Negative Ion MALDI Mass Spectrometry of Polyoxometalates (POMs): Mechanism of Singly Charged Anion Formation and Chemical Properties Evaluation

Abstract

Abstract. MALDI-MS has been developed for the negative ion mode analysis of polyoxometalates (POMs). Matrix optimization was performed using a variety of matrix compounds. A first group of matrixes offers MALDI mass spectra containing abundant intact singly charged anionic adduct ions, as well as abundant in-source fragmentations at elevated laser powers. A relative ranking of the ability to induce POM fragmentation is found to be: DAN > CHCA > CNA > DIT> HABA > DCTB > IAA. Matrixes of a second group provide poorer quality MALDI mass spectra without observable fragments. Sample preparation, including the testing of salt additives, was performed to optimize signals for a model POM, POMc12, the core structure of which bears four negative charges. The matrix 9-cyanoanthracene (CNA) provided the best signals corresponding to singly charged intact POMc12 anions. Decompositions of these intact anionic species were examined in detail, and it was concluded that hydrogen radical-induced mechanisms were not prevalent, but rather that the observed prompt fragments originate from transferred energy derived from initial electronic excitation of the CNA matrix. Moreover, in obtained MALDI mass spectra, clear evidence of electron transfer to analyte POM species was found: a manifestation of the POMs ability to readily capture electrons. The affinity of polyanionic POMc12 toward a variety of cations was evaluated and the following affinity ranking was established: Fe 3+ > Al 3+ > Li + > Ga 3+ > Co 2+ > Cr 3+ > Cu 2+ > [Mn 2+ , Mg 2+ ] > [Na + , K + ]. Thus, from the available cationic species, specific adducts are preferentially formed, and evidence is given that these higher affinity POM complexes are formed in the gas phase during the early stages of plume expansion.

TiO2 Nanocoral Structures as Versatile Substrates for Surface‐Assisted Laser Desorption/Ionization Mass Spectrometry

“…Among these, MS is an important analytical technique for accurately determining the structures and compositions of POMs. Both ESI and MALDI have been used as ionization techniques for POMs for MS analysis. Figure a–b shows negative ion SALDI mass spectra of dodecatungstosilicic acid (H 4 SiW 12 O 40 , SiW12) and phosphomolybdic acid (H 3 PMo 12 O 40 , PMo12), which are typical POMs, obtained by using TNC substrates.…”

The ongoing evolution of laser desorption/ionization mass spectrometry: Some observations on current trends and future directions

“…Of these, 9‐AA is reported to be “unequivocally the matrix of choice” for negative ion lipid studies. Other matrices used for proton extraction include 9‐cyanoanthracene (CNA) and an organic salt, N‐(1‐naphthyl) ethylenediamine dinitrate (NEDN) . Vaidyanathan and Goodacre developed a quantitative workflow for the analysis of metabolites using MALDI with 9‐AA as the matrix, but they reported a large variance in results.…”

Extensive generation and characterization of ion clusters by silicopolyoxometalate anions under matrix‐assisted laser desorption/ionization conditions in the gas phase

“…For example, Cronin and co‐workers reported that electrospray ionization and cryospray mass spectrometry combined with high‐resolution time‐of‐flight systems could be applied to unravel the complexities of inorganic and supramolecular self‐assembly in solution 15 . Cole and co‐workers reported the detection of intact POMs and corresponding fragmented clusters under negative MALDI conditions 16 . These authors systematically investigated the effects of laser energy, salt additives, and matrices with various chemical structures on cluster formation.…”