Challenges for the estimation of uncertainty of measurements made in situ
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GENERAL PAPER
Challenges for the estimation of uncertainty of measurements made in situ Michael H. Ramsey1 Received: 17 March 2020 / Accepted: 4 September 2020 © The Author(s) 2020
Abstract In situ measurements are made without the removal of a physical sample and have many advantages over traditional ex situ measurements, made on a removed sample usually in a remote laboratory. The quality of ex situ measurements is usually expressed primarily in terms of their measurement uncertainty, including that arising during the sampling process. However, estimates of uncertainty for in situ measurement values have not usually included this uncertainty from sampling (UfS). It is argued that the making of an in situ measurement inevitably includes the taking of an ‘undisturbed sample’ that generates UfS, which should be included in the estimate of measurement uncertainty. Because undisturbed samples are not prepared or mixed, as is usual for removed samples, the heterogeneity of the analyte concentration in the sampling target is the primary source of UfS. Existing methods for estimating UfS for ex situ measurements can broadly be applied to in situ measurements. However, four extra challenges that limit the design and uptake of uncertainty estimation for in situ methods are identified, and possible solutions and actions required are discussed. Examples of in situ measurements considered include Pb in top soil by hand-held PXRF, 137Cs at a nuclear site by portable gamma-ray spectrometry, and bilirubin in new-born infants by hand-held reflectance photometry. Keywords In situ measurement · Uncertainty · Sampling · Heterogeneity
Introduction This paper aims to describe in situ measurements in general, identify the challenges there are in estimating their uncertainty, and suggest possible solutions to these challenges. In situ measurements of chemical concentration are made at the original location of the test material without the removal of a physical sample. Such measurements are now becoming more prevalent than traditional ex situ measurements. In situ measurements cover an enormous diversity of analytes, targets and situations at a wide range of different measurement scales, from the macro (e.g. centimetre scale with PXRF [1]) down to the micro (e.g. microns with SIMS [2]), with Based upon the talk ‘The way forward for Uncertainty from Sampling’ presented by the author at the EURACHEM Workshop ‘Uncertainty from sampling and analysis for accredited laboratories’, November 2019, Berlin, Germany. * Michael H. Ramsey [email protected] 1
School of Life Sciences, University of Sussex, Brighton BN1 9QG, UK
the mass of the corresponding ‘undisturbed sample’ ranging from micrograms to picograms. It is widely accepted that when such measurements are made ex situ in a laboratory, be that remote or’on site’, then the analytical method needs to be validated. The key metric of the quality of any measurement value is its uncertainty, which can be used to judge whether a measurement is fit for its intended purpose (FFP). How
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