X-Ray Fluorescence in Biological Sciences. Группа авторов

conditions and the cross‐sections for the excitation. Figure 1.4 clearly shows the excitation efficiency for electrons and X‐rays that indicates that the cross section of electrons and X‐rays for exciting the K‐shell depends on the atomic numbers of the atoms [23]. This diagram reveals the high efficiency (high cross section) of electron excitation for lighter elements. In electron microscopes, lighter elements can easily be detected; even boron (B) or beryllium (Be) can be measured. However, heavy elements can be detected easily by excitation with X‐rays with better efficiency. Therefore, all the elements with atomic number greater than 20 (such as Ca) exhibit higher peak intensities and better sensitivities with X‐ray excitation [23]. This also results in better LODs in the case of heavy elements, as demonstrated in Figure 1.5 [23].

Figure 1.4 K‐shell cross sections for electron and X‐ray irradiation. Here X‐denotes the emission due to X‐rays.

      Source: Reproduced from Haschke and Boehm [23] with permission from Elsevier.

Figure 1.5 Limit of detection for electron and X‐ray excitation (for similar measurement conditions for electron and X‐ray excitation).

      Source: Reproduced from Haschke and Boehm [23] with permission from Elsevier.

      Use of X‐ray excitation makes it possible to obtain a deeper view of the sample that provides a more illustrative analysis for bulk materials [8, 23]. Using poly‐cap optics, μ‐XRF provides similar information as offered by SEM‐EDS the spot diameter and it can also be used to examine the coating systems with thicker layers and multiple layer systems [8, 23]. Overall, as compared to SEM‐EDS, μ‐XRF offers easier sample handling, experimental conditions, and other factors which provide fast analytical results and increases its applicability for a broad variety of materials.

      SEM‐EDS and μ‐XRF have a high potential for analyzing non‐homogeneous materials and thus μ‐XRF is considered a complementary analytical technique to SEM‐EDS. The other analytical capabilities of the two methods are shown in Figure 1.1 and tabulated in Table 1.1. Combined EDS and μ‐XRF provide complete sample characterization by combining the better light element sensitivity of electron excitation with the better trace element sensitivity of XRF. Simultaneous mapping with μ‐XRF and electron beam excitation combines the advantages of both the techniques, exciting the light elements (C to Na) using the electron beam and heavier elements by μ‐XRF [3, 8, 23].

      1.3.7.2 Combination of SEM‐EDS and μ‐XRF

      μ‐XRF can be combined with other well‐established methods and can provide some complementary information related to the material. Using this complementary information provided by μ‐XRF in combination with SEM‐EDS, the detection range of elements can be extended to lighter elements [24, 25]. This kind of combination of different techniques into a single instrument with sequential or even simultaneous data collection can be interesting for spatially‐resolved analysis. This would be useful for analyzing different parts of the sample, and it will also be very beneficial in terms of the use of the same sample chamber if the measurement requires a high effort such as high vacuum. A representative model of this kind of combination is the electron microscope, which generates mapping images and also reveals the elemental contents using SEM‐EDS and structural information using EBSD.

The combination of μ‐XRF and SEM‐EDS is possible and quite easily achieved by adding an X‐ray tube and focusing optics to the electron microscope. A detector is already available for monitoring the fluorescence radiation excited by electrons and/or by X‐rays. Few experimental set ups for XRF‐featured electron microscopes have already been built and offered by few reputed companies [8, 23]. The diagrammatic view of the combination of XRF with an electron microscope can be seen in Figure 1.6 [23] which shows the excitation by a μ‐XRF source in an SEM.

Figure 1.6 Excitation geometry in an SEM with a μ‐XRF source.

      Source: Reproduced from Haschke and Boehm [23] with permission from Elsevier.

      The X‐ray source (tube) with the optic is generally mounted on a flange of the SEM and the X‐ray beam is focused to the spot position of the electron beam (as can be seen in Figure 1.6). It is also important to state that the analyzed volume for both the techniques is quite different. For the electron beam, the analyzed volume is in the range of few μm³ but for the μ‐XRF it is in the range of few thousands of μm³ (information depth and excited area >20 μm).

      That caveat notwithstanding, both the techniques furnish the qualitative and quantitative chemical compositions of the materials. However, they have different sensitivities for different energy ranges. For light elements, SEM‐EDS provides better sensitivity. For heavier elements, μ‐XRF gives better sensitivity [8, 23]. Thus, combining both, a better elemental characterization of the material from lighter elements to heavier elements can be expected.

      In addition, it is possible to use the results of one technique for the refinement of the results of the other due to the different excitation probabilities of EDS and μ‐XRF. In μ‐XRF, the quantitative analysis of the elements is mostly carried out using fundamental parameter (FP)‐models with a normalization of 100%. But in the case of organic samples, lighter elements cannot be analyzed using XRF. In that case, using the results of EDS for lighter elements, the results of XRF analysis can be improved significantly [25]. This combination of μ‐XRF and SEM‐EDS is very beneficial for the exhaustive material characterization of samples such as organic materials and oxides that contain a lot of C, H and O, which are not possible to measure directly by μ‐XRF. This combination can be easily made possible instrumentally by an X‐ray tube with an X‐ray optic as an optional feature on electron microscopes.

1.4 Comparison of XRF and XRD

X‐ray diffraction (XRD) is a well‐established technique used for the investigation of crystalline materials, which provides information on the structures, phases, texture analysis, average particle size, and crystalline properties of the materials. The combination of XRF and XRD may be uniquely useful because information about the chemical composition of the materials obtained through XRF analysis may be used for their phase analysis with XRD. With that information, phase analysis of the materials is more accurate and faster because many phases with similar structures can be reduced given knowledge of the chemical composition of the materials. A few instrumental concepts combining XRD and XRF are available and an analysis area less than 100 mm has been achieved. This combination can be integrated in one instrument and is useful in cement industries for the analysis and characterization of raw materials [8, 23, 26, 27].

      XRD analysis is also position‐sensitive with the use of X‐ray optics in conventional instruments, such as use of capillary optics in combination with a monochromator, or synthetic multi‐layer