page_title: Total organic carbon
section_title: Non-dispersive infrared (NDIR)
hierarchical_section_title: Total organic carbon / Measurement / Detection and quantification / Non-dispersive infrared (NDIR)
context_page_description: Total organic carbon is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic carbon in soil, or in a geological formation, particularly the source rock for a petroleum play; 2% is a rough minimum. For marine surface sediments average TOC content is 0.5% in the deep ocean, and 2% along the eastern margins.
  A typical analysis for total carbon measures both the total organic carbon present and the complementing total inorganic carbon, the latter representing the amount of non-organic carbon, like carbon in carbonate minerals. Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the TIC 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.
context_section_description: The non-dispersive infrared analysis (NDIR) method offers the only practical interference-free method for detecting CO₂ in TOC analysis. The principal advantage of using NDIR is that it directly and specifically measures the CO₂ generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements.
  A traditional NDIR detector relies upon flow-through-cell technology, where the oxidation product flows into and out of the detector continuously.  A region of absorption of infrared light specific to CO₂, usually around 4.26 µm (2350 cm⁻¹), is measured over time as the gas flows through the detector. A second reference measurement that is non-specific to CO₂ is also taken and the differential result correlates to the CO₂ concentration in the detector at that moment.  As the gas continues to flow into and out of the detector cell the sum of the measurements results in a peak that is integrated and correlated to the total CO₂ concentration in the sample aliquot.
  A new advance of NDIR technology is Static Pressurized Concentration (SPC). The exit valve of the NDIR is closed to allow the detector to become pressurized.  Once the gases in the detector have reached equilibrium, the concentration of the CO₂ is analyzed.  This pressurization of the sample gas stream in the NDIR, a patented technique, allows for increased sensitivity and precision by measuring the entirety of the oxidation products of the sample in one reading, compared to flow-through cell technology. The output signal is proportional to the concentration of CO₂ in the carrier gas, from the oxidation of the sample aliquot. UV/ Persulfate oxidation combined with NDIR detection provides good oxidation of organics, low instrument maintenance, good precision at ppb levels, relatively fast sample analysis time and easily accommodates multiple applications, including purified water (PW), water for injection (WFI), CIP, drinking water and ultra-pure water analyses.
caption: Plot of atmospheric transmittance in part of IR region showing CO2 absorbing wavelengths
exif: {}