INVESTIGATION OF CATIONIC ORGANIC-DYES USED AS MOLECULAR PROBES BY LIQUID SECONDARY-ION MASS-SPECTROMETRY

The positive-ion mass spectra of twelve organic dyes used as molecular probes were measured using liquid secondary ion mass spectrometry (LS IMS). Nine of the twelve dyes were singly charged cations and the othe r three were doubly charged cations. The mass spectra of each of the d yes in m-nitrobenzyl alcohol contain abundant signals for the intact c ation, C+ (singly charged cation dyes), or for singly-charged forms of the doubly charged cation formed by proton loss, [C2+-H+](+), or halo gen counter ion attachment, [C2++X(-)](+). Fragmentation is usually mi nimal under the conditions used. However, the cations of five of the s ingly charged compounds appear to undergo charge-remote fragmentation. Collision-induced dissociation experiments on a hybrid mass spectrome ter of EBqQ geometry at collision energies up to 300 eV failed to acce ss this fragmentation pathway. In contrast to the LSIMS of many other doubly charged organic compounds, two of the dicationic dyes produced a doubly charged ion of reasonable abundance (2-20%) in the mass spect rum. When glycerol was used as a matrix solvent, the addition of the m atrix modifier trifluoroacetic acid increased the abundance of C2+.