Headspace GC-FID acetic acid

[amartins]

Hi!

Is there anyone working with HS-GC-FID to analyse acetic acid?

Thanks,
Anabela

[rb6banjo]

That's a tough one. Acetic acid is very soluble in water so it doesn't want to partition up into the headspace very much. What is the low end of the concentrations do you need to detect?

[GOM]

Hi

As rb6banjo says, what is the concentration and the sample and solute? Or is it neat acetic acid that you wish to analyse?

If you think about vinegar, which has a concentration of around 5%, there is plenty in the headspace to easily detect it just by sniffing so partition isn't really a problem

If it was me and the sample solution was aqueous I would

a) adjust the pH to 3 to ensure that it is fully dissociated and
b) add NaCl (saturated)to improve the partition into the headspace and
c) use a polar column (e.g. Innowax) to get a symmetrical peak and
d) quantify by standard addition

Cheers

Ralph

[osp001]

Looks like acetic acid boils at 118C, so if the rest of the sample has a relatively low boiling point, your samples might be good candidates for total evaporation (TE) headspace.

[amartins]

Thanks for your replies.

We are trying to implement ISO 17299-3: Textiles - Determination of deodorant property.
We need to measure the peak area of the FID chromatogram for testing gases in vials containing a solution of 1g/L acetic acid and 9g/L NaCl (with and without a textile sample).
It has to be static headspace, injecting 1 mL of the 'odour testing gas'.

We are trying to find the better conditions and any help is welcome.

[Peter Apps]

Tell us what you have already tried, then we do not waste time suggesting things that you already know will not work.

Also is this one of those official methods where you are not allowed to change anything ? What parameters are you able to adjust ?

Peter

[MSCHemist]

I'd also suggest total evap. No matrix effects to worry about. I do total evap for such compounds as diacetyl, trimethylamine, ethanol or other small polars on a wax column with GC/MS SIM I get down to 10ppm solution with an FID you can probably get 200 ppm or so.

[GOM]

In addition to Peter's questions I would also ask

Is this to do with a textile to reduce malodour?

If so,
Is acetic acid the model malodorant?
What is the textile? cotton, polyester or cotton/polyester - they have different adsorption/absortion properties.
Is this to do with foot, body or axilla malodour?

My questions are out of interest because I have some experience in this field having analysed treated Nike textiles

[amartins]

The norm is divided in two parts.
At part B the malodorant compounds is acetic acid (foot and sweat malodour), at part A (we didn't test yet) the compounds are indole, isovaleric acid and 2-nonenal.
We can adjust the GC conditions, not the sampling conditions.

For acetic acid we need to measure the peak area of the FID chromatogram corresponding to 1mL headspace gas of a 22mL vial containing 850uL of a 1g/L acetic acid and 9g/L NaCl solution (with and without a textile sample).
This vial has to be placed in an oven at 80ºC for 30 min before sampling.

[Peter Apps]

Do you have a headspace sampler ? If so, load the vial with the necessary ingredients, equilibrate under the specified conditions, and inject to a capillary (or even a packed) column with a polar phase.

Peter

[amartins]

No, I don´t. Manual injection unfortunately.

[Peter Apps]

No, I don´t. Manual injection unfortunately.

That is going to make your life very difficult I am afraid. Is there any way that you can not do this analysis ?, as in contract it out to a lab with the necessary hardware ?

The main problem is that you need to handle a gas-tight syringe heated to about 85C, quickly and accurately enough that it does not cool down while you take a headspace sample from the vial (with multiple pumps of the syringe) and inject to the GC. It can be done, but it's very difficult to do it well.

Peter

[GOM]

Hmmm it is a shame that you are stuck with a specified method.

For foot malodour the key molecule is IVA, for axilla sweat it is a thiol. Indole and acetic acid are less of a key molecule in either case and for general body sweat.

For the COOH and amine functional groups it has to be a polar column.


Is manual SPME headspace an option for you? Probably not

[MSCHemist]

For both amines and carboxylic acids I like using chloroformate derivatization/solvent extraction. For free fatty acids I use Isobutyl chloroformate in water/isobutanol/pyridine because isobutyl acetate is the only ester of acetic that can get clear of the reaction pyridine but it doesn't amino acids nor hydroxy acids very well. For amino acids and amines I use ethyl chloroformate.

The sample can be dissolved in either the alcohol or water and added to the reaction mixture and then it is extracted with either chloroform or hexanes.