Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 56

The theory of XRF has been already described in earlier chapters of this book and elsewhere in literature [5, 6]. However, for the sake of continuity a brief description of it is being given in this chapter. When high energy X‐rays (energy >1 keV) fall on a sample, they cause ejection of the core‐shell electrons depending on their binding energies. Thus, electron vacancies are created and the atoms become unstable. The atom stabilizes itself by filling the vacancies by transition of electrons from outer shells to the vacant positions.However, the outer shell electrons moving to inner shell to fill the vacancies created by incident X‐rays leave vacancies in outer shells which in turn are filled further from next outer shell electrons. This activity starts a chain reaction to fill the vacancies, thus created by incident X‐rays and electrons moving to inner shells from outer shells. This process results in emission of photons having energies equal to the difference in binding energies of the two shells involved in such electron transitions. The photon energies emitted due to such transitions are in the X‐ray energy region and have systematic nomenclature. The energies of these X‐rays are characteristic of the the particular element involved and hence these are called characteristic X‐rays. The energies of the characteristic X‐rays are related to the atomic number of the element by Moseley's Law [5]. The intensities of the elemental X‐ray lines, thus emitted, are proportional to the elemental concentrations in ideal situations and give information about the concentration of the elements present in the samples. After excitation of the samples with the X‐rays, the samples do not get destroyed and remain in same form. For this reason, XRF analysis is considered a non‐destructive and non‐consumptive analytical technique, though it requires some additional sample preparation steps involving pelletization, dissolution, or bead making. The sample specimen after XRF analysis remains available for further investigation and can be used for further studies by other methods or can be reused for repeating XRF measurements, in case of some doubt in analysis. In addition to the elemental analysis, XRF can be used to find out the distribution of the elements in samples such as bones, hairs, nails, and cancerous and normal tissues using a very small X‐ray beam spot of size of a few μm (the technique is then called μ‐XRF). The distribution of elements in the body parts such as bones, hairs, nails, etc. during treatment through medicines can be studied using μ‐XRF. The above description shows that XRF analysis is very simple technique and can produce the several insights in biological samples as elaborate above. However, it has some limitations as well. It's first limitation is its inability to detect lower concentrations of elements in the ppb level.This is due to high background resulting from scattering of the X‐rays penetrating deep into the sample and emitted electrons loosing their energies in form of X‐rays coming as spectral background. Due to this limitation, XRF requires a comparatively larger amount of sample during analysis, which is not feasible always especially in case of precious, toxic, or scarce samples, such as forensic and biological samples. In addition, the XRF analysis gives inaccurate results in absence of matrix matching standards due to severe matrix effects resulting from deep penetration of X‐rays into the samples. These two limitations, i.e. higher detection limit and severe matrix effects, limit the applicability of XRF in studying the trace and ultra‐trace elements in biological samples.