Hello fellow chromatographers. I'm trying to implement the Ph. Eur. method for Ethanol 96 % analysis, where a GC-FID system is being used. The problem I have is that the first peaks that elute (acetaldehyde, methanol and ethanol) display fronting and tailing at the same time (see pictures). The rest of the peaks seem to behave normally. Any clue why?
Carrier gas: Helium
Linear velocity: 35 cm/s
Split ratio: 1:20
Inlet temperature: 200 ºC
Detector temperature: 280 ºC
Injection amount: 1 uL
Column:
* 6 % cyanopropyl-phenyl – 94 % dimethylpolysiloxane
* 30 m long, 0.32 mm diameter, 1.8 uL film thickness
We are using solutions and injection volumes defined in Ph. Eur. so I assume that with their specifications this should not happen. I guess it must be something else.
Actually there are many reasons for tailing and fronting in chromatograms. Most prominent of all is because of column. First you should run just blank column in triplicate. Then, try to dilute your sample a little and inject and see. You can also increase the split ratio. I am sure these three trials will help you to get rid of your problem.
Update: seems like the only peak that is really giving problems is the methanol peak. The solvent peak will always be fronting because it's the main component of the sample. Acetaldehyde seems to be almost symmetric. The methanol peak is the one for which I didn't find a suitable explanation yet. May it be the high solubility of methanol in ethanol?
I think I finally found an explanation. Seems like methanol, because of such similar molecular structure, elute very closely to ethanol. In this case, ethanol is the solvent, and will interact with many active sites of the column. This means that, for components that elute close to ethanol will have way less active sites to interact with the column, hence the symmetry may be compromised.
Reference: https://blog.restek.com/?p=8674
Dear Michail,
have you checked the injector? If not, please check the right position of the column end in the injector liner. I also recommend cleaning /exchaning the liner, especially if you use a plug glass wool in the middle of it .
kind regards, Tomas
The chomatography (Peak shape and peak width) here is terrible. A longer, better column is necessary. Solvent washing of first 10 cm of column support has occurred. I have no access to your column description, but it must be checked for any of several deficiencies.
Dilute your samples. Maybe you'll have to work with 2 injections concentrations to quantify your analytes in their best detections parameters.
Anna, David, thank you for your suggestions. Sadly, I can't modify the sample preparation process neither the method parameters described in my post, as it is mandatory for my lab to follow European Pharmacopoeia monographs. There may be a problem of the column but we can't afford a new one right now, specially because the one we are using is technically new (we changed our He tank recently, so maybe there was some air that went through the column?). I will try to search for some performance tests for my column, hopefully that will clarify is the column is in good state.
Tomas, I've installed and reinstalled the column leaving 1 mm of column from the end of the ferrule. Liner was also exchanged for a new one without wool.
Is this a packed column or ultra thin-layer capillary? If excess hot water/methanol has been flushed through the front of your column, it can be damaged. We cannot see your protocol or details. What must you do to change this? reverse the column? if it is capillary, it does not care! If this is a Packed column, replace the first 5 cm of particulate packing.
It is a capillary column (it is an Macherey-Nagel Optima 624 LB). No water or methanol was flushed through the column. But air could've been a problem. So far, the peaks are separated, the only problem is the peak shape of methanol. If I get to solve this problem I will update this post with a comment.
Can you use a smaller sample volume? ie: use a smaller sample loop, or only partially-fill the injector loop - as a small 'bolus' of your over-large methanol standard, to prevent saturation of your column and detector?
This has worked for me, in the past.
A 624 phase is not a polar phase, and you are seeing classic symptoms of analyte/phase mis-match. You have, essentially, overloaded your column. You can increase your split ratio (say, 50:1 or 100:1), which will help, but the real solution is to move to a more polar phase (wax or FFAP). Your peak widths show this; they are much too wide for good capillary work, which is an indication that you are not refocusing your analytes on the head of your column. Again, classic symptoms of analyte/phase mis-match.
Try the following:
Start the oven 30-50 degrees lower than what you are using now
Increase your split ratio
Also, 1 mm is not enough space between your ferrule and the end of your column. Not sure where you got that from, but you should have 5 mm or so, depending on the type of liner you are using. You should stay with the liner with the glass wool plug since you are using a split injection.
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