But there are much bigger challenges than the decomposition behavior of the sample matrix.


PRODUCTIVITY BOOST IN SULPHUR (UVFD) ROUTINE ANALYSIS


The determination of total sulphur by means of combustion- coupled UV-fl uorescence detection is an integral part of the daily routine in refi neries and many other labs of


related industries dealing with process and quality control. Independent of analysing feeds, process streams or ultra-pure fi nal products, it is essential to have information about the “real” sulphur content in the shortest possible time.


This is to avoid any undesired effect like catalyst poisoning, corrosion of parts of installation, low quality of products, or exceeding legal limits. That´s why the sulphur content of many different samples of the production process has to be determined in close intervals. This and the fact that fewer and fewer lab technicians analyse more and more, and in some cases challenging, samples is producing enormous pressure to deliver timely and reliable measurement values. Delayed or incorrect results signifi cantly infl uence the performance and profi tability of the company.


Where time is money, a technique that ensures fast and reliable analysis, independent of matrix effects and the operator´s skills, is urgently needed.


Time pressure vs. reliability of results


In combustion-based organic elemental analysis, the very fi rst step of the process – sample digestion – already determines the quality of the fi nal result. Only a quantitative oxidation of all sample components can ensure trustable results. Sounds simple – but it isn´t.


Petro-matrices can consist of several hundred compounds, including different aliphatic and aromatic hydrocarbons, alcohols, esters, nitrogen-containing additives, traces of sulphur-and halogen-containing impurities, and many others. As different as their chemical structure is their digestion behavior – evaporation, ignition etc. Only the complete oxidation of all matrix components can ensure a fast release of the required SO2


.


Depending on the furnace confi guration used – vertical or horizontal mode – petro-matrices produce widely varying results. Regulatory bodies do not recommend the use of vertical mode for any matrices other than those with boiling point below 400 °C. This has a simple reason. Higher fractions mainly consist of compounds (long-chained, polyunsaturated or polycondensed structures), which cannot be evaporated under the given conditions. What happens, if this is attempted, is incomplete digestion with all its well-known facets – sticky pyrolysis residues, soot formation, increased system blank, instable base line and, last but not least, low-quality results. This normally makes the horizontal approach necessary, despite the drawbacks of remarkably longer processing times per analysis and the need for a skilled operator who is able to create an optimal program for the boat drive.


This is a dilemma! Either fast results, but with limited trustability and a remarkably increased maintenance effort, or reliable results, but with processing times that clearly reduce the lab’s effi ciency.


What is the best strategy?


The quartz-pyrolyzer is a valuable approach that benefi ts from the vertical advantages while avoiding its drawbacks. Even challenging samples (e.g. heavy diesel, pure plant oil) can be processed quickly, delivering correct results.


Figure 1: compEAct SMPO with LS 2 liquids sampler and EAvolution software


The sample introduction was carried out fully automatically by means of the LS 2. For all samples, a volume of 10 µl was injected into the compEAct analyser. The quantitative combustion took place at 1050 °C.


To ensure a fast and complete digestion independent of the analysed sample matrix, a specially designed quartz glass reactor, including a quartz-pyrolyzer, was used.


The sophisticated management of process gases allows the lighter sample components to evaporate quickly and safely in a pure inert carrier gas atmosphere. Heavier components are pyrolized quickly and controlled on the active surface of the quartz-pyrolyzer. In this way, only the gaseous components can enter the combustion zone in the fi rst stage of the process. In second stage, which immediately starts after injection was fi nished, the formed pyrolysis products as well as other heavier sample components, which were retained by the quartz-pyrolyzer, are digested quantitatively in presence of the suffi cient amount of pure oxygen.


To avoid any undesired effect on long-term stability and sensitivity, the combustion gases were cleaned and dried suffi ciently before entering the HiPerSens.


Before the formed SO2 is detected by means of UV fl uorescence,


Especially in fuel analysis, undesired interferences endanger the correctness of results reporting. With the increasing utilization of nitrogen-containing additives (e.g. 2-ethylhexyl nitrate), the risk of exceeding the legal limit becomes a real threat. During combustion, such materials form NO and other oxides. Due to the positive cross- sensitivity of NO to UV fl uorescence detection, the falsifi cation of the measured sulphur content will be more or less signifi cant. This effect is well documented in literature. Depending on the analyser type used, this results in 0.6 - 2 ppm of additional “false” TS per 100 ppm of “real” TN.


Considering that many fuels already have an actual TS content close to the specifi ed limit of 10 ppm, this results in serious problems.


While a classic UV fl uorescence analyser will give a falsifi ed, excessively high result, the MPO technique enables reliable analysis without any extra effort. Compared to approaches like matrix- related calibration, utilization of N-based correction factors, or the usage of a trap and release separation system, which remarkably extends the analysis time, the MPO is clearly superior.


Meeting all challenges A compEAct SMPO


elemental analyser, equipped with new


generation HiPerSens UVDF+ detection system and a LS 2 – liquids autosampler, was used for all tests.


The HiPerSens UVFD+ is including the patented Micro Plasma Optimizer (MPO), which ensures the correct sulphur analysis even in the presence of increased nitrogen content (e.g. diesel fuels with N-based cetane improver). A sample pretreatment, dilution of the higher concentrated samples or enrichment of the ultra-traces by trap and release technique, was not necessary due to the wide linear operation range of this detector. This ensures that the impact of operator mistakes will be lessened and the processing time of each sample will be remarkably shortened, independent of its concentration.


The MPO renders interfering nitrogen compounds harmless. This happens fully automatically as integral step of the analysis process.


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