Technological Approaches

1. The development and validation of an uncertainty DMT

The Decision Management Tool (DMT) will utilise well-established methods for the estimation of measurement uncertainty and will then employ novel Optimised Contaminated Land Investigation (OCLI) method¹ to optimise the uncertainty of on-site measurements to promote their effective use in contaminated land assessment.

Measurement uncertainty (the range with which the true value lies) arises from both the sampling procedure and the chemical analysis², and has both systematic and random components. The total uncertainty on individual on-site measurements can be estimated by combining the random and systematic components. 

The precision is an estimate of the random component of the uncertainty of the measurement process and can be estimated using the Duplicate Method³ often applied using a balanced design:

In this method a proportion of the sample measurements are taken in duplicate (e.g. 10%, but not less than 8). These duplicate samples are not taken in exactly in the same place, but separated by a distance that represents the ambiguity in the sampling protocol and surveying technology (e.g. 2m in a typical site investigation). The duplicate samples are then each analysed in duplicate. The data produced from this balanced design can then be analysed using the statistical technique called robust analysis of variance (ANOVA) to separate and quantify the contributions of sampling and analysis to the precision.

The systematic component of the uncertainty can be estimated by the bias of the on-site measurement technique. This will be estimated by a comparison between the on-site measurements and the lab-based analysis (MCerts accredited). These lab-based measurements are not true values, but also have estimates of uncertainty issued by the laboratory (e.g. MCerts accredited), and the measurements also have traceability, usually to certified reference materials (CRM). The estimation of bias is made using a regression of the on-site measurements against the lab-based values (using a maximum likelihood approach that allows for uncertainty on both axes).   

A sampling plan will be implemented to evaluate the uncertainty of the measurements made with the on site tools. It will require at least 20 sampling locations, and a sufficient amount of soil taken at each sampling location for on site Field Analytical Tests (FATs) and lab analysis. A second (duplicate) sample will be taken at 8 of the sampling locations at a distance that represents the ambiguity in the sampling protocol. The differences in concentration between the duplicate samples are used to estimate the variability caused by short-range heterogeneity, which is one component of the whole uncertainty of measurement Robust Analysis of Variance (RANOVA).

Once the on site measurements have been validated in the initial phase of investigations, routine quality control procedures will be devised and tested, for subsequent applications of the same ‘on site’ technique to new sites.

The ‘Optimised Contaminated Land Investigation’ (OCLI) Method can be used to assess the fitness-for-purpose of an investigation by looking at the costs of the investigation and the cost of misclassification caused by the measurement uncertainty. The method can be used to find the minimal expectation of loss and hence the optimal measurement uncertainty. Comparison of the optimal measurement uncertainty to the actual uncertainty estimated for the investigation will give an indication of the fitness-for-purpose of the investigation.

2. To develop an OUMCI field analytical tool

The field analytical immunoassay tool will be developed to effectively minimise uncertainty by reducing sources of errors. The major sources of error are quantitative (sensitivity of detection), qualitative (a lack of specificity producing false positives) and operator error (addition of assay reagent). Current immunoassay products used in the contaminated land sector allow the quantification of classes of compounds, for example (PAHs) and carcinogenic PAHs⁴. The technology is well established and validated but has significant limitations.

Current systems have the ability to detect toxins down to the parts per million (ppm) level, however, key-risk drivers such as benzo(a)pyrene (BaP), often require part per billion (ppb) detection. The existing detection system produces a colormetric reaction upon binding of the desired toxin to an antibody, but this system has limited sensitivity together with a lack of specificity of compounds detected. Additionally, the assay protocols require repeated rounds of user manipulation, a source of measurement uncertainty.

The technical approach implemented will utilise highly sensitive chemiluminescent detection methodologies. Chemiluminescent technologies have surpassed conventional colormetric methods of analysis due to their typically 100-fold increase in sensitivity, and greater rapidity. Essentially, a specific antibody to a toxin, labelled with alkaline phosphatase, catalyses the decomposition of the substrate, CDP-Star^® (Applied Biosystems Ltd.) resulting in light generation⁵. Light output will then be quantified using photodiode and photomultiplier analysers. The newly developed immunoassay technology will also utilise monoclonal antibodies, for the specific detection of key-risk drivers compounds.

¹ Taylor, P.D., Ramsey, M.H. and Potts, P.J. (2004)  Balanced measurement uncertainty against financial benefits: a comparison of in situ and ex situ analysis of contaminated land.  Environmental Science and Technology 38: 6824-6831

² Ramsey, M.H. (2004)  When is sampling part of the measurement process? Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement 9, 11-12: 727-728

³ Eurachem/Eurolab/Citac/Nordtest (2006) Estimation of measurement uncertainty arising from sampling. Draft for consultation

⁴ Chuang, J.C., Chou, Y-L, Nishioka, M., Andrews, K., Pollard, M. and Menton, R. (1997)  Field evaluation of screening techniques for Polycyclic Aromatic Hydrocarbons, 2,4-Diphenoxyacetic Acid, and Pentachlorophenol in air, house dust, soil and total diet.  United States National Exposure Environmental Protection Research Laboratory Agency, Triangle Park, NC 27711 EPA/600/SR-97/109

Back to top of page