Nanoscale secondary ion mass spectrometry

Generally, NanoSIMS uses an ion gun to produce a primary beam of ions. These primary ions erode the sample surface and produce atomic collisions, some of the collisions resulting in the release of secondary ion particles. These ions are transmitted through a mass spectrometer, where the masses are measured and identified.[5] The primary ion beam can raster across the sample and create a ‘map’ of the element and isotope distribution by counting the number of ions that originated from each point at up to a 50 nanometer (nm) resolution, 10-50 times greater than conventional SIMS.[6][7] This is achieved by positioning the primary probe in close proximity to the sample.[8] 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.

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.[9] 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.[5][7] 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.[9]

The ion beam must either be set to detect negative or positive ions, commonly completed by using a Cesium+ or Oxygen- beam, respectively.[10] This high mass resolution 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 ¹³C¹⁴N-, and ¹²C¹⁵N-), nearly identical molecular weights of 27.000 and 27.006 daltons, respectively, will be generated. Unlike other imaging techniques, where ¹³C¹⁴N and ¹²C¹⁵N cannot be independently measured due to nearly identical masses,, NanoSIMS can safely distinguish the differences between these molecules.[10]

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

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

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 electron volts (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 secondary ions between the sample surface and the 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 unit of surface area. The collision that occurs causes atoms to sputter from the sample surface, and of these atoms only a small amount will undergo ionization. These become secondary ions, which are then detected after transfer through the mass spectrometer. Each primary ion generates a number of secondary ions of an isotope that will reach the detector to be counted. The count rate is determined by

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

where I(ⁱM)is the count 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 unit of 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. [11]

One of the most critical steps in NanoSIMS use is sample preparation.[12] Specific protocols should be developed for individual experiments in order to best preserve the true spatial distribution and abundance of molecules based on the sample. In general, due to the design of the NanoSIMS machine, the sample must be vacuum compatible (ie, volatile free), flat, which reduces varying ionization trajectories, and conductive, which can be accomplished sputter coating with Au, Ir, or C. Biological samples, such as cells or tissue, can be fixed and embedded in a resin before sectioning in 100 nm slices, and placed on silicon chips or slides before viewing.[12]

NanoSIMS can capture the spatial variability of isotopic and elemental measurements of sub-micron areas, grains or inclusions fromgeological and biological samples.[13] This instrument can characterize the nanostructured materials with complex composition that are increasingly important candidates for energy generation and storage.