Ion association by time-of-flight mass spectrometry: a study of V-P-O catalysts

The oxidation catalysts (VO₂)P₂0₇ and β-VOPO₄ are studied by laser desorption time-of flight mass spectrometry. Mass spectra of both positive and negative ions are reported for these compounds. Individual scans are tested for ion association, that is, the consistent appearance of a given pair of ions in many scans. The structurally simpler {amalgamation}-VOPO₄ shows fewer associations than either (VO)₂P₂0₇ or an ¹⁸O-substituted analogue of _β-VOPO₄ even though the overall spectra from the three compounds do not differ greatly. These results indicate that the associations observed between ions may be sensitive to small differences in structure in the original specimen. Thus, the identification of ion association may provide information to supplement the mass spectrum of the sample.