ANALISIS DE RAYOS X FRX

Consideration of matrix effects in XRF analysis

X-Ray Fluorescence analysis (XRF) is a nondestructive physical method used for chemical elemental analysis of materials in the solid or liquid state. The specimen is irradiated by photons or charged particles of sufficient energy to cause its elements to emit (fluoresce) their characteristic x-ray line spectra. The detection system allows determining energies of the emission lines and their intensities. Elements in a specimen are identified by their spectral line energies or wavelengths for qualitative analysis, and intensities are related to concentrations of elements providing opportunity for quantitative analysis. Computers are widely used in this field, both for automated data collection and for reducing the x-ray data to weight-percent and atomic-percent chemical composition or area-related mass.

When a primary x-ray from an x-ray tube or a radioactive source strikes a sample providing excitation, the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the "photoelectric effect." During this process, if the primary x-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies lead to an instability of the atom. As the atom returns to its stable state, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic x-ray which energy is determined by the difference between the two binding energies of the corresponding shells. All elements emit X-rays at their own characteristic energies. These X-rays are called "K lines" if they are emitted by an electron filling the innermost shell called K, and "L lines" if they result from filling the next electron shell, the L shell. Sometimes, as the atom returns to its stable condition, instead of emitting a characteristic x-ray it transfers the excitation energy directly 1

to one of the outer electrons, causing it to be ejected from the atom. The ejected electron is called an "Auger" electron. This process is a competing process to XRF. Auger electrons are more probable in the low Z elements than in the high Z elements. Because each element has a unique set of energy levels, each element produces x-rays at a unique set of energies, allowing one to non-destructively measure the elemental composition of a sample. The process of emission characteristic x-rays is called "X-ray Fluorescence," or XRF. Analysis using x-ray fluorescence is called "X-ray Fluorescence Spectroscopy." In most cases the innermost K and L shells are involved in XRF detection. A typical x-ray spectrum from an irradiated sample will display multiple peaks of different intensities. X-ray energy spectrum is shown in Fig. 2.

Fig. 2. X-ray energy spectrum of Pb.

X-ray fluorescence technology (XRF) provides one of the simplest, most accurate and most economic analytical methods for the determination of the chemical composition of many types of materials. It is non-destructive and reliable, requires no, or very little, sample preparation and is suitable for solid, liquid and powdered samples. It can be used for determination of a wide range of elements, from potassium (19) to uranium (92), and provides detection limits at the ppm level; it can also measure concentrations of up to 100% easily and simultaneously.

Heavy and toxic elements determination in environmental samples (geological and ecological, plants, herbs, soil, etc.) using XRF. 26 elements like K, Ca, Ti, Cr, V, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Cs, Ba, La, Pb can be determinate simultaneously in the sample. Detection limits for the different elements are between 1 and 5 mg/g depending on the matrix and Z of the element. Relative errors between 1 and 10% are typical for trace element analysis. A certain advantage of the method is the relatively simple sample preparation procedure.

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Traditionally, all XRF spectrometers have been based on radioisotope excitation. However, recent advances now enable small X-ray tubes to generate X-rays in XRF spectrometers without the need for radioactive materials.

Sources of the characteristic X-ray

The characteristic X-ray of the elements will be excited by ring-shaped Cd-109 (Eγ = 22.16 keV, T1/2 = 453 days) or Am-241 (Eγ = 59.6 keV, T1/2 = 432.2 years) sources.

XRF detector

The samples will be analyzed by X-ray fluorescence spectrometer a with Si(Li) detector (having 30 mm2 surface, 3 mm thickness, energy

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