Sensitive High Resolution Ion MicroProbe - Reverse Geometry

Our SHRIMP RG incorporates a completely different mass analyzer to that used on the SHRIMP II models. So, what is reverse geometry, or for that matter, forward geometry?

Double-focusing mass spectrometers consist of an electrostatic analyser (ESA) and a magnetic sector. 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.

There are advantages and disadvantages for both geometries. In a reverse geometry mass spectrometer, mass separation occurs relatively early in the beam path 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 analyser 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.

Perhaps the biggest advantage of the SHRIMP RG is the larger magnet dispersion afforded by the reverse geometry. Given similar source and collector slits, SHRIMP RG should yield four times the mass resolution of the SHRIMP FG designs with the same sensitivity. The SHRIMP RG is based on a Matsuda (1990) design. Comparisons for various mass spectrometers of two figure of merit are given below.

C/X is the ratio of dispersion to magnification; the desired attribute for the mass spectrometer is to have as much dispersion as possible without magnifying the image. For example, any of the mass spectrometers above could separate two peaks by 2 cm (dispersion) but they will have very different mass resolutions. AF is the aberration limted transmission factor. Basically this is the ultimate resolution achievable with infinitely narrow (or at least 1 ion wide) slits. This is not an achievable goal because the transmission would be totally compromised. For the SHRIMP RG design, the dispersion is 3.0 x 10e6 microns and the magnification is 0.32. Hence, if the source slit is 100 microns, it will appear to be only 32 microns across at the collector.

The SHRIMP RG schematic shows the same primary column and source chamber as the SHRIMP II design, but a completely different mass analyzer. Upon exiting the source chamber, the secondary ion beam is spread horizontally by two quadrupole lenses (Q1, Q2) in the QQH chamber. The action of the quadrupoles is to compress in the vertical direction and diverge in the horizontal (as looking down the beam line). If only one quadrupole was used, the beam would diverge and all beam transmission would be lost; the pair of lenses is used such that a crossover is produced between them and the divergence is halted by the second of the quads (see the schematic of the vertical profile in the bottom of the figure). The beam is passed into the 46° magnet producing a momentum spectrum which can be imaged at a scannable slit in the beginning of the ESA. Immediately before the "energy" slit is a third quadrupole lens which is used in a similar position on the SHRIMP II design. After the ESA, is a fourth quadrupole which simply acts as a projector lens to magnify the image produced at the collector.

Steve Clement commenced construction-design for the SHRIMP RG in 1993 and fabrication of the ANU prototype began in 1995. A second SHRIMP RG was built in parallel for the Stanford - USGS consortium by Australian Scientific Instruments following the placement of an order in late 1994. The SHRIMP RG was delivered on April 2, 1998 and installed over the following ten days.