Chromatography Forum

Hello people.
I'm having trouble finding out any information regarding a question I have been given. Please help.

In a sample that contains; acetophenone, nitrobenzene, benzene, and toluene, which type of chromatography would give the best separation between either; high performance liquid chromatography, and gas chromatography?

Thank you in advanced.

I'd venture a guess that either would work, but I'd try HPLC first.

Both techniques will give you the separation, I would go for GC because I do not have an HPLC.

This is not a particularly well posed question. The examiner probably has a particular answer in mind - you need to ask yourself part of your knowledge the question is really trying to test.

Peter

I'm with Peter Apps. GC is the simplest and you can get the best resolution. I would always try that before anything else. If they're dissolved in a nice solvent (pretty much anything but water), then I'd inject directly. Otherwise, I'd recommend some type of headspace technique (I'm partial to SPME).

Obviously, if the molecules are too heavy for GC, then you'd go to HPLC first but in this case, I believe GC is the way you want to go.

Sounded like a pretty typical LC test mixture to me, hence my LC leaning.

This is not a particularly well posed question.

Peter nailed it.

Many textbooks would probably say something like "if it's volatile and thermally stable, GC is the first choice", but the fact is that it could be done either way. I haven't done any GC since 1977, so you can guess what I would use.

Hello people.
I'm having trouble finding out any information regarding a question I have been given. Please help.

In a sample that contains; acetophenone, nitrobenzene, benzene, and toluene, which type of chromatography would give the best separation between either; high performance liquid chromatography, and gas chromatography?

Thank you in advanced.

With GC you would could be able to get quite a bit of separation between those compounds with Benzene being very early and Acetophenone eluting very late if you choose the correct column and oven ramp.

Being as it is a class project I will point you in a good direction short of the final answer Check through the Chromatograms listed on the Restek site, they would have examples for both GC and HPLC with many different column types of each category and you should be able to find a good answer there.

[rant mode]
Very often we learn what techniques are appropriate for differtent target molecules, but then have to use whatever instrument is available, even if it's far from the best option.

That's fine, but I'm struck by how many people then try to make a virtue of it, and publish papers with titles like "Improved sensitivity and specificity of Vitamin D measurements using hyperthermic photon resonance hairdryer-plugholeometry" when the new method has nothing to recommend it apart from being new. It's very important, when looking at literature and deciding on methods to follow, to realise that the choice of technology is often 10% analytical-chemistry-based logic and 90% local socio-economic circumstance!
[end of rant]

Good points lmh. Every time a "new" technique comes along there is a tsunami of papers doing the old analyses in the new way - SPME has been applied to hundreds of different analyses in thousands of publications that all point to the same advantages and drawbacks. Then the variations on the theme start up, SPME with different coatings, SPME with different supports, all applied to water samples containing analytes that are so hydrophobic that they leap out of the sample if you take the top off the bottle. Whatever happened to papers that advance practice over a broad front ?

Peter

[rant mode]
Very often we learn what techniques are appropriate for differtent target molecules, but then have to use whatever instrument is available, even if it's far from the best option.

That's fine, but I'm struck by how many people then try to make a virtue of it, and publish papers with titles like "Improved sensitivity and specificity of Vitamin D measurements using hyperthermic photon resonance hairdryer-plugholeometry" when the new method has nothing to recommend it apart from being new. It's very important, when looking at literature and deciding on methods to follow, to realise that the choice of technology is often 10% analytical-chemistry-based logic and 90% local socio-economic circumstance!
[end of rant]

I agree. That is why I first consult places like column manufacturers to look for reference chromatograms instead of trying to search the scholarly publications. Those I fall back on when nothing else proves fruitful or to maybe gain more insight into how someone tried to process a sample. Usually reference chromatograms show good results for a given stationary phase under set conditions, while many literature references show peaks that are there if you use your imagination but they did give the results the researcher was expecting to achieve. The one that sells a product(a column) is usually of much better quality than one that just helps prove a hypothesis.

... confession time, and extending my rant (sorry, haven't been back here for a few weeks).

In the area of plant metabolism, there are a lot of papers publishing methods for flavonoids. I'm thinking of one particular example whose title is an enthusiastic claim that combined mass spectrometry and LC can identify flavonoids (wow! Harborne and his mates identified huge swathes of flavonoids without the mass spectrometry, and published the book in the year I was born, 46 years ago...).

I once accidentally put the solvent bottles the wrong way round when running one of our local flavonoids gradients, thereby running the gradient backwards, and I still got quite decent separation between the major components in the sample.

It seems fundamentally odd that you can publish a novel method to separate things that actually separate if you put them through any C18 column irrespective of what solvent you put through afterwards...

I would suggest -- GC if you have GC with capillary column option