MS analysis of cyclic peptides in negative ion mode has been a challenge, in particular if the peptide does not contain acidic functional groups. In this paper, we present a way to easily produce negative ions from anionic peptide adducts, utilising collision-induced dissociation (CID)-mediated elimination. Using two different mass spectrometers, we have generated series of adducts of three cyclic and one linear peptide with various anions. They were then isolated and subjected to CID with a range of collision energies. The deprotonation percentage was then calculated from the resultant spectrum, and compared between the spectrometers, as well as with an external reference-proton affinity values. The susceptibility to deprotonate by detaching a HX moiety is proportional to the proton affinity of the X