The LMCM National Ion Microprobe facility is part of the Muséum's Analytical Platform. The core instruments are the Cameca NanoSIMS N50 and IMS 3f ion microprobes. Auxiliary instruments include a JEOL JSM840 SEM equipped with Genesis XM2 micro-analyzer with imaging capabilities as well as facilities for sample preparation, sample coating with Au or C, optical microscopes equipped with digital cameras etc....
SIMS is the acronym for Secondary Ion Mass Spectrometry. The physical basis for ion micro-probe analysis is the ability to perform mass-spectrometry on secondary ions sputtered from a solid target by the impact of a primary beam of charged particles. Secondary ions are sputtered from the top few atomic monolayers of the sample. Therefore, although SIMS is, strictly speaking, a destructive analytical technique, the level of damage to the sample is usually considered negligible. The yield of secondary ions from a given sample depends critically on a number of parameters, such as the chemistry of the sample, the crystal structure and conditions of sputtering. However, in general, SIMS provides high (in some cases extremely high) sensitive for most elements in the periodic table. Detection limits in the ppb range are not uncommon.
The NanoSIMS N50 is an ion microprobe that can deliver a primary beam Cs+ or O- to a sample surface, focused to a spot size of ~50 nanometers and ~150 nanometers, respectively. (On non-conducting materials, bombardment with Cs+ causes strong charging effects. Such positive charge build-ups are compensated by electrons, which can be delivered to the sample surface by an electron gun.) Secondary ions sputtered from the sample surface and charged opposite to the primary beam are transferred with high transmission to the high mass-resolution, multi-collection mass-spectrometer. The NanoSIMS instrument therefore combines high spatial resolution on the sample surface with high mass-resolution mass spectrometry and high analytical sensitivity. Ion images of the sample surface are created by a precisely controlled raster of the primary beam across the sample surface. This technology enables high-resolution imaging of variations in major, minor, and trace element distributions, as well as isotopic composition, in both conductive and insulating solids, including biological materials, on length scales much smaller than one micrometer.
The NanoSIMS is equipped with a multi-collector system that allows simultaneous collection of up to five different isotopes, i.e. five different images can be simultaneously recorded from the same sputtered volume. This capability can, for example, be used to create images or maps of elemental and isotopic variation within a sample. Such images can be generated from the lightest elements, such as H (e.g. D/H ratios), C (13C/12C ratios), N (15N/14N ratios), O (17O/16O and 18O/16O ratios) and S (e.g. 34S/32S ratios) to the heaviest elements in the periodic table, including uranium.
The NanoSIMS is developed to produce images of large chemical or isotopic variations in solid samples, when high spatial resolution is needed to resolve sub-micrometer structures with relatively modest analytical precision. The NanoSIMS is therefore the perfect analytical instrument in conjunction with, for example, biological labeling experiments, where high spatial resolution is required and high precision is not a requirement.
In the current configuration, the LMCM NanoSIMS N50 multi-collector system is equipped with 4 movable and one fixed detector. All five detector-positions are equipped with electron multipliers, which register the arrival of individual secondary ions up to about 300,000 counts per second. Additionally, two of the movable detector positions are equipped with a Faraday cup, which allows the detection of much larger secondary beams. This can result in higher precision analyses under favorable conditions.
NanoSIMS N50 - schematics:
The 3f IMS is a conventional, single collector ion microprobe, which is capable of delivering primary beams of O- and Cs+ to a sample surface with a spatial resolution around 10 micrometers. The moderate mass-resolution of the mass-spectrometer allows a multitude of interesting analyses to be carried out. Isotopic measurements (e.g. D/H ratio) and trace-element studies are routinely carried out on this type of instrument when the high spatial resolution of the NanoSIMS is not essential. As a general rule, analyses that do not require the high spatial resolution of the NanoSIMS will be preferentially carried out with the 3f IMS instrument.
The National Ion MicroProbe Facility also includes a JEOL JSM840 SEM equipped with Genesis XM2 micro-analyzer with imaging capabilities. This instrument is a standard tool for sample selection, characterization and mapping prior to ion microprobe analysis.