Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Sensitive high resolution ion microprobe

The sensitive high resolution ion microprobe (SHRIMP) is a large-diameter, double focusing secondary ion mass spectrometer (SIMS) instrument produced by Australian Scientific Instruments in Canberra, Australia. The SHRIMP microprobe uses a beam of ions to produce secondary ions, which are separated by mass and energy by the mass spectrometer and measured by a collector.

Applications
The SHRIMP is primarily used for geological and geochemical applications. It can rapidly measure the isotopic and elemental abundances in minerals at a scale as small as 5 μm and is therefore particularly well-suited for the analysis of complex minerals, as often found in metamorphic terrains, some igneous rocks, and detrital minerals in sedimentary rocks. The most common application of the instrument is in U-Th-Pb geochronology, although the SHRIMP can be used to measure other isotopic and elemental abundances. The second generation instrument (SHRIMP II) is currently commercially produced by Australian Scientific Instruments in Canberra, in association with the Australian National University.

Isotope dating
For U-Th-Pb geochronology a beam of primary ions (O2-) are mass analyzed, collimated and accelerated towards the target, and used to sputter “secondary” ions from the sample. These secondary ions are accelerated along the instrument where the various isotopes of uranium, lead and thorium are measured successively, along with reference peaks for Zr2O+, ThO+ and UO+. Since the sputtering yield differs between ion species and relative sputtering yield increases or decreases with time depending on the ion species (due to increasing crater depth, charging effects and other factors), the measured relative isotopic abundances do not relate to the real relative isotopic abundances in the target. Corrections are determined by analysing unknowns and standard material (matrix-matched material of known isotopic composition), and determining an analytical-session specific calibration factor.

SHRIMP Instruments around the World
Fifteen SHRIMP instruments have been installed, primarily in Australia and Asia. Six instruments are in Australia and three of these are at the Australian National University.