ASTM D6481-99
(Test Method)Standard Test Method for Determination of Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray Fluorescence Spectroscopy
Standard Test Method for Determination of Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray Fluorescence Spectroscopy
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 and health practices and determine the applicability of regulatory limitations to use. (See Note 1.)
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An American National Standard
Designation: D 6481 – 99
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 D 6481; 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 (e) indicates an editorial change since the last revision or reapproval.
TABLE 1 Elements and Range of Concentrations Determined
1. Scope
Element Concentration Range
1.1 This test method covers the quantitative determination
Phosphorus 0.02 to 0.3 mass %
of additive elements in unused lubricating oils, as shown in
Sulfur 0.05 to 1.0 mass %
Table 1.
Calcium 0.02 to 1.0 mass %
1.2 This test method is limited to the use of energy
Zinc 0.01 to 0.3 mass %
dispersive X-ray fluorescence (EDXRF) spectrometers em-
ploying an X-ray tube for excitation in conjunction with the
ability to separate the signals of adjacent elements.
calibration curve using empirical interelement correction fac-
1.3 This test method uses interelement correction factors
tors and ratio to backscatter.
calculated from empirical calibration data.
2.2 The EDXRF spectrometer is initially calibrated using a
1.4 This test method is not suitable for the determination of
set of prepared standards to collect the necessary intensity data.
magnesium and copper at the concentrations present in lubri-
Each calibration line and any correction coefficient are ob-
cating oils.
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.
3. Significance and Use
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 3.1 Some oils are formulated with organo-metallic addi-
routine test method for production control analysis.
tives, which act, for example, as detergents, antioxidants, and
1.7 This standard does not purport to address all of the antiwear agents. Some of these additives contain one or more
safety concerns, if any, associated with its use. It is the
of these elements: calcium, phosphorus, sulfur, and zinc. This
responsibility of the user of this standard to establish appro-
test method provides a means of determining the concentra-
priate safety and health practices and determine the applica- tions of these elements, which in turn provides an indication of
bility of regulatory limitations to use. (See Note 1.)
the additive content of these oils.
3.2 This test method is primarily intended to be used at a
2. Summary of Test Method
manufacturing location for monitoring of additive elements in
2.1 A specimen is placed in the X-ray beam, and the
lubricating oils. It can also be used in central and research
appropriate regions of its spectrum are measured to give the
laboratories.
fluorescent intensities of phosphorus, sulfur, calcium, and zinc.
4. Interferences
Other regions of the spectrum are measured to compensate for
varying background. If the detector does not completely
4.1 The additive elements found in lubricating oils will
resolve all the elements in a single measurement, then to affect the measured intensities from the elements of interest to
improve selectivity, there is a combination of sequential and
a varying degree. In general, for lubricating oils, the
simultaneous measurements employing primary and secondary X-radiation emitted by the element of interest can be absorbed
beam filters. There can be correction of measured intensities
by itself (self-absorption) or by the other elements present in
for spectral overlap. Concentrations of the elements of interest the sample matrix. Also the X-radiation emitted from one
are determined by comparison of these intensities against a
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 %,
This test method is under the jurisdiction of ASTM Committee D02 on
for the lighter elements. Enhancement effects can be minimised
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee
by selective excitation. The measured concentration for a given
D02.03 on Elemental Analysis.
Current edition approved Nov. 10, 1999. Published January 2000. element can be mathematically corrected for self-absorption
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6481
and for interelement effects by other elements present in the 6. Reagents and Materials
sample matrix. If an element is present at significant concen-
6.1 Reagent grade chemicals shall be used in all tests.
trations and an inter-element correction for that element is not
Unless otherwise indicated, it is intended that all reagents
employed, the results can be low due to absorption or high due
conform to the specifications of the Committee on Analytical
to enhancement.
Reagents of the American Chemical Society where such
4.2 If a sample containing barium as an additive above 0.03
specifications are available. Other grades can be used, pro-
mass % is measured against a calibration derived from stan-
vided it is first ascertained that the reagent is of sufficiently
dards without barium, then results will be low.
high purity to permit its use without lessening the accuracy of
4.3 If a sample containing chlorine as an impurity above
the determination.
0.03 mass % is measured against a calibration derived from
6.2 Helium, at least 99.5 % purity, for the optical path of the
standards without chlorine, then the results can be affected.
spectrometer.
4.4 There can be spectral overlap of one element onto
6.3 Diluent Solvent, a suitable solvent free of metals,
another, especially for phosphorus on sulfur, and the instru-
phosphorus, and chlorine and containing less than 10 ppm of
ment must include correction procedures for any such overlaps.
sulfur (for example, deodorized kerosene, white oil, or mineral
oil).
5. Apparatus
6.4 Calibration Standard Materials:
5.1 Energy Dispersive X-ray Fluorescent Analyzer—Any 6.4.1 Certified concentration solutions of liquid organo-
metallic salts, each containing calcium or zinc, or both. The
energy dispersive X-ray fluorescent analyzer can be used if its
design incorporates at least the following features. solutions shall be sulfur free or the certificate shall state the
concentration of sulfur. Alternatively, the following standard
5.1.1 Source of X-ray Excitation, X-ray tube with palla-
dium, silver, or rhodium target programmable between 4 and at materials can be used.
6.4.1.1 Calcium 2-Ethylhexanoate, approximately 12.3
least 25 keV for preferential excitation to simplify the sample
spectra. (Warning—Operation of an analyzer using an X-ray mass % calcium, with a certified value.
6.4.1.2 Zinc Cyclohexanebutyrate, approximately 16.2 mass
tube source is to be conducted in accordance with the manu-
facturer’s safety instructions and federal, state, and local % zinc, with a certified value.
6.4.2 Bis(2-Ethylhexyl)Hydrogen Phosphate, 97 % purity
regulations)
5.1.2 X-ray Detector, gas filled proportional counter with (9.62 mass % phorphorus).
6.4.3 Di-n-butyl Sulfide, 97 % purity, (21.9 mass % sulfur).
high sensitivity and a resolution value not to exceed 1300 eV
at 5.9 keV. 6.4.4 Stabilizers, 2-ethylhexanoic acid, 2-ethylamine. Also,
proprietary stabilizer/chelating solutions are available commer-
NOTE 1—The limited data from instruments with solid state detectors in
cially. Stabilizers shall be free of the additive element.
the inter-laboratory precision study did not support their inclusion in the
method.
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
5.1.3 Primary Beam Filters, to make the excitation more
prepared from materials similar to the samples being analyzed, provided
selective.
the calibration standards to be used have previously been characterized by
5.1.4 Secondary Beam Filters—When a proportional
independent primary (for example, gravimetric or volumetric) analytical
counter is used, these are necessary as a means of discriminat-
techniques to establish the elemental concentration mass % levels.
ing between an analyte’s X-rays and other analytes and the
7. Preparation of Calibration Standards
spectrum from the X-ray tube.
5.1.5 Multi-channel Analyzer, for discrimination between
7.1 To ensure complete solution of all components, prepare
an analyte’s X rays and background X rays.
calibration standards by precisely weighing the organo-
5.1.6 Optional Helium Purgeable Optical Path.
metallic solutions and phosphorus and sulfur solutions with the
5.2 Sample Cells, providing a depth of at least 6 mm and
diluent solvent along with the appropriate stabilizer. Table 2
equipped with replaceable X-ray transparent film. Suitable
lists suggested concentrations when determining empirical
films include polypropylene and polycarbonate with thickness
influence coefficients. Complete sets of standards based on
from 3.5 to 8 μm.
Table 2 are commercially available.
5.3 Instrument Setting Up Samples (Elemental Reference
8. Calibration
Samples), to quantify spectral overlaps. These are required
when the instrument’s software does not include reference
8.1 Spectrometer Settings—Follow the manufacturer’s rec-
spectra to deconvolute spectra.
ommendations and set up a series of measurement conditions,
5.4 Drift Correction Monitors, to correct for instrumental
(X-ray tube voltage, X-ray tube current, primary beam filter,
drift. A
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