Total Organic Carbon in Soil: A Comparison between Solid Sample Combustion and Suspension Methods

Significance of the topic

Organic carbon content in soil and sediments is a key indicator of ecosystem health, reflecting organic matter turnover, contamination levels, and soil quality. Accurate total organic carbon (TOC) measurement is essential for environmental monitoring, agricultural management, and pollution assessment.

Aims and overview of the study

This work compares two established TOC measurement approaches for soil samples: the Solid Sample Combustion (SSC) method and the Suspension (SP) method. The primary goals are to evaluate precision, accuracy, and operational efficiency, and to assess whether the SP method enhanced with a High Suspension Kit can match SSC performance.

Methodology

• SSC method: Dried, sieved soil is combusted in an oxygen stream at 980 °C using a cobalt oxide/platinum catalyst in the SSM-5000A unit. Resulting CO₂ is quantified by NPOC detection on the TOC-LCSH analyzer.
• SP method: About 200 mg of soil is suspended in 0.22 M HCl and homogenized at 18 000 rpm. The suspension is measured by TOC-LCSH using the High Suspension Kit and External Sparging Kit at 680 °C with standard platinum catalyst.

Instrumentation used

• TOC-LCSH Total Organic Carbon analyzer
• SSM-5000A solid sample combustion unit
• High Suspension Kit and External Sparging Kit
• IKA T 18 ULTRA-TURRAX dispersion device

Main results and discussion

Both SSC and SP methods produced highly linear calibration curves. TOC values for clay and sandy soil samples differed by less than 5% between methods. The SP method, when combined with the High Suspension Kit, achieved precision below 2% RSD, outperforming the SSC method’s labour-intensive single-combustion approach.

Benefits and practical applications

• The SP method allows multiple injections per sample, reducing sample inhomogeneity effects and enabling statistical reliability.
• High Suspension Kit minimizes particle sedimentation, ensuring uniform sample injection and reducing carryover.
• Automation with an autosampler increases throughput compared to SSC.

Future trends and potential applications

Future developments may integrate automated on-line sample preparation, extend suspension techniques to diverse matrices (e.g., sludge, compost), and combine TOC analysis with other geochemical parameters for comprehensive environmental and agricultural assessments.

Conclusion

The suspension method enhanced by specialized sampling kits offers a reliable, precise, and efficient alternative to traditional solid combustion for soil TOC determination. It matches SSC accuracy while improving throughput and reducing labour requirements.

References

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