Double cation adduction in matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry of electron deficient anthraquinone derivatives

Abstract

Six anthraquinone derivatives were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Clear (pseudo) molecular ions were observed for all the compounds. Interestingly, for some derivatives, strong ions with double cation adduction were also recorded in the positive mode. It is remarkable that all these ions are singly charged. In this work, possible mechanisms for the double cation adduction were investigated and discussed. It appears that the double cation adduction was due to the electron deficient nature of the derivatives, and formed by taking up two singly charged cations and one electron. Substituents on the anthraquinone ring were found to have a significant effect on the double cation adduction. In contrast, no considerable influence of the acidity of MALDI matrix/solution was observed, even on the double proton adduction. Furthermore, it was demonstrated that double cation adduction might occur in the MALDI gas-phase plume. In addition to the anthraquinones, three more electron deficient compounds of different types, i.e. a perylene bisimide derivative (PB), 3,7-decanoylamino-4,8-dihydrobenzo[1,2-b:4,5-b']dithiophene-4,8-dione (TQ) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), were also analyzed with MALDI TOF MS. The results indicate that the 'abnormal' double cation adduction might be a 'normal' phenomenon in the MALDI TOF MS analysis of many electron deficient compounds.

A Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Method for the Identification of Anthraquinones: the Case of Historical Lakes

“…The collected spectra are discussed in relation to the different ionization modes and the best one for each anthraquinoid dye has been assessed. Interestingly, the generation of singly charged double cation adducts has been observed in positive mode in agreement with the study of Lou et al [28], in which the mechanism of this phenomenon has been thoroughly investigated. The analyses of freshly prepared paint model systems allowed us to evaluate the role played by the presence and the kind of paint medium on anthraquinones identification.…”

“…This phenomenon has already been observed in a previous study carried out on the MALDI-MS analysis of anthraquinone derivatives used as potential precursors for urea-anthraquinone-based foldamers [28]. It is important to highlight that Lou et al [28] worked on pure, standard compounds; thus the behavior of anthraquinones in complex samples has not been assessed yet.…”

“…This phenomenon has already been observed in a previous study carried out on the MALDI-MS analysis of anthraquinone derivatives used as potential precursors for urea-anthraquinone-based foldamers [28]. It is important to highlight that Lou et al [28] worked on pure, standard compounds; thus the behavior of anthraquinones in complex samples has not been assessed yet. The mechanism of the formation of [M + 2H] + is unlikely ascribable to a reduction of the compound becuse the antharquinones have a very stable conjugated backbone, while it is more probable that the compound takes up first one electron upon laser irradiation and then two protons realeased by the photonexcited matrix or protonated ions.…”

“…The mechanism of the formation of [M + 2H] + is unlikely ascribable to a reduction of the compound becuse the antharquinones have a very stable conjugated backbone, while it is more probable that the compound takes up first one electron upon laser irradiation and then two protons realeased by the photonexcited matrix or protonated ions. According to the literature [28], this unusual behavior occurs in these compounds because of the electron-deficient nature of anthraquinones, which enhances their electron capture ability. Accordingto Lou et al [28], the kind of matrix used does not influence the phenomenon as long as it is a protic one.…”

“…MALDI-ToF-MS analyses of standard anthraquinones and derivatives are described in the literature [28,29] but the anthraquinones typical of red lakes have never been investigated by this means, and no study on the possible interferences from the matrix is available. In particular, the possible influence of the complex painting matrix on anthraquinone identification in unknown samples has not been evaluated yet.…”