The series of works from the UT Analytical chemistry group on measuring and predicting ionization efficiency in the electrospray (ESI) ion source of MS and LC-MS has reached a new milestone: for the first time an ionization efficiency scale for the atmospheric pressure chemical ionization (APCI) source has been established.
The work led by Dr Riin Rebane (photo on the left) resulted in APCI ionization efficiency scale containing 40 compounds with widely ranging chemical and physical properties and spanning 5 orders of magnitude of ionization efficiency. Analysis of the resulting data challenges the common knowledge about APCI as ionization method. Contrary to the common knowledge, ionization efficiency order in the APCI source is surprisingly similar to that in the ESI source and most of the compounds that are best ionized in the APCI source are not small volatile molecules. Large tetraalkylammonium cations are a prominent example. These findings suggest that the atmospheric pressure chemical ionization mechanism can be more complex than generally assumed and most probably several ionization mechanisms operate in parallel and a mechanism not relying on evaporation of neutral molecules from droplets has significantly higher influence than commonly assumed.
See the original publication Anal. Chem. 2016, 88, 3435-3439 for more information.
(Photo: Andres Tennus)