TOC Analysis Moving from Laboratory Testing to On-line Analysis

combined with a ‘main’ and ‘standby’ UV lamp arrangement, so that on detecting the ‘main’ UV lamp
failure, the analyser can automatically switch to the ‘standby’ UV lamp and continue to monitor on-line
TOC whilst raising an alarm so that the maintenance team can replace the failed UV lamp in a timely
manner.
VERIFYING ON-LINE TOC ANALYSERS
Unlike the membrane-based analysers used for laboratory TOC analysis the on-line TOC analyser must
be robust, have a stable calibration and be capable of consistent, repeatable analysis under varying
sample temperatures, pressure and flow-rates.
Figure 4: TOC analysers use temperature-compensated conductivity measurement
TOC analysers are basically temperature-compensated differential conductivity instruments using UV light
to oxidise the organic material to CO2 and utilising before-and-after-conductivity measurements as a
surrogate measurement for TOC. They must be able to accurately monitor the temperature of the sample
as it is analysed to ensure that the final conductivity measurement is compensated for temperature
change, otherwise the small change in conductivity caused by the CO2 evolved from the oxidised TOC will
be swamped by the change in conductivity due to the temperature change in the sample.
Hence it is important to verify:
a) the calibration of conductivity meter used to measure TOC
b) the calibration of the temperature compensation sensor
c) the TOC meter calibration against USP-traceable TOC standards
d) system suitability
e) the UV lamps
a) Verifying the calibration of the conductivity meter used to measure TOC
Guidance was given in USP30 regarding the calibration verification conductivity meters. This
was split into two parts: meter calibration and cell constant. Hach Ultra provides an SOP for carrying
these tests out on the Anatel PAT700 TOC analyser: SOP 700-4.
Meter calibration verification is made by disconnecting the measurement cell and replacing it with a
known-value NIST-traceable resistor. Cell constant is verified by running a certified conductivity
sample through the measurement cell.
TOC Analysis – Moving from laboratory to on-line monitoring.
March 2008 by Hach Ultra
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