Total Organic Carbon Analyzer | Theory of Operation |
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2.3 The UV/Heated Persulfate Method of Analysis
In the UV/Heated Persulfate Method as used by the Model 6750, the sample is initially mixed with acid and directed to the sparger. For TOC-Direct (NPOC) analysis, the CO2 converted from the inorganic carbon is sparged out of solution by the carrier gas and vented to atmosphere. Any volatile organic loss would occur at this point, exiting the sparger as HC gas. For TOC-True, the TIC-related CO2 is measured by the NDIR CO2 detector, which is blind to the HC. For NPOC analysis, the liquid, carbonate-free sample is then directed to the UV reactor, where the remaining organic carbon is oxidized to CO2 and measured by the NDIR as a "TOC-Direct" (or NPOC) analysis.
NDIR analysis of CO2 is specific and interference-free and is used in all critical and regulatory applications. The TOC-Direct method is the most accurate TOC analytical method, as determined by the EPA and other governmental agencies. Teledyne also offers conductivity detection but it is of limited use in these applications and is not proposed as a primary detection method for TOC analysis, especially in critical industrial applications where multiple or unknown analytes are present. Figure 2-5 compares both CO2 detection methods.
For the TC analysis, the combined liquid/gas sample is sent directly to the reactor, where all carbon (including the volatile HC) is converted to CO2. The NDIR thus measures the TOTAL CO2 (including that
generated from the TIC, volatile HC and NPOC) and reports it as "TC."
The important thing is that all measurements are made on the same sample, eliminating potential sample introduction errors and multiple sampling inaccuracies.
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