Analytical Sciences

Detailed Investigation on the Possibility of Nanoparticles of Various Metal Elements for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Tetsu YONEZAWA,* Hideya KAWASAKI,** Akira TARUI,** Takehiro WATANABE,** Ryuichi ARAKAWA,** Toshihiro SHIMADA,* and Fumitaka MAFUNE***

*Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo, Tokyo 113-0033, Japan
**Department of Applied Chemistry, Faculty of Engineering, Kansai University,3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan
***Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan

In this paper, we describe systematic detailed considerations of the feasibility of using various metal nanoparticles for organic-matrix-free surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). In order to avoid the influence of organic molecules on the nanoparticles, stabilizer-free bare nanoparticles of Ag, Au, Cu and Pt were prepared by laser ablation. Although all metal nanoparticles absorbed N₂ laser light (337 nm) energy, the performance of desorption/ionization of a representative peptide, angiotensin I, strongly depended on the metal element. Citrate buffer was used as a proton source; it reduced the amount of alkali cation adducts present. Then, protonated molecules of analytes predominated in the mass spectra when Au and Pt nanoparticles were used. Pt nanoparticles showed the highest performance in SALDI-MS, owing to their smaller heat conductivity and higher melting temperature. The selective desorption of a cationic surfactant with longer alkyl chains and a peptide with methionine was also observed.

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