Wavelength Dispersive of X-ray

The development of WD spectrometers goes back long before ED detectors became widely available in the early 1970s.

  The first electron probe microanalyzer (EPMA) was developed during the 1940s and used an optical microscope to observe the position and focus of the electron beam on the sample. Later, WD spectrometers were fitted to SEMs which allowed the specimen to be positioned more precisely under the electron beam and also made possible a visual picture of the distribution of a chosen element – the X-ray map.

On early commercial WDS spectrometers, all of the spectrometer mechanisms had to be moved by hand, and the operator had to physically exchange the crystals to cover the spectrum. The output from counters, recorded against time was sent to a chart recorder, and all the peak identification, peak and background measurements, and matrix corrections were done by hand.

Evolution of the WDS technique

The current generation of WDS spectrometers with their advanced control and analysis software make the technique considerably easier to use. Multi-crystal spectrometers now change crystals on-the-fly rather than first moving to a specified position on the Rowland circle, flipping to the appropriate crystal, and moving back to the correct position on the circle for analysis of the desired element. Software has been developed for quick, easy qualitative analysis by WDS. Operating parameters for the detectors have been optimized and entered into the software so that analysis setup is quick for the vast majority of samples. Comparing WD and ED spectra and combining WD and ED analysis is now routine and easy.

Evolution of the WDS technique

Wavelength Dispersive of X-ray

Wavelength dispersive spectrometers differentiate X-rays based on their wavelength.

They are mechanical systems consisting of two main components

1. An analyzing crystal

2. A detector.

Principle of OperationThe Wavelength dispersive X-ray spectroscopy

(WDXRF or WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging x-ray and the crystal's lattice spacings are related by Bragg's law and produce constructive interference if they fit the criteria of Bragg's law. Unlike the related technique of Energy dispersive X-ray spectroscopy (EDS) WDS reads or counts only the x-rays of a single wavelength, not producing a broad spectrum of wavelengths or energies. This generally means that the element must be known to find a crystal capable of diffracting it properly. The technique is often used in conjunction with EDS, where the general chemical make-up of an unknown can be learned from its entire spectrum. WDS is mainly used in chemical analysis, in an X-ray fluorescence spectrometer, or in an electron microprobe.

How it Works : They work on the principle of X-ray diffraction.

1. When the electron beam hits the sample, X-rays are produced.

2. A portion of those X-rays will hit the analyzing crystal in each of the spectrometers.

3. X-rays of specific wavelengths from the analytical crystal are passed on to the X-ray detector

4. For the diffracted X-rays to enter the detector the spectrometer must be properly aligned. This means that the detector, analyzing crystal, and the spot on the sample where the X-rays are generated, must all be on the circumference of a circle, called the “Rowland circle”.

5. Measure X-rays with different wavelength, the analyzing crystal and detector must be rotated around the Rowland circle such that the incident angle of the crystal changes, while maintaining position on the Rowland circle.

In practice ,the crystal is moved directly away from the sample and rotated, and the detector is moved around on the circumference the circle. As a result, the center position of the Rowland circle change, but its diameter stays the same. The position of the spectrometer can be given in terms of:1. The wavelength of X-rays it

collecting,2. The diffraction or incident angle,

or3. The distance between the

sample and the crystal(L-value)

The common feature of monochromators is the maintenance of a symmetrical geometry between the sample, the crystal and the detector. In this geometry the Bragg diffraction condition is obtained.

The X-ray emission lines are very narrow ,so the angles must be defined with considerable precision. This is achieved in two ways:

1. Flat Crystal with Soller collimators

2. Curved Crystal with Slit

Monochromators

 

1. Flat Crystal with Soller collimatorsThe Soller collimator is a stack of parallel metal plates, spaced a few tenths of a millimetre apart. To improve angle resolution, one must lengthen the collimator, and/or reduce the plate spacing. This arrangement has the advantage of simplicity and relatively low cost, but the collimators reduce intensity and increase scattering, and reduce the area of sample and crystal that can be "seen". The simplicity of the geometry is especially useful for variable-geometry monochromators.

Figure : Flat crystal with Soller collimators

Diffraction

 

Inside the spectrometer, analyzing crystals of specific lattice spacing are used to diffract the characteristic X-rays from the sample into the detector.

The wavelength of the X-rays diffracted into the detector may be selected by varying the position of the analyzing crystal with respect to the sample, according to

Bragg’s law : nλ=2d sin θ

Where n is an integer referring to the order of the reflection;

λ is the wavelength of the characteristic X-ray;

d is the lattice spacing of the diffracting material;

θ is the angle between the X-ray and the diffractor’s surface.

 

The Rowland circle geometry ensures that the slits are both in focus, but in order for the Bragg condition to be met at all points, the crystal must first be bent to a radius of 2R (where R is the radius of the Rowland circle), then ground to a radius of R. This arrangement allows higher intensities (typically 8-fold) with higher resolution (typically 4-fold) and lower background. However, the mechanics of keeping Rowland circle geometry in a variable-angle monochromator is extremely difficult. In the case of fixed-angle monochromators (for use in simultaneous spectrometers), crystals bent to a logarithmic spiral shape give the best focusing performance. The manufacture of curved crystals to acceptable tolerances increases their price considerably.

2. Curved Crystal with Slit

Identification of spectrally overlapped elements, such as S in the presence of Pb or Mo W or Ta in Si, or N in Ti Br in Al, common in semiconductor device failure

Detection of low concentration species (down to 100 or even 10 ppm) P or S in metals Contaminants in precious metal catalysts Trace heavy metal contamination Performance-degrading impurities in high temperature

solder alloys

Analysis of low atomic number elements Composition of advanced ceramics and composites B in BPSG films (sensitivity to 2000 ppm) Oxidation and corrosion of metals Characterization of biomedical and organically modified

materials

Applications include

WDS is used for non-destructive quantitative analyses of spots as small as a few micrometers, at detection levels as low as a few 10s of ppmw, and for elements from atomic number 5 (boron) and higher.

WDS works well in a variety of natural and synthetic solid materials, including minerals, glasses, tooth enamel, semi-conductors, ceramics, metals, etc.

The high spatial resolution of WDS not only allows quantitative analyses to be performed on small phases but also to detect chemical zoning on a small scale within a material (e.g. mineral).

Strengths

When the electron beam is rastered, the WD spectrometers can allow x-ray image maps of individual elements to be constructed.

When compared to EDS, WDS exhibits superior peak resolution of elements (Fig. 2) and sensitivity of trace elements (Fig. 3).

Strengths

Figure 2. Comparison of resolution of Mo and S spectral lines in EDS (yellow) vs. WDS (blue). In the EDS spectrum the molybdenum and sulfur lines are overlapped, but can be resolved in the WDS spectrum. Image modified from Oxford Instruments.

Figure 3. Comparison of EDS (left) and WDS (right) detection of trace Si.

Because WDS cannot determine elements below atomic number 5 (boron), several geologically important elements cannot be measured with WDS (e.g., H, Li, and Be).

Despite the improved spectral resolution of elemental peaks, some peaks exhibit significant overlaps that result in analytical challenges (e.g., VKα and TiKβ).

WDS analyses are not able to distinguish among the valence states of elements (e.g. Fe2+ vs. Fe3+) such that this information must be obtained by other techniques (e.g. Mossbauer spectroscopy).

The multiple masses of an element (i.e. isotopes) cannot be determined by WDS, but rather are most commonly obtained with a mass spectrometer (see stable and radiogenic isotope techniques).

Limitations