1、Designation: D7757 12Standard Test Method forSilicon in Gasoline and Related Products by MonochromaticWavelength Dispersive X-ray Fluorescence Spectrometry1This standard is issued under the fixed designation D7757; the number immediately following the designation indicates the year oforiginal adopti
2、on or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of totalsilicon by monochromatic
3、, wavelength-dispersive X-ray fluo-rescence (MWDXRF) spectrometry in naphthas, gasoline,RFG, ethanol and ethanol-fuel blends, and toluene at concen-trations of 3 to 100 mg/kg. The precision of this test methodwas determined by an interlaboratory study using representa-tive samples of the liquids des
4、cribed in 1.1 and 1.2. The pooledlimit of quantitation (PLOQ) was estimated to be 3 mg/kg.NOTE 1Volatile samples such as high-vapor-pressure gasolines orlight hydrocarbons might not meet the stated precision because of theevaporation of light components during the analysis.NOTE 2Aromatic compounds s
5、uch as toluene are under the jurisdic-tion of Committee D16 on Aromatic Hydrocarbons and Related Chemi-cals. However, toluene can be a contributor to silicon contamination ingasoline (see 4.4), thus its inclusion in this test method.1.2 Gasoline samples containing ethanol and other oxygen-ates may b
6、e analyzed with this test method provided the matrixof the calibration standards is either matched to the samplematrices or the matrix correction described in Annex A1 isapplied to the results. The conditions for matrix matching andmatrix correction are provided Section 5, Interferences.1.3 Samples
7、with silicon concentrations above 100 mg/kgcan be analyzed after dilution with appropriate solvent. Theprecision and bias of silicon determinations on diluted sampleshave not been determined and may not be the same as shownfor neat samples (Section 16).1.4 A fundamental assumption in this test metho
8、d is that thestandard and sample matrices are well matched, or that thematrix differences are accounted for (see 13.5). Matrix mis-match can be caused by C/H ratio differences between samplesand standards or by the presence of other interfering heteroa-toms; observe the cautions and recommendations
9、in Section 5.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to e
10、stablish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D4057 Practice for Manual Sampling of Petroleum andPetroleum ProductsD4177 Practice for Automatic Sampling of Petroleum andPetroleum Pr
11、oductsD4806 Specification for Denatured Fuel Ethanol for Blend-ing with Gasolines for Use as Automotive Spark-IgnitionEngine FuelD5798 Specification for Ethanol Fuel Blends for Flexible-Fuel Automotive Spark-Ignition EnginesD6299 Practice for Applying Statistical Quality Assuranceand Control Chartin
12、g Techniques to Evaluate AnalyticalMeasurement System PerformanceD6300 Practice for Determination of Precision and BiasData for Use in Test Methods for Petroleum Products andLubricants3. Summary of Test Method3.1 A monochromatic X-ray beam with a wavelength suit-able to excite the K-shell electrons
13、of silicon is focused onto atest specimen contained in a sample cell (see Fig. 1). Thefluorescent Ka radiation at 0.713 nm (7.13 ) emitted bysilicon is collected by a fixed monochromator (analyzer). Theintensity (counts per second) of the silicon X-rays is measuredusing a suitable detector and conve
14、rted to the concentration ofsilicon (mg/kg) in a test specimen using a calibration equation.4. Significance and Use4.1 This test method provides rapid and precise measure-ment of total silicon in naphthas, gasoline, RFG, ethanol andethanol-fuel blends, and toluene with minimum sample prepa-ration. T
15、ypical analysis time is 5 to 10 min per sample.4.2 Excitation by monochromatic X-rays reduces back-ground, simplifies matrix correction, and increases the signal/1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products and Lubricants and is the direct responsibility of
16、SubcommitteeD02.03 on Elemental Analysis.Current edition approved Jan. 15, 2012. Published March 2012. DOI:10.1520/D7757-12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, r
17、efer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.background ratio compared to polychromatic excitation used inconventional WDXRF techniques.34.3 Silicone oil defoamer can
18、be added to coker feedstocksto minimize foaming in the coker. Residual silicon in the cokernaphtha can adversely affect downstream catalytic processingof the naphtha. This test method provides a means to determinethe silicon content of the naphtha.4.4 Silicon contamination of gasoline, denatured eth
19、anol,and their blends has led to fouled vehicle components (forexample, spark plugs, exhaust oxygen sensors, catalytic con-verters) requiring parts replacement and repairs. Finishedgasoline and ethanol-fuel blends can come into contact withsilicon a number of ways. Waste hydrocarbon solvents such as
20、toluene can be added to gasoline. Such solvents can containsoluble silicon compounds. Silicon-based antifoam agents canbe used in ethanol plants, which then pass silicon on to thefinished ethanol-fuel blend. This test method can be used todetermine if gasoline and ethanol-fuel blends meet specifica-
21、tions with respect to silicon content of the fuel, and forresolution of customer problems.4.5 Some silicon compounds covered by this test methodare significantly more volatile than the silicon compoundstypically used for the preparation of the calibration standards.Volatile compounds (for example, h
22、examethyldisiloxane with aboiling point of 101C), which typically have boiling pointsbelow 170C can give higher silicon sensitivities than thestandard.5. Interferences5.1 Differences between the elemental composition of testsamples and the calibration standards can result in biasedsilicon determinat
23、ions. For fuels within the scope of this testmethod, the only important elements contributing to biasresulting from differences in the matrices of calibrants and testsamples are hydrogen, carbon, and oxygen. A matrix-correction factor (C) may be used to correct this bias; thecalculation is described
24、 in Annex A1. For general analyticalpurposes, the matrices of test samples and the calibrants areconsidered to be matched when the calculated correction factorC is within 0.95 to 1.05. No matrix correction is requiredwithin this range. A matrix correction is required when thevalue of C is outside th
25、e range of 0.95 to 1.05. For most testing,matrix correction can be avoided with a proper choice ofcalibrants. For example, Fig. 2 and the calculation inAnnexA1show that a calibrant with 87.5 mass % carbon and 12.5 mass% hydrogen can cover non-oxygen containing samples withC/H ratios from 5.0 to 11.0
26、, which corresponds to a correctionfactor range of 0.95 to 1.05.5.2 Fuels containing large quantities of ethanol, such as fuelethanol, denatured fuel ethanol, and gasoline-ethanol blends(see Specifications D4806 and D5798), can have a high oxygencontent leading to significant absorption of silicon K
27、a radiationand low silicon results. Such fuels may be analyzed using thistest method provided either that correction factors (see Table 1and Figs. 3 and 4) are applied to the results or by usingcalibration standards that are matrix matched to the testsample. For gasoline samples with oxygenates, up
28、to 3.1 mass% oxygen can be tolerated for test samples with the same C/Hratio as the calibrants.5.2.1 For test samples with high oxygenate content, such asfuel ethanol, denatured fuel ethanol, and gasoline-ethanolblends (see Specifications D4806 and D5798), ethanol basedcalibrants may be used provide
29、d the correction factors asdescribed in 5.1 are applied to the results. Table 1 and Figs. 3and 4 show the correction factor that should be applied to themeasurement results of the gasoline-gasoline ethanol blends ifthey are measured using either an isooctane or ethanol calibra-tion curve.NOTE 3Alcoh
30、ol based calibration standards may be preferred for testsamples containing a high oxygenate content.5.3 To minimize any bias in the results, use calibrationstandards prepared from silicon-free base materials of the sameor similar elemental composition as the test samples.3Bertin, E. P., Principles a
31、nd Practices of X-ray Spectrometric Analysis, PlenumPress, New York, 1975, pp. 115-118.FIG. 1 Schematic of the MWDXRF AnalyzerD7757 1225.3.1 When diluting samples, use a diluent with an elemen-tal composition the same or similar to the base material usedfor preparing the calibration standards.5.3.2
32、A base material for gasoline may be simulated bymixing 2,2,4-trimethylpentane (isooctane) and toluene in aratio that approximates the expected aromatic content of thesamples to be analyzed.6. Apparatus6.1 Monochromatic Wavelength Dispersive X-ray Fluores-cence (MWDXRF) Spectrometer4, equipped for X-
33、ray detec-tion at 0.713 nm (7.13 ). Any spectrometer of this type maybe used if it includes the following features, and the precisionand bias of test results are in accordance with the valuesdescribed in Section 16.6.1.1 X-ray Source, capable of producing X-rays to excitesilicon. X-ray tubes with a
34、power 20W capable of producingRh La,PdLa,AgLa,TiKa,ScKa, and Cr Ka radiation arerecommended for this purpose.6.1.2 Incident-beam Monochromator, capable of focusingwith an effective collection solid angle greater than 0.05 sr andselecting a single wavelength of characteristic X-rays from thesource on
35、to the specimen. A monochromatic beam is consid-ered to be monochromatic when it has an energy bandwidth(Full Width Half Maximum) less than 61.5% relative to theselected energy and containing more than 98% flux of thespectrum of the excitation beam which is incident on thesample.6.1.3 Optical Path,
36、designed to minimize the absorptionalong the path of the excitation and fluorescent beams using ahelium or vacuum atmosphere.6.1.4 Fixed-Channel Monochromator, suitable for dispers-ing silicon Ka X-ray photons with an effective collection solidangle greater than 0.3 sr.6.1.5 Detector, designed for e
37、fficient detection of silicon KaX-ray photons.4The sole source of supply of the apparatus known to the committee at this timeis XOS, Inc., 15 Tech Valley Drive, Suite 1, East Greenbush, NY 12061. If you areaware of alternative suppliers, please provide this information to ASTM Interna-tional Headqua
38、rters. Your comments will receive careful consideration at a meetingof the responsible technical committee,1which you may attend.FIG. 2 Matrix Correction for a Test Sample versus C/H and Total Oxygen Content Using Chromium Ka for the Excitation BeamTABLE 1 Gasoline-Ethanol Blend Correction FactorsIs
39、ooctane Calibration Curve Ethanol (E100) Calibration CurveEthanol in Test Sample, wt% Correction Factor CAGasoline in Test Sample, wt% Correction Factor CA0% 1.00 0% 1.002% 1.01 2% 0.994% 1.02 4% 0.996% 1.03 6% 0.988% 1.04 8% 0.9710% 1.06 10% 0.9620% 1.11 20% 0.9330% 1.17 30% 0.8940% 1.22 40% 0.8650
40、% 1.28 50% 0.82ACorrection factor is for gasoline-ethanol blend test sample measured on either an isooctane calibration curve or an ethanol calibration curve.D7757 1236.1.6 Single-Channel Analyzer, an energy discriminator tomonitor only silicon radiation.6.1.7 Removable Sample Cell, compatible with
41、the sampleand the geometry of the MWDXRF spectrometer. A disposablecell is recommended.6.1.8 X-Ray Transparent Film, for containing and support-ing the test specimen in the sample cell (see 6.1.7) whileproviding a low-absorption window for X-rays to pass to andfrom the sample. Use an X-ray transpare
42、nt film resistant tochemical attack. Follow manufacturers recommendations forappropriate film types.7. Reagents and Materials7.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended thatall reagents conform to the specifications of the Commi
43、ttee onAnalytical Reagents of the American Chemical Society wheresuch specifications are available.5Other grades may be used,provided it is first ascertained that the reagent is of sufficientlyhigh purity to permit its use without lessening the accuracy ofthe determination.7.2 Calibration-Check Samp
44、les, for verifying the accuracyof a calibration. The check samples shall have known siliconcontent and not be used in determining the calibration curve. Astandard from the same reliable and consistent source ofcalibration standards used to determine the calibration curve isconvenient to validate the
45、 calibration.5Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For Suggestions on the testing of reagents notlisted by the American Chemical Society, see Annual Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United Stat
46、es Pharmacopeiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,MD.FIG. 3 Matrix Correction for Gasoline-Ethanol Blend Samples Measured on an Isooctane Calibration CurveFIG. 4 Matrix Correction for Gasoline-ethanol Blend Samples Measured on an Ethanol (E100) Calibration
47、CurveD7757 1247.3 Octamethylcyclotetrasiloxane (D4), a high-purity mate-rial (typical purity 98%), is known to be suitable for makingsilicon calibration standards. Use the known silicon concentra-tion and the material purity when calculating the exact con-centrations of silicon in calibration standa
48、rds. (WarningOctamethylcyclotetrasiloxane is harmful if swallowed orabsorbed through skin. It is an eye irritant and may cause skinirritation.)7.4 Drift Correction Monitor(s) (Optional), to determineand correct instrument drift over time (see 11.4, 12.1, and13.1). Various forms of stable (with respe
49、ct to repeatedexposure to X-rays) silicon-containing materials are suitabledrift correction monitors; for example, liquid petroleum, solid,pressed powder, metal alloy, and fused glass. The count ratedisplayed by the monitor sample, in combination with aconvenient count time (T), shall be sufficient to give a relativestandard deviation (RSD) of less than 1 % (see Appendix X1).7.4.1 Calibration standards may be used as drift correctionmonitors. Because it is desirable to discard test specimens aftereach determination, a lower cost material is suggested for useover time. A
