Time-of-flight mass spectrometry with KeV and MeV projectiles using coincidence counting

Abstract

The goal of this study was to explore sample-projectile interactions by examining secondary ion (SI) yields in an event-by-event impact and detection mode. Cluster secondary ion mass spectrometry (SIMS) was evaluated in comparison to plasma desorption mass spectrometry (PDMS) and monoatomic SIMS. A cluster ion source was designed based on the spontaneous desorption (SD) process. The parameters affecting the SD were studied to optimize the cluster source performance. These factors were extraction grid vs. aperture, aperture size, source bias, source distance, and time. Under optimized SD conditions signal-to-noise ratios (S/N) up to approximately 27 were obtained for the production of CsI cluster ions. The ions of interest from the source were I' and (CsI)[n]I' clusters (where n is the cluster order). For our source a 3 mm gridless aperture 8 mm from the source foil produced the highest signal-to-noise ratio at 18 kV on the source. A signal-to-noise maximum was reached after the sample was biased for 12 hours. The SI yields from organic targets were compared for monoatomic keV ions, MeV ions (252Cf fission fragments), and polyatomic keV ions. The energy range of the clusters used was 18 to 27 keV. This comparison was made by calculating secondary ion yields for the molecular ions from a given sample. The organic samples used were epinephrine, γ-aminobutyric acid, phenylalanine, and valine. For cluster bombardment secondary ion yields ranged from 0.53% for epinephrine to 3.45% for phenylalanine. These yields are less than the molecular ion yields from PDMS which ranged from 3.09% for epinephrine to 7.00% for phenylalanine. The secondary ion yields for monoatomic bombardment were 0.05% for epinephrine and 0.29% for phenylalanine. The higher yields observed with cluster primary ions are attributed to an enhancement effect related to the number of constituents in a cluster...