Hi!
I'm currently working on a system where benzene and cyclohexane are two of the main analytes. (Alongside phenol, cyclohexanol and hexadecane.)
I have attempted to quantify my products by GC-MS, (Agilent 7890A GC equipped with a 19091S-433 column (30 m length, 0.250 mm inner diameter, 0.25 µm film))
Unfortunately, I am unable to separate the benzene from the cyclohexane: both peaks are merged (benzene comes a little sooner judging by the MS), and the retention time is very short (3-6 minutes, depending on GC settings).
I have tried changing the temperature, going as low as the GC allows (27°C), or changing the carrier gas flow rate (He, 0.55 to 1.2 ml/min). None of this appears to have helped.
Has anyone any advice on how to proceed? Will I have to get a new column, or is there another setting in the acquisition method I could try changing to separate those analytes?
Thanks!
You work on a mass spec. You should be able to distiguish between benzene (main mass m/z 78) and c-hexane (main masses m/z 56 and 84) and therefore be able to quantify on these main masses.
Furthermore you can use a more polar column then the 19091S-433 column, which is by the way better known as the (only slightly polar) HP-5ms GC Column. There is a number of possible choices, e.g. the Rxi-624Sil MS ( see https://www.restek.com/chromatogram/view/GC_GN1199 )
Please look at the following below links which may help you in your analysis:
https://www.ocf.berkeley.edu/~manraj/wp-content/uploads/2018/05/GC-MS-analysis.pdf
https://pubs.acs.org/doi/full/10.1021/ie960426f
Thanks
It should separate on PEG (carbowax), most ionic liquid stat. phases (IL100 or IL 111), as well as DB624.
Besides: what is rh3 concentration, injection volume and injection mode (split?). Can you post a chromatogram?
Regards
GB
Yes, of course. Benzene and Cyclohexane have different retention times in GC.
I assume that you are trying to deal with your problem only trying to interpret the chromatogram you obtained to quantify those compounds. As mentioned above, it is actually possible to elute them at different retention times by utilizing different columns or modifying your flow rate and oven gradient. If it is as I said and you are trying to solve the problem by only interpreting your chromatogram, and you are using a Mass analyzer, try to find your spectrum window and make a search for the m/z values of your compounds (the values Mr Gruber mentioned above). This will give you separated peaks for your compounds. Of course, eluting them at different times would provide a healthier result.
Regards.
You will be able to separate these two using DB-624 column which is a medium polar one.
You didn't mention your temperature gradient other than to say you varied the starting temperature. It is possible to use steps in your gradient to separate closely eluting molecules. Other than that the basic answer to you question is to go to the column manufacturers web pages and look for a a column that will do the job. Many allow you to input the molecule of interest.
I saw that NH3-Clinoptilolite can be used for the separation of benzene and cyclohexane... if you still have problems with commercial columns, it looks like a material that can be easily synthesized.
I had a similar problem using GC-MS. The most important challenge is to find a proper solvent. To my experience, analytical grade acetone works very well. Then, using DB-5 column, and GC-FID, you may get the separate peaks for each component. The initial temperature should be low enough as well.
Please refer to my latest article for more details:
Article Gas-phase hydrodeoxygenation of phenol over Zn/SiO2 catalyst...
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