Observation of alkali-metal-carboxylate radical anions in fast-atom bombardment.
The existence of a series of 32 molecular radical anions from carboxylic acids salts RCOO-Cat(-), where R = CH3, CH3CH2, CH3CH2CH2, (CH3)2CH, C6H5, o-CH3-C6H4, m-CH3-C6H4, and p-CH3-C6H4 and Cat = Li, Na, K, Rb, has been proven by the observation of their fragmentation in negative-ion fast-atom bombardment tandem mass spectrometry. These species occur at very low abundance and are not detected in the fast-atom bombardment spectra because they are hidden in the background. However, the collision-induced dissociation fragmentation of ions selected at the mass-to-charge ratio values that correspond to these species display characteristic signals that are completely different from the signals observed from pure matrix or after addition of corresponding metal hydroxide to the matrix. The main fragmentation observed is the loss of the neutral metal atom from RCOOCat(-·), followed by a loss of CO2 for the aromatic compounds. Neutral loss experiments also confirm the existence of these radical anions. Scans for the loss of a selected metal after addition of one of the carboxylic acid salts to the matrix display abundant peaks at mass-to-charge ratio values that correspond to the salt radical anions. Some weaker peaks appear at other mass-to-charge ratio values independent of the salt that is used and also are observed from the matrix when the corresponding metal hydroxide is added. When alkali metal salts from some deuterated acids are analyzed, the predicted shifts are observed. The loss of the neutral metal is more pronounced from RCOONa(-·), RCOOK(-·), and RCOORb(-·) than from RCOOLi(-·).
Spectrométrie de Masse, Université Catholique de Louvain, Place Pasteur 1, 1348, Louvain-la-Neuve, Belgium.
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de Hoffmann