Development and experimental application of a gold liquid metal ion source
Introduction
Effective high spatial resolution surface analysis of molecular materials by TOF-SIMS, particularly in the bio-sciences area requires that the secondary ion yield of molecular species be optimised. It has been known for some time that polyatomic or “cluster” primary ions can enhance the yield of secondary ions, when compared to monatomic projectiles [1], [2]. Over 10 years ago the Orsay group demonstrated that gold cluster ions, Aun+, delivered large non-linear increases in secondary ion yield of molecular ions from organic surfaces such as phenylalanine or fatty acid Langmuir Blodgett films [3], [4]. The origin of the non-linear effect may be partly due to an increase in sputtering due to ‘cooperative’ cascades from the simultaneous impact of the atoms of the polyatomic projectile. However, some significant increase in ion yield is also indicated. The potential benefits of greatly increased molecular ion yield during SIMS imaging of organic and bio-organic materials has stimulated the development of a highly focused liquid metal gold source able to deliver both monatomic Au+ and Aun+ primary ions.
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Experimental
A liquid metal ion source (LMIS) based on a gold germanium eutectic (Au:Ge) has been jointly developed between UMIST and Ionoptika Ltd. The main advantage of the Au:Ge LMIS is its reduced operating temperature (∼400 °C) when compared with a pure gold LMIS (>1000 °C). The benefits are reduced source substrate corrosion and reduced evaporation losses yielding extended source lifetime.
The Au:Ge LMIS was mounted in a modified Ionoptika IOG 25 gun. A primary pulse and beam chopper assembly was
Source characteristics
The Au:Ge LMIS is a rich source of primary ions. Fig. 1 shows those that have been identified on the basis of their flight time through the free-flight region using the pulse/chopper system. For this study, only Au+, Au2+ and Au3+ ions were employed as primary ions. Fig. 2 shows that good yields of 100 ns pulses of Au2+ and Au3+ primary ions are obtainable (about 25% of that of the monatomic Au+ ion).
Gramicidin and PET
The secondary ion spectra of the biological polypeptide gramicidin D and the polymer PET, were
Conclusions
The Au:Ge LMIS has proved to be long lasting (lifetime>1000 μAh) and reliable, and by selecting the appropriate primary ion, is capable of greatly improving molecular ion yields for biological and polymer materials. Future experiments will concentrate on further investigation of polymers, biomolecules and on biological imaging applications.
Acknowledgements
The Authors gratefully acknowledge the financial funding from EPSRC and the Leverhulme Trust.
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