Spectrum obtained toward Sgr B2(M) around 625 GHz with the APEX telescope in main beam brightness temperature. The red curve is the synthetic spectrum described in Sect. 4.1, the horizontal dotted line shows the continuum level, and the left and right vertical dotted lines mark the systemic velocity of the source (62 km s^-1) for HCl and H³⁷Cl, respectively. The strong emission close to the HCl line is likely due to a SO₂ line at 626 087.3 MHz.