by Akbar Huseynov
Last week, I attended the SwissSIMS winterschool in Lausanne to learn more about the SIMS approach, which I plan to use for my research project. In this blog post, I will describe its working principle, sample preparation, and disadvantages.
Secondary Ion Mass Spectrometry is a technique that is used for the examination of surfaces and thin films. SIMS is an abbreviation for this technique. In order for it to operate, a sample is first subjected to an onslaught of primary ions, which subsequently dislodge secondary ions off the surface of the sample. After that, the mass spectrometer is used to examine these secondary ions in order to identify their composition as well as the chemical composition of the sample.
It is the mission of the Swiss Society for Mass Spectrometry (SwissSIMS), an organization known simply as SwissSIMS, to encourage the application of mass spectrometry and other methods of analysis in Switzerland. “SwissSIMS winterschool” event that took place in Lausanne which is the center on the application of SIMS and other mass spectrometry techniques in the terms of both research and industry. On the other hand, the events consist of lectures and presentations given by specialists in the subject, hands-on training and workshops, chances for networking and collaboration, and other related activities. The content of the program is most likely center on the application of mass spectrometry and other related techniques, with a particular emphasis on secondary ion mass spectrometry (SIMS) and the applications it can be used for. It has the potential to be an excellent chance for participants to gain knowledge about the most recent advancements in the industry, enhance their skill sets, and network with other researchers and professionals working in the field of mass spectrometry.
The principle of operation of SIMS
The operation of Secondary Ion Mass Spectrometry (SIMS) is predicated on the primary ion beam sputtering a surface. Primary ions dislodge secondary ions from the sample’s surface, which are subsequently examined by a mass spectrometer to identify their chemical composition and sample’s chemical makeup.
The process begins by bombarding the sample surface with a primary beam of ions, typically with a high kinetic energy. The primary ions collide with the atoms in the sample, causing them to be ejected from the surface. These ejected atoms are known as secondary ions. The secondary ions are then extracted from the sample chamber and focused into a mass spectrometer where they are analyzed.
The mass spectrometer separates the ions based on their mass-to-charge ratio, allowing the determination of their chemical identity. This information can be used to create a chemical map of the surface, providing information on the composition and distribution of elements and compounds present on the surface.
The SIMS technique can be used to analyze a wide range of samples, including inorganic materials, biological samples, and thin films. It is particularly useful for the analysis of trace amounts of elements and compounds, and for the characterization of surfaces at the atomic scale.
Primary Ion Source of SIMS
The primary ion source is a key component of Secondary Ion Mass Spectrometry (SIMS) instrumentation. It is responsible for generating the primary ion beam that is used to sputter the sample surface and produce secondary ions for analysis.
There are several types of primary ion sources that can be used in SIMS, each with its own advantages and disadvantages. Some examples include:
- Gas field ionization (GFI) sources: These sources use a high voltage electric field to ionize a gas, such as argon or neon, and extract the ions to form the primary beam. GFI sources can produce highly focused beams with a high current density, but they have a relatively low ionization efficiency.
- Thermal ionization sources: These sources use a filament, heated by an electric current, to vaporize a metal, such as cesium or gold, which is then ionized to produce the primary beam. Thermal ionization sources have a high ionization efficiency and can produce intense beams, but they are relatively bulky and may not be suitable for all samples.
- Electron impact ionization sources: These sources use an electron beam to ionize a gas, such as argon or xenon, to produce the primary beam. Electron impact ionization sources can produce intense beams with high ionization efficiency, but they are relatively bulky and may not be suitable for all samples.
- Laser Ionization: In Laser SIMS, a laser beam is used to ionize the sample, the laser beam causes the atoms in the sample to be excited and ionized, producing a primary ion beam. The advantage of using laser ionization is that it allows for the creation of a highly focused beam with minimal sample damage.
The choice of primary ion source will depend on the type of sample, the information that is desired from the analysis, and the specific SIMS instrument that is being used.
Sample preparation for Secondary Ion Mass Spectrometry (SIMS) is an important step in the analysis process and can greatly affect the quality and accuracy of the results. The specific sample preparation methods used will depend on the type of sample and the information that is desired from the analysis.
Here are a few general steps that are commonly used in sample preparation for SIMS:
- Cleaning: The sample surface should be cleaned thoroughly to remove any contaminants that may interfere with the analysis. This can be done by using solvents, ultrasonic cleaning, or by sputtering the surface with a low-energy ion beam.
- Mounting: The sample should be mounted onto a sample holder that is compatible with the SIMS instrument. The sample should be securely mounted to prevent movement during the analysis.
- Polishing: If a cross-sectional analysis is required, the sample may need to be polished to obtain a flat and smooth surface. This is typically done using a diamond polisher.
- Sputtering: The sample surface may be sputtered with a low-energy ion beam before analysis to remove any surface contaminants or to create a fresh surface.
- Coating: If the sample is a non-conductive material or has a low secondary ion yield, the sample may need to be coated with a conductive material such as gold, silver or platinum.
- In situ analysis: Some samples may require in situ analysis, which means analyzing the sample in the environment in which it normally exists, such as a liquid or gas phase.
It’s important to note that these steps may vary depending on the sample type, and the specific instrumentation used for analysis. It is also important to note that the sample preparation procedure should be carefully optimized for each sample and for the specific analytical question to be addressed.
Disadvantage of SIMS
Secondary Ion Mass Spectrometry (SIMS) is a powerful analytical technique with many advantages, but it also has some limitations and disadvantages. Here are a few examples:
- Surface sensitivity: SIMS is a surface-sensitive technique, meaning that it can only provide information on the top few atomic layers of a sample. This can be a limitation when analyzing samples with buried interfaces or subsurface layers.
- Low sensitivity: SIMS has lower sensitivity compared to other mass spectrometry techniques such as inductively coupled plasma mass spectrometry (ICP-MS) or time-of-flight secondary ion mass spectrometry (TOF-SIMS). This means that it may not be able to detect very low concentrations of elements or compounds.
- Sample preparation: SIMS requires samples to be prepared in a specific way, which can be time-consuming and may introduce artifacts. This can be a limitation when analyzing samples that are difficult to prepare or that are sensitive to certain preparation methods.
- High cost: SIMS instrumentation is relatively expensive and requires a high level of technical expertise to operate. This can be a limitation for some research groups or organizations with limited resources.
- Damage to sample: Bombarding a sample with a primary ion beam can cause damage to the sample, which can be a limitation when analyzing sensitive or fragile samples.
- Complexity of data interpretation: The data obtained from SIMS can be complex and difficult to interpret, requiring expertise in the field and the use of specialized software.
It’s important to note that these disadvantages can be mitigated to some extent by using certain techniques such as low energy primary ion beam, or by combining with other techniques such as X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) to get a more complete characterization of the sample.