For the determination of low and trace concentrations, knowledge of blank values is indispensable. Blank valuesactually consist of different individual blank values. The instrument blank value is combined with residual concentrationin the solvent and in the reagents used. In TOC analysis, these influences are particularly significant as carbon containing compounds are present everywhere and a widely distributed contribution of carbon cannot be prevented. During careful samplepreparation and analysis, these blank values can be minimized and accurately determined.