Environmental Engineering Reference
In-Depth Information
Total Organic Carbon Analyzers
Total organic carbon (TOC) analysis is a well-defined and commonly used
methodology that measures the carbon content of dissolved and particulate
organic matter present in water. Many water utilities monitor TOC to deter-
mine raw water quality or to evaluate the effectiveness of processes designed
to remove organic carbon. Some wastewater utilities also employ TOC analy-
sis to monitor the efficiency of the treatment process. In addition to these uses
for TOC monitoring, measuring changes in TOC concentrations can be an
effective surrogate for detecting contamination from organic compounds (e.g.,
petrochemicals, solvents, pesticides). Thus, although TOC analysis does not
give specific information about the nature of the threat, identifying changes
in TOC can be a good indicator of potential threats to a system. TOC analy-
sis includes inorganic carbon removal, oxidation of organic carbon into CO
2
,
and quantification of the CO
2
. The primary differences among various online
TOC analyzers lie in the methods used for oxidation and CO
2
quantification.
The oxidation step can be high or low temperature. The determination of
the appropriate analytical method (and thus the appropriate analyzer) is based
on the expected characteristics of the wastewater sample (TOC concentrations
and the individual components making up the TOC fraction). In general, high-
temperature (combustion) analyzers achieve more complete oxidation of the
carbon fraction than do low-temperature (wet chemistry/ultraviolet) analyz-
ers. This can be important both in distinguishing different fractions of the
organics in a sample and in achieving a precise measurement of the organic
content of the sample. Three different methods are available for the detection
and quantification of CO
2
produced in the oxidation step of a TOC analyzer:
• Nondispersive infrared (NDIR) detector
• Colorimetric methods
• Aqueous conductivity methods
The most common detector that online TOC analyzers use for source water
and drinking water analysis is the nondispersive infrared detector.
Although the differences in analytical methods employed by various TOC
analyzers may be important in compliance or process monitoring, high lev-
els of precision and the ability to distinguish specific organic fractions from
a sample may not be required for detection of a potential chemical threat.
Instead, gross deviations from normal TOC concentrations may be the best
indication of a chemical threat to the system.
The detection limit for organic carbon depends on the measurement tech-
nique used (high- or low-temperature) and the type of analyzer. Because TOC
concentrations are simply surrogates that can indicate potential problems in
a system, gross changes in these concentrations are the best indicators of
potential threats; therefore, high-sensitivity probes may not be required for
security purposes. The following detection limits can be expected:
