ASTM D6481-14(2019)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D6481 − 14 (Reapproved 2019)
Standard Test Method for
Determination of Phosphorus, Sulfur, Calcium, and Zinc in
Lubrication Oils by Energy Dispersive X-ray Fluorescence
Spectroscopy
This standard is issued under the fixed designation D6481; 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.
1. Scope fluorescent intensities of phosphorus, sulfur, calcium, and zinc.
Other regions of the spectrum are measured to compensate for
1.1 This test method covers the quantitative determination
varying background. If the detector does not completely
of additive elements in unused lubricating oils, as shown in
resolve all the elements in a single measurement, then to
Table 1.
improve selectivity, there is a combination of sequential and
1.2 This test method is limited to the use of energy
simultaneous measurements employing primary and secondary
dispersive X-ray fluorescence (EDXRF) spectrometers em-
beam filters. There can be correction of measured intensities
ploying an X-ray tube for excitation in conjunction with the
for spectral overlap. Concentrations of the elements of interest
ability to separate the signals of adjacent elements.
are determined by comparison of these intensities against a
1.3 This test method uses interelement correction factors calibration curve using empirical interelement correction fac-
tors and ratio to backscatter.
calculated from empirical calibration data.
1.4 This test method is not suitable for the determination of 2.2 The EDXRF spectrometer is initially calibrated using a
set of prepared standards to collect the necessary intensity data.
magnesium and copper at the concentrations present in lubri-
cating oils. Each calibration line and any correction coefficient are ob-
tained by a regression of this data, using the program supplied
1.5 This test method excludes lubricating oils that contain
with the spectrometer.
chlorine or barium as an additive element.
1.6 This test method can be used by persons who are not
3. Significance and Use
skilled in X-ray spectrometry. It is intended to be used as a
3.1 Some oils are formulated with organo-metallic
routine test method for production control analysis.
additives, which act, for example, as detergents, antioxidants,
1.7 This standard does not purport to address all of the
and antiwear agents. Some of these additives contain one or
safety concerns, if any, associated with its use. It is the
more of these elements: calcium, phosphorus, sulfur, and zinc.
responsibility of the user of this standard to establish appro-
This test method provides a means of determining the concen-
priate safety, health, and environmental practices and deter-
trations of these elements, which in turn provides an indication
mine the applicability of regulatory limitations to use.
of the additive content of these oils.
1.8 This international standard was developed in accor-
3.2 Several additive elements and their compounds are
dance with internationally recognized principles on standard-
added to the lubricating oils to give beneficial performance
ization established in the Decision on Principles for the
(Table 2).
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical 3.3 This test method is primarily intended to be used at a
manufacturing location for monitoring of additive elements in
Barriers to Trade (TBT) Committee.
lubricating oils. It can also be used in central and research
2. Summary of Test Method
laboratories.
2.1 A specimen is placed in the X-ray beam, and the
4. Interferences
appropriate regions of its spectrum are measured to give the
4.1 The additive elements found in lubricating oils will
affect the measured intensities from the elements of interest to
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
a varying degree. In general, for lubricating oils, the
Subcommittee D02.03 on Elemental Analysis.
X-radiation emitted by the element of interest can be absorbed
Current edition approved May 1, 2019. Published July 2019. Originally approved
by itself (self-absorption) or by the other elements present in
in 1999. Last previous edition approved in 2014 as D6481 – 14. DOI: 10.1520/
D6481-14R19. the sample matrix. Also the X-radiation emitted from one
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6481 − 14 (2019)
TABLE 1 Elements and Range of Concentrations Determined
5.2 Sample Cells, providing a depth of at least 6 mm and
Element Concentration Range equipped with replaceable X-ray transparent film. Suitable
Phosphorus 0.02 mass % to 0.3 mass % films include polypropylene and polycarbonate with thickness
Sulfur 0.05 mass % to 1.0 mass %
from 3.5 µm to 8 µm.
Calcium 0.02 mass % to 1.0 mass %
Zinc 0.01 mass % to 0.3 mass %
5.3 Instrument Setting-Up Samples (Elemental Reference
Samples), to quantify spectral overlaps. These are required
when the instrument’s software does not include reference
spectra to deconvolute spectra.
5.4 Drift Correction Monitors, to correct for instrumental
element can further excite (enhance) another element. These
drift. At least two samples are necessary to correct both
interelement effects are significant at concentrations varying
sensitivity and baseline drifts. For each element and scatter
from 0.03 mass %, due to the heavier elements, to 1 mass %,
region, there shall be one providing a count rate similar to
for the lighter elements. Enhancement effects can be minimised
samples from the upper end of the calibration and another
by selective excitation. The measured concentration for a given
providing a count rate as if from a blank. This last can be a
element can be mathematically corrected for self-absorption
blank oil. For the high concentration of each element, a glass
and for interelement effects by other elements present in the
disk, XRF fusion bead, or pressed pellet have all been found to
sample matrix. If an element is present at significant concen-
be satisfactory. They can be the same samples as in 5.3.
trations and an inter-element correction for that element is not
employed, the results can be low due to absorption or high due
6. Reagents and Materials
to enhancement.
6.1 Reagent grade chemicals shall be used in all tests.
4.2 If a sample containing barium as an additive above
Unless otherwise indicated, it is intended that all reagents
0.03 % by mass is measured against a calibration derived from
conform to the specifications of the Committee on Analytical
standards without barium, then results will be low.
Reagents of the American Chemical Society where such
specifications are available. Other grades can be used, pro-
4.3 If a sample containing chlorine as an impurity above
vided it is first ascertained that the reagent is of sufficiently
0.03 % by mass is measured against a calibration derived from
high purity to permit its use without lessening the accuracy of
standards without chlorine, then the results can be affected.
the determination.
4.4 There can be spectral overlap of one element onto
6.2 Helium, at least 99.5 % purity, for the optical path of the
another, especially for phosphorus on sulfur, and the instru-
spectrometer.
ment must include correction procedures for any such overlaps.
6.3 Diluent Solvent, a suitable solvent free of metals,
phosphorus, and chlorine, and containing less than 10 ppm of
5. Apparatus
sulfur (for example, deodorized kerosene, white oil, or mineral
5.1 Energy Dispersive X-ray Fluorescent Analyzer—Any
oil).
energy dispersive X-ray fluorescent analyzer can be used if its
6.4 Calibration Standard Materials:
design incorporates at least the following features.
6.4.1 Certified concentration solutions, of liquid organo-
5.1.1 Source of X-ray Excitation, X-ray tube with
metallic salts, each containing calcium or zinc, or both. The
palladium, silver, or rhodium target programmable between
solutions shall be sulfur free or the certificate shall state the
4 keV and at least 25 keV for preferential excitation to simplify
concentration of sulfur. Alternatively, the following standard
the sample spectra. (Warning—Operation of an analyzer using
materials can be used.
an X-ray tube source is to be conducted in accordance with the
6.4.1.1 Calcium 2-Ethylhexanoate, approximately 12.3 %
manufacturer’s safety instructions and federal, state, and local
by mass calcium, with a certified value.
regulations.)
6.4.1.2 Zinc Cyclohexanebutyrate, approximately 16.2 %
5.1.2 X-ray Detector, gas filled proportional counter with
by mass zinc, with a certified value.
high sensitivity and a resolution value not to exceed 1300 eV at
6.4.2 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity
5.9 keV.
(9.62 % by mass phosphorus).
NOTE 1—The limited data from instruments with solid state detectors in
6.4.3 Di-n-butyl Sulfide, 97 % purity, (21.9 % by mass
the interlaboratory precision study did not support their inclusion in this
sulfur).
test method.
6.4.4 Stabilizers, 2-ethylhexanoic acid, 2-ethylamine. Also,
5.1.3 Primary Beam Filters, to make the excitation more
proprietary stabilizer/chelating solutions are available commer-
selective.
cially. Stabilizers shall be free of the additive element.
5.1.4 Secondary Beam Filters—When a proportional coun-
ter is used, these are necessary as a means of discriminating
Reagent Chemicals, American Chemical Society Specifications, American
between an analyte’s X rays and other analytes and the
Chemical Society, Washington, D.C. For suggestions on the testing of reagents not
spectrum from the X-ray tube.
listed by the American Chemical Society, see Analar Standards for Laboratory
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
5.1.5 Multi-Channel Analyzer, for discrimination between
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
an analyte’s X rays and background X rays.
MD.
5.1.6 Optional Helium Purgeable Optical Path. These certified concentrated solutions are commercially available.
D6481 − 14 (2019)
TABLE 2 Lubricants and Additive Materials
Element Compounds Purpose/Application
Calcium Sulfonates, phenates Detergent inhibitors, dispersants
Phosphorus Dithiophosphates, phosphates phosphites Anti-rusting agents, extreme pressure additives, anti-wear
Sulfur Base oils, sulfonates, thiophosphates, polysulfides a
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6481 − 14 D6481 − 14 (Reapproved 2019)
Standard Test Method for
Determination of Phosphorus, Sulfur, Calcium, and Zinc in
Lubrication Oils by Energy Dispersive X-ray Fluorescence
Spectroscopy
This standard is issued under the fixed designation D6481; 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.
1. Scope*Scope
1.1 This test method covers the quantitative determination of additive elements in unused lubricating oils, as shown in Table
1.
1.2 This test method is limited to the use of energy dispersive X-ray fluorescence (EDXRF) spectrometers employing an X-ray
tube for excitation in conjunction with the ability to separate the signals of adjacent elements.
1.3 This test method uses interelement correction factors calculated from empirical calibration data.
1.4 This test method is not suitable for the determination of magnesium and copper at the concentrations present in lubricating
oils.
1.5 This test method excludes lubricating oils that contain chlorine or barium as an additive element.
1.6 This test method can be used by persons who are not skilled in X-ray spectrometry. It is intended to be used as a routine
test method for production control analysis.
1.7 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations to use.
1.8 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.
2. Summary of Test Method
2.1 A specimen is placed in the X-ray beam, and the appropriate regions of its spectrum are measured to give the fluorescent
intensities of phosphorus, sulfur, calcium, and zinc. Other regions of the spectrum are measured to compensate for varying
background. If the detector does not completely resolve all the elements in a single measurement, then to improve selectivity, there
is a combination of sequential and simultaneous measurements employing primary and secondary beam filters. There can be
correction of measured intensities for spectral overlap. Concentrations of the elements of interest are determined by comparison
of these intensities against a calibration curve using empirical interelement correction factors and ratio to backscatter.
2.2 The EDXRF spectrometer is initially calibrated using a set of prepared standards to collect the necessary intensity data. Each
calibration line and any correction coefficient are obtained by a regression of this data, using the program supplied with the
spectrometer.
3. Significance and Use
3.1 Some oils are formulated with organo-metallic additives, which act, for example, as detergents, antioxidants, and antiwear
agents. Some of these additives contain one or more of these elements: calcium, phosphorus, sulfur, and zinc. This test method
provides a means of determining the concentrations of these elements, which in turn provides an indication of the additive content
of these oils.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.03 on Elemental Analysis.
Current edition approved Dec. 1, 2014May 1, 2019. Published January 2015July 2019. Originally approved in 1999. Last previous edition approved in 20102014 as
D6481 – 99 (2010).D6481 – 14. DOI: 10.1520/D6481-14.10.1520/D6481-14R19.
*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
D6481 − 14 (2019)
TABLE 1 Elements and Range of Concentrations Determined
Element Concentration Range
Phosphorus 0.02 to 0.3 mass %
Sulfur 0.05 to 1.0 mass %
Calcium 0.02 to 1.0 mass %
Zinc 0.01 to 0.3 mass %
TABLE 1 Elements and Range of Concentrations Determined
Element Concentration Range
Phosphorus 0.02 mass % to 0.3 mass %
Sulfur 0.05 mass % to 1.0 mass %
Calcium 0.02 mass % to 1.0 mass %
Zinc 0.01 mass % to 0.3 mass %
3.2 Several additive elements and their compounds are added to the lubricating oils to give beneficial performance (Table 2).
3.3 This test method is primarily intended to be used at a manufacturing location for monitoring of additive elements in
lubricating oils. It can also be used in central and research laboratories.
4. Interferences
4.1 The additive elements found in lubricating oils will affect the measured intensities from the elements of interest to a varying
degree. In general, for lubricating oils, the X-radiation emitted by the element of interest can be absorbed by itself (self-absorption)
or by the other elements present in the sample matrix. Also the X-radiation emitted from one element can further excite (enhance)
another element. These interelement effects are significant at concentrations varying from 0.03 mass %, due to the heavier
elements, to 1 mass %, 1 mass %, for the lighter elements. Enhancement effects can be minimised by selective excitation. The
measured concentration for a given element can be mathematically corrected for self-absorption and for interelement effects by
other elements present in the sample matrix. If an element is present at significant concentrations and an inter-element correction
for that element is not employed, the results can be low due to absorption or high due to enhancement.
4.2 If a sample containing barium as an additive above 0.03 mass %0.03 % by mass is measured against a calibration derived
from standards without barium, then results will be low.
4.3 If a sample containing chlorine as an impurity above 0.03 mass %0.03 % by mass is measured against a calibration derived
from standards without chlorine, then the results can be affected.
4.4 There can be spectral overlap of one element onto another, especially for phosphorus on sulfur, and the instrument must
include correction procedures for any such overlaps.
5. Apparatus
5.1 Energy Dispersive X-ray Fluorescent Analyzer—Any energy dispersive X-ray fluorescent analyzer can be used if its design
incorporates at least the following features.
5.1.1 Source of X-ray Excitation , Excitation, X-ray tube with palladium, silver, or rhodium target programmable between
44 keV and at least 25 keV 25 keV for preferential excitation to simplify the sample spectra. (Warning—Operation of an analyzer
using an X-ray tube source is to be conducted in accordance with the manufacturer’s safety instructions and federal, state, and local
regulations)regulations.)
5.1.2 X-ray Detector, gas filled proportional counter with high sensitivity and a resolution value not to exceed 1300 eV at 5.9
keV.1300 eV at 5.9 keV.
NOTE 1—The limited data from instruments with solid state detectors in the inter-laboratoryinterlaboratory precision study did not support their
inclusion in the this test method.
5.1.3 Primary Beam Filters, to make the excitation more selective.
TABLE 2 Lubricants and Additive Materials
Element Compounds Purpose/Application
Calcium Sulfonates, phenates Detergent inhibitors, dispersants
Phosphorus Dithiophosphates, phosphates phosphites Anti-rusting agents, extreme pressure additives, anti-wear
Sulfur Base oils, sulfonates, thiophosphates, polysulfides and Detergents, extreme pressure additives, anti-wear
other sulfurized components
Zinc Dialkyldithiophosphates, dithiocarbamates, phenolates Anti-oxidant, corrosion inhibitors, antiwear additives, detergents, crankcase
carboxylates oils, hypoid gear lubricants, aircraft piston engine oils, turbine oils, automatic
transmission fluids, railroad diesel engine oils, brake lubricants
D6481 − 14 (2019)
5.1.4 Secondary Beam Filters—When a proportional counter is used, these are necessary as a means of discriminating between
an analyte’s X rays and other analytes and the spectrum from the X-ray tube.
5.1.5 Multi-Channel Analyzer, for discrimination between an analyte’s X rays and background X rays.
5.1.6 Optional Helium Purgeable Optical Path.
5.2 Sample Cells, providing a depth of at least 6 mm and equipped with replaceable X-ray transparent film. Suitable films
include polypropylene and polycarbonate with thickness from 3.53.5 μm to 8 μm.8 μm.
5.3 Instrument Setting-Up Samples (Elemental Reference Samples), to quantify spectral overlaps. These are required when the
instrument’s software does not include reference spectra to deconvolute spectra.
5.4 Drift Correction Monitors, to correct for instrumental drift. At least two samples are necessary to correct both sensitivity
and baseline drifts. For each element and scatter region, there shall be one providing a count rate similar to samples from the upper
end of the calibration and another providing a count rate as if from a blank. This last can be a blank oil. For the high concentration
of each element, a glass disk, XRF fusion bead, or pressed pellet have all been found to be satisfactory. They can be the same
samples as in 5.3.
6. Reagents and Materials
6.1 Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the
specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available.
Other grades can be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without
lessening the accuracy of the determination.
6.2 Helium, at least 99.5 % purity, for the optical path of the spectrometer.
6.3 Diluent Solvent, a suitable solvent free of metals, phosphorus, and chlorine, and containing less than 10 ppm 10 ppm of
sulfur (for example, deodorized kerosene, white oil, or mineral oil).
6.4 Calibration Standard Materials:
6.4.1 Certified concentration solutions, of liquid organo-metallic salts, each containing calcium or zinc, or both. The solutions
shall be sulfur free or the certificate shall state the concentration of sulfur. Alternatively, the following standard materials can be
used.
6.4.1.1 Calcium 2-Ethylhexanoate, approximately 12.3 mass %12.3 % by mass calcium, with a certified value.
6.4.1.2 Zinc Cyclohexanebutyrate, approximately 16.2 mass %16.2 % by mass zinc, with a certified value.
6.4.2 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity (9.62 mass %(9.62 % by mass phosphorus).
6.4.3 Di-n-butyl Sulfide, 97 % purity, (21.9 mass %(21.9 % by mass sulfur).
6.4.4 Stabilizers, 2-ethylhexanoic acid, 2-ethylamine. Also, proprietary stabilizer/chelating solutions are available commer-
cially. Stabilizers shall be free of the additive element.
NOTE 2—In addition to the calibration standard materials identified in 6.4.1 – 6.4.3, single or multielement calibration standards can also be prepared
from materials similar to the samples being analyzed, provided the calibration standards to be used have previously been characterized by independent
primary (for example, gravimetric or volumetric) analytical techniques to establish the elemental concentration mass % levels.
7. Preparation of Calibration Standards
7.1 To ensure complete solution of all components, prepare calibration standards by precisely weighing the organo-metallic
solutions and phosphorus and sulfur solutions with the diluent solvent along with the appropriate stabilizer. Table 3 lists suggested
concentrations when determining empirical influence coefficients. Complete sets of standards based on Table 3 are commercially
available.
8. Calibration
8.1 Spectrometer Settings—Follow the manufacturer’s recommendations, and set up a series of measurement conditions, (X-ray
tube voltage, X-ray tube current, primary beam filter, secondary beam filter, measurement time, and multichannel analyzer region
of interest) to measure the •K spectrum of phosphorus, sulfur, calcium, and zinc. Include at least one region of backscatter.
8.2 Fill respective sample cells nearly full with the calibration standard solutions. Follow the manufacturer’s recommendation
to ensure that the cell is full enough such that X-ray count rate does not depend on the amount of sample in the cell. Make sure
that no wrinkles or bulges are present in the film. The film must be flat, and the cell shall be vented.
8.3 Place the sample cell in the X-ray beam to measure and record the intensity for each element and scatter region in each
calibration standard in accordance with the conditions set by following 8.1. Measure each standard two times, using a freshly
prepared cell for each measurement.
Reagent Chemicals, American Chemical Society Specifications, , American Chemical Society, Washington, D.C. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
These certified concentrated solutions are commercially available.
D6481 − 14 (2019)
TABLE 3 Recommended Concentrations for Standards
(all values mass %)
Standard Calcium Phosphorus Sulfur Zinc
1 0 0 0 0
2 0.005 0.005 0.050 0.05
3 0.600 0 0 0
4 0 0.300 0 0
5 1.00 0 1.00 0
6 0 0 0 0.300
7 0.005 0.250 0.800 0.300
8 0.500 0.150 0.500 0.150
9 0.010 0.200 0.100 0.250
10 0.050 0.010 0.400 0.075
11 0.100 0.150 0.200 0.200
12 0.200 0.200 0.800 0.100
13 0.400 0.005 0.800 0.300
14 0.600 0.100 0.500 0.050
15 0.800 0.010 0.050 0.100
16 1.00 0.300 1.00 0.150
17 0.400 0.050 0.600 0.250
8.4 Measure any instrument setting up samples necessary to quantify spectral overlaps. Use at least the measurement time of
the calibration standards
8.5 Measure and record the intensity for each element and scatter region of the drift correction monitors. Use at least the
measurement time of the calibration standards.
NOTE 3—An instrument setting up sample used in 8.4 can also be used as a drift correction monitor i
...