I am developing a method for the detection of chlorobenzene, p-chlorotoluene, biphenyl and 4-methylbiphenyl with various other catalysts and additive spikes however I am having problems actually getting peaks for these aromatics in my initial gradient on a 150x4.6 C18 column with MeCN/H2O mobile phase monitoring at 270 and 254nm. Should I be using a phenyl-hexyl column? Any other suggestions welcome.

You should get peaks with your C18, no problem. Tell us about your gradient! you may need to go to 100% acetonitrile, and you will find nothing in the first 60%, no matter what column you use.

got the peaks now, just all overlapping at around 90%MeCN. Thanks anon

I have not managed to resolve these peaks-still using the C18 column and the aromatics particularly the biphenyls overlap, to complicate things further my samples will also contain toluene from rxn solvent and benzene impurities (I have no way of removing these from my samples before analysis).
Using eg. 85-100%MeCN over 10 min at 1.2mL/min.
Considering cracking open this new (75x4.6) phenyl-hexyl column but need to know if it will do my separation any good before I do (have never used one and not clear on its advantages).
As i said, all suggestions welcome
cheers

Dear KIO;

The phenyl-hexyl column will give slightly different selectivity and therefore a better chance to resolve your compounds. If no success is achieved, then I recommend you try a PFP one from Keystone. In my experience it has been very selective resolving aromatic isomers.

Good Luck;

Benjamin

Now that you found the peaks, the best thing to do is to run a flatter gradient. Since you say they are eluting around 90%, maybe you want to start the gradient at 70%, then 80% to see what is better. Don't bother with another column yet.

why are you not using gc? the analysis would be trivial

That 150 x 4.6 column doesn't have a lot of plates. Try either a longer column, or better yet a smaller bore if it can handle the injection volume. A 150 x 2 mm is about equivalent to a 250 x 4.6 mm. The comment on the 'flat gradient' is right on. Extend the time to 15-20 minutes for the same 85-100, and see if it helps.

Start the gradient where the earliest peak elutes relatively quickly, and then flatten the gradient until you get separation, or can't deal with the long analysis time. I have one analysis I run from 70-90% over 20 minutes. The first peak elutes at about 6-7 minutes, the last at about 20. I'm separating cis/trans isomers of several compounds with it.

Maybe you should try a C8 (YMC ProC8, 150 x 4.6 mm, 3 µm or any other "good" C8) and run a Water(+0.5% 0.5M Sulfuric acid)/Acetonitril gradient (from 70/30 to 10/90 in 30 min. at a flow around 0.8 ml/min.). Never tried your seperations but this are the conditions I would start with...
First change would be a (normal) phenyl-phase to check for different selectivity.

BTW:
What phenyl-hexyl-column do you mean?
The Betasil Phenyl-Hexyl from Thermo Hypersil-Keystone or the Luna Phenyl-Hexyl from Phenomenex. These columns might have the same name, but differ in bounding.
Phenomenex Luna Phenyl-Hexyl is a hexyl-linked phenyl phase, while the column from Thermo Hypersil has both C6 and phenyl boundings.
Never hold the Betasil in my hands, but the Luna acts like a phenyl with some interesting C8 characteristics.

If someone has tested the Betasil phenyl-hexyl, I am interested in a rough overview.

Don't even think of putting sulfuric acid into your mobile phase for running neutral aromatics!

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