Molecular depth profiling of histamine in ice using a buckminsterfullerene probe.

We employ a buckminsterfullerene ion source to probe the distribution of histamine molecules at the water-ice/vacuum interface. The experiments utilize secondary ion mass spectrometry to detect molecular ions that are desorbed from a frozen aqueous histamine solution. The results show that this cluster ion probe induces an extraordinarily high sputter yield of 2400 ice molecules per impact event as determined by a quartz crystal microbalance. As a consequence of this high yield, we show that it is possible to produce molecular depth profiles of the top several hundred nanometers below the ice surface without destruction of the molecular ion signal by accumulation of beam-induced chemical damage. Similar profiles are reported for desorbed neutral molecular fragments by utilizing a high-power femtosecond-pulsed laser for photoionization. While this type of information could not be achieved using atomic projectiles, it is possible to remove the damage induced by such projectiles by subsequent cluster bombardment. These experiments are particularly important for organic surface analysis since they suggest that cluster ion probes may successfully be employed to remove overlayers that may mask the desired molecular information in static secondary ion mass spectral analysis.