Chromatography Forum

Hi all,

I am trying to analyse fatty acids using GC-MS. I bought a mixed set of standards from Supelco, but cannot differentiate between 9-octadecenoic and 7-octadecenoic acid. There are two peaks where the mass spectra are very similar (cannot see any difference). The peaks to not elute at the same time or in the same order as in the representative chromatogram from the company. The fatty acids are methylated for GC analysis.

Can anyone help? ,please!!!

Carolina

You'll need the specialty capillary column (also available from Supelco), one of the cyanopropyl silicones. Contact them directly for the most suitable column.

Its really, you need a polar column, as Mr Consumer Guy products say, but I supose you are working wtih that, its very dificult to know wich peak is wich, but if you ca see and homologe serie, in your standart I could suposse that the 7-octadecenoic acid, is the more polar, hence hsould be appear in and spected order.
Good luck.

thanks,

I will phone Supelco and I will see if they can help because I have the peaks but I don't know which compounds is which. If any of you have used this standard before and know the retention time of the two compounds or how to differentiate using the mass spectrum will be very useful for me.

Thank again,

Ca rolina

Supelco has a bulletin concerning the separation of the isomers you describe.

If you use hydrogen at 40cm/sec for the 100m 0.25mm 0.2µm SP-2560 column you can separate these isomers at 170-180°C in a realatively short time. Using Helium carrier gas does take about twice as much time.

Can´t you get single acid standards?

Nu Chek Prep is a very good source for fatty standards such as these.

Yes, you can if you have the money and the time. Sometimes if you need to show separation of different isomers it is a lot cheaper and easier to have them together in a mix rather than individually injected. You can also more quickly identify impurities and other peaks present.

If Carol had individual standards it would have about taken the time to write the original question to find the answer. How does the salary compare?

Carol

If you believe the separation is not as it is supposed to be, call Supelco Technical Service if you have not yet done so and you will receive help with the issue.

The peaks of interest will change in retention to other trans isomers depending upon temperature. It is not always easy to identify the peaks but the 'fingerprint' is usually quite reproducible. The help is free, please take advantage of it.

You are welcome to call me directly. There is also a Supelco Bulletin 855B and a Supelco Reporer article April 2004 you may wish to review.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com