interest do not change then changing the heating rate is not the way to go.
b) If the resolutions increase, you might be on the right path. Try higher heating rates. And, again, don't forget about the flow optimization and column dimensions (items #1 and #3)
c) If higher heating rate reduces the resolutions then lowering the rate would most likely improve the resolutions.
*************************
6. The most certain way to change the resolution(s) is to change the stationary phase type. Unfortunately, it is difficult to predict which way the resolution will go. Changing the stationary phase type might increase resolutions of some pairs while reducing the resolutions of others. No predictable advice here. Look for the expert advice. Try whatever you can. Good luck could helpful too. So, cross your fingers if it was helpful in other occasions.
*************************
7. Changing the film thickness should have a relatively minor effect on the resolutions of well retained peaks. Thicker film might improve the resolutions of the earlier eluting peaks, but that’s about it. Of course, if nothing else worked and you have plenty of time, you can try to play with the film thickness. I would rather try another stationary phase type.
Hope that these mumblings could be helpful in some way.
