The operating principle of SHRIMP is quite simple. A
high-energy (10kV) beam of ions (usually O2-) is focussed
onto a small area (<30 µm diameter) on the surface of
the target. The ion bombardment erodes (sputters) atoms
and molecules from the target, some of which are
themselves ionised. These secondary ions are gathered
using electrostatic lenses and transfered to a mass
spectrometer, by which they are separated according to
their relative masses. A limiting factor in the accuracy
of all ion probe analyses is the mass spectrometer's
ability to distinguish between (resolve) secondary ions
of atoms and molecules that are extremely similar in mass
(isobars).
In the secondary ion spectra of complex compounds,
isobars with fractional mass differences of <3 x 10-4
are common. The high mass resolution of SHRIMP is
achieved by the use of double-focussing mass spectrometer
(simultaneous energy and mass refocussing) with a very
large turning radius (magnet radius 1 m, electrostatic
analyser radius 1.27 m) The resulting instrument has a
beam line over 7 m long and weighs more than 12 tonnes.
The SHRIMP II Sensitive High Resolution Ion Micro Probe,
developed, manufactured, and marketed by Australian
Scientific Instruments (ASI), is a fully commercial
version of an instrument originally designed for
geological research purposes and constructed by the
Research School of Earth Sciences (RSES) at the
Australian National University (ANU).
The SHRIMP II ion microprobe is a high precision
scientific instrument in the broad category of Secondary
Ion Mass Spectrometers (SIMS). Ion microprobes make in
situ isotopic and chemical 'surface' analysis of
solid targets on the scale of a few thousandths of a
millimeter by bombarding the sample on a very fine scale,
and eliminate the difficult, time-consuming,
labour-intensive and potentially hazardous aspects of
sample preparation for conventional mass spectrometry.
SHRIMP II, like other ion microprobes, obtains material
for analysis by bombarding a sample surface with a finely
focused beam of high energy atomic particles (ions). The
ion bombardments slowly wear away a small portion of the
surface, producing electrically charged atoms and
molecules of the sample that can be separated by mass,
identified and counted by an attached mass spectrometer.
The quality of the instrument can be assessed by its
sensitivity (ability to detect trace elements present in
the target at low concentrations) and mass resolution
(ability to distinguish between ions of very similar
mass). SHRIMP II has the highest sensitivity at high mass
resolution of any commercial ion microprobe.
