The Cameca NanoSIMS 50L instrument installed in March 2010 at our department provides a recently developed technique of ion microprobe, optimizing SIMS analysis performance at high lateral resolution.
For SIMS, secondary ion mass spectrometry, the sample is continuously bombarded with energetic ions (primary ions), oxygen or cesium in our case. The ions sputter atoms and molecules out of the sample surface. A part of the sputtered atoms is ionized and can be detected with a mass spectrometer, distinguished by their mass to charge ratio.
Oxygen or cesium reactive primary ions are enhancing considerably (by several orders of magnitude compared to non-reactive primary ions) the secondary ions signal (ionization yield).
The Cameca NanoSIMS 50L allows a fine focusing of the primary ion beam. Using the Cs+ source, a spatial resolution down to 50 nm can be achieved, whereas the O- primary ions can be focused down to 150 nm.
The instrument uses a coaxial setup for the primary and secondary ion beam. The advantage of this setup is the high collection efficiency of secondary ions. Due to this design, only oppositely charged secondary ions can be measured.
The secondary ions are guided to a magnetic sector mass analyzer. Their trajectories get curved in the magnetic field according to their mass. The setup allows a high mass resolution, giving the possibility to differentiate between ionic species of the same mass number such as 12C15N- and 13C14N- where the mass difference is only about 0.007 amu.
Up to 7 selected mass images can be recorded simultaneously by 7 detectors with sensitivity in the ppm range, originating from the same sputtered volume of the sample. This enables the comparison of images of the distribution of different measured isotopes (e.g. 12C, 13C, 14N, 15N).
Theoretically masses from 1 amu (1H) to 400 amu can be detected with some constraints given by the configuration of the mass spectrometer: a mass range of factor 21 and a mass separation between adjacent detectors of Mmax/58, where Mmax is the highest mass to be measured.