Teledyne 6750 - Total Organic Carbon analyzers - Manual (Page 23)

Total Organic Carbon Analyzer Theory of Operation
Teledyne Analytical Instruments 11
2.4 Subsystems Principles of Operation
The Model 6750 TOC Analyzer is comprised of five subsystems:
1. Sample Handling
2. Inorganic Carbon Removal
– OR –
Inorganic Carbon Analysis/TOC-True Method
3. Oxidation
4. NDIR CO
2
Gas Detection
5. Electronic Signal Processing, Display and Control
The sample system is designed to accept the liquid sample and any
required reagents, transporting them through the analyzer to the
appropriate components. For TOC (actually, NPOC) analysis, the
sample is pumped initially to the sparger, where it is mixed with acid to
lower the pH between 2.0 and 3.0. At that pH, all inorganic
carbon/carbonates are converted to dissolved CO
2
gas, which is sparged
out of the liquid solution by the air/O
2
carrier gas. At this point, any
volatile organic carbon is also sparged out and lost for inclusion in the
“TOC” analysis, unless a TOC-True analysis is performed, as described
below.
2.4.1 Sample Handling
The sample is processed through the analyzer in two phases: liquid
and gaseous
2.4.1.1 L IQUID P HASE S AMPLE H ANDLING
Referencing Figures 2-6 and 2-7, the sample is introduced to the
analyzer with a self-priming peristaltic pump (P-4). It is then initially
directed to the Inorganic Carbon (IC) Sparger, where it is mixed with
acid delivered by pump (P-2). The pH of the solution is lowered to
approximately 2, converting the inorganic carbon to CO
2
, which is
sparged out by the carrier gas and measured as Total Inorganic Carbon
(TIC) by an infrared analyzer (NDIR-2) in the “TOC-True” mode.
The “TOC-True” mode is the preferred method of use if volatile
hydrocarbons are present, which would otherwise be lost in the sparging
process. Thus, through subsequent analysis of Total Carbon (TC) and