Total Organic Carbon: Measuring Organic Contamination in Samples

What is TOC?

Total Organic Carbon measures the amount of organic contamination within a sample. TOC analysis is constantly happening globally, and it is vital to the safety and success of humanity. Organic contamination can degrade ion-exchange capacity and cause undesired biological growth, making treated water unsafe for us to use. Not to mention, water treatment can produce harmful byproducts that the Environmental Protection Agency (EPA) regulates, so TOC analysis helps municipalities determine the effectiveness of their treatment process.

How do you measure TOC?

Simply put, the job of a TOC analyzer is to oxidize organic compounds into CO₂ and measure the amount of CO₂ produced. Regardless of the oxidation technique, TOC must be determined by either measuring or removing the various fractions found within the sample's total carbon (TC). These fractions are defined below:

Total Organic Carbon (TOC) — all carbon atoms covalently bonded in organic molecules

Inorganic Carbon (IC) — carbonate, bicarbonate, and dissolved CO₂

Dissolved Organic Carbon (DOC) — the fraction of TOC that passes through a 0.45 μm-pore-diameter filter

Suspended Organic Carbon (also called Particulate Organic Carbon) — the fraction of TOC retained by a 0.45- μm filter

Purgeable Organic Carbon (POC, sometimes called Volatile Organic Carbon) — the fraction of TOC removed from an aqueous solution via gas stripping under specified conditions

Nonpurgeable Organic Carbon (NPOC) — the fraction of TOC not removed by gas stripping. In most cases, inorganic carbon is purged and not "determined," in which case only NPOC is determined, and POC is assumed to be negligible.

This graphic offers a visual of how these fractions of TC are divided:

Other Important Terms:

Reagent Blank – The detector response is generated from an analysis sequence with reagents without introducing a sample or standard. This response is due to carbon contamination in the reagents, gas, digestion vessel, and/or tubing.

Standard – Any sample with a known amount of added carbon

Water Blank or Instrument Method Blank – Response of an analyzer to the carbon content of the water. This is measured by using water as the zero-concentration standard during calibration.  

Determining TOC

Total organic carbon consists of all carbon atoms that are bonded in organic molecules. Specifically, it is a measure of all the carbon in organic compounds that have been converted to carbon dioxide by oxidation after the inorganic carbon has been removed. The two most common techniques for determining TOC are TOC by subtraction and TOC by sparging.

In TOC by subtraction, IC is measured and subtracted from the measured TC:

The advantage of TOC by subtraction is that there is a minimal loss of POC. However, there are several disadvantages to this technique. TOC by subtraction has poor throughput compared to TOC by sparging because it requires two analyses for one TOC result. This technique should not be used for samples containing high IC, where IC content is more than 50% of the expected TOC result.

In TOC, by sparging, the IC is purged from the sample before any measurement takes place. TOC is equal to NPOC in this technique if POC is negligible. For this reason, TOC by sparging is sometimes referred to as the NPOC technique. 

TOC by sparging is faster than TOC by subtraction because it only requires one analysis to determine TOC. It has the best accuracy for samples containing high amounts of IC and better precision overall because there is no interference from the subtraction of IC. The disadvantage of this technique is that it does not measure POC.

The Main Steps

TOC Analysis can be broken down into four main steps: Sample Introduction, oxidation, detection, and display. There are several options by which each of these steps is completed; this ultimately depends on the TOC analyzer that is used.

Sample Introduction
A simple sipper system that analyzes one sample at a time could be used for sample introduction. Or perhaps there is a complimentary autosampler that allows the user to load multiple samples onto a sequence for analysis. If the TOC analyzer is an Online TOC analyzer, then it is connected directly to the water pump system and automatically collects samples for analysis at a set interval.  

Oxidation
Several approved Standard Methods are used to measure TOC, including wet chemical persulfate oxidation and high-temperature catalytic oxidation (combustion). Wet chemical TOC analysis involves oxidation of organic carbon to CO₂ via a reagent and heat or a UV lamp. These two subdivisions of wet chemical oxidation are heated persulfate and UV persulfate. The combustion method for TOC analysis involves the injection of the sample into a heated reaction chamber with an oxidative catalyst. Combustion TOC is covered in SM 5310B, while the heated persulfate and UV persulfate methods are covered in SM 5310C.

Detection
In TOC analysis, detection is typically achieved via a solid-state, non-dispersive infrared detector (NDIR) to measure the organic content of your sample. The NDIR works by shining an infrared beam through the cell containing the CO₂ sample gas. It then measures the amount of infrared absorbed by the sample at a specific wavelength range. For example, CO₂ absorbs an infrared wavelength range of 4.26 µm.

Display
And finally, display refers to how the data obtained in the detection step is visualized. This step has benefited tremendously from the technological revolution of the 21^st century. Many TOC analyzers have an internal computer and a touch screen interface, or they are controlled by software on an external PC.

Choosing an Oxidation Technique

Since both oxidation methods fulfill the same purpose in a TOC analyzer, it is essential to consider the advantages and disadvantages of each technique before purchasing one for your laboratory. The combustion technique is better suited to analyze organic carbon that contains suspended materials (such as humic acids, bacterium, vegetation, or specific high-molecular-weight molecules) or, more generally, for samples above 1 ppm C. This technique is less efficient at carbon levels lower than 1 ppm C because the hotter temperature of the combustion technique limits the amount of sample volume that can be injected into the system. The main drawback to the combustion technique is that there are generally higher system blanks (or a higher background) due to carbon memory effects on the catalyst.

The two wet chemical oxidation techniques perform significantly better when low-level detection is needed. The reaction occurs at a much lower temperature than combustion (95-100 °C, compared to >680 °C in combustion), so a larger volume of sample can be injected into the system without concern for rapid expansion. While both wet chemical techniques are more precise and reliable than the combustion technique, heated persulfate is the most reliable technique. In the heated persulfate technique, heat reacts with the reagent through convection, while in UV-persulfate, UV light activates the persulfate. As a result, turbid samples may reduce the intensity of the UV light that reaches the sample matrix, which would, therefore, reduce the oxidative capacity of the system.

Water Treatment

Measuring TOC in treated water is critical for ensuring the effectiveness of drinking water treatment processes. During treatment, TOC serves as an indicator of organic material present in source water, which needs to be reduced to meet regulatory standards. High levels of TOC are associated with increased formation of harmful disinfection byproducts (DBPs) like Trihalomethanes (THM) and Haloacetic Acids (HAA), regulated by the EPA under the Disinfectants and Disinfection Byproducts Rule (DBPR).

The first steps in a typical water treatment process include chemical coagulation, flocculation, sedimentation, and filtration, all aimed at removing organic carbon from the water. TOC measurement provides a reliable way to assess the organic load both before and after these steps. For surface water systems, the EPA mandates a specific percentage of TOC removal, depending on the TOC concentration in the source water. Typically, source water TOC levels range from 2 to 4 mg/L, and treatment plants target TOC levels below 2 mg/L in the finished water.

TOC also plays a key role in monitoring the formation of THMs, which are byproducts of chlorine disinfection. Chlorine reacts with organic carbon compounds in the water, and without proper TOC control, this can lead to the formation of THMs, which pose health risks such as liver, kidney, or central nervous system problems. Water treatment plants can reduce the potential for THM formation by closely monitoring TOC levels at various points in the treatment process, especially after filtration and prior to disinfection.

YSI’s 9220 Total Organic Carbon Analyzer, which utilizes heated persulfate oxidation with non-dispersive infrared (NDIR) detection, provides a highly reliable solution for monitoring TOC levels in drinking water treatment applications. The 9220 TOC analyzer meets EPA compliance standards (SM 5310C, EPA 514.3) and ensures accurate, real-time monitoring of TOC throughout the treatment process, helping to minimize DBP formation and safeguard public health.

To learn more, check out our webinar on TOC & THM Monitoring Solutions for Water Treatment and our blog on Managing TOC & THM Risks in Drinking Water Treatment.

Wastewater

Wastewater treatment facilities analyze the TOC of incoming wastewater to plan and streamline their treatment process. The industrial expansion of cities leads to increasing wastewater loads, which present challenges in determining just how much the increase in volume, organics, and oxygen demand will be. To prepare for this, wastewater treatment facilities can test for TOC and/or use biological oxygen demand (BOD) and chemical oxygen demand (COD) as a substitute for TOC to determine the organic load and oxygen demand.

Food and Beverage

Understanding the contents of our food and beverages is essential to leading a healthy lifestyle. TOC analysis is becoming more prevalent in the food and beverage space, including applications in food process control to determine product loss to effluent (common in the dairy industry) and quality control of pure and organic food and beverages to combat artificial additives (used on honey, maple syrup, and other naturally derived foods). This way, TOC analysis is helping companies increase their revenue and consumer confidence in their products.

Environmental

TOC analysis plays a vital role in environmental analysis, and many municipalities use contract environmental laboratories to test for contaminants in their water and wastewater. However, TOC is not just limited to water analysis, and OI Analytical offers several optional kits to expand the capabilities of traditional TOC analysis. The 1030S solids module has its own built-in furnace and crucible for sample combustion, but it uses the NDIR on either the 1030W or 1030D to test solid samples such as soils, sludges, or slurries.  

With the TN_b kit, the combustion process liberates both carbon and Nitrogen at the same time. A NOX converter is added after the combustion tube that converts the Nitrogen to Nitric Oxide. After this, the same oxygen carrier gas used for TOC transports both the carbon and the Nitrogen into the NDIR and then Nitrogen into an added electrochemical detector for analysis.

Research and Academia

Scientists worldwide are finding new and exciting ways to use established technologies. TOC analysis continues to be a part of this effort, including blood and plasma analysis for genetic research, analysis of contaminated water and soil for environmental impact research, and ultra-pure water analysis for agricultural and industrial applications. Applications in carbon-dating and environmental disaster response applications are also becoming more common in academia, especially as the threat of global warming increases in magnitude.

Conclusion

Total Organic Carbon (TOC) is a crucial parameter in water quality monitoring, influencing everything from regulatory compliance to the effectiveness of water treatment processes across various applications. Whether you’re treating drinking water, managing wastewater, or working in environmental or industrial settings, accurately measuring TOC can help ensure water safety and operational efficiency. With a variety of TOC analyzers and techniques available, it’s essential to choose the right solution for your specific needs.

Do you have questions about TOC or need help selecting a TOC measurement solution? Ask our experts or schedule a free virtual consultation today!