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Hi, I am using the Sirius decomp tool to predict molecular formulas, but I noticed the mass you are using for Iron is incorrect. There is a ~2 Da mass error in the predictions that contain an iron atom. I think you are using the mass of 54Fe : 53.93960899 instead of 56Fe: 55.93493633, which is the standard, monoisotopic mass.

Hey, thanks for reporting. Can you provide some example to reproduce this problem (I know it has been a while since you reported this).

"Fe": {
        "name": "Iron",
        "valence": 1,
        "average": 55.84515022389856,
        "isotopes": {
            "mass": [53.939615, 55.934942, 56.935399, 57.933280],
            "intmass": [54, 56, 57, 58],
            "abundance": [0.05845, 0.91754, 0.021191, 0.002819]
        }

I checked our periodic table and it seems to be correct. so the problem seems to be caused by something else....

Hey, no worries, yes all of this element info is correct.

I think it's more of a definition thing, because the monoisotopic mass of iron is 55.934942, but when Sirius does MFP it uses the lowest mass isotope 53.939615 instead of the most common monoisotopic isotope. I think this lead could to some errors in the isotopic composition, as Fe53 Fe55 should be seen as -2 & 0 and not 0 & +2 isotopes. Not many people use these kinds of elements in their predictions though.

Hmm, expected behaviour would be to use the most common isotope is used. Do you have some iron containing compound spectrum lying around we can use for testing?