FAMEs on standard polysiloxane column (Restek Rxi-5ms)?

[Arne]

Hi all,

we don't usually run FAME samples on our high resolution GCMS system but now we have a few samples. Has anyone successfully analyzed FAMES on a standard polyxiloxane column? If so, what kind of temperature program did you use?

We have a Restek Rxi-5ms 30m column installed. http://www.restek.com/catalog/view/6729

Thank you so much for your help!

Arne

[Consumer Products Guy]

Yeah, we do this all the time for FAMEs, on similar Rxi-1 and Rxi-5 columns. Just run a typical temperature program from about 60C to 280C, maybe at 10C/minute.

As opposed to the more polar capillaries typically used, on which the unsaturated FAME elutes AFTER the saturated FAME of same carbon number, on the Rxi-1 and Rxi-5 type columns the unsaturated FAME elutes BEFORE the saturated FAME of same carbon number.

So on Rxi-5, order of elution would be 18:2 then 18:1 then 18:0, as an example. Saturated FAMES will have distinctive peaks at 74 and 87 m/z.

You people just "knew" that I had experience with these!

[Arne]

Thank you so much for the good answer!

So you do not have trouble separating the similar FAMES, e.g., 18:0 and 18:1 or 16:0, chromatographically?

[Consumer Products Guy]

Thank you so much for the good answer!

So you do not have trouble separating the similar FAMES, e.g., 18:0 and 18:1 or 16:0, chromatographically?

No trouble, that's what temperature programming is for !!! You will not separate the FAMEs of elaidic acid and oleic acid though (the trans and cis forms of 18-1) with this column though, you'll need the high polarity for that..

[Arne]

ok, good to know. We'll get cracking today then! I don't need to separate positional or cis/trans isomers anyway. Thanks again!

[MSCHemist]

On a slightly related note I was considering doing some basic FAMES profiles using the alkyl chloroformates. I use ethyl or methyl chloroformate for a lot of stuff amino acids, Kreb's cycle acids, Carnosic Acid etc. Has anyone ever tried them and do fatty acid ethyl esters or fatty acid methyl esters give better separation?

http://alexandria.tue.nl/repository/fre ... 619857.pdf
http://www.ogeochem.jp/pdf/ROG_BN/vol27/v27_pp91_95.pdf

[Arne]

no idea on the methyl versus ethyl chloroformates, sorry.

Update on our separation: Try as we may we cannot get 18:3 and 18:1 to separate. I did implement your suggestion, Consumer Products Guy, of ramping from 60 to 280C at 10deg/min, and then zeroed in on a method to get 18:2 and 18:1 to separate, which eventually worked. However, 18:3 and 18:1 almost perfectly co-elute. You can't even tell that there are two compounds under that peak.

I attach my temperature program and a chromatogram.



Can someone give me some specific conditions under which you achieve the complete separation of Fames on a standard GC column?

Thsi is most likely a one-time analysis in our lab, but I may just have to spend the $500 and get a famewax column...

Any help is super appreciated as always!

[skunked_once]

DB-23 30m X 0.25 mm with He at 29 psi.
190 for 4 min then 190 to 220 at 15 deg/min, hold 1 min, then 220 to 240 at 25 deg/min.
Good for C16:0 through C24:0.

[lgchrom]

no idea on the methyl versus ethyl chloroformates, sorry.

Update on our separation: Try as we may we cannot get 18:3 and 18:1 to separate. I did implement your suggestion, Consumer Products Guy, of ramping from 60 to 280C at 10deg/min, and then zeroed in on a method to get 18:2 and 18:1 to separate, which eventually worked. However, 18:3 and 18:1 almost perfectly co-elute. You can't even tell that there are two compounds under that peak.

I attach my temperature program and a chromatogram.



Can someone give me some specific conditions under which you achieve the complete separation of Fames on a standard GC column?

Thsi is most likely a one-time analysis in our lab, but I may just have to spend the $500 and get a famewax column...

Any help is super appreciated as always!

If it's a one time analysis, why don't you send it out to a qualified lab for this analysis. It will cost far less than buying a new column and setting up the method.