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Nanoscale secondary ion mass spectrometry (nanoSIMS or nano secondary ion mass spectrometry) is a nanoscopic scale resolution chemical imaging mass spectrometer based on secondary ion mass spectrometry. It works based on a coaxial optical design of the ion gun and the secondary ion extraction, and on an original magnetic sector mass spectrometer with multicollection.

NanoSIMS not only refers to the technique used, but also the mass spectrometer specialized for this method. There are only 22 NanoSIMS devices in the world until end of 2014.

How it works

The magnetic sector mass spectrometer causes a physical separation of ions of a different mass-to-charge ratio. The physical separation of the secondary ions is caused by the Lorentz force when the ions pass through a magnetic field that is perpendicular to the velocity vector of the secondary ions. The Lorentz force states that a particle will experience a force

$\mathbf {F} =q\left[\mathbf {E} +(\mathbf {v} \times \mathbf {B} )\right]$

when it maintains a charge q and travels through an electric field E and magnetic field B with a velocity v. The secondary ions that leave the surface of the sample typically have a kinetic energy of a few eV, although a rather small portion have been found to have energy of a few keV. An electrostatic field captures the secondary ions that leave the sample surface; these extracted ions are then transferred to a mass spectrometer. In order to achieve precise isotope measurements, there is a need for high transmission and high mass resolution. High transmission refers to the low loss of of secondary ions between sample surface and detector, and high mass resolution refers to the ability to efficiently separate the secondary ions, or molecules of interest, from other ions and/or ions of similar mass. Primary ions will collide with the surface at a specific frequency per surface area. The collision that occurs causes sputtering of atoms from the sample surface, and of these atoms only a small amount will undergo ionization; these become the secondary ions, which are then detected after transfer through the mass spectrometer. From each primary ion comes a certain number of secondary ions of an isotope that will reach the detector. Those that do reach the detector are typically counted per second, such that this counting rate can be determined by

$I(^{i}M)=d_{\mathrm {b} }\times S\times Y\times X_{\mathrm {M} }\times A_{\mathrm {i} }\times Y_{\mathrm {i} }\times T$

where I(ⁱM)is the counting rate of the isotope ⁱM of element M. The counting rate of the isotope is dependent on the concentration, X_M and the element's isotopic abundance, denoted A_i. Because the primary ion beam determines the secondary ions, Y, that are sputtered, the density of the primary ion beam, d_b, which is defined as the amount of ions per second per surface area, will affect a portion of the surface area of the sample, S, with an even distribution of the primary ions. Of the sputtered secondary ions, there is only a fraction that will be ionized, Y_i. The probability that any ion will be successfully transferred from mass spectrometer to detector is T. The product of Y_i and T determines the amount of isotopes that will be ionized, as well as detected, so it is considered the useful yield.

Features

The mechanism of nanoSIMS is based on secondary ion mass spectrometry. This instrument can characterize the nanostructured materials with complex composition that are increasingly important candidates for energy generation and storage.

NanoSIMS is able to create nanoscale maps of elemental composition, Parallel acquisition of seven masses, isotopic identification, combining the high mass resolution, subparts-per-million sensitivity of conventional SIMS with spatial resolution down to 50 nm and fast acquisition (DC mode, not pulsed)

Applications

NanoSIMS combined with fluorescence microscopy can be used as a tool for subcellular imaging of isotopically labeled platinum-based anticancer drugs.

It also allows precise isotopic and elemental measurements of deep sub-micron areas, grains or inclusions from the different geological and spatial samples.

References

Category:Imaging Category:Mass spectrometry Category:Semiconductor analysis