Aliquot size and sample heterogeneity in environmental studies: consequences for isotope-based interpretation

Area of Science:

• Environmental science
• Geochemistry
• Analytical chemistry

Background:

• Advances in analytical techniques enable isotope and geochemical measurements on smaller samples.
• Heterogeneous environmental matrices (soils, sediments) pose challenges for microsampling representativeness.

Purpose of the Study:

• To assess the representativeness of small sample aliquots in heterogeneous environmental matrices.
• To determine the necessity and strategy for replication in isotope-based environmental studies.

Main Methods:

• Statistical simulations using a soil carbon-14 (¹⁴C) distribution.
• Empirical ¹⁴C and carbon-13 (δ¹³C) measurements on an Argentinian Acrisol soil sample.
• Analysis of data variability in relation to sample size and heterogeneity.

Main Results:

• Single measurements on small aliquots (∼100 μg C) often fail to represent the bulk sample's isotopic signal.
• Replication is crucial for improving representativeness, with the required number depending on matrix heterogeneity.
• Observed variability in δ¹³C replicates exceeded analytical error, highlighting heterogeneity issues.

Conclusions:

• For environmental microsampling, replication strategy is more critical than analytical precision for robust interpretation.
• Isotope-based environmental process interpretation, including biogeochemical cycling and environmental change assessments, requires careful consideration of sampling representativeness.