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Figure 2.4 EDXRF spectra obtained in the analysis of leaves of Betula Pendula species collected in a mining landfill and in a non‐polluted area (control sample).

      In the last years, the employment of plants to remediate areas polluted by metals (phytoremediation) has acquired popularity as an alternative or complementary technology to other more sophisticated remediation methods. Moreover, some vegetation species have also shown their potential use to avoid erosion, to stabilize wastes and to reduce effects of metal pollution (phytostabilization). To increase the efficiency of such technologies, it is important to learn more about the specific plant physiological processes involved.

      Usually these kinds of studies are performed in the laboratory by exposing the plant to the target metal for a predefined period of time. After the exposure, in addition to the determination of the metal content in roots and shoots, the localization and distribution of metals within the plant tissues as well as their chemical forms (metal ligand environment) is of paramount importance to understand metal uptake, translocation and tolerance mechanisms. Usually μ‐XRF and X‐ray absorption techniques in combination with synchrotron radiation are used for such purposes [41].

The incorporation of metals into plants is principally attained by uptake from the substrate through the roots, but it may be achieved also from deposition of meals on the leaves from aerosol or atmospheric particulate. For this reason, vegetation species are also extensively used in air pollution studies. In particular, mushrooms, mosses, and lichens are well‐suited for this purpose because of their lack of tissue development that causes a non‐barrier effect and, therefore, accumulation ability. A wide range of articles dealing with the use of mosses as bioindicators near roads with high density traffic and in industrial areas have been published using XRF as analytical technique [42]. Due to the low amount of moss available in some of these studies, TXRF is preferred since a multi‐elemental analysis using less than 10 mg of sample can be performed [23]. The advantages of this analytical approach have also been highlighted for multi‐elemental analysis of pollen as atmospheric pollution indicator [43].

      Finally, biofilms and algae have also been used to monitor the quality of the aquatic environment. In the case of biofilm analysis, TXRF plays an important role since the amount of sample required for the analysis is lower than with other XRF configurations and even the analysis can be performed directly on the support (quartz reflector) where the biofilm is grown [22]. Algae are also influenced by ambient environmental conditions and, in this respect, the distribution and occurrence of certain species may often reflect local water quality. As such, multi‐elemental analysis of this type of samples has been also the topic of research of several scientific contributions. In this sense, it is interesting to highlight the study of Turner and co‐workers, which used a portable XRF system to identify hot‐spots of contamination by analyzing fresh sample sections of coastal macro algaes [44].

      2.4.2 Nutritional and Agronomic Studies

      Vegetal foodstuffs are one of the principal sources of essential, major, and trace elements for human beings and for this reason a significant number of studies and scientific contributions have been made utilizing multi‐elemental analysis of foodstuff (i.e. cabbage, spinach, seaweed, tomatoes, mushrooms, and mulberries) for both nutritional and safety purposes [45, 46]. In addition to the total elemental content determined by conventional XRF systems, in some studies, the benefits of using μ‐EDXRF have also been discussed. For instance, the use of this technique allows studying the micronutrient distribution within the different parts of the rice grain which is of interest in view of the large consumption of this cereal worldwide [47]. Due to the high consumption of the species in many cultures, elemental composition of this type of sample has also been a topic of research [48]. In this regard, it is interesting to highlight the use of WDXRF systems for a reliable determination of light elements such as magnesium, sulfur and chlorine which are elements of interest for nutritional purposes, but are difficult to determine with conventional absorption and emission atomic spectroscopic techniques [6].

      The regular consumption of coffee, tea, and medicinal herbs may contribute to the daily dietary requirements of several elements. For instance, tea is an important source of manganese and the large amount of potassium in comparison with sodium could be beneficial for hypertensive patients. Different configurations of XRF systems have been used to get information about multi‐elemental composition of coffee grains [49], teas and medicinal herbs [50, 51] as well as their infusions [24]. In the latter case, TXRF systems which allow the analysis of both solid (by means of a previous suspension or digestion preparation) and liquid samples such as infusions could be of interest.

      Some other applications in the field of vegetable food analysis include for instance the study of specific elements or groups of elements that can be potentially toxic if consumed by humans. Within this category, Gupta and co‐workers studied the presence of heavy metals in cauliflower grown in contaminated and uncontaminated soils [52]. An interesting finding of this study was that similar elemental concentrations in the edible flower part were found irrespective of the soil type. A similar study was also performed about the determination of arsenic in onion plants growing in contaminated substrates using TXRF [53].

Finally, XRF instrumentation has also been widely employed as a fast and cost‐effective technique in agronomic studies with the aim of studying elemental composition of edible vegetal crops using different growing conditions. Due to the multi‐elemental capability of EDXRF systems, it was possible, for instance, to study the effect of different fertilization types and farming regimes on the nutritional mineral content of different vegetal foodstuffs (i.e. wheat, lentil, sunflower, chickpea, tomato plants) [54–56]. Similarly, the effect of irrigation with reclaimed wastewaters on different types of crops was also studied through the multi‐elemental analysis of the vegetation tissues [10]. Moreover, in this contribution, μ‐EDXRF analysis were also carried out in transversal and longitudinal carrot sections. Carrots were grown in experimental plots irrigated with fresh water and reclaimed wastewaters and it was demonstrated that the water used for irrigation purposes affects the concentration but not the spatial distribution of these elements within the vegetal tissue (see Figure 2.5). The benefits of obtaining information about the location and distribution of elements within the vegetal tissues of edible plants through μ‐EDXRF analysis have also been highlighted in some studies dealing with the addition of supplements (i.e. Zn, Fe) to the studied crops [57, 58].