SH/T 0797-2007 Determination of benzene content in spark-ignition engine fuels — Mid-infrared spectroscopy

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1. Method number / full name / English name / replacement relationship

• Method number: SH/T 0797—2007
• Chinese full name: Determination of benzene content in spark-ignition engine fuels — Mid-infrared spectroscopy
• English name: Standard test method for determination of benzene in spark-ignition engine fuels using mid infrared spectroscopy
• Replacement relationship: not provided in the available source (the foreword does not mention replacement of any standard)
• Adoption relationship: modified adoption of ASTM D6277-01 "Determination of benzene content in spark-ignition engine fuels (mid-infrared spectroscopy)", redrafted in accordance with ASTM D6277-01
• Main differences from ASTM D6277-01 (foreword):
  • Only the method in ASTM D6277-01 that uses a Fourier transform mid-infrared spectrometer for determination and classical least squares for calibration (CLS) is adopted (c method); the related content of the method using a filter-type mid-infrared spectrometer (a method) and the method using a Fourier transform mid-infrared spectrometer for determination and partial least squares for calibration (PLS) (b method) is not adopted
  • In ASTM D6277-01, 3.1.1.1 and 3.1.1.2 "Discussion" are changed to Note 1 and Note 2
  • In Chapter 5, add "5.4 In the application of this method, a pattern recognition method is used to compare the infrared spectral characteristics of the test sample with the spectral characteristics of the calibration set samples, to determine whether the model used is applicable to the sample."
  • In Chapter 7, add "Note: Other material instrument accessories that can achieve the same test requirements may also be used."
  • When converting benzene mass fraction to volume fraction, the density of the component at 20℃ is used, instead of the relative density of the component at ASTM D6277-01 in 15.56℃/15.56℃
  • In Annex A, add formulas for the pooled standard deviation of the validation set (PSEQ) and the degrees of freedom of the pooled standard deviation of the validation set (DOF)
  • Editorial modifications: rearrangement of chapters; wording of repeatability and reproducibility modified according to Chinese practice
• Annex A is a normative annex
• Publication/implementation: published on 2007-08-01, implemented on 2008-01-01
• Proposed by/administrated by: proposed by China Petrochemical Corporation; administrated by Sinopec Research Institute of Petroleum Processing
• Drafting units: Tianjin University of Commerce, North China Research Institute of Sinopec Sales Co., Ltd.
• Main drafters: Gong Yunlan, Zhan Yuechen, Chen Zhuanwang

2. Scope of application and non-applicability

• 1.1 specifies a method for the determination of benzene content in spark-ignition engine fuels, applicable to the determination of benzene content in the range of 0.1% (volume fraction) to 5% (volume fraction).
• 1.2 uses the International System of Units [SI] units.
• 1.3 does not purport to address all of the safety problems associated with its application; users should establish appropriate safety and protective measures before use and clarify the restricted scope of application.
• Not applicable: no explicit non-applicability clause is provided in the available source.

3. Method principle

• The spark-ignition engine fuel sample is placed in a liquid sample cell; after the beam emitted by the infrared source passes through the sample, it is detected by a detector, and the response signal of the detector is measured.
• For a full-wavelength measuring instrument, the spectral response is measured within the measurement band range corresponding to benzene.
• Analysis is performed based on the spectral absorption of the unknown sample in a specific region and an analytical model established by multivariate calibration methods, converting the detector response signal of the unknown sample into benzene content.
• Interference factors (Chapter 6): the interference factors in the measurement are toluene and other mono-substituted aromatic hydrocarbons; these interferences can be eliminated or reduced by reasonable selection of measuring instruments, spectral processing, and multivariate calibration techniques.

4. Reagents and materials (purity / preparation)

• 8.1 Calibration standards, validation standards, and quality control standards: the purity of the chemical reagents used to prepare the relevant standards shall not be lower than 99%; the purity of the standards shall be checked when necessary.
• Warning: These reagents are flammable and harmful if inhaled or ingested.
• 8.1.1 Benzene
• 8.1.2 1,3- Xylene (m-xylene)
• 8.1.3 Ethylbenzene
• 8.1.4 3- Ethyltoluene
• 8.1.5 n-Hexane (with water as reference, at 250nm, using a 1cm sample cell, absorbance less than 0.1AU) [requires confirmation against the source standard (source-text extraction): the text here is incomplete in the source text]
• 8.1.6 2,2,4- Trimethylpentane (isooctane) [requires confirmation against the source standard (source-text extraction): the text after "2,2," is incomplete in the source text]
• 8.1.7 n-Pentane (at 250nm, using a 1cm sample cell, absorbance less than 0.1AU)
• 8.1.8 Propylbenzene
• 8.1.9 Toluene
• 8.1.10 1,3,5- Trimethylbenzene
• Note: Some reagents are used in each calibration and instrument validation process.
• Calibration set sample composition (Annex A.1.1, Table A.1, unit % (mass fraction)): benzene concentration range 0% to 6% (mass fraction); the table shows columns for components such as benzene, toluene, isooctane, n-hexane (source-text extraction the table is incomplete, the specific row-by-row composition requires confirmation against the source standard (source-text extraction)).
• Background correction mixture (Annex A.1.2, Table A.2, unit % (mass fraction)): prepared from six substances: toluene, 1,3- xylene, 3- ethyltoluene, 1,3,5- trimethylbenzene, ethylbenzene, and propylbenzene; the specific composition is shown in Table A.2 (source-text extraction the table is incomplete, requires confirmation against the source standard (source-text extraction)).

5. Instruments and equipment

• 7.1 Fourier transform mid-infrared spectrometer (FTIR): shall be equipped with an infrared source, an infrared transmission sample cell or a liquid attenuated total reflection sample cell, an interferometer, a detector, and a sample introduction device. Instrument performance specifications (including the use of a ATR sample cell) requirements:
  • Scan range: 4000cm⁻¹ to 600cm⁻¹
  • Resolution: 4cm⁻¹
  • 674cm⁻¹ signal-to-noise ratio S/N: >300:1 (RMS)
• A single-beam spectrum generated by air or nitrogen may be used as a reference to measure the instrument noise signal level and background spectrum.
• The background and sample spectra may be obtained by averaging multiple scans, but the scan time shall not exceed 60s.
• If there is interference from H₂O or CO₂ in the air during background measurement, dry air or N₂ may be used to eliminate it.
• The measurement of the sample spectrum must be performed under the same conditions as the background spectrum.
• In the range of 700cm⁻¹ to 664cm⁻¹, the noise level of the spectral measurement shall not be greater than 0.0015AU (absorbance units).
• 7.2 Sample cell: may be a transmission cell or an attenuated total reflection cell (ATR).
• 7.2.1 Transmission cell: the salt window may be potassium bromide, zinc selenide, or other materials that transmit in the range of 712cm⁻¹ to 660cm⁻¹; the transmission cell path length is 0.025mm ± 0.005mm. To eliminate interference phenomena, a wedge-shaped transmission cell with a path length of 0.025mm ± 0.005mm may also be used.
• 7.2.2 ATR Sample cell specifications (listed in the source text):
  • ATR Material: ZnSe
  • Condensing lens component: conical afocal integrated cell body
  • Element structure: circular cross-section conical structure
  • Angle of incident light on the sample surface: 60°
  • Element length: 39.37mm
  • Element diameter: 3.175mm
  • Maximum range of incidence angle: 53.8° ± 1.5°
  • Standard absorption (cm⁻¹ acetone): 0.38AU ± 0.02AU
  • Structural material: 316 stainless steel
  • Seals: Chemraz or Kalraz O type rings
• Note: Other material instrument accessories that can achieve the same test requirements may also be used.

6. Recommended operating conditions

• Sample test temperature: 15℃ to 38℃ (see 9.2, 12.1)
• Spectral region: 4000cm⁻¹ to 600cm⁻¹ (see 12.6, A.2.3)
• Spectral region used by the calibration model: 710cm⁻¹ to 660cm⁻¹ (see A.2.4)
• Benzene net absorption peak fitting region: 691cm⁻¹ to 660cm⁻¹ (see 12.7.1.2, A.2.4.4)
• Resolution: 4cm⁻¹; scan time shall not exceed 60s (see 7.1)
• Other recommended operating conditions: not provided in the available source

7. Sampling and sample preparation

• 9.1 Sampling:
  • Volatilization of benzene or other components in the sample will affect the measurement of benzene content. Care should be taken during sample transfer or handling to prevent changes in sample composition to the greatest extent possible.
  • Sample according to the requirements of GB/T 4756 method.
  • Samples should be stored under ambient conditions of 0℃ to 5℃.
  • If leakage of the stored sample is found, the sample must be re-taken.
• 9.2 Sample preparation:
  • When analyzing samples by mid-infrared spectroscopy, the sample test should be kept under conditions of 15℃ to 38℃. If the sample is stored in a refrigerator before analysis, it should be allowed to stand until it reaches 15℃ to 38℃ before measurement.
  • After analysis, if the sample needs to be preserved, it should be placed again in a refrigerator or ice bath at 0℃ to 5℃.

8. Analysis procedure

• 12.1 Before analysis, the sample temperature should be maintained at 15℃ to 38℃.
• 12.2 Sample cell cleaning: the sample cell and the tubing between the inlet and outlet should be rinsed thoroughly with sufficient pentane, then air or nitrogen should be passed through to completely evaporate the residual pentane. Ensure that no residual sample remains in the sample cell, so as to obtain a baseline spectrum of the empty sample cell.
• 12.3 If necessary, the baseline spectrum may be obtained using the method set by the instrument manufacturer.
• 12.4 Before analyzing unknown samples, the instrument should undergo a quality control check. Rinse the sample cell at least three times with QC standard, ensure the sample cell is filled with sample, measure the spectrum of the quality control standard and calculate the analysis result, and compare the measured value of the QC standard with the known value.
• 12.5 Introduce the sample using the method set by the instrument manufacturer, and rinse the sample cell at least three times with the sample to ensure the sample cell is filled with sample.
• 12.6 Measure the spectrum of the sample using a FTIR type spectrometer to obtain the absorption spectrum data of the sample in the region from 4000cm⁻¹ to 600cm⁻¹.
• 12.7 Determine the benzene concentration [% (volume fraction)] according to the calibration equation established in Annex A.
• The 12.7.1 FTIR type instrument uses the classical least squares fitting method for calibration, fitting the absorption spectrum in the spectral region from 710cm⁻¹ to 660cm⁻¹ (K matrix method). The fitting matrix must include the spectra of toluene, 1,3- xylene, 3- ethyltoluene, 1,3,5- trimethylbenzene, ethylbenzene, propylbenzene, etc. (determined according to A.2.4.2).
• 12.7.1.1 To eliminate spectral line overlap, the contribution of the relevant aromatic hydrocarbon components (toluene, 1,3- xylene, 3- ethyltoluene, 1,3,5- trimethylbenzene, ethylbenzene, propylbenzene) to the benzene spectrum should be subtracted from the sample spectrum. This contribution can be obtained by multiplying the corresponding coefficients by the corresponding spectral absorbances (the coefficients are the results obtained by fitting the absorption spectrum using the classical least squares method). What remains is the net absorption peak of benzene.
• 12.7.1.2 In the spectral region from 691cm⁻¹ to 660cm⁻¹, fit the net absorption peak of benzene using a Lorentzian band shape function with linear background correction (determined according to A.2.4.4), and measure the peak height of the net absorption peak of benzene.
• 12.7.1.3 Substitute the peak height of the net absorption peak of benzene measured in 12.7.1.2 into formula (A.1) (determined according to A.2.4.4) to measure the concentration of benzene in the sample [% (mass fraction)].
• 12.7.1.4 Determine the density of the sample according to the method of GB/T 1884 or SH/T 0604.
• 12.7.1.5 Use formula (1) of Chapter 13 to convert the benzene content from mass fraction to volume fraction.
• Quality control check (Chapter 11):
  • 11.1 Before measurement each day, according to the operating procedure of Chapter 12, measure the benzene content of at least one quality control sample with known benzene content (preparation method see 11.1.1 and 11.1.2) to check the validity of the instrument and calibration model.
  • 11.1.1 Prepare standard samples with known benzene content according to the provisions of Appendix A, and convert them to volume fraction using the measured density (see Chapter 13). Prepare at least one standard sample with a benzene mass fraction of 1.2% ± 0.2%, i.e., a volume fraction of approximately 1.0%; other concentration standard samples may also be prepared for quality control checks. Quality control standard samples (QC standard samples) should be sufficient in number; standard samples prepared from the same batch of raw materials should at least satisfy the measurements for 30 quality control checks. Store the quality control standard samples sealed for future quality control check analyses.
  • 11.2 If a quality control standard sample with a benzene mass fraction of 1.2% is used for measurement, and the measured volume fraction differs from the known value by more than 0.12% (volume fraction), it indicates that there is a problem with the measurement system. Before analyzing the cause of the problem and making corrections, benzene content determination is not allowed.
  • 11.3 If solving the problem requires repairing the instrument or recalibrating, then before the system measures the benzene content of samples, the performance of the instrument should be re-verified as described in A.3.
• Instrument calibration and verification (Chapter 10):
  • 10.1 Instruments used by users must be calibrated according to the provisions of Appendix A. Calibration of the instrument may be completed by the instrument manufacturer when the instrument is provided. After instrument maintenance, calibration must be performed again.
  • 10.2 Before use, the instrument must be verified according to the provisions of Appendix A. Instrument verification must also be performed after instrument initialization, recalibration, or repair.

9. Qualitative and Quantitative and Calculation Formulas

• 13.1 Calculate the volume fraction of benzene in the sample according to formula (1) (%):
  • φ = c₀ × (ρ₂₀ / 0.8789) ………………(1)
  • Where:
    • φ—— Volume fraction of benzene in the sample, %;
    • c₀——Mass fraction of benzene in the sample, %;
    • ρ₂₀——Density of the sample at 20℃, determined according to the method of GB/T 1884 or SH/T 0604 and converted according to GB/T 1885;
    • 0.8789—— Density of benzene at 20℃, g/cm³.
• Appendix A Calibration:
  • A.2.1 Establish a calibration model separately for each device. Strictly follow the infrared spectroscopy multivariate calibration analysis standard ASTM E1655 to establish the calibration, continuous frequency spectral region.
  • A.2.2 Before analysis, all samples should be placed in a laboratory at 15℃ ~ 38℃. Fill the sample cell with standard sample according to the method of ASTM E168 or according to the instrument manufacturer's design requirements.
  • A.2.3 Use a FTIR type spectrometer to measure the spectrum of the sample to be tested, obtaining the absorption spectrum of the sample in the region from 4000cm⁻¹ to 600cm⁻¹.
  • A.2.4 Establish a corresponding calibration model for each sample. The calibration model uses the spectral region from 710cm⁻¹ to 660cm⁻¹.
  • A.2.4.1 In the spectral region from 710cm⁻¹ to 660cm⁻¹, measure the spectra of n-hexane and the six background correction mixtures prepared according to Table A.2. Subtract 0.8 times the n-hexane spectrum from the mixture spectrum; the result is the spectrum of the individual aromatic hydrocarbon.
  • A.2.4.2 Use the classical least squares fitting method (K matrix method) to fit the absorption spectrum in the spectral region from 710cm⁻¹ to 660cm⁻¹. The fitting matrix should include the spectral contributions of toluene, 1,3- xylene, 3- ethyltoluene, 1,3,5- trimethylbenzene, ethylbenzene, propylbenzene, etc.
  • A.2.4.3 To eliminate spectral line overlap, the contribution of the relevant aromatic hydrocarbon components (toluene, 1,3- xylene, 3- ethyltoluene, 1,3,5- trimethylbenzene, ethylbenzene, propylbenzene) to the benzene spectrum should be subtracted from the sample spectrum. This contribution can be obtained by multiplying the corresponding coefficients by the corresponding spectral absorbances (the coefficients are the results obtained by fitting the absorption spectrum using the classical least squares method). What remains is the net absorption peak of benzene.
  • A.2.4.4 In the spectral region from 691cm⁻¹ to 660cm⁻¹, fit the net absorption peak of benzene using a Lorentzian band shape function with linear background; the Lorentzian band shape function equation L(w) is given in formula (A.1):
    • L(w) = A·Γ² / [Γ² + (w₀ − w)²] + h·w + d ………………(A.1)
    • Where (visible in the original text): peak height; peak width; center wavenumber; wavenumber; linear background slope; linear background intercept. [Requires confirmation (source-text extraction): the correspondence between symbols and variables is incomplete in the original text]
    • Use the least squares method to fit the parameters A, Γ, w₀, h and d.
  • A.2.4.5 The net peak height of benzene determines the calibration parameters (parameter A corresponds to the absorption of benzene) [Requires confirmation (source-text extraction): the numbering and text of this clause are incomplete in the original text]
• Instrument verification (A.3):
  • After instrument calibration, the instrument must be verified with verification standard samples to ensure accurate measurement of benzene in spark-ignition engine fuels. High-content mono-substituted aromatic hydrocarbon components generally have interfering effects. This verification step must be performed after instrument initialization, recalibration, or repair.
  • A.3.1 Verification standard sample preparation: Prepare multi-component benzene verification standard samples (mass fraction). The preparation of verification standard samples is similar to the calibration set standard samples used to establish the model, but not exactly the same. When preparing verification standard samples, vary the content of benzene and interfering components within a certain range, i.e., the concentration span of the verification set standard samples is at least…… [Requires confirmation (source-text extraction): the text here is incomplete in the original text]. The required number of verification standard samples is recommended to refer to the relevant provisions of chemometrics multivariate calibration, generally the number of independent variables in the calibration equation…… [Requires confirmation (source-text extraction)]. For calibration methods using classical least squares spectral line fitting, at least 20 verification standard samples are required.
  • A.3.2 Obtain verification data
  • A.3.3 Measure the benzene content of the verification set standard samples according to the test procedure required in Chapter 12, expressed as volume fraction. Determine the validity of the instrument according to the relevant provisions of method ASTM E2056.
  • Note: Because this method was established before the ASTM E2056 method, this method cannot provide all the data required by ASTM E2056. The following steps are consistent with the purpose of the ASTM E2056 method.
  • A.3.4 The calculation of the standard deviation of the verification set is given in formula (A.2) [Requires confirmation (source-text extraction): the formula body is incomplete]
  • The calculation of the pooled standard deviation of the verification set (PSEQ) is given in formula (A.3) [Requires confirmation (source-text extraction): the formula body is incomplete]
  • The calculation of the degrees of freedom of the pooled standard deviation of the verification set (DOF) is given in formula (A.4) [Requires confirmation (source-text extraction): the formula body is incomplete]
  • A.3.4.1 For each type of instrument, calculate F = SEQ/PSEQ, compare this F value with the critical value of F, F critical = g/DOF (PSEQ). [Requires confirmation (source-text extraction): the original text is incomplete]
  • A.3.4.2 The PSEQ value and degrees of freedom DOF (PSEQ) value of the FTIR type spectrometer (CLS method) are shown in Table A.3, and the critical value of F is shown in Table A.4.
  • A.3.4.3 If the F value is less than or equal to the critical value of F in the table, the instrument is considered to have reliable testing performance. If the F value is greater than the critical value of F in the table, the instrument is considered to have unreliable testing performance.
  • Table A.3 FTIR type spectrometer (CLS method) PSEQ value: PSEQ = 0.0812; DOF (PSEQ) = 270
  • Table A.4 F critical values (partially listed in the original text): denominator DOF (PSEQ) as columns 20, 25, 30, 35, 40, 45, etc., numerator as rows 389, 306, 270, etc., values such as 1.60, 1.53, 1.49, 1.45, 1.43, 1.40, 1.38, 1.54, 1.50, 1.46, 1.41, 1.39, 1.61, 1.55, 1.47, 1.44, 1.42, 1.40, etc. (source-text extraction the table is incomplete, the cell-by-cell correspondence requires confirmation (source-text extraction)).

10. Precision

• 14.1 The precision of this standard is determined through statistical examination of test results from multiple laboratories (confidence level of 95%). As shown in Table 1.
• 14.2 Repeatability:
  • 14.2.1 For benzene content in the range 0.1% ~ 1.8% (volume fraction), the difference between two results obtained by the same operator, using the same instrument, on the same test sample, in repeated determinations shall not exceed the value of formula (2):
    • r = 0.047 + 0.043X ………………(2)
    • Where: X—— the average of two determinations of benzene content, % (volume fraction).
  • 14.2.2 For benzene content at 4% (volume fraction), the difference between two results obtained by the same operator, using the same instrument, on the same test sample, in repeated determinations shall not exceed 0.18% (volume fraction).
• 14.3 Reproducibility:
  • 14.3.1 For benzene content in the range 0.1% ~ 1.8% (volume fraction), the difference between two independent results obtained by different laboratories, different operators, on the same test sample shall not exceed the value of formula (3):
    • R = 0.099 + 0.031X ………………(3)
    • Where: X—— the average of two determinations of benzene content, % (volume fraction).
  • 14.3.2 For benzene content at 4% (volume fraction), the difference between two independent results obtained by different laboratories, different operators, on the same test sample shall not exceed 0.23% (volume fraction).
• 14.4 Bias: Since no suitable reference material is available, no statement regarding bias has been made.
• Table 1 Repeatability and reproducibility (unit: % (volume fraction)):
  • Benzene content: 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.8, 4
  • Repeatability: 0.05, 0.06, 0.07, 0.08, 0.09, 0.09, 0.10, 0.11, 0.12, 0.18
  • Reproducibility: 0.10, 0.11, 0.11, 0.12, 0.13, 0.13, 0.14, 0.15, 0.15, 0.23

11. Report

• 13.2 Report the volume fraction of benzene in the test sample φ, with the result accurate to 0.1% (volume fraction).

12. Normative references

• GB/T 1884 Standard test method for density of crude petroleum and liquid petroleum products (hydrometer method) (GB/T 1884—2000, eqv ISO 3675:1998)
• GB/T 1885 Petroleum measurement tables (GB/T 1885—1998, eqv ISO 91-2:1991)
• GB/T 4756 Standard practice for manual sampling of petroleum liquids (GB/T 4756—1998, eqv ISO 3170:1988)
• SH/T 0604 Standard test method for density of crude petroleum and petroleum products (U oscillating tube method) (SH/T 0604—2000, eqv ISO 12185:1996)
• ASTM E168 General technical specification for quantitative analysis by infrared spectroscopy
• ASTM E1655 Standard for infrared multivariate calibration analysis
• ASTM E2056 Specification for verification of spectrometers and spectrophotometers used for multivariate analysis
• Adopted document: ASTM D6277-01

13. Source and verification

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14. Quick reference for questions and answers