NanoSIMS is a new generation of ion-microprobe analysis that combines high spatial resolution with high sensitivity, offering highly versatile imaging and analysis opportunities across the fields of biology, earth science and materials science. The technique employs secondary ion mass spectrometry (SIMS), where a high-energy ion beam ablates material from a sample surface and this material is then analysed in a mass spectrometer. The advantage of nanoSIMS is that a newcoaxial lens design allows the primary beam to be focused down to 50 nm, compared to the tens-of-micrometres typical in conventional SIMS.
NanoSIMS uses a rastered ion beam to scan across a sample surface in a manner similar to an SEM. The secondary ions that are sputtered from the sample are sorted by their atomic mass in a magnetic-sector mass spectrometer. The ions are collected and counted by electron multipliers at the exit of the mass spectrometer. Multiple detectors allow parallel mapping of up to five ion species, simultaneously. The recorded data set simply consists of the number of counts recorded for each pixel of the scan, for each ion species. These data are then reconstructed to produce an image showing the surface composition of the sample.
As the mass spectrometer distinguishes between ions by mass, it is possible to distinguish different isotopes of the same element. Furthermore, the high mass resolution allows the separation of overlapping peaks on the same mass – for example, 13C and 12C1H at 13 atomic mass units. This allows the precise measurement of isotope ratios by tuning the detectors to measure, for example, 12C and 13C simultaneously.
The high-resolution ion imaging capability is a powerful and unique feature of nanoSIMS, and is highly suitable for elemental or isotopic mapping at the sub-micrometre scale. This is a particularly powerful technique for mapping the distribution of isotopic tracers (e.g. 15N or 13C in biological experiments). Additionally, high-resolution linescans can also be obtained, which can be used to show diffusion profiles across phase boundaries. As the technique is destructive, it can also be used for depth profiling, in particular where layers are too thin to be resolved though other techniques. Combining depth profiling with imaging allows 3-D images to be obtained. Due to the fine-scale beam, depth profiling is generally limited to less than a micrometre in depth.
The main advantage of nanoSIMS is also its chief limitation: as the size of the primary beam gets smaller, the sample interaction volume gets smaller and the number of secondary ions sputtered from the surface decreases. It can therefore take a long time to acquire an image, as each pixel requires a longer dwell time. Another major limitation is in quantification. Different elements have different secondary ion yields, and these ion yields change between different matrices. This makes it very difficult to calibrate the instrument for absolute concentrations, especially in fine-scale, multi-component systems. Where quantification is required, it is necessary to analyse a well-characterised standard with an identical matrix, concurrently.
The benefits of SIMS in general are that it is very sensitive, with a large dynamic range where major and trace elements can be measured simultaneously. It can also detect most of the elements in the periodic table, although some elements require a high degree of fine-tuning. Secondary ion yields are dependent on the ionisation potential or electron affinity of the desired element. In nanoSIMS, negative secondary ions (C–, O–, Si–, CN–, S–) can only be generated with a Cs+ primary beam, where as positive secondary ions (Na+, Ca+, Si+, Fe+, etc) can only be generated with a O- primary beam. The Cs+ primary can be focussed down to less than 50 nm, while the O- primary beam can only achieve around 200–150 nm, leading to a small discrepancy between positive and negative ions images.
Examples of analyses performed using nanoSIMS:
Mapping metals in cultured cells after treatment with anti-cancer compounds.
Mapping and measurement of isotopic labels (15N) in bacterial communities at the scale of individual microbes.
Measuring nutrient uptake by plant roots, fungi and bacteria in soil.
Sub-cellular distribution of metals in the leaves of hyper-accumulating plants.
Earth and planetary science
Distribution of ‘invisible’ Au in ore-bearing pyrite.
C and N mapping in ancient sedimentary structures to determine biogenicity.
Element distributions in primitive meteorites.
Variations in elemental ratios as climate proxies in biomineralised marine microorganisms.
Composition changes with depth through thin-films and doped semiconductors.
Composition changes around sulphide inclusions in stainless steel.
Composition of phases within stress-corrosion cracks in steel.
Samples must be vacuum-compatible (10-9 torr), solid, flat and polished. They must also be electrically conductive. Insulating samples can be coated with Au, Pt or C to provide conductivity, and an electron flood gun can be used to compensate charge build up when using the Cs+ primary beam. Sample charging is less of an issue when using the O– primary.
Sample holders can accommodate 25 mm, 12.5 mm and 10 mm discs, and it is usually possible to embed irregular shaped samples in resin or Wood's metal, and then grind and polish to the desired grade. Geological glass slides do not fit unless they are cut down to fit inside the 25 mm mount. Biological materials must be fixed and sectioned (500 nm to 1 µm sections work best), mounted flat on either 2.5 mm diameter Si wafer or Au-coated Al stubs. TEM samples mounted on standard 3 mm formvar-backed Cu grids can also be accommodated, although it is difficult to get a strong signal from very thin TEM sections (less than 100 nm).