摘要:

IntroductionA Collaborative Trial (or method performance study) is an internationally required method for assessing new or amended analytical methodologies using procedures developed by the International Harmonised Protocol.1It is an inter-organisational study in which portions of the same test sample are analysed in duplicate by the same method, in different laboratories (n⩾ 8). Variation within- and between-laboratories is used to determine the precision with which this test sample has been characterised. The results are used to assess the capabilities of the method, or to identify laboratories that may be recognised as a competent user of the method.Applied to sampling, the above approach requires a number of participants (called samplers) to take two sets of samples from a target using various interpretations of the same sampling protocol. Each sample is then analysed in duplicate, under randomised repeatability conditions, and hierarchical analysis of variance (ANOVA) used to decide whether within-sampler and between-sampler precision are within a specified fitness-for-purpose criterion.2Chemical analysis under repeatability conditions is required to avoid confusing analytical and sampling variations. The measurements of concentration are treated with ANOVA to estimate precision (as standard deviations) between-samplers (s2), within-samplers (s1) and between analytical duplicates (s0). The within-sampler variation is also called the sampling repeatability standard deviation3(s1=sr(s)) and refers to one sampler using the same procedure and equipment over a short period of time. Reproducibility is derived from the sum of squares of the within- and between-sampler standard deviations (√s21+s22) and refers to measurements made on a single or composite sample, collected by different participants using the same sampling protocol. The reproducibility standard deviation represents the uncertainty in measuring the mean concentration of an analyte using the selected protocol. If the uncertainty is found to be too large for particular investigations (i.e., not fit-for-purpose) then modifications to the protocol would be required,e.g., collecting composite rather than single samples.4The above methodology assesses the protocol in terms of precision, but makes no estimate of bias arising from the sampling methodology. The existence of this bias is a contentious issue, being questioned by some authors,5but recognised by others.6Such bias is usually difficult to estimate with respect to the true concentration of a contaminant within contaminated land, as the true concentration is never known. An alternative reference point for the estimation of the sampling bias is the consensus value from a substantial number of measurements made by different protocols4and/or independent samplers.2Previous applications of this methodology have taken no account of the spatial variability of the analyte in question, which is a parameter often required for assessing potentially contaminated areas for remediation.The present study therefore used a synthetic reference sampling target (RST), comprising a single hot spot of known concentration and position, to act as a reference value against which to assess the performance of the sampling process.7This collaborative trial in sampling (CTS), in addition to the objectives of previous trials, allows the first estimates of the bias from sampling to be obtained, these being traceable to a known mass of pure analyte. Such biases could arise from several causes, such as contamination from the sampling tools, inappropriate handling or selective sampling.8A new scoring method, based on the true hot spot characteristics, was required to assess the fitness-for-purpose of the sampling protocol to spatially delineate an area of contamination. The results of the CTS are processed to provide an assessment of the sampling protocol, and its application by each participant, in the form of a score derived from a novel adaptation of the International Harmonised Protocol.9The objectives of this study were therefore to determine: (1) whether it is possible to use a spatially resolved CTS to judge the fitness-for-purpose of a particular sampling protocol (e.g., herringbone pattern,n= 25); (2) whether the variation in spatial delineation by samplers was greater within-sampler or between-samplers; and (3) the measurement uncertainty caused by the precision and bias of the sampling and analytical methods at two selected sampling locations, where one location is on the hot spot and the other is on the background population of uncontaminated soil. The methodology for the estimation of spatial uncertainty from the regions of soil classified as contaminated will be described in a subsequent paper.

ISSN:0003-2654    

出版商:RSC    

年代:1999

数据来源: RSC