ASTM D7418-23

Standard Practice for Set-Up and Operation of Fourier Transform Infrared (FT-IR) Spectrometers for In-Service Oil Condition Monitoring

SIGNIFICANCE AND USE
4.1 This practice describes to the end user how to collect the FT-IR spectra of in-service oil samples for in-service oil condition monitoring. Various in-service oil condition monitoring parameters, such as oxidation, nitration, soot, water, ethylene glycol, fuel dilution, gasoline dilution, sulfate by-products and phosphate antiwear additives, can be measured by FT-IR spectroscopy (4-7). Changes in the values of these parameters over operating time can then be used to help diagnose the operational condition of various machinery and equipment and to indicate when an oil change should take place. This practice is intended to give a standardized configuration for FT-IR instrumentation and operating parameters employed in in-service oil condition monitoring in order to obtain comparable between-instrument and between-laboratory data.
SCOPE
1.1 This practice covers the instrument set-up and operation parameters for using FT-IR spectrometers for in-service oil condition monitoring for both direct trend analysis and differential trend analysis approaches.  
1.2 This practice describes how to acquire the FT-IR spectrum of an in-service oil sample using a standard transmission cell and establishes maximum allowable spectral noise levels.  
1.3 Measurement and integrated parameters for individual in-service oil condition monitoring components and parameters are not described in this practice and are described in their respective test methods.  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 75.160.20 - Liquid fuels
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.96.03 - FTIR Testing Practices and Techniques Related to In-Service Lubricants

Relations

• Refers: ASTM D4175-23a - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 15-Dec-2023
• Refers: ASTM D4175-23e1 - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Jul-2023
• Refers: ASTM D7844-12 - Standard Test Method for Condition Monitoring of Soot in In-Service Lubricants by Trend Analysis using Fourier Transform Infrared (FT-IR) Spectrometry
• Effective Date: 01-Dec-2012
• Refers: ASTM D4057-06(2011) - Standard Practice for Manual Sampling of Petroleum and Petroleum Products
• Effective Date: 01-Jun-2011
• Refers: ASTM E131-10 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 01-Mar-2010
• Refers: ASTM D7414-09 - Standard Test Method for Condition Monitoring of Oxidation in In-Service Petroleum and Hydrocarbon Based Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry
• Effective Date: 01-Jul-2009
• Refers: ASTM E1421-99(2009) - Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
• Effective Date: 01-Mar-2009
• Refers: ASTM E1866-97(2007) - Standard Guide for Establishing Spectrophotometer Performance Tests
• Effective Date: 01-Dec-2007
• Refers: ASTM E168-06 - Standard Practices for General Techniques of Infrared Quantitative Analysis (Withdrawn 2015)
• Effective Date: 01-Mar-2006
• Refers: ASTM E131-05 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 01-Sep-2005
• Refers: ASTM E168-99(2004) - Standard Practices for General Techniques of Infrared Quantitative Analysis
• Effective Date: 01-Feb-2004
• Refers: ASTM E1421-99(2004) - Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level Zero and Level One Tests
• Effective Date: 01-Feb-2004
• Refers: ASTM E131-02 - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 10-Sep-2002
• Refers: ASTM E131-00a - Standard Terminology Relating to Molecular Spectroscopy
• Effective Date: 10-Sep-2000
• Refers: ASTM D4057-95(2000) - Standard Practice for Manual Sampling of Petroleum and Petroleum Products
• Effective Date: 10-Apr-2000

Overview

ASTM D7418-23 is an international standard practice issued by ASTM International, specifically designed for the set-up and operation of Fourier Transform Infrared (FT-IR) spectrometers as applied to in-service oil condition monitoring. This standard ensures that users collect consistent FT-IR spectra of lubricating oil samples withdrawn from equipment, facilitating reliable detection of critical parameters such as oxidation, nitration, soot, water, ethylene glycol, fuel dilution, sulfate by-products, and phosphate antiwear additives. By implementing this standardized approach, laboratories and operators can confidently compare data between instruments and across facilities, supporting effective machine maintenance and asset management.

Key Topics

• Standardized FT-IR Set-up: Guidance on configuring FT-IR spectrometers, including recommended source, detector, beamsplitter, and sample cell requirements. Emphasizes using a standard transmission cell and establishing allowable levels of spectral noise.
• Spectral Acquisition Parameters: Defines acquisition parameters such as spectral resolution, data spacing, apodization, scanning range, and output format.
• Direct and Differential Trend Analysis:
  • Direct trend analysis: Involves direct measurement of the FT-IR spectrum of the in-service oil sample to monitor changes over time.
  • Differential trend analysis: Uses the spectrum of a new or reference oil to identify changes in the in-service oil by spectrum subtraction.
• Sample Handling and Preparation: Sampling recommendations, including filtering and proper loading of the transmission cell to avoid contamination and ensure representative sampling.
• Interference Management: Addresses potential analytical interferences from water, soot, ester-based compounds, or additive packages that may impact specific measurements. Provides optional procedures, such as sample dilution, to improve accuracy when interferences are present.
• Normalization and Reporting: Specifies that all measurements should be normalized to a 0.100 mm pathlength for comparability, and details standard reporting requirements for spectral data.
• Instrument Performance and Quality Checks: Recommends periodic performance checks and spectral noise assessments to maintain data quality.

Applications

ASTM D7418-23 is essential for industries relying on petroleum and hydrocarbon-based lubricants, such as power generation, transportation, manufacturing, and heavy equipment operations.

Practical applications include:

• Predictive Maintenance: Early detection of oil degradation, contamination, or wear-related changes, enabling timely oil changes and avoiding unscheduled downtime.
• Machine Health Diagnostics: Monitoring trends in oxidation, nitration, soot, and contaminant levels allows for root cause analysis and targeted maintenance.
• Asset Reliability: By adhering to a global standard, companies ensure repeatable and accurate oil condition monitoring data, supporting high equipment reliability and reducing overall maintenance costs.
• Laboratory Consistency: Ensures that laboratories and field service providers can produce comparable FT-IR results, fostering confidence in oil analysis findings across different sites and equipment fleets.

Related Standards

ASTM D7418-23 references several complementary ASTM standards that support comprehensive oil condition monitoring and FT-IR spectrometry:

• ASTM D4057: Practice for Manual Sampling of Petroleum and Petroleum Products
• ASTM D4175: Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• ASTM D7414: Test Method for Condition Monitoring of Oxidation by FT-IR
• ASTM D7624: Test Method for Condition Monitoring of Nitration by FT-IR
• ASTM D7844: Test Method for Condition Monitoring of Soot by FT-IR
• ASTM E131: Terminology Relating to Molecular Spectroscopy
• ASTM E168: Practices for General Techniques of Infrared Quantitative Analysis
• ASTM E1421: Practice for Describing and Measuring Performance of FT-MIR Spectrometers
• ASTM E1866: Guide for Establishing Spectrophotometer Performance Tests

Keywords: in-service oil condition monitoring, FT-IR spectrometer, oil analysis, lubricants, oxidation, nitration, soot, fuel dilution, ASTM D7418, spectral normalization, maintenance diagnostics, predictive maintenance, asset reliability.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7418 − 23
Standard Practice for
Set-Up and Operation of Fourier Transform Infrared (FT-IR)
Spectrometers for In-Service Oil Condition Monitoring
This standard is issued under the fixed designation D7418; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This practice describes the instrument set-up and operation parameters for using FT-IR spectrom-
eters for in-service oil condition monitoring. The following parameters are typically monitored for
petroleum and hydrocarbon based lubricants: water, soot, oxidation, nitration, phosphate antiwear
additives, fuel dilution (gasoline or diesel), sulfate by-products and ethylene glycol. Measurement and
data interpretation parameters are standardized to allow operators of different FT-IR spectrometers to
obtain comparable results by employing the same techniques. Two approaches may be used to monitor
in-service oil samples by FT-IR spectrometry: (1) direct trend analysis and (2) differential (spectral
subtraction) trend analysis. The former involves measurements made directly on in-service oil
samples, whereas the latter involves measurements obtained after the spectrum of a reference oil has
been subtracted from the spectrum of the in-service oil being analyzed. Both of these approaches are
described in this practice, and it is up to the user to determine which approach is more appropriate.
1. Scope* 1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This practice covers the instrument set-up and operation
ization established in the Decision on Principles for the
parameters for using FT-IR spectrometers for in-service oil
Development of International Standards, Guides and Recom-
condition monitoring for both direct trend analysis and differ-
mendations issued by the World Trade Organization Technical
ential trend analysis approaches.
Barriers to Trade (TBT) Committee.
1.2 This practice describes how to acquire the FT-IR spec-
trum of an in-service oil sample using a standard transmission 2. Referenced Documents
cell and establishes maximum allowable spectral noise levels.
2.1 ASTM Standards:
1.3 Measurement and integrated parameters for individual D4057 Practice for Manual Sampling of Petroleum and
in-service oil condition monitoring components and param-
Petroleum Products
eters are not described in this practice and are described in their D4175 Terminology Relating to Petroleum Products, Liquid
respective test methods.
Fuels, and Lubricants
D7414 Test Method for Condition Monitoring of Oxidation
1.4 The values stated in SI units are to be regarded as
in In-Service Petroleum and Hydrocarbon Based Lubri-
standard. No other units of measurement are included in this
cants by Trend Analysis Using Fourier Transform Infrared
standard.
(FT-IR) Spectrometry
1.5 This standard does not purport to address all of the
D7624 Test Method for Condition Monitoring of Nitration in
safety concerns, if any, associated with its use. It is the
In-Service Petroleum and Hydrocarbon-Based Lubricants
responsibility of the user of this standard to establish appro-
by Trend Analysis Using Fourier Transform Infrared
priate safety, health, and environmental practices and deter-
(FT-IR) Spectrometry
mine the applicability of regulatory limitations prior to use.
D7844 Test Method for Condition Monitoring of Soot in
In-Service Lubricants by Trend Analysis using Fourier
1 Transform Infrared (FT-IR) Spectrometry
This practice is under the jurisdiction of ASTM Committee D02 on Petroleum
Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-
mittee D02.96.03 on FTIR Testing Practices and Techniques Related to In-Service
Lubricants. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2023. Published May 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2007. Last previous edition approved in 2022 as D7418 – 22. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D7418-23. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7418 − 23
E131 Terminology Relating to Molecular Spectroscopy in-service oil is taken after a short period of operation
E168 Practices for General Techniques of Infrared Quanti- following top-off of the lubricant (see 3.2.4.1) and is employed
tative Analysis thereafter as the reference oil.
E1421 Practice for Describing and Measuring Performance
4. Significance and Use
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
eters: Level Zero and Level One Tests
4.1 This practice describes to the end user how to collect the
E1866 Guide for Establishing Spectrophotometer Perfor-
FT-IR spectra of in-service oil samples for in-service oil
mance Tests
condition monitoring. Various in-service oil condition moni-
toring parameters, such as oxidation, nitration, soot, water,
3. Terminology
ethylene glycol, fuel dilution, gasoline dilution, sulfate by-
3.1 Definitions:
products and phosphate antiwear additives, can be measured by
3.1.1 For definitions of terms used in this standard, see FT-IR spectroscopy (4-7). Changes in the values of these
Terminology D4175.
parameters over operating time can then be used to help
3.1.2 For definitions of terms relating to infrared spectros- diagnose the operational condition of various machinery and
copy used in this practice, refer to Terminology E131.
equipment and to indicate when an oil change should take
3.1.3 Fourier transform infrared (FT-IR) spectrometry, place. This practice is intended to give a standardized configu-
n—form of infrared spectrometry in which an interferogram is
ration for FT-IR instrumentation and operating parameters
obtained; this interferogram is then subjected to a Fourier employed in in-service oil condition monitoring in order to
transform calculation to obtain an amplitude-wavenumber (or obtain comparable between-instrument and between-
wavelength) spectrum. laboratory data.
3.2 Definitions of Terms Specific to This Standard:
5. Interferences
3.2.1 condition monitoring, n—field of technical activity in
5.1 High levels of water contamination will interfere with
which selected physical parameters associated with an operat-
-1 -1
measurements in the ranges 3600 cm to 3000 cm and
ing machine are periodically or continuously sensed, measured
-1 -1
1690 cm to 1575 cm . This can affect values determined for
and recorded for the interim purpose of reducing, analyzing,
oxidation by Test Method D7414 and nitration by Test Method
comparing and displaying the data and information so obtained
D7624 as the baseline for both procedures are in this affected
and for the ultimate purpose of using interim result to support
region. Any calibration models using this region may also be
decisions related to the operation and maintenance of the
affected.
machine. (1, 2)
5.2 Soot directly affects the ability of the laser to penetrate
3.2.2 direct trend analysis, n—monitoring of the level and
the sample and will cause increasing sample baseline absor-
rate of change over operating time of measured parameters (2,
bance due to decreasing transmittance.
3) using the FT-IR spectrum of the in-service oil sample,
directly, without any spectral data manipulation such as spec-
NOTE 1—Refer to Test Method D7844 Fig. 1 for a demonstration of the
tral subtraction.
effect of increasing soot concentration on sample transmittance.
3.2.3 differential trend analysis, n—monitoring of the level
5.2.1 It is recommended to use caution when interpreting
and rate of change over operating time of measured parameters
measurements for all parameters in samples with calculated
using the FT-IR spectra of the in-service oil samples, following
soot concentrations exceeding 2.5 Absorbance values.
subtraction of the spectrum of the reference oil.
5.3 High levels of ester-based, polyols, glycols, and alco-
-1
3.2.4 in-service oil, n—lubricating oil that is present in a
hols interfere with measurements in the range 1260 cm to
-1
machine that has been at operating temperature for at least one
1000 cm . This can affect sulfation and phosphate anti-wear
hour.
measurements and any calibration models using this region.
3.2.4.1 Discussion—Sampling an in-service oil after a short
5.4 Various additive packages containing carbonyl com-
period of operation will allow for the measurement of a base
pounds (aldehydes, ketones, and esters) absorb in the range
point for trend analysis; the minimum sampling time should be
-1 -1
1745 cm to 1680 cm . Their presence can affect oxidation
at least one hour after oil change or topping-off.
and nitration measurements and any calibration models using
3.2.5 reference oil, n—sample of a lubricating oil whose
this region.
spectrum is subtracted from the spectrum of an in-service oil
5.4.1 Additive packages containing esters and carboxylic
for differential trend analysis.
acids, such as some dispersants, viscosity index improvers,
3.2.5.1 Discussion—The most commonly employed refer-
pour point depressants, and rust inhibitors, can give false
ence oil is a sample of the new oil. It should be noted, however,
positives for oxidation. In addition, oils mixed with any
that the continued use of the same reference oil after any
synthetic ester-based oil products will also give very high
top-off of lubricant may lead to erroneous conclusions, unless
values for oxidation. In some oils the contributions from
the added lubricant is from the same lot and drum as the
additive packages and synthetic ester-based oils may be so high
in-service oil. This possibility is averted if a sample of the
that oxidation cannot be reliably measured.
5.4.2 Additive packages containing detergents (sulfonates,
phenates and salicylate), dispersants, demulsifiers will interfere
The boldface numbers in parentheses refer to a list of references at the end of
this standard. with the sulfate by-products measurement.
D7418 − 23
5.5 It is recommended to trend the in-service oil against the spectra of later samples. On the basis of this report, KCl and
new oil to help identify any interference and the influence on KBr windows should not be used with samples of gasoline or
measurements. In some oils the contributions from additive natural gas engine oils as well as other types of lubricants
packages and synthetic ester based oils may be so high that the where nitration by-products may form due to the combustion
desired parameter cannot be reliably measured. process or other routes of nitration formation.
6.2.1.1 When ZnSe is used as the window material, the
5.6 When the baseline of a sample is raised (as occurs with
reflections of the infrared beam that occur at the inner faces of
high soot content) or the presence of an interferent creates an
the windows cause fringes to be superimposed on the oil
intense peak in the same region of the desired parameter, the
spectrum; these must be minimized using physical or compu-
influence of the interferent may be reduced through dilution of
tational techniques as presented in Appendix X1. Because KCl
the sample as described in Section 11 and Appendix X3. In
and KBr have lower refractive indices than ZnSe, the use of
those cases, the result is corrected by the dilution factor and
these window materials avoids observable fringes in the oil
reported.
spectrum.
5.6.1 If differential analysis is required, the reference oil
6.2.2 Cell Pathlength—The standard cell pathlength to be
must also be diluted with the same dilution factor and material
employed for in-service oil condition monitoring is 0.100 mm;
prior to analysis. This diluted reference is then used in the
however, in practical terms, pathlengths ranging from
subsequent differential calculation.
0.080 mm up to 0.120 mm are suitable, with values outside this
range leading to either poor sensitivity or non-linearity of
6. Apparatus
detector response, respectively. The actual cell pathlength
6.1 Fourier Transform Infrared (FT-IR) Spectrometer—All
obtained can be determined from the interference fringes in the
FT-IR instruments suitable for use in this practice must be
spectrum recorded with an empty cell or by recording the
configured with a source, beamsplitter and detector suitable for
spectrum of a check fluid; details for calculating cell pathlength
-1
spectral acquisition over the mid-infrared range of 4000 cm
are presented in Appendix X2. The reporting units of the
-1
to 550 cm . FT-IR spectrometer’s IR source and interferom-
various in-service oil condition monitoring parameter test
eter should be in a sealed compartment to prevent harmful,
methods are based on a pathlength of 0.100 mm (see the
flammable, or explosive vapors from reaching the IR source
respective test methods). Accordingly, all data must be normal-
and air-cooled source.
ized to a pathlength of 0.100 mm, either by multiplying all data
6.1.1 Detectors—The standard configuration of detectors
points in the absorption spectra by a pathlength correction
include a room temperature deuterated triglycine sulfate
factor (spectral normalization) or by multiplying the results of
(DTGS), Silicon (Si), indium gallium arsenide (InGaAs),
the respective test methods by a pathlength correction factor
indium antimonide (InSb), lithium tantalate (LiTaO ), comple-
(see 12.2). The normalization procedure is usually part of the
mentary metal-oxide semiconductor (CMOS) array/linear vari-
software provided by instrument manufacturers.
able filters (LVF) or photoacoustic detectors.
6.2.2.1 Discussion—However, if sample dilution is em-
NOTE 2—Photoconductive detectors such as mercury cadmium telluride ployed (see Appendix X3), longer pathlengths may become
(MCT) should not be used owing to inadequate linearity of the detector
suitable. For example, for dilution with odorless mineral spirits
response.
(OMS) in a 2:1 OMS:oil sample ratio, a pathlength of
6.1.2 Beamsplitters—Beamsplitters can include potassium 0.200 mm has proven suitable.
bromide (KBr), germanium-coated potassium bromide (Ge/
NOTE 3—For purposes of interlaboratory comparison of results, spectral
KBr), cesium iodide (CsI), or zinc selenide (ZnSe).
normalization should be performed.
6.2 Sample Cell—The sample cell employed for in-service
6.3 Filter (optional)—The use of a particulate filter with a
oil condition monitoring is a transmission cell with a fixed
mesh size of 0.100 mm or less to trap any large particles
pathlength that can be inserted in the optical path of the FT-IR
present in the sample is strongly recommended to prevent cell
spectrometer. Cell window material and cell pathlength con-
clogging.
siderations are stated below.
6.4 Sample Pumping System (optional)—A pumping system
6.2.1 Cell Window Material—ZnSe is commonly used as
capable of transporting oil to be analyzed into the transmission
the window material for condition monitoring and is recom-
cell and of emptying and flushing the cell with solvent between
mended because of its resistance to water. Sample cells
samples may be used instead of manual cell loading. Commer-
constructed of materials other than ZnSe may be used;
cial vendors offer various pumping systems that may differ in
however, to address all the various methods associated with
the type of pump, tubing, and transmission cell. Depending on
condition monitoring, the window material should transmit IR
-1 -1
the sample handling system employed and the viscosity of the
radiation over the range of 4000 cm to 550 cm . KCl and
oils analyzed, a wash/rinsing solvent may be run between
KBr are common cell window materials that meet this require-
samples to minimize sample-to-sample carryover as well as
ment but these are water-soluble salts and should not be used
keep the cell and inlet tubing clean; commercial vendors may
if oil samples containing moisture are frequently run through
recommend specific solvent rinse protocols.
the cell, as contact with water will cause the windows to fog
and erode rapidly. In addition, Coates and Setti (8) have noted 6.4.1 Hydrocarbon Leak Alarm—When a sample pumping
that oil nitration products can react with KCl and KBr system is used, an independent flammable vapor sensor and
windows, depositing compounds that are observed in the alarm system is strongly recommended The purpose of this
D7418 − 23
alarm system is to alert the operator when a leak occurs in the significant changes may imply wear or contamination on the
tubing, connectors or transmission cell. cell windows and should prompt remedial action. To serve as
a check fluid, a solvent must have consistent spectral charac-
7. FT-IR Spectral Acquisition Parameters
teristics (lot-to-lot) and a measurable (on-scale) IR absorption
band for cell pathlength calculation; for more details, see X2.2.
7.1 The spectral acquisition parameters are specified below.
One IR manufacturer uses heptane, another uses OMS, and
Because the spectral resolution, data spacing, and apodization
4,5
other commercial products are available.
affect the FT-IR spectral band shapes, these specifications must
be adhered to:
10. Procedure for Collecting FT-IR Spectra
-1
Spectral resolution: 4 cm
-1
10.1 Background Collection—Collect a single-beam back-
Data spacing: 2 cm
ground spectrum at the beginning of each run and frequently
Apodization: Triangular
-1 -1
enough thereafter such that changes in atmospheric water
Scanning range: 4000 cm to 550 cm
vapor levels and other changing ambient conditions do not
Spectral format: Absorbance as a function of wavenumber
significantly affect the sample results (for example, every
7.2 The number of scans co-added and hence the scan time
30 min). Four methods may be used to collect single-beam
will depend on the desired spectral noise level (see Section 14),
background spectra: (1) collecting an air (open-beam) back-
whereby an increase in scan time by a factor of N will decrease
ground spectrum, (2) collecting a cell background spectrum,
1/2
the level of noise by a factor of N .
(3) collecting an air (open-beam) background spectrum and a
cell reference spectrum, or (4) air background with corrected
8. Sampling
cell reference. The background spectrum shall be acquired
8.1 Sample Acquisition—The objective of sampling is to
using the operating parameters specified in 7.1.
obtain a test specimen that is representative of the entire
NOTE 4—It should be noted that changes in atmospheric conditions,
quantity. Thus, laboratory samples should be taken in accor-
such as humidity and temperature, can change the background spectrum.
dance with the instructions in Practice D4057.
The frequency of background checks shall be determined by the individual
laboratory.
8.2 Sample Preparation—Filtering the sample using a filter
described in 6.3 prior to loading the cell with the sample is
10.1.1 Air Background—Collect a single-beam background
highly recommended. An exception to this recommendation spectrum with no cell in the sample compartment.
may be made when oil samples are diluted (see Appendix X3).
10.1.2 Cell Background—Collect a single-beam cell back-
ground with the clean empty cell in the sample compartment.
9. Preparation and Maintenance of Apparatus
10.1.2.1 To use an empty cell background, either physical or
computational fringe reduction methods (see Appendix X1)
9.1 Rinsing, Washing and Check Solvents—A variety of
must be employed so as to reduce the superimposition of
hydrophobic solvents may be used to clean the cell and rinse
fringes from the spectrum of the empty cell onto the sample
the lines between samples as well as serving as a check fluid to
absorption spectrum.
monitor pathlength. Typical solvents include hexanes,
10.1.2.2 To verify that the cell is empty and clean, an
cyclohexane, heptane or odorless mineral spirits (OMS).
absorption spectrum of the empty cell should be collected
Health and safety issues on using, storing, and disposing of
using a previously collected or archived single-beam air
check or cleaning/wash solvents will not be covered here.
(open-beam) spectrum as the background spectrum. Measure
Local regulations and Material Safety Data Sheets (MSDS)
-1 -1
the maximum peak height between 3000 cm and 2800 cm
should be consulted.
-1
relative to a baseline at 2700 cm . If this value is <0.2
9.2 Sample Cell and Inlet Filter—The cell should be flushed
absorbance units, then the cell is adequately clean for recording
with the designated rinse/wash solvent at the start and end of
an empty cell background. This spectrum may also be used to
analytical runs to clean the cell. Immediately following flush-
verify that the fringe reduction technique employed meets the
ing of the cell, an absorption spectrum of the empty cell (see
criterion of sample spectral peak-to-peak noise (see Section
10.1.2.2) should be recorded to check for build-up of material
14).
on the cell windows. If an inlet filter is used, the filter shall also
10.1.3 Air Background with Cell Reference—Collect a
be checked for particle build-up and its effect on sample flow
single-beam background spectrum with no cell in the sample
rate.
compartment. Obtain a cell reference spectrum by collecting a
9.3 Check Fluid and Pathlength Monitoring—The purpose
single-beam empty cell background spectrum, according to the
of a check fluid is to verify proper operation of the FT-IR
procedure outlined in 10.1.2, and ratioing it against the newly
spectrometer/transmission cell combination, as well as any
acquired air background spectrum to give the absorption
associated sample introduction and cleaning hardware. It is
spectrum of the empty cell. This absorption spectrum is then
recommended that an absorption spectrum of the check fluid be
subtracted in a 1:1 ratio from the absorption spectra of the
recorded when a new or re-assembled cell is initially used and
samples collected using an air background.
archived to disk as a reference spectrum against which subse-
10.1.4 Air Background with Corrected Cell Reference—
quent spectra of the check fluid may be compared. The
Collect a cell reference absorbance spectrum according to the
spectrum of the check fluid may also be used to calculate the
pathlength of the sample cell to normalize all data to 0.100 mm
Conostan Division, Conoco Inc. 1000 South Pine, Ponca City, OK 74602-1267.
and to monitor changes in the cell pathlength over time, where Thermal-Lube Inc., 255 avenue Labrosse, Pointe-Claire, QC H9R 1A3.
D7418 − 23
procedure outlined in 10.1.3. If any visible interference fring- Oxidation 5 130*
ing is present after fringe-reduction methods have been
Absorbance
average Absorbance v 2 BL v !
~ ~ ! ~ !
F G
employed, smooth the spectrum by convolution with a trian-
$ % 0.100 mm
1670→1800
gular function having full width at half-height (that is, FWHH)
(4)
-1
of 33 cm and unit area, or by an equivalent procedure. To
10.3.4 Baseline C
...

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Designation: D7418 − 22 D7418 − 23
Standard Practice for
Set-Up and Operation of Fourier Transform Infrared (FT-IR)
Spectrometers for In-Service Oil Condition Monitoring
This standard is issued under the fixed designation D7418; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This practice describes the instrument set-up and operation parameters for using FT-IR spectrom-
eters for in-service oil condition monitoring. The following parameters are typically monitored for
petroleum and hydrocarbon based lubricants: water, soot, oxidation, nitration, phosphate antiwear
additives, fuel dilution (gasoline or diesel), sulfate by-products and ethylene glycol. Measurement and
data interpretation parameters are standardized to allow operators of different FT-IR spectrometers to
obtain comparable results by employing the same techniques. Two approaches may be used to monitor
in-service oil samples by FT-IR spectrometry: (1) direct trend analysis and (2) differential (spectral
subtraction) trend analysis. The former involves measurements made directly on in-service oil
samples, whereas the latter involves measurements obtained after the spectrum of a reference oil has
been subtracted from the spectrum of the in-service oil being analyzed. Both of these approaches are
described in this practice, and it is up to the user to determine which approach is more appropriate.
1. Scope*
1.1 This practice covers the instrument set-up and operation parameters for using FT-IR spectrometers for in-service oil condition
monitoring for both direct trend analysis and differential trend analysis approaches.
1.2 This practice describes how to acquire the FT-IR spectrum of an in-service oil sample using a standard transmission cell and
establishes maximum allowable spectral noise levels.
1.3 Measurement and integrated parameters for individual in-service oil condition monitoring components and parameters are not
described in this practice and are described in their respective test methods.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This practice is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.96.03 on FTIR Testing Practices and Techniques Related to In-Service Lubricants.
Current edition approved Oct. 1, 2022May 1, 2023. Published October 2022May 2023. Originally approved in 2007. Last previous edition approved in 20212022 as D7418
– 21. DOI: 10.1520/D7418-22.22. DOI: 10.1520/D7418-23.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7418 − 23
2. Referenced Documents
2.1 ASTM Standards:
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D7414 Test Method for Condition Monitoring of Oxidation in In-Service Petroleum and Hydrocarbon Based Lubricants by
Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry
D7624 Test Method for Condition Monitoring of Nitration in In-Service Petroleum and Hydrocarbon-Based Lubricants by Trend
Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry
D7844 Test Method for Condition Monitoring of Soot in In-Service Lubricants by Trend Analysis using Fourier Transform
Infrared (FT-IR) Spectrometry
E131 Terminology Relating to Molecular Spectroscopy
E168 Practices for General Techniques of Infrared Quantitative Analysis
E1421 Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level
Zero and Level One Tests
E1866 Guide for Establishing Spectrophotometer Performance Tests
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this standard, see Terminology D4175.
3.1.2 For definitions of terms relating to infrared spectroscopy used in this practice, refer to Terminology E131.
3.1.3 Fourier transform infrared (FT-IR) spectrometry, n—form of infrared spectrometry in which an interferogram is obtained;
this interferogram is then subjected to a Fourier transform calculation to obtain an amplitude-wavenumber (or wavelength)
spectrum.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 condition monitoring, n—field of technical activity in which selected physical parameters associated with an operating
machine are periodically or continuously sensed, measured and recorded for the interim purpose of reducing, analyzing, comparing
and displaying the data and information so obtained and for the ultimate purpose of using interim result to support decisions related
to the operation and maintenance of the machine. (1, 2)
3.2.2 direct trend analysis, n—monitoring of the level and rate of change over operating time of measured parameters (2, 3) using
the FT-IR spectrum of the in-service oil sample, directly, without any spectral data manipulation such as spectral subtraction.
3.2.3 differential trend analysis, n—monitoring of the level and rate of change over operating time of measured parameters using
the FT-IR spectra of the in-service oil samples, following subtraction of the spectrum of the reference oil.
3.2.4 in-service oil, n—lubricating oil that is present in a machine that has been at operating temperature for at least one hour.
3.2.4.1 Discussion—
Sampling an in-service oil after a short period of operation will allow for the measurement of a base point for trend analysis; the
minimum sampling time should be at least one hour after oil change or topping-off.
3.2.5 reference oil, n—sample of a lubricating oil whose spectrum is subtracted from the spectrum of an in-service oil for
differential trend analysis.
3.2.5.1 Discussion—
The most commonly employed reference oil is a sample of the new oil. It should be noted, however, that the continued use of the
same reference oil after any top-off of lubricant may lead to erroneous conclusions, unless the added lubricant is from the same
lot and drum as the in-service oil. This possibility is averted if a sample of the in-service oil is taken after a short period of operation
following top-off of the lubricant (see 3.2.4.1) and is employed thereafter as the reference oil.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The boldface numbers in parentheses refer to a list of references at the end of this standard.
D7418 − 23
4. Significance and Use
4.1 This practice describes to the end user how to collect the FT-IR spectra of in-service oil samples for in-service oil condition
monitoring. Various in-service oil condition monitoring parameters, such as oxidation, nitration, soot, water, ethylene glycol, fuel
dilution, gasoline dilution, sulfate by-products and phosphate antiwear additives, can be measured by FT-IR spectroscopy (4-7).
Changes in the values of these parameters over operating time can then be used to help diagnose the operational condition of
various machinery and equipment and to indicate when an oil change should take place. This practice is intended to give a
standardized configuration for FT-IR instrumentation and operating parameters employed in in-service oil condition monitoring in
order to obtain comparable between-instrument and between-laboratory data.
5. Interferences
-1 -1 -1
5.1 High levels of water contamination will interfere with measurements in the ranges 3600 cm to 3000 cm and 1690 cm to
-1
1575 cm . This can affect values determined for oxidation by Test Method D7414 and nitration by Test Method D7624 as the
baseline for both procedures are in this affected region. Any calibration models using this region may also be affected.
5.2 Soot directly affects the ability of the laser to penetrate the sample and will cause increasing sample baseline absorbance due
to decreasing transmittance.
NOTE 1—Refer to Test Method D7844 Fig. 1 for a demonstration of the effect of increasing soot concentration on sample transmittance.
5.2.1 It is recommended to use caution when interpreting measurements for all parameters in samples with calculated soot
concentrations exceeding 2.5 Absorbance values.
-1 -1
5.3 High levels of ester-based, polyols, glycols, and alcohols interfere with measurements in the range 1260 cm to 1000 cm .
This can affect sulfation and phosphate anti-wear measurements and any calibration models using this region.
-1
5.4 Various additive packages containing carbonyl compounds (aldehydes, ketones, and esters) absorb in the range 1745 cm to
-1
1680 cm . Their presence can affect oxidation and nitration measurements and any calibration models using this region.
5.4.1 Additive packages containing esters and carboxylic acids, such as some dispersants, viscosity index improvers, pour point
depressants, and rust inhibitors, can give false positives for oxidation. In addition, oils mixed with any synthetic ester-based oil
products will also give very high values for oxidation. In some oils the contributions from additive packages and synthetic
ester-based oils may be so high that oxidation cannot be reliably measured.
5.4.2 Additive packages containing detergents (sulfonates, phenates and salicylate), dispersants, demulsifiers will interfere with
the sulfate by-products measurement.
5.5 It is recommended to trend the in-service oil against the new oil to help identify any interference and the influence on
measurements. In some oils the contributions from additive packages and synthetic ester based oils may be so high that the desired
parameter cannot be reliably measured.
5.6 When the baseline of a sample is raised (as occurs with high soot content) or the presence of an interferent creates an intense
peak in the same region of the desired parameter, the influence of the interferent may be reduced through dilution of the sample
as described in Section 11 and Appendix X3. In those cases, the result is corrected by the dilution factor and reported.
5.6.1 If differential analysis is required, the reference oil must also be diluted with the same dilution factor and material prior to
analysis. This diluted reference is then used in the subsequent differential calculation.
6. Apparatus
6.1 Fourier Transform Infrared (FT-IR) Spectrometer—All FT-IR instruments suitable for use in this practice must be configured
-1 -1
with a source, beamsplitter and detector suitable for spectral acquisition over the mid-infrared range of 4000 cm to 550 cm .
FT-IR spectrometer’s IR source and interferometer should be in a sealed compartment to prevent harmful, flammable, or explosive
vapors from reaching the IR source and air-cooled source.
D7418 − 23
6.1.1 Detectors—The standard configuration of detectors include a room temperature deuterated triglycine sulfate (DTGS), Silicon
(Si), indium gallium arsenide (InGaAs), indium antimonide (InSb), lithium tantalate (LiTaO ), complementary metal-oxide
semiconductor (CMOS) array/linear variable filters (LVF) or photoacoustic detectors.
NOTE 2—Photoconductive detectors such as mercury cadmium telluride (MCT) should not be used owing to inadequate linearity of the detector response.
6.1.2 Beamsplitters—Beamsplitters can include potassium bromide (KBr), germanium-coated potassium bromide (Ge/KBr),
cesium iodide (CsI), or zinc selenide (ZnSe).
6.2 Sample Cell—The sample cell employed for in-service oil condition monitoring is a transmission cell with a fixed pathlength
that can be inserted in the optical path of the FT-IR spectrometer. Cell window material and cell pathlength considerations are
stated below.
6.2.1 Cell Window Material—ZnSe is commonly used as the window material for condition monitoring and is recommended
because of its resistance to water. Sample cells constructed of materials other than ZnSe may be used; however, to address all the
various methods associated with condition monitoring, the window material should transmit IR radiation over the range of
-1 -1
4000 cm to 550 cm . KCl and KBr are common cell window materials that meet this requirement but these are water-soluble
salts and should not be used if oil samples containing moisture are frequently run through the cell, as contact with water will cause
the windows to fog and erode rapidly. In addition, Coates and Setti (8) have noted that oil nitration products can react with KCl
and KBr windows, depositing compounds that are observed in the spectra of later samples. On the basis of this report, KCl and
KBr windows should not be used with samples of gasoline or natural gas engine oils as well as other types of lubricants where
nitration by-products may form due to the combustion process or other routes of nitration formation.
6.2.1.1 When ZnSe is used as the window material, the reflections of the infrared beam that occur at the inner faces of the windows
cause fringes to be superimposed on the oil spectrum; these must be minimized using physical or computational techniques as
presented in Appendix X1. Because KCl and KBr have lower refractive indices than ZnSe, the use of these window materials
avoids observable fringes in the oil spectrum.
6.2.2 Cell Pathlength—The standard cell pathlength to be employed for in-service oil condition monitoring is 0.100 mm; however,
in practical terms, pathlengths ranging from 0.080 mm up to 0.120 mm are suitable, with values outside this range leading to either
poor sensitivity or non-linearity of detector response, respectively. The actual cell pathlength obtained can be determined from the
interference fringes in the spectrum recorded with an empty cell or by recording the spectrum of a check fluid; details for
calculating cell pathlength are presented in Appendix X2. The reporting units of the various in-service oil condition monitoring
parameter test methods are based on a pathlength of 0.100 mm (see the respective test methods). Accordingly, all data must be
normalized to a pathlength of 0.100 mm, either by multiplying all data points in the absorption spectra by a pathlength correction
factor (spectral normalization) or by multiplying the results of the respective test methods by a pathlength correction factor (see
12.2). The normalization procedure is usually part of the software provided by instrument manufacturers.
6.2.2.1 Discussion—However, if sample dilution is employed (see Appendix X3), longer pathlengths may become suitable. For
example, for dilution with odorless mineral spirits (OMS) in a 2:1 OMS:oil sample ratio, a pathlength of 0.200 mm has proven
suitable.
NOTE 3—For purposes of interlaboratory comparison of results, spectral normalization should be performed.
6.3 Filter (optional)—The use of a particulate filter with a mesh size of 0.100 mm or less to trap any large particles present in the
sample is strongly recommended to prevent cell clogging.
6.4 Sample Pumping System (optional)—A pumping system capable of transporting oil to be analyzed into the transmission cell
and of emptying and flushing the cell with solvent between samples may be used instead of manual cell loading. Commercial
vendors offer various pumping systems that may differ in the type of pump, tubing, and transmission cell. Depending on the sample
handling system employed and the viscosity of the oils analyzed, a wash/rinsing solvent may be run between samples to minimize
sample-to-sample carryover as well as keep the cell and inlet tubing clean; commercial vendors may recommend specific solvent
rinse protocols.
D7418 − 23
6.4.1 Hydrocarbon Leak Alarm—When a sample pumping system is used, an independent flammable vapor sensor and alarm
system is strongly recommended The purpose of this alarm system is to alert the operator when a leak occurs in the tubing,
connectors or transmission cell.
7. FT-IR Spectral Acquisition Parameters
7.1 The spectral acquisition parameters are specified below. Because the spectral resolution, data spacing, and apodization affect
the FT-IR spectral band shapes, these specifications must be adhered to:
-1
Spectral resolution: 4 cm
-1
Data spacing: 2 cm
Apodization: Triangular
-1 -1
Scanning range: 4000 cm to 550 cm
Spectral format: Absorbance as a function of wavenumber
7.2 The number of scans co-added and hence the scan time will depend on the desired spectral noise level (see Section 14),
1/2
whereby an increase in scan time by a factor of N will decrease the level of noise by a factor of N .
8. Sampling
8.1 Sample Acquisition—The objective of sampling is to obtain a test specimen that is representative of the entire quantity. Thus,
laboratory samples should be taken in accordance with the instructions in Practice D4057.
8.2 Sample Preparation—Filtering the sample using a filter described in 6.3 prior to loading the cell with the sample is highly
recommended. An exception to this recommendation may be made when oil samples are diluted (see Appendix X3).
9. Preparation and Maintenance of Apparatus
9.1 Rinsing, Washing and Check Solvents—A variety of hydrophobic solvents may be used to clean the cell and rinse the lines
between samples as well as serving as a check fluid to monitor pathlength. Typical solvents include hexanes, cyclohexane, heptane
or odorless mineral spirits (OMS). Health and safety issues on using, storing, and disposing of check or cleaning/wash solvents
will not be covered here. Local regulations and Material Safety Data Sheets (MSDS) should be consulted.
9.2 Sample Cell and Inlet Filter—The cell should be flushed with the designated rinse/wash solvent at the start and end of
analytical runs to clean the cell. Immediately following flushing of the cell, an absorption spectrum of the empty cell (see 10.1.2.2)
should be recorded to check for build-up of material on the cell windows. If an inlet filter is used, the filter shall also be checked
for particle build-up and its effect on sample flow rate.
9.3 Check Fluid and Pathlength Monitoring—The purpose of a check fluid is to verify proper operation of the FT-IR
spectrometer/transmission cell combination, as well as any associated sample introduction and cleaning hardware. It is
recommended that an absorption spectrum of the check fluid be recorded when a new or re-assembled cell is initially used and
archived to disk as a reference spectrum against which subsequent spectra of the check fluid may be compared. The spectrum of
the check fluid may also be used to calculate the pathlength of the sample cell to normalize all data to 0.100 mm and to monitor
changes in the cell pathlength over time, where significant changes may imply wear or contamination on the cell windows and
should prompt remedial action. To serve as a check fluid, a solvent must have consistent spectral characteristics (lot-to-lot) and
a measurable (on-scale) IR absorption band for cell pathlength calculation; for more details, see X2.2. One IR manufacturer uses
4,5
heptane, another uses OMS, and other commercial products are available.
10. Procedure for Collecting FT-IR Spectra
10.1 Background Collection—Collect a single-beam background spectrum at the beginning of each run and frequently enough
thereafter such that changes in atmospheric water vapor levels and other changing ambient conditions do not significantly affect
the sample results (for example, every 30 min). Four methods may be used to collect single-beam background spectra: (1)
collecting an air (open-beam) background spectrum, (2) collecting a cell background spectrum, (3) collecting an air (open-beam)
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D7418 − 23
background spectrum and a cell reference spectrum, or (4) air background with corrected cell reference. The background spectrum
shall be acquired using the operating parameters specified in 7.1.
NOTE 4—It should be noted that changes in atmospheric conditions, such as humidity and temperature, can change the background spectrum. The
frequency of background checks shall be determined by the individual laboratory.
10.1.1 Air Background—Collect a single-beam background spectrum with no cell in the sample compartment.
10.1.2 Cell Background—Collect a single-beam cell background with the clean empty cell in the sample compartment.
10.1.2.1 To use an empty cell background, either physical or computational fringe reduction methods (see Appendix X1) must be
employed so as to reduce the superimposition of fringes from the spectrum of the empty cell onto the sample absorption spectrum.
10.1.2.2 To verify that the cell is empty and clean, an absorption spectrum of the empty cell should be collected using a previously
collected or archived single-beam air (open-beam) spectrum as the background spectrum. Measure the maximum peak height
-1 -1 -1
between 3000 cm and 2800 cm relative to a baseline at 2700 cm . If this value is <0.2 absorbance units, then the cell is
adequately clean for recording an empty cell background. This spectrum may also be used to verify that the fringe reduction
technique employed meets the criterion of sample spectral peak-to-peak noise (see Section 14).
10.1.3 Air Background with Cell Reference—Collect a single-beam background spectrum with no cell in the sample compartment.
Obtain a cell reference spectrum by collecting a single-beam empty cell background spectrum, according to the procedure outlined
in 10.1.2, and ratioing it against the newly acquired air background spectrum to give the absorption spectrum of the empty cell.
This absorption spectrum is then subtracted in a 1:1 ratio from the absorption spectra of the samples collected using an air
background.
10.1.4 Air Background with Corrected Cell Reference—Collect a cell reference absorbance spectrum according to the procedure
outlined in 10.1.3. If any visible interference fringing is present after fringe-reduction methods have been employed, smooth the
-1
spectrum by convolution with a triangular function having full width at half-height (that is, FWHH) of 33 cm and unit area, or
by an equivalent procedure. To account for the increase in reflection losses seen with an empty cell, subtract 0.06
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