![]() In situ methods are used in a wide range of different situations. This is because the primary sample that has been removed can be prepared and mixed in the field lab, so the uncertainty contribution from in situ heterogeneity contributes no more than it does for remote ex situ measurements. In terms of uncertainty estimation and method validation, ‘on site’ measurement can usually be treated as an ex situ measurement. This class of measurement is clearly intermediate between those made in situ and ex situ. This paper will not be specifically discussed ‘on site’ measurements, where a physical sample is removed, but the ex situ measurement is made near the original location of the test material, typically in a field laboratory. Once realistic estimates of uncertainty are available, it should then be possible to rigorously validate the methods used to make sufficiently reliable in situ measurements. However, this rigorous approach has not been widely applied to measurements that are made in situ, for reasons that will be discussed below. 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). 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. microns with SIMS ), with the mass of the corresponding ‘undisturbed sample’ ranging from micrograms to picograms. centimetre scale with PXRF ) down to the micro (e.g. 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. Such measurements are now becoming more prevalent than traditional ex situ measurements. In situ measurements of chemical concentration are made at the original location of the test material without the removal of a physical sample. 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. 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. 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. Existing methods for estimating UfS for ex situ measurements can broadly be applied to in situ measurements. 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. 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. However, estimates of uncertainty for in situ measurement values have not usually included this uncertainty from sampling (UfS). The quality of ex situ measurements is usually expressed primarily in terms of their measurement uncertainty, including that arising during the sampling process. 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. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |