Carbon, Nitrogen, Sulfur and Chlorine Analyzer (CNSX)

Laboratories looking for reliable elemental analysis solutions for carbon, sulfur, nitrogen, and chlorine (C, N, S, X) find in our multi EA 5100 one of the most advanced and flexible analyzers in this field. Its sister device, the multi EA 4000, enables the determination of TOC/TIC, EC/RC, and BOC in solids.

Completing the combustion elemental analysis portfolio is the compEAct, a high-throughput, easy-to-use sulfur and nitrogen analyzer that can operate 24/7. An extensive range of accessories allows further customization of all devices to individual application requirements.

CNSX Analysis

Carbon, nitrogen, sulfur and chlorine (CNSX): These four non-metals, along with hydrogen and oxygen, make up the elementary building blocks of our world and occur in organic as well as inorganic compounds. Analytic chemistry was quick to recognize the great significance of these elements. This discipline of chemistry advanced elemental analysis as a method for measuring elemental ratios.

Its most important tasks – besides measuring the general composition of chemical compounds – is assessing the purity of materials. Here, elemental analysis is used to measure the levels of undesired impurities in compounds, many of which contain nitrogen, sulfur or chlorine. In NSX analysis, the aforementioned non-metals are not only detected (qualitative analysis), but also measured with respect to their quantity (quantitative analysis).

Underlying principle; methods

Measurement of CNSX elements relies on the principle of high-temperature combustion. In this process, the weight or volume of the sample under test must be known precisely before the sample is combusted in an oxygen-rich atmosphere. Depending on the measurement method and apparatus, this takes place at temperatures between 1,000 °C and 1,800 °C. Because of the higher temperatures, a catalytic converter is not strictly necessary. The nitrogen, sulfur and carbon compounds in the sample are transformed into their oxides – CO₂, NOx and SO₂. Meanwhile, halogen compounds are transformed into the corresponding hydrogen halides.

Combustion gases produced in the oxidation process are dried, purified and transported to the element-specific detection systems using a carrier gas. It is possible to conduct a quantitative measurement of the CNS element content based on the recorded areas under the curves (integrals) or, for Cl, the total electrical energy consumed, along with a predetermined calibration. Cutting-edge elemental analyzers such as the multi EA 5100 all-in-one solution have multiple methods at their command, making them compliant with international standards such as ASTM, EPA, DIN, ISO and EN. If the focus is on quick and cost-efficient measurement of total sulfur and total nitrogen in liquids, gases and LPG samples, then the versatile compEAct series is a compelling choice.

Below, we present the key measurement principles and detection systems. Certain detection systems can also be combined for specific applications, depending on the design of the analyzer.

• NDIR spectrometry (carbon, sulfur): A non-dispersive infrared sensor (NDIR sensor) is a spectroscopic gas sensor whose primary purpose is to measure the carbon content (represented by CO₂) and the sulfur content (represented by SO₂) of a gas. The sensor works by measuring the absorption of a specific wavelength in the infrared spectrum. The infrared radiation generated in the machine penetrates the gas and is restricted to a certain optical range using optical filters. The absorption can then be evaluated as an electrical signal.
• Chemiluminescence (nitrogen): Chemiluminescence exploits the effect of chemiluminescent radiation created when nitrogen monoxide (NO) reacts with ozone to produce an excited nitrogen dioxide (NO₂^*) molecule. A receiver can then pick up the radiation and, based on its proportion to the NO concentration, allow conclusions to be drawn about the nitrogen content in the sample under test. The most powerful detectors also work with other components such as converters that transform NO₂ to NO and powerful pumps that generate a vacuum.
• UV fluorescence (sulfur):  To measure the sulfur content in samples – for instance in fuel quality control – analytic chemists make use of UV fluorescence. One advantage of this method is its low relative detection limit of 5 µg/l. In order to achieve correct analytic results without interference from the matrix, powerful detectors are equipped with additional components to help eliminate cross-sensitivity to nitrogen oxides.
• Coulometry (sulfur, chlorine): One alternative to UV fluorescence, used when measuring the sulfur content, is coulometry. This method additionally allows for measurement of the chlorine content. Coulometry is based on the measurement of an electric charge converted at an electrode, which, according to Faraday's law, is proportional to the amount of substance converted. The relative detection limit for sulfur is around 600 µg/l, significantly higher than with UV fluorescence.

The measurement principles described here can be flexibly combined with one another in modern elemental analyzers. When doing so, however, the working ranges and detection limits of each detection system should be considered, as these should be suited to the intended application. Below, we provide an overview of the detection systems and detection limits for the multi EA 5100 CNSX analyzer:

Analyzer design

The underlying principle rests on the concept of combustion analysis: A gaseous, liquid or solid sample is combusted at a high temperature – either directly or after the sample has been prepared, depending on the application. The resulting emissions (e.g. No_x, CO₂, HCl, SO₂) are then routed to the detection systems described above where they are subjected to qualitative and quantitative measurement of carbon, nitrogen, sulfur and chlorine.

Depending on the characteristics of the sample, the combustion process can take place in either a vertical or a horizontal orientation. Vertical combustion has become established as the standard in analytic chemistry whenever there is a need to quickly and precisely analyze liquids and gases for trace levels. Meanwhile, the horizontal mode is better suited for samples with high volatility and viscosity. Modern analyzers unite the advantages of both combustion methods in one machine.

Depending on the feature set of the analyzer, it may also be possible to measure important environmental parameters in addition to total elemental content. One example is the multi EA 5100 analyzer, capable of reliably measuring the following sum parameters:

• TOC = Total Organic Carbon (total amount of carbon in organic compounds)
• AOX = Adsorbable Organic Halides (total organic halides, such as chlorine, that can be adsorbed on activated carbon)
• EOX = Extractable Organic Halides (total organic halides, such as chlorine, that can be extracted with a solvent)
• EC = Elemental Carbon
• BOC = Biodegradable Carbon (biodegradable organic carbon compounds)
• TIC = Total Inorganic Carbon (total amount of carbon in inorganic compounds that can be accessed with methods of wet chemistry, such as the addition of a non-oxidizing acid)

The sum parameters listed here play an important role in environmental analysis. AOX/EOX values, for example, are key parameters for assessing wastewater or sludge.

Challenges and limitations of CNSX analysis

CNSX analysis enjoys an established role in analytic chemistry as a reliable, versatile and cost-effective analytic method for trace elements such as sulfur, nitrogen, chlorine and carbon in solids, liquids and gases. The method's primary challenges lie in the high sensitivity of the measurement results to the smallest errors in sample preparation, combustion process or detection system. Thus, the key challenges for this analytic method include:

• Sample preparation: For CNSX analysis, sample preparation is not strictly necessary. It is seldom carried out when measuring total elemental levels in liquids and gases. With solids or heterogeneous liquids, sample preparation is always recommended whenever the sample either has particles that are too large or is heterogeneous. Cutting-edge analyzers such as the multi EA 4000 include a fully automatic, high throughput solid sampler as standard. Common sample preparations processes involve grinding, shredding, filtering, dilution or dissolving in solution with a suitable solvent. When measuring specific sum parameters, however, sample preparation is a standby in the analysis workflow. One example is the adsorption of halogen compounds in an aqueous sample using activated carbon (in preparation for AOX measurement). The choice of a sample preparation process that is suitable for the sample matrix is an important influencing factor on the quality and correctness of measurement results.
• Soot formation: Soot formation in the combustion process is one of the most common issues with conventional combustion analyzers, especially with heavy matrices such as oil, VGO, polymers and liquefied petroleum gas. Modern machines prevent soot with an integrated flame sensor. This ensures matrix-optimized, high quality combustion.
• Sensitivity: Depending on the analytic method or detection system used, measurement results may be distorted by matrix effects. A classic example is the measurement of total sulfur (TS) in fuel quality control: In this situation, matrix-related sensitivity effects with nitrogen oxides can cause incorrectly high measurement results. With the multi EA 5100 analyzer, Analytik Jena utilizes matrix-insensitive UV detection with micro-plasma optimization (MPO) to convert interfering NO molecules into benign species.

Besides the typical problems mentioned here, another challenge is cost-effect operation of machines. This is why modern machines utilize 24/7 high-throughput analysis and combine their wide spectrum of applications with an advanced level of automation and high operational reliability.

Areas of application for CNSX analysis

CNSX analysis is a mainstay in industrial testing laboratories. It gives companies in many different sectors reliable insight into relevant process parameters and material quality characteristics. In addition, this analytic method also has a stable role in contract laboratories and public authorities tasked with checking compliance with environmental requirements in the oil and gas industry, the petrochemical industry and the chemical industry.

The following application examples are representative for the method's wide range of applications:

• Oil and gas industry: The applications of CNSX range from inspection of raw inputs such as crude oil and natural gas, to quality inspection of fuels and heating oil, to the testing of lubricants, gear oils and hydraulic fluids.
• Conservation: Modern analyzers allow for measurement of environmental sum parameters such as TOX, AOX and EOX. These parameters are of crucial importance, for example in the testing of coolant water, process water and wastewater for their compliance with environmental requirements. And finally, classification and recycling of waste is another important area where CNSX finds application.
• Chemical industry: When it comes to aromatic or aliphatic hydrocarbons, or fatty acids, dyes, additives and polymers: CNSX analysis enables reliable quantitative and qualitative component analysis with all of these compounds.

And besides the examples of applications mentioned here, the list includes many more. Modern CNSX analyzers such as the multi EA 5100, multi EA 4000 and the machines of the compEAct series can be flexibly modified to suit custom, industry-specific needs, thanks to their modular construction.