LITMIR.BIZ

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L, M, H L, M, H L, M, H M, H M, H L, M, H Amount of sample few g few g few g n.a.^a few mg/μl n.a.^a Bulk analysis Yes Yes Yes Yes Yes No Element distribution information No No No No No Yes Limits of detection mg/kg‐μg/kg mg/kg mg/kg‐μg/kg mg/kg μg/kg‐ng/kg ng‐pg Critical moving parts Crystals None Targets No Incidence angle No Portability (in situ analysis) No No No Yes No No Capital costs High Medium High Low High/medium Medium Consumables Medium Low Medium Low Low Low/medium

      ^a n.a.: not applicable (direct analysis of the solid sample).

      WDXRF: Wavelength dispersive XRF, 2D‐EDXRF: Two‐dimensional energy dispersive XRF, 3D‐EDXRF: Three‐dimensional energy dispersive XRF or polarized energy dispersive XRF, TXRF: total reflection XRF, μ‐EDXRF: micro‐energy dispersive XRF.

      Usually in WDXRF spectrometers the analysis of different elements is carried out in a sequential way by changing synchronously the orientation of the goniometer‐controlled detection system to catch the discrete wavelengths corresponding to each chemical element. Therefore, they are not usually employed for multi‐elemental analysis of unknown samples and their use in vegetation samples analysis is not very common. Nevertheless, one of the benefits of WDXRF systems is the possibility to accurately determine light elements such as P, S, Cl which can play an important role in vegetation metabolism and are difficult to determine with other atomic spectroscopic techniques. For instance, WDXRF was used by Barua and co‐workers [6] to determine the concentration of P, K, S, Ca, Fe, Mg, Cl, and Na in seeds of chili for nutritional purposes. Other applications include the determination of specific elements (Nd, Pb, Th, and U) in fungi [7] or the combined determination of light and some trace elements in vegetation species collected in mining environments [3]. In general, quantitative analysis by WDXRF is performed by using the empirical calibration method. However, it is sometimes difficult to get sufficient reference materials with matrices similar to the target samples. In those cases, synthetic standards made of spiked cellulose with the elements of interest could provide a good option to simulate the vegetal matrix and to produce suitable calibration curves for quantification purposes [3].

Unlike WDXRF systems, conventional 2D‐EDXRF spectrometers only consist of two basic units, the excitation source and the detection system. Using this configuration, all the X‐rays emitted by the samples are collected at the same time in the detector and thus, 2D‐EDXRF systems can provide simultaneous multi‐elemental information of the sample. For this reason 2D‐EDXRF are widely used in environmental sample analysis, including vegetation samples [8]. Moreover, since the development of lower power ceramic micro‐focus X‐ray tubes and air‐cooled silicon drift detectors, low‐cost benchtop and even portable 2D‐EDXRF systems are commercially available and have been successfully used in the field of vegetation analysis [9, 10]. Nevertheless one of the main drawbacks of 2D‐EDXRF systems is the limited sensitivity for trace and ultra trace elements including some important environmentally‐relevant metals such as Pb and Cd. This is mostly due to the high spectral background arising from the elevated degree of scattering of the X‐ray beam by light organic matrices. This background can be decreased very significantly by using polarized EDXRF systems in which a secondary target is interposed between the X‐ray tube and the sample configuring a Cartesian geometry (tri‐axial, 3D) between source, sample and detector. With this geometry, a significant decrease of the background is achieved because the exciting radiation is polarized when scatter occurs through the right angle and cannot then be scattered a second time into the detector. Therefore, the sensitivity and detection limits for minor and trace elements are improved as compared with those associated with conventional 2D‐EDXRF spectrometers. In Figure 2.2, as an example, spectra obtained in the analysis of a leaf sample using a 2D‐EDXRF spectrometer (W X‐ray tube) and a 3D‐EDXRF spectrometer (W X‐ray tube and Mo secondary target) are displayed. An additional advantage of the use of secondary targets is that they can be used to modify the energies of the beam impinging the samples but in a simpler way. With the right target, we can selectively excite the elements of the sample and, sometimes, act as a near‐monochromatic secondary source of somewhat higher energy than the absorption edges of the analytical lines of the requested analytes.

Figure 2.2 Spectra acquired in the analysis of a leaf sample collected in a contaminated mining area using (a) 2D‐EDXRF system(W X‐ray tube) and (b) 3D‐EDXRF system(W X‐ray tube and Mo secondary target).

      Therefore, with the combination of polarization and the use of different secondary targets, a system can be designed to produce a range of excitation conditions adequate for different groups of elements. For example, for the determination of light elements (Na, Mg, Al, Si, P, and S) in spruce needles it was adequate to use a highly oriented pyrolytic graphite (HOPG) crystal as a secondary target [11]. Meanwhile, for determination of elements such as Pb, Fe, Cu, and Zn in different vegetation specimens, a secondary target made of Zr proved to be a good choice [12]. Targets made of pure metals (i.e. Mo, Zr, Co) have proven to be adequate for the excitation of specific elements or a reduced group of neighboring elements. The use of an Al₂O₃ target (Barkla scatter) in a 3D‐EDXRF system was also successful for the determination of low amounts of Cd (<1 mg/kg) in vegetation samples. However, in this application the use of a Gd X‐ray tube and a Ge semiconductor detector was necessary in order to allow the determination of Cd through the Cd‐K lines overcoming in this way the reduced sensitivity and the spectral interferences issues that occur when using L

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