Ionics Instruments 2004 Page 10-10 DLM 30007-08 Rev. A
IMPORTANT FACTS
Negative TOC values are usually the result of measuring low levels of TOC
with relatively high levels of IC, and can be statistically relevant results on a
validated and properly functioning TOC analyzer, however, many customers
find this undesirable. In an effort to mitigate this situation, the following
procedure is designed to fine tune both the IC and TC channels of the
analyzer.
In the case of off-line water samples, the TOC may be low, and the IC
relatively high (off-line samples absorb carbon dioxide from the atmosphere).
Off-line samples may equilibrate over a 250-400 ppb IC range. Depending on
the extent to which samples have equilibrated with the atmosphere, there
could be different levels of IC for the same water samples if they are analyzed
at different times.
TC ZERO OFFSET
If it has been determined by an Ionics Instruments Service Representative to
make an adjustment to the TC zero offset, perform the following:
Note: Be sure to record all TC zero offset adjustments and communicate the
history to the Ionics Instruments Service Representative assisting you. If you
have made more than two adjustments to the TC zero offset in the last six
months, there may be underlying issues with your water system or analyzer.
Please inform our representatives accordingly.
Modification of procedure in the TC Zero Offset section in Chapter 9:
1. Obtain a 1 L or larger flask (preferably a beaker to offer a large area of
exposure to the air) and fill the container approximately half-full with low
TOC water. Leave the sample open to the atmosphere overnight so that
it may equilibrate with room CO
2
levels (200-400 ppb typically).
2. If necessary, loosely cover the mouth of the flask with parafilm (being
careful to leave openings) to minimize organic contamination that may
be present in the lab.
3. Turn the UV lamp off.
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