TOC Analysis Moving from Laboratory Testing to On-line Analysis

LABORATORY WIDE-RANGING MEMBRANE CONDUCTIVITY TOC ANALYSERS
Laboratory-based TOC analysers utilise membrane conductivity (MC) with a CO2 permeable membrane
and two conductivity cells to measure conductivity before and after oxidation of the organic material.
Typically, the TOC analyser will have up to three conductivity probes; one for reporting conductivity and
two to make the before- and after-oxidation measurements.
MC technology was developed to provide a lab-based general purpose TOC analyser capable of
measuring TOC in a wide range of water types. Interference from other salts and chemicals present in
non-GMP waters is avoided as they cannot pass through the membrane, allowing the MC analyser to only
report increases in conductivity from the evolved CO2 from the oxidised TOC. However, they are a flowthrough
design and full oxidation the TOC is rarely achieved. Instead factors are used to extrapolate the
TOC from the slight changes in conductivity as the TOC is partially oxidised on it’s way through the
analyser.
This technology does not have a stable calibration and, if used for on-line analysis, is affected by
temperature, pressure and flow rate changes in the PW or WFI system. Also, the CO2 transfer rate
through the membrane is dependant on the pH and temperature of the water system. Slight drift in the
calibration of the multiple conductivity cells used can also contribute to inaccurate TOC analysis. In fact,
when this technology is used in the laboratory, a calibration check is frequently recommended for each
batch of TOC samples run to ensure that the analyser calibration has not drifted.
TOC Analysis – Moving from laboratory to on-line monitoring.
March 2008 by Hach Ultra
Figure 1: Lab-based Membrane Conductivity TOC Analysis
ON-LINE DIRECT CONDUCTOMETRIC-BASED TOC ANALYSERS
Direct Conductometric (DC) based TOC analysers, such as the Anatel PAT700, utilise a single
conductivity meter to measure the increased conductivity of a water aliquot after all the organic material in
the aliquot has been completely oxidised to carbon dioxide (CO2) using ultra-violet light.
Because the water treatment system creating PW and WFI is designed to GMP-compliant quality water,
there must be no potentially interfering contaminants present, so the membrane technology is not an
advantage and, being much more stable and robust in design than membrane flow-through TOC
analysers, these DC TOC analysers are not affected by the temperature, pressure and flow changes
expected on a typical PW or WFI system.
As shown in Figure 2 below, the WFI or PW aliquot is trapped in the analysis cell by valves, the oxidising
UV lamp is turned on and the organic materials are fully oxidised. The difference in conductivity created by
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