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 of tests that would be acceptable for possible standardisation. Expert consultations and discussions are based on the selection of existing extraction schemes, e.g. EDTA, DTPA, calcium chloride etc. which have to be first accepted as candidate standard tests (on the basis of their scientific significance), then demonstrated to be applicable to various matrices (easiness of use, ruggedness) and possibly tested by expert organisations. This approach requires extensive consultations and possibly interlaboratory testing of the selected candidate standard procedures.

One of the principles of standardisation is to 'write what is done' and to 'do what is written'. A standard may be defined as a reference text, which has been elaborated by a recognised – national or international – organisation (e.g. AENOR in Spain, AFNOR in France, DIN in Germany, BS in the United Kingdom etc., CEN for the European Union and ISO at the world scale) after agreement of all interested parties. Figure 1 recalls the process of preparation of a standard.

Support to standardisation is achieved through the funding and organisation of interlaboratory studies, which aim to demonstrate the applicability of standards and to establish minimum technical requirements. Collaborative testing may indeed help standardisation organisations to take the decision to adopt or not a given procedure as an international standard. The approach followed by the SM&T programme and its successor (the European Commission) is a good example of such supporting activity. It should be stressed that the SM&T programme funded research and feasibility studies (including interlaboratory testing) but was not responsible for the adoption of standards nor for their implementation. This type of RTD programme should hence be seen as a tool for standardisation bodies but not as standardisation organisation itself.

1.3 SM&T Activities for the Harmonisation of Extraction Schemes

The SM&T programme (also referred to as BCR throughout the text) has often launched projects for the improvement of the quality of analytical measurements in a stepwise manner, i.e. starting by small-scale projects and developing them into wide interlaboratory programmes. Some of these projects dealt with the harmonisation of single and sequential extraction procedures for soil and sediment analysis with the aim of providing laboratories with reference schemes that could later become international standards and of preparing related certified reference materials (CRMs) to provide valuable tools for validating methods and for quality control.

The stepwise approach of the SM&T programme consisted of feasibility studies, collaborative testing of the selected schemes and undertaking certification campaigns. Under this framework, different expert groups on soils and sediments in both organic and inorganic pollutants have been created and worked out together different schemes in a collaborative way. The steps considered in the SM&T programme approach were:

• Selection of the different schemes to be tested, based on a literature search and consultations of European experts2 for choosing extractants which led to good results for decision-making related to sediment and soil analyses, taking into consideration the extractable capacity of the agent in relation to the studied matrices.

• Collaborative testing of the selected schemes: the analysis of the results obtained in these exercises highlighted that comparable results were only obtained when the protocols were thoroughly applied, which implied that the procedures had to be carefully written, including all operational details. Calibration errors were actually found to be the main sources of error in both single and sequential extraction schemes.4-24.2s

• Appropriate selection of the test and candidate certified samples: in particular complying with the requirements of homogeneity and stability, representativeness of the studied matrices (calcareous, siliceous, organic, etc.) and with measurable analyte contents (i.e. not too close to the detection limits of the selected analytical techniques). For both single and sequential extraction schemes, interlaboratory studies were designed and conducted with soil and sediment reference materials originating from the Joint Research Centre of Ispra, Italy.

• Validation of extraction methods: the concept of method validation in the case of operationally-defined procedures may be understood by the acceptance and testing of common schemes, possibly proposed as standardised methods to official standardisation organisations. Validation can be carried out as described above, i.e. through the organisation of interlaboratory studies in which a group of laboratories receive 'real case' samples to be analysed following a strict protocol. The experience has shown that, while the tests carried out in some schemes resulted in a satisfactory agreement (e.g. EDTA, DTPA, acetic acid), this was not always achieved for other procedures, which demonstrated the inadequacy (or the lack of 'maturity') of the scheme(s) at the time it was tested, e.g. ammonium acetate and also weak extractants such as calcium chloride or sodium nitrate, owing to difficulties in applications (the low extractable contents resulting in a wide spread of results). Another route for validating extraction methodologies is to make available certified reference materials that are certified on the basis of the schemes in question, i.e. for extractable forms of elements.

The findings, proposed procedures and certified reference materials obtained from this programme (Table 1) are presented in detail in the following chapters of the book.

1.4 The Variety of Extraction Methods and the Need for Harmonisation

As stressed above, the variety of existing extraction schemes does not allow data to be compared world-wide, which may create problems for data interpretation by regulatory bodies. Efforts are being made to harmonise the approaches used in environmental management. One aspect is collaborative testing (with possible standardisation), as described above, but the consensus for a range of extraction procedures to be possibly used as a common approach by laboratories working in different areas (e.g. soils, sludges, composts) is far from being achieved. Some laboratories recognise the suitability of specific schemes based on scientific arguments but they are pressed by national or regional regulations to use other procedures, which creates major confusion. As an example, various tests used in soil surveys, prospecting, characterisation etc. are listed below. They focus e.g. on 'pseudo-total' contents to assess the extent of pollution in top soils, 'forms' of metals for the evaluation of their mobility, plant availability etc.; the list below has been established as a result of consultations of laboratories from different fields:

• aqua regia ('pseudo-total' contents) for risk assessment prior to spreading sludge onto agricultural soils;

• EDTA, DTPA, acetic acid, for studies of trace metal mobility, soil-plant transfers, study of physico-chemical processes;

• weak extractants (e.g. calcium chloride, calcium nitrate) for plant uptake studies, soil deficiency assessment and remediation, fertility studies, risk assessment;

• column tests, availability batch tests, for environmental risk assessment;

• sodium nitrate, ammonium acetate, for risk assessment and evaluation of soil multi-functionality;

• ammonium chloride, acid oxalate, for the differentiation of lithogenic and anthropogenic origin of some critical elements in soils.

With respect to sediment analysis, extractable forms are determined to assess the heavy metal mobility. Analyses generally concern the 'active' sediment layer and aim at studying the metal remobilisation and fluxes. The concept of obtaining information on (i) easily mobilisable fractions (e.g. water or neutral electrolyte solution with or without complexing abilities), (ii) slowly mobilisable fractions (e.g. with EDTA or DTPA) and (iii) immobile fractions (e.g. with aqua regia or real total, using HF) follows in part the approach developed in waste research (total, available, actual leaching). Among the various sequential extraction schemes used nowadays, the three-step sequential extraction protocol developed by BCR is finding increasing international recognition.

1.5 Trends and Conclusions

As recently discussed, the coordination between 'harmonisation' (adoption of a common protocol based on scientific grounds, without official mandate) and 'standardisation' activities is not fully satisfactory. This is illustrated by the BCR work on single and sequential extraction schemes for soil and sediment analysis that led to internationally adopted schemes backed-up with CRMs but which, however, were not considered by ISO or CEN as working possible candidates for international standards. There are certainly efforts to be made to establish standards not only on the basis of regulatory requirements but also on the outcome of the results of international collaborative efforts. Furthermore, there is an obvious need to pursue validation activities of existing schemes, e.g. by applying them to a wide variety of soil/sediment matrices. It is timely to avoid the multiplication of protocols and critically evaluate newly developed schemes versus existing ones. Questions related to their usefulness etc. should be thoroughly investigated. Too many procedures are used as a result of regulations without being strongly supported by practical considerations such as easiness, user-friendliness, scientific soundness, robustness etc. One of the main trends is certainly the new approach followed by a group of EU experts who collaborate to harmonise various schemes applied to a variety of matrices (waste, soil, sludge, compost, sediment, construction material etc.). Such a network, piloted by the SM&T programme, establishes clearly where collaborations are possible among disciplines for adopting common strategies for environmental risk assessment. Standardisation may be the final goal for the establishment of a range of tests 'fit-for-purpose' for various objectives, along the lines of the scheme shown in Figure 2.

Another trend, which is a continuation of the efforts of harmonisation of leaching and extraction tests, is the necessary production of reference materials certified for operationally-defined parameters; this aspect is thoroughly described in this book.