Chromatography Forum

Dear all,

What would be the best column for analysing small molecules (10-50 amu) with GC/MS?

We have a carboplot column but we do not see any peak in the MS unit.

Regards

What kind of polar molecules?

This columns is used for measurement of N2, O2, CO, CO2, and very light hydrocarbons, more non-polar related compounds. Also, Plot columns are not usually the best for MSD is that they lose particles over time and could dirty the source quickly.

If "small, polar, volatile" means acids and alcohols up to C5 or C6, or diols in the same range a wax type polyethylene glycol stationary phase will do the trick. I use Restek's Stabilwax and RtxWax.

Peter

Hi Natalie & Peter,

I mean what Peter has pointed out, i.e. acids, aldehydes and alcohols up to C5 in the gas phase. I did not see anything using the carboplot column nor the Al2O3/KCl column and I see alcohols using a Poraplot Q column, however acid peaks are sluggish and I don't think I will be able to quantify.(Using a MS unit).
On another matter, I am not using headspace chromatography, but just injecting gas samples using normal syringes and a mundane split/splitless injector. Is this a lost battle then?

Peter, do you inject gas samples as well? what injection method do you use?

Regards

As Paracelsus said in another context; it is all in the concentration. If you have high concentrations you should be able to detect and quantify the compounds that you are interested in, if they are at trace levels you are going to struggle. A lot also depends on what other compounds from the samples are likely to inteferfere with the analysis. An MS is not particualrly selective when working with those small molecules, and indeed in another thread it is pointed out that it is pretty well impossible to detect methanol in the presence of air, you would do better with an FID which has a lower limit of detection and a wider linear range than an MS, and does not see water.

I usually work with gas phase samples purged from solids, and then cold-trapped, using a home made inlet setup. This is a very specialised application, and the hardware is not commercially available, although you can get similar hardware off the shelf. This prings up the other major term in the equation - how much time and money do you have to throw at this problem ?

Peter

Thanks Peter,

I see methanol in air. My MS resolves 0.01 amu and methanol is above 32 amu, whereas oxygen is below 32 amu. With regard to resources, I have to say that we have money but not too much time...

Your best bet is a wax phase column, 30 m long, 0.25 or 0.32 mm diameter, 0.5 of 1 um film. If it was me doing it: http://www.restek.com/catalog/view/1433, or http://www.restek.com/catalog/view/1414, or http://www.restek.com/catalog/view/1414 or an equivalent.

Use an inlet liner with no packing.

Peter

Peter,

Can we talk? I am on a new job and am going to try to measure small molecules outgassing from liquid/solid waste. I have a neslab chiller and was thinking of putting a short gc column into it to try to collect volatiles at 5C then put the GC in the oven and run a slow ramp up. Curious to hear what you are doing.

Thanks,
Marc (mfoster@micromidas.com)

I can see easily acids and alcohols with the WAX column but the aldehydes have terrible peak shapes...
I thinking whether it is better to try to derivatize the molecules...

Peter,

Can we talk? I am on a new job and am going to try to measure small molecules outgassing from liquid/solid waste. I have a neslab chiller and was thinking of putting a short gc column into it to try to collect volatiles at 5C then put the GC in the oven and run a slow ramp up. Curious to hear what you are doing.

Thanks,
Marc (mfoster@micromidas.com)

Hi Marc

If the samples have water in them then any kind of cold trapping is cursed by condensation. Fortunately the African wild dog scent marks that I am working on can be dried before sampling.

In your place I would look seriously at SPME or stir bar SPME (once again concentration is the critical parameter). This requires the miimum extra hardware. If your concentrations are high you might be able to use equilibrium headspace, but to do that properly you need an automated headspacer. For very low concentrations from wet samples purge and trap is indicated - this can also be automated, but one-shot systems can work very well.

Have a look at the literature - there are dozens of applications out there - and choose what best suits your needs, hardware, budget and time constraints. A search of this forum will also throw up some valuable advice.

Peter

Tremenda,

Peter's comments about the inlet liner and no packing are an excellent place to start troubleshooting. I have seen methanol jump back up with change of wool or change of sample filter. You may also consider the differences in waxes Nukol, Stabiliwax, DB-Waxter, etc.... as a potential solution to your problem. I don't think all waxes are created equally. Out of curiousity, what MS are you using?

sepscientologist,

Have you talked to Gerstel or SGE about cryofocusing? If nothing else, they can probably add a great deal to Peter's comments about the water headache.

Best regards,

AICMM