X-ray fluorescence spectroscopy is one of the most advanced and adaptable methods used for analysing and characterising material. X ray fluorescence is becoming more and more sophisticated as its development in recent years has perceived the idea of just been a bench instrument. In 1895 in the University of Wurzburg, Wilhelem Roentgen discovered x-rays while performing experiments with discharge cathode tubes. However to Roentgen unfortunate lack of scientific evidence, he could not say for certain that the rays he found were actually x-rays.
In 1905 Barkla, discovered the nature of the waves of x-rays when passed through a body. The x-rays scattered and never followed their original direction within a body; this was later known as polarization of x-rays.
In 1912 Laue, Freidrich and Knipping performed advanced studies on Roentgen discovery which essentially lead to the theory of x-ray diffraction by a crystal, the crystal in a sense acted as a grating giving a 3D diffraction grating. In 1913, an English physicist Bragg demonstrated by means of previous work performed by Laue, Freidrich and Knipping, how the x-ray diffraction experiments showed how the radiation had sharply defined wavelengths.
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Bragg's theory led to the technique which we know today as crystallography.
Also in the same year, 1913 an English physicist Henry Gwyn Jeffrey's Moseley demonstrated that wavelengths were not only characteristic to that element been analysed, but they also had the same atomic number sequence. This discovery played a very vital part in the determination of atomic numbers unambiguously for the first time.
Later it was discovered that secondary fluorescent x-rays were excited in any material exposed with primary x ray beams, this lead to an investigation into the possibilities of x-ray fluorescence spectroscopy as a means by which elemental analysis could be determined quantitatively and qualitatively (Assmus, 1995).
Principles behind x-ray fluorescence
X-rays are part of the electromagnetic spectrum, which displays electromagnetic waves of extremely short wavelengths, approximately 100A to 0.1A.
X-ray fluorescence is a widely used analytical technique for quantitative and qualitative analysis of elements. X-ray fluorescence has more advantages to its technique when compared to more competitive techniques like inductively coupled plasma spectroscopy (ICPS), Atomic absorption spectroscopy (AAS), and neutron activation analysis (NAA). X-ray fluorescence is generally non destructive, fast, cost effective and can analyse multi elements. It produces a uniform detection limit across a large section of the periodic table and is adaptable to a wide range of concentrations, from a 100% concentration to parts per million concentrations. Its main disadvantage is its ability to analyse elements heavier than fluorine. (Mermet et al. 1997).
X- Ray fluorescence is a form of photoluminescence an analytical technique in which molecules of an analyte are excited. This excitation gives rise to an emission spectrum of the analyte, the result of which provides vital information for quantitative and qualitative analysis. Fluorescence and phosphorescence are very similar techniques, however in fluorescence an inner shell electron is excited by primary x-rays. The x-rays cause some of the inner electrons in the analyte to dislodge and are ejected as a result, causing vacancies in the inner shell. Electrons in the outer higher energy orbital's jump into these vacancies, the difference in energy between the two shells in the process by which an emission of secondary x-rays is given off, this secondary emission is known as fluorescence. The x-ray spectrum obtained during the emission process shows a number of characteristic peaks, these characteristic peaks lead to the identification of the elemental metals present in the sample. The intensity of the characteristic peaks provides relevant concentration of the element (Stephenson, 2010).
X-ray fluorescence instrumentation
The x-ray fluorescence instruments are categorised as follows
The wavelength dispersive x-ray fluorescence spectroscopic instrument was first introduced in the 1950's and has developed significantly since the mid 1970's. The wavelength dispersive spectrometers are available as single channel or multi channel. The single channel is used for routine and non routine testing of ferrous and non ferrous alloys, ores, minerals, and oils. The single channels are adaptable somewhat, but compared to the multi channel spectrometers, the single channel spectrometers are slow. The multi channels are more commonly employed for routine analysis because of their accuracy, speed and resolution.
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The energy dispersive spectrometers show more advantages over the wavelength dispersive, the energy dispersive has the ability to display the relative information on elements under investigation at the same time, and also the energy dispersive is more adaptable in the quality industry but also in the field of forensic science as it is less prone to troubleshooting issues.
The primary source
The primary source consists of a very stable, high voltage generator which provides up to, and around 3kW of power at a potential of 60-80kV, and a sealed x-ray tube. The x-ray tube has an anode which delivers an intense source of continuous radiation which is projected onto the analyte which is been examined, where radiation is generated by the analyte, which is characteristic to the radiation generated by the anode. A part of the characteristic radiation is retained by the spectrometer where a beam is passed via a slit onto the surface of analyzing crystal, where elemental wavelengths are diffracted in accordance to Bragg's law. A photon detector usually in the form of a gas flow counter, is then used to convert the diffracted photons into a electrical pulses which are integrated and displayed as a measure of characteristic line intensity. There are many different types of sources used for the excitation of the characteristic x-radiation; some include electrons, γ-radiation and synchrotron radiation. Today the most commonly used source is an x-ray photon source. The x-ray photon source is used as a primary mode in wavelength and energy dispersive fluorescence. The γ-radiation is a radioisotope that is employed directly or in a mode similar to that in secondary fluorescence mode in energy dispersive spectrometry. Wavelength dispersive x-rays use a higher powered x-ray source e.g. bremsstrahlung source. The energy dispersive can use either a high or low power primary source depending if the spectrometer is used in either the primary or secondary mode. The primary source is a stable, high energy generator enabling a potential of 40 to 100kV. The current from the generator is fed the filament of the x-ray tube which is normally tungsten wire. The applied potential causes the tungsten filament to glow emitting electrons. Some of the electron cloud is pushed to the anode of the x-ray tube, which is a cooled copper block with the necessary anode material plated to its surface. The electrons produce x-radiation which, a percentage of which passes through a small beryllium window to the sample (Jenkins 2000).
X-ray detector is a transducer for the conversion of the x-ray photon energy into electrical pulses. The detectors work on a basis of photoionization, which occurs between the photon of the x-ray and the detector which produces electrons. The electrical current produced by the electrons is converted into a pulse by means of a capacitor and a resistor. A digital pulse is formed for the x-ray photon. The photon energy is sensitive and as a result is suitable for a range of wavelengths. For every x-ray photon entering a detector, a pulse is produced, where the size of the pulse is proportional to the energy of the photon and the detector is proportional to that pulse of energy (Jenkins 2000).
The energy dispersive x-ray fluorescence shows no physical discrimination of secondary radiation that leaves a sample and enters the detector. The photon of all energies in the secondary beam interacts with the detector. The detector and its associated signal processing chain has limited capacity to process events and results in a range of 1 to 50Kcps.