Sodium adduct formation efficiency in ESI source.

Formation of sodium adducts in electrospray (ESI) has been known for long time, but has not been used extensively in practice, and several important aspects of Na(+) adduct formation in ESI source have been almost unexplored: the ionization efficiency of different molecules via Na(+) adduct formation, its dependence on molecular structure and Na(+) ion concentration in solution, fragmentation behaviour of the adducts as well as the ruggedness (a prerequisite for wider practical use) of ionization via Na(+) adduct formation. In this work, we have developed a parameter describing sodium adducts formation efficiency (SAFE) of neutral molecules and have built a SAFE scale that ranges for over four orders of magnitude and contains 19 compounds. In general, oxygen bases have higher efficiency of Na(+) adducts formation than nitrogen bases because of the higher partial negative charge on oxygen atoms and competition from protonation in the case of nitrogen bases. Chelating ability strongly increases the Na(+) adduct formation efficiency. We show that not only protonation but also Na(+) adduct formation is a quantitative and reproducible process if relative measurements are performed.