Dr. Stefan Huber
How LC-OCD works
Organic Carbon Detector (OCD)
The ”heart” of the LC-OCD system is the OC-Detector (OCD). The main components of the OCD are the so-called Graentzel Thin-Film Reactor and non-dispersive IR-Detector.

The Graentzel thin-film reactor works as following (see also figure below): Carrier gas (nitrogen) enters the reactor in the upper third of the reactor. The gas is split, about two third is flowing upwards and picks up inorganic carbon dioxide, i.e. carbon dioxide released from carbonic salts upon acidification. This gas is released through an outlet hidden behind the reactor. The other one third of the carrier gas flows downwards and picks up the organic carbon dioxide, i.e. the carbon dioxide released upon oxidation. This gas leaves the reactor at an outlet found at the lower end of the reactor (Organic Carbon Flow, OC).

Both gas flows are then entering special Vigreux condensers to remove a defined amount of moisture. For the OC flow this is necessary because moisture has a slight cross-sensitivity in the infrared cuvette. For the IC-flow this is necessary to prevent condensation of moist, which would cause increase in back pressure, which then would automatically affect the OC flow. Both gas flows are defined by capillary restrictors to maintain a reactor inner pressure of about 10 mbar (about 10 cm liquid height in the siphon). To maintain a constant flow of gas entering the reactor, a mass-flow controller (MFC) is used. The flow is pre-set at 180 - 240 sccm.

We have now described the gas system, now we come to the liquid system. The mobile phase solution and the acidification solution enter the reactor at the upper end where they become mixed by means of Teflon pins placed inside a rotating quartz cylinder. The upper third of the reactor is UV-shielded and the counter-current IC flow strips all carbon dioxide which was either already present, or was formed by acidification from carbonate salts, hence all inorganic carbon. The residence time of the solution in this area is about 20 seconds. The solution continues its way down the inner side of the reactor mantle (gravity-driven) and enters the UV-exposed area where the carrier gas in co-flow direction downwards picks up carbon dioxide released by oxidation (hence organic carbon, OC). The exact position of the gas inlet is such that in a small area the co-flow direction is within the UV-shielded area to prevent a break-
through of IC into the OC flow). This carbon dioxide is quantified in the NDIR cuvette. The residence time in the UV-exposed area is about 60 sec. The thinness of the liquid film and the active stirring by the Teflon pins guarantees a rapid oxidation and stripping process. This is the unique feature of the Graentzel reactor, that both reactions are performed quantitatively and in relatively short time. This is necessary to be able to use the system as a chromatographic detector. Another very important feature is that the reactor operates with constant pressure, temperature and volume. No valves are used. This means that the residence time of the measuring gas in the IR cuvette is extremely stable. This allows high-sensitivity analysis even in the sub-ppb-range.

After oxidation the liquid passes a siphon. The height of the siphon compared to the sump is about 10 cm. This defines the pressure inside the reactor, which is, hence, about 10 mbar or 1,01 bar absolute. The waste is sent to a ”condensate” pump (it is of course not a condensate which is removed) to remove the waste via an outlet at the rack side wall. This outlet should is connected to a sink or a tank. The level of the sink can be more than 2 m above the reservoir.

Owing to the large inner gas volume of the Graentzel reactor it takes relatively long time to fully expel all ”OC” carbon dioxide from the oxidation process. Thus, narrow peaks are not possible, but for the chromatograms this disadvantage is of less importance because the diffusion process in the chromatographic separation process leads to peak-broadening which exceeds the peak-broadening caused by the OCD.

The OCD is designed for continuous operation, it may run for weeks and month without interruption.