Enhancing Secondary Ion Yields in Time of Flight-Secondary Ion Mass Spectrometry Using Water Cluster Primary Beams

Abstract

Low secondary ion yields from organic and biological molecules are the principal limitation on the future exploitation of time of flight-secondary ion mass spectrometry (TOF-SIMS) as a surface and materials analysis technique. On the basis of the hypothesis that increasing the density of water related fragments in the ion impact zone would enhance proton mediated reactions, a prototype water cluster ion beam has been developed using supersonic jet expansion methodologies that enable ion yields using a 10 keV (H2O)1000+ beam to be compared with those obtained using a 10 keV Ar1000+ beam. The ion yields from four standard compounds, arginine, haloperidol, DPPC, and angiotensin II, have been measured under static+ and high ion dose conditions. Ion yield enhancements relative to the argon beam on the order of 10 or more have been observed for all the compounds such that the molecular ion yield per a 1 μm pixel can be as high as 20, relative to 0.05 under an argon beam. The water beam has also been shown to partially lift the matrix effect in a 1:10 mixture of haloperidol and dipalmitoylphosphatidylcholine (DPPC) that suppresses the haloperidol signal. These results provide encouragement that further developments of the water cluster beam to higher energies and larger cluster sizes will provide the ion yield enhancements necessary for the future development of TOF-SIMS.

Matrix effects in biological SIMS using cluster ion beams of different chemical composition

“…8,9,[18][19][20] The results indicate that in the singlecomponent samples, the ionization of both molecules is enhanced with the (H 2 O) n þ ion beam, particularly with larger water clusters (Fig. 2).…”

Reduce the matrix effect in biological tissue imaging using dynamic reactive ionization and gas cluster ion beams

“…They have shown orders of magnitude improvement in ionization efficiency as well as reduced matrix effects associated with proton transfer processes. 6 Other types of projectiles have been examined with varying degrees of success. Our strategy is to utilize Ar GCIB's mixed with a low concentration of reactive small molecules, inducing chemical ionization directly at the point of impact of the projectile.…”

Comparing C60+ and (H2O)n+ clusters for mouse brain tissue analysis

“…[14] Another approach proposed the use of a water vapour electrospray beam as the primary ion beam to explore its effect on secondary ion yields. [15] In our lab in Manchester, a prototype water cluster ion beam developed by Ionoptika Ltd has been investigated and was successfully applied to the analysis of standard compounds as shown by Sheraz et al [16] Herein, we discuss the application of this newly developed water cluster ion beam prototype for mouse brain tissue analysis and its effect on ionisation efficiency. The principles of its operation can be found elsewhere.…”

“…The principles of its operation can be found elsewhere. [16] Experimental A whole mouse brain was obtained using ethically approved procedures from the Faculty of Life Sciences, the University of Manchester. It was sectioned in a Cryo-Microtome (Wolfson Molecular Imaging Centre, The University of Manchester, UK) at À20°C.…”