NanoSIMS (nanoscale secondary ion mass spectrometry) is an analytical instrument manufactured by
CAMECA which operates on the principle of
secondary ion mass spectrometry
Secondary-ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. ...
.
The NanoSIMS is used to acquire nanoscale resolution measurements of the elemental and isotopic composition of a sample. The NanoSIMS is able to create nanoscale maps of elemental or isotopic distribution, parallel acquisition of up to seven masses,
isotopic identification, high mass resolution,
subparts-per-million sensitivity with spatial resolution down to 50 nm.
![NanoSIMS50 instrument diagram](https://upload.wikimedia.org/wikipedia/commons/a/af/NanoSIMS50_instrument_diagram.png)
The original design of the NanoSIMS instrument was conceived by Georges Slodzian at the
University of Paris Sud in
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
. There are currently around 50 NanoSIMS instruments worldwide.
[Li, K., Liu, J., Grovenor, C. R. M., & Moore, K. L. (2020). NanoSIMS Imaging and Analysis in Materials Science. Annual Review of Analytical Chemistry, 13, 273-292. https://doi.org/10.1146/annurev-anchem-092019-032524]
How it works
The NanoSIMS uses an ion source to produce a primary beam of ions. These primary ions erode the sample surface and produce atomic collisions, some of these collisions result in the release of secondary ion particles. These ions are transmitted through a mass spectrometer, where the masses are measured and identified.
The primary ion beam is rastered across the sample surface and a ‘map’ of the element and isotope distribution is created by counting the number of ions that originated from each pixel with at best a 50 nanometer (nm) resolution, 10-50 times greater than conventional SIMS.
This is achieved by positioning the primary probe in close proximity to the sample using a coaxial lens assembly.
The primary ion beam impacts the sample surface at 90°, with the secondary ions extracted back through the same lens assembly. This allows for the isotopic composition of individual cells to be distinguished at parts per million (ppm) or parts per billion (ppb) range. The main drawback of this set up is that the primary and secondary ion beams must be of opposite polarity which can limit which elements can be detected simultaneously.
NanoSIMS can detect minute mass differences between ions at the resolution of M/dM > 5000, where M is the nominal mass of the isotope and dM is the mass difference between the isotopes of interest.
The high mass resolution capabilities of NanoSIMS allows for different elements and their isotopes to be identified and spatially mapped in the sample, even if very close in mass. The mass spectrometer is capable of multicollection, meaning up to 5 (NanoSIMS 50) or 7 (NanoSIMS 50 L) masses can be simultaneously detected, from hydrogen to uranium, though with limitations.
The relatively large number of masses helps eliminate measurement errors as possible changes in instrumental or sample conditions that may occur in between runs are avoided.
The ion beam must either be set to detect negative or positive ions, commonly completed by using a cesium+ or oxygen- beam, respectively.
The high mass resolution achievable is particularly relevant to biological applications. For example, nitrogen is one of the most common elements in organisms. However, due to the low electron affinity of the nitrogen atom, the production of secondary ions is rare. Instead, molecules such as CN can be generated and measured. However, due to isotope combinations, such as the
isobars 13C
14N-, and
12C
15N-, nearly identical molecular weights of 27.000 and 27.006 daltons, respectively, will be generated. Unlike other imaging techniques, where
13C
14N and
12C
15N cannot be independently measured due to nearly identical masses, NanoSIMS can safely distinguish the differences between these molecules allowing isotopic spiking experiments to be conducted.
The physics of NanoSIMS
The magnetic sector mass spectrometer causes a physical separation of ions of a different
mass-to-charge ratio
The mass-to-charge ratio (''m''/''Q'') is a physical quantity relating the ''mass'' (quantity of matter) and the ''electric charge'' of a given particle, expressed in units of kilograms per coulomb (kg/C). It is most widely used in the electrody ...
. The physical separation of the secondary ions is caused by the
Lorentz force
In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
when the ions pass through a magnetic field that is perpendicular to the velocity vector of the secondary ions. The
Lorentz force
In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
states that a particle will experience a force
: