SM&T Activities in Support of Standardisation of Operationally-defined Extraction Procedures for Soil and Sediment Analysis

Ph. QUEVAUVILLER

European Commission, DG Research, Brussels, Belgium

1.1 Introduction

The environmental ecotoxicity and mobility of heavy metals is strongly dependent upon their specific chemical forms or way of binding. Consequently, their toxic effects and biogeochemical pathways can only be studied on the basis of the determination of these forms. The determination of chemical species (e.g. organometallic compounds) is often difficult in soil and sediment matrices and, to date, only very few compounds have been reported to be accurately determined in sediment (e.g. tributyltin, methylmercury), using e.g. hyphenated techniques involving a succession of analytical steps (extraction, separation, detection). In practice, environmental studies on soil and sediment analysis are often based on the use of leaching or extraction procedures (e.g. single or sequential extraction procedures), enabling broader forms or phases to be measured (e.g. 'bioavailable' forms of elements), which are in most instances sufficient for the purpose of environmental policy. This type of determination is often referred to as 'speciation' although, strictly speaking (see the recent IUPAC definition of 'speciation',) this term should not be applied to operationally-defined procedures. Speciation would cover the determination of well-defined chemical species (e.g. organometallic compounds, metals with different oxidation states etc.), whereas the extracted 'forms' should be only related to the extractant used, e.g. EDTA-extractable element, and not as e.g. 'bioavailable', 'mobile' etc. forms, which are interpretations of data rather than results of actual measurements. This type of measurement is also referred to as 'fractionation'.

For heavy metals, the development and use of extraction schemes started at the end of the 1970s and aimed to evaluate the metal fractions available to plants (and thus estimate the related phytotoxic effects) and the environmentally accessible trace metals (e.g. mobility of metals from a soil and potential groundwater contamination); these schemes have been adapted and are still widely used for soil and sediment studies as reflected by the number of recently published papers dealing with their applications to various environmental studies. Similarly, extraction schemes were developed for studying the release of phosphorus from lake sediment in relation to eutrophication studies.1 Finally, operationally -defined procedures are increasingly considered for environmental studies related to the mobility of trace organic compounds^! which are strongly matrix-dependent and require complex procedures involving different analytical steps (extraction, clean-up, separation, detection).

Besides the usefulness of these schemes, however, it was recognised that the lack of uniformity in the procedures used did not allow the results to be compared world-wide nor the methods to be validated. Indeed, the results obtained are 'operationally-defined' which means that the 'forms' of pollutants are defined by the determination of extractable contents using a given procedure and that, therefore, the significance of the analytical results is highly dependent on the extraction procedures used.

Results are useful and usable only if they correspond to well-defined and accepted procedures. In other words, the only means for achieving sound interpretation and basis for decisions is to achieve comparability of results, which is closely linked to a consensus with respect to the used procedures, followed by their validation, and their possible implementation as a standard.

This introductory chapter describes an approach followed by the Standards, Measurements and Testing programme (formerly BCR) of the European Commission for harmonising single and sequential extraction procedures for soil and sediment analyses, with the aim to provide laboratories with reference schemes that could later become international standards. Details on the different selected and tested schemes for trace metals (EDTA, DTPA, acetic acid for soil analysis and three-step sequential extraction scheme for sediment analysis) and phosphorus forms (sequential extraction scheme) are given in Chapters 2–4 of this book. As discussed later, the collaborative testing of these schemes, along with the preparation of related Certified Reference Materials (CRMs), has a clear effect on their world-wide use which is increasingly reflected by the literature. The schemes are not standardised sensu stricto (i.e. they were not adopted as official standards by an international standardisation organisation) but they fulfil the same role in enabling data comparability in this analytical field.

1.2 Standardisation

Many discussions have arisen on the risks that standardisation might 'fossilise' progress in analytical science in a wide range of cases. It is, however, generally accepted that the only way to achieve comparability when using operationally-defined procedures is to standardise them and apply them, following very strictly the written protocols. This does not mean that improvements should not be investigated to ensure progress in the use and result interpretation of these schemes; in this case standardisation offers scientists a possibility to speak the same language and decision-makers a way to identify better possible strategies for environmental risk assessment. Extraction tests are widely used for the assessment of the release of inorganic contaminants from soils, sludges and sediments. In many instances, these schemes are included in national (or sometimes regional) regulations. The International Standardization Organization (ISO) is coordinating working groups on soil quality (e.g. ISO TC/190) with the aim to identify a range...