Hi Everyone,
I am trying toquantify Acetic Acid in empty capsules by GC (both Head Space and Direct injections)
Any suggestions anyone?
I am tryng to dissolve it in warm DI Water.
Thanks in advance...

Acetic acid has a boiling point of 118 C, so headspace analysis may be difficult. It is also highly soluble in water.

Direct injection GC would be much easier. You will also need a column designed for polar compounds, and tolerant of water injections. Carbowax (e.g., DB-Wax) or the Free Fatty Acid (FFAP) phases should work. Check with the GC column manufacturers for a recommendation.

Also, remember that this is a weak acid, and the ionic (acetate) form is not volatile. So you will need to acidify your water with something before injection. Formic acid might work. HCl is also a possibility, but use it in small quantities, as it is corrosive to metal parts in the system.

Write back with other questions.

As I read your post again, if you want to do headspace, use only the capsules, do not add water. Then you could do a headspace extraction at above 100 C.

Again, this assumes that all of the acetic acid is in the neutral (volatile) form.

Part of the question you need to ask, is if I am interested in solely what may be trapped within the air within the capsule of what is interstitial to the capsule material.

If I was interested in solely what is trapped in within the empty capsule, then a heated headspace procedure will likely serve you best. You;ll need to come up with some means of braking the capsule in a closed system if that's the case.

Depending upon the sensitivity you need, you could also try to simply pierce the capsule itself with a syringe for a true sampling of the gas within the capsule.

If you're interested in what may be present interstitially (within the walls of the capsule), and assuming the capsules are the typical material used for drug delivery, regular DI water is slightly acidic (due to CO2 dissolving to form carbonic acid), and should be sufficient to keep from forming any acetate ions (which won't go through your GC). My guess is that they would slowly dissolve. Some slight heat may be necessary to hurry the process along.

As mentioned, pick a column well suited for highly polar organic acids. Most manufacturers now have pretty good reference info in their catalogs and decent technical support if you wish to make a call to double-check your choice.

Thank You everybody. Today I tried HS injections for both std and sample in DI water. I think it is working, I would be able to confirm it in a few days.
Yes, I am using FFAP column, and thermostatting time of 25 min at 85 degrees.
Hope it works...

My recovery for capsule testing is only 43%. Is there any way to improve recovery results by adjusting GC parameters? I run on a PE Clarius 600 machine with FID.

I would start with your headspace parameters first.

If you are using only capsules, and no water, then increase the temperature to around 110 C and try again. Make sure the transfer lines and everything else are hot enough to prevent condensation.

Have you done a spike recovery? Add a known amount to a capsule sample and try extracting. I doubt this is a GC problem.

Hi Everyone,
I am having lots of trouble analyzing empty capsules by Head space GC for Acetic acid using DI water and Sodium sulfate salt as the matrix.
1. There is a big peak in the blank(DI water+sodium sulfate) at the same RT.
2. Recovery at 100% level is poor with 0.50g capsules, but better a when 0.25g sample is weighed, which I think is due to matrix problem.
3. Linearity range is too narrow(40%-100%)


Anybody working on something similar?

One thing you can try is to muffle the sodium sulfate (try (400C for a few hours should be fine). This will ensure that there isn't any organics sneaking into your analysis that way.

Greg

Thanks, Greg
I will try it.
I am also trying direct injection testing.

i think better u try to use Flash chromatography becoz its mainly used for compounds purification and quantification process so u try me be the best