In fact it is impossible to predict with 100% certainty what ions you will see from any analyte, especially inorganic ones. You must be aware that even though you may see a molecular ion under certain conditions and using a certain ion source and employing a certain ionization method on a certain instrument, you may well be able to see a completely different picture even on the same instrument just by tweaking a parameter or two. Ion sources vary even with the same manufacturer and you're never guaranteed you will achieve the same result with the same analyte. For example, if I run iodobenzene in the + mode on a Quattro Ultima (Micromass, now part of Waters), I can vary the amount of molecular cation and protonated species if I control the atmosphere around the source. In a Synapt G2 (also Waters), I could never get the protonated species, no matter what I tried. On the same Synapt, I've seen a number of Pd peaks in the negative mode from, say, PdCl2, including its molecular anion (which has nothing special about things like stability or whatever). There is simply too much going on in even the simplest mass spectrometer, to be able to predict with certainty what will happen when an analyte is injected. In that sense, the word "only" in your question is a risky one; while it's certainly a very interesting question, you may well be able to achieve the same results in completely different settings, or completely different results if you change your current settings, and still wonder why that happened. Our group is gradually accumulating evidence on the topic; you may want to consult some of our latest papers.

Mr. Zakharchenko,

The proton accepting ability cannot be changed depending on the ionization conditions. This is property of the analyte.

The proton affinity is a constant, but the availability of protons can be changed.

Thank you for the answers!