Localize where a chemical modification sits on a known structure from a pair of MS/MS spectra — the known compound (with its SMILES) and a modified analog. Combinatorial fragmentation, peak annotation, spectral alignment, and per-atom scoring all run in your browser; your structure and spectra never leave your machine (only a USI, if you choose to resolve one).
Enter the task, then give a feature scan (row ID) in either spectrum below and click Load feature. Peaks resolve indirectly through the metabolomics-USI service; if that's unreachable, the aligned-features MGF is pulled from the host directly.
= most likely site. Shading is per-atom likelihood; deeper red is more probable.
Known spectrum up, modified analog down (each scaled to its own base peak). Blue = matched at the same m/z (unshifted); red = matched across the Δ-mass shift (shifted — these carry the modification); grey = unmatched — the same colors ModiFinder uses. Hover a peak for m/z & match detail; click one to select it (green); clicking a shifted peak also marks its partner (olive).