Secondary ion mass spectrometry measurements under ambient and humid conditions using MeV ions

Abstract

Secondary ion emission with MeV heavy ions provides a unique opportunity for insight into ion collisions and material analysis. MeV-energy ion beams excite electrons near the surface and enhance the ionization of high-mass molecules, enabling the acquisition of secondary ion mass spectrometry (SIMS) spectra of ionized molecules with suppressed fragments. The SIMS technique with MeV-energy heavy ions (MeV-SIMS) has opened new possibilities for investigating the chemical composition and structure of organic and biological materials, as well as for their imaging. The ambient analysis system is essential for analyzing solid–liquid interfaces, because liquid materials evaporate easily in a vacuum. It is difficult to measure volatile liquid (wet) samples using conventional SIMS. 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. By contrast, evaporation of liquid materials in He at atmospheric pressure is suppressed and samples containing liquid can be measured using the MeV-SIMS technique without the need for preparing dry samples. Recently, the authors developed a humidity-controlled ambient MeV-SIMS system because the water evaporation rate strongly depends on humidity. To obtain imaging mass spectra of wet samples, the wet sample surface needs to be maintained under ambient and humid conditions for a prolonged time. At 0.04 MPa, under 100% humidity, a sample of aqueous solution of benzoic acid produced water droplets that remained in the target chamber for more than 30 min, and clusters and compounds of benzoic acid with water were detected with high sensitivity by the humidity-controlled ambient MeV-SIMS system.