I am estimating the amount of ethylene in my standard, using headspace sampler coupled with GC-MS, where I am willing to plot a graph by varying ethylene concentration. I am saturating the first vial with std ethylene and then transferring 1ml from the vial into the headspace of the next vial, sort of serial dilution. In the same respect, I am getting highest hit in the NIST library for the first vial, while for the next vial, first hit is Nitrogen (90-95% probability) while succeeding (1-2% probability) hits are for ethylene. Secondly, AUC for both are almost the same. So although the ethylene is reducing in concentration due to serial dilution, AUC remains constant and I cannot use this AUC parameter for getting the standard plot. In such case, whether am I supposed to multiply the AUC by SOME factor, such that ONLY ethylene will be considered or there is some other way to calculate the same like retention index, match/reverse match scores in NIST library?
Though intensity of the base peak in the mass spectra may be considered for calibration calculations, it is not recommended to calibrate manually through NIST lib/ GCMS TIC/ full scan mode. Why not you can try GCMS - SIM mode?
You really, desperately, need training on how to use a mass spectrometer. If you thought even a little bit about what you posted you would realize that the largest peak in your chromatogram from your "serial" dilutions is clearly nitrogen. That is precisely why you have to actually separate the ethylene from the nitrogen; it's called chromatography, and you rather clearly skipped that part of GC-MS. Find someone, anyone, who knows at least a little bit about the proper use of GC-MS and get some lessons before you do real damage to the instrument.
Remember, the 'heart' of chromatography is separation. MS is NO replacement for .....!
I completely agree with Mark and Bruce. You seem to have skipped the separation step completely and to be trying to use the mass spectrometer to try and disentangle an unseparated mixture of ethylene and nitrogen. This is impossible. The separation needs to happen in the GC BEFORE anything gets to the mass spec. Then you should be able to see separate peaks for the two gasses. In this context a mass spectrometer is only used as a detector for the output of the GC and cannot be used to separate two compounds.
Thanks Mark Krause , Valerie Steele , Bruce Neagle and Srinivasan Krishnamoorthy for your inputs. I have worked upon GC-MS before for analysis of volatile compounds and derivatized biomolecules, but 'light gases' like ethylene was a new avenue to explore, because majority of the research on ethylene uses GC-FID. Ultimately, we learn from our shortcomings, and suggestions from peers and expertise.
Therefore, ultimately I had a word with NIST and agilent expertise, and few chemical engineers (offline) as well, especially who are working on ethylene. In my comprehensive view, NIST does offer a probability platform for narrowing down our spectrum for the compounds, but for better resolution and calculations AMDIS can help. On the other hand, considering instrumentation, one can use another column (GC-GasPRO) which can separate ethylene and nitrogen or GC-FID. Third way out is use of GC-MS and CNS elemental analyzer to estimate ratios of C:N and relate it to the peak area obtained in GC to ultimately get the amounts of individual gases in the mixture.
Thanks again.
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