Crowded peaks in gas chromatography (FID)
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What we analyzed mainly was cyclohexylamine. The order of the peaks appearing was as follows: cyclohexane, benzene, toluene, cyclohexylamine, cyclohexanol, and dicyclic compounds. After switching to a different capillary column, cyclohexane, benzene, and toluene appeared together as one peak. How can these three peaks be separated? I thought a lot, but I still couldn’t solve the problem
This post was last edited by dalian*aochao on 2011-9-12 at 10:59. For the existing columns, the following can be done: 1. Increase the injection split ratio to reduce the amount of sample injected; 2. Reduce the flow rate of the carrier gas; 3. Lower the temperature of the chromatography column; 4. Use programmed temperature control to separate the sample peaks. If these methods don’t work, replace the capillary column – for the same model, a longer column can be used, or another model can be chosen as well. We use DB-1701 to separate the peaks.
Which gas’s flow rate should be reduced? Total flow rate of carrier gas or pressure before the column?
Of course, it’s to reduce the flow rate entering the column; if you use diversion, the total flow rate can be decreased while keeping the diversion ratio unchanged
It may be that the column capacity of the replaced capillary column has decreased, or the thickness of the coated film has become thinner, resulting in insufficient separation of the components. It is not recommended to reduce the carrier gas flow rate and the split ratio, as this will lead to a decrease in sensitivity and poor repeatability. It is better to choose a capillary column with a higher column capacity, or a thick-film capillary column, in order to ensure good separation and reliable analysis results. Is this an analysis for the production of nylon monomers?
I’m not sure if the columns chosen are the right ones; some of them have poor resolution.
If the column is fine, adjusting the program temperature or flow rate will help.
The peak emergence in chromatography is related to the boiling point of the substance; in cases of peak crowding, it is recommended to reduce the injection temperature.
What is the temperature of your chromatography column? It is recommended to use a column temperature of 70°C for 15 minutes, followed by a programmed increase in temperature to 220°C, or 250°C, with another 15-minute holding period at that temperature. This will definitely allow these three to be separated. Ours are like that too; they can definitely be separated.
Our company also does this; we recommend increasing the temperature gradually or lowering the column temperature. We use DB-1 – you might want to give that a try
Is the similarity the same when you change to another column? It would be best if the original poster provided detailed information on the model of the chromatography column before and after the replacement, as well as the analysis conditions. Everyone here is willing to help you; generally, the situation described in point 2 can resolve the problems you’re facing. But will the column you replaced be suitable for your sample?