Laurent Houssiau oral presentation (FN1-Mon3-2-5)
MS/MS Dissociation of Molecular Cluster Ions Produced by Ar Cluster Ion Beams SIMS Analysis of DSPC and Amino Acids
1 University of Namur, rue de Bruxelles 61, 5000 Namur, Belgium
2 Kyoto University, Uji campus, 611-0011 Uji, Japan
3 Quantum Science and Engineering Center, Kyoto University, Gokasho, 611-001 Uji, Japan
4 SENTAN, Japan Science and Technology Agency (JST), Chiyoda, 102-0075 Tokyo, Japan
The SIMS spectra obtained with Arn+ cluster ion beams on small organic molecules [M] often contain protonated clusters of these molecules [Mn+H]+. The recent development at the Kyoto University of an MS/MS collision cell coupled to a SIMS system allows a deeper understanding of the structure and cohesion of these molecular clusters. Three amino acids thin films (arginine, phenylalanine and tyrosine) and a phospholipid (DSPC) were analyzed with a DC 10 keV Arn+ cluster ion beam. In the MS/MS mode, precursor ions were first selected with a quadrupole mass analyzer, then accelerated with energies ranging from 0 to 100 eV and collided with 10 keV Ar gas in a travelling wave (T-wave) collision cell, resulting in the dissociation of precursor ions (collision induced dissociation). The generated product ions were subsequently analyzed with an orthogonal ToF mass spectrometer. The collision gas pressure was around 0.8 Pa, which ensures a multiple collision regime.
The method was first applied on dimer DSPC ions [DSPC2+H]+, which are abundant in the SIMS spectrum. Surprisingly, these dimers did not dissociate for collision energies with Ar gas up to 40 eV, which indicates a strong binding between the two DSPC molecules. Tetramers [DSPC4+H]+ were even found to remain intact up to 70 eV collision energy, then dissociate preferably into dimers. On arginine, the SIMS spectra contained several cluster ions [Argn+H]+, up to n=11. The MS/MS dissociation as a function of the collision energy was followed for clusters with n=1…6 and n=10. Again, a fairly strong binding of the molecules in the clusters was observed, which can be explained by the existence of multiple hydrogen bonds between carboxylic and guanidine groups of the Arginine molecules. A systematic sequential dissociation from [Argn+H]+ to [Argn-1+H]+ was observed as the collision energy increases (Fig. 1.). Many clusters were also observed on the phenylalanine and tyrosine films, but they already dissociate at a collision energy of only 1 eV, indicating a much weaker cohesion than in the arginine clusters.
In conclusion, we will show the first MS/MS analysis of organic clusters generated by SIMS. Insights on the binding energy and the structure of molecular clusters will be inferred from collision induced dissociation data.