GE Sievers 900 Series - Page 56

GE Analytical Instruments ©2009 56 of 236 DLM 90488-01 Rev. A
When the TOC concentration in the sample is low (<1 ppm), complete oxidation can usually be
achieved using only the hydroxyl radicals from the photolysis of water (eq. 1) without the addition of
persulfate.
The IC stream passes through a delay coil, which is designed to make the total transit time of the IC
stream through the Analyzer the same as the transit time of the TC stream through the Analyzer.
When the TC stream exits the oxidation reactor and the IC stream exits the delay coil, each stream
moves to its respective CO
2
Transfer Module. The CO
2
Transfer Module is a patented design,
utilizing a gas-permeable membrane that allows the transfer of CO
2
across the membrane. The
membrane separates the sample side of the Analyzer from the DI side. The DI side of the Analyzer is
a closed loop, and consists of two conductivity cells—one for the TC stream and one for the IC
stream—a DI water pump, DI water reservoir, and ion exchange resin (resin bed).
CO
2
from the sample passes through the membrane into the DI water supplied by the integrated DI
Loop, while interfering compounds and other oxidation by-products are blocked by the membrane
and remain on the sample side. The CO
2
forms carbonic acid upon reaction with water, and the
carbonic acid disassociates into hydrogen ions and bicarbonate ions:
CO
2
+ H
2
O ↔ H
2
CO
3
↔ H
+
+ HCO
3
-
(5)
DI water is continuously pumped through the DI side of the Analyzer, collecting the H
+
and HCO
3
-
ions and H
2
CO
3
and CO
2
molecules from the CO
2
transfer modules, delivering it to the conductivity
cell for measurement. Then the ion exchange resin removes the HCO
3
-
and other ions. The water is
then pumped back to the CO
2
transfer module to repeat the sequence.
The TC and IC conductivity cells each contain a thermistor, and all conductivity readings are
temperature corrected. The CO
2
from the TC and IC sample streams are measured by the
respective conductivity cells, and the conductivity readings are used to calculate the concentration
of TC and IC. Once the values are measured, TOC is calculated as the difference:
TOC = TC - IC (6)