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Realised by ALMS™
developer of the AIDS-HIV Reference project
Abstract No.: MoP-055
Session: Fundamentals: Theory and Experiment
Presentation date: Mon, Aug 28, 2006
Presentation time: 09:50 – 11:20

Electrospay Droplet Impact: A New High-sensitive Matrix-free SIMS

Kenzo Hiraoka1, Daiki Asakawa1, Kunihiko Mori1

1 University of Yamanashi, Kofu, Japan

Correspondence address: Kenzo Hiraoka, University of Yamanashi, Clean Energy Research Center, Takeda-4, Kofu, 400-8510 Japan.

Keywords: Collision(s), Surface; Ionization, Desorption; Ionization, Surface; SIMS.

Novel aspect: Electrospray droplet impact is a soft and high-sensitive method. This technique may merge as a versatile surface-analysis method in many applied field.

 

Introduction
Enhanced secondary ion yields result by increasing the mass of the incident projectile (i.e., "cluster ion sources" are more effective at desorbing molecules). Massive-cluster impact, which utilizes multiply charged glycerol clusters, satisfies the above features and also can afford soft desorption conditions for peptides and proteins. The subject of this paper is the electrospray droplet impact (EDI) ionization technique. EDI is much simpler and robust in operation compared to massive-cluster impact.

Experimental Method*
The charged liquid droplets formed by electrospraying 1 M aqueous acetic acid are sampled through an orifice with a 400 micrometer diameter into the first vacuum chamber, transported into a quadrupole ion guide, and accelerated by 10 kV after exiting the ion guide. The electrospray droplets impact on a dry solid sample that is deposited on a stainless steel substrate. The secondary ions formed are transported into a second quadrupole ion guide and mass-analyzed by an orthogonal TOF-MS.

Results
EDI is found to be applicable to almost all kinds of compounds such as peptides, proteins, sugars, synthetic polymers, (endohedral metal) fullerenes, polycyclic aromatic compounds, drugs, pigments, etc. without using matrices. EDI can sputter only a few monolayers of the top-surface sample film without damaging the sample underneath. Dry 10 fmol gramicidin S deposited on the SUS target could be detected for longer than 30 min. EDI is totally contamination-free because it utilizes the volatile projectile composed of mainly water. One of the characteristic features of EDI is its ability to generate negative ions as well as positive ions. For example, about equal abundances of C60+ and C60- ions for C60, and (Arg+H)+ and (Arg-H)- for arginine can be observed. These results suggest that the ionization mechanism is cooperative, i.e., 2C60 = C60+ + C60-. The velocity of the projectile is about 14 km/s, which is approximately one order of magnitude larger than the velocity of sound in solid. With this projectile velocity, the surface collision takes place in a time range of 10-13 to 10-14 s. Owing to the occurrence of collision limited in sub picosecond time domain, the sudden extremely high pressure is realized. Such a collective and coherent collision results in the formation of phonon-dressed excited states that leads to electronic excitation, ionization, and desorption of molecules in the colliding interface. EDI is promising for the direct analysis of biological samples because peptides can be detected with the presence of many orders of magnitude higher concentrations of salts.