<i>In Situ</i> Geochemical Analysis and Age Dating of Rocks Using Laser Ablation-Miniature Mass Spectrometer
- Thursday, 01 March 2012
Instrument offers a more quantitative assessment of elemental composition than techniques that detect laser-ionized species produced directly in the ablation process.
A miniaturized instrument for performing chemical and isotopic analysis of rocks has been developed. The rock sample is ablated by a laser and the neutral species produced are analyzed using the JPL-invented miniature mass spectrometer. The direct sampling of neutral ablated material and the simultaneous measurement of all the elemental and isotopic species are the novelties of this method.In this laser ablation-miniature mass spectrometer (LA-MMS) method, the ablated neutral atoms are led into the electron impact ionization source of the MMS, where they are ionized by a 70-eV electron beam. This results in a secondary ion pulse typically 10–100-μs wide, compared to the original 5–10-ns laser pulse duration. Ions of different masses are then spatially dispersed along the focal plane of the magnetic sector of the miniature mass spectrometer and measured in parallel by a modified CCD (charge-coupled device) array detector capable of detecting ions directly. Compared to conventional scanning techniques, simultaneous measurement of the ion pulse along the focal plane effectively offers a 100% duty cycle over a wide mass range. LA-MMS offers a more quantitative assessment of elemental composition than techniques that detect laser-ionized species produced directly in the ablation process because the latter can be strongly influenced by matrix effects that vary with the structure and geometry of the surface, the laser beam, and the ionization energies of the elements. The measurement of high-precision isotopic ratios and elemental composition of different rock minerals by LA-MMS method has been demonstrated. The LA-MMS can be applied for the absolute age determination of rocks. There is no such instrument available presently in a miniaturized version that can be used for NASA space missions. Work is in progress in the laboratory for geochronology of rocks using LA-MMS that is based on KAr radiogenic dating technique.
Besides the MMS that has a double focusing focal plane geometry and a modified-CCD array ion-detector, the major components of the instrument include the laser, pumping, and its housing subsystem. These have been miniaturized for other NASA missions and when combined with the LA-MMS, make the instrument deployable on a rover or lander.