Paper AS-TuP8
Real-time Monitoring of Surface Reactions by Means of Cluster-induced Desorption/Ionization Mass Spectrometry

Tuesday, November 8, 2016, 6:30 pm, Room Hall D

The reactions of biomolecules on surfaces are of great interest both from a fundamental point of view as well as with respect to applications such as surface functionalization. However, the chemical information obtained by standard surface analysis tools is limited. Recently, we have shown that desorption/ionization induced by neutral clusters (DINeC) is a soft and matrix-free ion source for mass spectrometry of biomolecules. DINeC employs molecular clusters of 10³ to 10⁴ SO₂ molecules; the clusters do not only provide the energy necessary for desorption but, due to the high dipole moment of SO₂, also serve as a transient matrix in which the desorbing molecule is dissolved during cluster-surface impact. Thus desorption takes place at comparably low cluster energies (< 1eV/molecule); shattering of the clusters during and after surface impact furthermore leads to a rapid redistribution of the system's energy. As a consequence, desorption takes place without fragmentation of the desorbing molecules.

In this contribution, we demonstrate that DINeC can be used for real-time monitoring of surface reactions of larger molecules such as porphyrins and oligo-peptides. The quantitative nature of the method was demonstrated using angiotensin II molecules individually adsorbed on gold substrates by means of electrospray ion-beam deposition. These adsorbates were desorbed and detected as intact molecules down to a coverage of 10^-13 mol/cm² (0.001 ML); a linear relationship between surface coverage and signal intensity was observed over three orders of magnitude. Real-time monitoring of surface reactions is demonstrated for isotope-exchange experiments with angiotensin II and different types of porphyrins. Dosing D₂O led to a broadening of the isotopic pattern and a continuous shift towards higher m/z values. When the D₂O pressure was chosen high enough, the H/D exchange rate was determined by the exchange process itself. The results were modeled by means of Monte Carlo simulations taking into account reaction and back-reaction of H/D exchange. Several different rate constants could be extracted and are assigned to the different types of functional groups in the respective molecules.