TOC/TNb Analysis

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TOC analysis / TN_b analysis

TOC/TN_b analysis is a method for measuring the total organic carbon (TOC) and total bound nitrogen (TN_b) in a sample. This method is suitable for analyzing liquid as well as solid materials. It is mainly used in environmental analysis to assess water or soil quality, in the pharmaceutical industry to assess ultrapure water quality (or for cleaning validation) and in various other industries from power generation and the chemical industry to the oil and gas sector.

Sum parameters and their significance in chemical analysis

TOC/TN_b analysis is one method of sum parameter analysis. Under defined analysis conditions, groups of substances are quantified in summary form without measuring the individual parameters in each group. The main advantage of this measurement method compared to single substance analysis is that it's fast: Instead of quantitatively detecting the individual substances in a labor-intensive manner, TOC/TN_b analysis only measures the total amount of organic carbon and bound nitrogen. For example, water can contain several million types of organic molecules. Instead of identifying and quantifying these individually, they can be measured collectively using TOC/TN_b analysis. It is thus possible to reliably assess water quality for environmental analysis in just a few minutes.

The TOC sum parameter

TOC (total organic carbon) or DOC (dissolved organic carbon) plays a key role in environmental analysis when judging quality standards or identifying undesirable foreign substances in samples. This sum parameter indicates total organic carbon in the sample. Inorganically bound carbon is not a component of the TOC sum parameter. TOC can be measured for liquid samples consisting of drinking water, groundwater, wastewater or surface water as well as for solid samples from soils, sediments or waste.

Analytical measurement of the TOC value follows certain technical standards that specify a procedural guideline. Water analysis follows the requirements of the DIN EN 1484 standard, which contains instructions for TOC measurements in drinking water, groundwater, surface water, seawater and wastewater in the range from 0.3 mg/l to 1,000 mg/l. Among other things, the standard contains stipulations for sample pretreatment and measurement procedures for water samples. In the international context, the DIN EN ISO 20236 standard also comes into play. It is likewise aimed at water analysis and specifies requirements for measuring TOC and TN_b as well as dissolved organic carbon (DOC) and dissolved bound nitrogen (DN_b) after catalytic oxidative high-temperature combustion. However, when it comes to measuring the TOC sum parameter in sludge, treated biowaste, soil and waste, the DIN EN 15936 standard takes precedence.

Oxidation methods in TOC measurement

Various methods for determining the TOC value have become established in analytical chemistry. What they all have in common is that the organic compounds in the sample are oxidized to carbon dioxide and the resulting CO2 is recorded and quantified. With TOC measurement methods, a distinction is made between:

• Wet chemical UV oxidation: The sample is mixed with an oxidizing agent and oxidized in a reactor at around 80 °C while a carrier gas is passed through. Alternatively, the sample can be irradiated with UV light, which produces OH radicals. These ensure that organic substances are converted to CO₂. In many modern TOC analyzers, the two methods are combined to achieve a higher oxidation rate, even for matrix-rich samples.
• Catalytic combustion oxidation: The sample is burned in an oxygen-containing atmosphere at high temperatures and converted to CO₂. This process uses temperatures between 800 °C and approx. 1000 °C in the presence of suitable catalysts. In this process, the sample is inserted either by direct injection or by flow injection. With direct injection, the sample is dosed directly into the combustion system without additional valves. With flow injection, a syringe driver, dosing valve and hoses are used to inject the sample.

These two processes are used in different applications due to their characteristic advantages and disadvantages. The following overview shows typical applications and properties of each process at a glance:

Detection of CO2 content and methods for measuring TOC

After oxidation, the CO₂ content of the sample must be quantified; the TOC is then calculated based on this measurement. Infrared spectrometry, characterized by high selectivity and sensitivity, is the preferred method. The CO₂ produced during oxidation is carried evenly by a carrier gas through a non-dispersive IR detector (NDIR detector). The measured CO₂ concentration is recorded as a function of time and the resulting area integral ultimately allows quantification of the total carbon dioxide released from the sample.

There are three different calculation methods for quantification:

• Differential method (TOC = TC – TIC): The differential method is particularly suitable for large TOC concentrations, for example in wastewater analysis. Two measurements are carried out: The first measures all carbon compounds (total carbon, or TC) and the second only the inorganic carbon (total inorganic carbon, or TIC). The difference between the two values gives the TOC concentration.
• Direct method: In the direct method, inorganic carbon compounds are first removed from the sample by acidification. This leaves only the organic component to be quantified. This method has become established in many areas of TOC analysis due to its ease of implementation, speed and reliability. It should be noted that volatile organic compounds can escape from the sample during TIC removal. For this reason, the resulting measured value is referred to as non purgeable organic carbon (NPOC). However, since such compounds do not occur in most water samples, the NPOC should be set equal to the TOC.
• Additive method: If the highly volatile compounds described above occur in the sample, they can be quantified separately (POC = purgeable organic carbon). Adding POC to NPOC then gives the TOC value. This method is more theoretical in nature and is rarely used in practice.

The TN_b sum parameter

TN_b stands for total bound nitrogen. This sum parameter includes nitrate, nitrite, ammonium and organic nitrogenous compounds. Elemental nitrogen (N₂) is not included in this.

Similar to TOC analysis, the TN_b value is measured in a single analysis run without analyzing the individual components. This analytic method is standardized: The standard in question is DIN EN ISO 20236, which replaces the older DIN EN 12260. It describes how to measure total organic carbon (TOC), dissolved organic carbon (DOC), bound nitrogen (TN_b) and dissolved bound nitrogen (DN_b) after catalytic oxidative high-temperature combustion.

Catalytic combustion oxidation and detection for TN_b measurement

Similar to TOC measurements, analytic chemistry also makes use of a combustion process in TN_b measurements. Here, nitrogen-containing substances in the sample are converted into nitrogen monoxide at a temperature of over 720 °C. The nitrogen monoxide can then be detected and measured quantitatively.

In chemical analysis, the chemiluminescence method has become established for detecting bound nitrogen. In this process, the nitrogen monoxide (NO) produced during combustion is oxidized with ozone. The resulting NO₂ is at first energetically excited, then falls back to its ground state, emitting light quanta. The light quanta can be detected in a photomultiplier, in which the measured radiation intensity is proportional to the NO concentration in the sample gas and thus to the nitrogen concentration in the sample. In addition to this method, electrochemical detection is also widely used. With electrochemical detection, the resulting NO dissolves in a solid-phase electrolyte. The changing cell potential is "titrated back" to its initial state by the formation of electrons. The current which thus flows is also proportional to the nitrogen concentration in the sample. This method is characterized above all by lower overhead costs compared to CLD detection.

Advantages and limitations of TOC/TN_b analysis

In modern analytic chemistry, the TOC/TN_b analysis described above can be excellently combined into a versatile and economical analytic method. Modern machines such as the multi N/C Series are able to reliably measure sum parameters in the shortest possible time and in compliance with standards. In addition to the TOC and TN_b parameters described above, these machines can measure other meaningful parameters such as NPOC, DOC, POC, TC and TIC.

Compared to the complex analysis of individual substances, sum parameter analysis offers many advantages. The method not only saves a significant amount of time and money, but also scores points for its ease of use and the high degree of comparability of the measurements obtained.

In practice, sum parameter analysis often represents the preliminary stage before other, more specific analytic methods. The method allows potential hazards to be identified quickly, allowing it to serve double duty as an early warning function, for example to flag hazards caused by contaminated water.

The main limitations of TOC/TN_b analysis lie in the fact that both parameters are parameters of convention: In practice, the lack of detailed information about the individual components can be misleading as to the composition and hazard potential of substances within. Single substance analysis must therefore also be included as a further, specific analytic method depending on the application.

Applications and industries

TOC/TN_b analysis has become firmly established in various applications and industries as a fast, reliable and economical method. Some typical fields of application for this method of sum parameter analysis, with selected examples:

• Chemicals and materials: TOC measurement in various material streams such as nickel, cobalt, manganese and lithium salts for the production of cathodic materials – for instance in lithium-ion batteries or in product quality monitoring of hydrogen peroxide and phosphoric acid.
• Geology, mining and metals: TOC measurement in oil shale is used as a relevant quality parameter for evaluating the source rock in oil and gas production.
• Power plants and energy technology: TOC measurement in ash, slag and filter dust from coal combustion or in boiler feed and cooling water in thermal power plants.
• Food and agriculture: TOC measurement plays a role in agricultural soils, dried manure and sediments, and provides quality assessment of drinking water.
• Environment: TOC/TN_b measurement is used in municipal wastewater treatment plants and in industrial wastewater (e.g. the pulp and paper industry); TOC/TN_b and DOC/DN_b analysis is used to assess surface water and characterize waste for landfill classification.
• Oil & gas: TOC/TN_b measurement in refinery wastewater, TOC measurement in brine samples from the crude oil desalting process.

In conclusion, TOC/TN_b analysis is suitable for a wide range of liquid and solid substances. In environmental engineering and water resources management, this method can be applied to drinking water, groundwater, surface water, seepage water and wastewater, to name just some examples. In process applications, it has numerous uses in the analysis of cooling water and boiler feed water, pure water in the semiconductor industry, and electroplating baths.

In the field of solids analysis, the method has proven itself above all in waste recycling and the testing of contaminated sites, where soils, sediments, building rubble, sludge, filter cakes, ash and household waste all must be scrutinized.