TOF-SIMS Machine and software.Presentation Transcript
Fragmentation of Organic Molecules Desorbed from a Surface using Secondary-Ion Mass Spectrometry Alger Pike Nicholas Winograd
Secondary-Ion Mass Spectrometry SIMS
Imagine shooting a bullet into sand.
Shrink the bullet and sand to atomic scale.
SIMS in Combinatorial Chemistry An optical image of a bead-holder specially fabricated for 60 m polystyrene beads (capacity: 10,000 beads/cm 2 ) The TOF image of Sasrin-Biotin beads loaded into the bead-holder. The molecular mass of biotin 245.3 amu is shown in green. R. M. Braun, A. Beyder, J. Xu, M. C. Wood, A. G. Ewing and N. Winograd, Anal. Chem. 71, 3318 (1999).
Why Study Fragmentation?
To learn what factors contribute to fragmentation patterns seen in SIMS spectra
To establish control over these factors in order to allow easier interpretation of spectra and assay of complex organic mixtures
To expand applications for imaging SIMS of organic molecules – i.e. screening combinatorial libraries
Types of Fragmentation 1) Direct Desorption 2) Desorb then Fragment 3) Ion-Beam Induced
The ARTOF-SIMS Machine
ARTOF-SIMS Sample Manipulator a) Azimuthal Rotation: Sample rotates like the hands of a clock, around the center of the crystal. b) Polar Rotation: Sample rotates like a revolving door, around the axis perpendicular to the azimuthal axis.
Simulation vs. Real
Easy to use Graphical User Interface (GUI)
Most common controls on main window
Redundancy for heavily used features
High-speed data throughput
Optimized I/O is ~100 times faster
Device driver allows application level I/O
real-time Windows NT/2000
Interrupt driven / multithreaded code
High level of automation
“ Go button” ease of use
CD 3 S-Cu (100): The Model System 1) Chemisorption to a four-fold hollow 3) No domains 2) No tilt angle due to short R group 4) Simplified mass spectra Imanishi, S. Takenaka, T. Yokoyama, Y. Kitajima and T.Ohta J. PHYS. IV FRANCE 7 (1997)
FCC (100) Surface 1) Red atom is an ejecting atom 2) Blue atoms are blocking atoms 3) SIMS yield decreases with blocking
Azimuthal Distribution of Cu(100) 50 0
1200 L CD 3 SH on Cu(100) High Mass
1200 L CD 3 SH on Cu (100) Low Mass
Overall Fragmentation of CD 3 SH (total fragments signal) (total monolayer signal) F =
Direct Desorption Fragmentation Low energy cascade that desorbs an ion which does not have enough energy to fragment.
Direct Desorption Fragmentation
Fragmentation After Desorption High energy cascade leads to desorbed species which have enough energy to further fragment.
Fragmentation After Desorption
Ion-Beam Induced Fragmentation The ion beam directly breaks a bond which causes desorption of the fragment into vacuum.
Ion-beam Induced Fragmentation (fragment signal) (total monolayer signal) C =
Two-Body Model Simulation
Ion Trajectories at 35 º Incidence Angle
Specialized TOF-SIMS machine built
Designed with computer to optimize parameters
Fully automated: computer manages the system
Fragmentation shows orientation effects
CuM + yield is higher with normal primary ions
Primary ion interactions create fragmentation
Three distinct fragmentation mechanisms
Direct desorption of CuM +
CD 3 + metastable decay of high energy M +
C + and D + fragments created by primary ions
Winograd and Garrison Groups
ONR, NIH, NSF
Further Evidence for Cu 2 SCD 3 + Ejection
No SCD 3 + ions seen in SIMS
Small amounts of CuSCD 3 + are seen
Cu 2 + itself is formed by recombination i.e. there are no ejected Cu dimers
Peak width of Cu 2 SCD 3 + is consistent with dimer formation not trimer
Further Evidence for CD 3 + Formation XSCD 3 + XS + CD 3 +