GB/T 8120-1987 Determination of Purity of High-Purity n-Heptane and Isooctane (Capillary Chromatography)

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Card type: Test method

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

• Standard number: GB 8120-87
• Chinese full title: Determination of Purity of High-Purity n-Heptane and Isooctane (Capillary Chromatography)
• English name: High-purity n-heptane and isooctane — Determination of purity — Capillary gas chromatography
• Replacement relationship: not provided in the available source
• Drafting organization: drafted under the responsibility of the Research Institute of Petroleum Processing; technical jurisdiction: Research Institute of Petroleum Processing
• Main drafters: Li Jinglan, Sun Jiahe
• Reference standard: ASTM D2268-84 "Determination of Purity of High-Purity n-Heptane and Isooctane (Capillary Gas Chromatography)"
• Approval/implementation: approved by China Petrochemical Corporation 1987-05-12, implemented on 1988-08-01

2. Scope of application and non-applicability

• Applicable to the analysis of high-purity n-heptane and isooctane (2,2,4- trimethylpentane), with purity greater than 99.5% (volume).
• n-Heptane and isooctane are the main standard oils for determining the octane number of fuel oils.
• This method can also detect hydrocarbon impurity components at concentrations less than 0.01%.
• Non-applicable scope: not provided in the available source.

3. Method principle

The sample is injected into a fused silica capillary chromatographic column of length 50 m, internal diameter 0.25 mm, coated with OV-101. The sample is carried by the carrier gas through the column and separated into individual components. Each component elutes from the column and enters a hydrogen flame ionization detector for detection, and the chromatographic peaks are recorded on a data processor. Using cyclohexane as the internal standard, after measuring the volume percentage of each hydrocarbon impurity component, the purity of n-heptane and isooctane is obtained by the subtraction method.

4. Reagents and materials (purity / preparation)

• 3.1 Dichloromethane: analytical reagent.
• 3.2 Cyclohexane: purity greater than 99.5% (volume), used as internal standard.
• 3.3 Stationary liquid: OV-101.
• 2.2 Materials: ordinary air, hydrogen, and high-purity nitrogen.
• Internal standard preparation (Section 5): Place 15~20 mL standard fuel oil (n-heptane or isooctane) in a pre-weighed 50 mL volumetric flask and weigh; then use a 50 µL syringe to take 50 µL cyclohexane and add it to the volumetric flask containing n-heptane (or isooctane), then weigh again; dilute to the mark with n-heptane (or isooctane) and weigh. When preparing the sample, weigh accurately to 0.0002 g. From the weights and densities of cyclohexane and n-heptane (or isooctane), calculate the volume percentage of the internal standard cyclohexane to 0.001 according to formula (2).
• 20 ℃ densities: cyclohexane 0.7786 g/cm³, n-heptane 0.8838 g/cm³, isooctane 0.6919 g/cm³.
• The volume percentage of internal standard cyclohexane Vₛ is calculated according to formula (2): Vₛ = [ W₁ / ρ₁ ] / [ W₁/ρ₁ + W₂/ρ₂ ] × 100 Where: W₁—weight of cyclohexane, g; W₂—weight of n-heptane (or isooctane), g; ρ₁—density of cyclohexane, g/cm³; ρ₂—density of n-heptane (or isooctane), g/cm³. (Note: The original formula (2) source-text extraction is garbled in the source text; the above is organized according to the original symbols and textual description, and the formula form requires confirmation against the source standard (source-text extraction))

5. Instruments and equipment

• 2.1.1 Chromatograph: should be capable of installing a capillary column, with split and makeup devices, and equipped with a hydrogen flame ionization detector (FID), amplifier, and data processing system. Instrument sensitivity: for a concentration of 0.1% (volume) cyclohexane/n-heptane, the peak height of the cyclohexane chromatographic peak on the recording paper should be one tenth of full scale.
• 2.1.2 Microsyringe: 1.0 µL and 50 µL.
• 2.1.3 Pipette: 20 mL.
• 2.1.4 Volumetric flask: 50 mL.
• 2.1.5 Capillary column: fused silica capillary column, length 50 m, internal diameter 0.25 mm, or other capillary columns that can achieve the same effect.

6. Recommended operating conditions

• Chromatographic column preparation (4.1): A fused silica capillary chromatographic column prepared by the static method, with the inner wall coated with OV-101 stationary liquid, after aging at 280 ℃ for 8 h, can be used for determination of resolution.
• Resolution (4.2): Under the selected chromatographic conditions, separate cyclohexane and n-heptane, and the resolution R must be better than 10. R is calculated according to formula (1): R = 2(d₁ − d₂) / (Y₁ + Y₂) Where: d₁—retention time of n-heptane, s; d₂—retention time of cyclohexane, s; Y₁—peak width at baseline of the n-heptane chromatographic peak, s; Y₂—peak width at baseline of the cyclohexane chromatographic peak, s. Original example: R = 2(10.65 − 8.65) / (0.2 + 0.2) = 10
• Table 1 Chromatographic operating parameters:
  • Column length: 50 m
  • Column internal diameter: 0.25 mm
  • Column temperature: 40 ℃
  • Detector temperature: 150 ℃
  • Vaporization chamber temperature: 150 ℃
  • Carrier gas: nitrogen, pressure 1.4×10⁵ Pa
  • Injection volume: 0.15~0.20 µL
  • Split ratio: 200:1
  • Fuel gas: hydrogen, linear velocity 30 cm/s (the correspondence between the original table header and values requires confirmation against the source standard (source-text extraction))
  • Combustion-supporting gas: air, pressure 2.1×10⁵ Pa (original value 2.1×10° requires confirmation against the source standard (source-text extraction)), flow rate 500 mL/min
  • Makeup gas: nitrogen, flow rate 30 mL/min
  • Paper speed: 1 cm/min

7. Sampling and sample preparation

• Sampling: not provided in the available source.
• Sample preparation: not provided in the available source (for internal standard preparation see Section 4).

8. Analysis procedure

• 6.1 Adjust the chromatographic operating conditions according to the operating parameters in 4.3. After the conditions are stable, inject the sample 0.15~0.20 µL.
• 6.2 Various hydrocarbon impurity components present in n-heptane and isooctane samples can be subjected to chromatographic analysis under the same chromatographic conditions, and identified by relative retention time. Appendices A and B are typical chromatograms of standard n-heptane and isooctane chromatographic analysis.
• Table 2: Relative retention times of various hydrocarbon components on a fused silica capillary chromatographic column coated with OV-101 at 40 ℃ (n-heptane = 1.00).
• Table 3: Common main impurity components in n-heptane and isooctane.

9. Qualitative and quantitative determination and calculation formulas

• 7.1 The peak areas of the internal standard and each impurity peak are obtained from the data processing system, and the results are calculated as volume percentages.
• 7.2 The volume percentage of each hydrocarbon impurity component Vᵢ is calculated according to formula (3): Vᵢ = (Vₛ × PAᵢ) / (PAₛ × Sᵢ × (100 − Vₛ)) × 100 Where: Vₛ—volume percentage of internal standard cyclohexane, %; PAᵢ—peak area of the impurity component being measured; PAₛ—peak area of internal standard cyclohexane; Sᵢ—response value per unit volume of the component being measured relative to cyclohexane. (Note: The original formula (3) source-text extraction is garbled in the source text; the above is organized according to the original symbols and textual description, and the formula form requires confirmation against the source standard (source-text extraction))
• 7.3 Table 4 is the response value per unit volume of various light hydrocarbon components relative to cyclohexane on FID.
• 7.4 Sum the volume percentages of each impurity component, and then calculate the volume percentage concentration of n-heptane and isooctane by the subtraction method.
• Table 4 FID Relative response data (response value per unit volume relative to cyclohexane / cyclohexane = 1): the original lists isopentane, n-pentane, 2,2- dimethylbutane, cyclopentane, 2,3- dimethylbutane, 2- methylpentane, 3- methylpentane, n-hexane, 2,2- dimethylpentane, methylcyclopentane, 2,4- dimethylpentane, benzene, 2,2,3- trimethylbutane, 3,3- dimethylpentane, cyclohexane, 2- methylhexane, 2,3- dimethylpentane, 1,1- dimethylcyclopentane, 3- methylhexane, 1- cis-3- dimethylcyclopentane, 1- trans-3- dimethylcyclopentane, 3- ethylpentane, 1- trans-2- dimethylcyclopentane, 2,2,4- trimethylpentane, n-heptane, 1- cis-2- dimethylcyclopentane, 2,2- dimethylhexane, 1,1,3- trimethylcyclopentane, methylcyclohexane, 2,5- dimethylhexane, ethylcyclopentane, 2,4- dimethylhexane, 2,2,3- trimethylpentane, 1- trans-2- cis-4- trimethylpentane, 3,3- dimethylhexane, 1- trans-2- cis-3- trimethylcyclopentane, 2,3,4- trimethylpentane, 2,3,3- trimethylpentane, etc. (specific values are misaligned in the source-text extraction table and require confirmation against the source standard (source-text extraction)).

10. Precision

Judge the reliability of the test results according to the following provisions (95% confidence level; the original source-text extraction is "8% confidence level", requires confirmation against the source standard (source-text extraction)).

• 8.1 Repeatability: For two test results obtained by the same operator using the same sample, the same chromatograph, and operating according to the experimental conditions specified in this method, the difference shall comply with the following specified value:

  Composition	Concentration (volume %)	Repeatability (volume %)
  n-Heptane	greater than 99.5	0.010
  Isooctane	greater than 99.5	0.010
  (Note: In the original text, the concentration column source-text extraction is “greater than 9.5”; according to the context it should be “greater than 99.5”; requires confirmation against the source standard (source-text extraction))

• 8.2 Reproducibility: For the same test sample by different operators in different laboratories, operating under the test conditions specified in this method, the difference between two determination results shall comply with the following specified values:

  Composition	Concentration (volume %)	Reproducibility (volume %)
  n-Heptane	greater than 99.5	0.054
  Isooctane	greater than 99.5	0.054
  (Note: In the original text, the concentration column source-text extraction is “greater than 99.5” and “greater than 9.5”; requires confirmation against the source standard (source-text extraction))

11. Report

Take the arithmetic mean of two replicate determination results as the sample result.

12. Referenced Documents

• ASTM D2268-84 “Determination of Purity of High-Purity n-Heptane and Isooctane (Capillary Gas Chromatography)” (developed by reference)
• Appendix A: Typical chromatogram of standard n-heptane (reference item)
• Appendix B: Typical chromatogram of standard isooctane (reference item)
• Appendix C: Precautions (supplementary item)
  • C.1 Cyclohexane, n-heptane, isooctane: flammable, harmful if inhaled. Keep away from heat sources, sparks and open flames. Keep sealed. Use with adequate ventilation.
  • C.2 Air, nitrogen: compressed gases under high pressure; a pressure reducing valve must be used during use. Protect from sunlight and keep away from heat sources. Never drop the gas cylinder; ensure it is upright, supported and secured. Pay attention to cylinder markings to ensure they meet usage requirements.
  • C.3 Hydrogen: Danger! Flammable gas, flammable under pressure. When replacing the gas cylinder, the pressure in the cylinder must not be lower than 980.7 kPa (10 kgf/cm²). Use with adequate ventilation. Other precautions are the same as C.2.

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 Q&A