Dear Mrinmoy,

what do you mean with NST hits? I am a little unsure about it, if it is just a typing error.

Did you mean your performed a database search with the NIST database for your peaks, got several possible substances for one single peak and want to know how to decide which substance it really is?

If this is not what you meant and NSt isn't a typing error, than you would have to excuse me as I cannot help you, because in this case I do not know the problem nor could I give you any advise.

Kind regards,

Kristina

I apologize for over simplifying the sentence. you are correct to understand that I have a database search in NIST with so many possible compounds for each peak. How do I select which is the right one?

Thank you for clarifying that bit.

So there are different ways to determine which substance is the right one for your peak. (And be sure that your ionization energy used is the same as in the database)

First of all most database searches give you some kind of score that gives an indication about how good a proposed compound's MS spectrum fits your generated spectrum. So with that you can presort your results. Everything with a fit worse than 50% you can immediately disregard. Above that value it is complicated. Normally I would say that 70% match is also a rather bad result but depending on your matrix and how complex your background is 70% match can still be the most you will get. Nevertheless, if you have matches with 90% score or higher I would only look at them for a start.

Secondly you will have to look at the spectra personally. Compare your measures mass spectrum with the one in the data base for each proposed compound. Look at them with some common sense. If the proposed compound for example contains bromine or chlorine but your measured spectrum does not show isotope patterns, I would question this database proposal.

Another tricky thing I experienced with databases such as NIST is that they have listed some compounds separately under different synonyms. So you get results, both very good matches, you look through the spectra and they both seem to fit very well…and they are actually the same. Different names, e.g. the IUPAC and one common name. So be sure to check the CAS numbers or look up compounds you do not know for synonyms.

After that hopefully you will have narrowed your hits down to a few. If you still do not get conclusive results you have to measure your sample more often and see if the proposed compounds stay the same or if you get only one compound that is suggested every time.

If this still won’t give you clarity you should repeat your experiment and look for an overlap in suggested compounds.

The last thing you could do if only two or three possible compounds are left and you still do not know which one it is, you could try getting references for these compounds and see at which retention time they elute in your GC method. Hopefully only one will show the matching retention time as your peak.

Of course if you already have reference substances you could perform this measurement earlier.

I hope this will help you with your work.

Kind regards,

Kristina

Thanks a lot for your time.