Application Note · compEAct N

Determination of Ultra Low Nitrogen in
Aromatic Hydrocarbons by Oxidative Combustion
and Reduced Pressure Chemiluminescence
Detection according to ASTM D7184

Challenge
Fast and reliable analysis of
N contents in the ultra-trace
range in different types ofliquid aliphatic and aromatichydrocarbons and their mixtures.

Solution
Optimized vertical combustion
combined with HiPerSens
chemiluminescence detection forconcentration-independent TN
determination.

Introduction
Liquid aliphatic and aromatic hydrocarbons play an important role in refining
processes, in the petrochemical, chemical and polymer industry, either as raw
materials, process intermediates, or as end products. Regardless of their source or
their further use, they all have in common that they need to be ultra-pure. 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 nitrogen-containing compounds not only affect theperformance and economic life of catalysts used in the production process(catalyst poisoning), but also form unwanted by-products and thereby decrease
the quality of the end products. Therefore their content has to be kept as low as
possible, preferably below 100 ppb. To make this possible, the exact knowledgeof the present nitrogen (TN) is required. Combustion-based elemental analysis
coupled with a pressure-reduced chemiluminescence detector is the only methodthat has proven to be suitable for this challenging task.
The compEAct N is an analysis system which has been specifically optimized forthe fast and trouble-free determination of nitrogen contents in the ultra-trace
range. Combining catalyst-free high-temperature combustion and highly sensitive

2TN Determination in Aromatic Hydrocarbons by Oxidative Combustion and Chemiluminescence Detection according to ASTM D7184

HiPerSens detection, it allows the determination of nitrogen at concentrations ranging from 15 μg/l up to 10,000 mg/l with
one and the same device and method.
Materials and Methods
Samples and Reagents■■
Different aliphatic and aromatic hydrocarbons and their mixes (e.g. benzene, isooctane, naphtha)■■
Isooctane (C8H18), Suprasolv®, GR for gas chromatography (Merck Art.-No.: 1.15440.1000)■■Pyridine (C5H5N), GR for analysis (Merck Art.-No.: 1.09728.0100)■■
Extended calibration standard kit Nitrogen (0–25 mg/l) (Analytik Jena, Art.-No.: 402-889.076)
Sample Preparation
The samples are light volatile, have a low viscosity, and contain TN 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 nitrogen standard solutions based on pyridine (N) in
isooctane in the range of 0 to 4,000 ng N. Figure 1 and 2 depict typical calibration curves and performance parameters forultra-trace applications.

Fig.1: TN calibration for ultra-trace range

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

The calibration was checked with different concentrated standards.
Instrumentation
The measurements were performed using a compEAct N, equipped with HiPerSens CLD detection for the determination ofnitrogen. Sample introduction was carried out fully automatically using the LS 2 liquids sampler to ensure a high 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 guarantees highest operationalsafety (particle and aerosol trap) and a complete transfer of the formed NOx into the CLD after a sufficient drying of thereaction gases. The compEAct N enables a detection limit of as low as 15 μg/l N.

TN Determination in Aromatic Hydrocarbons by Oxidative Combustion and Chemiluminescence Detection according to ASTM D7184

Method Parameters
The standard method ASTM D7184 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 NParameter

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 volume40 µl

Injection

0.5 µl/sEvaluation Parameters
Standard method settings were applied. The parameter settings are summarized in the following table.
Table 2: Detection parameters CLDParameter

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 TN analysis
Measurement

TNSD

Isooctane

42.6 ppb± 5.60 ppb

Xylene (mix of isomers)

53.6 ppb± 5.23 ppb

Benzene59.8 ppb± 1.08 ppb

Toluene572 ppb± 3.95 ppb

Ethanol

173 ppb± 6.29 ppb

Cumene119 ppb± 3.74 ppb

Naphtha489 ppb± 5.92 ppb

TN Standard (c = 725 ppb)

726 ppb± 16.3 ppb

TN Standard (c = 73.2 ppb)

72.4ppb< 1 ppb3

4

TN Determination in Aromatic Hydrocarbons by Oxidative Combustion and Chemiluminescence Detection according to ASTM D7184

Fig. 3: TN analysis curve for sample “Toluene”

Fig. 4: TN analysis curve for sample “Isooctane”

Fig. 5: TN analysis curve for TN standard “73.2 ppb N”

Fig. 6: TN analysis curve for TN standard “725 ppb N”

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
the sample processing time and thereby generates a higher sample throughput. The analysis results received and theirreproducibility depict the high quality of the sample combustion. The proper performance of the analysis system wasconfirmed by analyzing standard materials with known N contents (see Table 3).
Conclusion
The compEAct N is extremely well suited for the measurement of widely varying nitrogen contents in versatile aliphatic andaromatic hydrocarbons and their mixes (e.g., naphtha, toluene, benzene, ethanol). 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 15 μg/l of nitrogen.
The optimal sample digestion and the efficient Auto-Protection system enable excellent reproducibility, independent of theTN concentration or digestion characteristics and composition of the sample analyzed. A high sample throughput is easily
achieved by using the LS 2 liquids sampler.
The analyzer can easily be extended to include the analysis of gaseous and liquefied pressurized hydrocarbons by adding oneof the available application-optimized sample supply systems

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