X-ray fluorescence spectroscopy (XRF):
A technique of material characterization.
X-ray fluorescence spectroscopy (XRF) is a popular method for the qualitative and quantitative determination of the elemental composition of a material sample. XRF is similar to atomic absorption spectrometry and optical emission spectrometry except that the sample is not required to be dissolved in a solution to be analyzed. The sample for XRF analysis may be in the form of a solid, liquid, powder or emulsion.
XRF methods are of two types, viz. energy-dispersive X-ray spectroscopy (EDXRF) and wavelength-dispersive X-ray spectroscopy (WDXRF). Energy dispersive spectrometers measure x-ray intensity as a function of energy and are faster and less expensive. On the other hand, wavelength dispersive spectrometers are more sensitive, accurate and offer higher resolution.
Principle of XRF spectroscopy method:
For chemical analysis by XRF the sample under investigation is exposed to x-rays or gamma rays of high energy and is excited. In the process electrons from the inner shells of the sample atom are knocked off. The vacancy so created is filled up by the electrons from the outer electron shells. The energy in the form of fluorescence, corresponding to the difference between the energies of the two shells, is given off as an x-ray fluorescence having characteristic energy. The released energy is then detected by a fluorescence detector. As the electron energy levels are characteristic of the atom, in turn the energy of the emitted photon is also characteristic of the atom of each element. The x-ray spectrum acquired during the above process reveals a number of characteristic peaks corresponding to the different elements present.
A modern XRF instrument as shown in Fig.2 consists of an x-ray excitation source, a sample chamber, a silicon (lithium) detector, amplifiers, and a multi-channel pulse height analyzer.
Fig.2: Schematic of XRF method
The source of x-ray radiation can be a radioisotope such as Fe-55, Co-57 or Cd-109 or an x-ray tube capable of emitting x-rays of definite energy. X-ray tube for XRF spectrometer is a diode (vacuum tube) which consists of the filament generating thermo- electrons and the anode (target) generating x-rays. An electrically heated filament type cathode emits electrons. X-rays are generated when the accelerated electrons bombard on to the sample i.e. anode. This generates the fluorescent radiations. The fluorescent x-rays excited from the sample strike a detector. An energy dispersive detector such as a Si(Li) detector measures the energy distribution of the fluorescence radiation. The Si (Li) semiconductor detector is a diode with Li drifted over a high-purity single Si crystal, cooled by liquid nitrogen down to a temperature of 77K, and maintained in a vacuum. Finally, a multi-channel electronics circuit processes the measured signals. The measured spectrum shows lines or peaks that are characteristic for the chemical elements in the sample. In this spectrum the energy of the peaks (i.e., the location of the peak on the x-axis) enables the identification of the elements present in the sample (i.e. provide qualitative analysis of the material), whereas the intensity of the peaks (as given by the height of the peak/fluorescence count) provides the elemental concentration (semi-quantitative or quantitative analysis). Figure 3 gives a typical XRF spectrum.
Fig. 3: Typical XRF spectrum
For quantitative analysis several samples of known element concentration are subjected to measurements using XRF and a correlation is developed between the intensity of the measured element’s fluorescent X-rays and its concentration in the sample. Based on the calibration curve drawn, the concentration of various elements present in an unknown sample is found out.
Advantages and limitations of XRF:
The main advantage of XRF spectroscopy method is that it is a non-destructive, multi-elemental, fast and cost-effective method for elemental analysis of various materials including metals and alloys. The XRF method needs very simple & minimum sample preparation unlike atomic absorption spectroscopy (AAS) or flame photometry. Some samples require sample preparation such as pelletizing. It takes short measurement times. The XRF spectra are relatively simple and the peak positions are almost independent of the chemical state of the analyte. XRF method is applicable over a wide range of concentrations. The detection range covers all elements from boron to uranium and the concentration can range from 100% down to ppm level. Not only this, the measurement accuracy is of the order of around 0.1% of the major elements present.
The major limitation of XRF method is that the X-ray penetration of the sample is limited to the top 0.01 – 0.1 mm layer of the sample. Besides this the inter element (MATRIX) effects may become substantial and require computer correction.
Applications of XRF spectroscopy:
- Manufacturing and process industry: Quality control of raw materials, production processes and final products
- Aerospace industry
- Power industry
- Geology and mineralogy: Qualitative and quantitative analysis of soils, minerals, rocks, etc.
- Jewelry: Measurement of precious metals concentrations
- Food chemistry: Determination of toxic contaminants in food items
- Environment and agriculture: Measurement of undesirable metals such as lead, arsenic, cadmium & trace impurities in soils, sediments, water
- Archaeological studies related chemical analysis