Total organic carbon (TOC) is a sum parameter in water and wastewater analysis and reflects the pollution of water with organic substances. The concentration of the total organically bound carbon in the water is determined and usually measured in automated measuring procedures. In the area of large TOC concentrations (e.g. municipal wastewater), the "difference method" is often used. In the first step, the total of all carbon compounds ("TC" = Total Carbon) is determined and then, in a second measurement, the proportion of inorganic carbon compounds (e.g. carbonates) ("TIC" = Total Inorganic Carbon) is determined. The TOC value is then obtained by subtracting the TIC from the TC.  

(Quelle: https://www.chemie.de/lexikon/Gesamter_organischer_Kohlenstoff.html)