ASTM D7757-22

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: D7757 − 17 D7757 − 22
Standard Test Method for
Silicon in Gasoline and Related Products by Monochromatic
Wavelength Dispersive X-ray Fluorescence Spectrometry
This standard is issued under the fixed designation D7757; 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 determination of total silicon by monochromatic, wavelength-dispersive X-ray fluorescence
(MWDXRF) spectrometry in naphthas, gasoline, gasoline-ethanol blends, reformulated gasoline (RFG), ethanol and ethanol-fuel
blends, and toluene at concentrations of 3 mg ⁄kg to 100 mg ⁄kg. The precision of this test method was determined by an
interlaboratory study using representative samples of the liquids described in 1.1 and 1.2. The pooled limit of quantitation (PLOQ)
was estimated to be 3 mg ⁄kg.
NOTE 1—Volatile samples such as high-vapor-pressure gasolines or light hydrocarbons might not meet the stated precision because of the evaporation
of light components during the analysis.
NOTE 2—Aromatic compounds such as toluene are under the jurisdiction of Committee D16 on Aromatic Hydrocarbons and Related Chemicals. However,
toluene can be a contributor to silicon contamination in gasoline (see 4.45.4), thus its inclusion in this test method.
1.2 Gasoline samples containing ethanol and other oxygenates may be analyzed with this test method provided the matrix of the
calibration standards is either matched to the sample matrices or the matrix correction described in Annex A1 is applied to the
results. The conditions for matrix matching and matrix correction are provided Section 56, Interferences.
1.3 Samples with silicon concentrations above 100 mg ⁄kg can be analyzed after dilution with appropriate solvent. The precision
and bias of silicon determinations on diluted samples have not been determined and may not be the same as shown for neat samples
(Section 1617).
1.4 A fundamental assumption in this test method is that the standard and sample matrices are well matched, or that the matrix
differences are accounted for (see 13.514.5). Matrix mismatch can be caused by C/H ratio differences between samples and
standards or by the presence of other interfering heteroatoms; observe the cautions and recommendations in Section 56.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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 prior to use.
1.7 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 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 July 1, 2017Nov. 1, 2022. Published July 2017December 2022. Originally approved in 2012. Last previous edition approved in 20122017 as
D7757 – 12.D7757 – 17. DOI:10.1520/D7757-17.DOI:10.1520/D7757-22.
*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
D7757 − 22
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
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4806 Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel
D5798 Specification for Ethanol Fuel Blends for Flexible-Fuel Automotive Spark-Ignition Engines
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
D6300 Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products, Liquid Fuels, and
Lubricants
D7343 Practice for Optimization, Sample Handling, Calibration, and Validation of X-ray Fluorescence Spectrometry Methods
for Elemental Analysis of Petroleum Products and Lubricants
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer to Terminology D4175.
4. Summary of Test Method
4.1 A monochromatic X-ray beam with a wavelength suitable to excite the K-shell electrons of silicon is focused onto a test
specimen contained in a sample cell (see Fig. 1). The fluorescent Kα radiation at 0.713 nm (7.13 Å) emitted by silicon is collected
by a fixed monochromator (analyzer). The intensity (counts per second) of the silicon X-rays is measured using a suitable detector
and converted to the concentration of silicon (mg/kg) in a test specimen using a calibration equation.
5. Significance and Use
5.1 This test method provides rapid and precise measurement of total silicon in naphthas, gasoline, gasoline-ethanol blends, RFG,
ethanol and ethanol-fuel blends, and toluene with minimum sample preparation. Typical analysis time is 5 min to 10 min per
sample.
FIG. 1 Schematic of the MWDXRF Analyzer
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.
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5.2 Excitation by monochromatic X-rays reduces background, simplifies matrix correction, and increases the signal/background
ratio compared to polychromatic excitation used in conventional WDXRF techniques.
5.3 Silicone oil defoamer can be added to coker feedstocks to minimize foaming in the coker. Residual silicon in the coker naphtha
can adversely affect downstream catalytic processing of the naphtha. This test method provides a means to determine the silicon
content of the naphtha.
5.4 Silicon contamination of gasoline, gasoline-ethanol blends, denatured ethanol, and their blends has led to fouled vehicle
components (for example, spark plugs, exhaust oxygen sensors, catalytic converters) requiring parts replacement and repairs.
Finished gasoline, gasoline-ethanol blends, and ethanol-fuel blends can come into contact with silicon a number of ways. Waste
hydrocarbon solvents such as toluene can be added to gasoline. Such solvents can contain soluble silicon compounds.
Silicon-based antifoam agents can be used in ethanol plants, which then pass silicon on to the finished ethanol-fuel blend. This test
method can be used to determine if gasoline, gasoline-ethanol blends, and ethanol-fuel blends meet specifications with respect to
silicon content of the fuel, and for resolution of customer problems.
5.5 Some silicon compounds covered by this test method are significantly more volatile than the silicon compounds typically used
for the preparation of the calibration standards. Volatile compounds may not meet the stated precision from this test method
because of selective loss of light materials during the analysis.
6. Interferences
6.1 Differences between the elemental composition of test samples and the calibration standards can result in biased silicon
determinations. For fuels within the scope of this test method, the only important elements contributing to bias resulting from
differences in the matrices of calibrants and test samples are hydrogen, carbon, and oxygen. A matrix-correction factor (C) may
be used to correct this bias; the calculation is described in Annex A1. For general analytical purposes, the matrices of test samples
and the calibrants are considered to be matched when the calculated correction factor C is within 0.95 to 1.05. No matrix correction
is required within this range. A matrix correction is required when the value of C is outside the range of 0.95 to 1.05. For most
testing, matrix correction can be avoided with a proper choice of calibrants. For example, Fig. 2 and the calculation in Annex A1
show that a calibrant with 87.5 % by mass carbon and 12.5 % by mass hydrogen can cover non-oxygen containing samples with
C/H ratios from 5.0 to 11.0, which corresponds to a correction factor range of 0.95 to 1.05.
FIG. 2 Matrix Correction for a Test Sample versus C/H and Total Oxygen Content Using Chromium Kα for the Excitation Beam
Bertin, E. P., Principles and Practices of X-ray Spectrometric Analysis, Plenum Press, New York, 1975, pp. 115–118.
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6.2 Fuels containing large quantities of ethanol, such as ethanol fuel blends, denatured fuel ethanol, and gasoline-ethanol blends
(see Specifications D4806 and D5798), can have a high oxygen content leading to significant absorption of silicon Kα radiation
and low silicon results. Such fuels may be analyzed using this test method provided either that correction factors (see Table 1 and
Table 2) are applied to the results or by using calibration standards that are matrix matched to the test sample. For gasoline samples
with oxygenates, up to 3.1 % by mass oxygen can be tolerated for test samples with the same C/H ratio as the calibrants.
6.2.1 For test samples with high oxygenate content, such as denatured fuel ethanol and ethanol fuel blends (see Specifications
D4806 and D5798), ethanol-based calibrants may be used provided the correction factors as described in 5.16.1 are applied to the
results. Table 1 and Table 2 show the correction factor that should be applied to the measurement results of the gasoline-ethanol
and ethanol fuel blends if they are measured using either an isooctane or ethanol calibration curve.
NOTE 3—Alcohol based calibration standards may be preferred for test samples containing a high oxygenate content.
6.3 To minimize any bias in the results, use calibration standards prepared from silicon-free base materials of the same or similar
elemental composition as the test samples.
6.3.1 When diluting samples, use a diluent with an elemental composition the same or similar to the base material used for
preparing the calibration standards.
6.3.2 A base material for gasoline may be simulated by mixing 2,2,4-trimethylpentane (isooctane) and toluene in a ratio that
approximates the expected aromatic content of the samples to be analyzed.
7. Apparatus
7.1 Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) Spectrometer , equipped for X-ray detection at
0.713 nm (7.13 Å). Any spectrometer of this type may be used if it includes the following features, and the precision and bias of
test results are in accordance with the values described in Section 1617.
7.1.1 X-ray Source, capable of producing X-rays to excite silicon. X-ray tubes with a power >20 W capable of producing Rh Lα,
Pd Lα, Ag Lα, Ti Kα, Sc Kα, or Cr Kα radiation are recommended for this purpose.
7.1.2 Incident-beam Monochromator, capable of focusing with an effective collection solid angle greater than 0.05 sr and selecting
a single wavelength of characteristic X-rays from the source onto the specimen. A monochromatic beam is considered to be
monochromatic when it has an energy bandwidth (Full Width Half Maximum) less than 61.5 % relative to the selected energy and
containing more than 98 % flux of the spectrum of the excitation beam which is incident on the sample.
TABLE 1 Correction Factors for Gasoline-Ethanol and Ethanol Fuel Blends Measured on an Isooctane Calibration Curve
NOTE 1—Determine the correction factor in the table below by finding the known ethanol content of the test specimen (for example, 15 % by mass)
as the sum of the value in the first column and the value in the first row (for example, 15 = 10+5). The intersection of these two values is the correction
factor (for example, 1.0844). Apply the correction according to 13.514.5. Refer to 7.78.7 and 11.112.1 for isooctane calibration.
Ethanol, 0 % 1 % 2 % 3 % 4 % 5 % 6 % 7 % 8 % 9 %
% by mass
0 % 1.0000 1.0056 1.0112 1.0169 1.0225 1.0281 1.0337 1.0394 1.0450 1.0506
10 % 1.0562 1.0619 1.0675 1.0731 1.0787 1.0844 1.0900 1.0956 1.1012 1.1069
20 % 1.1125 1.1181 1.1237 1.1294 1.1350 1.1406 1.1462 1.1519 1.1575 1.1631
30 % 1.1687 1.1744 1.1800 1.1856 1.1912 1.1969 1.2025 1.2081 1.2137 1.2194
40 % 1.2250 1.2306 1.2362 1.2419 1.2475 1.2531 1.2587 1.2644 1.2700 1.2756
50 % 1.2812 1.2868 1.2925 1.2981 1.3037 1.3093 1.3150 1.3206 1.3262 1.3318
60 % 1.3375 1.3431 1.3487 1.3543 1.3600 1.3656 1.3712 1.3768 1.3825 1.3881
70 % 1.3937 1.3993 1.4050 1.4106 1.4162 1.4218 1.4275 1.4331 1.4387 1.4443
80 % 1.4500 1.4556 1.4612 1.4668 1.4725 1.4781 1.4837 1.4893 1.4950 1.5006
90 % 1.5062 1.5118 1.5175 1.5231 1.5287 1.5343 1.5400 1.5456 1.5512 1.5568
The sole source of supply of the apparatus known to the committee at this time is XOS, Inc., 15 Tech Valley Drive, Suite 1, East Greenbush, NY 12061. If you are aware
of alternative suppliers, please provide this information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend.
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TABLE 2 Correction Factors for Gasoline-Ethanol and Ethanol Fuel Blends Measured on an Ethanol (E100) Calibration Curve
NOTE 1—Determine the correction factor in the table below by finding the known ethanol content of the test specimen (for example, 85 % by mass)
as the sum of the value in the first column and the value in the first row (for example, 85 = 80+5). The intersection of these two values is the correction
factor (for example, 0.9460). Apply the correction according to 13.514.5. Refer to 7.68.6 and 11.112.1 for ethanol calibration.
Ethanol, 0 % 1 % 2 % 3 % 4 % 5 % 6 % 7 % 8 % 9 %
% by mass
0 % 0.6400 0.6436 0.6472 0.6508 0.6544 0.6580 0.6616 0.6652 0.6688 0.6724
10 % 0.6760 0.6796 0.6832 0.6868 0.6904 0.6940 0.6976 0.7012 0.7048 0.7084
20 % 0.7120 0.7156 0.7192 0.7228 0.7264 0.7300 0.7336 0.7372 0.7408 0.7444
30 % 0.7480 0.7516 0.7552 0.7588 0.7624 0.7660 0.7696 0.7732 0.7768 0.7804
40 % 0.7840 0.7876 0.7912 0.7948 0.7984 0.8020 0.8056 0.8092 0.8128 0.8164
50 % 0.8200 0.8236 0.8272 0.8308 0.8344 0.8380 0.8416 0.8452 0.8488 0.8524
60 % 0.8560 0.8596 0.8632 0.8668 0.8704 0.8740 0.8776 0.8812 0.8848 0.8884
70 % 0.8920 0.8956 0.8992 0.9028 0.9064 0.9100 0.9136 0.9172 0.9208 0.9244
80 % 0.9280 0.9316 0.9352 0.9388 0.9424 0.9460 0.9496 0.9532 0.9568 0.9604
90 % 0.9640 0.9676 0.9712 0.9748 0.9784 0.9820 0.9856 0.9892 0.9928 0.9964
7.1.3 Optical Path, designed to minimize the absorption along the path of the excitation and fluorescent beams using a helium or
vacuum atmosphere.
7.1.4 Fixed-Channel Monochromator, suitable for dispersing silicon Kα X-ray photons with an effective collection solid angle
greater than 0.3 sr.
7.1.5 Detector, designed for efficient detection of silicon Kα X-ray photons.
7.1.6 Single-Channel Analyzer, an energy discriminator to monitor only silicon radiation.
7.1.7 Removable Sample Cell, compatible with the sample and the geometry of the MWDXRF spectrometer. A disposable cell is
recommended.
7.1.8 X-Ray Transparent Film, for containing and supporting the test specimen in the sample cell (see 6.1.77.1.7) while providing
a low-absorption window for X-rays to pass to and from the sample. Use an X-ray transparent film resistant to chemical attack
that does not contain a listed silicon impurity. Follow manufacturer’s recommendations for appropriate film types.
8. Reagents and Materials
8.1 Purity of Reagents—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 may 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.
8.2 Calibration-Check Samples, for verifying the accuracy of a calibration. The check samples shall have known silicon content
and not be used in determining the calibration curve. A standard from the same reliable and consistent source of calibration
standards used to determine the calibration curve is convenient to validate the calibration.
8.3 Octamethylcyclotetrasiloxane (D4), a high-purity material (typical purity 98 %), is known to be suitable for making silicon
calibration standards. Use the known silicon concentration and the material purity when calculating the exact concentrations of
silicon in calibration standards. (Warning— Octamethylcyclotetrasiloxane is harmful if swallowed or absorbed through skin. It
is an eye irritant and may cause skin irritation.)
8.4 Drift Correction Monitor(s) (Optional)—, to determine and correct instrument drift over time (see 11.412.4, 12.113.1, and
13.114.1). Various forms of stable (with respect to repeated exposure to X-rays) silicon-containing materials are suitable drift
Reagent Chemicals, American Chemical Society Specifications,ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference
Materials, American Chemical Society, Washington, DC. For Suggestionssuggestions on the testing of reagents not listed by the American Chemical Society, see
AnnualAnalar 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.
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correction monitors; for example, liquid petroleum, solid, pressed powder, metal alloy, and fused glass. The count rate displayed
by the monitor sample, in combination with a convenient count time (T), shall be sufficient to give a relative standard deviation
(RSD) of less than 1 % (see Appendix X1).
8.4.1 Calibration standards may be used as drift correction monitors. Because it is desirable to discard test specimens after each
determination, a lower cost material is suggested for use over time. Any stable material meeting the recommendations of 7.48.4
or 7.4.18.4.1 may be used for monitoring of drift on a given day when samples are being analyzed.
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NOTE 4—The effect of drift correction on the precision and bias of this test method has not been studied.
8.4.2 Drift correction may be done automatically if the instrument embodies this option, although the calculation may be readily
done by conventional methods of data reduction and processing.
8.5 Quality-Control (QC) Samples, for use in establishing and monitoring the stability and precision of an analytical measurement
system (see Section 1516). Use homogeneous materials, similar to samples of interest and available in sufficient quantity to be
analyzed regularly for a long period of time.
8.5.1 Verification of system control through the use of QC samples and control charting is highly recommended.
NOTE 5—Suitable QC samples can be prepared by combining retains of typical samples.
8.6 Ethanol, use a high purity grade and account for its silicon content when calculating the silicon concentrations of the
calibration standards. (Warning—Ethanol is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause
skin irritation.)
8.7 2,2,4-Trimethylpentane (Isooctane), use a high purity grade and account for its silicon content when calculating the silicon
concentration of the calibration standards. (Warning—Isooctane is flammable and harmful if swallowed or inhaled. It is an eye
irritant and may cause skin irritation.)
8.8 Toluene, use a high purity grade and account for its silicon content when calculating the silicon concentration of the calibration
standards. (Warning—Toluene is flammable and harmful if swallowed or inhaled. It is an eye irritant and may cause skin
irritation.)
8.9 Helium, (for units that require helium per manufacturer), minimum purity 99.9 %, for use as an optical path.
9. Hazards
9.1 (Warning—Exposure to excessive quantities of X-ray radiation is injurious to health. The operator needs to take appropriate
actions to avoid exposing any part of his/her body, not only to primary X-rays, but also to secondary or scattered radiation that
might be present. The X-ray spectrometer should be operated in accordance with local and national regulations governing the use
of ionizing radiation.)
9.2 Consult current health and safety regulations such as OSHA, suppliers’ Material Safety Data Sheets and local regulations for
all materials used in this test method.
10. Sampling and Sample Handling
10.1 Sample fuel according to the procedures in Practices D4057 or D4177.
10.2 Use the utmost care in sampling and handling gasoline to prevent evaporation of light ends which could change the
concentration of silicon in the sample. Store gasoline in a leak tight container at 0 °C to 4 °C until ready for analysis. If possible,
maintain at this temperature throughout any transfer and handling processes. Allow samples maintained at 0 °C to 4 °C to come
to room temperature before testing, and expose these materials to ambient conditions only as long as necessary to obtain a sample
for analysis. Analyze test specimens as soon as possible after sub-sampling from bulk container. Do not allow bulk container to
remain uncovered any longer than is needed to obtain desired sub-samples.
10.3 For specimen preparation, see 10.211.2.
10.3.1 Because impurities and thickness variations can occur in commercially available transparent films and vary from lot to lot,
use calibration-check samples (see 7.28.2) to verify calibration integrity after starting each new batch of film or if the type and
thickness of the window film is changed.
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10.4 When reusable sample cells are used, thoroughly clean and dry cells before each use.
10.4.1 Disposable sample cells shall not be reused.
11. Preparation of Apparatus and Specimens for Analysis
11.1 Analyzer Preparation—Ensure that the MWDXRF analyzer has been installed and put into operation according to
manufacturer’s instructions. Allow sufficient time for instrument electronics to stabilize. Perform any instrument checkout
procedures required. When possible, the instrument should be run continuously to maintain optimum stability.
11.1.1 Use the count time (T) recommended by the instrument manufacturer for the lowest silicon concentration expected. The
typical time for each measurement is five to ten minutes.
11.1.2 Alternatively, determine T expected for a desired count precision by following the procedure in Appendix X1.
11.2 Specimen Preparation—Prepare a specimen of a test sample or a calibration standard as follows:
11.2.1 Carefully transfer a sufficient portion of the liquid to fill a sample cell above a minimum depth beyond which additional
liquid does not
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