Evidence for laser-induced redox reactions in matrix-assisted laser desorption/ionization between cationizing agents and target plate material: a study with polystyrene and trifluoroacetate salts

- The results shed new light on MALDI-MS and provide a new analyte ionization strategy.
- Laser-induced redox reactions occur between target plate and cationizing agents.
- They depend on the species' standard reduction potentials and ionization energies.
- The cationizing agent's ability to form salt clusters and/or adducts is decisive too.
- Peak assignment reliability at high salt concentration requires critical evaluation.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is often applied to assess the dispersity and the end groups of synthetic polymers through the addition of cationizing agents. Here we address how these cation adducts are formed using polystyrene (PS) as a model polymer. We analyzed PS by MALDI-MS with a 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as the matrix and a range of trifluoroacetate (TFA) salts as cationizing agents on a range of different target plate materials (copper, 1.4301 stainless steel, aluminum, Inconel 625, Ti90/Al6/V4 and chromium-, gold- and silver-plated stainless steel). It was found that on a stainless steel substrate the metal cations Al+, Li+, Na+, Cu+ and Ag+ formed polystyrene adducts, whereas K+, Cs+, Ba2+, Cr3+, Pd2+, In3+, or their lower oxidation states, did not. For the copper and silver substrates, PS and DCTB adduct formation with cations liberated from these target plate materials was observed upon addition of a cationizing agent, which indicates the occurrence of redox reactions between the added TFA salts and the target plate material. Judging from their standard electrode potentials, these redox reactions would not normally occur, i.e., they require an additional energy input, strongly suggesting that the observed redox reactions are laser-induced. Furthermore, copper granules were found to successfully sequester PS from a tetrahydrofuran (THF) solution, consistent with the view complex formation with the copper target plate can take place prior to the MALDI-MS measurement.

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