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Mass spectrometry is an analytical methods that uses the mass-to-charge ratio to characterize compounds. During a mass spectral analysis it is common that the compound breaks down in smaller fragments, resulting in multiple peaks. The masses of the fragments or compound specific. But even then, with just a mass spectrum it is not easy to identify a compound.
The monoisotopic mass is the mass of a compound with all atoms having a single, major isotopic mass for each element. For example, the monoisotopic mass of hydroxyurea is 76.02728.
You can experiment with the range in the advanced search. Making it slightly larger increases the number of search hits. Try searching for the range 76.0 to 76.5.
Lysine has the molecular formula C₆H₁₄N₂O₂. If the mass spectrum gives us a [M+H]+ peak matching it molecular formula C₆H₁₅N₂O₂+ then it could be lysine. But with only the chemical formula, we do not know how the atoms are bound to each other. That is, we do not know what the correct stereoisomer is. In fact, we do not even know what the correct constitutional isomer is.
Given a chemical formula, say C₂H₆O, you can figure out how many chemical structures have that formula.
But this number goes up quickly. Toluene has many other chemical structures with the same
chemical formula. A tool called Surge can efficiently
enumerate them. Not all structures it generates are chemically stable, but many are. PubChem gives
you some idea, but this includes isotopic variants.
Not part of this practical, but if you try Surge, use the -S option to output SMILES, which
you can render as 2D structures with CDK Depict
for up to 500 SMILES strings in one go.
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