SIMS is a desorption mass spectrometry technique. This is a surface chemical analysis and imaging technique with molecular specificity. SIMS instruments (also known as ion microprobes) use an internally generated beam of either positive (e.g., Cs) or negative (e.g., O) ions (primary beam) focused on a sample surface to generate ions, which are then transferred into a mass spectrometer across a high electrostatic potential and are known as secondary ions. In a similar technique, a beam of high-speed neutral atoms (e.g., Ar) can be employed in place of the primary ion beam, which is largely utilized for surface investigation of organic molecules and has few uses in geosciences. MaTestLab is one of the best testing service providers, with the best network of testing laboratories in the USA to carry out SIMS tests for our clients.
Principle and Methodology of Secondary Ion Mass Spectrometry (SIMS)
The contact of the primary ion beam with the sample (in vacuum) generates enough energy to ionize several elements. If the primary beam contains positively charged ions, the resulting ionization favors the synthesis of negative ions; primary beams containing negatively charged ions favor the generation of positive ions. Although the majority of atoms and molecules ejected from the sample by the primary beam’s interaction with the sample surface (known as sputtering) are neutral, some are ionized. A mass spectrometer accelerates, focuses, and analyses these ions.
Instrumentation of SIMS
SIMS instruments are commercially manufactured with applications in geosciences, such as Cameca’s 1280, 7f, NanoSIMS, ASI’s SHRIMP and SHRIMP RG, and EAG’s ToF SIMS. These instruments control the intensity, energy, and orientation of the primary beam, generating ions that form the secondary beam. These ions are then transmitted to a mass spectrometer. The configuration of the mass spectrometer varies but all use magnetic and electrostatic analyzers. Forward geometry design allows multiple ion beams to be measured simultaneously, while reverse geometry design improves mass resolution but loses the ability to measure multiple ion beams simultaneously.
Applications of Secondary Ion Mass Spectrometry
In the field of material sciences, SIMS is a popular instrument in geochemistry for various analytical tasks.
- Large radius forward and reverse geometry instruments can measure trace elemental and isotopic compositions in individual minerals with a spatial resolution of 10 microns, such as U-Th-Pb geochronology of zircon and other accessory minerals.
- These instruments can also measure the isotopic composition of low atomic number elements like O with similar spatial resolution.
- Smaller radius, double focusing instruments offer high sensitivity for trace element analyses, 10x the sensitivity of microprobes using electron beams.
- Some SIMS use other types of mass spectrometers for surface characterization, molecular analysis, and depth profiling.
Strengths and limitations of Secondary Ion Mass Spectrometry
Strengths
- SIMS is a non-destructive analysis method that uses very little sample, allowing for high sensitivity to analyze samples with low concentration levels.
- It is used to determine trace element abundances in meteorites, interplanetary dust, and non-conducting substrates.
- It also allows depth profiling of elemental and molecular abundances and isotopic ratios.
- In situ analysis eliminates the need for complex sample preparation, allowing minerals to be analyzed directly as grain mounts or thin sections. This makes SIMS a valuable tool in various industries.
Limitations
- The sputtering of samples produces both atomic and molecular species, making it difficult to quantitatively analyze all elements in substrates. For instance, Lu-Hf in zircon faces unresolvable isobaric interferences that cannot be resolved by forward geometry multi-collection or reverse geometry high-resolution instruments.
- SIMS instrumentation is expensive.
