Figure 13
Diagram of the prototype NanoSims50 used in MIMS. The main components of the instrument include: the primary column (PC) with a cesium primary-ion source (IS) used to sputter the surface of the sample and to enhance the yield of negative secondary ions and a series of lenses (L0, L1, L2 and L3) to shape the primary beam; the objective column (OC), where the same ion optic is used in a coaxial manner both to focus the primary beam on the sample (S) and to collect the secondary ions (the primary-ion beam is focused on the sample with the objective lens (EOP) and aperture limited with the diaphragm (D1), the secondary ions are collected with the secondary-ion focusing lens (EOS)); the secondary column (SC) where the secondary-ion beam is shaped to match the acceptance of the spectrometer (the secondary column contains an entrance slit (ES), a corrector (CY) and deflectors (P2 and P3) to center the secondary-ion beam in the entrance slit); an aperture slit (AS) to reduce the angular aberration of the secondary-ion beam; and the mass spectrometer, made up of the association of an electrostatic prism (EP) and a magnetic prism (MP), which enables the focusing of the secondary ions as narrow lines along the focal plane (FP) of the magnet. Chromatic aberrations are minimized with the quadrupole (Q) and the slit lens (LF4). Energy aberration is reduced with the energy slit (WS). The transmission at high mass resolution is improved by correcting the main second-order aperture aberrations with the hexapole (HX) placed in front of the electrostatic prism, tilting the entry and exit faces of the magnetic prism, and focusing in the vertical section using additional lenses placed between the electrostatic and magnetic prisms. The deflectors (C4) help to center the masses in the detectors. In the multi-collection chamber (MC), four detectors can be moved along the focal plane. Each detector is made up of deflection plates (DP) followed by a selection slit (SS) and a miniature electron multiplier (EM). The deflection plates, which permit scanning of a small portion of the spectrum, greatly improve the final tuning of mass lines. This instrument provides both parallel detection and high mass resolution with little sacrifice in secondary-ion transmission from sample to detector. As a particularly striking example, in Figure 15, the 12C- secondary ions were detected at 90% relative transmission with a mass resolution of 1 part in 11,825.