Chromatography Forum

First of all, I must apologize for being not as proficient on GC than on LC.

I'm trying to analyse ppm level styrene in a sample with DMF as solvent using a 30m AT-WAX column. I found the styrene eluted just before the DMF solvent front. This is due to the high polarity of the column.

Now, with expert help here, I would think this method is not suitable for analytical work since there is no retention. Plus, other unretained species may co-elute with the styrene, so quantitation would be unreliable.

Is this line of thinking okay? Or am I missing something more?

Thanks in advance for any assistance rendered.

I've been away from the LC forum for some time now, due to other work commitments. But it's great to be back.....hopefully staying around longer.

Whenever someone wants to measure at ppm levels in a matrix one usually uses a selective detector like Mass Spec to avoid measuring co-eluting analytes with the peak of interest. This is not usually possible due to costs for a process analyzer, but may be acceptable for a research center.

Another alternative is to use multidimensional chromatography which will separate an analyte from other interferences with a high reliability if the application is designed properly and the known possible interferences are fully documented.

Companies like Dupont and Dow will purchase applications and analyzers as well as preparing their own applications if they wish to keep proprietary information secret. Styrene at ppm levels is not a difficult application and you may even find details available on the web.

A simple backflush application may work for your analysis and a two or three column heartcut application most certainly will work effectively.

best wishes,

Rodney George
consultant

Thanks for your quick response and suggestions Rodney....much appreciated.

I guess I'm really thinking straight....hehehe.

sfe-co2,

I would never have believed it if I had not looked at it myself on NIST webbook! Amazing little problem.

Even a heartcut would, I think, be hard pressed for this separation but an MS would do it based on the higher ions of the styrene. Doesn't help if you don't have one sorry...

Best regards.

Don't forget MM that when selectivity is required, a capillary column is not always the best option.

best wishes,

Rodney George
consultant

Hi AICMM, yes it is interesting. And Rodney's suggestions are reasonable. However, like you mentioned, one is restricted without the hardware. Obviously a change in selectivity is needed, so a DB-5 or equivalent column....

Have you tried to drop the oven temperature and/or slowing the program rate to improve the separation.

I would think the separation is possible with a wax column.

best wishes,

ROdney GEorge

Are you sure the styrene isn't being retained? Just because it elutes before the solvent doesn't mean it's unretained. DMF is probably polar enough that it's retained also.

It's shoot some methane, this peak location will tell you the unretained volume/time of the system. If the styrene is 2-3X times this retention, you may be OK. If the styrene/DMF separation is sufficient.

Having said all that, I'd try a less polar column also. You're not really going to know until you see some samples with the impurities. Then you're probably going to have to spike a few with some styrene to see if you have the separation that you need.

The really surprising thing to me is the Kovat's for DMF both non-polar and polar. Fascinating.

Best regards.

That is why temperature is so important as Kovat values will change with temperature.

Of course, there are many ways to skin a cat, and that is why packed columns offer so many cats compared to the handful of capillary cats.

A similar problem arose some years ago when I had to separate 1,2-DCE from benzene in less than 60 seconds at ppm levels in the presence of 50 some chlorinated hydrocarbons as well as gasoline fumes. The two separate easily on a methyl silicone column, but not in 60 seconds, and not from all the other interfences. No other capillary column gave any better selectivity and separation.

It pays to have a lot of cats on hand.

best wishes,

Rodney George
consultant