Comparing loss-on-ignition, acid decarbonation and combustion elemental analysis for measuring organic carbon concentrations – recommendations for best practice

Measurements of sedimentary carbon concentrations, including total carbon (TC) and the organic (OC) and inorganic (IC) fractions, support research across the Earth and environmental sciences, and are critical for regulatory monitoring, reporting and verification of carbon stocks. Multiple methods are used across disciplines, but some have poor accuracy (producing incorrect results) or precision (large run-to-run variance). This research addresses these methodological inconsistencies, by rigorously testing three common protocols using a combination of certified reference materials and real-world samples. Loss-on-Ignition (LoI) converts measured organic matter loss into an organic carbon concentration, and none of the previously published conversions tested in this study were accurate. Total carbon measured by elemental analysis (EA) was accurate and precise but differentiating OC and IC using the elemental analyser required pre-treatment – two were tested. Removal of carbonate via acid washing led to significant organic carbon loss, giving poor accuracy and low precision. Combustion of organic matter prior to EA gave precise and accurate IC and OC measurements, which were independently confirmed using an alternative method − temperature-dependent combustion. Paired EA measurements using both acid washing and combustion pre-treatment techniques showed the magnitude of organic carbon lost. Across > 150 intertidal sediment samples from around England, acid decarbonation-EA reported OC values 37% ± 8% lower. Overall, we recommend that loss-on-ignition or acid washing are not used to determine organic carbon contents, especially in recent sediments.