A new matrix of MALDI-TOF MS for the analysis of thiolate-protected gold clusters

Abstract

Mass spectrometry (MS) can play an important role in determining the number of gold atoms per ligand molecule in thiolated Au clusters (CLs). However, the precise determination of the formulae of thiolated Au CLs is not a routine analysis and is still technically challenging. In this study, we report a new matrix for the matrix assisted laser desorption/ionization (MALDI) technique for thiolated Au CLs based on a commercially available α,β-diphenylfumaronitrile (DPF) matrix. This MALDI-time of flight (TOF) MS involving a DPF matrix allowed detection of intact Au₂₅(SCH₂CH₂Ph)₁₈ and Au₂₅(SC₁₂H₂₅)₁₈ ions with less fragmentation (i.e. softer ionization process) and stronger ion peak intensities compared to similar techniques containing a typical trans-2-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene] malononitrile (DCTB) matrix for Au CLs. The DPF compound exhibited an absorbance maximum at about 336 nm, thereby making this matrix suitable for common N₂ lasers (337 nm). The DPF matrix was suggested to function as an electron transfer agent from DPF⁻˙ to the Au CLs, leading to effective production of intact radical anions from Au CLs in negative ion operating mode.