Development of Spectrometers

X-rays are used to separate chemical components of a sample into their characteristic spectral lines for identification and determination of concentration.

How does this work?

http://fp.okstate.edu/catlos/eprobe/working.htm

High velocity electrons are generated from a filament, in this case, tungsten.  These electrons are focused through condenser lenses into a narrow beam.  When the electron beam strikes the same a characteristic spectral is obtained.  This is used to obtain chemical compositions.

http://fp.okstate.edu/catlos/eprobe/working.htm

This figure above shows how the incident beam of electrons affects the sample and causes it to emit x-rays which dependent on the composition of the sample will by unique.  The amount of x-rays emitted is a direct reflection of the concentrations of elements in a sample.

http://fp.okstate.edu/catlos/eprobe/working.htm

The image above is an example of an x-ray spectra of albite.  Each peak on the spectrum represents a transition with a characteristic energy. Every element in albite has its own "fingerprint" of peaks.   One can see the characteristic peaks for different elements and the amplitude of the peaks indicates the concentration of the respective element.  For example one can deduce from this spectra that not only is Silicon present in this sample but it is the most abundant element as well.

After discussion the source of the electrons and the data they provide, it is necessary to discuss the detector which makes it possible to view the data.  There are two forms of detectors wavelength dispersive detectors and energy dispersive detectors. Energy dispersive measures the energy of all wavelengths simultaneously, which causes poorer spectral line resolution.  This makes energy dispersive detectors of limited use when trying to determine quantitative information about a sample.  Wavelength dispersive detectors on the other hand, are quite useful in this regard.

http://fp.okstate.edu/catlos/eprobe/working.htm

A wavelength dispersive spectrometer collects data when the source electron beam hits a diffraction crystal, which disperses X-rays by Bragg reflection. Most electron microprobes are equipped with several different crystals to allow analysis of a wide range of X-ray wavelengths lithium fluoride, pentaerythritol and thallium acid phthalate are common crystals used. These crystals can measure all X-ray wavelengths generated by elements from ¹¹Na to ⁹²U. For lighter elements, wider d-spacing is needed. In this case, soap films can be used.

Current Instruments:

Below are examples of instruments used in x-ray spectroscopy


www.amptek.com/x123.html

This is an example of data of Multi-Element Fluorescence from Cd-109


www.amptek.com/x123.html