A typical analysis for TOC measures both the total carbon present and the so-called “inorganic carbon” (IC), the latter representing the content of dissolved carbon dioxide and carbonic acid salts. Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the IC portion first and then measuring the leftover carbon. This method involves purging an acidified sample with carbon-free air or nitrogen prior to measurement, and so is more accurately called non-purgeable organic carbon (NPOC).

Virtually all TOC analyzers measure the CO₂ formed when organic carbon is oxidized and/or when inorganic carbon is acidified. Oxidation is performed either through Pt-catalyzed combustion, by heated persulfate, or with a UV/persulfate reactor. Once the CO₂ is formed, it is measured by a detector: either a conductivity cell (if the CO₂ is aqueous) or a non-dispersive infrared cell (after purging the aqueous CO₂ into the gaseous phase). Conductivity detection is only desirable in the lower TOC ranges in deionized waters, whereas NDIR detection excels in all TOC ranges.