Paper AS+BI-TuA12
Fundamental Metrology for Tissue Imaging by SIMS - A Study of Cholesterol and Determination of the Argon Cluster Sputtering Yield

Tuesday, October 20, 2015, 6:00 pm, Room 212D

Secondary Ion Mass Spectrometry (SIMS) has become an invaluable tool to study organic and biological samples. An important biological application is in the analysis of mammalian cellular membranes. Considerable contribution to the field comes with the use of large cluster ion beams, and in recent years the application of argon gas cluster ion beam has emerged as the prevailing method.

Cholesterol, as a key component of nearly all mammalian cell membranes, is of particular interest. It alters the physical properties of the membranes, interacts with neighbouring lipids and proteins and is involved in numerous biomolecular processes. Being able to detect, identify and characterise the distribution of cholesterol in biological samples has vast implications in medical sciences. To do this, we need to underpin the basic metrology involved. It is important to evaluate cholesterol sputtering yields for argon cluster sputtering over a range of energy and cluster sizes so that a general description of the molecule behaviour may be established.

In this study, we compared the use of C₆₀⁺⁽⁺⁾ and Ar_n⁺ as sputtering ions for depth profiling of cholesterol thin films. Films of different thicknesses were prepared by thermal evaporation and the sputtering yields of cholesterol were measured from depth profiles made using 2.5 to 20 keV Ar₁₀₀₀⁺ and Ar₅₀₀₀⁺ and 10 and 20 keV C₆₀⁺⁽⁺⁾ sputtering beams. We show that, at room temperature, the C₆₀⁺⁽⁺⁾ ions caused significant damage but gave a well-behaved depth profile whereas Ar_n⁺ gas clusters left the material undamaged but the very clean layer readily restructured making the profiles much more complex. This restructuring does not occur at room temperature normally but results from the actions of the beams in the sputtering process for profiling in SIMS. The sputtering yields from these restructured films are up to twice that for material not so restructured. Good profiles may be made by reducing the sample temperature. This is likely to be necessary for many lower molecular weight materials (below 1000 Da) to avoid the movement of molecules. The yields for both C₆₀⁺⁽⁺⁾ and Ar_n⁺ fit the universal yield equation [1]. Our results show that considerable differences can occur between the measurements performed with the two ion clusters, affected, in addition, by factors such as sample temperature or exposure to light. These will be discussed.

[1] M.P. Seah, J. Phys. Chem. C, 2013, 117 (24), pp 12622–12632