Paper AS-TuP3
Estimation of Useful Yield of Electrospray Droplet Impact/Secondary Ion Mass Spectrometry

Tuesday, October 30, 2012, 6:00 pm, Room Central Hall

The electrospray droplet impact secondary ion mass spectrometry (EDI/SIMS) has been developed as cluster SIMS[1]. EDI utilizes charged water droplets generated by ambient electrospray. The typical droplet is represented as [(H₂O)_90,000 + 100H]⁺ with mass of ～10⁶ u. The kinetic energy of droplets is about 10⁶ eV with the velocity of 12 km/s. EDI/SIMS has the atomic/molecular level etching abilities[1][2]. EDI/SIMS was found to be applicable to many kinds of inorganic and organic material [1][3]. In spite of the shallow surface etching, relatively high secondary ion yields can be obtained by EDI.

The high ionization efficiency may be due to the occurrence of supersonic collision taking place between the droplet and the sample surface. In order to estimate the useful yield (i.e., total ions generated divided by the total molecules desorbed), mass spectra were measured for binary mixtures of C₆₀/Rhodamine B (1:1) and C₆₀/Aerosol OT (1:1). The equimolar samples were crushed and mixed in mortar and deposited on a stainless steel target as thin films using a spatula. This method was adopted from solvent-free MALDI. By assuming that (1) the desorption efficiency is the same for C₆₀, Rhodamine B and Aerosol OT, and (2) the desorbed ionic compounds directly give secondary ion signals, the useful yield was crudely estimated to be ～0.1. This high value explains the high ionization efficiency of EDI/SIMS.

Reference

[1] K. Hiraoka, D. Asakawa, S. Fujimaki, A. Takamizawa, K. Mori, Eur. Phys. J. D38, 225 (2006)

[2] D. Asakawa, K. Mori, K. Hiraoka, Appl. Surf. Sci. 255 (2008) 1217

[3] K. Hiraoka, K. Mori, D. Asakawa, J. Mass. Spectrom. 41 (2006) 894