Page 18 - 5_TOC in daily practice

Sum Parameter – Total Organic Carbon

TOC – Measurement principle

Catalytic combustion at 680°C
SCA-130-509

■TOC-Measurement principle Salt interference at the detection cell from the
The organic carbon compound is oxidized by salt melt products may impact the quality and
combustion to carbon dioxide. The carrier gas accuracy of the data. Maintenance time is
(transporting the CO 2) is cooled and also increased due to the extended cool
dehumidified and passed through a halogen down and reheating time required based on
scrubber into the cell of the NDIR (Non the higher combustion temperature.
Dispersive Infrared) detector where the CO 2
is detected. The NDIR outputs a detection ■ Shimadzu TOC method
signal which generates a peak. Shimadzu developed the catalytically
oxidation at 680°C and uses peak area for
integration. This temperature is lower as the
melting points of some salts:

Compound Melting point
NaCl 801 °C
KCl 773 °C
Na 2SO 4 888 °C
MgCl 2 782°C
CaCl 2 782 °C
Tab. Melting points of different salts

The deactivation of the catalyst and the
corrosion of the combustion tube are
minimized. In total the maintenance request
is lower as using higher temperature. On the
other hand the platinum catalyst ensures a
complete oxidation of all organic compounds.

■ Peak detection
In the past high temperatures (up to 1000°C)
were necessary because the first TOC
instruments use the peak height for
integration. Due to this the conversion to CO 2
must be instantaneous to keep the peak as
narrow and sharp as possible.

■ Disadvantage of high temperature
The very high combustion temperature has
the disadvantage of high levels of
maintenance (deactivation of catalyst,
corrosion of combustion tube and detector
cell) due to the salt melt products.