Application Note · compEAct N

TN Determination in Bio-based Fuels According
to ASTM D4629 – Standard Test Method forTrace Nitrogen in Liquid Hydrocarbons bySyringe/Inlet Oxidative Combustion andChemiluminescence Detection

Challenge
Ultra-trace analysis - Ensuring
reliable results in accordance withASTM D4629 for bio-based fuelsof diverse source materials.

Solution
Fast and optimized combustion
process for reliable andreproducible ultra-trace analysis
independent of the type of thebiofuel.

Introduction
The production of alternative fuels becomes increasingly popular, as they provide
a way to respond to both, the growing demand for energy and the goal to reducecarbon emissions. In addition to technologies based on the fermentation of starchand sugar, and the transesterification or hydrocracking of plant and animal oil(first generation biofuels), processes based on the conversion of cellulose-richbiomass and waste appear to be promising (second generation biofuels). Withthe use of biogenic materials for fuel production, their inherently high contentof harmful hydrocarbons containing nitrogen, sulfur and chlorine has to bemonitored. This is not only due to the adherence of given regulations and legallimits (Tier I–IV), but also to avoid negative effects on the production process,such as catalyst poisoning, and the environment. Even lowest levels of nitrogencompounds can seriously affect the performance and service life of catalysts,
hence their ratio must be kept as low as possible. The desired TN content is far
below 1 ppm, in general it should be around 100 ppb N. For this reason, a strict
monitoring of the intermediate and final products is inevitable.
In ultra-trace analysis many factors influence the quality of analysis and therebyof course the final result. Besides the sensitivity of the detection system used,factors like the cleanliness of the apparatus used (system blank), the skills of

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TN Determination in Bio-based Fuels According to ASTM D4629

the responsible operator (method development and adaptation to each sample matrix), the quality and suitability of thecalibration used and, of course, the combustion quality (enabling sharp peak modulation) are highly important.
To eliminate or minimize all these possible error sources, a fast and catalyst-free high-temperature combustion in verticaloperation mode and the use of the extremely sensitive HiPerSens detection technology is required. This eliminates matrixrelated method optimization which often leads to a confusing multitude of methods. Thanks to a quartz pyrolyzer, evenmatrices containing traces of heavier components can be digested completely without soot formation. This reduces themaintenance effort to a minimum.
Materials and Methods
The different bio-based fuels were analyzed for their nitrogen content by using a compEAct N elemental analyzer. Equippedwith the HiPerSens detection system, it enables determination of the smallest traces as low as 0.6 ng N abs.
The analyses have been run in vertical operation mode. All samples were dosed directly into the evaporation zone of thequartz glass combustion tube. This process was carried out fully automatically by means of a LS 2 high-throughput liquids
autosampler. The catalyst-free, bi-phasic combustion process is carried out at temperatures of up to 1050 °C. In the firstprocess 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
and formed 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. Afterwards, reaction gases were dried and transferred into the HiPerSens
chemoluminiscence detector.
Sample Preparation
The analyzed sample matrices are a representative spectrum from the field of bio-based fuels. Samples 1–4 are different end
products (bio-based fuels). They are light volatile, have a low viscosity, and contain TN in the ultra-trace level. The samples
were analyzed directly without a sample preparation step.
Calibration
Liquid calibration standards based on pyridine (N) in isooctane were used to calibrate the analysis system in the appropriate
concentration ranges.

Fig.1: Calibration range 1 – ultra-trace

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

Liquid calibration standards based on pyridine in isooctane were used to calibrate the analysis system. For evaluation of thegained measurement results the calibration range 1 (ultra-trace) was used. Figure 2 shows the calibration curve for the CLD
detector. The calibration was checked with different concentrated standards.

TN Determination in Bio-based Fuels According to ASTM D4629

Instrumentation
A compEAct N equipped with CLD detection for the determination of nitrogen was used. For the automated sample
introduction and transfer into the analyzer the system was equipped with the LS 2 high-throughput liquids sampler.
For the determination in the ultra-trace range, sample volumes of 40 µl were used.
The sample digestion is carried out by efficient catalyst-free high-temperature combustion in a quartz tube. The process
is split into two phases. In the first phase evaporation of light components and pyrolysis of the heavier components takes
place within an inert argon atmosphere. The resulting gaseous products are converted in the pure oxygen atmosphere of
the combustion zone. In the second phase the system switches completely to oxygen and the remaining components arecombusted quantitatively.
The implemented Auto-Protection System guarantees highest operational safety (particle and aerosol trap) and a complete
transfer of the formed NOx into the CLD. The compEAct N enables a detection limit as low as 15 µg/l N.
Method Parameters
From the compEAct’s integrated method library module, the standard method ASTM D4629 was used for all analyses. The
following table summarizes the parameter settings for the combustion process.
Table 1: Process parameters compEActParameter

Specification

Furnace temperature

1050 °CSecond 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

0.5 µL/s

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

Specification NSMax. integration time

300 s

Start (N)

1 cts

Stop (N)

1 cts

3

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TN Determination in Bio-based Fuels According to ASTM D4629

Results and Discussion
The examined samples are a representative spectrum from the field of biofuel. The results given in Table 3 are averages ofthree replicate analyses.
Table 3: Results of the TN analysis
MeasurementTNSD

Biodiesel (corn oil)

0.87 ppm± 0.02 ppm

B-t-L** (tall oil)5.33 ppm± 0.08 ppm

Bioethanol (cellulose)

0.18 ppm< 0.01 ppm

Bio-based diesel

0.34 ppm± 0.02 ppm

Standard 0.5 mg/l N

0.5 mg/l± 0.01 mg/l

Standard 0.05 mg/l N

0.05 mg/l± 0.01 mg/l** B-t-L = bio mass to liquid fuelDue to the optimal process conditions a threefold determination is generally sufficient to reach results within 3 % RSD. Thisis remarkably affecting the sample processing time and thereby generates a higher sample throughput. The analysis resultsreceived and their reproducibility depict the high quality of the sample combustion. The proper performance of the analysis
system was confirmed by analyzing standard materials with known N contents (see Table 3).
Typical measuring curves for samples and standards are depicted in Figures 3–6.

Fig. 3: TN measuring curves for bio-based diesel fuelFig. 4: TN measuring curves for bioethanolFig. 5: TN measuring curves for standard 0.05 µg/l NFig. 6: TN measuring curves for standard 0.5 µg/l N

TN Determination in Bio-based Fuels According to ASTM D4629

Conclusion
The compEAct together with the LS 2 autosampler provides a fast and reliable solution for the precise determination ofultralow TN contents in bio-based fuels. Thanks to the highly sensitive HiPerSens CLD, even smallest differences in theppb range can be distinguished exactly. The optimal sample digestion and the efficient Auto-Protection system enable anexcellent reproducibility. A high sample throughput is easily achieved using the LS 2 liquids sampler. If needed, the analysis
system can be extended for the determination of pressurized gases and LPG by adding the suited sampling system.
Further time optimization can be achieved by decreasing the sample quantities for highly concentrated sample matrices. The
same process and detection parameters can be applied.
The HiPerSens detection system used for nitrogen allows analyzing samples in the widest possible concentration. That also
positively affects the processing time of samples.

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