Comparison of TOC Determination Methods in Soil Samples Application Note · multi EA 4000

Introduction

The carbon contained in soil has different sources and effects on the soil‘s quality. Total inorganic carbon (TIC), mainly carbonate and hydrogen carbonate, is a natural part of soil but can also derive from artificial addition and other sources. Total organic carbon (TOC) is generated by organic matter that is present from natural sources (plants, animals, degradation processes) and contaminations caused by humans and industrial and agricultural processes (oil residues, waste, overfertilization, etc.). Its content is influenced by erosion, biological decomposition (e.g., by bacteria), and farming. The TOC affects soil properties (e.g., color, fertility, toxicity). This makes TOC an important parameter in environmental protection, agriculture, waste management, and landfills, needing to be regularly monitored. For this purpose elemental analysis techniques based on sufficient high-temperature combustion of all sample components have proven best suited, followed by NDIR detection of the formed CO2 (e.g., DIN EN 13137).

Depending on the soil type and its TOC/TIC ratio, two different TOC determination strategies are available. The direct method is preferred in case of low TOC compared to the present TIC. It requires an extensive pretreatment of the sample to completely remove the interfering TIC (wet chemical acid digestion and drying of the treated samples). This method is also difficult to automate and negatively affects the hardware of the analyzer in terms of consumables and maintenance. Thus the difference method is preferred whenever possible (equivalent or higher content of TOC compared to TIC). This method requires two analyses (TC and TIC determination) to obtain results. Still, additional measurement time is minimal compared to the pretreatment efforts for direct determination.