Calibration of acetaldehyde HELP

[tavernello]

Hi, I'm new of this forum.

I want to find a method to calibrate acetaldehyde. I use a Finnigan Trace 2000 GC with TCD detector.

I already tried with liquid injection of very little volumes (0.1 to 1 microliters). The areas was very different at the same volume injection.

After I read in this forum about the possibility to made a solution and quantify acetaldehyde not by volume but by mass (tare a flask with little amount of solvent and after add some drops of acetaldehyde). Also with this metod it is impossible to obtain similar areas with the same volume injected.

It's also difficult for me to use headspace technique because I can't inject a great amount of acetaldehyde without damage the coloumn.

Someone have experience with this kind of calibration? Can you help me?

Thank you in advance.

[rb6banjo]

Welcome to the forum. What you are trying to do is not clear. Are you saying that you tried to inject neat acetaldehyde? If yes, that will be very difficult as it is almost a gas at room temperature (b.p. = 21 °C). The warmth of the syringe will vaporize that small volume instantaneously. This is why most of us drop a known mass into a flask containing solvent. Once the AA dissolves in the solvent, you have essentially trapped it. Once it's in a solvent, it's much easier to handle. With correct choice of your solvent, you can make your stock standard nearly as concentrated as you want (just not neat AA).

How concentrated is the AA in your samples?

[tavernello]

Sorry for bad explanation. As you suggest in other posts I try to inject AA in EtOH and calculate it by mass.

Yesterday I tried to build a calibration curve and it seems to be ok. Now I will try to do another calibration with another solution (made by same technique) and I'll made a compare between the two curves.

The problem Is that my unknown samples are less then 1 micromolar , I have to inject very low volume of solution or ​​a very dilute solution . In Both cases I made ​​big errors , probably I can solve with multiple injections of same quantities?

[rb6banjo]

1 µM is 44 ppb (how I think about things). That's a very low concentration - especially difficult if your matrix is substantially aqueous. AA is very soluble in water so it won't want to partition out of the water to any great extent. You might try trapping it with a derivatizing agent. I had very good success recently getting formaldehyde at low concentration in a substantially aqueous matrix using o-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine to derivatize and trap it on a solid-phase microextraction fiber. It would work for AA too (you get cis- and trans- isomers of the derivative).

My procedure is basically a modification of what was done in Martos and Pawliszyn, Anal. Chem., (1998) 70, 2311-2320.

Briefly, you load up a fiber with the reagent (headspace above a very concentrated water solution of the reagent). Expose the loaded fiber to the headspace above the sample (I heated the sample), and then after a specified time, make your injection. The problem is that if you have lots of other materials in the sample that will react with your reagent. You have to make sure that you see plenty of unreacted reagent in your chromatogram so that you are sure you didn't deplete the reagent during sampling.

Yours is a tough problem. AA is just so soluble it's difficult to get it out of the matrix.

[mckrause]

Derivatize it with DNPH and extract it from the aqueous matrix. USEPA method 8315; works quite well for acetaldehyde.

Direct injection is pretty hopeless; it's just too reactive in the injector port unless you derivative.

[CharapitsaS]

"Sorry for bad explanation. As you suggest in other posts I try to inject AA in EtOH and calculate it by mass".

May be this method http://inp.bsu.by/calculator/vcalc.html will help for your measurements.

Regards,
Siarhei