Removal of Nitrogen Interferences for Sulfur Determination in Fuels
Thu 15 Apr, 2021
Elimination of Nitrogen Interferences for the Detection of Sulfur in Fuels
Total sulfur (TS) is a routine parameter in the quality control of fuels. However, analysis can prove challenging as nitrogen interferences can distort results. Now, innovative technologies are available that suppress these interferences and save laboratories a lot of time and effort in the analysis process.
UV fluorescence is usually used to measure TS. The cross-sensitivity involved with nitrogen, however, means that there are interferences in this method that can affect results. This article explains how you can easily avoid these and thus speed up your analyses.
Cross-sensitivity of nitrogen and UV fluorescence
In the production of motor fuels through the hydrogenation of coal or plant oils and the production of conventional mineral oil-based fuels with biodiesel additives, the ignition properties of the end products may be impaired. In these cases, special additives – generally referred to as cetane improvers – are used to improve the fuel’s ignition properties (cetane number). Nitrogen compounds such as amyl nitrite, cyclohexyl nitrite, triethylene glycol dinitrate or 2-ethylhexyl nitrate (2-EHN) have proven to be especially suitable and cost-effective. These substances not only have a positive effect on ignition properties but also increase the nitrogen content of the end product. When measuring TS content, this is a problem, as the nitrogen can produce false-positive results. Many fuels classified as sulfur-free already exhibit an actual TS content close to the statutory limit of 10 ppm (ASTM D4814, D6751//DIN EN 590, DIN EN 14214). If a nitrogen-based cetane improver is used, the sulfur content measurement may misleadingly exceed the specified limit, in which case the fuel will no longer satisfy requirements and a complex treatment process becomes necessary.
“Micro Plasma Optimization” provides a solution to the problem of interferences
However, this additional effort can be avoided if a matrix-independent UV detection method is used that enables differentiation between sulfur and nitrogen interference. One effective technology that achieves this is micro plasma optimization (MPO), an evolution of the conventional UV fluorescence from Analytik Jena. It guarantees reliable results within a very short time, even despite interferences. MPO converts the interfering NO molecules into harmless species. This eliminates the need for extra additives such as catalysts, multiple injectors or matrix separation using trap & release methods. Analytik Jena uses MPO technology in solutions such as the combustion elemental analyzers of the compEAct and multi EA 5100 series, which are frequently used in the oil and gas industry for sulfur measurement in fuels.
Reliably measuring of total sulfur in fuels using the compEAct S
An example measurement shows here how truly effective MPO technology is. Here, various fuel samples with nitrogen additives as well as sulfur and nitrogen-based standard solutions are analyzed using the compEAct SMPO . Thanks to the integrated HiPerSens detector, the sample does not need to undergo complex preparation. Specially developed for fuel applications, this elemental analyzer enables reliable analysis of sophisticated matrices and simplifies routine tasks by reducing the risk of operating errors and substantially shortening the processing time of each sample regardless of concentration.
Measurement of fuel samples
To calibrate the measurement system for the relevant concentration range, liquid standards based on dibenzothiophene (S) in iso-octane are used. Specific calibration strategies are not required, as the compEAct performs combustion in a manner that is optimized for the matrix. For all samples, a volume of 10 μL was injected directly into the combustion tube. Quantitative combustion occurred at 1050 °C. For this process to be performed quickly and in a matrix-independent manner, a specially designed quartz glass reactor, including a quartz pyrolyzer, was used. The intelligent management of the process gases enables lighter sample components to evaporate quickly and safely in a pure, inert carrier gas atmosphere. Heavier components are pyrolyzed quickly in a controlled fashion on the active surface of the quartz pyrolyzer. This ensures that only the gaseous components reach the combustion zone in the first stage of the process. In the second stage, the pyrolysis products and other heavier sample components that are retained by the quartz pyrolyzer undergo a quantitative reaction in the presence of pure oxygen. Before the combustion gases reach the MPO, they are purified and dried. All interfering nitrogen compounds are then removed by the powerful cold plasma ionization. The remaining SO2 is then detected using UV fluorescence.
Results
The analysis results obtained by these means and their reproducibility are evidence of the quality of the exploratory process. The overall performance of the compEAct SMPO has been validated by means of an analysis of a TS standard with a known concentration. The analyzer enabled rapid and interference-free measurement of sulfur content in fuels in the presence of nitrogen-based additives.
Table: Results of the sulfur determination with and without utilization of MPO technique
| Measurement | TS ± RSD with MPO | TS ± RSD without MPO | TN * |
| Diesel A | 6.2 ppm ± 1.12 % | 6.7 ppm ± 2.06 % | 26 ppm |
| Diesel B | 9.8 ppm ± 1.17 % | 10.6 ppm ± 0.22 % | 75 ppm |
| FAM Normal Gasoline | 93.8 ppb ± 7.91 % | 94.1 ppb ± 7.82 % | 363 ppb |
| Gasoline | 969 ppb ± 2.17 % | 973 ppb ± 2.69 % | 1.60 ppm |
| Standard 5 ppm S | 5.03 ppm ± 2.38 % | 4.99 ppm ± 1.23 % | n. a. |
| Standard 100 ppm N | 0.02 ppm ± 4.55 % | 1.03 ppm ± 1.67 % | 100 ppm |
* TN content determined by means of elemental analysis (CLD)
The complete and detailed documentation of the measurement can be found in our related application note.
Left: Diesel B without MPO;
Right: Diesel B with MPO
The highly sensitive detector enables injection of smallest volumes and thus fast analysis results in approximately 3 minutes, independent of the analyzed sample or the sulfur content.
Left: 100 ppm N without MPO with faulty TS peak
Right: 100 ppm N with MPO without TS peak
The analysis curves of the sulfur-free 100 ppm N standard prove the effectiveness of the MPO technique as a valuable tool to eliminate the interfering NO completely before analyzing the TS content.
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