Total organic carbon

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The total organic carbon or TOC value (English total organic carbon ) is a sum parameter in environmental analysis and indicates the sum of the total organic carbon in a sample. It is the measure of the organic carbon content in a water, soil or air sample. The TOC thus excludes inorganically bound carbon, such as carbonates in soil samples or CO 2 in air samples. As a rule, the TOC in water and air allows conclusions to be drawn about contamination with foreign substances. For soils, the TOC content is closely related to soil fertility.

TOC in air or exhaust

TOC is determined in air or exhaust gas samples with a flame ionization detector (FID) or FTIR spectrometer . Permitted TOC contents in exhaust gases are currently regulated in Germany via the Technical Instructions for Air Quality Control (TA-Luft) and in the future via the ordinance on medium-sized combustion, gas turbine and combustion engine systems . They depend on the type of system. For example, engines that run on biogas will be limited to 1.3 g / m³ from January 1, 2025.

TOC in water samples

To determine the TOC content, the concentration of all organically bound carbon in the water is determined and usually determined using automated measuring methods.

Clean spring water has a TOC content of 1–2 mg / l. Rivers and streams with low levels of pollution show values ​​around 2–5 mg / l. In mesotrophic lakes values ​​around 5–10 mg / l are already reached, in productive carp ponds typically 15–25 mg / l. In heavily polluted waters, the value can rise to over 100 mg / l. In addition to other sum parameters, the TOC is used to estimate the water quality .

The method is based on the oxidation of the carbon compounds contained in the water and the subsequent determination of the resulting CO 2 ( carbon dioxide ). The oxidation usually takes place through thermal combustion or alternatively through UV persulfate digestion of the water sample, the subsequent detection of the CO 2 by means of infrared photometry .

The determination of the TOC in water chemistry is in a competitive relationship with the determination of the chemical oxygen demand (COD). The advantages of the TOC determination lie in the increased accuracy, the lower sample volume required and the better automation. Another advantage compared to the COD determination is that there is no sulfuric acid or wastewater highly contaminated with heavy metals ( Hg , Ag , Cr ). The disadvantage is the much greater expenditure on equipment. The main difference between the two methods, however, lies in the subject of the measurement: When determining the COD, the mean oxidation level of the currently present carbon compounds plays the decisive role. All other oxidizable water constituents are also recorded or have to be masked (e.g. chloride, Cl - ) through special effort. The TOC determination, on the other hand, is selective, only carbon compounds are recorded, regardless of the oxidation level of the carbon.

The measurement of the TOC has prevailed above all in the examination of drinking water and surface water, since in these cases a COD determination would often be too imprecise. In wastewater analysis, on the other hand, the COD value in combination with the BOD value ( biochemical oxygen demand ) is widely used as a measure of the organic load, especially with regard to the pollution and / or performance of sewage treatment plants .

In the meantime, the TOC value is also permissible as a measure of organic pollution in the case of wastewater discharge . However, the conversion into COD / BOD data is problematic and only reasonably reliable if a constant oxidation level is assumed in most municipal wastewater. On the other hand, very different factors apply to the conversion between the two quantities for industrial wastewater from different sectors.

Analytical methods

The standardization (e.g. DIN EN1484) describes three different methods for determining the TOC content. This takes into account the fact that, on the one hand, all relevant connections should be recorded and, on the other hand, interference with the measurement due to matrix effects should be avoided as far as possible. Modern analysis devices can be operated according to all three methods. The selection of the suitable method depends on the composition and concentration of the carbon compounds and on the expected interfering substances.

NDIR detection

To determine the carbon dioxide content in the carrier gas (expelled TIC / TOC), mostly non-dispersive infrared sensors (NDIR) are used, through which the carrier gas flows constantly or reproducibly. The concentration measured by the NDIR detector is recorded as a function of time. The resulting integral of the CO 2 concentration over time (often referred to as the peak area) is a measure of the carbon C released from the sample.

Combustion method

Modern TOC analyzer
An example of a modern TOC analyzer based on catalytic high-temperature combustion for solid and liquid samples.

In the combustion method, the sample is completely burned in a heated reactor and the carbon contained in the sample is oxidized to carbon dioxide CO 2 .

Solid samples are burned at about 900 ° C or higher. For liquid samples, the combustion temperature is usually approx. 700 to 1,000 ° C, with higher temperatures generally favoring the digestion of solid and liquid samples. The combustion and complete conversion of the carbon contained in the sample in the gas phase to CO 2 is supported by the use of catalysts (e.g. copper oxide, cerium dioxide or platinum-containing catalysts). If liquid samples are burned at 1,200 ° C, they do not need an oxidation-promoting catalyst, as the carbon present in the sample is completely converted into CO 2 .

The combustion gases are transported with a stream of carrier gas (mostly synthetic air or pure oxygen), dehumidified / dried and fed to the detector. The determination of the carbon dioxide is carried out in a non-dispersive infrared detector (NDIR).

Since June 2009, the draft standard DIN EN 15936 describes the test methods for determining the TOC content in solids. The draft is intended to replace the DIN EN 13137 standard and describes combustion in a separate solids analyzer as well as a new suspension method for determining solids in liquid analyzers.

In parallel to the TC or NPOC, the TN b (total bound nitrogen) can be determined with a separate, nitrogen monoxide -specific detector (e.g. chemosensor, electrochemical detector (ECD), chemiluminescence detector or IR detector).

In order to obtain the TIC, the sample is acidified in the combustion devices (usually in a separate, unheated reactor) and the carbon dioxide released is expelled. The determination of the carbon dioxide takes place again in the NDIR detector.

A frequent problem with the combustion method is the salts and acid or alkali components contained in many samples, which can lead to premature wear, mainly of the combustion tube (mostly quartz glass) and inactivation of the catalyst. On the part of the device manufacturers, there are various concepts for bypassing this problem in some cases quite effectively (e.g. matrix deposition, ceramic reactors, improved process management or processes without catalysts).

Wet chemical method "UV persulfate method" (wet chemical)

The sample is introduced into a heated decomposition vessel. Acid is added and the inorganic carbon (TIC) is converted into carbon dioxide. The carbon dioxide is expelled with nitrogen and measured as TIC in the NDIR detector. Persulfate is added to the sample, it is irradiated with UV light and the organic carbon (TOC) is converted into carbon dioxide in the heated reactor. The carbon dioxide is expelled with nitrogen and measured as TOC in the NDIR detector. Solid sample components (including suspended particles) cannot be completely digested. This method is therefore unsuitable for samples containing particles. Samples containing salt are also problematic because with increasing salt content (especially chloride) the persulfate enters into more side reactions and is no longer available for the oxidation of the carbon. There is a risk of poor results.

In a modified form, this method can also be operated continuously. The conductivity of the medium is measured before it is then irradiated with UV in a thin tube. The carbon dioxide formed forms carbonic acid and thus increases the conductivity. The organic carbon content (TOC) can be calculated from the difference in the conductivity of the medium before and after the UV irradiation.

Range of application of the different methods

The wet chemical method "UV persulfate method" is preferably used for the detection of the smallest TOC contents, for example in pharmaceutical water (ultrapure water, WFI, ...) because a large sample quantity (up to approx. 20 ml) can be used because of the high sensitivity that can be achieved. In addition to the unsuitability for particle-containing samples, the disadvantage of this method is that some very stable organic compounds (e.g. barbituric acid ) may not be completely broken down and there is a risk of poor results.

For the field of environmental analysis (in addition to wastewater, drinking water and surface water), the combustion method (injection quantity up to approx. 2 ml) is preferably used. The high-temperature digestion is a method that reliably digests compounds and particles that are difficult to oxidize. Modern devices achieve a high measuring sensitivity with the combustion method and can therefore be used as an alternative to the UV method, for example for the analysis of pharmaceutical water.

Calculation method

Difference procedure (TOC = TC - TIC)

In the area of ​​high TOC concentrations (e.g. in the case of municipal wastewater), the "difference method" is often used. In the first step, the totality of all carbon compounds (“TC” = Total Carbon) is determined and then in a second measurement the proportion of inorganic carbon compounds (e.g. carbonates ) (“TIC” = Total Inorganic Carbon) is determined. The TOC value is then obtained by subtracting the TIC from the TC.

Direct process (TOC as NPOC)

Since both the TC and the TIC are subject to a measurement inaccuracy, the difference method often leads to inaccurate results for samples with a TOC content that is small compared to the TIC. The so-called "direct method" is therefore used in the field of drinking water analysis. For this purpose, the sample is acidified before the measurement in order to convert the inorganic carbon content "TIC" into carbon dioxide (CO 2 ) via carbonic acid . The resulting CO 2 is then blown out of the sample with a stream of inert gas . However, volatile acids (formic acid from acidified formate, acetic acid from acidified acetate etc.) and all highly volatile organic substances (e.g. fuel components) are lost for the measurement. This is why the TOC obtained in this way is also referred to as "NPOC" (Non Purgeable Organic Carbon). Nevertheless, this method is the method of choice in relatively TIC-rich (= hard) waters, as it provides the best accuracy and reproducibility.

Addition method

Highly volatile organic compounds are not included in the direct method. If their share in the TOC cannot be neglected, the "addition method" is used. For this purpose, the blown gas stream is freed from CO 2 and the volatile compounds contained are then oxidized and determined. The “POC” (Purgeable Organic Carbon) obtained in this way is then added to the NPOC to form the TOC.

TOC for concentration analysis

If there is only one organic substance in the sample to be determined, the TOC method can also be used to determine the concentration of the substance. The concentration of the sample in the substance can be determined as follows:

This equation determines the content of a substance by comparing the measured concentration of organic carbon via the ratio of the molar masses of carbon and the substance and the number of carbon atoms in a molecule of the substance. This is possible because every carbon atom in a molecule of the substance creates a CO 2 molecule during analysis .

literature

Individual evidence

  1. ^ Fritz Scheffer, Paul Schachtschabel: Textbook of soil science. 15th edition. Stuttgart 2002, ISBN 3-8274-1324-9 .
  2. ↑ DIN EN 12619 emissions from stationary sources, determination of the mass concentration of the total organically bound carbon in low concentrations in exhaust gases, continuous process using a flame ionization detector
  3. ↑ § 16 44th BImSchV - single standard. Retrieved March 9, 2020 .
  4. ↑ Elementar Analysensysteme GmbH: TOC analyzers. Retrieved August 13, 2020 .