Studying mass spectrometric behaviors of {Au₆Ag₂(C)[PPh₂(4‐CH₃‐Py)]₆}(BF₄)₄ and {Au₈[(PPh₃)₂O]₃(PPh₃)₂}(NO₃)₂ by electrospray time‐of‐flight mass spectrometry and electrospray ion trap mass spectrometry

Abstract

RATIONALE: Mass spectrometry (MS) has been recognized as a powerful technique to detect accurate chemical information about metal clusters. Maintaining metal clusters intact, which is a great challenge in MS analysis, was achieved in this work by choosing a suitable mass analyzer and carefully optimizing analysis parameters. METHODS: Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) and electrospray ion trap mass spectrometry (ESI-IT-MS) were applied to characterize the synthesized ligand-protected metal clusters [Au 6 . Three kinds of buffer gas (helium, mass: 2; nitrogen, mass: 28; argon, mass: 40) and various radiofrequency (RF) amplitudes (from 70 to 330) were chosen to study the fragmentation rate during the "collision cooling" process in the ion trap analyzer. RESULTS: In the ESI-TOF-MS analysis, metal clusters 1 and 2 were mainly observed as intact clusters, which were Au 6 Ag 2 (C)(L 1 ) 6 (BF 4 ) 2 2+ , Au 6 Ag 2 (C)(L 1 ) 6 (BF 4 ) 3+ , Au 6 Ag 2 (C)(L 1 ) 6 4+ for 1 and Au 8 (L 2 ) 3 (L 3 ) 2 2+ for 2. While, in the ESI-IT-MS analysis, only fragments could be found, such as, Au 6 (L 2 ) 3 2+ for 2. It is obvious that the two kinds of mass analyzers caused different MS behaviors of metal clusters. In the ion trap (IT) mass analyzer, particularly, "collision cooling" was contributing to further dissociation of fragile compounds, in which a higher RF amplitude and a larger mass buffer gas led to more fragmentation. CONCLUSION: In this work, intact metal clusters were obtained in ESI-TOF-MS, instead of ESI-IT-MS, in which the "collision cooling" process caused more cluster dissociation. It was concluded that the analyzer in ESI-TOF-MS is "softer" than that in ESI-IT-MS for metal clusters. Copyright © 2016 John Wiley & Sons, Ltd.Metal clusters composed by metal cores and organic ligands have brought about an upsurge in research because of their interesting properties, such as catalysis, [1][2][3] chirality, [4,5] electrical conductivity, [6,7] luminescence, [8][9][10] C-H activation, [11] and so on. The properties of metal clusters depend on their structures, chemical compositions, and bondings. Recently, more and more metal clusters are studied by mass spectrometry (MS), since MS not only provides the exact molecular mass and isotopic patterns to characterize their composition, [12][13][14][15][16][17] but also gives insights into their chemical behaviors in solution, such as ligand exchange and the growth mechanism. [18][19][20] However, due to the weak non-covalent interactions, such as coordination bond and metal-metal bond, metal clusters are easily decomposed in MS analysis, and it is challenging to obtain information about their intact clusters. It is well known that fragments of metal clusters can be produced in the ionization processes. The ionization methods causing less degrees of dissociation of the analyzed compound are always reported as being ''soft'' ionization methods, including electrospray ionization (ESI), [12,13,17,19,20] matrix-assisted laser desorption (MALDI), [...