Michael J Eller oral presentation (PB1-Tue4-1-3)
Secondary Ion Mass Spectrometry with Individual Projectiles: A Complete Mass Spectrum from a Single Impact?
Texas A&M University, Department of Chemistry, TX 77843-3144 College Station, United States
Secondary ion mass spectrometry, SIMS, with individual projectiles is a variant of SIMS where instead of a beam of projectiles, projectiles are separated in time and space. This allows for the ejecta from each projectile impact to be collected and mass analyzed. As each projectile emits ions from a nano-volume, this allows for nano-scale mass spectrometry analysis, and allows to test nano-scale molecular homogeneity. For this to be practical multiple ions must be emitted by each impact. We describe here a novel projectile Au2800+8 for use in SIMS with individual projectiles, which generates more than 30 ions on average per projectile impact, enabling nano-scale characterization. The gold clusters, e.g. Au2800+8, are generated by a gold liquid metal ion source, LMIS, and are mass selected using a Wien filter, and accelerated to 120 qkV. The primary ion beam is pulsed to ~1000 Hz and a time of flight, TOF, measurement electronically selects the projectile based on the emission/detection of e- or H+ from each impact. The secondary ions are mass analyzed with a reflection TOF mass analyzer and collected with a multi-anode microchannel plate based detector. Thus, for each impact event a mass spectrum is collected and stored.
A set of neat samples (glycine, phenylalanine, cysteine, cholesterol and gramicidin S) deposited onto silicon were analyzed with 520 keV Au400+4, and 1040 keV Au2800+8. From each sample the number and type of secondary ions emitted per projectile impact was recorded. It was found that with impacts of 1040 keV Au2800+8 on glycine ~4.4 molecular ions are detected per projectile impact, an increase of >2X compared to 520 keV Au400+4. Correcting for the transmission of the mass spectrometer, corresponds to ~14.9 molecular ions ejected per impact. For Gramicidin S the yield of the molecular ion increased ~3X. In all cases the emission of molecular species was promoted over fragment ions. For impacts of 1040 keV Au2800+8 on glycine, on average 30 ions were detected per impact, allowing for a nearly complete mass spectrum to be collected from a single impact. A mass spectrum from a single impact corresponding to the most probable emission event (containing 30 ions) is shown in figure 1. Here 6 characteristic fragment species (4 CN- and 2 CNO-) and 6 Glycine-H- ions were detected. Work supported by NSF grant CHE-1308312.