Total Reflection X-Ray Fluorescence
Total Reflection x-ray fluorescence (TXRF) and the fundamentally related Grazing
emission x-ray fluorescence (GEXRF) rely on scatter properties near and
below the Bragg angle to reduce background intensities and improve detections
limits an order of magnitude or more over more traditional XRF instruments.
If light is directed at a smooth surface at a very small angle (typically
less than 0.5 degree for x-rays) virtually 100% of the light will be reflected
at an equally small angle. This is the same principle relied on for polycapillary
optics. A few x-rays will excite atoms immediately at the surface, and
those atoms emit their characteristic radiation in all directions. Because
there is virtually no backscatter into the detector, extraordinary detections
limits can be achieve.
GEXRF turns the theory around
and takes advantage of the fact that when x-rays are directed at
a surface they will not be scattered at an angle below the Bragg
angle. A detector that only detects x-rays coming off a surface at
an angle less than the Bragg angle, will only detect fluorescence
x-rays and not background scatter.
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TXRF instruments are usually very sophisticated and expensive pieces of equipment
with finely tuned optics. The x-ray tubes are usually very high in power,
several kilowatts, and must have a small spot size on the anode. A long
collimator or wave-guide is needed to restrict the angle to less than the
Bragg angle. Using multilayers in the wave-guide can improve the efficiency.
The sample needs to be finely and reproducibly polished and positioned
precisely with respect to angle and height. A detector is positioned above
the surface. Given the sophistication of these systems, Si(Li) or other
high resolution detectors are used in most systems.
Some people prefer the GEXRF
variation. The x-ray tube can be directed at the sample with little
regard to spot size or angle. This saves on a lot of hardware expense.
A detector and collimator assembly is positioned so that only x-rays
coming from less than the Bragg angle are counted. back
While these techniques can achieve amazing performance, they are seldom used.
The principle problems are that only a few products are suitable for TXRF analysis
without a substantial amount of sample preparation. The other problem is that
the optical alignment is so critical that minor vibrations and temperature
changes make it necessary to re-align the optics, and/or calibrate the instrument.
These problems, in addition to the high cost of most existing systems, have
limited the use of these techniques to date. back