Trace Element Analysis of Plants: Applications, Sampling, and Analytical Methods
Plants (including vegetables and fruits) have developed mechanisms to absorb various substances from soil and water sources so they can adapt to the different chemical properties of the environment. They absorb both nutrients and trace elements that plants may or may not need for their growth and survival. These trace elements include:
· Heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), and arsenic (As), antimony (Sb), tin (Sn)
· Precious metals such as gold (Au) and silver (Ag)
· Rare earth elements such as are cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb) and yttrium (Y)
Because plants are able to absorb the elements mentioned above, scientists have found applications of trace element analysis of plants in:
· Pollution monitoring
· Hydrogeochemical analysis
· Mineral Exploration
Agriculture and Pollution Monitoring
Trace elements (especially if present in high levels) might have an effect to the healthy development of plants. For example, plants can accumulate toxic concentrations of cadmium in their roots. Cadmium comes from fertilisers, sewage sludges, and other sources. The element affects the plants’ health by interfering with cell division and new plant tissue development.
Consumer health and awareness is also one of the reasons trace element analysis in plants is important. Consumption of plants and plant-products (including vegetables and herbal supplements) can lead to accumulation of heavy metals into the bodies of humans and animals. Indirectly or directly, accumulation of trace elements in animals and humans over a short or long time period will lead to diseases.
This concern becomes especially important in the regular use of herbal supplements. Their use became popular because people now prefer “natural” methods for them to become healthier. However, FDA said “natural” does not mean “safe.” Contaminants (including trace elements) and other chemicals might be present in those herbal supplements. It is important that chemical analysis is performed before processing plant parts into herbal supplements to help in ensuring the safety levels of plants.
As mentioned earlier, plants have developed mechanisms to adapt to their chemical environment. They have also adapted to environments with high concentrations of toxic elements. As a result, they are able to absorb these toxic elements and reduce the amount of contaminants in soil.
The process is called phytoremediation and it could be useful in reducing the environmental effects of mining and ore processing. Plants use different phytoremediation techniques depending on the contaminants present in the soil.
For example, lead (Pb) can be absorbed by plants through their root systems or be deposited in their leaves and stems. Pb is a common pollutant that comes from various industrial practices such as fossil fuel combustion and emissions. It is one important reason many laboratories perform routine plant tissue analysis for lead.
Hydrogeochemical analysis and mineral exploration
Aside from agriculture, pollution monitoring, and phytoremediation, trace element analysis in plant samples has also found applications in hydrogeochemical analysis and mineral exploration. In hydrogeochemical analysis, the plants serve as a direct link to knowing the elements or contaminants in surface water or groundwater.
In mineral exploration, plant analysis is an easier, less invasive, and more economical method than soil analysis. This is important in areas where inaccessibility is a major problem. Because many plants have deep roots, they serve as the above-ground extension of the ground underneath. This means analyzing plant samples can help professionals assess the economic potential of a land area in terms of mining. Often sampling of the vegetation is used as a complimentary tool for drill targeting together with other geochemical or geophysical methods of exploration.
Why choose trace element analysis of plants?
As discussed earlier, mineral plant analysis is:
· More economical
· Less invasive
· Easier (especially when it comes to sampling)
Sampling for Mineral Plant Analysis
One clear advantage of plant analysis over soil analysis is ease of sampling. In hydrogeochemical analysis and mineral exploration, getting samples beneath the ground cover is laborious, expensive, and invasive.
Acquiring samples from the leaves, barks, flowers, seeds, and twigs is easier. Analysts can choose one or two of the most popular plants that grow in the region or those species that are required for analysis. For example, in arid regions such species as Eucalyptus, Acacia, Tea Tree, Black Oak, Spinifex and Monterey pine can be sampled for mineral exploration purpose. It is advisable to choose plants that are deep-rooted and have a tendency to hyperaccumulate substances of interest.
For example, Hybanthus Floribundus is known to accumulate more than 0.1% of nickel. For other plant species, that concentration is toxic. This information is useful in mineral prospecting because plants having anomalous amounts of a certain metal could mean economic potential or a sign of soil contamination.
During sampling, analysts are careful especially when taking samples from different parts of the plants. To avoid cross-contamination, analysts often use different gloves and other tools for each sampling activity. After sampling, care must also be taken during storage. Degradation (due to microbial activity and other factors) will affect the results of the analysis.
Methods for Mineral Plant Analysis
First, analysts prepare the samples by drying, milling, and digestion before the actual chemical analysis. The common analytical methods used for the determination of trace elements in plants are:
· Flame Atomic Absorption Spectrometry (FAAS)
· Inductively Coupled Plasma - Mass Spectrometry (ICP-MS)
· Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES)
· Microwave Plasma - Atomic Emission Spectrometry (MP-AES)
· X-Ray fluorescence
Analysts often choose a method according to costs, their objectives, and sensitivity of the method. For example, if analysts and other concerned parties are more interested in the biogeochemical interpretation of an area, they will choose a method that is capable of multi-element analysis.
Plant samples usually have very low concentrations of elements. It is one of the reasons analysts also consider the detection limits of a particular method. In all the methods mentioned above (FAAS, ICP-MS, ICP-OES, MP-AES and XRF), they require a reference material with known analyte content. This is to establish the relation between the readings and analyte concentration.
For example, a multi-element biogeochemical reference material suitable for mineral exploration and environmental monitoring is the 25 g Eucalyptus Leaf. This reference material was analysed for over 60 elements including gold, rare earths, and base metals (Download this Data Sheet if you want to learn more).
Summary and conclusion
Trace element analysis of plants has applications in agriculture, environmental monitoring, phytoremediation, research, hydrogeochemical analysis, and mineral exploration. The analytical methods only require less invasive and more economical means, especially when it comes to sampling.
The choice of reference material and analytical method also play a huge role in obtaining reliable data that are useful for the analysts, scientists, and other stakeholders. In the future, mineral plant analysis might play a bigger role in various fields especially in agriculture, mineral exploration, and environmental monitoring.
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