Chromatography Forum

Hi

When analysing for xylenes in gasoline range organics, how can one tell apart the retention times for ortho and para xylenes since they come paired and which one comes first. My user library isn't quite clear on peak identification.

Thank you

By running a sample only containing one of the xylenes.

What type is your column?

You're going to have trouble separating them all on a nonpolar column (like a 5-phase). m- and p- isomers coelute on that phase. The o- isomer comes out later on the 5-phase. You can separate them all on a wax. The difficulty comes in identification because you can't get a standard for each that is pure. At least I haven't been able to find each component in pure form. If you're using a wax phase, you might want to check an app note from the column vendor to see if they call them out individually. Is it imperative that you know this information? For me, the fact that they're xylenes is enough.

Hi

RB makes some good points.

1. The column manufacturers do have example chromatograms with their ids

2. in this analysis do you really need to separate those isomers? I did some analyses on car exhaust gas/beer related studies (see below) -but that's another interesting story and didn't need that degree of separation





3. A long (60m) polar column (Innowax or equivalent ) or a thick film( 1 micron) will separate all the isomers

Regards

Ralph

sigma aldrich sell samples of separate xilenes at least with a good grade of purity to determine the retention time. They are not useful a standards for quantification, but you can get some information

You're going to have trouble separating them all on a nonpolar column (like a 5-phase). m- and p- isomers coelute on that phase. The o- isomer comes out later on the 5-phase. You can separate them all on a wax. The difficulty comes in identification because you can't get a standard for each that is pure. At least I haven't been able to find each component in pure form. If you're using a wax phase, you might want to check an app note from the column vendor to see if they call them out individually. Is it imperative that you know this information? For me, the fact that they're xylenes is enough.

Hi Rb

Well I needed to know it for the sake of re-creating my method and I had a problem with one of the xylenes not being identified in due to overlapping. I reduced the RT window period and it sort of helped.

Thanks

By running a sample only containing one of the xylenes.

What type is your column?

Hey Forse

I use a WCOT

ortho-Xylene and para-Xylene are well separated on almost any capillary column. Do you mean meta-Xylene and para-Xylene? Unless you use a very polar column (wax, etc) you will not separate the meta- and para- isomers.

As far as ortho- and meta/para-, the ortho- isomer elutes significantly later than the meta/para- on most capillary columns.