Teledyne 6750 - Total Organic Carbon analyzers - Manual (Page 26)

Theory of Operation Model 6750
Teledyne Analytical Instruments 14
TOC analysis in a continuous, on-line TOC analyzer.
The CO
2
generated by the oxidation of carbon in the reactor is read
by the NDIR as a percentage concentration of the CO
2
in the carrier gas.
CO
2
(TOC)
CO
2
(TOC) + Carrier
Any increase or decrease of the carrier flow rate will have a
corresponding inverse effect on TOC reported value, (eg. double the
carrier flow rate, reported TOC will approximate ½ of actual sample
TOC concentration).
The Model 6750 uses a computer-controlled mass flow controller
rather than a less accurate system consisting of pressure regulation with
a flow meter in a capillary system.
The inorganic carbon removal system (sparger) flow rate is
controlled by needle valves (V1 & V2) set at the factory. The operator
will note bubbles through the liquids in the sparger, which is stripping
the CO
2
converted from the carbonates to eliminate the potential
interference from inorganic carbon in TOC analysis.
The carrier gas directs the CO
2
and other products of oxidation
from the reactor to the Gas/Liquid Separator (GLS), where liquid waste
is directed to drain and gases containing CO
2
and other gaseous products
of oxidation are directed to the CO
2
specific NDIR. The CO
2
concentration is directly related to sample TOC concentration and is
reported accordingly.
2.4.2 Inorganic Carbon Removal/Analysis
The choice of either TOC –Direct (NPOC) or TOC-True analysis
involves a method of either measuring the TIC (for a TOC-True
analysis) or not (for a Non-Purgeable TOC analysis only). The Model
6750 Analyzer can perform either method. The TOC-True method
measures the inorganic carbon and is able to detect volatile organic loss
in the sparger. The NPOC analysis ignores the volatile organics for a
less precise NPOC analysis if organic volatiles are present.
The sparger used in the Model 6750 is a gas/liquid counter-flow
stripper. Actual acid addition depends on the amount of sample
buffering and pH. Factory set-up is for a nominal sample pH of 7.0, to