Strong-field ionization applied to sputtered neutral molecules from varied chemical environments

Open Access
Lerach, Jordan Oswald
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
March 05, 2015
Committee Members:
  • Nicholas Winograd, Dissertation Advisor
  • Barbara Jane Garrison, Committee Member
  • John B Asbury, Committee Member
  • Matthew Scott Fantle, Committee Member
  • SIMS
  • Mass Spectrometry
  • Laser post ionization
  • depth profile
  • C60
  • secondary ion
  • secondary neutral mass spectrometry
  • laser
The advent of cluster ion sources and improved function of time-of-flight mass spectrometers has helped usher in the use of secondary ion mass spectrometry (SIMS) for exceedingly complex biological samples, however further advancements are needed to enable viable quantitative analysis of in situ biomolecules. Specifically, ionization matrix effects due to the local chemical environment makes quantitation with SIMS very difficult. Similarly, even qualitative measurements may be hindered due to ionization enhancement or suppression as an effect of the local chemical environment. The use of laser post-ionization (LPI) for the ionization of sputtered neutrals enables SIMS ionization mechanisms to be decoupled from the sputtering event. The LPI scheme used in this research utilizes an ultra-short pulse of high power laser radiation in the infra-red region to photoionize sputtered neutral molecules which have been released from the sample’s surface following primary ion bombardment. In this process, the SIMS sputtering event is decoupled from the SIMS ionization event. For samples where matrix effects are present, observing neutral species via photoionization may provide more quantifiable information. The ionization process during LPI analysis creates ions which are not determined by the local chemical environment. Employing LPI concurrently with SIMS experimentation affords researchers the ability to negate matrix effects and acquire data which more accurately depicts the chemical environment, particularly for samples whose SIMS ionization mechanisms are affected by matrix effects. Research is presented which utilizes various approaches to 1) understand the effect of local chemical environment for the formation of SIMS molecular ions, 2) how LPI can be used in conjunction with SIMS to increase sensitivity and overcome matrix effects associated with SIMS.