Total elemental analysis has reached an irreversible turn towards that of elemental speciation. During the previous century analytical techniques have been refined to the point where picogram or lower amounts of nearly every element can be measured with good precision. Although very valuable, such information has only a limited importance. All knowledge about the molecular structure of the elements is lost through dissolution and/or mineralization of the samples. There was a change for the better by the 1980's when analysts started to develop adequate sample treatment followed by hyphenated chromatographic techniques. From then on big efforts were made towards species identification.
Through many decades of research by toxicologists, biochemists, environmental scientists and last but not least analytical chemists, we learned that metals and non-metals occur in a large variety of species. These may be simple ions, inorganic- or organometallic complexes, complexes with humic acids and elements bound to proteins. (The latter recently gave rise to a new discipline called metallomics, a subdivision of proteomics.) Every element is present under a specific form in a sample. Each form or species is expected to exert unique properties with regard to essentiality, toxicity, bioavailability or inaccessibility in living systems and the environment.
A few examples will illustrate the need to be “species minded”. There are first of all the compounds of anthropogenic origin that make up a major new category of species to be monitored. The most ill famed are the trialkylated organo-tin compounds, intensively used as broad-spectrum biocides. Only after being intensively used worldwide were they identified as severe endocrine-disruptors, causing important biological disorders in exposed organisms. The abundance of these compounds in the aquatic environment has triggered a number of serious adverse effects in fresh and marine water ecosystems. Although it has been hitherto claimed that organotin compounds in food pose only a negligible risk, it may be anticipated that the concentrations of these compounds in fish and seafood will continue to increase. Also, there is no scientific basis for the statement that there is “only a negligible risk to humans”. The only certainty we have is that before their introduction by man into the environment, they were non-existent.
It is also possible that an element is leached into the environment at low concentrations, but on a large scale, in a form with low toxicity, similar or identical to that in which it occurs in nature. The element may be subsequently transformed into one or more toxic forms through the intermediary of living organisms at the bottom of the food chain, only to reach dangerously toxic levels in the top predators. The best example of this is mercury. Inorganic mercury is methylated through the action of enzymes and becomes highly toxic methyl mercury.
Trace element species came into the limelight mainly because of their detrimental effect as impurities in the environment or the food chain. However, there are also beneficial effects, in particular from some species of essential trace elements in food. A very puzzling element is selenium, as illustrated by the number of research papers trying to identify a potent cancer-protecting selenium compound. The most enigmatic element is chromium, because of its alleged beneficial characteristics in sugar metabolism, referred to as the glucose tolerance factor of hitherto unknown composition. Last but not least it is necessary to speak about arsenic because of the baffling fact of the non-toxicity of a substantial number of arsenic species, contrary to the general belief that arsenic is a toxic element. There can be no doubt that the inorganic forms of arsenic (i.e., arsenite and arsenate) are toxic species. Humans, other mammals, fish, bivalves, algae - to name just these - apply a detoxifying mechanism for inorganic arsenic through successive methylation steps. The resulting species are monomethylarsonic- and dimethylarsinic acid in mammals, additionally arsenobetaine and arsenocholine in fish and seafood and a display of arsenosugars, especially in algae. These organic arsenic compounds are considered harmless. Some products, such as fish gelatin, may have a total arsenic concentration exceeding the legal norm, but should not be rejected because of the non-toxicity of the arsenic species that are present.
It should also be emphasized that elemental speciation is being treated with great attention in occupational hygiene and medicine since many decades. The problem is very well documented in the nickel, chromium, platinum and semi-conductor industries. The chemical form of the element on the surface of the inhaled particulates in the workplace is considered decisive for the short- and long-term injuries they may inflict.
Today, analysts have a duty to directly address the species or chemical forms of the elements, as these determine most of their properties. This trend has become an important issue in a wide variety of fields: food, environment, health, product-quality, etc. It is no longer a purely academic subject: various industrial sectors, government and legislative bodies are all concerned.
Industries are more and more focussing on cost effective solutions, since a substantial amount of money has to be invested in analytical measurements. The list is not exhaustive when we mention process control and optimization, characterization of raw materials, intermediates and products and last but not least for controlling the workplace and the emissions into the environment. These measurements are meant to provide vital information concerning product quality and safety for the consumer, process efficiency with respect to raw materials, energy and waste production, safety in the production plant and the workplace environment, compliance with rules and legislation (especially concerning emissions) and effective risk assessment decisions. Unfortunately, traditional analysis techniques cannot provide the necessary information, since the chemical and physical characteristics, biological activity, toxicity, mobility, bioavailability, …, depend solely on the type and concentration of chemical species and not on total element concentrations. During the previous European Thematic Network activity “Speciation 21”, it already became obvious that industrial partners did not have the technical equipment nor the required knowledge to carry out the minimum analytical investigations on chemical species in their products, by-products, effluents, waste, etc. The need to develop rapid but robust speciation methods has already been identified within the Speciation 21 Network, with an emphasis on retrieving as much information as possible from complex signals with a minimum of sample pre-treatment. Such methods are intended for direct measurement during production in an on-line mode. This will improve process control and ameliorate quality with respect to the chemical forms of the elements.
Sustainable development is a key issue for the future and will only be effective when based on accurate and reliable information. Metal and metalloid species determinations are coming more and more into the limelight. The EC, WHO, FAO, EPA and other international bodies are issuing (or are on the brink of issuing) recommendations about the concentrations of species rather than total element built-up. This will allow improving health-related issues, inducing sound environmental monitoring, enhancing knowledge of chemical reactivity and improving modelling of trace elements. Finally we anticipate that at the industrial level such an approach will be translated either into cost-effective solutions or in targeted risk assessments. At present, we cannot afford to bypass information related to elemental species.
Recently instrument manufacturers have started to market robust and fit-for-purpose instrumentation allowing routine elemental speciation for a number of elements and their species, including appropriate quality control.
It is the mission of EVISA (European Virtual Institute for Speciation Analysis) to integrate, promote and facilitate on a commercial basis the accessibility of state-of-the-art speciation knowledge. The scope of this Institute is to provide an interface between the academic world and the user, creating an on-line transfer of knowledge and a swift provision of services related to elemental species. EVISA also wants to be instrumental in the organization of training courses and in e-learning.
We believe that EVISA will prove an efficient and essential platform for all elemental speciation-related activities.
Laboratory for Analytical Chemistry,
University of Gent,
B-9000 Gent, Belgium;
Olivier F.X. Donard, University of Pau
and CNRS (LCABIE-France),
University of Muenster, Institute for Inorganic & Analytical Chemistry, Corrensstr. 33,
Münster, Germany, e-mail: Wolfgang.Buscher@uni-muenster.de
| Michael Sperling, |
University of Muenster, Institute for Inorganic & Analytical Chemistry, Corrensstr. 33, Münster, Germany,