Ambient Analysis of Liquid Materials with Wet-SIMS

(1) Department of Nuclear Engineering, Kyoto University, Uji, 611-0011 Kyoto, Japan.
(2) Quantum Science and Engineering Center, Kyoto University, Uji, 611-0011 Kyoto, Japan.
(3) Department of Electronic Science and Engineering, Kyoto University, Nishikyo, 615-8510 Kyoto, Japan.

A measurement system that operates at ambient conditions is required for the analysis of biological samples and solid–liquid interfaces, because the samples contain relatively large amounts of volatile materials, such as water, alcohols, and fatty acids. Particle induced X-ray emission (PIXE) has been used to analyze such samples in air, but this technique does not provide molecular information. Recently, an ambient secondary ion mass spectrometry (SIMS) system has been developed, but volatile liquid (wet) samples are difficult to measure using conventional SIMS, because samples must be dried and introduced into a high vacuum chamber. The mean free path of ions with energy in the keV range is very short in low vacuum and these ions cannot penetrate the surface. In contrast, wet samples can be measured using heavy ions in the MeV-energy range without dry sample preparation because of the high transmission capability of these ions at low vacuum pressure. MeV-energy ion beams also excite electrons near the surface and enhance the ionization of high-mass molecules and thus fragment-suppressed SIMS spectra of ionized molecules can be obtained. We have developed an ambient analysis system with secondary ion mass spectrometry for wet samples (Wet-SIMS) that operates from low vacuum to atmospheric pressure using MeV-energy heavy ion beams. The system is equipped with fine nozzles that avoid vacuum degradation at both the primary beam incidence side and the secondary ion measurement side, even when the target chamber is filled with gaseous He at atmospheric pressure. Water evaporation was suppressed in He at atmospheric pressure and a solution of benzoic acid could be measured using MeV-energy heavy ions without dry sample preparation.