analyses of free fatty acids in water (C2-C7)

[Helena]

Analyses of free fatty acids in water by GC-FID on a AT-Aquawax column (25m x 0.25mm ID x 0.25µm df) give some problems with reproducibility.

All comment about this problem is welcome

[MikeD]

Sometimes underivatized free fatty acids are better done by ion chromatography if you have it. That would be my choice for acetic and propionic at least, but we don't know your application or LoD objectives.

[CE Instruments]

Water and GC are not a good mix. You provide no details regarding injector type and conditions. Are you using an autosampler ? How bad are the reproducability problems ?

Water does not vaporize easily (think water spilt on a hot plate) and expands more than most other solvents. To get good reproducability you need to look at a high residence time in the injector to allow full evaporation and sufficient volume in the liner to cope with the injected volume. It is usually possible to buy special "water" liners for most GCs are you using one ?

With more info I am sure someone will be able to give even better advice

[Bill Tindall]

This analysis can be done with great precision and accuracy down to ppm levels with UV detection and various LC techniques. Many reversed phase columns do the job as well but will require a gradient. And, ion exclusion columns such as Biorad 87 H will do the separation with sulfuric acid as eluant.

At moderate to high levels it can also be done by GC given the right column. You do need to be sure your sample is acidified as the salts of these acids aren't amenable to GC.

[Helena]

For the analyse of the fatty acids we use a PTV injector in the CT split mode. The temperature of the injector is 250°C and the split flow is 50 ml/min. We work with a constant flow of 1.2 ml/min. The injectionvolume is 1 µl. We don't use a special "water" liner.
Besides we use a CE instrument "TRACE GC" (thermoquest)
All samples and/or standards are acidified.

Hopefully someone can give more specified comments or solutions.
Thanks!

[chromatographer1]

The problem lies in absorption most likely.

You are getting NO peaks? or tailing peaks? or irreproducible peaks?

What analytes are you having difficulities with?

What kind of liner are you using?

What acid are you adding to your samples?

How fast are you injecting it?

Have other samples been used for this injector and column?

Are you injecting manually or with an autosampler?