We are trying to quantify monoacylglycerol (MAG) from plant lipid extracts via GC-MS (after silylation) or LC-MS. In both methods we find a high background of 16:0 MAG and 18:0 MAG in the solvent controls and blanks, which makes the quantification of plant MAG with the same acyl chains impossible. Does anyone have experience with putative sources for these contaminants? We have tried to reduce the backgound by rigorous cleaning of the machines and all consumables but the contamination is very persistant. Any ideas how to solve this problem would be highly appreciated!
Thank you both very much for your input! We also see peaks for C16:0 and C18:0 after transesterification, but they are of much lower abundance than the fatty acids we usually measure in our samples. However for MAG analysis the background is persistantly higher than the MAGs we extract from plant tissue. Using Eppendorf tubes instead of other suppliers is something we can definitely try. However we usually do all our extraction and analyses in glass ware. Do you have any recommendations for contaminant-free glass GC vials? Thanks again!
I assume that you are using high purity solvents and reagents. These should be HPLC or Analar grade at the very least. I also assume that you are triple cleaning all your glassware, syringes etc with solvents before use (we use DCM). We don't have any problem with the GC/LC vials themselves, so don't usually bother cleaning them, but we always buy them direct from Agilent and make sure that we never handle them except with nitrile gloves on. Do you use plastic pipette tips? These are another potential source of contamination and we always use cleaned microsyringes/pasteur pipettes instead. If you are using nitrogen blowdown to concentrate your samples this may also be a source of contamination. Do you ever get clean blanks for either LC or GC by the way? If so, can you identify what was different about those batches?
The only other thing I can suggest is that you prepare a set of blanks, using your usual method, and analyse a portion of the blank after each stage of the sample prep so that you can identify where the contamination is getting into your samples. Then you can look at the process where the contamination occurs and may be able to eliminate it.
I would like to thank you all for your numerous suggestions! We have isolated at least one source for the contamination right now, which is indeed new glassware (8 ml glass tubes though, not the GC vials). Now, after using old, rigorously cleaned tubes at least we get clean blanks in the GC. We will now backtrack all the steps of the extraction procedure to see if there are any additional sources of MAG contamination.
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