Chromatography Forum

I'm not sure if this is the right thread. Or if I should post to GC?

FAMEs on GCFID.

In the past, we added a surrogate/recovery standard (methyl tridecanoate) to our samples before transesterification (TE).
After transesterification and before running on instrument, we added our internal standard (pentadecane) to all samples, Calibration Verification Standard (CVS), standard curve.

Now, we do not use pentadecane, but we have migrated to using just an internal standard throughout, adding methyl tridecanoate to our samples, running through transesterification, and running an aliquot of that through the GCFID. Thus, for our standard calibration curve, our internal standard check is also methyl tridecanoate. The responses between the two - with no TE (standard curve) and with TE - varies. No TE - 8736971.55; with TE - 7440853.522

I don't feel like the methyl tridecanoate alone is enough of a check, aside from having QC samples (the procedure is done with TE) and a CVS (the procedure is done just like the standard curve).

Is the method we run now, ok. Or should I add a surrogate/recovery standard to my unknown samples? If so, how can I use excel to determine that these surrogate/recovery standard numbers are acceptable and consistent?

I have an excel sheet that calculates for the method now and would prefer not to make a huge change to it. Can I just run some methyl ester alone with the internal standard and check for variability there?

Any suggestions would be great.

Thanks in advance,

MaryL14

FAMEs on GCFID.

In the past, we added a surrogate/recovery standard (methyl tridecanoate) to our samples before transesterification (TE).
After transesterification and before running on instrument, we added our internal standard (pentadecane) to all samples, Calibration Verification Standard (CVS), standard curve.

Mary, I'm glad you dumped the pentadecane as internal standard, seems convoluted to me.

Here's what I would do. If my sample was methyl esters (like biodiesel), then I would add methyl tridecanoate as internal standard (after I made sure that the methyl tridecanoate had no other FAME that would interfere with my sample peaks).

If my sample was soaps or fatty acids, I would add trdecanoic acid instead.

If your sample is triglyceride, we would typically esterify in two steps: saponify first using NaOH-methanol, then esterify with sulfuric acid or BF3 in methanol, then extract esters into petroleum ether and inject. So maybe in that case I'd also use methyl tridecanoate.

Mary, I'm glad you dumped the pentadecane as internal standard, seems convoluted to me.

Here's what I would do. If my sample was methyl esters (like biodiesel), then I would add methyl tridecanoate as internal standard (after I made sure that the methyl tridecanoate had no other FAME that would interfere with my sample peaks).

If my sample was soaps or fatty acids, I would add trdecanoic acid instead.

Thank you!

I prefer the mild methanolysis/HT methanolysis protocol.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817593/

I do a basic fames profile very seldom so keeping reagents arround like BF3 methanol which have a very short shelf-life is not cost effective. OTOH I always have methanol and 35% HCl arround. Their method works pretty well for me. It even works for acids like citric malic tartaric.

I agree I use either glutaric or one of the odd chain fatty acids as an ITSD preferably the acid.