Comment on "Non-linear photoelectron effect contributes to the formation of negative matrix ions in UV-MALDI".

Alonso and Zenobi (AZ) recently claimed "a comprehensive theoretical description of negative ion formation in UV-MALDI" (Phys. Chem. Chem. Phys., 2016, 18, 19574). Emphasizing photoelectrons, it is found to be unphysical in several respects, including violation of charge and mass conservation, and in the treatment of ablation, expansion and electron capture. It is not internally consistent, and ions created by the photoelectron mechanism are given artificial preference. Although AZ claimed the "first proposal for a comprehensive theoretical description of negative ion formation in UV-MALDI", the Coupled Physical and Chemical Dynamics model has successfully reproduced a number of phenomena relevant to negative ion production over many years.