TOC INDICATOR OPERATION
The Milli-Q® Reference A+ system uses an integrated
185/254 nm UV lamp to oxidize neutral organics into
charged molecules and facilitate their removal by the
mixed bed ion-exchange resin in the polishing cartridge.
The TOC Indicator employs this same UV lamp to indicate
TOC value by measuring the difference in ultrapure water
resistivity upstream and downstream from the UV lamp.
Previous attempts to use this method resulted in devices
with an important drawback: they did not take into account
the fact that all organic molecules are not oxidized at the
same speed and that the nature of organics in feed water
may change depending on the geographical location and
the time of year. These TOC indicators were calibrated in
factories using feed water containing a specific mix of
organic substances—and they performed reasonably well—
with the same feed water. However, they were unable to
adapt their operation to feed water variations, and often
delivered erratic results when the mix of neutral organic
molecules in the feed water was changed. Consequently,
these devices could very well indicate low product water
TOC in cases where the TOC value was actually high.
On the other hand, the
TOC Indicator
in the
Milli-Q®
Reference A+ system
uses an operating process that
includes a patented “TOC Curve Check” phase, which
adjusts the TOC analysis results each night to the evolution
of the mix of organics in the system’s feed water. The same
patented process allows taking into account possible
variations of UV light intensity that might be due to
deposits on the quartz sleeve surrounding the lamp.
Merck Millipore’s patented process ensures that the TOC
values displayed by the Milli-Q® Reference A+ system are
reasonably accurate for TOC levels within 1 and 10 ppb.
The system will also indicate any increase in TOC values
above this level.
Millipore, Milli-Q, A10, Q-Gard and Quantum are registered trademarks of Merck KGaA Darmstadt Germany. The M Mark and Merck
Millipore are trademarks of Merck KGaA.
Lit. No. PF2954EN00.
© 2012 EMD Millipore, Billerica, MA. U.S.A. All rights reserved.
TOC INDICATOR AND MONITOR
The differences between a Merck Millipore TOC Indicator
and a Merck Millipore TOC Monitor (A10) are listed in the
table below.
Parameter
TOC
Indicator TOC Monitor
Monitoring
Frequency
Once per
second
Once every 5 minutes
- N.B. This is not very
important as TOC
values typically do not
change very quickly
Accurate TOC
Detection Range
1 - 10 ppb 1 - 999 ppb
TOC Values Display 1 -
999 ppb
1 - 999 ppb
Calibration to
reference solutions NO
YES
Certificate
of Calibration
NO
YES
TOC Curve Check
YES
NO - The TOC curve
check process is not
required as the TOC
monitor is calibrated
to reference solutions
Suitability test
as required by USP NO
YES
Cost
Low
Medium
2
TOC INDICATOR
During normal system operation, water at a
resistivity of 18.2 MΩ·cm @ 25 °C flows through
the 185/254 nm UV lamp. Water conductivity and
temperature are measured at the outlet of the UV
lamp whenever water is recirculated or delivered.
Oxidation by the UV lamp of the neutral organics
into charged molecules generates a conductivity
increase whose value is converted to a TOC value
by an algorithm. To ensure that the algorithm takes
into account any variations in the nature of the
water’s organic compounds or possible decrease in
UV light intensity, a “TOC Curve Check” is performed
every day.
TOC CURVE CHECK
Once a day, or when the user requires, the system
performs a “TOC Curve Check.” During this operation,
the water flow is diverted from the UV lamp. This
allows the system (1) to check that the water resistivity
entering the lamp is still at 18.2 MΩ·cm @ 25 °C and
(2) to capture a water sample inside the UV lamp.
The first sample will be oxidized for 20 seconds, then
released and its conductivity measured. This process is
repeated several times with increasing oxidation times
(the second sample is oxidized for 30 seconds before
release and measurement, the third for 40 seconds,
etc.). The data collected are used to build a curve of
the conductivity as a function of oxidation time.
During the day, this curve is used to generate valid
TOC measurements based on the small conductivity
variation resulting from the passage of the water
through the UV lamp.
