Fig. 4

Relative cumulative frequency of \(\overline{\Delta m/z}\) values for all blank-subtracted series detected in positive (a) and negative (c) ionization modes, stacked top-down for individual STP samples in order of their IDs 1–10 (cp. Table S6 in Additional file 9). Solid red lines indicate masses of three common homologue units at z = 1, red dashed ones at z = 2. Solid gray lines denote \(\overline{\Delta m/z}\) values of possible multiples, additions or subtractions thereof. Gray dashed lines indicate isotopologue shifts of some of these masses, equal to ¹²C vs. ¹³C mass transitions. Moreover, gray bars in b and d show STP counts from a moving \(\overline{\Delta m/z}\) window (±5 μ) over all stacked distributions for the positive and negative mode, respectively. In contrast, blue bars were derived after omission of non-monoisotopic series