Energy-Dispersive X-ray Spectroscopy
Introduction:
Energy Dispersive X-ray Spectroscopy (EDS) is an addition to FESEM devices used to determine the elemental composition of solid samples. This analysis can identify the type of element and its atomic or weight percentage using the unique energy emitted by the sample. This method can detect light elements such as oxygen, carbon, and halogens, making it particularly advantageous for analyzing organic samples. It allows for the observation of elemental distribution at high magnification and can reveal concentration variations between different areas of the sample (commonly referred to as line scan analysis). Additionally, measurements can be performed over an area of the sample surface (mapping).
Principle of Operation:
EDAX-EDS is an advanced technique used in materials science, forming part of the Scanning Electron Microscope (SEM) system. When an SEM is equipped with an EDS detector, it can extract data about the sample’s elemental composition while also providing high-resolution imaging of the sample surface.
EDAX-EDS technology is based on the principle of X-ray energy spectroscopy. When a sample is bombarded with electrons, X-rays are emitted due to radiation interactions. The EDS detector measures these emitted X-rays, enabling the identification of the chemical elements present in the sample.
An EDS or EDAX sensor is typically connected as an accessory to an SEM. In this setup, the sample is placed inside the SEM, allowing both imaging and elemental analysis from targeted regions. It also facilitates mapping and line scan analyses. If imaging is not required, the SEM can still provide vacuum conditions and electron radiation for analysis, eliminating the need for gold coating on non-conductive samples. In such cases, exposure to electron beams generates characteristic X-ray emissions.
Since X-ray energy is much greater than visible light, electron transitions can produce X-rays only when an electron drops to the first or second energy level of an atom. Generally, transitions to higher levels do not generate X-rays but may produce visible or ultraviolet light, which is not useful for EDS analysis.
Additionally, the energy levels of electrons in an atom depend on the nuclear charge. The greater the nuclear charge (atomic number), the larger the energy gap between inner and outer electron shells. As a result, when an electron transitions to the first shell of heavier atoms (higher atomic number), it produces higher-energy X-rays, whereas lighter atoms produce lower-energy X-rays.
First-period elements (hydrogen and helium) cannot generate X-rays due to the very low energy of their first level. Similarly, elements in the second period (such as carbon, nitrogen, and oxygen) produce low-energy X-rays. In heavier elements, the second shell can also generate characteristic X-rays alongside the first shell. However, X-ray energy from the second shell is lower than that of the first shell. Consequently, light elements produce characteristic X-rays with a single energy level, while heavier elements exhibit multiple energy levels.
In the spectrum generated by EDAX analysis, peaks are labeled based on the element, the energy level to which the electron transitions, and the number of levels involved in the transition.
Benefits of EDS Measurement:
1. Elemental Analysis: EDS can analyze a wide range of chemical elements, providing comprehensive information about a sample’s chemical composition.
2. Elemental Distribution: It helps determine how elements are distributed within a sample, aiding in understanding material composition.
3. Diverse Applications: This technique is widely used in materials science, metallurgy, chemistry, and environmental sciences, offering high-precision analysis of samples.
EDS technology provides rich information about a sample’s chemical composition, enabling researchers to conduct detailed studies on material properties, chemical interactions, and material behavior under specific conditions.
Measurement Conditions:
1. Sample Preparation:
When analyzing large samples such as rocks, it is preferable to convert them into powder to improve measurement accuracy.
2. Elemental Analysis Limitations:
EDS cannot detect hydrogen and helium.
It has significant challenges in accurately measuring second-period elements (e.g., carbon, nitrogen, oxygen).
3. Using Line Scan:
When performing line scan analysis, the points to be analyzed must be specified, for example, from the edge to the center of the sample, as well as the target elements.
4. Using Mapping:
For mapping analysis, the specific area and dimensions to be examined must be determined. This helps generate a precise image of the elemental distribution in the sample.
Interpretation of Results:
1. Interpreting EDX Results
Generated Spectrum:
The EDX spectrum contains peaks corresponding to the chemical elements in the sample. Each peak represents a specific element and can be identified based on its energy.
Peak Intensity:
The intensity of a peak reflects the element’s concentration in the sample. Higher peaks indicate higher concentrations, while lower peaks suggest lower concentrations.
Element Identification:
Elemental energies must be compared with reference tables to ensure accurate identification. Specialized software can be used for automated data matching.
2. Interpreting Mapping Results
Distribution Image:
The mapping results provide a color-coded image showing the distribution of elements in a specific area of the sample. Each color represents a different element.
Elemental Distribution:
The image helps determine how elements are distributed across the sample, aiding in understanding material composition and chemical interactions.
Region-Specific Analysis:
Specific regions in the image can be analyzed to determine local elemental concentrations, providing insights into the chemical structure.
3. Interpreting Line Scan Results
Line Plot:
The line scan results are represented as a graph showing element concentrations along a predefined path in the sample. This graph reveals variations in element concentrations across a given distance.
Detecting Variations:
The plot can help identify regions with high or low concentrations of specific elements, providing insights into their distribution.
Elemental Comparison:
Concentrations of different elements can be compared within the same plot to understand their relationships and coexistence in the sample.
Interpreting EDX, Mapping, and Line Scan results requires a thorough understanding of the extracted data. By analyzing spectra, elemental distribution, and concentration variations, researchers can gain a detailed understanding of material composition. For further interpretation and clarification, experts from Photon Center can be consulted.