Total carbon

A variety of different terms are used to identify the different forms of carbon present at different levels of detail.

Relationship between forms of carbon

• Total carbon (TC) – all the carbon in the sample, including both inorganic and organic carbon

Carbon compounds can be distinguished as either organic or inorganic, depending on their composition. Organic carbon forms the backbone of key component of organic compounds such as – proteins, lipids, carbohydrates, and nucleic acids. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO₂, H₂CO₃, HCO₃⁻, CO₃²⁻ respectively).

• Total organic carbon (TOC) – material derived from decaying vegetation, bacterial growth, and metabolic activities of living organisms or chemicals.
• Total inorganic carbon (TIC) – often referred to as inorganic carbon (IC), carbonate, bicarbonate, and dissolved carbon dioxide (CO₂).

• Volatile organic compounds (VOC) – also referred to as purgeable organic carbon (POC); organic carbon that has been removed from a neutral, or acidified sample by purging with an inert gas. These are usually determined by purge and trap gas chromatography–mass spectrometry.
• Non-purgeable organic carbon (NPOC) – commonly referred to as TOC; organic carbon remaining in an acidified sample after purging the sample with gas.

Marine carbon is further separated into particulate and dissolved phases. These pools are operationally defined by physical separation – dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.

• Dissolved organic carbon (DOC) – organic carbon remaining in a sample after filtering the sample, typically using a 0.45 micrometer filter.
• Particulate organic carbon (POC) – also called suspended organic carbon; the organic carbon in particulate form that is too large to pass through the filter.

• Dissolved inorganic carbon (DIC) – inorganic carbon remaining in a sample after filtering the sample, typically using a 0.45 micrometer filter.
• Particulate inorganic carbon (PIC) – also called suspended inorganic carbon; the inorganic carbon in particulate form that is too large to pass through the filter.

• Elemental carbon (EC) – charcoal, coal, and soot. Resistant to analytical digestion and extraction, EC can be a fraction of either TOC or TIC depending on analytical approach.[1]