Our IDLE (Interstellar Dust Laser Explorer) laboratory is home to two time-of-flight secondary ion mass spectrometers (TOFSIMS) which we have built ourselves. Building the instruments ourselves not only saved money but also made it possible to customize the instruments for our needs.
These instruments are able to study the elemental and isotopic compositions of very small samples such as presolar grains found in meteorites, dust collected from comets and solar wind collected by the Genesis mission. Studying these kind of samples allows us to learn more about the different materials found in our Solar System and understand how and where they formed and what sort of conditions they have experienced.
The TOFSIMS technique analyses the composition of solid surfaces by sputtering the surface of the specimen with a focused ion beam and collecting and analysing ejected secondary ions. The IDLE instruments have a primary ion gun to deliver an ion beam onto the sample which is usually pulsed to generate secondary ions on impact. The extracted secondary ions leave the sample surface at the same time and so can be separated by mass according to their time-of-flight (heavy ions with the same energy will travel slower and arrive later than lighter ions).
The time-of-flight analyser has a reflectron for higher mass resolution and an ion counting detector which delivers pulses to a multi-stop acquisition card in a PC. By rastering (scanning) the beam over the sample surface and building up a mass spectrum at each point that the beam hits, we can “map” elemental distribution images and also quantitatively analyze individual regions-of interest.
The ionization of sputtered particles is very low, typically below 1%, making secondary ion mass spectrometry relatively inefficient. To make use of secondary neutrals, one of the two IDLE instruments is additionally equipped with a laser for non-resonant post-ionization of secondary neutrals which enhances the sensitivity for most elements. Using an F2-excimer laser with 157nm wavelength, around 60% of all elements can be directly ionized with a single photon. This acquisition of all secondary ions in parallel makes these instruments very efficient and keeps sample consumption to a minimum which is essential for samples like presolar grains which are only a few microns in size.
Samples have to be quite small, typically below 500micron, and preferably flat to get the best results. Samples are usually prepared under clean conditions to minimize contamination.
For a more detailed description of the original IDLE instrument see Henkel, T. et al.; Review of Scientific Instruments 2007; 78: 055107.