From bottom to top, panels a) to c) show absorption spectra taken with the APEX telescope with the CHAMP+ receiver array. Spectra a) and b) were taken with the central pixel of CHAMP+ toward Sgr B2(M) (see Fig. 5). Spectrum b) was obtained toward a position (− 20.4″,1.4″) offset from Sgr B2(M) with the CHAMP+ pixel 2. a): H³⁵Cl 1−0 absorption spectrum. b): SH⁺ 1₁–0₁ absorption spectrum. c): ¹³CH⁺ 1–0 absorption spectrum. In panels a)–c), the red curve is the synthetic spectrum described in Sect. 4.1 and the horizontal dotted line shows the continuum level. d) and e): same as b for the c-C₃H₂ 2₁₂–1₀₁ and H¹³CO⁺ 1–0 transitions observed toward Sgr B2(M) with the IRAM 30 m telescope, respectively. Panel f) shows the optical depth of the 21 cm HI line vs. LSR velocity as derived by the procedure described in Sect. 5.1.2. In panels d) and e), the red curve is the synthetic spectrum of the molecule, the green curve is the predicted spectrum including in addition all molecules identified in our complete 3 mm line survey so far, and the horizontal dotted line indicates the zero level (after baseline removal because the observations were performed in position-switching mode with a far OFF position, which yields a very uncertain baseline level). In all panels, the spectra are plotted in main-beam brightness temperature scale, the vertical dotted line marks the systemic velocity of the source (62 km s^-1).