C o M e C S - Project

Comet and Meteorite Materials - Studied by Chemometrics of Spectroscopic Data

COSIMA TOF-SIMS Principle

The TOF-SIMS used >99.9% isotope-pure indium ¹¹⁵In for the production of primary ions (energy 8 keV, 3 ns pulses with about 1000 ions, repetition rate 1.5 kHz). The area (footprint) hit by the primary ion beam was about 35 mm × 50 mm (full width at half maximum intensity).

The secondary ions were extracted by 2 kV into a 54 cm drift tube at 1 kV; a two-stage ion reflector compensated for varying kinetic energies of ions with identical mass-to-charge ratios (m/z). After passing a 51 cm drift tube the ions were counted by a detector and the flight-time measured with a time resolution of 1 ns. A typical mass spectrum required approximately 2.5 minutes acquisition time with 225,000 primary ion shots.

The mass-to-charge ratio m/z is related to the flight time t by t = a + b (m/z)^0.5; instrumental parameters a and b define the mass calibration. The mass resolution was about m/Dm = 1400 at mass m/z 100 (Dm is the full width at half maximum peak height), which allows a separation of elemental ions from H-rich CHNO ions of the same nominal (integer) mass up to about mass 100.

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