NanoSIMS is an ion imaging technique that allows the analysis of elemental and isotopic composition of a solid sample at a sub-micrometer spatial scale (down to 50 nm). Although originally “NanoSIMS” was the name of an instrument that makes such an analysis possible (NanoSIMS 50 or 50L, produced by Cameca), it is now commonly used as a synonym for the technique itself. An informative overview of the NanoSIMS technique, together with many examples of applications ranging from cell biology to microbial ecophysiology to cosmochemistry, can be found on the Cameca website or in a number of review articles, e.g., Lechene et al. (2006), Wagner (2009), Hoppe et al. (2013).
NanoSIMS analysis uses a finely focused beam of ions (ion probe) to erode the sample surface. The secondary ions (atomic and molecular) that are produced in this process are then analyzed by a mass spectrometer.
Although the basic principle is rather simple, the technical realization for achieving high sensitivity, mass resolution and spatial resolution requires some very sophisticated technology. This technology is put together in an instrument called NanoSIMS 50 or 50L, produced by Cameca. Aspects of the instrument that are important from the perspective of the sample preparation and measurements performed at the Utrecht University are briefly discussed below.
Since any material will be eroded when hit by high-energy ions, NanoSIMS can be used to analyse practically any kind of solid material. On the other hand, because of this erosion, the technique is destructive. For example, when analysing mineral particles of about 100 nm in size, the particles will be eroded away in a few seconds. When analysing microbial cells, which are typically 1-2 microns in size, they will be eroded away on the scale of minutes.
Spatial resolution of ~50 nm is achieved by using a focused ion beam that hits the sample surface perpendicularly to its surface. By rastering such a beam over the sample surface, high-spatial resolution images can be obtained. Clearly, the sample position must be highly stable to achieve images with accurate spatial information. Here, temperature control of the laboratory is essential to minimize distortions of the images, which is important especially when taking multiple planes of the same field of view.
The mass spectrometer used for the analysis has a very high mass resolution (M/dM>5000). This allows discrimination between ions of very close masses. For example, when analyzing organic material enriched in 15N, one must be able to distinguish the molecular ion 12C15N- (mass 27.000) from the molecular ions 13C14N- (mass 27.006) and 12C14N1H- (mass 27.011), as these are all likely eroded simultaneously from the sample. To be able to do this, mass resolution (M/dM) of better than 27/0.006=4500 is required.
NanoSIMS 50/50L uses a magnet to separate the masses. According to the law of electromagnetism, namely the Lorenz magnetic force, the trajectory of a charged particle traveling in a magnetic field is curved, with the curvature proportional to the ratio between the particle's mass and charge (m/q). After passing through the magnet, particles with different m/q ratios will emerge at different locations, where they can be detected by sensitive detectors. NanoSIMS 50L has 7 detectors, which makes it possible to study co-localization of 7 masses in the sample.
In principle, atomic or molecular ions with masses ranging from hydrogen to uranium and beyond can be detected by NanoSIMS. However, there are some restrictions with respect to the simultaneous detection of multiple masses.
Experiments showed that the efficiency of emission of ionized particles differs if the surface bombardment is done with different primary ions. The best efficiency is achieved by using Cs+ for detecting negative ions and O- for detecting positive ions. Although both primary ion sources are available with the NanoSIMS 50/50L instrument, only one can be used at a time. The fact that switching to a different source is a lengthy procedure imposes some constrains on the experimental design and the subsequent NanoSIMS analysis.
To achieve maximum sensitivity of the instrument, many components of its ion optics need to be fine-tuned. Since a number of these optimization steps need to be done frequently, even when moving from one field of view on the sample to another, the total analysis of a batch of samples (replicates+treatments) from a given experiment may take some time (several days of continuous measurement time). Clearly, the experimental design should consider this very carefully.
Since the detection by NanoSIMS is achieved through bombardment with ions, net charge would be deposited onto the sample during the measurement if the charge was not removed. Since most of the analyzed samples will not be sufficiently conductive, samples should be coated with a thin conductive layer (e.g. Au, C, Au-Pd), which will facilitate removal of the excess charge and thus prevent problems associated with sample charging. For samples that are put on a support substrate (e.g. cells or tissues on a filter or a glass slide), the conductive layer deposition can be done before or after the sample deposition. A gold-coating instrument is available at Utrecht University.
Experiments showed that the detected signal is affected by minute changes in topography of the sample surface. To minimize these artifacts, samples should be prepared as flat as possible. For geological samples, polishing with a 30-50 nm powder is advised. Polishing instruments are available at Utrecht University.
As mentioned above, NanoSIMS is a surface probing technique, with the thickness of the probed surface in the range of a few nanometers. This is important to realize when analyzing thicker samples such as tissues; features seen in a microscope, obtained for example in a transmission or fluorescence mode, may not be “visible” to NanoSIMS if they are buried deeper in the sample. The only way to get to them is by eroding the layers of the sample above them. Although this can be, in principle, done by NanoSIMS itself, clearly this would not be the most efficient way to measure. Therefore, the sample should be prepared preferably in thin sections.
Because the detection of ionized particles eroded from the sample surface is based on their acceleration through a high electric potential, it is essential that the whole process occurs in a high vacuum (<10-7 Pa, <10-9 mbar) to avoid arching. This means that the analyzed samples must withstand such vacuum. Specifically, they should not contain any volatile substances.