If you have GC-MS software and MS library you can use it to find the expected mass spectra of your analytes. Select two m/z values for each analyte set the MS in SIM program and that is all.

Remember about proper GC condtions - splitless injection (about 280-300 deg.C).

Check these papers:

Article Sample preparation procedure using extraction and derivatiza...

Article Method for the determination of carboxylic acids in industri...

regards,

G

thank you so much Grzegorz Boczkaj for your valuable suggestion.yeah we have GC-MS software and MS library .

You can check also the literature for product of the derivatization, maybe somebody already selected the optimal conditions (SIM) for its detection/quantification.

regards,

G

To analyse a range of acids above C20:0 you do not want to use a SIM method as they will be very hard to identify. I have successfully analysed silylated fatty acids up to C34:0 using a DB-5ms column and a generic analytical programme. A 15mx2.5mmx2.5μm column (or a 30m version with a lot longer run time) is suitable and should allow the longer acids to elute. I usually use a split/splitless injector at 250-300° or programmed to follow the oven temperature; gas flow should be set up for the length of column; oven temperature programme 2 to 5 mins isothermal at 50°C (depending on how much delay you have to allow for the solvent peak to pass before starting to collect data), 10° per minute up to 340-350° (depending on the top temperature of your column), 10 mins isothermal hold.

The mass spectra of silylated fatty acids are very distinctive with a large peak at 117 (all acids) and a large peak at [M-15]+; the molecular ion is usually present but small. For C20:0 [M-15]+ is 369 and you can easily work up or down from C20:0 by adding/subtracting 14 for each extra or less carbon atom.