tailieunhanh - X-ray Spectrometry

The characteristics lines in X-ray spectra result from electronic transitions between inner atomic orbitals. The X-ray spectra for most heavy elements are much simpler than the UV/Vis spectra observed in ICP-OES, for example. (Only a few lines!!!) Big difference between X-ray and UVVis: The radiation is ionizing, and doesn’t just excite electrons to higher levels. Moseley’s law: Predicts the basic relationship of atom number and the frequency of the characteristic lines | a W VILLANOVA UNIVERSITY X-ray Spectrometry Lecture Date February 4th 2008 Notes See Chapters 12 and 21 mostly Chapter 12 of Skoog This lecture covers both atomic and molecular applications of X-ray spectrometry X-ray diffraction is only briefly discussed here - it is covered in its own lecture along with its applications to crystallography and solid-state structural analysis Surface analysis and microscopy is also briefly discussed in advance of its own lecture 1 Outline X-ray absorption fluorescence processes - Auger electron emission - Photoelectron emission Excitation of X-rays - X-ray fluorescence X-ray emission X-ray Detection and Spectrometer Design - Energy-dispersive ED spectrometers - Wavelength-dispersive WD spectrometers Methods and Applications Topics mentioned here but discussed in detail during the Surface Analysis and Microscopy Lecture - Scanning electron microscopy - an X-ray emission microprobe - Auger electron spectrometry electron energy - X-ray photoelectron spectrometry again electron energy 2 X-rays What are X-rays High energy photons. - Note gamma rays are just high-energy X-rays Advantages of X-ray spectrometric methods - The X-ray spectrum is not very sensitive to molecular effects or chemical state or excitation conditions This is because core electrons are usually involved in X-ray transitions - physical and chemical state have only minute effects . gas vs solid oxide vs. element . - Atomization is not necessary for elemental analysis - Precision and accuracy are good spectra are simple - Surface-sensitive penetration of 100 um at most Disadvantages of X-ray methods - Surface-sensitive if you want bulk analysis often not a problem - Modest limits of detection compared to other elemental methods . AA ICP-OES ICP-MS X-ray Production X-ray are commonly produced by bombarding a target with electrons The target emits a spectrum with two components - Characteristic radiation - Continuous radiation also called white radiation .

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