Application Note · compEAct SMPO

Determination of Sulfur Content of Automotive
Fuels – Ultraviolet Fluorescence Method According to EN ISO 20846 – Removal of Nitrogen
Interferences by N-based Cetane Improvers

Challenge
Avoiding the exceedance of given
legal limits for total sulfur (TS)impurities in fuels caused by thepositive cross sensitivity of NO tothe UV-fluorescence technique.

Solution
Elimination of the interfering
N-compounds by cold plasm
ionization (MPO technology).

Introduction
During the production of motor fuels by hydration of coal or vegetable oil, as wellas during the production of traditional fuels based on mineral oil with biodiesel
addition, end products with affected ignition characteristics can occur. In this casespecial additives — generally known as cetane improver — are used to enhance theignitability (cetane number). Nitrogen compounds have proven to be especiallysuitable and cost efficient, such as amyl nitrate, cyclohexyl nitrite, triethylene
glycol dinitrate, or 2-ethylhexyl nitrate (2-EHN). Besides the positive effect on theignition characteristics these substances also lead to increased nitrogen content inthe final product. This is a serious problem because the nitrogen can lead to false
positive results when determining the TS content in fuels. Considering the factthat many fuels classified as sulfur-free already have an actual TS content close to
the statutory limit value of 10 ppm (ASTM D4814, D6751//DIN EN 590, DIN EN14214), the sulfur content determined can exceed the specified threshold easily, ifnitrogen-containing cetane improvers are used. The fuel no longer complies withthe requirements and a costly post-treatment is needed. However, this effort canbe avoided when using a matrix-independent UV detection technique that makes
it possible to differentiate between sulfur and the nitrogen interference.

2Interference-free TS Determination by UV-Fluorescence in Fuels with N-containing Cetane Improvers

The Micro Plasm Optimization (MPO), a patented technique developed as an enhancement of the classic UV-fluorescence,guarantees reliable results within a short measurement time. It converts the interfering NO molecules to harmless species.
There is no need for additional auxiliary materials (e.g., catalysts), multiple injections, or matrix separation by trap-andrelease approaches.
Materials and Methods
Samples and Reagents
Different fuel samples with and without N-containing additives as well as S and N standard solutions have been analyzed.
Sample Preparation
The samples were analyzed directly. No sample preparation was required.
Calibration
Liquid calibration standards based on dibenzothiophene (S) in isooctane were used to calibrate the analysis system in theappropriate concentration range. Matrix-related calibration strategies are not required since the combustion is optimized andinterfering components are eliminated before detection.
Instrumentation
The fuel samples have been analyzed on a compEAct SMPO elemental analyzer, equipped with HiPerSens UVFD detection
system and an autosampler LS 2 for fully automated sample introduction.
The HiPerSens UVFD includes the patented MPO, which ensures the correct sulfur analysis even in the presence of elevatednitrogen contents. The MPO fully automatically renders interfering nitrogen compounds harmless as an integral step of
the analysis process. For all samples a volume of 10 µL was injected directly into the combustion tube. The quantitative
combustion took place at 1050° C.
To ensure that this process is fast and completely matrix-independent, a specially designed quartz glass reactor including aquartz pyrolyzer was used. The sophisticated management of process gases allows lighter sample components to evaporate
quickly and safely in a pure inert carrier gas atmosphere. Heavier components are pyrolyzed quickly and controlled on theactive surface of the quartz pyrolyzer. This way only the gaseous components enter the combustion zone in the first stageof the process. In the second stage, the formed pyrolysis products as well as other heavier sample components, which wereretained by the quartz pyrolyzer, are digested quantitatively in the presence of pure oxygen.
Before entering the MPO, the combustion gases are cleaned and dried. Afterwards, all interfering nitrogen compounds areremoved by the powerful cold plasm ionization. The remaining SO2 is detected by means of UV fluorescence.

Interference-free TS Determination by UV-Fluorescence in Fuels with N-containing Cetane Improvers

Method Parameters
Standard method settings for vertical operation provided in the method library are applied. The parameter settings for thecombustion process are summarized in Table 1. The evaluation parameters for the detection of sulfur are given in Table 2.
Table 1: Process parameters
Parameter

Specification

Furnace temperature

1050 °C

Second combustion

60 s

Ar flow (first phase)

100 mL/min

O2 main flow

200 mL/min

O2 flow (second phase)

100 mL/min

Draw up

2 µL/sInjection

0.5 µL/s

Table 2: S detection parameters
Parameter

Specification

Max. integration time

300 s

Start

0.1 ppb

Threshold0.11 ppb

Stability

7Results and Discussion
The results of the two diesel fuel samples and the standards summarized in Table 3 show the averages of three replicate
measurements. Due to the matrix-optimized combustion a threefold determination is sufficient to achieve results within
3 % RSD. This remarkably affects the sample processing time and allows a higher sample throughput. The analysis resultsreceived and their reproducibility prove the quality of the digestion process. The overall performance of the analysis system
was validated by analyzing a TS standard material with known concentration, results are given in Table 3.
Table 3: Results of the sulfur determination with and without utilization of MPO technique
Measurement

TS ± RSDwith MPO

TS ± RSD
without MPO

TN *Diesel A

6.2 ppm ± 1.12 %6.7 ppm ± 2.06 %26 ppmDiesel 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)

34Interference-free TS Determination by UV-Fluorescence in Fuels with N-containing Cetane Improvers

Typical measuring curves for the two applied techniques for sample “Diesel B“ and the “Standard 100 ppm N“ solution are
shown in Figures 2a–b and 3a–b.
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.

Fig. 2a: Diesel B without MPO

Fig. 2b: Diesel B with MPO

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.

Fig. 3a: 100 ppm N without MPO Fig. 3b: 100 ppm N with MPO
with faulty TS peakwithout TS peakConclusion
The compEAct SMPO facilitates fast and interference-free determination of sulfur contents in the presence of nitrogencontaining additives. Especially designed for fuel applications, it enables reliable analysis of challenging matrices andsimplifies routine work.
The wide liner operation range of the HiPerSens detector makes sample pretreatment, e.g., dilution of higher concentrated
samples or enrichment of ultratraces by trap-and-release technique a thing of the past. It thus reduces the risk of operator
errors and remarkably shortens the processing time of each sample independent of the concentration.

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