Ionics Instruments 2004 Page 10-9 DLM 30007-08 Rev. A
EFFECT of INORGANIC CARBON on INSTRUMENT ACCURACY
The Sievers 800 total organic carbon (TOC) analyzers determined TOC by
measuring both the amount of total carbon (TC) and inorganic carbon (IC) in
samples, and then calculating the difference [TOC= TC - IC]. The
concentration of IC in samples is therefore an important consideration for
obtaining accurate results. As stated in the 800 operation/maintenance
manual (system specifications), the on-line accuracy of the 800 analyzer is
±3% of the total carbon value. The error in measurement then, is dictated by
the total amount of carbon in the sample, of which the IC is a component. The
off-line error of the 800 will be higher due to sample handling/preparation and
exposure to air, and for those samples, an additional 2-3% error is expected
based on data we have collected.
As an example, IC levels in municipal tap water often exceed 20 ppm. In such
cases, the error would be ~1 ppm (from: (20,000 ppb) x (.05) = 1000 ppb).
The 800 can indicate that the IC level is high relative to the TOC by
illuminating “W42” on the display. Warning 42 notifies the user that the ratio of
IC: TOC is greater than 10:1, and that the accuracy of the TOC measurements
may be compromised. Since the accuracy error is directly proportional to the
amount of IC in a sample, it is recommended that as much IC as possible be
removed before analysis to obtain the most accurate results possible. One
method that can be used to reduce IC levels is the acidification and mixing of
the samples prior to sampling. Acidification reduces IC levels by liberating
much of the dissolved CO
2
from the sample. This method is time consuming
though, and can also introduce contamination into the sample from both the
acid and additional handling. This procedure should not be performed unless
materials and techniques can be ensured not to alter sample integrity.
Note: For samples with high IC, Ionics Instruments recommends the use of an
inorganic carbon removal (ICR) module. The ICR accessory effectively
removes CO
2
(IC) from samples without additional handling.
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