Application Note · compEAct S

Determination of Total Sulfur in Aromatic
Hydrocarbons and Related Chemicals by
UV Fluorescence according to ASTM D7183

Challenge
Fast and reliable analysis ofsulfur contents in the ultratrace range in different typesof liquid aliphatic and aromatichydrocarbons and their mixtures.

Solution
Optimized vertical combustion
combined with HiPerSens UVfluorescence detection forconcentration-independent TS
determination.

Introduction
Liquid aliphatic and aromatic hydrocarbons play an important role in refinery
processes, the petrochemical, chemical and polymer industry, either as raw
materials, process intermediates, or as end products. Regardless of their sourceor their further use, they all have in common that they need to be ultrapure. Thismeans that they have to be free of undesired compounds, as these could affect theproduction process and the quality of the final products.
Impurities such as sulfur-containing compounds not only affect the performanceand economic life of the used catalysts (catalyst poisoning), but also form
unwanted by-products and thereby decrease the quality of the end product.
Therefore their content has to be kept as low as possible, preferably below 100
ppb. To make this possible, the exact knowledge of the present sulfur (TS) isrequired. Combustion-based elemental analysis coupled with a UV fluorescence
detector is the only method that has proven to be suitable for this challenging
task.
The compEAct S is an analysis system which has been specifically optimized
for the fast and trouble-free determination of sulfur contents in the ultra-trace
range. Combining catalyst-free high-temperature combustion and highly sensitive
HiPerSens detection, it allows the determination of sulfur at concentrations
ranging from 5 µg/l up to 10,000 mg/l with one and the same device.

2TS Determination in Aromatic Hydrocarbons and Related Chemicals by UV Fluorescence according to ASTM D7183

Materials and Methods
Samples and Reagents■■
Different aliphatic and aromatic hydrocarbons and their mixes (e.g. toluene, isooctane, naphtha)■■
Isooctane (C8H18), Suprasolv®, GR for gas chromatography (Merck Art.-No.: 1.15440.1000)■■
Dibenzothiophene (C12H8S), GR for synthesis (Merck Art.-No.: 8.20409.0025)■■
Extended calibration standard kit Sulfur (0–10 mg/l) (Analytik Jena, Art.-No.: 402-889.061)
Sample Preparation
The samples are light volatile, have a low viscosity, and contain TS in the ultra-trace level. This made a pretreatment step
redundant. The samples were analyzed directly.
Calibration
Prior to the actual determination, the system was calibrated using sulfur standard solutions based on dibenzothiophene (S)in isooctane in the range of 0 to 200 ng S. Figure 1 and 2 depict typical calibration curves and performance parameters forultra-trace applications.

Fig.1: TS calibration for ultra-trace range

Fig.2: Wide-range calibration curve of the HiPerSens UVF detector

The calibration was checked with different concentrated standards.
Instrumentation
The measurements were performed using a compEAct S, equipped with HiPerSens UV fluorescence detection for thedetermination of sulfur. Sample introduction was carried out fully automatically using the LS 2 liquids sampler to ensure ahigh sample throughput.
The analyses have been run in vertical operation mode. The samples were dosed directly into the evaporation zone of thequartz glass combustion tube. This process took place fully automatically by means of the LS 2 high-throughput autosampler.
The catalyst-free, bi-phasic combustion process is carried out at temperatures of up to 1050 °C. In the first process phase,
evaporation of volatile sample components in an inert gas stream takes place, followed by the combustion of the formed
gaseous products in an oxygen-rich atmosphere. In the second phase the heavier, nonvolatile sample components andformed pyrolysis products are quantitatively oxidized in pure oxygen. Thereby the quartz pyrolyzer ensures a uniform
evaporation, modulates the combustion process, and prevents incomplete combustion. This establishes the best conditions
for a reproducible and fast ultra-trace analysis. The implemented Auto-Protection System (particle and aerosol trap)
guarantees highest operational safety and a complete transfer of the formed SO2 into the UVFD, after a sufficient drying ofthe reaction gases. The compEAct S enables a detection limit of as low as 5 µg/l S.

TS Determination in Aromatic Hydrocarbons and Related Chemicals by UV Fluorescence according to ASTM D7183

Method Parameters
The standard method ASTM D7183 from the method library of compEAct’s EAvolution software was used for all analyses.
The following table summarizes the parameter settings for the combustion process.
Table 1: Process parameters compEAct SParameter

Specification

Furnace temperature

1050 °C

Second combustion

60 s

Ar flow (first phase)

150 ml/min

O2 main flow

200 ml/min

O2 flow (second phase)

150 ml/min

Draw up

2 µl/sInjection volume

40 µl

Injection

0.5 µl/s

Evaluation Parameters
Standard method settings were applied. The parameter settings are summarized in the following table.
Table 2: Detection parameters UVFD
Parameter

Specification

Max. integration time

300 sStart

1 cts

Stop

1 cts

Results and Discussion
The examined samples are a representative spectrum of hydrocarbons from refinery applications, the petrochemical andchemical industry as well as in polymer production. The results given in Table 3 are averages of three replicate analyses ofsamples and test standards. For all samples and standards an injection volume of 40 µl was used. Figures 3–6 show typicalmeasuring curves for selected samples resp. standards.
Table 3: Results of the TS analysis
Measurement

TSSD

Isooctane

48.9 ppb± 1.22 ppb

Xylene (mix of isomers)

272 ppb± 2.02 ppb

n-Hexane

36.3 ppb± 1.42 ppb

Toluene51.1 ppb< 1 ppb

Acetone42.7 ppb< 1 ppb

Naphtha (light)

632 ppb± 3.27 ppb

TS Standard (c = 725 ppb)

724 ppb< 1 ppb

TS Standard (c = 74.8 ppb)

72.7ppb± 2.14 ppb

TN Standard (c = 73.2 ppb)

72.4ppb< 1 ppb34

TS Determination in Aromatic Hydrocarbons and Related Chemicals by UV Fluorescence according to ASTM D7183

Fig. 3: TS analysis curve for sample “Naphtha”

Fig. 4: TS analysis curve for sample “Xylene”

Fig. 5: TS analysis curve for TS standard “74.8 ppb S”

Fig. 6: TS analysis curve for TS standard “725 ppb S”

Due to the optimal process conditions a three- to fivefold determination, with injection volumes between 20 and 40 µl per
replicate analysis, is generally sufficient to reach satisfying results for ultra-trace applications. This is remarkably affecting thesample processing time and thereby generates a higher sample throughput. Utilization of huge injection volumes and timeconsuming strategies like trap and release are redundant. The analysis results received and their reproducibility depict thehigh quality of the sample combustion. The proper performance of the analysis system was confirmed by analyzing standard
materials with known sulfur contents (see Table 3).
Conclusion
The compEAct S is extremely well suited for the measurement of ultra-trace sulfur concentrations in versatile aliphatic andaromatic hydrocarbons and their mixes (e.g., naphtha, toluene, xylene, acetone). The detector, with its unique HiPerSens
technology, achieves a measuring range of up to 10,000 mg/l starting at a limit of detection as low as 5 µg/l of sulfur.
The optimal sample digestion and the efficient Auto-Protection system, including a high-capacity membrane dryer, enable
excellent reproducibility, independent of the TS concentration or digestion characteristics and composition of the sample
matrix analyzed. A high sample throughput is easily achieved by using the LS 2 liquids sampler. For lower throughput
demands the LS 1 liquids sampler resp. the AI auto injector can be used alternatively.
If required, the analyzer can be extended to include the analysis of gaseous and liquefied pressurized hydrocarbons at any
time by adding a matrix-optimized sample supply system.

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