Diesel Fuels and Fuel Additives – Interferencefree Determination of Sulfur Traces by UV Fluorescence Detection with MPO according to ASTM D5453 Application Note · multi EA 5100

Introduction

The fast and correct determination of total sulfur is an integral part of process and quality control in refineries. The most reliable method to do this is combustion of the sample followed by UV fluorescence detection of the produced SO2. Independent of the sulfur content, the majority of matrices can be analyzed with excellent results. But some of them give too high sulfur readings due to the presence of interfering components, such as nitrogen compounds. This effect is caused by a cross-sensitivity to NO, which fluoresces in the same wavelength range as SO2. Depending on the nitrogen content of the sample, this can lead to too high results of approx. 0.6%–2% of the TN content on the TS result. As an example, for a matrix, with 500 ppm TN, the TS value will be falsified by approximately 3–10 ppm.

Refinery products contain a multitude of additives, amongst them cetane improvers. They are used to improve the ignitability (cetane number) of fuels. For this purpose compounds containing nitrogen, such as amyl nitrate, cyclohexyl nitrite, triethylene glycol dinitrate, 2-ethoxyethyl nitrate, or 2-ethylhexyl nitrate, have proven to be especially suitable and cost efficient. Besides the positive effect on the ignition characteristics these substances also lead to increased nitrogen contents in the final product.

Depending on the amount of cetane improver this can have a significant influence on the sulfur determination. This is a serious problem, considering the fact that some fuels already have an “actual” TS content close to the specified maximum limit of 15 ppm resp. 10 ppm (ASTM D4814, D6751//DIN EN 590, DIN EN 14214). If the sulfur content is analyzed after the add-on of such additives the result can be higher than allowed. The fuel exceeds the specified threshold (e.g. ULSD, grade S15) and does no longer comply with the requirements. This means that a time and cost intensive “after treatment” is required (e.g. blending with a more pure fuel). However, these unnecessary costs and efforts can easily be avoided by using interference-free working detection techniques.

While conventional sulfur analyzers fail in presence of nitrogen-containing additives, the patented Micro Plasma Optimization (= MPO) technology overcomes this problem. Correct TS readings are achieved, regardless of the composition of the test matrix. This is realized by the conversion of interfering components into harmless species by means of direct ionization of the reaction gas stream.