Spectra observed at the frequencies of the ground-state transitions of CH⁺ (black), ¹³CH⁺ (red), and SH⁺ (blue) in the direction of DR21(OH), G34.3+0.1, W31C, W33A, W49N, W51, SgrA*+50, and SgrB2(N). All spectra have been normalized to the continuum temperatures. To make the absorption features discernible, the ¹³CH⁺ and SH⁺ signals were mutiplied by a factor of 2 towards SgrA*+50, and SgrB2(N), and by factors of 2 and 3, respectively, towards the other sources. For more clarity the ¹³CH⁺ and SH⁺ spectra were shifted from the CH⁺ spectra by 0.5 and 1.0.