GE
Water & Process Technologies
Analytical Instruments
Technical Bulletin
The Sievers* Inorganic Carbon Remover (ICR)
In measuring Total Organic Carbon (TOC), all analyzers
must deal with the presence of Inorganic Carbon (IC). IC
is defined to be carbon from CO2, HCO3-, and CO3-2.
Sources of IC include dissolved limestone and absorption
of CO2 from air. Almost all water samples contain both
organic and inorganic carbon. Combined, these components are commonly referred to as Total Carbon (TC).
Total Carbon (TC) = Organic Carbon (TOC) + Inorganic Carbon (IC)
Because it is not analytically possible to measure only
the organic carbon when inorganic carbon is also present, most TOC analyzers work by measuring the TC content of the sample, the IC content of the sample, and
then calculate the difference as TOC.
Total Carbon (TC) - Inorganic Carbon (IC) = Organic Carbon (TOC)
Measured
Measured
Calculated
Alternatively, a TOC analyzer may operate by sparging
out the inorganic carbon, and the subsequent carbon
measurement is assumed to be 100% free of inorganic
carbon. In this case, Total carbon is measured and is
reported as the TOC of the sample.
TC = TOC
Some TOC analyzers can both measure and remove IC,
giving the operator flexibility depending on the level of IC
in the sample. When IC is relatively small compared to
the TOC level of a sample, accurate measurements can
be obtained without removing IC; it can simply be measured and subtracted from the TC value.
Figure 1
imagination at work
However, when IC is very high and TOC is low (e.g., IC =
10x TOC), TOC measurements can become unstable if IC
is not removed or reduced. In the example where an
instrument is measuring both TC and IC to calculate TOC,
both the TC and IC values will be very large (since IC is a
component of TC), and the stated instrument error in
measuring TC and IC can be a significant percentage of
the final calculated TOC value. In this case, improved performance will be realized by first eliminating or reducing
the IC prior to conducting the analysis.
Take for example a sample with 100 ppb of TOC and 1900
ppb of IC. We will assume that the instrument measures
TC and IC with 2% accuracy. In one case IC is not
removed, in the other it is reduced to 100 ppb (Figure 1).
In cases where IC is high, and TOC is low, removal or
reduction of IC will result in improved analytical performance. In general, for Sievers* TOC analyzers, we recommend reduction/removal of IC when it becomes 10 times
higher than the expected TOC value. The 900 Series TOC
Analyzers automatically detect this situation, and can
inform the user when IC removal is recommended.
IC Removal and Reduction Methods
Some TOC analyzers remove the IC by sparging the sample with gas, and what remains is organic carbon, which
is then measured. While sparging can be an effective
means of IC removal, there are several concerns:
1. Purity of the sparge gas (so as to avoid
contamination of the sample with organics)
2. Loss of volatile organics
3. If IC removal is not 100%, the remaining IC may
be reported as TOC, introducing a bias into the
analytical system.
4. Sparge gases increase the cost of ownership
5. Increases sample preparation/analysis time. In the
proposed EPA TOC Method 415.3, “Determination of
Total Organic Carbon and Specific UV Absorbance at
254 nm in Source Water and Drinking Water,” the
USEPA specifies a sparging time of 20 minutes, with
100-200ml/min of gas flow.
The Sievers technology utilizes a gas-free ICR (Inorganic
Carbon Remover) to reduce IC content. This method is
patented and approved for use for USEPA compliance
monitoring.
ICR Theory of Operation
IC is removed by first acidifying the sample to force all IC
into the form of CO2. IC exists in ionic and non-ionic forms;
the ionic forms are carbonate and bicarbonate, and the
non-ionic form is CO2. The ratio of ionic to non-ionic forms
is pH-dependent. By acidifying the sample, all IC is forced
into the form of CO2, which can then be degassed.
CO2
Low
→
←
HCO3pH
→
←
CO3-2
High
Acidification is accomplished automatically by the
Analyzer which detects the presence of the Inorganic
Carbon Remover unit, and using the same acid that is
used for normal TOC analysis, so no additional reagents
are necessary. The acidified sample is then passed
through a small membrane module in the ICR; a vacuum
is created on the outside of the membrane, and the inorganic carbon (CO2) is pulled out of the sample. Air that is
passed across the membrane is first 'scrubbed' by a builtin chemical trap that removes any organics from the air
and ensures no sample contamination. The IC removal
process is approximately 95-99% efficient. One hundred
percent IC removal is not required since Sievers TOC analyzers still measure any remaining IC and subtract that
value from the TC value to calculate TOC.
Because the IC level is greatly reduced, improved analytical performance is realized. There are several advantages to this method of IC reduction/removal.
1. No sparge gases are necessary, keeping costs low
and IC removal simple
2. The sample is degassed inline with the analysis
sample stream, so there is no additional time
required for IC reduction / removal.
3. Volatile Organic Carbon is not lost during the process;
this can be important as lost VOC from influent
samples may result in lower TOC removal efficiency
calculations between influent and effluent samples.
4. Because the process is automated by the analyzer,
no operator action is necessary.
The ICR can be easily bypassed via its switching valve,
allowing a fast and easy transition to normal monitoring
modes when IC removal is neither required nor desired.
Application Recommendations
An ICR should be considered when IC levels exceed 10x
TOC. This can be common in raw surface and ground
waters. In some cases, finished drinking water samples
may also benefit from IC reduction / removal.
For online, continuously monitoring applications, the ICR
should be turned on and remain on for all samples. The ICR
is housed within the Sievers 900 On-line TOC Analyzer's
case for maximum environmental protection; status lights
and the bypass valve are on the outside of the 900's case.
For laboratory monitoring with an autosampler, a standalone ICR is used, and can be controlled by the autosampler software to be utilized only on those samples where
IC removal is recommended or necessary.
* Trademark of General Electric Company; may be registered in one or more countries.
For more information, visit www.GEinstruments.com. Find a sales partner near you through the “Contact Us” Section.
USA
GE Analytical Instruments
6060 Spine Road
Boulder, CO 80301-3687 USA
T +1 800 255 6964
T +1 303 444 2009
F +1 303 444 9543
[email protected]
www.geinstruments.com
Europe
Unit 3 Mercury Way
Mercury Park Estate
Trafford Park
Manchester, UK M41 7LY
T +44 (0) 161 866 9337
F +44 (0) 161 866 9630
[email protected]
©2005, General Electric Company. All rights reserved.
300 00109 Rev A
MC05-194