FT-IR Analysis of Used Lubricating Oils General ConsiderationsMichael C. Garry, John Bowman, Thermo Fisher Scientific, Madison, WI, USA

ApplicationNote: 50731

Introduction

Infrared spectroscopy is a widely used technique for theanalysis of lubricating oils. The Thermo Scientific Nicolet

FT-IR spectrometer, coupled with OMNIC Integra

software, provides modern laboratories with an intuitivepackage for routine infrared analysis of liquid samples.This application note details the application and theparameters used to derive the highest quality data.

Used Lubricant Analysis Using FT-IR

Infrared analysis of used lubricants provides useful molecularinformation about the changes in a lubricant and themechanical compartment being lubricated. This makes thetechnique an effective trending and screening tool in modern maintenance programs. Information from infraredanalysis can be used to complement data obtained by elemental analysis techniques, such as atomic emission orinductively coupled plasma (ICP) spectroscopy. In addition,it may reduce or replace the analysis requirements for certain traditional techniques that are tedious and timeconsuming. It must be stressed that infrared analysis is atrending and screening tool for multiple parameters ofinterest in lubricants and mechanical systems.

The parameters typically analyzed using FT-IR are listedin Table 1. This table presents the parameters of interestand information on obtaining and using them. The spectralplot in Figure 1 shows a used oil difference spectrum andlabels the peaks of interest indicated in the table.

Key Words

Antifreeze

AntiwearAdditive

Diesel Fuel

Gasoline

Lubricants

Nitration

Oil Analysis

Oxidation

Soot

Sulfation

Figure 1: Used oil difference spectrum

Spectral Parameter Location Type of Traditionalof Interest (approx. cm-1) Measurement Measurement

Soot 2000 Trending carbon load Total insolubles,(diesel engines) Thermogravimetric

analysisOxidation 1700 Trending oil Total base number, (carbon) degradation Total acid number,

Viscosity*Nitration 1630 Trending oil Total base number,

degradation Total acid number, (engines only) Viscosity*

Sulfation 1150 Trending oil Total base number, degradation Total acid number, (engines only) Viscosity*

Water 3400 Contaminant Crackle test, screening Karl Fischer

Diesel fuel 800 Contaminant Flash point, screening Viscosity,

Gas chromatographyGasoline 750 Contaminant Flash point,

screening Viscosity, Gas chromatography

Antifreeze 880 Contaminant Colorimetric assay, (glycol) screening Gas chromatographyAntiwear 960 Additive depletion Elemental zincadditive

Table 1

* This analysis relates to viscosity, but is not a replacement for this measurement.

Analysis Considerations

As a trending and screening tool, FT-IR analysis of usedlubricants has limitations and must be applied with appropriate measures to assure proper generation andinterpretation of data. First and foremost, numeric resultsfrom all of the parameters being determined (with theexception of soot), are affected by the molecular fingerprintof the base lubricant and additive package. That is to say,the additives and base oil have spectral features that interferewith the peaks that develop as the lubricant is contaminatedor oxidized during use. The magnitude of the peaks thatdevelop are typically smaller than, or of similar magnitudeto, those found in the unused oil. This can be seen inFigure 2, which displays a plot of a new oil, the same oilused and the subtraction result (difference spectrum). Thedifference spectrum is shown with a much expanded y-scale.

Figure 3 shows four different new oils. The plot shows the differences in spectral features resulting fromthe variation in base oils and additives used to make thelubricants. These differences must be taken into account,or they will cause errors in the numeric values obtainedfrom the analysis. Because of this, spectral subtraction ofthe proper new oil reference spectrum is applied to obtaincorrect used oil analysis results.

OMNIC Integra software provides advanced featuresfor the creation and maintenance of custom, quantitativenew oil reference libraries. In addition, when the new oilis not known, the autoreference feature is available tosearch a library for the best selection before reference subtraction. However, this option should not be used routinely since it may generate incorrect results, particularlyfor antifreeze, fuel and water, depending on the referencelibrary contents. It should only be used if a reference spectrum is unobtainable.

Table 2 provides a relative sensitivity rating of theanalysis components to the correct reference oil beingused. Components with higher sensitivity will give a moresignificant error in the numeric values obtained when theincorrect reference is used. In the case of water, antifreeze,diesel fuel and gasoline, using the incorrect reference may produce false positive indications, or cause thesecomponents to not be detected. This is particularly truewhen autoreferencing is used because an unknown usedoil with a contaminant is being searched against a cleanoil library. The presence of the contaminant can make thespectrum of the unknown used oil appear more likeanother type of lubricant than the correct new oil reference.

2100 1900 1700 1500Wavenumber

New Oil

Used Oil

Difference

1300 1100 900 700

Figure 2: New oil reference spectrum subtraction

1850 1650 1450 1250Wavenumber

1050 850 650

Figure 3: Infrared spectra of four different new oils

Component Relative Sensitivity

soot insensitiveoxidation fairly insensitivenitration fairly insensitivesulfation fairly insensitivewater fairly sensitiveglycol very sensitivefuel extremely sensitivegasoline extremely sensitiveZDDP extremely sensitive

Table 2

Soot Particles

Suspended soot is the result of the incomplete combustionof fuel. This is usually only a consideration in dieselengines but could be indicative of carburetor or injectorproblems with other fuel systems. While it has no specificfrequency of absorption in the infrared spectrum, sootcauses a shift in the baseline of the spectrum due toabsorption and scattering of light. Since there are no otherspectral features in the region around 2000 cm-1, this areais used to assess the level of soot in a sample. The baselineshift caused by soot is affected by the amount of sootpresent and the effective particle size. The effective particlesize is a resultant of the nature of the combustion systemand the dispersants in the oil. This makes it difficult todirectly assess the quantity of soot, so factors that relatethe amount of soot to the infrared value must be establishedwith the engines and lubricants of interest.

Carbonyl Oxidation Products

The broad feature centered around 1730 cm-1 is due to thepresence of carbonyl-containing degradation products ofoil. These have been identified as lactones, esters, aldehydes,ketones, carboxylic acids and salts, as well as others. Thebroadness of the peak is because of the wide variety ofmaterials present. The point of maximum intensity willvary as the oil and conditions of its use are changed. Theincrease in peak height that occurs as the number of hoursthe oil has been run in the engine increases has a greatersignificance in the measurement of degradation than totalacid number (TAN) or viscosity.

Nitrogen Oxidation Products

The sharp feature at 1630 cm-1 is the result of nitrogenoxide fixation into the oil. The materials that create thisfeature are due mostly to nitrate esters. Nitrogen fixationproducts are most significant in gasoline and natural gasengines and some diesel systems that use exhaust gas recirculation.

Sulfur Oxidation Products

Another broad spectral feature, centered around 1150 cm-1,is the result of sulfate compounds as well as overlap withoxidation products. Sulfate material is a result of theintroduction of sulfur from fuels or from the oxidation ofsulfur from the base oil and additives. This band is a fairlyspecific measure of over basing additive consumption andrelates to total base number (TBN).

Fuel Residue

The measurement of fuel residues or raw fuel is difficultby any method. The main difference between the fuel andbase oil is in molecular weight or boiling range and thepercentage of aromatic materials. Fuel has a lower boilingrange and significantly higher percentage of aromaticmaterial. Conventional methods make use of the lowerboiling range, while the infrared approach examines thearomatic content to indicate if fuel is present. The spectralfeatures around 800 cm-1 are used for this purpose.

The accuracy of measurement by any technique isstrongly affected by the standard material used to calibratethe instrument and the level to which the fuel is evaporatedwithin the engine crankcase. The infrared technique hasthe additional requirement of needing to match the new-oil reference properly, as discussed earlier. It is the changein base oil aromatic level that is the major factor leadingto errors from reference mismatch. Gasoline contamina-tion detection is less problematic than detection for dieselfuel because of the higher aromatic content of gasoline.

Even though calibrations are provided in the OMNICIntegra software, it is best if fuel is calibrated by the labo-ratory using the system. In any case, it is important to useat least one other independent test, such as viscosity, flashpoint, or gas chromatography to confirm an indication offuel presence by FT-IR analysis.

Water and Glycol Contamination

These two contaminants have characteristic absorptions inthe infrared that make their detection possible near the0.1% level. While this is not as sensitive as some othertechniques, it is at a level where problems from the presenceof these contaminants in the crankcase could begin. Wateris detected as a broad feature, centered around 3400 cm-1

that is caused by the hydroxyl (-OH) group. Glycol has acharacteristic peak around 880 cm-1 as well as peaks at1040, 1080, and 3400 cm-1. The peak at 880 cm-1 is usedto quantify because it is not subject to interferences to thesame extent as the other bands. The other peaks in thespectrum related to glycol are used to confirm its presence.

Glycol will interfere with the quantification of thewater level when present, but the converse is not true.Therefore, when glycol is present, water cannot be reliablyquantified using FT-IR. This is not considered a problembecause the presence of glycol has a greater significance toengine performance. As with fuel, it is extremely importantto use at least one other test to confirm that water or glycolare present in a used-oil sample. Water can be confirmedby the crackle test or the Karl Fischer test. Glycol presencecan be confirmed by a colorimetric test, and corroboratedwith elemental analysis results of sodium and boron.

Considerations for Specic Analysis Parameters of InterestPlease refer to Figure 1 and Table 1 for this discussion.

Reporting of Data

The values determined from the infrared spectrum for thecomponents discussed in the preceding section are reportedin different ways because of the nature of the materialspresent. The following values are reported by OMNICIntegra software:

Oxidation, nitration and sulfation components are notreported as concentration values, because there aremany different chemical compounds formed that contribute to the measured absorption. Since no singleproduct is formed, standard materials are not availableto generate the calibration curves needed to relateabsorbance to concentration. Absorbance values relatedto the thickness of sample exposed to light are reported.The units used for reporting are absorbance per 0.1 millimeter (ABS/0.1 mm), which relates directly to thepeak intensities observed in the difference spectrum of a used oil. The information for these components ismost useful in trend analysis.

The carbon particles that form soot do not cause localized absorption. Instead, an offset of the spectralbaseline due to light scattering and absorption isobserved. The magnitude of this offset is determined bythe particle size of the soot as well as its concentration.The particle size observed is affected by the engine typeand lubricant. ABS/0.1 mm values related to the thicknessof sample exposed to light are reported as well.

Antiwear additive, typically various forms of zincdithiodiphosphate, is reported in ABS/0.1 mm the sameas the above oxidation components. However, the antiwear additive is reported as a negative value becausein the used-oil difference spectrum the peak goes in theopposite direction of other peaks in the spectrum. Thisis due to the depletion of the additive compared to theoriginal reference oil.

Fuel, water, glycol, and gasoline are reported in percentvalues from calibrations with standards. The calibrationsthat are provided in the software for diesel fuel andgasoline may not accurately reflect the fuels in use in aparticular region of the world. Because of this, thesecomponents should be calibrated with locally generatedstandards. In the case of water and glycol, these materialsare typically very consistent. These components shouldnot need to be routinely calibrated locally because of thedifficulty of making and analyzing standards.

In practice, the approach taken to make use of theabsorbance values for the uncalibrated components is tonote their increase over time within an engine. From aplot or table of the change in absorbance with time, judgments about when to change the lubricant can bemade. This is where experience with used lubricant analysiscomes into play.

Table 3 provides an explanation of the units reportedby the software with some typical and high results.

ReferenceUsed Lubricating Oil Analysis manual, P/N 269-069403, Thermo FisherScientific, 2003.

Limit of Typical Component Units Detection Result High Result

soot ABS/0.1 mm NA 0.22 0.70oxidation ABS/0.1 mm 0.02 0.1 1.0nitration ABS/0.1 mm 0.02 0.1 1.0sulfation ABS/0.1 mm 0.02 0.1 1.0antiwear* ABS/0.1 mm -0.02 -0.05 -1.0diesel fuel weight % 2.0 NA 6.0gasoline weight % 1.0 NA 6.0glycol weight % 0.1 NA 0.3water weight % 0.1 NA 0.3

Table 3

* Antiwear is reported as a negative value since it represents the depletion ofa material. NA means not applicable

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