X-Ray Fluorescence in Biological Sciences. Группа авторов

are different, however, XRD is also considered a complementary technique to XRF [28]. For example, in a crystalline sample, XRF is used to determine the total concentration of Ca and Fe. On the other hand, XRD provides information on the phases or compounds in crystalline materials such as oxide materials, rocks, and minerals. It also furnishes information about the content of Ca phases such as CaO, CaCO₃, Ca(OH)₂, or the contents of Fe phases such as FeO, Fe₂O₃, Fe₃O₄, or Fe₃C [28]. Therefore, combination of both the techniques will provide better and more complete characterization of crystalline samples. So, the integration of innovative XRD systems allows both techniques to be included in the same instrument, which would bring significant advantages to the users such as:

       cost effectiveness,

       only one sample introduction,

       single user interface for both XRD and XRF,

       both phase and elemental information in single analysis,

       space minimization, and

       reduction of water cooling systems.

      Combination of XRF and XRD techniques is useful for numerous applications, which includes the analysis of element in all kinds of materials, free lime in clinker and slags, clinker phase, measurements of Fe²⁺ in sinters, iron phases in direct reduced iron processes, phases related to electrolysis of Al, thin films, etc. [28].

1.5 Comparison of XRF and Raman Spectroscopy

      Raman spectroscopy is an excellent spectroscopic analysis technique which is used to produce information related to the chemical structure, phase, polymorphism, crystallinity, and molecular interactions of the sample. This information, obtained by Raman spectroscopy, can be added with XRF to obtain better elemental information of the samples [23, 29]. In Raman spectroscopy, an intensive beam from mono‐energetic optical light source is used for inelastic scattering from the sample molecules. During the scattering process, the energy loss depends on the energy levels of the scattering molecule. Therefore, the Raman analysis method is used for the investigation of the energy levels of outer electron shells [23]. These levels provide information on the atomic and molecular compositions of the materials. Using laser radiation sources of different energy ranges, Raman spectroscopy (in particular, μ‐Raman) provides the possibility of focusing the light on small sample areas, which allows study of the sample materials identical to μ‐XRF [23].

A model for combining XRF and Raman spectroscopy has been prepared to test the possibility for the measurement of the constituents of sample materials [8, 23]. The μ‐XRF setup was created using a micro‐focus tube with an Rh‐target, polycapillary optics, and SDD. It was possible to mount the sample on a three‐dimensional motorized stage, and simultaneously observe the sample by using an optical microscope [8, 23]. It was also used to focus the incident laser light on the sample surface (particularly 633 and 785 nm). The laser light and the Raman‐spectrometer were connected to the microscope using optical fibers. This model can be seen in details elsewhere [23] and can be used for several kinds of analysis to investigate pigments and to analyze geological samples [23, 29, 30]. The methods used here are quite different in terms of measurement strategies and data interpretation, which requires thorough and deep understanding in order to get correct analytical results. Their use will certainly expand into all sorts of research areas. In addition, these instruments can be relatively affordable while also being non‐destructive and easy to use.

1.6 Conclusion and Prospects

      We have discussed a few analytical techniques including XRF and μ‐XRF used for the elemental characterization of materials. A concise visual chart has been included, from the literature sources for the reference, for most of the analytical techniques to compare the detection limits and analytical resolutions for materials characterization. Some important parameters that distinguish the analytical capabilities of the techniques such as elemental range, imaging possibility, depth resolution, and instrumental effort is summarized for better understanding. We have briefly discussed a comparative point of views for few analytical techniques (LIBS, XRF, LA‐ICP‐MS, SEM‐EDS, PIXE) and possibilities for their combination (XRD, SEM‐EDS, LIBS, Raman spectroscopy) for better analysis of materials. Differences among the analytical techniques such as LIBS, XRF, Micro‐XRF, LA‐ICP‐MS, and SEM‐EDS are discussed in terms of detection limit, elemental detection range, spatial and depth resolution, sample handling, experimental conditions, sample stress, and excitation sources. The advantages and limitations of some of the techniques are also elaborated.

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