1
Introduction

Katarzyna Chojnacka

Wroclaw University of Science and Technology, Faculty of Chemistry, Department of Advanced Material Technologies, Wroclaw, Poland

1.1 Introduction

Trace elements (TEs), although present in low quantities, can have significant effects in living organisms. Although the role of trace elements in the human body is not yet fully understood, it is known that their effect on human health can be essential, neutral, or detrimental [1]. Trace elements play a role in many chemical, biochemical, and enzymatic reactions; biological and physiological, catabolic and metabolic processes of living organisms [2]. Their role relies on the unique property of them forming complexes and binding with macromolecules (e.g., proteins) [1]. Frequently mentioned trace elements or micronutrients are: Cr, Co, Cu, F, I, Mn, Mo, Se, V, and Zn. The main sources of these elements for humans are drinking water, food and food supplements, and the general environment. There are trace elements that are essential, but there are also those that are non-essential or potentially toxic: Al, As, Cd, Hg, and Pb [2].

Humans are exposed to trace elements from atmospheric suspended particles in street and house dust to soil and are exposed through different routes such as inhalation, ingestion, or dermal adsorption. The establishment of emission standards for trace elements is important when considering the potential impact on society from urban areas, taking into account toxicity and the degree of human exposure [3].

1.2 Definition of Trace Elements (TEs)

Trace elements were first described at the beginning of the twentieth century as elements present at very low levels in different matrices. In actual fact, different branches of science (e.g., geochemistry, medicine, agriculture, and chemistry) have different understandings of TEs. The word "trace" is usually related to abundance, and includes elements with different chemical properties: elements and metalloids, including the micronutrients group, essential elements, and toxic elements. In geochemistry, TEs are chemical elements that occur in the earth's crust in amounts less than 0.1% and to biological sciences TEs are elements present in trace concentrations in living organisms [4]. The result of these differences is that, until now, no precise definition of TEs has been provided. Elements that are trace in biological materials are not necessarily trace in terrestrial environments (e.g., iron) [4]. Early research theorized that these elements do not play important functions due to their low abundance [1] but, more recently, it has been shown that this is not the case.

1.3 Sources of Trace Elements for Humans

There are beneficial effects of TEs in food. However, in some cases, impurities in the food chain and in the general environment has been observed to have detrimental effects [1]. The relation between bioavailability and speciation in food is an important factor here, especially concerning iron, selenium, or chromium [1].

Vincevica-Gaile et al. [2] reviewed the trace metal content in foods from plant (vegetables: carrots, onions, potatoes) and animal origin (cottage cheese, eggs, honey). Environmental factors (e.g., geographical location or seasonality), botanical origin, agricultural practices, product processing, and storage were all found to influence the content of TEs. The level of TEs in food depends on the environmental conditions of specific sites such as the composition of soil and water [2].

Tea plants contain high levels of TEs because they are grown in acidic soils where metal ions are more available for uptake by the root system. Some of the TEs (Al, Cu, Cd, Cr, Mn, and Ni) are beneficial; others are harmful for human health and are transferred through tea infusion. The content of tea has been assessed and found to show nutritional value, but also adverse health effects [5]. Tea contains 4-9% of inorganic matter, 30% of which is extracted. Polyphenolic compounds (flavonoids) bind metal ions, especially Fe and Cu [5]. The reported TE contents in fresh tea leaves are as follows (mg/kg): for example, Chinese tea [6]: Al 2034-3322, Cd 0.03-0.08, Cu 9.68-18.82, As 0.024-0.066, and Pb 0.31-3.42 [7] and Turkish tea: Mn 2617-3154 and Ni 6.60-11.7 [5, 8].

A good dietary source of TEs (Fe, Cu, Zn, and Mn) comes from seaweed. For instance, Porphyra vietnamensis can be added to foods to improve the content of essential minerals and trace elements. The strong flavor of seaweeds is related to the presence of TEs, the content of which is higher than in terrestrial vegetables. An example content of TEs in seaweed is: Fe 1260 mg/kg and Cu 7.46 mg/kg. The consumption of 8 g of green, brown, or red seaweed contains more than 25% of a daily Dietary Recommended Intake [9].

1.4 Analytical methods

Pollution of the environment with trace metals has generated the need for finding suitable analytical methods that are sensitive, rapid, effective, and reliable. Several analytical techniques; inductively coupled plasma-atomic emission spectrometry (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS), x-ray fluorescence (XRF), total reflection x-ray fluorescence (TXRF) spectroscopy, and neutron activation analysis (NAA) have been developed to analyze and monitor trace elements in environmental and food samples, as well as in the human body [10]. The determination methods ICP-OES, NAA, and ICP-MS are techniques with high sensitivity and multi-element capability [11]. TXRF is a quantitative analysis technique for liquid samples which can be deposited as thin films on clean reflectors. The sensitivity and detection limits of TXRF are better than XRF [10].

The unique chemical properties and coherent behavior of TEs means that their environmental distribution reflects geographical location and aquatic factors (e.g., source input and water-rock interaction). Similarities between trace metals and their very low concentrations do, however, make determination difficult. Problems appear if a particular element is evaluated in a mixture with other elements as interferences and coincidences can occur [11]. The matrix and elements that are to be analyzed dictate the how difficult an analysis may be. For example, the direct determination of REEs (Rare Earth Elements) in high-salt groundwater, because the concentrations of REEs are close to the detection limit of ICP-MS and there are high concentrations of matrix ions (K, Na, Ca, and Mg) which defocus the extracted ion beam due to space charge effects, means that significant losses of analyte sensitivity are produced [11].

For this reason, pre-concentration techniques are used and separation from the matrix elements is required before ICP-MS analysis takes place. Solid phase extraction (SPE) or solvent extraction (SE) techniques are employed for the pre-treatment of high-salt samples (e.g., seawater). This removes the matrix components and enriches the samples with analytes. Of course, this can generate a new matrix and new interferences [11].

Speciation of TEs is important in the analysis of food, quality of products, health, and environment. Mobility, bioavailability, storage, retention, and toxicity of TEs depends on their chemical form. Biochemical and geochemical pathways depend on speciation [1]. Of particular importance is characterizing speciation of TEs in samples related to the chemistry of life. This requires the elaboration of separation techniques, sensitive enough to determine elements, as well as the identification of metallo-compounds [12]. The problem with speciation analysis is related to the low total concentration of TEs, for example, ng/L in serum. The level of given species can even be several times lower. Another problem lies within non-covalent bonds that are formed by TEs in different matrices such as tissue, blood, urine, sediment, water, and sludge, that are unstable especially after sampling [1].

1.5 Toxicity

The toxicity of TEs depends not only on their concentration (dose), but also on their speciation. Safe and adequate daily intake (SAI), and acceptable daily intake (ADI) have been defined as important toxicological measures. Table 1.1 summarizes the important toxicological issues related to TEs together with guidelines for drinking water and daily intake.

Table 1.1 Trace elements and their toxicity [5, 13].