X-ray Fluorescence Flashcards
What is x-ray?
X-rays are short wavelength (10-5 A to 100 A) EM produced by the deceleration of high –energy electrons or by electronic transitions of electrons in the inner orbitals of atoms.
List the four main sources of “analytical” x-ray
Bombardment of metal target with a beam of high-energy electrons
Electrons produced at cathode and accelerated toward a high potential anode. Collision electrons decelerated and x-ray spectrum is produced. This is dependent only on the accelerating voltage and is independent of target material. Energy of photon is equal to difference in KE before and after collision.
X-ray fluorescence by a material irradiated by x-rays
The absorption of x-rays produces electronically excited ions, when the ion returns to it’s ground electronic state. Characteristic lamba are produced.
Cutoff l form the primary x-ray source must be less (greater in energy) than the “absorption edge” of the analyte.
Use of an radioactive source
X-ray radiation can be produced by radioactive species. g-rays are high energy EM that is indistinguishable from X-rays
Synchrotron radiation source (highly specialized facility not discussed)
Describe this spectra.
- X-rays are absorbed by materials through an photoelectron effect process.
- Observed lamba is characteristic of the element and is independent of its’ chemical state. Inner electron far removed from valence electrons
- Sharp discontinuities are call absorption edges
- “K” reflects the difficulty in removing electron from that orbital. For example, it is more difficult to remove electron from Pb with 82 protons than Ag with 47 electrons. Therefore, 82Pb has a much lower wavelength (higher energy) K band than does 47Ag.
Write the equation of beer’s law in relation with mass absorption coefficient
Label the x-ray spectrometry instrumentation diagram.
Names the sources for x-ray spectrometry instrumentation
- X-ray tube: These are the most commonly used sources for analytical work
- Radioisotopes: The nature of the radiation material used. Many produce line spectra. Since absorption sensitivity is related to the proximity to specific absorption edges, specific sources are more applicable to specific analysis.
- Secondary fluorescence: this can be quite useful, as discrete lines are produced without the underlying continuum of x-ray tube. However, a primary x-ray tube or radioisotope souce is required to stimulate fluorescence.
Describe the different type of x-ray transducers for x-ray fluorescence spectroscopy.
- Gas-filled transducers: inert gases such as argon, xenon, or krypton are enclosed metal tube equipped with electrodes that have a high potential applied across them. When x-rays ionized the gas, a current is produced, the nature of which is dependent on the magnitude of the applied potential. (see image)
- Geiger tube
- Proportional counters
- Ionization chamber
- Scintillation counters: radiation striking a phosphor produces luminescence that can be monitored and amplified with a photomultiplier tube.
- Semi-conductor transducers: semiconductor based detectors have a roughly analogous mode of operation to gas filled detectors.
Name the two type of signal processor and its function.
Pulse-height selector: only signal with a preset range of intensities are collected. See figure 12-13.
Pulse-height analyzers: signals with specific energy range have a distinct energy, so scanning energy range is comparable to scanning frequency (or wavelength)
List the advantages and disadvantages of x-ray fluorescence:
- Advantages:
- Simple spectra
- Spectral interferences limited
- Non-destructive technique (for the most part)
- Many sample types and sizes
- Very rapid and convenient
- Disadvantages:
- Low sensitivity (0.01 to 100%)
- Less applicable for lighter elements (elements below vanadium A#23)
- Cost of $5000 to $500 000
