This study constrains protocol-dependent uncertainty in elemental carbon (EC) quantification by thermal–optical analysis (TOA) using a novel solvent-extraction-based mass balance framework. To eliminate organic particulate matter interference, PM<sub>2.5</sub> samples underwent sequential water and organic solvent extraction. A backup filter was strategically employed to account for EC redistribution during the extraction process, which was found to involve 37 ± 6 % of the total EC. The resulting solvent-extracted EC, corrected for redistribution, served as an operational reference largely independent of thermal charring artifacts. Comparative analysis revealed that EC determined by the IMPROVE protocol was consistently higher, whereas the default NIOSH protocol yielded systematically lower values than the reference. By reducing the maximum OC analysis temperature (OC4) in the NIOSH protocol to 650 °C, the EC values showed improved agreement with the reference (ratio = 1.08 ± 0.13). Furthermore, a logarithmic regression of the solvent-extracted EC to bulk EC ratio as a function of OC4 temperature identified a unity condition at 615 °C, defined here as the “KRISS temperature”. This framework provides a robust, reproducible basis for OC4 temperature selection and enhances inter-protocol comparability by explicitly constraining protocol-dependent uncertainties.