Actic Acid analysis in GC

[vengom]

Hello My name is Venkat. I am a chemical engineering student in NMSU, USA. I am working with fatty acids and I am trying to detect them using a GC. Our GC is equipped with FID detector and I am using a column Hayesep QD (stainless steel column, Alltech) for analyzing the fatty acids. I started calibrating the GC with acetic acid (420 PPM in deionized water) by injecting 1 microlitre but I am unable to see any consistent peak in the GC. I could see more than a peak in the FID. Injector / Column oven temperature is 160 Deg C, FID temp is 220 Deg C. Carrier gas nitrogen. Kindly let me know your suggestions

[chromatographer1]

First shooting into a SS column is not recommended.

Second shooting low levels (<1000ppm> onto a Porapak Q is not recommended.

Third shooting samples of strong organic acids in water is not recommended.

Why?

Because they won't give you good results !

Has been tried before many times and was PROVEN not to be reliable 20 years ago at a major pharmaceutical company I worked for.

Either use a capillary column FFAP or SP-1000, or a glass or FS lined SS column of Supelco 11825-U packing.

Nitrogen is OK. Matrix effects can be daunting but you did not mention this important aspect.

Good luck

Rod

[Victor]

I agree with what Rod is saying. The analysis of free fatty acids is one of the most difficult that can be accomplished by GC.

In addition to the columns Rod recommended, Supelco used to market a Carbopack column coated with 4% Polyethylene glycol which works well (not sure why Rod did not recommend this, because he works for Supelco). Maybe that is what 11825 is? You also need to do on-column injections-that is with the syringe needle sticking into the column. Then you can avoid those losses which might occur by transfer from the vaporisation chamber-e.g the glass liner in a flash vaporiser system.

Other precautions-use a small sample volume; water makes a lot of vapour which can backflush from the column
Try using quartz wool to retain the packing instead of glass wool.fused silca wool may be available from some suppliers. Glass wool may be a source of adsorption.
Some people found better results adding oxalic acid to their sample solutions. This decomposes to formic acid on vaporisation which may block any adsorption sites.

[MikeD]

Venkat

Given a free choice, ion chromatography with conductivity detection of acetate is the way to go. Without a free choice, GC/FID of acetic acid is a struggle for the inexperienced, as you have been informed above. If sensitivity is an issue then IC with conductivity detection is about 10X more sensitive than GC/FID working at its best (specifically for acetic acid that is).

[chromatographer1]

MikeD is quite right.

I know of even LC-UV methods that can perform a better analysis than usually seen with GC packed columns.

However, if you acidify your sample with free formic acid and use a FFAP capillary column you can measure acetic acid cleanly as it elutes before formic acid.

I assume you are doing a student project and have limited resources. You could make a methyl or propyl ester of the acid and then use a porapak Q to separate the methyl or propyl acetate for your project.

Good luck,

Rod

[HW Mueller]

Victor, how does formic derive from oxalic? Via some sort of reduction with H2?

[Victor]

HWM- I thought formic acid was just produced by thermal decomposition of oxalic acid.

I read this a long time ago in a paper by Henderson J. Chromatogr. 244 (1982) 337. However, I do not have access any more to this original reference, neither can I confirm by my own results whether these assertions by the authors were correct.