X-Ray Fluorescence in Biological Sciences. Группа авторов

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Название X-Ray Fluorescence in Biological Sciences
Автор произведения Группа авторов
Жанр Химия
Серия
Издательство Химия
Год выпуска 0
isbn 9781119645580



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to be used depends strongly on the vegetal matrix, the amount of sample used to prepare the pellet, and the pellet size. For instance, to make solid pellets of 3.2 cm in diameter, it was necessary to add 150–200 mg of cellulose to 500 mg of savannah grass powder [31]. To press the pellets usually requires a hydraulic or manual press and an adequate die set, which includes a die body, base, a plunger and two polished metal disks. These disks are usually made of a metal alloy such as tungsten carbide which proved to be useful for pressing this type of samples. At present, there are a broad variety of analytical methods in the literature to prepare vegetation pressed pellets involving different sample amounts (usually 0.5–5 g) and pellet dimensions (10–40 mm).

      If the amount of plant sample available is limited (i.e. roots), the option to make a stable pellet is unpractical and in this case the vegetation sample can be presented to the XRF spectrometer as a loose powder by packing it in a cell or by distributing the powder on an adhesive tape or X‐ray foil (made of polyester, polypropylene, polycarbonate, etc.). Using this latter approach, a plant layer ranging from 0.8–2.5 mg/cm2 can be obtained [32]. However, in view of the inherent X‐ray absorption by the foil, the sensitivity for low Z elements such as Na is reduced in comparison with the analysis of pressed pellets. Another shortcoming of loose powder preparation is the worse precision of the results obtained due to the bulk density and grain size differences between replicate measurements. In any case, depending on the analytical purpose, the loose powder preparation method can be a fast analytical alternative above all when the amount of sample is limited and WDXRF, 2D‐EDXRF, and 3D‐EDXRF systems are used.

      Another interesting alternative when the amount of vegetation sample to be analyzed is scarce is the use of TXRF systems. As it was discussed in the previous section, to perform analysis by TXRF the sample should be presented as a thin film by deposition of a small amount of sample (a few μg or μl) on a reflective carrier. In terms of sample preparation procedure, an additional advantage of TXRF is the possibility to prepare easily the vegetation sample by suspending several milligrams of the powdered sample (10–50 mg) in an adequate disperser and depositing a few μl (5–20 μl) of the suspension on a reflective carrier. This approach has demonstrated to be effective, for example, in multi‐elemental analysis of vegetal foodstuff [24]. Nevertheless, it is interesting to remark that a careful study of all the parameters affecting suspension preparation and TXRF analysis (i.e. amount of sample, suspension concentration, dispersant type, sample deposition volume on the reflector, etc.) should be evaluated in order to obtain reliable results. Another important aspect to be considered to prepare homogeneous solid suspensions and to minimize the particle size agglomerations caused by the presence of particle superficial electrostatic forces when using suspension preparation, is to grind the sample to a particle size <100 μm and use high energy ultrasound treatments before sample deposition on the reflector. Finally, TXRF analysis is especially suited when calibration standards with a similar matrix to the analyzed samples are not available, since quantification can be performed by means of internal standardization. Using this approach, acceptable results in terms of accuracy and precision were obtained for the determination of mid‐high Z elements (Mn‐Sr) in vegetal foodstuff. But empirical calibration using several plant reference materials was necessary to correct absorption effects and obtain reliable results for low Z elements such as K and Ca.

      Finally, the direct analysis of the plant material is necessary in some applications, above all when dealing with the study element distribution within plant tissues or in in vivo analysis. In this latter case, the specimen is directly analyzed by the XRF system and any sample treatment is not required. For example, the study of the intake of mineral nutrients by time‐resolved XRF analysis proved to be useful in the investigation of plant diseases due to nutrient deficiency and excess [36].

      Usually, when performing μ‐XRF analysis to study element distribution within plant tissues a minimum sample treatment is necessary to ensure a flat and smooth surface of the vegetation sample which can affect the quality and reliability of μ‐XRF spectra collected. It has been demonstrated in different studies that dehydration of plant tissues can alter the in‐situ location of dissolved components [25]. For this reason, vegetation tissues are usually sandwiched between two thin films transparent to X‐rays to prevent the sample from drying, oxidizing and dehydrating [37]. More sophisticated sample treatments for μ‐XRF analysis include sectioning of the plant tissue. Different approaches are used for such purposes including the sectioning of the fresh tissue under cryogenic conditions or after embedding the tissue in a hard resin or soft paraffin wax. In the latter case, it is possible to obtain ultrathin (100 nm to 1 μm) sections that are very well‐suited for elemental distribution and speciation analysis of plant tissues using synchrotron radiation [38].

      It is worth to mention that in p‐EDXRF systems vegetation samples can be measured in‐situ in the field without any sample treatment directly by pointing the handheld window (of approximately 2 cm2) to the vegetal tissue. Even under field‐moist conditions, it has been demonstrated, for instance, that p‐EDXRF systems can be reasonably used for the determination of Zn and Cu in different vegetation species [18]. However, a significant improvement of the results can be assessed if drying the vegetation sample or if using additional sample treatment such as the fabrication of pressed pellets from the powdered plant material [19].

      2.4.1 Environmental Studies