X-Ray Fluorescence Study Notes

TOPIC 4: X-RAY FLUORESCENCE

1. Introduction to X-Ray Fluorescence

  • Definition:

    • X-ray fluorescence (XRF) refers to the phenomenon where the absorption of X-rays results in the re-emission of radiation at different energies (generally lower).

    • The absorption of X-rays produces electronically excited ions that return to their ground state through electronic transitions, leading to fluorescence.

  • Process of Absorption and Emission:

    • When an atom (e.g., lead) absorbs radiation with wavelengths shorter than 0.14 Å, it becomes excited, creating an ion with a vacant K shell.

    • After a brief period, this ion returns to its ground state through a series of electronic transitions where X-radiation is emitted (fluorescence).

    • Comparison of Absorption and Emission:

    • Wavelengths of emitted fluorescence lines are always greater than the corresponding absorption edge wavelengths due to the difference between ionization (complete removal of an electron) and emission (transition of an electron from a higher energy level).

    • Example:

      • K absorption edge for silver: 0.485 Å

      • K emission lines for silver: 0.497 and 0.559 Å.

2. Instrument Components in X-Ray Fluorescence

  • Key Functional Components:

    • There are five main components involved in X-ray analytical instruments, analogous to optical spectroscopic measurement:

    1. Source of radiation

    2. Wavelength restriction device

    3. Sample holder

    4. Radiation detector or transducer

    5. Signal processor and readout

    • While the specific details of these components differ from optical counterparts, their functions are similar and are combined in similar configurations.

2.1 X-Ray Sources

  • Types of X-Ray Sources:

    1. X-ray Tubes

    2. Radioisotopes

    3. Secondary fluorescent sources

2.1.1 The X-Ray Tube

  • Description:

    • The most common source of X-rays for analytical work.

    • A highly evacuated tube containing:

    • Tungsten filament cathode

    • Massive anode (typically a heavy copper block with metal target plated or embedded)

    • Target Materials:

    • Common metals used include tungsten, chromium, copper, molybdenum, rhodium, scandium, silver, iron, and cobalt.

  • Operations:

    • Separate circuits heat the filament and accelerate electrons to the target.

    • Heater Circuit: Controls the intensity of emitted X-rays.

    • Accelerating Potential: Affects the energy and wavelength of the emitted X-rays.

    • For quantitative work, power supplies must maintain a control tolerance of 0.1% relative.

2.2 X-Ray Transducers and Signal Processors

  • Early Detection Methods:

    • Photographic emulsions were initially used for detection and measurement of X-ray radiation.

  • Modern Transducers:

    • Current instruments utilize transducers that convert radiant energy into electrical signals for convenience, speed, and accuracy.

    • Types of transducers include:

    1. Gas-filled transducers

    2. Scintillation counters

    3. Semiconductor transducers

3. X-Ray Fluorescence Method

  • Excitation Mechanism:

    • X-ray emission spectra can be excited by irradiating the sample with an external X-ray beam from a tube or radioactive source.

    • The sample absorbs the primary beam, leading to the emission of characteristic fluorescent X-rays from its constituent elements.

  • Applications and Advantages of XRF:

    • XRF is widely employed for qualitative element identification of elements with atomic numbers greater than oxygen (atomic number > 8).

    • Also applicable for quantitative elemental analysis.

    • Key Advantage: Non-destructive analysis of the sample.

4. Underlying Principle of X-Ray Fluorescence as an Analytical Tool

  • Ionization Process:

    • Exposure to short-wavelength X-rays or gamma rays can lead to the ionization of component atoms by ejecting one or more tightly held electrons, provided the radiation energy exceeds the ionization potential of the atom.

    • An electron being ejected leaves a vacancy in the atomic orbital, causing higher-energy electrons to transition down to fill the vacancy, emitting energy as fluorescence in the process.

  • Energy Release:

    • Energy released during electron transitions results in photons emitted, with energies characteristic of the atom's electronic structure.

4.1 Analysis Methods

  • Types of Analysis:

    • The fluorescent radiation can be analyzed via:

    1. Energy-dispersive analysis: Sorting the energies of emitted photons.

    2. Wavelength-dispersive analysis: Separating the wavelengths of emitted radiation.

  • Relation of Intensity to Element Amount:

    • The intensity of each characteristic emitted radiation is directly related to the amount of each element present in the material, forming the basis of XRF as a powerful analytical tool.

4.2 Energy Dispersive Spectrometry (EDX/EDS)

  • Detector Technology:

    • Uses detectors based on silicon semiconductors in lithium-drifted silicon crystals or silicon wafers for photon energy determination.

  • Schematic Layout:

    • Arrangement includes:

    • High-purity X-ray source

    • Detector

    • Electronics

    • Computer

    • Sample holder

  • Figures Overview: Illustrations provided show schematic arrangements of EDX spectrometers and X-ray spectrometers.

5. Applications of X-Ray Fluorescence

  • Use Cases:

    • Qualitative and quantitative elemental analysis in:

    • Metals

    • Glass

    • Ceramics

    • Building materials

    • Utilized in various fields:

    • Geochemistry

    • Forensic science

    • Archaeology

6. Units of X-Ray Fluorescence

  • Measurement Unit:

    • The intensity of fluorescence is typically measured in counts per second (cps).