An electrostatic quadrupole doublet focusing system for MeV heavy ions in MeV-SIMS

Abstract

The importance of imaging mass spectrometry (MS) for visualizing the spatial distribution of molecular species in biological tissues and cells is growing. In conventional SIMS with keV-energy ion beams, elastic collisions occur between projectiles and atoms in constituent molecules. The collisions produce fragments, making acquisition of molecular information difficult. In contrast, MeV-energy ion beams excite electrons near the surface and enhance the ionization of high-mass molecules, hence, fragment suppressed SIMS spectrum of ionized molecules can be obtained. This work is a further step on our previous report on the successful development of a MeV secondary ion mass spectrometry (MeV-SIMS) for biological samples. We have developed an electrostatic quadrupole doublet (EQ doublet) focusing system, made of two separate lenses, Q1 and Q2, to focus the MeV heavy ion beam and reduce measurement time. A primary beam of 6MeV Cu4+ was focused with this EQ doublet. We applied 1120V to the Q1 lens and 1430V to the Q2 lens, and the current density increased by a factor of about 60. Using this arrangement, we obtained an MeV-SIMS image of 100×100pixels of cholesterol-OH+ of cerebellum (m/z=369.3) over a 4mm×4mm field of view, with a pixel size of 40μm within 5min, showing that our EQ doublet reduces the measurement time of current imaging by a factor of about 30.