We used the TOFMS to measure the emissions from a waste incinerator. The precise molecular weight were got by TOFMS. There are so many matters with this weight (0.2 difference). How do I make sure which is the true matter in this weight?
eg. A measured matter with molecular weight at 74.103. With in 0.2 difference, its elemental formula may be C1H6N4, C1H4O1N3, C1H2O2N2, C1O3N1, C2H8N3, C2H6O1N2, C2H4O2N1, C2H2O3, C3H10N2, C3H8O1N1, C3H6O2, C4H10O1,C5N1, C6H2. The high molecular weight wil be more possible matters.
Could anyone help me on this topic?
The molecular weight alone is not sufficient to ientify the substance. Even if the weight is more precise, there is no method known to me that allows identification based on this single value. Since you work with a ToF, I suppose you use EI with 70eV as an ion source. This allows to use standard fragmentation patterns used by many libraries such as NIST.
If you have ms data of your substance you should be able to identify it with these libraries. However if you could provide more information, we might be able to give some additional input.
Hope that helps,
Moritz
Don't you have any standards of these substances that you could inject to see what differences they have between them?
As Moritz Heil wrote, you should add what kind of experimental set you use. Is it direct injection or do you use gas chromatography before your ToF? What kind of ionization source is used? If you look up in libraries you should be sure that the library covers the same kind of instrument and the same kind of ionization source you employed.
Another point: What is the nitrogen source in your waste? I mean you should look for some chemical information about your precursors in waste before icineration. This could elicit what are the probable structures you would obtain after icineration.
You're going to have to apply additional tests in order to get more information before you can make any kind of determination. There are specific chemical tests for degree of oxygenation, prevalence of double vs. single bonds, average chain length, and a whole battery of tests for determining the breakdown of nitrogen valences. What kind of waste is this and why would you want to know much more than the heating value and moisture content? If you send a sample to a competent laboratory you should know that there's no such thing as "send me a list of everything that's in this sample by tomorrow morning." That only happens in the movies. You must request specific tests in order to get specific results and they can be very expensive when performed correctly. You need to have some idea what groups of compounds are present or which ones you care about too.
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