Detection of Impurities in Premium Diesel Fuel via Terahertz Frequency Domain Spectroscopy

. Long-chain hydrocarbons, petroleum and diesel, have long been used as source of energy for locomotives. Unlike it’s short -chain counterpart petroleum, diesel fuel is considered dirtier due to black soot particulates it emits that poses greater health hazard. Here, we attempt to measure the absorbance spectra of premium diesel fuels, neat and adulterated, using terahertz (THz) frequency domain spectroscopy to determine the level of impurities that can further exacerbate its emission. Two broad absorption peaks at 6.42 THz and 7.75 THz as well as narrow peaks at 13.07 THz, 13.88 THz, and in the range of 16-17 THz, characterized the premium diesel. These spectral features are well identifiable in the adulterated samples but their intensities vary depending on the type of impurities. Decrease in the absorbance is observed with water contaminant, increase in isopropanol, while sulfur and methanol contaminants did not influence the absorbance spectra. This technique demonstrates initial but promising results in probing adulteration in petrochemical products.


Introduction
Diesel fuels, similar to petroleum or gasoline, are longchain hydrocarbon molecules that when combusted translates its thermal energy to locomotion via internal combustion engine.Diesel engines are more rugged, produces larger torque, and has higher fuel economy.However, it also emits soot, i.e., black carbon particles from incomplete combustion that are greater risk to health than its petroleum counterpart.Moreover, it is estimated that the nitrogen dioxides (NOx) released into the atmosphere by diesel engines is about seven times higher than the limits set by Euro 6 standard. 1As such, any additional contaminants that may be present in diesel fuels could lower its combustion efficiency and be translated to even higher soot and NOx emissions.
In this work, we attempt to detect contaminants in commercially available diesel fuels using the terahertz (THz) frequency domain spectroscopy technique.The THz radiation lies between the infrared and the microwave regimes.In contrast to microwave and infrared, THz does not have a lot of applications as it can be easily absorbed by hydrogen-bonded molecules.This is due to the fact that its energy is similar to O-H breaking and making, e.g., water has very high absorption in the THz frequencies.However, this unique characteristic is quite suitable in probing long-chain molecules as they have low frequency intra-and inter-molecular vibration modes. 2 In the work of Zhao et al., the absorption coefficient of diesel/biodiesel * Corresponding author: ponseca.c@gust.edu.kwmixture was obtained in the THz frequencies.Using a nonlinear multivariate model, they were able to correlate the cetane number and solidifying point of bio-diesel blends with absorption coefficient, demonstrating simple quantitative analysis of fuel properties. 3Arik et al., on the other hand, reported significant differences in relaxation times of molecules in diesel and gasoline, which attributed to the differences in their intermolecular forces.This was obtained by analyzing the real and imaginary parts of THz permittivity spectra of the fuels. 4In the recent work of Lapuerta, et al., the optical and dielectric properties (absorption coefficient, refraction index and dielectric constant) of alcohol blends with diesel and biodiesel blends were determined in the THz region.From these properties, the relaxation dynamics were modelled for blends, and nonlinearities in their physical properties were explained. 5These reports show that THz spectroscopy is an effective tool in understanding optical properties of diesel fuel.This work extends the investigations of diesel fuels using higher (2-18 THz) frequency-domain THz spectroscopy technique rather than the conventional time-domain approach whose frequency spectra is limited to 0.1 -1.5 THz only.Two broad absorption peaks at 6.42 THz and 7.75 THz as well as narrow peaks at 13.07 THz, 13.88 THz, and in the range of 16-17 THz, characterized the premium diesel.These spectral features, not reported elsewhere before, are well identifiable in the adulterated samples but their intensities vary depending on the type of impurities.

Experiment
Premium diesel was obtained from a gasoline station Neste.A neat sample of diesel is measured as standard while other four blends were prepared with 0.5% water, 0.5% isopropanol, 0.5% methanol, and 0.5% sulfur to represent contamination.The schematic diagram of Frequencydomain THz spectrometer/interferometer is shown in Figure 1.A heated blackbody source emits THz radiation from 2-18 THz (gray area) and is redirected by mirror 3 (M3) to the beam splitter, where part THz is reflected and the other passes through.The reflected THz radiation by the beam splitter bounces to mirror 2 (M2) while that that passed through will be reflected by mirror 1 (M1).M1 is stepwise adjusted to generated interference.The interfering THz signals goes to mirror 4 and is redirected to the cuvette containing the samples, where part of the terahertz radiation is absorbed and redirected to the detector.The detector senses the interfering signal with the optical information of the sample.A continuous wave laser (red trace) is also made to interfere in the setup which gives the time resolution or the time delay between the two paths and is then correlated to the detector time of the THz radiation.Initially, the cuvettes were empty.After allowing THz to pass through the aperture, we obtain an interferogram of the aperture which was used as reference for comparison.Next, empty bags were mounted on the cuvette and the THz radiation is shot into the aperture of the cuvette resulting to another interferogram that was also used as a reference.The bag was then filled with neat premium diesel.A 0.5% (by volume) of water, isopropanol, methanol, and sulfur were added to liquid samples but ensuring that the total volume is identical to that of the neat diesel.Fast Fourier transformation (FFT) of the interferogram signals yield the absorbance spectra which are plotted in Figure 2.

Results and Discussion
The absorbance spectra of neat premium diesel (black trace) is characterized by two broad absorption peaks at 6.42 THz and 7.75 THz, narrow peaks at 13.07 THz, 13.88 THz, and several peaks in the range of 16-17 THz.Note that the sharp peaks around 13.88 THz and between 16-17 THz may have been experimental artifacts as the signal to noise ratio of the spectrometer are lower at this region.Adding 0.5% of water (red trace) into the diesel did not change the global shape of the absorbance spectra nor modify the intensity ratios of any of the peaks.This indicates that detected spectra solely come from diesel.However, it is noticeable that the overall absorbance intensity spectra are lower than neat diesel.This suggests that the reduction is due to water alone.On one hand, for sulfur and methanol, both the shape and intensity of the absorbance spectra are almost identical to neat diesel, meaning that the THz interferometer is blind to these contaminants.On the other hand, while the isopropanol's absorbance intensity from 2-12 THz is higher than neat diesel, they are rather lower from 12-18 THz.Moreover, the two absorption peaks at 6.42 THz and 7.75 THz appears less discernable from each other, which can be surmised as one broad peak.These variations in the absorbance spectra may imply that isopropanol is detected spectrally rather than just in its intensity.

Summary
Neat and adulterated diesel fuels were investigated by frequency-domain THz spectroscopy technique.The absorbance spectra showed rich absorbance peaks between 2-18 THz.While water and isopropanol contaminants were detected, sulfur and methanol are indiscernible.Even lower concentration of contaminants will be further investigated to resolve the detection limits of this technique.