Temperature-Dependent Determination of Total Carbon (TC), Total Organic Carbon (TOC), Residual Oxidizable Carbon (ROC), and Total Inorganic Carbon (TIC) in Soil

Significance of the topic

Total organic carbon (TOC) and its thermally differentiated fractions are core indicators of soil quality, fertility, and long‑term carbon sequestration. Temperature‑dependent differentiation that separates TOC, residual oxidizable carbon (ROC) and total inorganic carbon (TIC) provides a practical, reagent‑free alternative to acid digestion methods. This approach supports agronomy, ecological monitoring, carbon accounting and regenerative agriculture by distinguishing labile versus stable carbon pools and enabling assessment of management impacts on soil carbon dynamics.

Objectives and overview of the application note

• Describe a temperature‑ramped combustion method, compliant with ISO 17505, for determining TC, TOC, ROC and TIC in soils using the LECO CM812 instrument.
• Provide method parameters, sample preparation and calibration procedures to achieve reproducible differentiation of carbon fractions by combustion temperature ranges.
• Show typical performance data using certified or proficiency test soils and synthetic reference materials.

Methodology

The temperature‑differentiation approach exploits distinct thermal decomposition/oxidation windows:

• Organic carbon (TOC) oxidizes primarily between 150 °C and 400 °C.
• Residual oxidizable carbon (ROC) oxidizes largely between 400 °C and 600 °C and represents more stable organic forms (char, lignin, humified material, biochar).
• Inorganic carbonates (TIC) thermally decompose between ~600 °C and 900 °C producing CO2.

Key operational sequence used (ISO 17505 style, as implemented on the CM812):

• Preheat and bake sample boats to remove adventitious carbon (recommend 1100 °C; cool in desiccator; re‑bake if not used within 24 h).
• Dry representative soil samples at 105 °C for 1 h and store in a desiccator until analysis.
• Instrument ramping sequence: ramp from 150 → 400 °C (TOC) with a hold of 240 s; ramp to 600 °C (ROC) with a hold of 100 s; ramp to 900 °C (TIC) with a hold determined by peak detection.
• Oxygen is used as the combustion gas; afterburner temperature typically at 850 °C; purge and analytical flows are controlled to maintain reproducible peak shapes and detector response.
• Peak integration windows are defined for TOC, ROC and TIC to calculate fraction masses; TC = TOC + ROC + TIC; difference between TC and carbonate carbon can be used alternatively when acid digestion is applied.

Used instrumentation

• LECO CM812 multiphase carbon and moisture determinator with a variable ramp furnace and CO2 detector suitable for temperature differentiation.
• Combustion boats: quartz (LECO 781‑335) or nickel (LECO 625‑505‑430), pre‑baked at 1100 °C.
• Reference materials: LECO LCRM/LRM and/or NIST or other matrix‑appropriate CRMs for carbon calibration and verification.

Method parameters and quality control

• Typical sample mass: 0.10–0.25 g for calibration; 0.15–0.25 g for routine soil samples (reported on a dry basis).
• Gas flows: purge flow ~4.00 LPM, analytical flow ~0.75 LPM; purge 15 s before run.
• Furnace ramp rates: 70 °C/min for TOC and ROC ramps; 120 °C/min for TIC ramp to 900 °C.
• Afterburner setpoint ~850 °C; peak find enabled for TIC step to ensure complete carbonate decomposition detection.
• Calibration strategy: low‑range linear force‑through‑origin and high‑range full regression using synthetic carbon reference materials (e.g., 0.53–5.00 % C standards). Verify calibration with independent CRM and reduced sample mass checks.
• Blanks and replicate analysis: run three or more blank replicates for calibration and sample blanks; run reference material replicates for calibration verification and instrument performance monitoring.

Main results and discussion

The example data show reproducible differentiation of carbon fractions across different soil proficiency samples. Representative outcomes:

• Sample set with ~4.13 % TC: TOC ≈ 2.38 %, ROC ≈ 0.76 %, TIC ≈ 1.00 % (TOC+ROC ≈ 3.14 %), low standard deviations show good repeatability.
• Lower‑carbon sample (~1.19 % TC): TOC ≈ 0.97 %, ROC ≈ 0.19 %, TIC ≈ 0.02 % demonstrating method sensitivity at low carbon contents.
• Calibration and verification using LECO synthetic LCRM standards produced linear response across the tested range and acceptable recovery for verification materials and NAPT proficiency samples.

These results illustrate that the temperature‑ramped approach can separate labile and more stable organic carbon pools and quantify inorganic carbonate content without acid reagents. Method robustness depends on careful sample drying, consistent boat handling, appropriate calibration materials and properly set integration windows.

Benefits and practical applications

• Reagent‑free workflow: eliminates acid digestion hazards, waste and time associated with carbonate removal methods.
• Operational efficiency: unattended sample loading (autoloader) and defined furnace programs enable higher throughput.
• Informative carbon partitioning: provides actionable data for agronomy, soil restoration, carbon sequestration accounting and biochar characterization by quantifying labile vs. stabilized carbon fractions.
• Applicable for proficiency testing and routine QC when paired with certified reference materials and appropriate blanks.

Future trends and potential uses

• Integration with automated sampling and cloud‑based QAQC will increase throughput and traceability in large monitoring campaigns.
• Coupling thermal‑differentiation data with spectroscopic or chromatographic analyses could improve source attribution of organic matter and biochar characterization.
• Standardization and inter‑laboratory exercises will refine uncertainty estimates for ROC and TOC fractions to support carbon markets and soil carbon reporting frameworks.
• Method adaptations to wider matrices (sediments, waste materials) and lower detection limits may expand applicability in environmental monitoring.

Conclusion

The temperature‑ramped combustion method on the LECO CM812, following ISO 17505 principles, provides a robust, reagent‑free pathway to quantify TC, TOC, ROC and TIC in soils. When combined with appropriate sample preparation, baked combustion boats, reference materials and calibration/blank protocols, the approach delivers reproducible fractionation of labile and stable carbon pools and reliable quantification of carbonate carbon. This enables practical soil health assessment, carbon sequestration studies and routine laboratory workflows without chemical digestion.

References

• ISO 17505: Soil and waste characterization — Temperature dependent differentiation of total carbon (TOC400), residual oxidizable carbon (ROC) and total inorganic carbon (TIC900).
• LECO Corporation. Application note: Temperature‑Dependent Determination of TC, TOC, ROC and TIC in Soil using the 812 Series (LECO CM812). LECO Corporation, St. Joseph, MI, USA. Form No. 203‑821‑728, 2026.
• North American Proficiency Testing (NAPT) Program soil samples and informational values as used for method verification in the application note.