The compounds in my oil mix have very close boiling points and they are co-eluting in gas chromatography. I tried to play around with the ramp temperature, but that didn't help. I have given it a thought to use a longer column, but I am already using a 120 metres long column. Can anyone suggest any other way of getting around with this situation?
Decreasing the flow rate might help or if you are using nitrogen as a carrier gas than use helium. Also if your peak respose is good than increasing the split ratio will decrease the peak response and thus the co-eluting peak might get seperated.
You have two parameters to adjust in order to improve the resolution. One of them you have already "touch". It is the temperature ramp. this parameter may vary the resolution but in very small amount. The parameter who adjust the resolution more drastically is the column flow. It depends not only of the flow value but also depends on the carrier gas. The best carrier gas is hydrogen, and the best compromise is Helium. Also, if you are using a FID detector, one important parameter to be taken into account is the makeup gas flow. This parameter is a flow value from the same carrier you are using to analyze your samples declared for the detector.
Thank you for the responses! I am using hydrogen as the carrier gas and its a GC-MS system. I will try changing the flow rate and/or split ratio and see how that goes. Changing the column is not a very feasible option I am looking at.
Split mode in place of splitless could be a solution. Already done with success in GCMS.
This question would be much easier to answer if you provided details of your current operating conditions. Lacking those I can give only general advice.
With hydrogen as carrier gas the linear flow velocity for maximum resolution is 50 cm/s. Increasing the flow will decrease resolution slightly, but will save time. Decreasing the flow will degrade resolution and waste time, so is pointless.
The optimum programme rate for a 120m column is 1 C/min - you should see peak widths at half height the same for late peaks as for early ones when the programme rate is optimal.
A column delivers the maximum number of separated peaks when the peaks are more or less evenly spread across the chromatogram. If your peaks are bunched together but you have areas of empty baseline also then you are not using all the separating power of the column. If you are trying to separate compounds with similar boiling points on a column whose selectivity is based on molecular weight, volatility and boiling point, then you will inevitably have bunched, overlapping peaks. The only solution to this is a change in stationary phase - and a solution is not guaranteed; you would need compounds with structural differences for the different selectivity of another stationary phase to make any difference to their relative retentions.
In a crowded chromatogram, no matter what column you use, there will always be peak overlaps - the best that you can hope for is to get clean peaks for the subset of components that you are particularly interested in.
Since increased peak widths at baseline compromise separation you need to eliminate extraneous causes of peak distortion - concentration overloading, dirt in the inlet, poor choice of solvent, wrong inlet liner etc etc.
Dear Nirupama
you can decrease the flow rate, increase the split ratio ad "play" with temperature: you can alternate ramp temperature with isotermal eluition!
good luck
The previous answers covered most of the possible solution but how about if you play with the volume that you inject
I agree with Peter. We need more details. Flow rates, column dimensions, ramp rates, injection volume and type of injection.
That said, hydrogen is a very unconventional carrier gas for MS. It leads to issues in the source that are not readily remedied, some of which can manifest themselves as "co-elution". Have you tried switching to He? For GC-MS it is vastly preferable.
Ramp rate on a 120m column won't have much effect - it is VERY long!! Probably way more column than you need.
I agree with Mark. In most of the laboratories GC-MS is normally configured for Helium gas as carrier gas which gives the best possible results. But as Hydrogen is a cheaper gas and its sources are not limit like Helium, it is a good option. Changing from Hydrogen to Helium will not be that easy one has to change the hardware part of the instrument and has to recalibrate the system, is it right ? somebody please comment.
Just to note that there is no mention of mass spectrometry in the original question, so the comments about helium vs hydrogen as a carrier gas are speculative. If the detector is an FID then hydrogen is clearly the best choice of carrier gas.
She added both the hydrogen as carrier and the GC-MS as a comment, Peter. See the full list of comments.
You are right, Mark. I missed that. The maximum resolution available with helium is the same as with hydrogen, it just takes longer to generate it so hydrogen is a strange choice.
As you are using GC-MS, you may not need perfect separation, depending on your analytes and on what you actually want to do. You can try deconvolution with e.g. AMDIS, which is free.
If you have the budget and the possibility to change the configuration of your GC, try heartcutting or GCxGC.
You can also tell us more about your analytes and maybe share a chromatogram or mass spectra. That might trigger some new suggestions.
Unless you are running with a gas flow much faster than the optimum, then decreasing the flow will make resolution worse not better, and will waste time by making the elution times longer. It might look as if the peaks are better resolved because they are further apart, but they are also wider, and so overlap more than they do at optimum flow.
To improve the resolution of peak is very important. At first I would like to say you that the peak separation and resolution are confused sometimes. Separation is the time or distance between two peaks. Peak widths are not factored into this measurement. Peak resolution takes into account the amount of peak separation and the widths of the peaks. Changes in resolution are due to changes in peak separation and/or peak width. Decreasing column temperatures usually increase peak separation but often with a corresponding increase in peak width. If the increase in peak separation is greater than the increase in peak width, improved peak resolution occurs. This is usually the case for earlier eluting peaks (i.e., partition ratios [k] of around 5 or less) in the original chromatogram. If the peak separation increase is less than the increase in peak width, peak resolution is degraded. This is usually the case for later eluting peaks (i.e., partition ratios [k] of around 10 or more) in the original chromatogram. Lowering column temperature is a common method to improve peak resolution, especially for early eluting peaks. An increase in peak separation is expected; however, in rare cases, this does not occur. This has occurred for the solvent chromatograms in question. The peak widths did not increase, but the peak separation decreased. This caused the loss of resolution.
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