Chromatography Forum

People say that is not possible separate m- and p- xylene using 100%PDMS (5um film) columns.
However I performed this analysis for a long time and I had 3 peaks when injectin xylene standard.
Someone knows if this was a problem of old columns and the actual ones can separate this critical pair?


Thanks

hello,
People is OK, it is not possible to separate the M and the P xylene because they have a very close boiling point, thus impossible to separate on a phase 100% PDMS; on the other hand on a PEG 20000 (CPWAX or DBWAX) it's working very well, because you make the separation on the difference of polarity aspect.
You should test to see whether you obtain 4 peaks: 3 xylenes and...... your impurity!!!!

Fabiano,

Commercial xylene always in my experience contains ethylbenzene, p/m-xylene, o-xylene eluting in that order, which gives you 3 well resolved peaks on typical -1 phase capillary columns. Old methylsilicone packed columns would not give baseline separation of the ethylbenzene. A -10 phase 50 m x 0.2 mm thin film capillary just shows a trace of shoulder for the p- m- separation. It is only a 0.1% retention time difference. Clearly you could do it given enough resolving power and time, but you need a polar capillary to separate them in a reasonable time, as in the message above.

I will agree that on a normal DB-1 ( 100% methyl siloxane ) phase we cannot separate m & P-xylene. and we get 3 peaks for Ethyl benzene, m+p-Xylene and o-xylene. But the film thickness (5µ) can do something. I think you should check minutely by injecting the pure isomers whether they separate or not. Of course on a polar column they get separated.

Rgds
Shashi Kant Tiwari

Another column both in packed and SCOT capillary versions is a bentone 34 which separates these isomers.

But I agree for percent level separations to be done quickly for ID purposes a Carbowax capillary is probably the best choice.