Trace elements in soil: Their determination and speciation

Summary The different types of soil analysis are reviewed in-outline and some recent developments and methodologies are discussed.For the determination of the total trace element content of soils, conventional, multi-element, solid sample methods including d.c. arc optical emission and spark source mass spectrometric procedures are briefly considered together with the potential of current X-ray fluorescence, solid sample graphite furnace atomic absorption and glow discharge mass spectrometry.The use of strong acid digestion, with for example aqua regia, for the determination of pseudo-total concentrations of heavy or toxic metal accumulations in soil is described.The limitations of solution methods for multi-element analysis of soils are outlined together with the prospects for the use of soil slurries to eliminate the sample preparation and dilution problems associated with the dissolution of soils. The difficulties in taking reproducible and representative samples of inhomogeneous materials such as soils are highlighted.Trace element speciation can be defined as the identification and quantification of the different forms or phases in which they occur in soils. Some examples of such procedures and extractants for both essential and toxic elements in soils are presented. The difficulties of trace element speciation in soils as distinct from soil extracts or soil solutions are illustrated briefly.     

Use of radiochemical methods as tools for speciation purposes in environmental and biological sciences.

Chemical speciation for a few elements can be facilitated to a great extent by incorporating a suitable radioisotope into the system and measuring the radiation of the isolated species. This radiospiking can be applied to in vitro and in vivo labelled experiments. Radionuclides are, however, also present as an anthropogenic contaminant from various nuclear fission activities. The radiotracer should be added under such conditions that it behaves in exactly the same way as the isotopes it represents. It should possess an adequate radioactive half-life, and preferably be a gamma-emitter because of the ease of detection. Radiotracer labelling is now widely used to study speciation problems of many essential and toxic elements in body fluids and tissues. It can be used to trace the different locations where the element is metabolized and stored, and subsequently to detect the element in the isolated biocomponents. The determination of the location of a radiotracer in a cell by autoradiography proved to be impractical because of the lack of resolution. Radiochemistry is similarly very useful for investigating particular aspects of the speciation of heavy metals as they occur in the ecosystem, and to follow the fate and effects of fission nuclides in the environment as they are carried around by the water and air masses. However, in certain circumstances the behaviour of fission products appears to be different from that of their stable analogues. For the actinides they simply do not exist. Radiochemical methods are a major tool for identifying and quantifying the nuclides in the different species.     

Determination of Iron: Electrochemical Methods

Iron is an abundant element in the environment which plays an important role in environmental and biological systems. In particular, its essential function in photosynthesis has been seen as a limiting factor for phytoplanktons in ocean waters. Thus, sensitive speciation and determination of iron is of major interest, and many techniques have been established for analytical purposes. Electrochemical methods have been commonly explored due to their inexpensive, simple and rapid nature, with adsorptive stripping voltammetry (AdSV) being widely used due to its ability to complex and preconcentrate iron ions for ultrasensitive detection. This paper aims to present a review of recent determinations of trace iron using electrochemical methods.     

Analysis of trace elements and their physico-chemical forms in natural waters

Trace elements in natural waters can be present in different physico-chemical forms, varying in size, charge and density properties. Knowledge of speciation is essential for understanding the transport, distribution, and biological uptake of trace elements in the environment. The development of techniques to provide reliable information on physico-chemical forms has, therefore, become a challenge within Analytical Chemistry.When selecting analytical methods for the determination of total concentrations or fractions of trace elements in natural waters, no exclusion of species should occur, or at least it must be accounted for. Furthermore, the determination limits must be sufficiently low to allow the actual concentrations to be determined with reasonable precision and accuracy. For very low concentrations, preconcentration techniques are applicable, provided the chemical yield of the spike represents that of the original species present. For methods meeting these criteria, the suitability for routine analysis should be considered.When the physico-chemical forms of trace elements are to be determined, the fractionation should take placein situ or shortly after sampling. As the concentrations involved in speciation studies may be extremely low, there is an increasing awareness of potential sources of errors influencing analytical results. Sample collection and separation/fractionation/concentration procedures prior to analysis are, therefore, essential within Analytical Chemistry, and the whole procedure must be taken into account when interpreting the results. There are, however, several requirements which should be met by techniques applicable for speciation purposes. In general, size fractionation techniques (e.g.in situ hollow fibre ultrafiltration) should be applied prior to the addition of any chemical reagents (charge fractionation techniques).     

Recent trends in nanomaterial-based microanalytical systems for the speciation of trace elements: A critical review

Trace element speciation in biomedical and environmental science has gained increasing attention over the past decade as researchers have begun to realize its importance in toxicological studies. Several nanomaterials, including titanium dioxide nanoparticles (nano-TiO₂), carbon nanotubes (CNTs), and magnetic nanoparticles (MNPs), have been used as sorbents to separate and preconcentrate trace element species prior to detection through mass spectrometry or optical spectroscopy. Recently, these nanomaterial-based speciation techniques have been integrated with microfluidics to minimize sample and reagent consumption and simplify analyses. This review provides a critical look into the present state and recent applications of nanomaterial-based microanalytical systems in the speciation of trace elements. The adsorption and preconcentration efficiencies, sample volume requirements, and detection limits of these nanomaterial-based speciation techniques are detailed, and their applications in environmental and biological analyses are discussed. Current perspectives and future trends into the increasing use of nanomaterial-based microfluidic techniques for trace element speciation are highlighted.     

High Sensitivity Hyphenation System of Capillary Electrophoresis Combined Inductively Coupled Plasma Mass Spectrometer for Elemental Speciation Analysis

The proper functioning of life is critically dependent on trace elements in a number of different ways. Some trace elements are highly toxic whereas others, considered essential, are needed for the accomplishment of life process. A surge of evidence during the past 20 years has been leading to the conclusion that it is not the total element content but that of a particular species that should be determined if valid information on the essentiality or toxicity of a given element is to be obtained^[1-5]. Thus, speciation analysis has become one of the fastest developing areas of analytical chemistry towards the close of the 20^th century^[5]. The advantages of Capillary Electrophoresis include rapid analysis, low sample requirements, high separation efficiency and low operation costs^[6]. The separation potential of CE,with superior detection capability of ICPMS make CE-ICPMS a powerful tool for metal speciation.     

Determination of chromium,manganese, lead and cadmium in biological samples including hair using direct electrothermal atomic absorption spectrometry

Direct analysis of solid samples employing a laboratory assembled electrothermal atomic absorption spectrometer is demonstrated to be a feasible approach for determination of trace elements in plant tissue and hair samples for special applications in plant physiology and biomedical research. As an example, the kinetics of Cr uptake by cabbage and its distribution have been measured as a function of chromium speciation in the nutrient solution. Further, longitudinal concentration gradients of Cr, Pb and Cd have been measured in hair of various population groups exposed to different levels of these elements in ambient and/or occupational environments. The techniques are validated for the determination of these trace elements by neutron activation analysis, dissolution atomic absorption spectrometry and by analysis of certified reference materials. Slurry sample introduction is found appropriate for routine trace element determination and in homogeneity testing. Direct sample introduction is indispensable in the analysis of very small (< 1 mg) tissue biopsy samples in the determination of trace element distributions.     

Applications of nuclear analytical techniques in environmental research. Plenary lecture.

Among nuclear analytical techniques, neutron activation analysis (NAA) is particularly useful for environmental studies. It affords low detection limits for many elements, high specificity and few sources of systematic error, which means that high accuracy is attainable. Neutron activation analysis is particularly useful for trace and ultra-trace analysis of environmental samples (water, soils, rocks and biological material). In trace element work associated with pollution, instrumental NAA is a powerful technique for multi-element surveys, in particular when combined with other spectroscopic techniques. Nuclear techniques, as with most analytical techniques, cannot be used to distinguish between different physico-chemical forms of an element per se. When used in combination with appropriate separation techniques, however, nuclear techniques can provide valuable information about trace element speciation in environmental and biological systems. From dynamic tracer experiments, i.e., addition of chemically well defined labelled compounds to environmental systems, valuable information can be obtained on the distribution of species and on microchemical processes influencing the physico-chemical forms. In these laboratories, speciation studies on trace elements in natural waters have been carried out by using instrumental NAA in combination with physical separation techniques, such as dialysis and ultrafiltration, in situ and in the laboratory. Dynamic radiotracer experiments have provided important information about processes influencing the speciation of trace elements in aquatic systems. Sequential extraction techniques have proved to be useful in studies on sediments and soils when combined with NAA. Sequential extractions also provide significant information about the physico-chemical behaviour of radionuclides supplied to natural soils from the Chernobyl accident.     

The importance of trace element speciation in biomedical science

According to IUPAC terminology, trace element speciation reflects differences in chemical composition at multiple levels from nuclear and electronic structure to macromolecular complexation. In the medical sciences, all levels of composition are important in various circumstances, and each can affect the bioavailability, distribution, physiological function, toxicity, diagnostic utility, and therapeutic potential of an element. Here we discuss, with specific examples, three biological principles in the intimate relation between speciation and biological behavior: i) the kinetics of interconversion of species determines distribution within the organism, ii) speciation governs transport across various biological barriers, and iii) speciation can limit potentially undesirable interactions between physiologically essential elements. We will also describe differences in the speciation of iron in states of iron overload, to illustrate how speciation analysis can provide insight into cellular processes in human disease.     

Liquid chromatography-inductively coupled plasma mass spectrometry

It is known that while many elements are considered essential to human health, many others can be toxic. However, because the intake, accumulation, transport, storage and interaction of these different metals and metalloids in nature is strongly influenced by their specific elemental form, complete characterization of the element is essential when assessing its benefits and/or risk. Consequently, interest has grown rapidly in determining oxidation state, chemical ligand association, and complex forms of a many different elements. Elemental speciation, or the analyses that lead to determining the distribution of an element’s particular chemical species in a sample, typically involves the coupling of a separation technique and an element specific detector. A large number of methods have been developed which utilize a multitude of different separation mechanisms and detection instruments. Yet, because of its versatility, robustness, sensitivity and multi-elemental capabilities, the coupling of liquid chromatography to inductively coupled plasma mass spectrometry (LC–ICP–MS) has become one of the most popular techniques for elemental speciation studies. This review focuses on the basic principles of LC–ICP–MS, its historical development and the many ways in which this technique can be applied. Different liquid chromatography separations are discussed as well as the factors that must be considered when coupling each to ICP–MS. Recent applications of LC–ICP–MS to the speciation of environmental, biological and clinical samples are also presented.     

Size fractionation techniques combined with INAA for speciation purposes

In natural waters trace elements, especially trace metals may be present in a variety of physicochemical forms. They may be associated with forms ranging from simple ions and molecules via hydrolysis products and colloids, pseudocolloids and organic or inorganic particles. The transition between categories is gradual. The presence of species differing in size, charge and density will influence on the transport, mobility and bioavailability of the trace element in question. Fractionation techniques which do not influence the distribution patterns are therefore required for speciation purposes. In the present work dialysis in situ and large membrane (hollow fibers) ultrafiltration are used for fractionation of low molecular weight species, colloids, pseudocolloids and particles. Due to the presence of foreign components transformation processes influence the distribution patterns of trace elements of interest. Sorption to foreign surfaces, complexation with agents present and aggregation of colloids (e.g., increasing ionic strength) result in a shift towards higher dimensions while desorption and dispersion processes mobilize the trace elements. Information on several components is therefore needed in speciation studies and a multielemental method of analysis having low determination limits must be applied. Instrumental neutron activation is appropriate to this kind of study because of its high sensitivity for simultaneous determination of a great-number of elements. Size fractionation techniques combined with INAA for the characterization of trace element species in natural waters will be discussed.     

Recent on-line processing procedures for biological samples for determination of trace elements by atomic spectrometric methods

Few of the elements present in nature play a metabolic role in living organisms. According to their abundance, these elements are classified as macro-, micro- or trace elements, representing 93%, 5% and around 1% respectively, of the total body weight. The remaining percentage could be attributed to those elements with unknown biological functions, to others which are present only because of the exposure to polluted environment or to those intentionally introduced into the body for a special treatment. This review summarizes and discusses the most recent publications related to the on-line processing of biological samples for trace element determination using atomic spectrometry-based detectors. Preconcentration/separation procedures based on solid phase or cloud point extractions, electrochemical deposition, microdialysis, as well as chemical vapor generation are the common practice for improving the sensitivity and selectivity of the available atomic spectrometric techniques. The advantages of using isotope dilution mass spectrometry in speciation studies are also emphasized. Digestion or leaching in oxidizing acidic mixtures aided by heat or by ultrasound or microwave radiation, performed off- or on-line, is necessary to previous steps when processing solid biological samples. The most relevant analytical figures of merit such as detection limits, enrichment factors and sample throughput as well as some aspects related to the on-line system configurations and accuracy assessments are critically presented.     

Biochemical speciation analysis by hyphenated techniques

The elucidation of mechanisms that govern the essentiality and toxicity of trace elements in living organisms is critically dependent upon the possibility of the identification, characterization and determination of chemical forms of these elements involved in life processes. The recent progress and the state-of-the-art of biochemical species-selective trace element analysis are critically evaluated with particular emphasis on the use of techniques combining the high selectivity of high performance liquid chromatography (HPLC) with the elemental or molecular specificity of mass spectrometry [using inductively coupled plasma (ICP) or electrospray ionization (ESI)]. The potential and limitations of hyphenated techniques as a tool for speciation of metals and metalloids in biological materials is discussed using a number of examples drawn from the latest research in the authors’ laboratory.     

The role of element speciation in environmental and occupational medicine

In order to evaluate the interaction with the environment or to assess absorption, binding mechanisms, reactivity and excretion of elements in humans, element speciation can provide more information than the analysis of element as a whole. Some examples that confirm the importance of speciation depend on the choice of the most appropriate indicator or representative matrix. The determination of As(III), As(V), monomethylarsonic and dimethylarsinic acids can be used to evaluate occupational exposure to As. Exposure to inorganic Hg should be measured by its content in urine, whereas in the case of exposure to alkyl Hg, blood and hair should be considered. Speciation may also be useful in studying element toxicokinetics, since it is well known that hexavalent Cr is taken up more than the trivalent form, and that species of the same metal are differently partitioned in blood. Pentavalent forms of As are absorbed more than trivalent forms, and the organic species of elements are excreted faster than inorganic species. In addition, speciation can play an important role in assessing element toxicodynamics. The toxicity of the three oxidation states of Hg differs considerably; for As a decreasing toxicity from arsenite to dimethylarsinic acid is proposed; for organotin compounds, higher toxicity for ethyl groups than for phenyl groups is reported. However, speciation in biological media is difficult when applied to other elements because of the lack of information on the existence and significance of species whose determination could be valuable. Furthermore, there may be no analytical methods that allow an accurate measurement of the species. The feasibility of speciation in occupational and environmental medicine depends mainly on our capability to solve some problems related to the identification and determination of species and on the demonstration that species measurement represents a clear improvement compared to total element determination.     

Interaction of chemical species with biological regulation of the metabolism of essential trace elements

Variations in the chemical speciation of dietary trace elements can result in the provision of different amounts of these micronutrients to the organism and might thus induce interactions with trace-element metabolism. The chemical species of Zn, Fe, Cu, and Mn can interact with other components of the diet even before reaching the site of absorption, e.g. by formation of poorly soluble complexes with phytic acid. This might considerably modify the amount of metabolically available trace elements; differences between absorptive capacity per se toward dietary species seems to be less important. Homeostasis usually limits the quantities of Zn, Fe, Cu, and Mn transported from the gut into the organism, and differences between dietary species are largely eliminated at this step. There is no homeostatic control of absorption of Se and I, and organisms seem to be passively exposed to influx of these micronutrients irrespective of dietary speciation. Inside the organism the trace elements are usually converted into a metabolically recognizable form, channeled into their biological functions, or submitted to homeostatically controlled excretion. Some dietary species can, however, be absorbed as intact compounds. As long as the respective quantities of trace elements are not released from their carriers, they are not recognized properly by trace element metabolism and might induce tissue accumulation, irrespective of homeostatic control.     

Lower limits of detection in speciation analysis by coupling high-performance liquid chromatography and chemical-vapor generation

Speciation studies are much more important than total element determination because toxicity of many elements depends on their chemical forms. Nobody can claim that a foodstuff is very dangerous to eat by determining total arsenic due to the possibility that the arsenic could be present in non-toxic forms. Hence, speciation studies are crucial in any matrix relevant to human beings.Trace-element speciation requires sufficiently sensitive procedures to monitor each species at trace levels. One way to increase the sensitivity for elements forming volatile species is coupling high-performance liquid chromatography (HPLC) with chemical-vapor generation (CVG). This review aims to highlight not only development of HPLC-CVG techniques for ultratrace-elemental speciation in a variety of matrices but also their application. In addition, we discuss the advantages and the disadvantages of these techniques.     

Epigenetics: an important challenge for ICP-MS in metallomics studies

Trace metal analysis has been long regarded as one of the principle tasks in areas of chemical analysis. At the early stage of instrumental development, total concentration was assessed in a variety of samples, yielding results, among others, for environmental, biological, and clinical samples. With the power of newer analytical techniques, such as inductively coupled plasma mass spectrometry (ICP-MS), accurate quantitative results can now be obtained at ultra-trace levels not only for metals, but also for metalloids and several non-metals. Even though the importance of trace elements in many biological processes is widely accepted, the elucidation of their biological pathways, understanding specific biological functions, or possible toxicological aspects is still a challenge and a driving force to further develop analytical methodology. Over the past decades, the scientific interest has moved from total element determination to include speciation analysis, which provides quantitative information of one or more individual element species in a sample. More recently, metallomics has been introduced as a more expanded concept, in which the global role of all metal/metalloids in a given system is considered. Owing to the multi-elemental focus of metallomics research, the use of ICP-MS becomes indispensable. Furthermore, considering the biological role of metals/metalloids and the use of elements as internal or external molecular tags, epigenetics should be considered as an important emerging application for metallomics studies and approaches. Among a variety of epigenetic factors, essential nutrients, but also environmental toxins, have been shown to affect DNA methylation, modification of histone proteins, and RNA interference, all of them being implicated in cancer, cardiovascular disease, and several inherited conditions. Recent studies suggest that epigenetics may be a critical pathway by which metals produce health effects. In this Trends article, the basic epigenetic concepts are introduced, followed by the early applications of ICP-MS classified as: (i) detection of ³¹P as a natural element tag for DNA, (ii) analysis of DNA adducts with metal-based drugs, (iii) element species as epigenetic factors.     

Multielemental trace analysis of biological materials using double focusing inductively coupled plasma mass spectrometry detection

The analytical potential of double focusing-inductively coupled plasma-mass spectrometry (DF-ICP-MS) for total elemental analysis in clinical samples (serum, blood, urine and other biological fluids), tissues and food products is illustrated by reviewing typical applications recently published. Also, the use of DF-ICP-MS as specific detector for trace element speciation in biological samples is discussed. After adequate separation of interferences in the chromatographic column, low resolution measurements (R = 300) can be used to provide enhanced sensitivities of more than 100 times compared with quadrupole-inductively coupled plasma-mass spectrometry (Q-ICP-MS). This capability is extremely valuable in speciation studies. Also, the use of DF-ICP-MS at low resolution could provide very precise isotope ratio measurements for isotope dilution analysis due to the ‘flat topped’ peaks obtained at this resolution. Unfortunately, the literature on these last two issues is rather scarce so far, in spite of their extremely high analytical possibilities for biological research. Moreover, the bright future of DF-ICP-MS as a most powerful multielemental detector for trace element applications in biological systems will be highlighted. Apart from applications detailed above other important application fields can be envisaged. In particular, we will speculate on its possible use to confirm/establish ‘reference values’ of trace element content in ‘normal’ populations and so to help to diagnose health and disease status, related with trace element total content or their speciation in clinical specimens.     

Electrothermal vaporisation ICP-mass spectrometry (ETV-ICP-MS) for the determination and speciation of trace elements in solid samples - A review of real-life applications from the author's lab

The use of electrothermal vaporisation (ETV) from a graphite furnace as a means of sample introduction in inductively coupled plasma mass spectrometry (ICP-MS) permits the direct analysis of solid samples. A multi-step furnace temperature programme is used to separate the vaporisation of the target element(s) and of the matrix components from one another. Sometimes, a chemical modifier is used to enable a higher thermal pre-treatment temperature, by avoiding premature analyte losses (stabilisation) or promoting the selective volatilisation of matrix components. In almost all instances, accurate results can be obtained via external calibration or single standard addition using an aqueous standard solution. Absolute limits of detection are typically ~1 pg, which corresponds to 1 ng/g for a typical sample mass of 1 mg. Real-life applications carried out in the author's lab are used to illustrate the utility of this approach. These applications aim at trace element determination in industrial and environmental materials. The industrial materials analysed include different types of plastics - Carilon, polyethylene, poly(ethyleneterephtalate) and polyamide - and photo- and thermographic materials. As samples from environmental origin, plant material, animal tissue and sediments were investigated. Some applications aimed at a multi-element determination, while in other, the content of a single, but often challenging, element (e.g., Si or S) had to be measured. ETV-ICP-MS was also used in elemental speciation studies. Separation of Se-containing proteins was accomplished using polyacrylamide gel electrophoresis (PAGE). Subsequent quantification of the Se content in the protein spots was carried out using ETV-ICP-MS. As the volatilisation of methylmercury and inorganic mercury could be separated from one another with respect to time, no chromatographic or electrophoretic separation procedure was required, but ETV-ICP-MS as such sufficed for Hg speciation in fish tissue.     

一种改进的同步辐射X射线荧光原位分析人肝细胞胞质溶胶内金属蛋白分布的方法

对同步辐射X射线荧光分析技术(SBXRF)原位测定蛋白质经十二烷基硫酸铀．聚丙烯酰胺凝胶电泳(SDS-PAGE)分离后条带中微量元素的方法做了重要改进。通过增加一个于胶步骤，使得本底材料对光源的康普顿散射明显减弱，本底信号降低到约为湿胶的10％，从而可以用来测定蛋白条带内比锌轻的微量元素的相对含量。所提出的方法有望成为一种具有广泛应用前景的生物样品中微量元素的种态分析方法。对人肝细胞胞质溶胶内金属蛋白分布的研究结果表明：在15-95kDa的范围内有6个含锌蛋白，相对分子量分别为17．5、20．5、27、35、55及63kDa，其中63kDa蛋白为主要的含锌蛋白；此外至少还有4种分子量分别为20、23、43和83．5kDa的含铁蛋白分布于这个分子量范围内。