Problem with residual solvents analysis by GC-FID

Discussions about GC and other "gas phase" separation techniques.

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I am participating in the transfer of an analytical method (through comparative testing) for the analysis of residual solvents in a pharmaceutical raw material (methanol, ethanol, IPA, ethyl acetate, acetonitrile, N,N-DMF and acetic acid) using GC-FID.

I have prepared a standard (system suitability) containing the above solvents at levels of approximately 500-5000 ppm in DMSO.

The chromatography looks great except one problem - my acetic acid peak isn't showing up or is just barely visible in the baseline.

As it's a method transfer I can't vary any of the method parameters, however I will list the parameters here FYI:

manual split injection (5/1 ratio), Helium carrier gas, injection port @ 180 oC, FID @260 oC, DB-624 column (new), temperature program as follows 6 min @ 40 deg oC, 9 minutes @ 100 oC and 10 minutes @ 240 oC with 15 oC/min ramps, 2.5 ml/min flow rate, FID @ 40ml/min H2 and 400 ml/min Air.

I am thinking that the acetic acid is reacting with something in the solvent or in the GC liner, or that my standard is somehow degrading before it gets injected into the GC. Maybe I should try a new batch of DMSO solvent? The DMSO batch I have is new and straight from Aldrich (>99.9% pure confirmed by GC-HS analysis). I have re-prepared my standard numerous times from the same batches of chemicals to no avail.

Anyone have any ideas as to what's going on?

Thanks
The most likely explanation is adsorption of the acetic acid to active sites. What kind of inlet liner are you using, and does it have glass wool in it ?

Less likely is that the acid is reacting with the alcohols to make esters. Do you see any extra peaks, do you still see ethyl acetate if you leave it out of the mix ?

Peter
Peter Apps
Peter Apps wrote:
The most likely explanation is adsorption of the acetic acid to active sites. What kind of inlet liner are you using, and does it have glass wool in it ?

Less likely is that the acid is reacting with the alcohols to make esters. Do you see any extra peaks, do you still see ethyl acetate if you leave it out of the mix ?

Peter


Not likely to be the acid reacting with the alcohols to make esters as I have analysed the acid in the absence of the other solvents and don't see any peak.

RE the inlet liner, yes the liner is packed with wool. What difference does this make?

Regards
Hello

I'd check liner type. It is very important if it has glass wool and also position of the glass wool plug (bottom or in the middle).
To increase evaporation surface I'd use liner with glass wool in the middle of liner. It will help to evaporate all compounds without discrimination.

Regards

Tomasz Kubowicz
Chemist0241 wrote:
Peter Apps wrote:
The most likely explanation is adsorption of the acetic acid to active sites. What kind of inlet liner are you using, and does it have glass wool in it ?

Less likely is that the acid is reacting with the alcohols to make esters. Do you see any extra peaks, do you still see ethyl acetate if you leave it out of the mix ?

Peter


Not likely to be the acid reacting with the alcohols to make esters as I have analysed the acid in the absence of the other solvents and don't see any peak.

RE the inlet liner, yes the liner is packed with wool. What difference does this make?

Regards


If you had told us at the beginning that you cannot see the acid when it is the only component in the mix, then we could have saved one speculative step.

The possibly problem with the glass wool is that it has active sites on it that adsorb the acid. Are you using pre-packed liners, or packing them yourself ? What happens with a brand new liner ?

Peter
Peter Apps
Monitor Acetic Acid with HPLC
Mr. G wrote:
Monitor Acetic Acid with HPLC



How???
maliheh.1367 wrote:
Mr. G wrote:
Monitor Acetic Acid with HPLC



How???


Look for an Organic Acids column and there should be parameters for the chromatography listed with it.
The past is there to guide us into the future, not to dwell in.
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