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 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 catalyst and heat or a UV lamp. These two subdivisions of wet chemical oxidation are heated persulfate and persulfate-UV. 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 persulfate-UV 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 persulfate-UV, UV light is the heat source. 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 of treated water is essential as it helps to ensure that the processes used to remove contamination from these facilities are working properly. The disinfectants that are used in water treatment can create byproducts that the EPA regulates in the Disinfectants and Disinfection Byproducts Rule (DBPR). Common byproducts such as Trihalomethanes (THM) and Haloacetic Acids (HAA) can be reported using TOC analysis. Therefore, TOC measurement is crucial in determining that our water is safe to use and drink.

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.