**3.1.4. Inductively coupled plasma spectrometry**

[21],[22],[23],[24] and the effect of substitutions in the apatite structure [25]. Neutron diffrac‐ tion data enable to explain the oxygen over-stoichiometry in the structure of La9.67(SiO4)6O2.5 apatite [17]. Since neutrons make possible the accurate determination of the thermal factors and provide the visualization of the diffusion paths in ionic conductors, powder neutron diffraction is also used for the characterization of solid oxide fuel cell materials [26]. This method is also used to investigate apatite in hard tissues where it provides the evidence about the deficiency of hydroxyl ion in bone apatites [27] and reconfirms that the inorganic por‐

**3.1.3. X-ray fluorescence analysis and total-reflection X-ray fluorescence analysis**

X-ray fluorescent analysis (XRF) is a method for the determination of sample composition [29], [30]. The origin of characteristic X-ray spectra can be described as follows. When sufficient energy is introduced into the atom, the electrons may be knocked out of one the inner shells. The atom is then in an excited (ionized) state and returns to the ground state within 10−8 s. The place of the missing electron is filled by an electron from a neighboring other shell, the place of which, in turn, is filled by an electron from more outer shell. The atom then returns to the ground state in steps. In every step, i.e. in every electron jump, the electron from a higher energy level goes into a lower energy level emitting excess energy in the form of an X-ray quantum. The energy of emitted radiation is characteristic for the atomic number of emitting element as well as for particular electron transition taking place within the electron shell of the atom. By measuring the energy or the wavelength of emitted radiation, the particle element

The energy that is necessary for the atom to get to excited state can be introduced either by the collision with a high-energy electron (sample is bombarded by electrons which are accelerated by high-voltage) or by the absorption of an energy-rich photon, i.e. the X-ray quantum (sample is irradiated by X-ray or gamma rays). In modern X-ray fluorescence analysis, the sample is irradiated by polychromatic radiation from an X-ray tube. In analogy to the optical case, this technique is referred to as fluorescence, which is responsible for the name X-ray

There are two types of instruments (**Fig. 1**) used for X-ray fluorescence spectrometry [32],[33]: **1. Wavelength**-**dispersive XRF (WDXRF) or total reflection XRF (TRXRF):** the method is also often abbreviated as XRF. X-rays impinge on the sample (**Fig. 1**(**a**)) and generate fluorescent X-rays. These are then diffracted on a crystal. A goniometer selects the geometry between the crystal and detector that controls the detection of X-ray from the element of interest. Different crystals have different sensitivities. Many of commercial WDXRF instruments have two detectors and up to six crystals to optimize the condi‐

**2. Energy-dispersive XRF (EDXRF):** the method is also abbreviated as EDX. The EDXRF instruments use much less energetic X-ray tube. Emitted X-ray radiation from the sample impinges directly on a detector, typically Si(Li), which generates pulses on an incident beam. These pulses are sorted and counted by a multichannel analyzer (**Fig. 1**(**b**)).

fluorescence analysis as the technique of spectrochemical analysis with X-rays [31].

tion is basically a hydroxylapatite-like material [27],[28].

114 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

can be identified unambiguously [31].

tions for each element.

Prior to inductively coupled plasma6 (ICP), the flame, direct current-arc and controlledwaveform spark were used for the atomization (i.e. decomposition of sample to individual

<sup>6</sup> Plasma is defined as an electrically neutral gas which consists of positive ions and free electrons. Plasma have sufficiently high energy to atomize, ionize, and excite virtually all elements in the periodic table, which are intentionally introduced into it for the purpose of elemental chemical analysis [45].

atoms) and excitation of sample in elemental analysis. ICP denotes the technique that uses atmospheric pressure argon inductively coupled plasma7 (ICP) for the atomization and excitation of sample. This plasma is a highly energetic media consisting of inert ionized gas with equivalent temperatures from 7000 to 10,000 K. Inductively coupled plasma are formed by coupling energy produced by RF generator (typically 700–1500 W) to the plasma support gas with an electromagnetic fields [45]. The cross-section of typical ICP torch is shown in **Fig. 2**.

**Fig. 2.** Plasma with torch assembly and load coil [45].

The treatment of sample before the quantification includes vaporization, atomization, excitation and ionization.8 The introduction of analyzed sample into inductively coupled plasma was applied in analytical techniques including [29],[45],[46],[47],[48]:


<sup>7</sup> Although there are many types of plasma, such as direct current, microwave induced, etc., the ICP is considered the most useful technique for analytical spectroscopy [45].

<sup>8</sup> Each element has characteristics first and second ionization potential, which depends on specific electronic structure of given element. Higher ionization potential means that more externally applied energy is required for ionization (thermal radiation, collision with other ion or electron, or exposure to high-energy photons) [45].

analyte species in original sample solution. Ions produced by ICP are representa‐ tively sampled and extracted from the plasma; next they are separated and meas‐ ured by a quadrupole or time-of-flight mass spectrometer

The method known as laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) is a coupling technique of laser ablation with ICP-MS technique [49],[50],[51],[52], [53]. Multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) was applied as the benchmark method for isotopic analysis [54] and for the determination of heavy rare earth elements in apatites [55],[56],[57].
