In pesticide analysis I used MRM mode in MS method, and it asks about precursor and product. I used default values as precursor at 272 and product at 222 m/z for analyte I used Benomyl (a fungicide) which has M.W = 390.32, but its gives no peak. How we will get its precursor and product value to get accurate selective peak.
Hi,
I'm not an expert in MRM as such, but I think the precursor is the mass you're selecting in Q1, which in your case would be m/z 391.32 ([M+H]+). The product is referring to the mass of the fragment you're interested in as a result of the dissociation of Benomyl. So, I think it is logical you will not get a signal using the default values you mention.
To check what the best/most intense (sensitive) fragment is, you would have to run some MS/MS experiments, I guess.
Hope this helps!
Hi,
First, I need to say that, you had chosen a very difficult compound (Benomyl) to be tested. Benomyl is a very unstable compound and it is easily degraded to carbendazim which is also a pesticide. You need to be sure that Benomyl is prepared freshly and I should emphasize the LC-MS system is more amenable for quantifying both Benomyl and Carbendazim rather than GC systems.
I assume, you are using Electron impact ionization (EI) thus pseudomolecular ion (M+H) is not common for EI and you need to expect lower Molecular weigh mass spectrum as a characteristic for targeted compound since EI is a kind of hard ionization technique. As I suggest if you switch the system to ESI soft ionization (LC-MS-API ionization), you probably observe +1 mass shifted precursor as M+H.
You may use NIST MS/MS library for targeted GC-MS/MS analysis, you can obtain reference spectrums for precursor and daughter (product) ions. If you are not capable to implement these information then you can perform compound tuning (scanning) at Q1 and Q2(Q3) stages for identifying the precursor and daughter ions which resulting from dissociation of the selected precursor. Then you may complete your MS acquisition library for each pesticide.
Good Luck...
Emir
Please look at the following below link which may reply to your question.
https://ibbr.cnr.it/ibbr/facilities/gc-ms-ms
Thanks
Assuming you are using EI at -70ev then this is a very aggressive form of ionisation compared with and LC ESI for example, this can be fantastic as it allows the GC analyst to ionise very nonpolar compounds which just isn't doable on a traditional ESI however because of the aggresive nature you will see much more fragmentation during your initial ionisation.
When dealing with new compounds it is important to either run a full scan on Q1 or analyse a scan from the internet, this will give you the most abundant ions and will also show you how much fragmentation is going on, the more fragments you see the lower your response will be at the end. Then once you have decided on 1 or 2 ions they are your precurso ions and you can run these in a product ion scan using different ce, this will then give you another scan showing the fragment from your precursor after going through your collion cell. These fragments are now your product ions. You can then choice a few of these and look to optimise ce running in MRM and can then evaluate using some extracts incase a particular transition gives problem peaks.
I have never looked at benomyl myself but as an GC analst who has evaluted many new compounds myself i would give this straight to an LC analyst to deal with. If you run a full scan and still dont see a peak then your method is not compatible with this compound and if LC is not available then a unique approach might be needed.
hope this helps
Thanks Ismail Emir, Isam Eldin and Josh Luther for your valuable recommendations
Before setting up your MRM method, you do a full scan to find your precursor ion (usually the most intense ion) and then you do a product ion scan (of this precursor ion) with different collision energy to find the best (specific and high intensity) product ion and collision energy for quantitation. Only a few hours are required for this procedure so do this step yourself even if you can find many published methods for the same compound.
There are different scan modes in MS/MS like Product Ion scan, Precurson Ion scan, Neutral Loss scan, MRM scan etc. Precurson ion is the one you select in Q1 and this is further fragmented in CID by colliding with a gas and this results in the formation of Product ions that are scanned in Q3.
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