Chromatography Forum

Hi
I am new in fatty acids analysis, and I want to screen for fatty acids in oil spill contaminated water. Can I use the SIM mode with help from NIST to preliminary identify (qualitative analysis) the FAMEs in water sample.
Any idea on what fatty acids should I consider in the analysis (there are many), and if there is method specialized for FAMEs in water or petroleum in general.

Thanks

Lots of questions to be answered. I assume you have a method for transformation of the fatty acids to the FAME's. If they're in water, you'll need to extract them from the water. I use solid-phase extraction to isolate them. I also use a pretty generic procedure and MeOH:BF3 from Supelco (http://www.sigma-aldrich.com) to convert the acids to esters.

I would certainly start using the scanning mode of detection in the mass spectrometer. You'll get good sensitivity with scanning mode. If you're new to all of this, you may not know that you can collect a total ion chromatogram and then sort by the masses that you want during analysis of the data. If you truly go SIM, you only get that one mass. I have found that SIM is required only when I need extra sensitivity. To me, it sounds like you're are probably not trying for "gnat's hind end" sensitivity.

I would certainly focus on the most common fatty acids found in nature (C16's - C22's, saturated and unsaturated). Methyl esters tend to give a great m/z=74. This makes them pretty easy to pull out of a complex chromatogram. Your ability to detect them will depend a lot on how complicated your chromatography is after your extraction/derivatization.

Good luck!

We analyze fatty acids in our lab all the time and i don't usually use sim, just a scan. As a matter of fact we almost always just use GC for ID, mostly because you can get a standard from sigma aldrich that contains many of the Fatty acids. Using the mass spec and Nist can be helpful, but i find using the standards even easier.

Thank you very much

The reason of SIM is that I am dealing with quite complex sample which makes the analysis quite difficult. By focusing on targeted fatty acids using either standard or NIST, I was thinking the monitor will be much easy. I assume these fatty acids to be at low concentrations, while other cyclic acids at high concentrations.
Thanks for the method, I am going to use the generic method, and obtain some standards for method development

I understand. Just know that if you go SIM, you will lose all other mass spectrometric data. If you scan, you can sort the data at the end by the masses you want. Your chromatography will get much simpler when you sort by specific masses.

You'll still need a standard to match up the retention times.

Is m/z 74 that characteristic of fatty acids or will it appear in your background peaks ? Identifying peaks based on a single mass is not good practice if in a complex unknown mixture. How can you be sure you do not have a huge co-eluting peak that just happens to have mass 74 as part of it's spectra. At the very least you should be using two masses and the ratio of them for peak conformation. IMO you should be looking at full scan data and selected ion chromatograms to identify your peaks. If you can't get the sensitivity in full scan but can in single ion then get a TOF which should have almost single ion senstivity for full scan data and you can also add GC x GC capability to remove background interference.

Many fatty acid methyl esters will have characteristic ions at 74 and 87.

I'm still not exactly clear if the original poster has a petroleum-type "oil" or a triglyceride-type "oil" spill.