Determination of the Degree of Consumption ( DoC ) of Lube Engine Oils Using Fluorescence Spectroscopy

The accreditation of a fast, inexpensive, and simple way to discriminate between different kinds of oils and their efficacy “degree of consumption (DoC)” has been developed. The fluorescence spectroscopy provides a reliable method for oil inspection without resorting to tedious separation. Different new and used oil samples available in the local Iraqi market were investigated. While the challenge is to build a directory containing data of all the oils available in the local market. This method expected to control the falsified (forged) trademarks of motor oils and to discriminate between different oils. The excitation-emission spectra of oil samples were determined in the range of 200 – 600 nm. The effect of the presence of trace metals on the fluorescence intensity of oils was considered by adding few milligrams of (Cu, Al, Fe) to the diluted oil solution. No major effect noticed on fluorescence intensity. The research suggests installing a simple Spectrofluorometer into vehicles to check the DoC of the oil regularly and to notify the driver exactly when to replace the engine oil. The obtained results indicate the applicability to execute such gadget to be installed in the vehicles for routine detection of the engine oil quality and its degree of consumption DoC. As well as demonstrate the potential of the technique in oil identification and could be further developed.


Introduction
The lube oil composition is a complex mixture of hydrocarbons with different molecular masses.The lube oil producers generally use the same base-stock then add different additives up to 5% by weight [1] to improve the oil performance [2].Lube oils are designed to lubricate the moving parts of internal combustion engines to reduce friction, provide oxidation resistance, improved deposit protection, better wear protection, better low-temperature performance over the life of the oil [2] and protect the engine from malfunctions [3].Thus, oil testing is important to examine lubricant's quality and to discriminate between new and used oils [4].Because of the complexity of lube oils and the many factors affecting their compositions, almost all analyses been partial and not very accurate [5].Exhausted motor oils are hazardous wastes to the environment [6] and could reach the sources of drinking water and crops irrigation [7].Most of the lubricants absorb UV or visible light, while few are fluorescent [8].Organic molecules such as oils, with extended π-electron systems as for aromatic and some unsaturated aliphatic compounds, often exhibit fluorescence efficiency [9].Compounds containing fused-rings usually exhibit high molecular fluorescence [10].Rigid molecules or multiple ring systems tend to have large quantum yields of fluorescence while flexible molecules generally have lower quantum yields [11].Fluorescence detectors are very selective and are about three orders of magnitude more sensitive (0.001-0.01 ng) than UV detectors as they measure the fluorescence of the analyte against an almost zero background [12].There are many other applications for fluorescence spectroscopy, not limited to; determination of thermal stability of biocatalysts, Characterizing bio labels for live cell imaging, Hydrocarbon mixtures in petroleum oils, and Characterizing GPCR (G proteincoupled receptors) oligomerization [13].Nevertheless, many other applications in Nanoparticle characterization, Surface chemistry research, Analytical chemistry, Pharmacology, Biotechnologies, and in crime investigation [14].Most fluorescence measurements use to carry out in liquid media and the solutions must be very dilute in fluorimetry.Otherwise, the results may not comply with the Lambert-Beer law and linearity cannot be achieved [15].This leads to the apparently paradoxical result that the fluorescence can diminish even though analyte concentration increases.Molecules that naturally fluorescence inactive can convert by chemical derivation or by reacting with a fluorescent molecule to become fluorescent.The interaction of a fluorescent molecule with the solvent medium will affect both the energy and intensity of fluorescence spectra.Effects of polarization and hydrogen bonding, viscosity effects, heavy atom effect, compound formation and photo-reaction have a critical influence on the resultant fluorescence [16].In polar media, fluoresce molecules may solvate by dipolar attraction.These effects produce differences in the equilibrium configurations of the ground and excited states [17].The solvent can also interact with fluorophores to form excited state complexes that do not fluoresce.Selection of a solvent to minimize this effect can enhance fluorescence.In addition to the absorption and emission, Scattering may happen either due to Rayleigh scattering or by small particles in colloidal suspension (Tyndall scattering) [18].Some of the incident energy transferred to the solvent molecules in the form of vibrational and rotational energy.Then reemitted in longer wavelength and less energy than the excitation radiation.This is called Raman scattering, which is 100 to 1000 times weaker than Rayleigh's [19].Unlike absorption -emission fluorimetry is synchronous fluorescence scan (SFS), in which both monochromators moves simultaneously [20].The synchronous technique allows the stronger peaks to be increased selectively by use of a suitable stoke shift Δλ [21].Unlike UV/visible spectroscopy, fluorescence may undergo quenching, which is a reduction in fluorescence intensity [22].One reason for quenching is the molecular interaction when a fluorophore is in contact with another molecule.Other reasons lead to a loss of emission from the fluorophore include energy transfer, charge transfer reactions or photochemistry [23].The final goal of this study is to develop a sensing system uses a simple fluorometer, which can be installed in the vehicle to determine the DoC and to notify the driver when to replace the engine oil.This will ensure changing the oil only when fully depleted, which reduces costs and preserve the environment.The reliance on the car mileage counter to replace the engine oil is not accurate enough since the consumption of oil (DoC) depends on many factors not limited to oil quality, climate, driving habit, engine efficiency, etc.Nevertheless, discrimination the quality of lube oils is of great importance as well. Benzene (97%, BDH), Cyclohexane (99.7%, BDH), Chloroform (99.4%, Merk), Dichloromethane (98.8%,Fluka), Ethanol absolute (99.9%,BDH), n-hexane (99.9%,Scharlau), Toluene (99.5%, Himedia).

Materials and reagents
 Heavy metals (Cu, Fe, Al) as fine powders, obtained from local workshops.

Optimization of Fluorescence Measurement
Different conditions been examined as a function of the fluorescence intensity as an arbitrary unit (a.u.) to optimize the test method and increasing its sensitivity.Conditions of Emission and Excitation Wavelengths pairs (EEWs), slit width of both the excitation and the emission monochromators, adjustment of the emission path length, temperature, and the contamination by the presence of wear metals in engine oil.

Degree of Consumption (DoC) for lube oil
DoC has been determined for some lube oils by measuring the fluorescence intensity at a certain EEWs for each pair of the fresh/consumed oil at different proportions.

Sampling
Eight out of 16 oil samples collected in pairs (new/used).Solvents like strong acids, strong bases or even acetone were avoided while cleaning the quartz cuvettes as they might attach to their walls [20].

Factors affecting Fluorescence Measurements
A number of parameters have been studied intensely to determine the optimized conditions.

Setting up EEWs pairs
Finding the right EEWs pairs is of great importance to maintain sensitivity.EEWs found automatically using the function "Prescan" available in the spectrofluorometer software.
While manual recognition of the EEWs was accomplished by setting the excitation monochromator to the of the maximum wavelength (λmax) UV/Vis absorption spectrum.Then the emission monochromator is set to the highest emission intensity obtained, and scan the excitation radiation again to specify the best corresponding excitation wavelength.Both approaches gave almost the same EEWs results.Emission intensities have been compared for different couples of new/used oil samples under identical measurement conditions, as appears in Figure (1)(2)(3)(4)(5).

Setting up the radiation slits widths
The best slit width of both the excitation and the emission monochromators were tested by changing the slit width of both monochromators individually.Better sensitivity obtained with wider slit widths.However, if the priority is to selectively discriminate a specific analyte in a mixture then, the usage of narrower slits will be necessary [17] [24].As it is clear from the figure (6); that the optimized value for the excitation-slit width was 10 nm while the best emission-slit width was 5 nm.However, the emission intensity was out of scale when both excitation and emission slits widths were set to 10 nm.

The influence of emission path length
The emission path length is the perpendicular distance between the excitation beam passing through the sample and the cuvette facet facing the detector.Longer paths may lead to self-quenching caused by the unexcited molecules between the fluorescent analytic and the emission detector those may absorb the emitted light [23].
When an engine oil consumed, it become darker, then a shorter path length will be a good solution to reduce the optical density.A Varian Peltier Multicell 4 Position Cell Holder, figure (7), used to control the emission path length so that the incident beam passes closer to the cuvette face in front of the detector.This will shorten the thickness facing the detector, figure (8).
From the Figure (9-10), the fluorescence intensity of the oil samples is inversely proportional to the emission path length facing the detector.This been expected, as the number of the unexcited molecules existing in the emission path those causing quenching by absorbing the fluorescent light is reduced.

Degree of Consumption (DoC) of lube oil
Direct measurement of lube oil without the need for any sample preparation with no residues generated and a very short analysis time is of great significance.The fluorescence intensity was examined for 1.5 mL of new oil samples after spiked by different aliquots of consumed oil of the exact same brand and type.These tests were carried out in optimized conditions to test the DoC.table (4), figures (22)(23)(24)(25)(26)(27)(28)(29)(30)(31).For each oil sample, the fluorescence intensity has been drawn against the added volume portions of the peer used oil.The slope and the linearity obtained from the sketch of the second order polynomial function for the curve represents DoC and the oil quality, i.e. higher slope and R2 values means better oil quality, figure (22).However, the suggested relation between DoC and the sample composition became clearer by plotting the value of logarithm to the base ten of the DoC against the added volume of consumed oil, then getting the exponential trend line of the curve, figure (23).
The resultant slope and linearity values represent the oil quality since high values mean the oil preserve lubricating properties and can endure larger volumes of the consumed residues without changing its properties.Therefore, the next graphs conclude that the oil coded R14 has the best quality than the other samples since it has the highest values for slope, table (5).

Conclusion
The experimental findings have indicated that fluorescence spectroscopy is easy, affordable, sensitive, fast, and an efficient way to identify and discriminate between different lube oils.As the measured fluorescence intensity of the of the new and used oil samples is proportional to the degree of consumption DoC of the oil.In addition, DoC is applicable and can be of great economic and environmental benefit.The major obstacle in this study is the lack of an updated indexed library implying data of all oils available in the local market.Such data is vital for oil comparison and verification.


Several motor oils from different producers available in the Iraqi market over the period Oct 2016 to May 2017, Error!Not a valid bookmark self-reference.1.

Figure ( 23 )
Figure (23): A comparison for the logarithmic scale to the base 10 of fluorescence intensity of some fresh oils (1.5 ml each) versus an added volume of its peer consumed oil

Table ( 1): Codes and details of different motor oil samples examined in this study Sample Code Trademark Origin
SL  SAE: The Society of Automotive Engineers API: American Petroleum Institute  * Samples have a used peer.Used oil samples followed by the suffix "u".