Can in situ geochemical measurements be more fit-for-purpose than those made ex situ?

It is argued that the selection of the most appropriate geochemical measurement technique should be based upon the fitness of its measurement results for any specified purpose, regardless of whether the measurement are made in situ or ex situ. Using this approach, in situ measurements made in the field are shown to have some definite advantages over those made ex situ in a laboratory. A case study is used to show that there are cases where in situ measurements can be more fit-for-purpose than their ex situ equivalents. This is primarily because the uncertainty of both types of measurement is usually limited by the uncertainty arising from the field sampling process. That uncertainty is mainly caused by small-scale heterogeneity (in space or time) in the analyte concentration within the environmental material (e.g. soil, water or air).

► We compare measurements made without removing a sample (in situ) and those made in a lab (ex situ). ► In situ values are more uncertainty, but reduce overall costs and enable reliable interpretation. ► A field study demonstrates this for the measurement of arsenic concentration in soils. ► The best criterion by which to compare measurement methods is the fitness-for-purpose of the measurements.