Surrogate Measurement for Chemical, Bacterial Contamination
A mainstay of environmental and quality control chemistry, total organic carbon (TOC) analysis measures the carbon content of dissolved and particulate organic materials in water. The carbon measured in TOC analysis may arise from any combination of living or dead organisms and chemical contamination.
Water utilities monitor TOC to determine raw water quality, to measure the effectiveness of organic carbon removal, and to monitor the efficiency of treatment processes. For example, water utilities use TOC to monitor by-products of chlorination or ozonation. TOC often serves as a surrogate for more difficult measurements, for example, contamination from petrochemicals, solvents, pharmaceuticals, chlorinated industrial chemicals, and pesticides. It can also act as a screen for additional analysis. For example, pharmaceutical manufacturers might use liquid chromatography-mass spectrometry to analyze water samples containing unacceptable TOC values.
As a quality measure, pharmaceutical regulatory authorities in the U.S., Europe, and Japan require TOC analysis of ultrapure water used in biotechnology, to ensure the absence of contaminating bacteria.
TOC analysis is nonspecific, meaning it tells how much organic carbon is present without identifying the contaminant.
How it’s done
The two main approaches to TOC measurement involve either initial removal of inorganic carbon (mostly carbonate) followed by TOC measurement, or the subtraction of inorganic carbon from total carbon present. The four steps in TOC measurement are acidification to remove inorganic carbon, purging to release volatile organics (which are measured separately), oxidation of the remaining carbonaceous material, and detection.
The latter two operations form the heart of TOC analysis. Several types of oxidation may be used: high- or low-temperature combustion, catalytic oxidation, photo-oxidation, thermochemical oxidation, or electrolytic oxidation. “Each has its pros and cons,” says Jeff Lane, a TOC specialist at OI Analytical (College Station, TX).
Detection is carried out through conductivity measurements, by nondispersive infrared (NDIR) spectroscopy of the strongly absorbing carbon dioxide oxidation product, or colorimetrically. The most common online TOC analyzers used for drinking water analysis use NDIR. “CO2 has a unique signature in the infrared,” notes Lane. “It’s possible to tune in specifically for it and rule out everything else.”
Detection limits for TOC depend on the measurement technique used and the type of analyzer. High-temperature (up to 950ºC) oxidation produces a sensitivity of 0.1 mg/L of carbon, while low-temperature methods (below 100ºC) are five times as sensitive, to about 0.02 mg/L. Response times for TOC analyzers vary widely, but instruments generally take five to fifteen minutes to report stable readings.
Online TOC analyzers are capable of continuous, unattended operation, but regular calibration, inspection, and maintenance by skilled technicians is required for reliable operation.
Purchasers of TOC analyzers value ease of use, throughput, and automation features. “They want to be able to set up a group of samples and walk away while the instrument does its thing,” says Lane. Another desirable feature is some sort of reporting and/or control function, which can be achieved by connecting analyzers to LIMS (laboratory information management systems), or for critical monitoring applications to a SCADA (supervisory control and data acquisition) or Ethernet network whose supervisory functions will close down a plant or process when serious excursions occur.
TOC analyzers are reasonably priced as lab instruments go. A basic unit costs approximately $20,000. Autosamplers will add to the cost, as will the addition of detectors for nitrogen or isotopic carbon. “Tandem detection systems are more common in research settings than in industrial laboratories,” Lane says.
“Over the past decade, the popularity of TOC analysis has been driven by regulations,” notes Steve Poirier, VP of business development at GE Power and Water (Boulder, CO). “Every pharmaceutical company that ships drugs into the U.S. and Europe is required to measure TOCs to certain specifications.” In the drug industry, high-purity water is used both for cleaning and in sterile drug products.
The second regulatory front is the environment. According to Poirier, every municipality of greater than 10,000 population is required to control TOC to specified levels in drinking water.
“Additionally, some companies have demonstrated equivalency between TOC measurements and other tests and use TOC as the primary regulatory assay for releasing wastewater,” Poirier adds. The advantages are that TOC analysis is straightforward and does not require the specialized skill set of chromatography.
User-friendliness is a desirable software feature, but Poirier notes that data management capabilities differ significantly among industries. “It’s important that software serve the specific needs of the application.” While LIMS capability is now common for TOC analyzers, drug companies, for example, will demand compliance with CFR Part 11, the FDA’s mandate for electronic records.
A TOC analyzer’s reliability and uptime are important factors, both for busy analytical labs and for online instruments that operate continuously. “But in the end, most purchase decisions are based on productivity, and that comes down to analysis time or throughput,” Poirier says.
For more information on TOC analyzers, please visit www.labmanager.com/toc-analyzers
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