Chromatography Forum

Hi all,

my sample typically contains the volatile liquids such as ethanol, butanol, acetic acid and butyric acids. During the preparation of the samples for GC-FID, I was told to acidify with 25ul of 1M H2SO4 to 475ul of my samples.

Can I please ask why do we need to acidify the sample when we run the GC-FID test?
Is it related to the organic acid groups?

Thanks in advance for your replies!

cheers,
the lost one

If there are any cations in your sample when you heat the acids in the sample they may form salts which may not be volatile and make your measurement in error.

By adding the strong mineral acid as a scavenger of the cations, you will make sulfate salts instead and all the volatile organic acids will volatilize and enter your column for analysis.

Rod

my sample typically contains the volatile liquids such as ethanol, butanol, acetic acid and butyric acids. During the preparation of the samples for GC-FID, I was told to acidify with 25ul of 1M H2SO4 to 475ul of my samples.

I may have to quanitate such organic acids as well and would like to do it by GC/FID rather than HPLC because of equipment usage issues.

Would adding that amount of H2SO4 to samples to aqueous samples to be injected via an Agilent headspace sampler with a silcosteel flow path (HS oven in the 70-80C range, loop 175C, Transfer Line 185C) be a problem in the long term for the equipment?


Thanks,
- Karen

I don't think so. The sulfuric acid has a low volatility from water, especially at these temperatures. And acids tend not to damage silicosteel coated components (while bases certainly do).

I would want to check the pH of the solutions after the addition of acid. If the solution was neutral or basic then I would not expect quantitative analysis of the organic acids.

Caveat:

As I have pointed out repeatedly on this forum, short chain free acids are best measured via ion chromatography, not by GC analysis, even though GC is near and dear to my heart. The scientist in me always wants to do the analysis that has the fewest issues and possibilities for error, especially if it is my job to pass results to those who have the responsibility to make decisions that may have bearing on the health, wealth, or safety of others.

Best wishes,

Rod

As I have pointed out repeatedly on this forum, short chain free acids are best measured via ion chromatography, not by GC analysis, even though GC is near and dear to my heart. The scientist in me always wants to do the analysis that has the fewest issues and possibilities for error, especially if it is my job to pass results to those who have the responsibility to make decisions that may have bearing on the health, wealth, or safety of others.

I like to do good science... Always have... and I agree using something like the Aminex 87H would be best - but I think the samples may require cleanup before that so maybe Ion paring on a RP column or just a mobile phase at pH 2 would be OK for the acids.

But sometimes when you need fast turn around, best is trumped by good enough. This is not for product release. For what the data is being used for it may be good enough... and I do plan to do spiking experiments to know how good the method is.

- Karen

I concur with your comments completely, Karen.

Rod

If there are any cations in your sample when you heat the acids in the sample they may form salts which may not be volatile and make your measurement in error.

By adding the strong mineral acid as a scavenger of the cations, you will make sulfate salts instead and all the volatile organic acids will volatilize and enter your column for analysis.

Rod

Hi,

Is H2SO4 the best acid to acidify these kind of samples for GC-FID? If not, how to choose an appropriate acid to do the acidification?

Thanks

It is commonly used. Best? opinions differ, due to availability, cost, effectiveness.

Check the literature. I have seen formic, nitric, and sulfuric acids used.

best wishes,

ROd

For what it's worth, I use phosphoric for acidifying headspace samples- absolutely zero chance of it getting into the vapour phase.

Peter