Friday, 6 December 2013

COMPARISON BETWEEN ED-XRF AND WD-XRF

We have discussed about  ED-XRF & WD-XRF in the earlier posts . Now I would like to highlight the major differences between the two X-ray techniques . The most important point of comparison are listed below ::

1. RESOLUTION :

It describes the width of the spectra peaks. The lower the resolution number the more easily an elemental line is distinguished from the nearby X-ray line intensities.

a) The resolution of the WD-XRF system is dependent on the crystal and optics design,particularly collimation, spacing and positional reproducibility. The effective resolution of a WD-XRF system may vary from 20 eV in an inexpensive bench top to 5 eV or less in a laboratory instrument. The resolution is not detector dependant.

Advantage of WD-XRF: High resolution means fewer spectral overlaps and lower
background intensities

b)  The resolution of ED-XRF system is dependent on the resolution of the detector. This can vary from 150 V or less for a liquid nitrogen cooled Si(Li) detector, 150 – 220 eV for various solid state detectors, or 600 eV or more for gas filled proportional counter.

Advantage of ED-WRF: WD-XRF crystal and optics are expensive, and are one more failure mode.

2. SPECTRAL OVERLAPS:

Spectral deconvolutions are necessary for determining net intensities when two spectral lines overlap because the resolution is too high for them to be measured independently.

a) With a WD-XRF instrument with very high resolution (low number of eV) spectral
overlap corrections are not required for a vast majority of elements and applications.
The gross intensities for each element can be determined in a single acquisition.

Advantage WD-XRF: Spectral deconvolutions routines introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the error

b) The ED-XRF analyzer is designed to detect a group of elements all at once. The some type of deconvolutions method must b used to correct for spectral overlaps. Overlaps are less of a problem with 150 eV resolution systems, but are significant when compared to WD-XRF. Spectral overlaps become more problematic at lower resolutions.

3. BACKGROUND

The background radiation is one limiting factor for determining detection limits, repeatability, and reproducibility.

a) Since a WD-XRF instrument usually uses direct radiation flux the background in the region of interest is directly related to the amount of continuum radiation within the region of interest the width is determined by the resolution.

b) The ED-XRF instrument uses filters and/or targets to reduce the amount of continuum radiation in the region of interest which is also resolution dependant, while producing a higher intensity X-ray peak to excite the element of interest.
Even, WD-XRF has the advantage due to the resolution. If a peak is one tenth as wide it has one tenth the background. ED-XRF counters with filters and targets that can reduce the background intensities by a factor of ten or more.

4. EXCITATION EFFICIENCY:

Usually expressed in PPM per count-per-second (cps) or similar units, this is the other main factor for determining detection limits, repeatability, and reproducibility. The relative excitation efficiency is improved by having more source x-rays closer to but above the absorption edge energy for the element of interest.

a. WDXRF generally uses direct unaltered x-ray excitation, which contains a continuum of energies with most of them not optimal for exciting the element of interest.

b. EDXRF analyzers may use filter to reduce the continuum energies at the elemental
lines, and effectively increasing the percentage of X-rays above the element absorption edge. Filters may also be used to give a filter fluorescence line immediately above the absorption edge, to further improve excitation efficiency. Secondary targets provide an almost monochromatic line source that can be optimized for the element of interest to achieve optimal excitation efficiency.

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