ASTM D2798-21

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Designation: D2798 − 20 D2798 − 21
Standard Test Method for
Microscopical Determination of the Vitrinite Reflectance of
Coal
This standard is issued under the fixed designation D2798; 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
1.1 This test method covers the microscopical determination of both the mean maximum and mean random reflectances measured
in oil of polished surfaces of vitrinite and other macerals present in coals ranging in rank from lignite to anthracite. This test method
can be used to determine the reflectance of other macerals.
1.2 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.3 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.
1.4 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. Referenced Documents
2.1 ASTM Standards:
D121 Terminology of Coal and Coke
D388 Classification of Coals by Rank
D2797/D2797M Practice for Preparing Coal Samples for Microscopical Analysis by Reflected Light
3. Terminology
3.1 Definitions—For definitions of terms, refer to Terminology D121.
3.2 Abbreviations:
3.2.1 R max—mean maximum reflectance measured in oil.
o
3.2.2 R ran—mean random reflectance measured in oil.
o
This test method is under the jurisdiction of ASTM Committee D05 on Coal and Coke and is the direct responsibility of Subcommittee D05.28 on Petrographic Analysis
of Coal and Coke.
Current edition approved Feb. 1, 2020May 15, 2021. Published March 2020June 2021. Originally approved in 1969. Last previous edition approved in 20192020 as
D2798 – 11aD2798 – 20.(2019). DOI: 10.1520/D2798-20.10.1520/D2798-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2798 − 21
3.2.2.1 Discussion—
Other organizations may use other abbreviations for mean maximum and mean random reflectance.
4. Summary of Test Method
4.1 The reflectance of the maceral vitrinite or other macerals is determined in this test method by illuminating a polished surface
of a section of coal in immersion oil using a microscopic system that photometrically measures the amount of light reflected from
the surface. The reflected light is recorded in percent reflectance after calibration of photometric equipment by measuring the
reflected light from standards of reflectance as calculated from their refractive indices.
5. Significance and Use
5.1 The mean maximum reflectance of the vitrinite component in coal as determined by this test method is often used as an
indicator of rank as presented in Classification D388, independent of petrographic composition, and in the characterization of coal
as feedstock for carbonization, gasification, liquefaction, and combustion processes.
5.2 This test method is for use in scientific and industrial research.
6. Apparatus
6.1 Microscope—Any microscope equipped for reflected light microscopy (such as a metallurgical or opaque-ore microscope) can
be used, provided the lens combination of objective and eyepieces permits examination of the specimen at a magnification between
400x and 750x, such that particles of 1 μm 1 μm can be resolved. The objectives shall be constructed so that samples can be
examined in oil with plane-polarized light and have the highest quality of antireflection coatings. The microscope shall be able to
project an image at similar magnification to a photomultiplier tube and to support the photomultiplier tube housing. Means shall
be provided to position the tube housing laterally to obtain maximum response. The microscope shall have a circular stage that
is capable of rotating a specimen through 360°. The mechanical stage attached to the microscope stage shall enable the analyst to
move the specimen accurately (within 0.1 mm) to a given field location. A combination of objective and circular stage shall permit
centering. The viewing eyepiece shall be supplied with a crosshair or grid to be used as a reference to locate precisely the area
sampled by the phototube. During measurement, no light shall be permitted to enter the observer’s end of the viewing eyepiece.
6.2 Polarizer and Illuminator—The light incident on the vertical illuminator of the microscope shall be plane-polarized by a prism
or sheet polarizer. The vertical illuminator can contain a Berek prism, a Smith illuminator, or high-quality glass plate. The polarizer
shall be oriented at 45° when using a Berek prism or at 0° when using a Smith illuminator or glass plate.
6.3 Photomultiplier Tube—In combination with the microscope optical system, light source, and filter used, the photomultiplier
photometer shall be capable of detecting the minimum light reflected from the limited portion of the coal sample (see 6.8). The
high voltage supplied to the photomultiplier tube must be within the prescribed range to obtain linearity of response. This is usually
from 300 V to 1100 V for side-window tubes and from 1000 V to 1500 V for end-window tubes. Photodiode arrays, channeltrons,
or other light-measuring devices are acceptable alternatives providing that sufficient gray levels obtainable will enable reliable
differentiation of signal equivalent to 0.01 % reflectance and that the system is linear in the range of the reflectance measured.
6.4 Photometer Amplifier—The signal from the photomultiplier tube shall be amplified and displayed by a galvanometer, digital
meter, or recorder. When adjusted for operation, the amplifier and meter shall be capable of reliably distinguishing differences in
signal equivalent to 0.01 % reflectance and shall be linear in the range of reflectance measured.
6.5 Recorder or Meter—The recorder or meter used shall have a response time at full scale of no more than 1 s to detect the
maximum reflectance level during rotation of the microscope stage.
6.6 Light Source—The light source shall have a regulated power supply to provide for stable output. Some photometers and
recorders require supplemental voltage-stabilizing transformers if the line voltage fluctuates.
6.7 Filters—The light shall be made approximately monochromatic green by passage through an interference filter or combination
of filters with peak transmittance of 546 nm 6 5 nm and a half-peak transmittance bandwidth of less than 20 nm. Insert the filter
into the light path after the sample and before the photomultiplier tube.
D2798 − 21
6.8 Limiting Aperture—A limiting aperture made of nonreflecting and opaque material shall be placed approximately in the focal
plane of the eyepiece at its central axis to restrict light to the photomultiplier tube window so that only a small area of the
reflectance standard or sample is sensed. The diameter of the aperture shall be selected to provide an effective field of measurement
(sensed spot) of about 5 μm diameter or about 20 μm area.
6.9 Calibration Standards—Prisms constructed of high-index glasses or synthetic minerals shall be used as standards to calibrate
the photometer for reflectance measurement. These standards must be durable, isotropic, resistant to corrosion, free from internal
flaws or fractures, and have negligible light absorption. A prism with sides that form a 30°-60°-90° triangle is the most effective
shape, with the side between the 30° and 90° angles highly polished and used as the reflectance-measuring surface. The prisms
shall be enclosed, except for the polished surface, in a durable, light absorbent, water- and oil-resistant mount; polyester or epoxy
resin, made light absorbent with a dye or filler, serves adequately. It is desirable to have a number of different standards with
reflectances near those of the vitrinite studied; these also serve to check the linear response of the photometer. The reflectance of
each standard shall be calculated to the nearest 0.001 % by means of the following equation:
2 2
R 5 100 n 2 1.5180 / n 11.5180 (1)
~ ! ~ !
s g g
where:
R = standard reflectance in oil of the glass, % and
s
n = refractive index of the glass at 546 nm wavelength, to the nearest 0.0001 index value.
g
R = standard reflectance in oil of the glass, %,
s
n = refractive index of the glass at 546 nm wavelength, to the nearest 0.0001 index value, dimensionless,
g
1.5180 = a constant based on the refractive index of the immersion oil at 23 °C and 546 nm wavelength, dimensionless, and
100 = conversion factor to convert the refractive index from dimensionless to percent, %.
NOTE 1—Most coal laboratories in North America use the following Bausch and Lomb Co., Klein and Becker Co., or Schott Co. optical glasses (the
reported refractive index at 546 nm and the calculated standard reflectance in oil are given in parentheses):
Bausch and Lomb Schott
689 309 (1.6935; 0.299 %) SF8-689-312 (1.6945; 0.303 %)
751 278 (1.7566; 0.532 %) SF13-714-276 (1.7477; 0.496 %)
827 250 (1.8351; 0.895 %) LaF12-836-423 (1.8400; 0.921 %)
850 324 (1.8543; 0.996 %) LaSF9-850-322 (1.8567; 1.009 %)
915 213 (1.9235; 1.390 %) LaSF18-913-325 (1.9273; 1.413 %)
980 222 (1.9907; 1.817 %) LaSF6-961-249 (1.9670; 1.662 %)
Other standards available that can be used include the following:
Leucosapphire (1.77; 0.59 %)
Yttrium aluminum garnet, YAG (1.84; 0.92 %)
Gadolinium gallium garnet, 3G (1.98; 1.73 %)
Silicon carbide (2.663; 7.52 %)
These theoretical standard values should be periodically checked in relation to reliable reference standards such are available from the manufacturer
or from commercial laboratories and scholarly organizations.
6.10 Immersion Oil—The oil shall be a nondrying, noncorrosive type that will not react with coal, does not contain carcinogens,
and has a refractive index within the range from 1.515 to 1.519 at 546 nm and 25 °C. Within the specified range, the refractive
index of the oil is not critical provided the specified value of 1.5180 is used in calculating reflectance of standards as specified in
6.9. Periodic checking of the refractive index of the oil is discretionary.
6.11 Sample-Leveling Press—A conventional manual leveling device can be used to level sample briquettes and glass standards
when they are mounted on microscope slides with modeling clay.
Taylor, G. H., Teichmuller, M., Davis, A., Diessel, C. F. K., Littke, R., and Robert, P., Organic Petrology, Gebruder-Borntraeger, Berlin, Stuttgart, ISBN 3-443-01036-9,
pp. 372 – 373.
D2798 − 21
7. Test Specimen
7.1 Prepare the sample briquette in accordance with Practice D2797/D2797M.
8. Setting Up and Calibrating the Apparatus
8.1 Turn on the photometer and light source and allow equipment to warm up for at least 30 min.
8.2 Mount the glass standards and a polished briquette containing the sample on slides using modeling clay and a leveling press
or use a leveling briquette holder.
8.3 Place the mounted briquette on the stage, apply immersion oil, and verify leveling of the mount and stage by checking that
there is no systematic focus change when the briquette is moved laterally on the stage. Use Köhler illumination. To minimize glare,
restrict the illuminated field by means of the field diaphragm so that the diameter is about one third or less than the size of the
full field. Adjust any other provisions of the illuminator to reduce scattered light in the system.
8.4 Verify the position of the limiting aperture of the photometer with respect to the field of view. This can be done by moving
a small bright object of the sample across the position of the crosshair or reticle that marks the photometer-sensed spot, ascertaining
that readings are highest when the bright object is within the sensed area or by using back-lit illumination of the measuring aperture
if so equipped.
8.5 Using a small, distinctive feature of the sample as a guide, adjust the microscope so that the axis of rotation of the stage is
coincident with the photometer-sensed spot. This is accomplished by adjusting the centering screws of the objective or stage. The
purpose is to eliminate movement of the object grain or area from the sensed spot when the stage is rotated.
8.6 Adjust the polarizer to a 45° position when using a Berek prism or 0° when using a Smith illuminator or glass plate. Place
a glass standard covered with clean immersion oil on the microscope stage and focus on the polished surface.
8.7 With no light reflected from the standard to the phototube, adjust the photometer zero setting or dark current.
8.8 Place on the microscope stage a briquette of opaque resin that has a hole 5 mm in diameter and 5 mm deep which is filled with
immersion oil. Measure the reflectance of the hole to ensure that a reflectance of 0.00 % 6 0.03 % is obtained thereby ensuring
that parasitic reflectances of the objective are minimal. If the reflectance of the hole exceeds the stated limits, then another objective
having a higher quality anti-reflection coating shall be used.
8.9 Allow the reflected light to impinge on the tube. Adjust the photometer amplifier or the illumination to obtain a meter or
recorder scale setting that conveniently represents the calculated reflectance of the glass standard (see Appendix X1).
8.10 Without changing the settings, measure the reflectance of one or more additional standards to check that the photometer
system measures correctly in the range to be studied.
8.10.1 The photometric system cannot give a linear response to a wide range of light flux, therefore, standards with reflectance
values close to that of the c
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