The non-dispersive infrared analysis (NDIR) method offers the only practical interference-free method for detecting CO in TOC analysis. The principal advantage of using NDIR is that it directly and specifically measures the CO 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 CO, usually around 4.26 µm (2350 cm), is measured over time as the gas flows through the detector. A second reference measurement that is non-specific to CO is also taken 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 CO 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 CO 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 CO 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.