Chromatography Forum

Hi everybody.

can anybody please help me to distinguish xylene and ethyl benzene using GCMS, Both are having same mass. I am unable to identify them eventhough with Nist Library search.

First, you need to try to separate both using a non-polar column such as HP-1 in which ethyl benzene elutes first and m/p-xylene second. A sequence is shown as follow, 1. solvent, 2. ethyl benzene, 3. m/p-xylene, 4. both. Second, it is then confirmed by NIST library search.

We use DB-624 and the order is ethylbenzene:m,p-Xylene;o-Xylene

almost any silicone or PEG column will separate EB from Xylenes.

The non-polar separation order is EB m-Xy p-Xy 0-Xy

The moderate polarity separation order is EB, m/p-Xy, o-Xy

The PEG separation order is EB, p-Xy, m-Xy, o-Xy.

The more plates you have the better the separation you will have. The best for the EB from Xylenes separation is probably the non-polar 100% methyl silicone column.

I believe the question is using mass spectrometry to verify compound identity. The short answer is you can't. The identification of ethylbenzene and the xylenes must be done on the basis of the retention time. There are some differences in the ion ratios of fragments in the mass spectra of the xylenes and ethylbenzene, but the library matching will generally give almost identical match scores for the compounds.

That´s surprising, I would have guessed that ethylbenzene would give a very prominent benzyl and/or phenyl fragment, while the xylenes would not.
Any explanations?

Thanks Ron

Your Thinking is correct, i want to identify the componds in an unknown sample. I am getting both the hits(ethylbenzene and xylene) in NIST Library which was installed in my system.Now i am trying to separate them by injecting the standards of both to get different retention times.One more thing i am only having GCMS for doing fragmentation my system is not suitable.

I knew the spetra between Ethylbenzene and xylenes are similar, but never really looked into it. I printed out the spectra for each compound and found some difference between them.

1) Ratio of m/z 106 to m/z 91 is bigger for xylenes than ethylbenzene.

2) Ratio of m/z 105 to m/z 107 is significently different for xylenes and ethylbenzene. I think you can make the call based on this ratio.

unfortunately, the real world issues are in that:


very few sample of p- or m- xylene are completely without EB and few samples of EB are without xylenes, ... at some level, .....which may confuse the spectra, and make it difficult to quanitate amounts of either.

Oh, that it were so, because p-xylene purity is a big issue in industry.