3
$\begingroup$

Thermo Fisher website explains that "WDXRF uses crystals to disperse the fluorescence spectrum into individual wavelengths of each element, providing high resolution and low background spectra for accurate determination of elemental concentrations." Additionally, it goes on to say

WDXRF systems are based on Bragg’s law, which states that crystals will reflect x-rays of specific wavelengths and incident angles when the wavelengths of the scattered x-rays interfere constructively. While the sample position is fixed, the angles of the crystal and detector can be changed in compliance with Bragg’s law so that a particular wavelength can be measured. Only x-rays that satisfy Bragg’s law are reflected.

However, I am still confused on how the crystal, in correspondence with Bragg's law, disperse the spectrum to show each residual wavelength. Because there is a large variation of wavelengths coming from the sample, wouldn't this mean that passing it through the crystal would cause some of them to destructively interfere? How does this help with analyzing the spectra?

Hopefully, someone can provide a more in depth explanation on how this technique is used to analyze fluorescence spectra.

Improve this question
$\endgroup$
5
  • 4
    $\begingroup$ Different 'colors' (energies) of x-rays will diffract at different angles from a crystal. That is all. Either have a position sensitive detector, or rotate the crystal to sample the different energies at one fixed detector. $\endgroup$
    – Jon Custer
    Commented Dec 4, 2021 at 20:33
  • 2
    $\begingroup$ Do you recall optical (visible) diffraction grating? Let us say, wavelegnths from 400-700 nm pass through a transmission grating. What do you see? A rainbow. This means that all wavelengths passed through the diffraction grating, but at different diffraction angles. A grating does not annhilate energy during destructive interference. $\endgroup$
    – ACR
    Commented Dec 4, 2021 at 21:36
  • $\begingroup$ "X-ray monochromators are analogous to grating monochromators and spectrometers in the visible portion of the spectrum. If the lattice spacing for a crystal is accurately known, the observed angles of diffraction can be used to measure and identify unknown X-ray wavelengths. $\endgroup$
    – ACR
    Commented Dec 4, 2021 at 21:36
  • $\begingroup$ Because of the sensitive wavelength dependence of Bragg reflection exhibited by materials such as silicon, a small portion of a continuous spectrum of radiation can be isolated. Bent single crystals used in X-ray spectroscopy are analogous to the curved line gratings used in optical spectroscopy. The bandwidth of the radiation after it has passed through a high-resolution monochromator can be as narrow as Δλ/λ = 10−4, and, by tilting a pair of crystals with respect to the incident radiation, $\endgroup$
    – ACR
    Commented Dec 4, 2021 at 21:37
  • $\begingroup$ the wavelength of the diffracted radiation can be continuously tuned without changing the direction of the selected light. (From Encyclopedia Britannica). $\endgroup$
    – ACR
    Commented Dec 4, 2021 at 21:37

0

Reset to default