Teledyne 6750 - Total Organic Carbon analyzers - Manual (Page 18)

Theory of Operation Model 6750
Teledyne Analytical Instruments 6
To perform correct TOC analysis, the analysis system must be
capable of measuring all constituents of organic carbon present in the
sample: Non-Purgeable Organic Carbon (NPOC) and Purgeable
Organic Carbon (POC). Correct TOC analysis must also exclude the
Total Inorganic Carbon (TIC) interference.
Figure 2-1 is a basic, commonly used technique to measure the
NPOC, sometimes referred to as “TOC-Direct”.
Figure 2-1: NPOC or TOC – Direct Method
In this method, acid is added to the sample, lowering its pH to
approximately 2.0, at which point the carbonates present in the sample are
converted to dissolved CO
2
. In the sparger, the carrier gas strips (sparges)
the CO
2
converted from the TIC and vents it, along with any purgeable
(volatile) organics, leaving only NPOC in the sample. The resultant NPOC
is then oxidized to CO
2
in the reactor and measured by the CO
2
detector as
NPOC in the sample, often referred to and reported erroneously as “TOC”.
In Figure 2-2 a preferred method of performing a “TOC- True”
analysis is shown.
Figure 2-2: TOC – True Method
In this case, acid is added to the sample, lowering its pH to
approximately 2.0, at which point the carbonates present in the sample
are converted to dissolved CO
2
. In the sparger, the carrier gas strips