GB/T 33646-2017 Determination of ester 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 33646—2017
• Chinese name: Determination of ester compounds in motor gasoline by gas chromatography
• English name: Determination of the ester compounds in motor rasoline—Gas chromatography (as in the source text, suspected to be an error of gasoline for source-text extraction, requires confirmation against the source standard (source-text extraction))
• 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/TC280)
• Drafting organizations: Shenzhen Institute of Metrology and Quality Inspection, Petrochemical Research Institute of PetroChina Company Limited, China National Offshore Oil Corporation Refining and Chemical Company Limited, National Petroleum Product Quality Supervision and Inspection Center (Shenyang)
• Main drafters: Zhao Yan, Lin Haoxue, Wang Lin, Xu Dongyu, Ji Ming, Zhang Shiyuan, Huang Weilin, Ouyang Kechuan, Lü Huanming, Li Dong

2. Scope of application and non-applicability

• Applicable to: determination of ester compounds in motor gasoline (including ethyl acetate, sec-butyl acetate, dimethyl carbonate).
• Determination concentration range: 50 mg/L～2000 mg/L; beyond this range, this method can also be used for determination, but the precision has not been investigated.
• Not applicable to: alcohol-containing gasoline.
• Interference: certain alcohol compounds (such as methanol, ethanol, tert-amyl alcohol and sec-butanol) and ketone compounds (such as acetone and butanone) may interfere with the determination of ester compounds.
• Warning: personnel using this standard should have regular laboratory work experience; this standard does not point out all possible safety issues, and users are responsible for taking appropriate safety and health measures and ensuring compliance with the conditions specified in relevant national laws and regulations.

3. Method principle

The sample is directly introduced into a gas chromatograph equipped with series dual columns for determination. The sample first passes through a column with a non-polar stationary phase, and the components are separated in order of boiling point. After n-octane elutes, the non-polar column is backflushed to backflush out components with boiling points greater than n-octane. n-Octane, ester compounds and light components then pass through a column with a strongly polar stationary phase to separate the ester compounds. The eluted components are detected with a thermal conductivity detector or a hydrogen flame ionization detector and recorded with a recorder, the peak areas are measured, and the concentration of each component is calculated by the external standard method.

4. Reagents and materials (purity / preparation)

• 5.1 Ethyl acetate: purity 99% or above.
• 5.2 sec-Butyl acetate: purity 99% or above.
• 5.3 Dimethyl carbonate: purity 99% or above.
• 5.4 Preparation of standard stock solution: respectively weigh 2.0 g (accurate to 0.1 mg) of ethyl acetate (5.1), sec-butyl acetate (5.2), and dimethyl carbonate (5.3), place in a 100 mL volumetric flask, dilute to volume with isooctane-xylene (volume ratio 5:1) solution, shake well, and set aside.
• 5.5 Preparation of standard working solutions: pipette an appropriate volume of the standard stock solution (5.4), and dilute stepwise with isooctane-xylene (volume ratio 5:1) to prepare standard working solutions with a concentration range of 50 mg/L～2000 mg/L, and shake well. The concentration of the standard working solutions should cover the concentration of ester compounds in the sample to be tested, and the standard working solution series should contain at least 5 concentration levels.
• 5.6 n-Nonane: analytical reagent.
• 5.7 n-Octane: analytical reagent.
• 5.8 Isooctane: analytical reagent.
• 5.9 Xylene: analytical reagent.
• 5.10 Compressed air: provides valve power, and the pressure should meet the power requirements for valve switching (if the device provides power to the valve by other means, this material may not be prepared).
• Note: If the detector of the device is of another type, corresponding materials need to be prepared according to the instrument requirements. For example, if the detector is a hydrogen flame ionization detector, air, nitrogen, and hydrogen need to be prepared.
• 5.11 Quality control (QC) sample: used to monitor the reliability of the method; through analysis of the quality control sample, verify whether the precision of the results is within the range of the method. The QC sample may be a stable sample that can represent the characteristics of the sample to be tested, and may be prepared from or purchased as a motor gasoline sample with a matrix similar to the compounds to be tested. The QC sample should be sealed and stored at low temperature, and remain unchanged during storage. The QC sample is checked for the effectiveness of the test process according to the method described in Chapter 10.

5. Instruments and equipment

• 6.1 Gas chromatograph: at least includes a vaporization chamber, a temperature-controlled column oven, a valve switching device, a thermal conductivity detector (or hydrogen flame ionization detector) and a chromatography workstation; the valve switching backflush system is shown in Appendix A.
• 6.2 The chromatographic columns include the following two types:
  • a) Non-polar stationary phase chromatographic column: 0.8 m×3.2 mm, packed with Chromosorb W [containing dimethylpolysiloxane with a mass fraction of 10% (such as OV-101), 180 μm～250 μm];
  • b) Strongly polar chromatographic column: 4.6 m×3.2 mm, packed with Chromosorb P [containing 1,2,3- tris(2- cyanoethoxy)propane (TCEP) with a mass fraction of 20%, 150 μm～180 μm].
• 6.3 Injection device: either an automatic or manual injection device equipped with a 10 μL injection syringe is acceptable; an automatic injection device is recommended.
• 6.4 Electronic balance: accurate to 0.0001 g.
• 6.5 Volumetric flask: 100 mL.
• 6.6 Sample vial: use a small glass vial with a crimp cap or screw cap on top, and with a rubber sealing gasket whose outer layer is polytetrafluoroethylene-faced in the cap.
• 6.7 Pipettes: 0.5 mL, 1.0 mL, 2.0 mL, 5.0 mL.

6. Recommended operating conditions

• 8.1 Gas chromatography conditions: different instruments may have different optimal analysis conditions, so it is impossible to give universal parameters for gas chromatographic analysis. The set parameters should ensure effective separation and determination of the components to be tested. The parameters given in Appendix A have been proven feasible.
• Table A.1 Instrument operating conditions:
  • Carrier gas (helium, purity > 99.999%) flow rate: 20 mL/min
  • Inlet temperature: 250℃
  • Column temperature: 110℃
  • Valve switching time (open time): 2.5 min
  • Valve switching time (close time): 20 min
  • Injection volume: 1 μL
  • Detector temperature (TCD detector): 200℃
  • Non-polar chromatographic column: see 6.2
  • Polar chromatographic column: see 6.2
• Key points for establishing the valve backflush system in Appendix A:
  • A.1.1 Adjust the valve to the forward purge position, adjust flow control valve A so that the flow reaches the flow set value in Table A.1, and measure the flow on the sample side of the detector outlet.
  • A.1.2 Adjust the valve to the backflush position, and measure the flow on the sample side of the detector outlet; if the flow changes compared with Table A.1, adjust flow control valve B so that the flow change is within ±1 mL/min.
  • A.1.3 Switch the valve from the forward purge to the backflush position several times and observe the baseline; the pressure change initially caused by valve switching should not produce baseline changes and drift; if the baseline changes, slightly adjust flow control valve B to make the baseline stable.
  • A.2 Determination of backflush time: prepare an n-nonane solution containing n-octane at a volume ratio of 5%, inject 1 μL of this solution into the system in the forward purge state, and record the chromatogram until n-nonane elutes and the recorder pen returns to the baseline; measure the time from injection until the recorder pen returns to the baseline between the n-octane and n-nonane peaks (s); half of this measured time can be approximately taken as the “backflush time”, and should be finely adjusted between 30 s～60 s; repeat the operation, switch to the backflush state according to the determined “backflush time”, and the n-nonane peak in the obtained n-octane chromatogram should be very small or invisible; if necessary, further experimentally adjust the “backflush time” until all n-octane has peaked and there is little or no n-nonane peak; the “backflush time” required for actual valve operation should be applied to all subsequent calibration and analysis.

7. Sampling and sample preparation

• 7.1 Sampling should be carried out in accordance with GB/T 4756 or other equivalent methods.
• 7.2 From the time the laboratory receives the sample, before completing any subsampling, the original container should be cooled to 0℃～5℃ for storage.
• 7.3 If necessary, transfer the cooled sample to a pressure-sealed container and store at 0℃～5℃ until analysis is required.
• 8.2 Injection: directly take about 1.5 mL of sample into a 2.0 mL sample vial (6.6), manually or with an autosampler inject 1.0 μL of test specimen into the gas chromatograph (6.1), and perform chromatographic analysis according to the optimized method conditions. If the content of ester compounds in the sample exceeds the test range specified in Chapter 1, the sample needs to be appropriately diluted (such as with an isooctane-xylene solution at a volume ratio of 5:1).

8. Analysis procedure

• 8.1 Gas chromatography conditions: see Section 6.
• 8.2 Injection: see Section 7.
• 8.3 Qualitative analysis: determine the standard working solution (5.5) and the test specimen under the same instrument analysis conditions, and perform qualitative analysis based on the retention times of the chromatographic peaks of the ester compounds. For the reference retention times of the three ester compounds, see Appendix B.
• 8.4 Quantitative analysis: inject the series of standard working solutions separately into the instrument for determination, plot with the concentration of the standard working solution as the abscissa and the peak area as the ordinate to obtain the regression equation of the standard working curve, and the linear correlation coefficient γ is not less than 0.995. Substitute the peak area of the ester compounds in the test specimen into the standard working curve to calculate the concentration of the ester compounds in the test specimen.
• Appendix B Reference retention times (Figure B.1): ethyl acetate 8.527 min; sec-butyl acetate 11.977 min; dimethyl carbonate 16.147 min.

9. Qualitative and Quantitative Determination and Calculation Formula

• 9.1 Calculate the concentration of each ester compound c_i from the standard working curve, and substitute it into formula (1) to obtain the concentration of each ester compound in the test sample c.
  • c = c_i × K …………………………(1)
  • Where:
    • c——— concentration of each ester compound in the test sample, in milligrams per liter (mg/L);
    • c_i——— concentration of the ester compound read from the standard working curve, in milligrams per liter (mg/L);
    • K——— dilution factor of the test sample; usually K=1 (see 8.2), if the test sample has been diluted (see 8.2), then K is the volume dilution factor.
• 9.2 If the result is required in units of mass fraction, convert according to formula (2):
  • w = c / (10 × ρ) …………………………(2)
  • Where:
    • w——— mass fraction of various ester compounds in the test sample, %;
    • c——— concentration of various ester compounds in the test sample, in milligrams per liter (mg/L);
    • ρ——— density of the test sample determined according to GB/T 1884, GB/T 1885 or SH/T 0604, in kilograms per cubic meter (kg/m³).
• 9.3 Report the concentration of ester compounds c, with the result accurate to 1 mg/L. Report the mass percentage concentration of ester compounds w (%), with the result accurate to 0.0001% (mass fraction).

10. Precision

• 10.1 Overview: According to the statistical method of GB/T 6683, the precision of this standard is statistically given based on test results from 5 cooperative laboratories, 10 different gasoline samples, with ester compound concentrations in the range of 50 mg/L～2000 mg/L. Table 1 lists the precision values calculated by the formula.
• 10.2 Repeatability (95% confidence level): For the same operator, using the same instrument, under the same test conditions, correctly operating according to the standard, the difference between two determination results for the same test sample shall not exceed the repeatability limit (r) in Table 1.
• 10.3 Reproducibility (95% confidence level): For different operators, using different instruments, in different laboratories, correctly operating according to the standard, the difference between two single and independent determination results obtained for the same test sample shall not exceed the reproducibility limit (R) in Table 1.
• Table 1 Repeatability limit and reproducibility limit:

• Note: X is the arithmetic mean of the two results.

11. Report

• Chapter 11 Quality assurance and control: Ensure the instrument is in normal condition or the detection process is correct by analyzing QC samples. A new standard working curve should be established each time the instrument is turned on, and after running 20 samples, verify the standard working curve once or analyze QC samples. Recalibrate the standard working curve with the middle calibration concentration. If the linear correlation coefficient meets the requirements, the standard working curve verification passes; otherwise, the source of error should be determined, and necessary checks on the method operation and instrument status should be carried out. The difference between the determination result and the reference value of the QC sample should be less than the method reproducibility requirement; otherwise, the source of error should be determined, and necessary checks on the method operation and instrument status should be carried out. If the laboratory has already established a QC or quality assurance (QA) protocol, it can be used when confirming the reliability of test results.
• Chapter 12 Test report: The test report shall at least give the following contents:
  • a) Description of the test sample;
  • b) Standard used;
  • c) Test results (see 9.3);
  • d) Differences from the specified analysis procedure;
  • e) Abnormal phenomena observed during the test;
  • f) Test date.

12. Normative references

• GB/T 1884 Standard test method for density of crude petroleum and liquid petroleum products (hydrometer method)
• GB/T 1885 Petroleum measurement tables
• GB/T 4756 Standard practice for manual sampling of petroleum liquids
• GB/T 6683 Standard practice for determining precision data for test methods of petroleum products
• SH/T 0604 Standard test method for density of crude petroleum and petroleum products (U-shaped oscillating tube method)

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 Q&A Reference