Fig. 2

SOM for series pairs from STP sample with ID = 1, positive ionization mode. Coloring of the top and bottom panels show \(\overline{\Delta m/z}_{x}\) and \(\overline{\Delta m/z}_{y}\) values at the SOM nodes, respectively. Sizes of the black dots in the top panel indicate frequencies of monoisotopic series pairs mapped onto the nodes. In turn, black squares in the bottom panel either indicate the three nodes with highest frequencies at low intersection angles (θ < 0.08π, nodes 1–3) or nodes with highest mapping frequencies containing 50% of all monoisotopic series pairs at larger intersection angles (θ ≥ 0.08π, nodes 4–11). The series mapped onto the latter nodes 4–11 are shown in Fig. 3. Moreover, crosses highlight the mapping nodes of the unknown superjacent series shown in Figure S6 (Additional file 15)