I am using GC FID to analyze fatty acid methyl ester from fish oil. I don't have automatic sample injection so I find out that the peak areas of solvent (hexane) are different from one to another chromatograms. So, can I used the peak areas of the solvent to correct my sample volume and then the whole peak areas of each component?
Hi,
In the situation where you need to make manual injections, irreproducible results could typically be observed and internal standard (IS) usage (for GC-FID analysis analogue molecules are the most suitable ones) could be the solution. A minor changes at solvent areas can also be detected even you are using autosamplers but if your analytical standard mixture (e.g. FAME mix in hexane solvent for calibration) is also giving unrepeatable areas than you may consider the usage of internal standard to compensate this problem.
Short pre-validation applying system repeatability test may be performed conducting the several injections of FAME mix or one component standard further evalution of repeatability %CV / std deviations. Targeted compound's areas can explain whether that is a problem of irreproducible injection volumes or any systemic problem depending from GC system /method or sample pretreatment. Retentive molecules like IS is the most appropriate correction strategy for the sample volume rather than the flow through molecules resulting solvent peaks. ,
Thanks
My experience with GC analysis of FAMEs was the solvent peak was usually so large that it was essentially unquantifiable. There is so much solvent coming through that you don't know if you have measured it all or some has escaped without being burnt in the FID.
You said in your question that peak areas are different sizes (for the same injection volume), so attempting to quantify peaks of interest based on an unknown solvent volume is not really practical or particularly good science. If asked to referee a manuscript using that technique, would you consider it worthy of publication?
You are much better off using some sort of internal standard as İsmail Emir Akyildiz suggests, as you can adjust the ISTD to match the quantities you are trying to measure, rather than use an irreproducible solvent peak as a comparator.
Purchase 2 compounds that you are fairly sure don't occur in your samples, and if one of them doesn't help (or turns up in your samples), use the other one.
I think you need to optimize your solvent volume. Try using different solvent volumes starting with one you were already using, and gradually decreasing the volume until you get a a peak not too large and acceptable by you. Note this volume and repeat the experiment for reproducibility. Make sure your column is not dirty and has been conditioned by the solvent in the GC at least overnight. The variability you said you were observing with your solvent may either be non suitable, old or dirty column. You need to use internal solvent for the fatty acids analysis as suggested by the first 2 contributors, thank you.
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