XRF is a widely used method for measuring the elemental composition of rocks, soil, and minerals sample. Standard reference materials are constantly needed in XRF analysis to ensure the reliability of analytical results. They play an important role in during the development of new analytical techniques, methodologies, and new sample preparation procedure. It also has a significant role in assessing short and long term stability of instrumentation, in detection of random and/ or systematic errors during routine analysis, for cross-calibration of different analytical techniques and methodologies and in laboratory inter-calibrations. Basic components of XRF:
All the spectrophotometers are basically composed of a sample holder, a source for excitation of atoms and detection system.
The primary function of a source in spectrophotometers is to irradiate a sample, to excite the atoms present in the sample. The emitted radiations are measured by the detector. In XRF instrument the excitation source is an x-ray tube. Spectrometer system is generally of two groups: energy dispersive system (EDXRF) and wave length dispersive system (WDXRF). Detection system is different in these two. In EDXRF spectrometer the detector measure different characteristics energy that are emitted directly from the sample. The detector can set apart the radiation from the sample into the radiation emitted from their respective elements that are present in the sample that is what called dispersion. Detection range of elements is from sodium to uranium and detection limit is good for heavy elements.
In WDXRF spectrometer there is an analyzing crystal which disperses different energies. The radiation strikes the crystal and diffracts into different direction. The detection range of elements is from Beryllium to uranium. Detection limit is good for both lighter and heavier elements.
Working principle of XRF:
The primary x-ray beam from x-ray tube falls on the sample, interact with the atoms in the sample. The electrons disposed off from the inner shells of atom. The displacement of electrons from the inner shell is due to difference in energy between primary x-ray beam emitted from the analyzer and the binding energy of electrons in their shell which hold them in their proper orbits. In this whole process the energy of x-ray is higher than the binding energy of electron. The knocked out electron leave behind a vacancy which make the atom unstable. The electrons from higher energy orbits jumps down to fill that vacancy with the release of radiations called secondary x-ray beam/ flourescence. The amount of energy lost is equivalent to the difference between two electrons shell i.e., ΔE= E2-E1.
E2= energy of higher shell
E1= energy of inner shells.
ΔE= difference between two energy level
The amount of energy lost is unique to each element present in the sample and can be used to identify the elements. The individual energies are calculated by the detectors. The quantity of each element be measured from the proportions of these individual energies.
Time required for XRF analysis:
Time taken by the sample for measurement depends on the nature of sample and levels of interest i.e., what type of elements are to be analyze and in which form sample is to feed in XRF. High percentage elements required a few seconds while part-per-million levels take a few minutes. The process of excitation and the emission of secondary x-rays during de-excitation process of atoms present in the sample occur in a small fraction of seconds. Modern handheld XRF can be design in a matter of seconds for such measurement. Generally the measurement time is in the range of seconds to 30 minutes and depends on the number of elements to be analyzed. After measurement analysis of sample takes a few seconds. For detection of major elements, sample is loaded in the form of glass bead/ fusion bead which takes approximately eight seconds.
Importance of XRF:
XRF has become a popular method in elemental analysis in geological investigation. This is because of following reasons.
Sample preparation for XRF analysis is quite easy and cheaper.
Measurement of elements in sample is rapid.
It does not require highly experienced analyst. Even a trained assistant can run and handle the machine.
XRF method is not so expensive like ICP method.
Unlike ICP method it’s not a sample destructive method.
This method is highly accurate and precise.
Detection limit is very good.
Application of XRF:
XRF method for sample measurement has broader applications. It can accurately measure all metals, cement, oils, polymer and plastics. The applications also include environmental analysis of water, waste materials, rocks and soil analysis.
Limitations of XRF:
XRF cannot measure organic samples.
XRF is not helpful in measuring isotopes of elements.
Types of sample preparation:
Several methods have been used for the analysis of powdered samples, such as rocks, minerals and ceramics by using x-ray fluorescence (xrf). These methods includes the pellet (briquette) method (tertian and claisse, 1982; feret and Jenkins, 1998; matsumoto and fuwa, 1979; guevara and verma, 1987) and the glass bead (fusion) method (Tertian and Claisse, 1982; Feret and Jenkins, 1998;norrish and Thompson,1990; hua and yap,1994) for normal amounts of sample. for small amounts of sample, the filter cake method (stankiewivez et al., 1996) which consists in filtering the suspension of powder or precipitate is used. The most widely used method among these is pressed powder method. Because pressed powder method is simple, takes less time in preparation and offcourse non-destructive. Both solids and liquids can be measured analytically with the help of xrf. Mostly samples are in circular disks with a radius between 5 to 50 mm. the sample is placed in a sample cup which is placed in the spectrometer. For the analysis of powder and liquids special supporting films are used.
Functions OFA binder
The functions of a binder are;
It holds the particles together after the pellet is dried and before it is finally hardened.
During drying process, the binder holds the particles of sample together while the water is removed; it continues to bind particles together until the pellet is heated sufficiently.
Classifications of binding systems
Binders can be defined as anything that cling the particles together and form a mass. Some binders are specific to particular type of material. So, they cannot all be used in all possible application. Therefore, binders are categorize into following five groups (Holley 1982):
The binder adhere the particles of sample together by forming a sticky layer over the particles. The binding forces may be adhesive or cohesive. Binding is reversible in case of inactive film binders.
The binder makes a film over the surface of the particles of sample then hardens after passing through a chemical reaction. It is irreversible type of binding.
A continuous matrix is formed by such binders in which particles of the sample are embedded. Such binders require high pressure that forced the particles to compact. The binders when heated emulsified to form fluid. On cooling it becomes hard and dry. It includes tar, pitch or wax type of materials. Binding when heated is reversible.
A continuous matrix is formed by such binders. As its name indicates, it undergoes through a chemical reaction that cause it to harden. Binding is irreversible.
The binder undergoes a chemical reaction with the sample material and form a strong bond with it. Such binders are specific for particular materials only and binding is irreversible. Widely used binder is inactive film because it holds the particles without chemical reaction and compaction pressure and is effective at low dosage.
The samples are weighed together with the binder with ratio 10:1. Subsequently the specimen is pressed with 20 ton feet pressure for one minute. MiniPress model PW4020/00 hydraulic press is used for pressing of sample powders. A PW2404 (Philips, Netherlands) AXIOS Advance wavelength dispersive PANalytical type spectrometer (WDXRF) with a SuperQ software was operated with a Rh tube at a maximum 60 kV and 125 mA.
Eivindson. T, and 0yvind Mikkelsen, looked over the problems by using pressed powder for XRF analysis for ferrosilicon alloys. Due to heterogeneity of the solidified ferrosilicone the problem aroused. Different distinct crystallographic phases with varying X-ray absorption and grinding properties are formed from the molten metal. It results in large particle size effects. Which in turn affect the accuracy and precision of the XRF measurement and analysis. Despite of these, the stability of powder pellets also got affected. To have a pellet that is more stable and less affected by radiation, the choice of a binder is necessary. To minimize the problems arise by using pressed powder it is important to strictly control the sample preparation routines.
Ahadnejad, et al, tried to set a simple and fast analytical standard for x-ray fluorescence spectrometry. 15 typical samples from granitic rocks of Malayer granitoid complex western iran, were selected to obtain a standard representative of the mineral samples. Their Biotites were separated and then analyzed by using ICP and use them as new standards. The separated minerals were evaluated as candidates for refrence materials for major element composition. The minerals were analyzed by using ICP method in AMEDL laboratories, Australia. Their results were used for making calibration curves to xrf method at tarbiat moallem university of Tehran in order to check their performance as reference materials. Same samples are analyzed with the help of new xrf application. Both the results are similar showing that xrf technique is simple and effective as well.
The majority of iron ores must be ground to a fine particle size to allow the iron oxides they contain to be concentrated, and the concentrate must then be agglomerated back into large enough particles that they can be processed in blast furnaces. The most common agglomeration technique is pelletization, which requires the use of binders to hold the iron oxide grains together so that the agglomerates can be sintered into high-strength pellets. Although bentonite clay is the most commonly used binder, there are many other possibilities that could be competitive in a number of situations. This article reviews the numerous types of binders (both organic and inorganic) that have been considered for iron ore pelletization, including discussion of the binding mechanisms, advantages and limitations of each type, and presentation of actual pelletization results, so that the performance of the various types of binders can be compared and evaluated.
Sample Preparation for Pressed Pellets
Herzog SHM 250 grinding machine is used for grinding the sample to a fine particle size. Specifically chosen Binders were mixed with 20g of powdered sample in a mixing vessel. Using stainless steel rings and disc and briquetting presser (Mekawa testing machine), powder is forced to press under high pressure of 20tf. The final product is a solid pellet or tablet. This is a common method for preparing samples for XRF analysis. The consideration of particle size of the sample, the choice of binder, the dilution ratio, the amount of pressure applied to the sample, thickness of the final pellet and sample contamination is important while designing a sample preparation protocol.