Introduction to SHRIMP-RG
The Stanford-USGS SHRIMP-RG was built by Australian Scientific Instruments following the design of a prototype instrument for ANU. The instrument was delivered on April 10, 1998. Ten hours after delivery, the main components were in place and three days later the vacuum pumps were turned on. Within two weeks of delivery, an ion beam was focused into the collector.
The SHRIMP-RG ion microprobe is cooperatively managed by Stanford University and the U.S. Geological Survey. It is a large format sector mass spectrometer that utilizes a single collector EPT® discrete-dynode electron multiplier operated in pulse counting mode and an off-axis Faraday cup. The SHRIMP-RG incorporates a completely different mass analyzer to that used on the SHRIMP II models, or other large format ion microprobes (e.g., CAMECA 1270 or 1280). The SHRIMP-RG is a double-focusing secondary ion mass spectrometer (SIMS), which consists of an electrostatic analyzer (ESA) and a magnetic sector (see figure below). The purpose of the ESA is to remove velocity dispersion from the mass filtered beam by producing an equal and opposite dispersion to that produced in the magnet. In other words, the magnet produces momentum dispersion and we only want mass dispersion. The double-focusing refers to the refocus of the ion beams of a single mass without any dispersion from the angular trajectory of the ion beams or the velocity of the ions. When the ESA precedes the magnet, it is referred to as forward geometry. Thus, magnet before ESA is reverse geometry.
In a reverse geometry mass spectrometer, mass separation occurs relatively early in the beam path (magnet) and so only one mass is passed through to the collector. Hence, multiple collection is not an option on the SHRIMP-RG. However, because the RG design is only transmitting a single mass ion beam, the abundance sensitivity (the degree to which scattered ions interfere with the peak of interest) is much reduced because any neighboring intense ion beam is rejected well out of sight of the collector. Based on this design, the SHRIMP-RG provides higher mass resolution than the standard forward geometry of the SHRIMP I and II (Clement and Compston, 1994), as well as other large-format ion microprobe instruments. The SHRIMP-RG geometry allows measurements to be performed without the need to energy-filter the secondary beam, an approach necessary for smaller mass spectrometers to improve the mass resolving power but which reduces the transmission of secondary ions by 50 to 90% and sensitivity of the instrument.
The SHRIMP-RG can be operated with maximum secondary ion transmission at a mass resolutions, of M/deltaM = 7500-8500 (10% peak height), which fully separates interfering molecular species for Pb, U, and Th isotopes. Mass resolution is also referred to as mass resolving power (MRP). Application of trace element measurements are typically performed with a mass resolutions of M/deltaM = ~10,000-15,000 (10% peak height), which fully eliminates interfering molecular species, particularly for 45Sc, 48Ti, and REE. Heavy rare earth elements (HREE) are often measured as oxides because the metal ions contain isobaric interferences that cannot be fully resolved, and which are not present for the oxide at higher mass. This higher mass resolutions can be achieved while retaining 75-50% of the secondary ion transmission. Analyses of isotopes like 93Nb in zircon requires mass resolution of ~14,325 (10% peak height) to fully resolve it from 92Zr1H, and therefore, requires further reduction of the secondary ion-beam by closing defining slits or utilizing energy filtering to help minimize isobaric interferences (see mass-scan figure below). Because the measurements are being performed on an electron multiplier, the backgrounds for most measurements are very low (<0.05 cps), resulting in a very high signal to noise ratio (generally »1000).
Spatial Resolution and Sensitivity
The SHRIMP-RG ion microprobe uses a focused beam of primary ions (typically O-, O2-, or Cs+) to sputter secondary ions from the target material. The primary beam can be focused to a diameter of approximately 10 to 40 microns. The analytical pit depth varies depending on the composition of the sample material (i.e., sputtering efficiency) and the duration of the analysis.
In general, pit depth are on the order of 1 to 5 microns. Compared to the volume of material analyzed by laser ablation pits, ion-probe analyses typically sample an order of magnitude less material. A typical zircon U-Pb analysis on the SHRIMP-RG may take 10 to 20 minutes, and generates sputtered pit that is 25 microns in diameter, and 2 microns deep (1-2 picograms). Due to the high spatial resolution of ion microprobe analytical spots, the SHRIMP-RG is best suited for in-situ analyses. The sputtering depth of the SHRIMP permits analyses of minerals within thin sections, or depth profiling to target sub-micron scale features on mineral surfaces.
During the course of an ion-probe analysis, primary ions are embedded into the sample surface, changing the chemical structure at the nano- to femto-meter scale. Secondary ions sputtered from the samples surface, which include metal ions (e.g., Zr+, Nb+), molecular ions (e.g., ZrO+, ZrH+), multiply charged ions (e.g., Zr++), as well as secondary electrons and ions from the primary beam (O-). The high-mass resolution of the SHRIMP-RG allows most isobaric interferences (different ions or molecules with similar mass-to-charge ratio) to be easily resolved, allowing analyzed in-situ without the need for chemical separation or purification. Although sputtering is a relatively in-efficient process in terms of extraction of secondary ions, the SHRIMP-RG is highly sensitive to ions that are accelerated into the mass spectrometer and reach the pulse-counting electron multiplier detector.
For an excellent review of SIMS instruments, including SHRIMP-RG, can be found in Ireland, (2014).
Another excellent paper comparing U-Th-Pb geochronology by ID-TIMS, SIMS, and LA-ICPMS is in Schaltegger et al., (2015).