Basics of Total Organic Carbon

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    Basics of Total Organic Carbon (TOC)

    What is "TOC"?

    Total organic carbon (TOC) indicates the total amount of carbon from organic material present in a sample. The advantages of TOC analysis are the fast analysis time of a few minutes, the exact and matrix-independent quantification and the very low consumption of chemicals. Since this is a sum parameter, the method is not suitable for identifying individual organic components. TOC is mostly determined in liquids where it serves as a representative index for water quality but can also be measured in solids.

    Due to the sheer number of known organic compounds, biochemical oxygen demand (BOD), chemical oxygen demand (COD) and permanganate consumption tests have been used in the past as indices for the collective measurement of all organic substances, regardless of their nature.

     

     

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    How is TOC measured?

    Carbon Species and Determination Methods

    Carbon Species

    The total amount of all carbon present in a sample is referred to as “total carbon” (TC). It can be further distinguished into two major groups, total organic carbon (TOC) and inorganic carbon (IC). Total organic carbon can be further classified as either non-purgeable organic carbon (NPOC) or purgeable organic carbon (POC).

    With regard to the solubility of organic substances in water, a distinction can be made between dissolved organic carbon (DOC), which are substances that pass through a filter with a pore size of 0.45 µm, and particulate organic carbon.

    Two main TOC determination methods are used:

    Difference method: TOC is determined by subtracting results for TC and IC (TOC = TC - IC).
    Direct method: TOC is determined by measuring NPOC, in other words TC after IC removal (TOC = NPOC).

    TOC determination methods

    Measuring IC

    For TOC measurement, IC refers to the total sum of inorganic carbon contained (where CO₂ indicates dissolved carbon dioxide, HCO₃‾ bicarbonate ions, and CO₃²‾carbonate ions). The quantity of dissolved carbon dioxide, bicarbonate ions, and carbonate ions in water are kept in an equilibrium that depends on the pH level of the water, according to the expression below.

    Measuring IC

    With decreasing pH, the equilibrium moves to the left side of the diagram above. At a pH of 3 or lower, almost all IC becomes dissolved carbon dioxide, which is easy to strip from water.
    Based on that principle, IC is measured by acidifying the sample to pH < 3 and then measuring the CO₂ extracted from the sample by stripping with CO₂-free air.

    Using Direct and Difference Methods

    Both the Difference Method (TC - IC) and the Direct Method (TOC = NPOC) are used to measure TOC. However, the optimal method must be selected based on sample characteristics.
    The difference method requires two separate analyses and is therefore subject to a larger measurement error than the direct method due to error propagation. As a guideline, the TOC content of the sample must also be greater than the IC content, otherwise the measurement uncertainty becomes unacceptable for the purpose of the analysis.
    For samples that are prone to foaming, or that have significant volatile contents, for example, the TC - IC method is used because the NPOC method can result in loss of purgeable organic carbon (POC) from samples during the stripping step, or in general due to foaming ingredients.

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    TOC Oxidation methods

    TOC analyzers are, in general terms, CO₂ gas analyzers with an upstream oxidation stage and sample preparation system. Regardless of which TOC determination method is used, TOC (also TC) is measured by the oxidation of organic carbon and subsequent quantification of the resulting CO₂ using an infrared detector. There are different oxidation methods for conversion to CO₂, of which two have become established; combustion oxidation and wet oxidation.

    Combustion oxidation method

    The sample is injected into a high-temperature (650 to 1,200 °C) combustion furnace to incinerate all organic carbon in the sample and measure it as fully oxidized carbon dioxide. Due to the simplicity of using heat/combustion as the principle for oxidation, the method requires no reagents for pretreatment or posttreatment processes. One of the main features of this method is its ability to efficiently oxidize organic carbon matter that is otherwise resistant to decomposition, such as particulate matter or macromolecular organic substances. In the past, high temperatures (1000 °C and more) were required because the first TOC instruments used peak height for integration. There, the conversion to CO₂ had to be extremely fast so that the signal was recorded as sharp as possible to achieve the best possible resolution.
    Very high combustion temperatures lead to the formation of salt melts in the analyzer, which in turn cause increased maintenance due to deactivation of the catalyst, corrosion of the combustion tube and the detector cell. Salt interference on the detector cell from the molten salt products can affect the quality and accuracy of the data. In addition, maintenance time is extended due to the longer cooling and reheating time required because of the higher combustion temperature.
    Shimadzu has developed the high-temperature catalytic oxidation (HTCO) at 680 °C. While the platinum catalyst ensures complete conversion of all carbon components, the combustion temperature is below the melting points of common salts. Thus, problems caused by salt are minimized while excellent recovery rates are achieved for all organic components. TOC combustion oxidation can easily be expanded to include the determination of an additional sum parameter for nitrogen, Total bound Nitrogen (TNb).

    Combustion oxidation method

    Wet Oxidation method

    With this method, an oxidizing agent is added to samples to chemically decompose carbon in organic matter for measurement as carbon dioxide. Though heat (up to 100 °C) or ultraviolet irradiation can be applied to promote the oxidation reaction, the ability of the chemical reaction to oxidatively decompose matter is weaker than combustive oxidation, which tends to result in lower carbon recovery rates from suspended or other particulate organic matter, or persistent substances. It does however allow the injection of comparably larger amounts of sample to reach lower limits of detection.

    Due to its superior oxidative reaction, the combustion oxidation method is commonly used to measure TOC levels in environmental water, factory effluent, and similar samples, where water samples often contain large amounts of insoluble organic carbon.

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    TOC in Drinking Water

    Confirming the safety of public drinking water

    Public drinking water is supplied by using a water treatment based on the water quality of the given river, lake, groundwater, or other water source. However, the water quality of public drinking water can vary due to changes in the water quality or usage rate of the river or lake.
    Therefore, it is important to regularly inspect the safety of treated water.

    Reactions between organic matter and disinfectants used for water treatment are said to generate substances that are harmful to humans. Therefore, measuring TOC in public drinking water provides an important index for confirming the safety of public drinking water.
    The TOC level is also said to affect the taste of public drinking water, so it can be used as an index for how good public drinking water tastes.

    Water Treatment Management

    A variety of processes are used to eliminate microorganisms and organic matter from water at water treatment plants.
    Measuring the TOC level at each process step can be used to confirm that each process is functioning properly. (pH and turbidity values are also measured in addition to TOC.) In addition to confirming water treatment functions, measuring TOC can also help optimize water treatment. Adjusting the amount of chemicals used based on the TOC values measured at each process step can also help reduce water treatment costs.

    Water Treatment Management

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    Frequently Asked Questions

    What is TOC?
    TOC is a sum parameter in chemical analysis. The total concentration of carbon originating from organic compounds is indicated in a single analytical value. "TOC" stands for "total organic carbon".

     

    What is a sum parameter?
    In a sum parameter, different compounds of a substance group or compounds with the same properties are recorded together as a sum (one analytical value).

     

    In which substances is TOC measured?
    TOC is determined in liquids, primarily in water, but also in various solids, such as soils or wastes. It is considered a measure of contamination by organic components in the respective matrix.

     

    How is TOC determined?
    In general, organic compounds are oxidized to carbon dioxide, and the resulting CO2 is detected with a suitable detector. Two different oxidation techniques have become established: wet chemical UV oxidation and catalytic combustion oxidation. Besides the different oxidation techniques, there are three different TOC determination methods: the difference method, the addition method, and the direct method (also called the NPOC method).

     

    How does wet chemical UV oxidation work?
    In wet chemical UV oxidation, the sample is irradiated with UV light in a reactor in the presence of persulfate ions at an elevated temperature (e.g., 80°C). OH radicals are formed, which convert the organic substances to CO2.

     

    How does catalytic combustion oxidation work?
    In catalytic combustion oxidation, the sample is burned in an oxygen-containing atmosphere at high temperatures (e.g., 680°C) on a catalyst (e.g., platinum catalyst) and converted to carbon dioxide.

     

    How is CO₂ detected in TOC analysis?

    The most commonly used detection method to detect CO2 in TOC analyzers is the NDIR technique (non-dispersive infrared). An NDIR detector consists of three important components:

    a.) The light source that emits IR light.

    b.) The measuring cell through which the measuring gas flows.

    c.) The measuring sensor.

    NDIR detector

     

    Why are there different TOC determination methods?

    When determining organic carbon (TOC), the inorganic carbon content must either be considered (mathematically) or eliminated before determination. If it is eliminated, e.g., by acidifying the sample and then degassing (carbonates and bicarbonates are driven off as CO2), it must be considered that there are also easily purgeable organic substances that can escape during sample preparation. This results in the following TOC carbon model:

    TC (total carbon) is the sum of organic (TOC) and inorganic (IC) carbon compounds (note: elemental carbon is recorded as TOC in the "organic fraction" in TOC determination).

    TOC is the sum of non-purgeable organic carbon compounds (NPOC) and purgeable organic carbon compounds (POC).

    TOC determination methods

     

    What do the abbreviations of the parameters in the TOC carbon model mean?

    TC (total carbon): The sum of organically and inorganically bound carbon and elemental carbon.

    IC (inorganic carbon) or TIC (total inorganic carbon): The total amount of carbon from carbon dioxide, carbon monoxide, cyanides, cyanates, and thiocyanates. TOC analyzers usually only detect the salts of carbonic acid (carbonates and bicarbonates) and dissolved CO2 for TIC.

    TOC (total organic carbon): The total amount of organic carbon in dissolved or suspended matrix, as well as elemental carbon, cyanates, and thiocyanates.

    NPOC (non-purgeable organic carbon): Non-purgeable organic carbon.

    POC (purgeable organic carbon): Purgeable organic carbon.

     

    How does the difference method for TOC determination work?

    In the difference method, the two different parameters TC and IC are determined separately. TOC is calculated by forming the difference: TOC = TC - TIC.

    TC: The determination of the total carbon content is done by oxidation (thermal or wet-chemical) and subsequent detection of the resulting carbon dioxide.

    TIC: The determination of the inorganic carbon content is done by acidifying the sample with a mineral acid at room temperature and subsequent detection of the driven-off carbon dioxide.

     

    What are the limitations of the TOC difference method?

    The proportion of inorganic carbon (TIC) should not be too high compared to TOC. Due to error propagation, the calculated TOC value can have too high an uncertainty. EN 1484 recommends that the TOC value should be greater than or equal to the IC value when using the difference method (TOC ≥ TIC).

    Example:

    TC content = 100 mg/l (RSD = 2%) ± 2 mg/l (98 – 102 mg/l)

    TIC content = 98 mg/l (RSD = 2%) ± 1.96 mg/l (96.04 – 99.96 mg/l)

    TOC = 2 mg/l ± 3.96 mg/l (-1.96 – 5.96 mg/l)

    Due to error propagation, the total error in this example is ± 3.96 mg/l. According to the difference method, the error of the total result is greater than the calculated TOC content.

     

    How does the addition method for TOC determination work?

    In the addition method, the two parameters POC and NPOC are determined separately - one after the other. TOC is calculated by adding both results: TOC = POC + NPOC.

    For POC determination, the sample is acidified and then purged with a carrier gas. In this step, both CO2 from carbonates and bicarbonates and purgeable organic substances (POC) are driven off. A CO2 trap (e.g., filled with LiOH) binds the CO2 from the gas mixture (originating from TIC), and the volatile organic substances pass through the trap and reach the catalyst, where they are oxidized to carbon dioxide and then detected (= POC). In the next step, an aliquot of the acidified and purged sample is injected onto the catalyst. The resulting CO2 corresponds to the NPOC. The sum of both concentration values gives the TOC.

     

    How does the direct method or NPOC method for TOC determination work?
    When using the direct or NPOC method, the assumption is that there are no or no significant amounts of volatile or purgeable organic compounds in the sample. TOC is directly determined as NPOC under this assumption. To do this, the sample is acidified with a mineral acid and degassed. Carbonates and bicarbonates are completely converted to carbon dioxide. Then, carbon dioxide is driven off from the sample solution by a purge gas. The direct measurement of NPOC (like TC) is done by oxidation to CO2 and subsequent detection. TOC corresponds to NPOC: TOC = NPOC.

     

    What role does combustion temperature play in TOC determination?
    During catalytic combustion oxidation, water evaporates, and some compounds, such as nitrogen and carbon components, are converted to gaseous compounds, while salts like chlorides or sulfates remain on the catalyst and accumulate there. The "salting" of the catalyst or combustion tube is one of the most common disturbances in TOC analysis. Depending on the combustion temperature, the salts can melt and block the active sites of the catalyst or damage the combustion tube. Therefore, it is advisable to use combustion temperatures below the melting points of common salts (e.g., 680°C) and combine oxidation with a highly efficient catalyst (e.g., platinum).

     

    What is the difference between TOC and COD?

    COD indicates the amount of oxygen required to oxidize oxidizable substances. In TOC determination, the total concentration of carbon from organic compounds is measured. In the reaction equation "carbon and oxygen react to carbon dioxide," the difference between TOC and COD can be easily illustrated:

    The determination of COD is also significantly more time-consuming. The triplicate determination of TOC using the NPOC method takes about 10-15 minutes, depending on the degassing time. The oxidative COD digestion takes 120 minutes according to the DIN method without preparation. Additionally, there is the time for titration or photometry.

     

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