Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 25

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.