GB/T 33649-2017 Determination of oxygenates and aniline compounds in motor gasoline by gas chromatography

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Card type: Test method

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

• Standard number: GB/T 33649-2017
• Chinese full name: Determination of oxygenates and aniline compounds in motor gasoline by gas chromatography
• English name: Determination of oxygenates and aniline compounds in motor gasoline—Gas chromatography
• Replacement relationship: not provided in the available source (the foreword only states that it was drafted in accordance with GB/T 1.1—2009, and does not mention any replaced standard)
• Publication/implementation: published in 2017-05-12, implemented in 2017-12-01
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants (SAC/TC 280)
• Responsible drafting organization: Sinopec Research Institute of Petroleum Processing

2. Scope of application and non-applicability

• Applicable to the determination of the content of dimethoxymethane (also known as methylal), ethyl acetate, sec-butyl acetate, aniline, N--methylaniline, o-methylaniline, m-methylaniline and p-methylaniline in motor gasoline (including ethanol gasoline).
• It can also be used to determine the content of oxygenate components such as methyl tert-butyl ether and methanol in motor gasoline.
• Mass fraction ranges for the determination of added components:
  • Methyl tert-butyl ether: 0.05%~15%
  • Methanol, sec-butyl acetate, dimethoxymethane, ethyl acetate and aniline: 0.05%~10%
  • N--methylaniline, o-methylaniline, p-methylaniline and m-methylaniline: 0.01%~10%
• Note: This standard can qualitatively detect dimethyl carbonate, but does not include quantitative determination.
• When necessary, it can also be used to determine the content of oxygenates such as C2~C4 alcohols, ethyl tert-butyl ether, and tert-amyl methyl ether, but this standard does not provide precision data for these components; the content of these oxygenates can be determined according to the method of NB/SH/T 0663.
• Trace ketone compounds such as acetone and butanone that may be present in motor gasoline do not interfere with the determination.

3. Method principle

• 3.1 The test sample and the internal standard ethylene glycol dimethyl ether (or 2--hexanone) are introduced together into a chromatographic system equipped with a microfluidic flow control central cutting (Deans switch) assembly and two capillary chromatographic columns. The components first enter the non-polar chromatographic column and are separated in order of boiling point. Through switching of the solenoid valve of the central cutting assembly, only the components with boiling points lower than 2--hexanone are allowed to flow out of the non-polar column and then enter the strongly polar chromatographic column connected to it, while the remaining components pass directly to the detector for detection. The components entering the strongly polar chromatographic column are separated on this column into hydrocarbon components and oxygenates, detected by a hydrogen flame ionization detector, and quantified by the internal standard method. It can quantitatively determine the content of dimethoxymethane, ethyl acetate, and sec-butyl acetate in gasoline fractions, and can also simultaneously determine the content of oxygenates such as methyl tert-butyl ether (MTBE) and methanol contained in the sample.
• 3.2 A second injection analysis is performed, adjusting the switching time of the solenoid valve of the central cutting assembly so that the aniline components enter the strongly polar analytical column together with the hydrocarbon components, and on the analytical column the aniline compounds are separated from the hydrocarbon components, detected by a hydrogen flame ionization detector, and quantified by the internal standard method, allowing quantitative determination of the content of aniline, N--methylaniline, o-methylaniline, m-methylaniline and p-methylaniline.

4. Reagents and materials (purity / preparation)

• 4.1 Unless otherwise specified, the reagents used in this method are of analytical grade; the use of other reagents of higher purity is permitted.
• 4.2 Carrier gas: nitrogen, purity not less than 99.99%. Warning—high-pressure compressed gas.
• 4.3 Fuel gas: hydrogen, purity not less than 99.99%. Warning—high-pressure compressed gas.
• 4.4 Air: dry air, free of organic compounds.
• 4.5 Carrier gas purifier: used to purify the carrier gas.
• 4.6 Reagents used for qualitative and quantitative analysis, including: methanol, ethanol, isopropanol, n-propanol, isobutanol, sec-butanol, tert-butanol, methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), diisopropyl ether (DIPE), tert-amyl methyl ether (TAME), dimethoxymethane, ethyl acetate, sec-butyl acetate, aniline, N--methylaniline, o-methylaniline, m-methylaniline, p-methylaniline, 2--hexanone, n-heptane, isooctane, xylene.
• 4.7 Quality control check sample: a sample used for routine monitoring of the reliability of the chromatographic system, a gasoline test sample containing dimethoxymethane, sec-butyl acetate and aniline at mass fractions of 1% respectively, prepared by quantitatively adding the above components to a gasoline test sample that does not contain these components, or purchased. The quality control check sample shall be sealed in ampoules and stored at 0℃~4℃, and its composition shall remain unchanged during storage. (The temperature number source-text extraction is missing at this point in the source text, requires confirmation against the source standard (source-text extraction))

5. Instruments and equipment

• 5.1 Gas chromatograph: equipped with a split/splitless inlet, dual hydrogen flame ionization detectors (FID) and a central cutting (Deans Switch) assembly; any chromatograph that can be operated under the conditions given in Table 1 may be used. Figure 1 is a schematic diagram of the system connections.
• 5.2 Chromatographic columns: Column 1 is a non-polar column, with a stationary phase of cross-linked 100% methylpolysiloxane, column length 30 m, internal diameter 0.32 mm, film thickness 1.0 μm; Column 2 is a strongly polar column, of type DB-OxyPLOT or CP-Lowox (see note), column length 10 m, internal diameter 0.53 mm, film thickness 10 μm. Other chromatographic columns that can achieve the separation effect shown in 8.1 may also be used.
• 5.3 Recorder: electronic integrator or computer capable of recording chromatograms. The electronic integrator or computer can determine components at a mass fraction of 0.01% and has a satisfactory signal-to-noise ratio.
• 5.4 Microsyringe: capacity 5 μL or 10 μL.
• 5.5 Balance: accurate to 0.0001 g.
• 5.6 Volumetric flask or stoppered vial: 10 mL, 100 mL.
• 5.7 Graduated pipette: 1 mL, 10 mL.

6. Recommended operating conditions

Table 1 Typical instrument conditions:

• Column oven: program 100℃ hold 5 min, then ramp at 5℃/min to 210℃, hold 8 min; run time 35 min.
• Front inlet: split mode; split ratio 80:1~100:1; vaporization chamber temperature 250℃; carrier gas (N2) constant pressure 68.95 kPa; total flow 33 mL/min; split flow 30 mL/min.
• Chromatographic columns: Column 1 non-polar column DB-1, column length 30 m, internal diameter 0.32 mm, film thickness 1.0 μm; Column 2 strongly polar column OxyPLOT or Lowox, column length 10 m, internal diameter 0.53 mm, film thickness 10 μm.
• Pressure control module (PCM): constant pressure 29.51 kPa.
• Front/rear detector (FID): temperature 280℃; hydrogen flow 30 mL/min; air flow 350 mL/min; makeup flow 25 mL/min.
• Run time table (first injection): 0.1 min solenoid valve position 1 open; 4.0 min solenoid valve position 1 closed.
• Run time table (second injection, using OxyPLOT chromatographic column): 0.1 min solenoid valve position 1 open; 11.0 min solenoid valve position 1 closed.
• Run time table (second injection, using Lowox chromatographic column): 3.2 min solenoid valve position 1 open; 11.0 min solenoid valve position 1 closed.
• Injection volume: 0.2 μL.
• Note: The table gives typical run time tables; suitable run time tables are determined according to the requirements of 7.3.

7. Sampling and sample preparation

• 6 Sampling: Sampling shall ensure that the sample taken is representative. When sampling from a storage tank or production line, sample according to the recommended methods of GB/T 4756 or GB/T 27867. Samples shall be stored at 0℃~4℃.
• 9.1 Preparation of test sample solution: Take 0.5 mL of internal standard and transfer to a 10 mL volumetric flask or stoppered vial. Record the net mass of internal standard added (W.), accurate to 0.1 mg. Reweigh the volumetric flask or stoppered vial, add 9.5 mL of cooled motor gasoline test sample to the volumetric flask or vial, cap, and record the net mass of test sample added (W&). Mix thoroughly. If an autosampler is used, transfer a portion of the solution to a glass vial for gas chromatography. Seal the glass vial for gas chromatography with an aluminum cap with a polytetrafluoroethylene liner. If the test sample solution is not analyzed immediately, it shall be sealed and stored at 0℃~5℃.

8. Analytical procedure

• 7 Instrument preparation
  • 7.1 Establishment of instrument configuration and conditions: The chromatographic system connections are shown in Figure 1. A microfluidic flow path controlled center-cutting (Deans switch) system is used to achieve switching of components between two different types of chromatographic columns. The inlet of column 1 is connected to the split injection port, and the outlet is connected to column 2 through a Deans switch connection assembly. The outlet of column 2 is connected to the detector. The damping column is a section of empty quartz capillary column, whose resistance matches that of column 2. By changing the solenoid valve switch position, the flow direction of the carrier gas exiting column 1 is changed, so that components exiting column 1 selectively enter column 2 for further separation or directly pass through the damping column to the detector for detection. Column 1 is a non-polar capillary chromatographic column, and column 2 is a strongly polar chromatographic column with special retention ability for oxygen-containing compounds. At the start of injection, the solenoid valve is set to the "open" state. After the sample is injected, the components are separated on column 1 in order of boiling point and sequentially enter column 2 [Figure 1a)]. When hydrocarbon and non-hydrocarbon components with boiling points lower than 2- hexanone enter column 2, the solenoid valve is closed, and the carrier gas flow direction is changed, so that components with boiling points higher than 2- hexanone pass through the damping column to detector 1 for detection [Figure 1b)], thereby eliminating the influence of heavy hydrocarbon components on oxygen-containing compounds. Light hydrocarbon components and oxygen-containing compounds enter column 2 for separation and are detected by detector 2.
  • 7.2 Column aging: New columns should be aged at 300℃ for more than 10 h; columns that have not been operated for a long time should be aged at 210℃ for more than 4 h before analysis, and if necessary, the aging temperature should be increased to 300℃.
  • 7.3 Determination of valve cutting time
    • 7.3.1 Prepare a n-hexane solution containing about 5% each of 2- hexanone and m-methylaniline. Place the valve in the closed state, inject 0.2 μL of this solution into the chromatographic system, and simultaneously start the chromatograph and record the chromatogram. At this time, all components pass through column 1 and then through the damping column to detector 1 (FID1) for detection. On the chromatogram, the time when the 2- hexanone peak ends plus 0.1 min is the valve cutting time T1, and the time when the m-methylaniline peak ends plus 0.1 min is the valve cutting time T2.
    • 7.3.2 For the first injection, set the solenoid valve open at 0.1 min and closed at T1 time, so that all components with retention time on column 1 not greater than T1 enter column 2, allowing oxygen-containing compounds and hydrocarbon components to be separated and enter detector 2 for detection; all components with retention time greater than T1 exit column 1 and then pass through the damping column to enter detector 1 for detection. For the second injection, set the solenoid valve open at 0.1 min and closed at T2 time, so that all components with retention time on column 1 not greater than T2 enter column 2, allowing aniline compounds and hydrocarbon components to be separated and enter detector 2 for detection; all components with retention time greater than T2 exit column 1 and then pass through the damping column to enter detector 1 for detection. For the case of using column Lowox, the time for opening the solenoid valve in the second injection can also be selected as the time before sec-butyl acetate flows out of column 1; the determination of the time can refer to the relevant content in Appendix A.
  • 7.4 Confirmation of column resolution: Prepare a n-hexane solution containing 1% each of sec-butyl acetate and isobutanol. Analyze this mixed solution under the determined chromatographic conditions, switch the valve using the determined valve switching time for the first injection, and record the chromatogram. Calculate the resolution (Res) of the sec-butyl acetate and isobutanol chromatographic peaks according to formula (1), which should not be less than 1.5. Otherwise, re-aging or replace the analytical column.
    • Formula (1): Res = 1.699(tR(A) − tR(B))/(Wh(A) + Wh(B))
    • Where: tR(A) retention time of sec-butyl acetate, min; tR(B) retention time of isobutanol, min; Wh(A) half-peak width of sec-butyl acetate, min; Wh(B) half-peak width of isobutanol, min.
• 8 Calibration
  • 8.1 Qualitative: Prepare a n-heptane mixed standard containing about 1% each of C1~C4 alcohols, MTBE, ETBE, TAME, DIPE, dimethoxymethane, ethyl acetate, sec-butyl acetate, ethylene glycol dimethyl ether (or 2- hexanone), aniline, N- methylaniline, o-methylaniline, m-methylaniline, and p-methylaniline. Inject and analyze under the conditions in Table 1, and open or close the solenoid valve according to the conditions in Table 1 or the times determined in 7.3. Qualitatively identify the components based on the retention times of the component peaks. When using column OxyPLOT as the analytical column, the chromatogram obtained from the first injection is shown in Figure 2, and the chromatogram of aniline compounds on column 2 obtained from the second injection is shown in Figure 3. When using column Lowox as the analytical column, the chromatogram on column 2 obtained from the first injection is shown in Figure 4, and the chromatogram of aniline compounds on column 2 obtained from the second injection is shown in Figure 5. The solenoid valve opening time selected for the second injection is the time before sec-butyl acetate elutes (3.2 min). Due to possible slight differences in the polarity of the strongly polar column used, and because the column easily absorbs moisture causing changes in separation, the qualitative identification of components should be determined based on the actual separation situation of the column used. When necessary, center-cutting can be used by setting different solenoid valve switching times to cut target components onto the analytical column for qualitative and quantitative analysis, or to prevent certain interfering components from entering the analytical column thereby eliminating their interference with qualitative and quantitative analysis. For detailed instructions, see Appendix A.
  • 8.2 Preparation of calibration standards
    • 8.2.1 Accurately weigh and mix each component in order of volatility from low to high to prepare a standard containing multiple components at the required concentrations. The components include dimethoxymethane, sec-butyl acetate, ethyl acetate, aniline, N- methylaniline, o-methylaniline, m-methylaniline, and ethylene glycol dimethyl ether. When using column Lowox, replace ethylene glycol dimethyl ether with 2- hexanone. The weighing order is methylaniline, aniline, sec-butyl acetate, ethyl acetate, and dimethoxymethane. For each component to be determined, use at least 5 calibration points and ensure that the concentration of each component is within the calibration range. To minimize mutual interference between components, standards of the required components can also be prepared individually or in groups. When necessary, standards with different concentrations containing C1~C4 alcohols, MTBE, TAME, etc. can also be prepared for quantitative analysis of these components. The mass fraction range of components in the standard is 0.01%~10%. For different components to be determined, calibration standards of different concentrations can be selected according to the actual range of added content in the sample. Note: When the mass fractions of aniline and N- methylaniline are higher than 5%, their chromatographic peaks will overlap significantly. Preparing standards of the two components separately can eliminate mutual interference caused by high component content.
    • 8.2.2 Before preparing standards, determine the purity of each component by capillary gas chromatography and correct the reagent purity. Reagents with purity not less than 99.5% should be used.
    • 8.2.3 Follow the steps below to transfer a certain volume of the component to be determined using a pipette, dropper, or syringe into a 100 mL volumetric flask or a 100 mL stoppered vial to prepare the standard. Cap the bottle and record the mass of the empty volumetric flask or vial, accurate to 0.1 mg. Open the cap and carefully add the components to be determined into the bottle starting from the least volatile component. Cap the bottle and record the net mass of the added component to be determined (W_i), accurate to 0.1 mg. Repeat this addition and weighing step for each component to be determined. The volume fraction of all added components to be determined should not exceed 50%. Similarly, add 5 mL internal standard (ethylene glycol dimethyl ether or 2- hexanone), record its net mass (W_s), accurate to 0.1 mg. Dilute each standard to the mark with isooctane-xylene mixed solution (volume ratio 1:1). When not in use, store the capped calibration standards sealed at 0℃~4℃.
  • 8.3 Calibration procedure
    • 8.3.1 Analyze each calibration standard according to the conditions in Table 1. For the first analysis, perform the solenoid valve switching operation at 0.1 min and time T1. For the second analysis, perform the solenoid valve switching operation at 0.1 min and time T2.
    • 8.3.2 Analyze the calibration standards according to the steps in 8.3.1. Measure the peak areas of oxygen-containing compounds and the internal standard from the first analysis. Measure the peak areas of the internal standard and aniline compounds from the second analysis. Determine the response ratio (rsp_i) and mass ratio (amt_i) for each component in each standard according to formula (2) and formula (3).
      • Formula (2): rsp_i = A_i / A_s; where: A_i peak area of the component to be determined; A_s peak area of the internal standard.
      • Formula (3): amt_i = W_i / W_s; where: W_i mass of the component to be determined, g; W_s mass of the internal standard, g.
    • 8.3.3 Using the response ratio (rsp_i) as the Y axis and the mass ratio (amt_i) as the X axis, construct the calibration curve for each component to be determined by the least squares method. An example is shown in Figure 6. The calibration curve of a component can be expressed according to equation (4): rsp_i = (a_i)(amt_i) + b_i; where: rsp_i is the response ratio of the compound to be determined i (y axis); a_i is the slope of the linear equation of the compound to be determined i; amt_i is the mass ratio of the compound to be determined i (x axis); b_i is the y axis intercept.
    • 8.3.4 Calculate the correlation coefficient r value of the calibration curve for each component by the least squares method. The r value should not be less than 0.990. If r does not obtain the expected value, rerun the calibration process or check the instrument parameters and hardware equipment.
• 9 Test procedure
  • 9.1 Preparation of sample solution (see Section 7).
  • 9.2 Chromatographic analysis: Using the same injection technique and injection volume as used in the calibration analysis, introduce 0.2 μL representative sample solution (9.1) into the chromatograph. Perform two chromatographic analyses of the sample solution, and perform the opening and closing operations of the solenoid valve at the times determined according to 7.3. The first analysis uses times 0.1 min and T1 to open and close the solenoid valve. The second analysis uses times 0.1 min and T2 to open and close the solenoid valve.

9. Qualitative and quantitative determination and calculation formulas

• 10.1 Mass fraction of components: After peak identification, measure the peak areas of oxygen-containing compounds and the internal standard from the first analysis. Measure the peak areas of aniline compounds and the internal standard from the second analysis. According to the slope and axis intercept of the equation already obtained in equation (4), use the response ratio of the component to be determined to the internal standard (rsp_i) to calculate the mass of each component to be determined in the motor gasoline sample (W_i). The calculation formula (5) is as follows:
  • Equation (5): W_i = (A_i / A_s) × W_s / (a_i × (A_i / A_s) + b_i) (the original formula source-text extraction is incomplete, requires confirmation against the source standard (source-text extraction))
  • Where: W_i is the mass of the component to be determined i, g; A_i is the peak area of the component to be determined i; A_s is the peak area of the internal standard; W_s is the mass of the internal standard added, g; a_i, b_i are the slope and intercept of the equation obtained according to equation (4).
• 10.2 Calculate the mass fraction c_i of each component to be determined according to equation (6), %:
  • Equation (6): c_i = (W_i / W_g) × 100; where: W_g is the mass of the motor gasoline sample, g.
• 10.3 Report the mass fraction w_i of the component to be determined, with the result accurate to 0.01%.

10. Precision

• 11.1 Overview: The precision of this standard was determined by statistical calculation from the test results of 6 laboratories on 22 gasoline samples containing different additive components, in accordance with the requirements of the GB/T 6683 method (95% confidence level).
• 11.2 Repeatability (r): The difference between two results obtained by the same operator using the same equipment under stable operating conditions on the same sample, repeated determinations, shall not exceed the repeatability limit in Table 2.
• 11.3 Reproducibility (R): The difference between two single, independent test results obtained by different operators in different laboratories on the same sample shall not exceed the reproducibility limit in Table 2.
• Table 2 Repeatability limit and reproducibility limit (% mass fraction):

• Note: X is the average of two determination results.
• 11.4 Bias: Because there is no suitable reference material, the bias of this method has not been determined.

11. Report

• 13 The test report shall at least provide the following aspects:
  • Test object;
  • The standard used (including the year of publication or issuance);
  • Test results;
  • Observed abnormal phenomena;
  • Test date.

12. Referenced documents

• GB/T 4756 Petroleum liquids — Manual sampling
• GB/T 6683 Petroleum products — Determination of precision data in relation to methods of test
• GB/T 27867 Petroleum liquids — Automatic pipeline sampling
• NB/SH/T 0663 Determination of alcohols and ethers in gasoline — Gas chromatography

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick reference for questions and answers