I am having trouble with setting up standard curves for volatile fatty acids with the Varian machine GC-MS. There is around 20% difference in curves of the same concentration? I make these standards up myself very carefully. Has anyone any experience with buying in these mixes? Some papers I have read also use internal standards also. Is it worth having standards in every run as well as spiking samples with a known mix also?
I think you should calibrate your system with std mix of fatty acids. Then run your samples and match it.
Gudluck
:)
I think your headspace method would be VASTLY improved with an internal std. Ran into the same thing trying to do a headspace method for hexanal in food. Solid phase micro extraction (SPME) may also be an option, but only if you need to run this method often (extra consumeables = lots of extra $).
Thank you. Yes I have ordered a standard fatty acid mix from Sigma. We also plan to make up sets of standard curves also. But further I think we should have the mix as an internal standard. Do you think it is necessary to do this for each sample? Also would you run standards for your standard curve within each run? What had been done in the lab previously was making standards curve files and then using these as reference for subsequent runs. However, I think it would be better practise to have standards in every run? SPME is not an option for us right now so it has to be this method I'm afraid, however, I think once we have the protocol well calibrated it will work well. Again thank you for your answers, there are so many different methods it seems each lab has a different practice with regard VFA analysis.
Clara,
It is a good Idea to include an IS in every run. If You use a pressure balanced system it is possible to have an RSD below 2% without IS but if it's possible include one. Your IS should be functionally related to the substances of interest , specially with HS/MS. And only one compound as IS for all Your runs in one experimental group is a good approach.
In our lab a part of the calibration is always repeated in each sequence, we never use old calibration curve's, samples and calibration are mixed.
Some thought's:
In static headspace if the composition of the liquid phase changes the response for your compound may also change even with the same concentration.
Do You have the same problems with an FID detector? Long and short term drift on an MS/EI is described
:https://www.researchgate.net/publication/51172598_Improving_quantitative_gas_chromatography-electron_ionization_mass_spectrometry_results_using_a_modified_ion_source_demonstration_for_a_pharmaceutical_application?ev=prf_pub
Article Improving quantitative gas chromatography-electron ionizatio...
Hi Kris,
Yes that what I was thinking. The IS will be related to the substances of interest, we also have purchased a mix of the volatile fatty acids that we are looking at. So I'm going to try that also and see how the results look. Yes I think so due to the column and system there will be changes but that's why I wanted to make sure that I have the process well calibrated. A service engineer has serviced our machine in August and the system is working perfectly. Thank you I will read that paper now, I will hopefully start using the machine next week once the standard mix is delivered and I will have more of an idea about the IS then.
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