What ions to expect in mass-spec?

One thing that we often do in Testing Centre of University of Tartu is the analyses of the synthesis products for different labs/companies. Usually these compounds are brand new, without any standard substance available. The aim of these analyses is to confirm the identity of the product. What is often puzzling about this type of analyses is that usually it can only be assumed what kind of ions would be formed of the compounds in electrospray. For example will it be a protonated molecular ion, sodium adduct, dimer or something else? Though there are some general ideas and also intuition helping mass spectrometrist’s there is no general guidance. My student Piia focused on the ions formed in negative ESI mode in her first study. More precisely differentiating between compounds forming anions and dianions. Altogether she studied 29 compounds with at least two acidic sites, but only 9 of these gave doubly charged ions in mass spectra. Interestingly the compounds giving multiply charged ions was not related to the acidity of the compounds: there were strong acids not giving multiply charged ions and week acids giving multiply charged ions. The mechanism for this interesting phenomena was revealed by the help of linear discriminant analyses. It turned out that to produce multiply charged ions the compound has to have a suitable combination of acidity and hydrophobicity. For example the strong acids that are quite hydrophilic (and become even more hydrophilic after dissociation) do not reach the ESI droplet surface and therefore can not be ejected to the gas phase for analyses with mass spectrometer. The mistery between ions observed in positive ESI/MS mode (molecular ions, different adducts and multiply charged compounds) is yet to be solved. 

DOI: 10.1007/s13361-016-1384-2