Method Overview

A water sample initially contains two types of carbon:

  • QbD1200 sliced imageTotal Inorganic Carbon (TIC) (from CO2 gas dissolved in H2O and dissolved carbonates in the water)
  • Total Organic Carbon (TOC) (from organic species)

To measure TOC, first remove TIC. Then convert organic species into CO2 gas, measure the gas on detector, and convert the result into a TOC value.

The Measurement Strategy:

The TOC Measurement Strategy

Steps:

  1. Remove TIC. In presence of acid H3PO4, all dissolved carbonates are converted into CO2 gas. Blow carrier gas through reaction chamber to remove all CO2 gas derived from inorganic carbon.
  2. Convert TOC into CO2 gas. In presence of UV light and powerful oxidizer (NH4)2S2O8, organic carbon species are converted into CO2 gas by oxidation. Blow carrier gas through reaction chamber to push all CO2 gas through NDIR detector (step 3).
  3. Detect CO2 gas as it goes through NDIR detector. TOC is quantified by integrating the area under the curve.
  4. Calculate TOC. Based on instrument calibration, convert CO2 gas signal (area under the curve) into TOC.

CO2 NDIR Detector Signal vs. Time

Note that the area under the curve for TOC can also be referred to as ‘NPOC’ (Non-Purge able Organic Carbon). If the initial water sample contained a volatile organic, the volatile organic would likely be purged during the step to remove TIC. Thus, what remains after TIC removal is non-purge able organic carbon. Because the QbD1200 is designed for clean water applications where high concentrations of volatiles are not present, this should not be a concern.

UV / Persulfate / NDIR Method

The general TOC analysis method described here has been widely used for many years in a variety of applications and conforms with numerous regulatory guidelines such as USP, EP, JP, and is also an approved method (5310c) under the US EPA guidelines. The QbD1200 is unique in that it combines all required reagents (acid, oxidizer, and dilution water) into a single reagent instead of requiring multiple reagents.

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