NB/SH/T 0713-2023 Determination of benzene and toluene content in gasoline by gas chromatography

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Card type: Test method

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

• Standard number: NB/SH/T 0713—2023
• Chinese name: Determination of benzene and toluene content in gasoline by gas chromatography
• English name: Test method for determination of benzene and toluene in gasoline―gas chromatography
• Replacement relationship: Replaces SH/T 0713—2002 "Determination of benzene and toluene content in motor gasoline and aviation gasoline (gas chromatography)". This document is the first revision; 2002 first published as SH/T 0713—2002.
• Publication/implementation: Published on 2023-12-28, implemented on 2024-06-28.
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants, Subcommittee on Petroleum Fuels and Lubricants (SAC/TC 280/SC1).
• Drafting organizations: Sinopec Research Institute of Petroleum Processing Co., Ltd., Petrochemical Research Institute of PetroChina Company Limited, Shenzhen Institute of Metrology and Quality Inspection.
• Main drafters: Li Changxiu, Qian Qin, Zhao Yan, Wang Chunyan, Xue Huifeng, Liu Yifei, Wang Botao.
• Main technical changes compared with the 2002 edition (foreword a to o): name changed; added system 1 measurement range and changed the applicability statement for alcohol-containing gasoline; added Table 1 system 1 analysis conditions, and the original system 1/2/3 was sequentially changed to system 2/3/4; added Chapter 3 terms and definitions; added system 1 connection diagram (Figure 1) and typical chromatogram (Figure 10); added system 1 capillary column specifications (see 6.2); the purity of MEK and MIBK was changed from greater than 99.9% to not less than 99.5% (see 7.2); added resolution confirmation reagents (see 7.4); added content for establishing system 1 and determining backflush time (see 11.3.1, 11.4.2); added separation performance confirmation for system 1 (see 11.5); added "Quality control check" (Chapter 15); report precision changed from 0.1% to 0.01% (Chapter 16); added repeatability and reproducibility requirements for system 1 (Chapter 17); added Annex A and Annex B; chapter, clause, table, and figure numbering sequentially revised.

2. Scope of application and non-applicability

• Applicable objects: determination of benzene and toluene content in spark-ignition engine fuels such as motor gasoline, motor ethanol gasoline, motor methanol gasoline, and aviation piston engine fuel (Chapter 1).
• Four analysis systems:
  • System 1: dual capillary chromatographic columns.
  • System 2, System 3: dual packed columns.
  • System 4: the pre-cut column uses a packed column, and the analytical column uses a capillary chromatographic column.
• Measurement range:
  • System 1: benzene volume fraction 0.12% to 5.2%; toluene volume fraction 0.4% to 19.7%.
  • System 2, System 3, System 4: benzene volume fraction 0.10% to 5.0%; toluene volume fraction 2.0% to 20.0%.
• System 1 and System 4 (analytical column is a capillary column with polyethylene glycol type stationary phase) are applicable to: gasoline not containing oxygen-containing compounds, gasoline containing ether compounds (such as MTBE, ETBE, TAME), gasoline with methanol volume fraction not higher than 10% (M10) and ethanol volume fraction not higher than 20% (E20); they can also be used for the determination of toluene content in motor methanol gasoline (M85) and motor ethanol gasoline E85, but are not applicable to the determination of benzene content therein.
• System 2 and System 3 (analytical column is a TCEP packed column) are applicable to: gasoline not containing oxygen-containing compounds and gasoline containing ether compounds; not applicable to gasoline containing ethanol, and methanol may also cause interference.
• Warning: The use of this document may involve certain hazardous materials, operations, and equipment, but not all safety issues have been addressed; users should establish appropriate safety precautions before use and determine the applicability of relevant regulatory limitations.

3. Method principle

Add 4-methyl-2-pentanone (MIBK) (System 1 and System 4) or butanone (MEK) (System 2 and System 3) to the sample as an internal standard, then introduce it into a gas chromatograph with series dual columns. The sample first passes through a chromatographic column packed with a non-polar stationary phase polydimethylsiloxane, and the components are separated in order of boiling point. After toluene has completely eluted from the non-polar chromatographic column, backflush the non-polar chromatographic column or switch the valve to switch components with boiling points greater than toluene to a damping column or backflush them out of the chromatographic system. The light components then pass through a chromatographic column packed with a strongly polar stationary phase polyethylene glycol (System 1 and System 4) or 1,2,3-tris(2-cyanoethoxy)propane (TCEP) (System 2 and System 3) to separate aromatic and non-aromatic compounds. The eluted components are detected with a flame ionization detector (or thermal conductivity detector), the peak areas are recorded and measured, and the content of each component is calculated with reference to the internal standard (Chapter 4).

4. Reagents and materials (purity / preparation)

• Solvents:
  • Methanol: analytical reagent (7.1.1).
  • Trichloromethane: analytical reagent (7.1.2).
  • Dichloromethane: analytical reagent, used for cleaning columns (7.1.3).
  • Acetone: analytical reagent, used for cleaning columns (7.1.4).
• Internal standards:
  • Butanone (MEK): purity not less than 99.5%, used for System 2 and System 3 (7.2.1).
  • 4-methyl-2-pentanone (MIBK): purity not less than 99.5%, used for System 1 and System 4 (7.2.2).
• Reagents for calibration:
  • Benzene: purity not less than 99.5% (7.3.1).
  • Isooctane: purity not less than 99.5% (7.3.2).
  • Toluene: purity not less than 99.5% (7.3.3).
  • n-Nonane: purity not less than 99% (7.3.4).
• Reagents for method resolution confirmation:
  • sec-Butanol: purity not less than 99% (7.4.1).
  • Isobutanol: purity not less than 99% (7.4.2).
  • n-Butanol: purity not less than 99% (7.4.3).
  • n-Propanol: purity not less than 99% (7.4.4).
  • Ethanol: purity not less than 99% (7.4.5).
• Materials:
  • Carrier gas: helium, hydrogen, or nitrogen, all with purity not less than 99.999%; a hydrogen generator may be used. Note: When hydrogen is used as the carrier gas, for safety reasons, excess hydrogen must be directed from the split outlet to a safe area or a dedicated exhaust outlet (7.5.1).
  • Air: dry air meeting the requirements for use of the chromatograph; an air generator with a purifier may be used (7.5.2).
  • Hydrogen: purity not less than 99.999%, a hydrogen generator may be used (7.5.3).
  • Makeup gas: nitrogen, purity not less than 99.999% (7.5.4).
  • Support: diatomaceous earth support, acid-washed, 60 mesh to 80 mesh and 80 mesh to 100 mesh (7.5.5).
  • Stationary phase: 1,2,3-tris(2-cyanoethoxy)propane (TCEP) and polydimethylsiloxane (7.5.6).
• Column packing preparation (Chapter 9):
  • Mass fraction 10% polydimethylsiloxane/Chromosorb W packing: weigh 45g Chromosorb W and pour into a 500mL flask; dissolve 5g polydimethylsiloxane in about 50mL trichloromethane; pour the solution into the flask, attach a rotary evaporator, connect a vacuum source, start the rotary evaporator and turn on the infrared lamp until the packing is thoroughly mixed and completely dry.
  • Mass fraction 20% (or 15%) TCEP/Chromosorb P packing: weigh 80g (or 85g) Chromosorb P and pour into a 500mL flask; dissolve 20g (or 15g) TCEP in 200mL methanol; pour the solution into the flask, attach a rotary evaporator, connect a vacuum source, start the rotary evaporator, turn on the infrared lamp until the packing is thoroughly mixed and completely dry (the packing must not be heated above 180℃).
• Standard sample preparation (12.1, Table 4): prepare seven standard samples, including benzene at volume fraction 0% to 5% and toluene at 0% to 20%; use a pipette to measure certain volumes of benzene and toluene respectively into a 100mL volumetric flask, and dilute to the mark with isooctane; all mixtures and glassware are maintained at room temperature.
  • Table 4 Volumes of benzene and toluene required for preparing standard samples:

    Serial number	Benzene volume fraction/%	Benzene volume/mL	Toluene volume fraction/%	Toluene volume/mL
    1	5.00	5.00	20.00	20.00
    2	2.50	2.50	15.00	15.00
    3	1.25	1.25	10.00	10.00
    4	0.67	0.67	5.00	5.00
    5	0.33	0.33	2.50	2.50
    6	0.12	0.12	1.00	1.00
    7	0.06	0.06	0.50	0.50

• Calibration mixture (12.2): use a pipette to accurately measure 1.0mL MIBK (System 1 and System 4) or MEK (System 2 and System 3) into a 25mL volumetric flask, and add standard sample No. 1 to the mark; prepare the remaining calibration mixtures according to this procedure.

5. Instruments and equipment

• Gas chromatograph (6.1): Any gas chromatograph equipped with a split/splitless or packed column inlet, a backflush system or a Deans switch center-cut system, a flame ionization detector (FID), a thermal conductivity detector (TCD), and meeting the typical instrument parameters in Table 1 or Table 2 may be used. Figures 1 to 4 show the configurations and connections of four systems. Figure 1 is a schematic diagram of a system using dual capillary columns and a Deans switch center-cut system, corresponding to system 1 in Table 1; this system requires dual FID detectors; system 1 may also use pressure backflush, equipped with one or two FID detectors, and the connection method is given in Appendix A. Figure 2 is a pressure backflush system, and Figure 3 is a valve switching system, both corresponding to the backflush systems when system 2 and system 3 in Table 2 use dual packed columns. Figure 4 is a valve switching system with a packed pre-cut column and a capillary analytical column, corresponding to system 4 in Table 2. Any of these systems may be used, but the applicability of different systems to different samples differs slightly.
  • Note: Deans switch center cutting refers to the flow switching process in which a certain component or components within a certain retention time range are cut from one capillary chromatographic column to another capillary chromatographic column by means of pressure changes through microfluidic flow path control.
• Chromatographic columns (6.2):
  • Pre-cut column (column A): a polydimethylsiloxane column of length 30m, internal diameter 0.25mm, and film thickness 0.5μm; a stainless steel column of length 0.8m or 1.0m and outer diameter 3.2mm, packed with Chromosorb W containing polydimethylsiloxane at a mass fraction of 10%, 60 mesh to 80 mesh (6.2.1).
  • Analytical column (column B): a polyethylene glycol (polar Wax) column of length 60m, internal diameter 0.32mm, and film thickness 1.0μm; a stainless steel column of length 4.6m and outer diameter 3.2mm, packed with Chromosorb P (80 mesh to 100 mesh) containing TCEP at a mass fraction of 15% or 20%; a modified polyethylene glycol (FFAP) fused silica capillary chromatographic column (6.2.2) of length 50m, internal diameter 0.25mm, and film thickness 0.3μm.
  • Damping column: a fused silica damping column tube (6.2.3) of length 57.2cm and internal diameter 0.1mm.
• Chromatographic data processing system (CDS): a chromatographic workstation (6.3) capable of recording chromatograms in real time and integrating chromatographic peaks.
• Microsyringe: capacity 5μL, minimum scale 0.2μL (6.4).
• Pipettes: graduated pipettes of volumes 0.5mL, 1mL, and 2mL, with a minimum scale of 0.01mL; a graduated pipette of volume 5mL, with a minimum scale of 0.1mL; single-mark pipettes of volumes 5mL, 10mL, 15mL, and 20mL. Note: Other types of manual or automatic solution measuring and diluting devices that meet the accuracy requirements may also be used (6.5).
• Volumetric flasks: volumes 25mL and 100mL (6.6).
• Electronic oscillator (6.7).
• Vacuum source (6.8).
• Vacuum rotary evaporator (6.9).
• Flask: round-bottom, short-neck, 24/40 joint, capacity 500mL, suitable for connection to a rotary evaporator (6.10).
• Infrared lamp (6.11).

6. Recommended operating conditions

• Table 1 Typical instrument parameters for a dual capillary column system (system 1):

  Item	Condition
  Column - pre-cut column	Polydimethylsiloxane column of length 30m, internal diameter 0.25mm, film thickness 0.5μm
  Column - analytical column	Polyethylene glycol column of length 60m, internal diameter 0.32mm, film thickness 1.0μm
  Damping column	Fused silica capillary, length 57.2cm, internal diameter 0.1mm
  Temperature - vaporization chamber/℃	250
  Temperature - detector/℃	250
  Temperature - column oven/℃	Initial temperature 75℃, hold 8min; first-order rate 5℃/min, first-order final temperature 85℃, hold 3min; second-order rate 40℃/min, second-order final temperature 150℃, hold 15min
  Carrier gas	Nitrogen
  Pre-cut column flow rate/(mL/min)	1.4
  Analytical column flow rate/(mL/min)	2.4
  Sample amount/μL	0.2 to 0.5
  Split ratio	100:1
  Total analysis time/min	30
  Backflush time/min	6.02 (footnote a: The appropriate valve switching or backflush time should be determined for each column system)

• Table 2 Typical instrument parameters for packed column systems:

  Item	System 2	System 3	System 4
  Detector	Thermal conductivity detector	Flame ionization detector	Flame ionization detector
  Pre-cut column - dimensions	Stainless steel column, column length 0.8m, outer diameter 3.2mm, internal diameter 2.2mm	Stainless steel column, column length 1.0m, outer diameter 3.2mm, internal diameter 2.2mm	Stainless steel column, column length 1.0m, outer diameter 3.0mm, internal diameter 2.0mm
  Pre-cut column - stationary phase	Polydimethylsiloxane, mass fraction 10%	Polydimethylsiloxane, mass fraction 10%	Polydimethylsiloxane, mass fraction 10%
  Pre-cut column - support	Chromosorb W, 60 mesh to 80 mesh	White silanized, 60 mesh to 80 mesh	White silanized, 60 mesh to 80 mesh
  Analytical column - dimensions	Stainless steel column, column length 4.6m, outer diameter 3.2mm, internal diameter 2.2mm	Stainless steel column, column length 4.6m, outer diameter 3.0mm, internal diameter 2.0mm	Fused silica capillary column, column length 50m, internal diameter 0.25mm, film thickness 0.3μm
  Analytical column - stationary phase	TCEP, mass fraction 20%	TCEP, mass fraction 15%	FFAP
  Analytical column - support	Chromosorb P, 80 mesh to 100 mesh	Chromosorb P, 80 mesh to 100 mesh	Reference column: any column or damping valve
  Temperature - vaporization chamber/℃	200	250	250
  Temperature - detector/℃	200	250	250
  Temperature - column oven/℃	145	90	80
  Carrier gas	Helium	Hydrogen or nitrogen	Nitrogen
  Carrier gas 1 flow rate/(mL/min)	About 30	About 40	About 10
  Carrier gas 2 flow rate/(mL/min)	About 30	About 40	About 60
  Pre-column pressure/kPa	About 200	About 350	About 350
  Analytical column flow rate/(mL/min)	About 30	About 40	About 0.8
  Split outlet flow rate/(mL/min)	—	—	59
  Sample amount/μL	2	0.5	0.5
  Total analysis time/min	8	15	15
  Backflush time/min	About 0.75 (footnote a)	About 2.5 (footnote a)	About 0.75 (footnote a)

  • Footnote a: The backflush time should be determined for each column system.
• Column conditioning (11.1):
  • Conditioning principle: Capillary chromatographic columns are conditioned before use according to the requirements of the commercial column; TCEP packed columns are conditioned using conditioning condition 1 or conditioning condition 2 according to the maximum operating temperature limit of the stationary phase.
  • Conditioning condition 1: Install column A and column B as shown in Figure 2 or Figure 3; before the column is properly conditioned, the outlet of column B should not be connected to the detector; pass helium or nitrogen through the column at a flow rate of about 40mL/min; condition the column according to the temperature and time schedule for conditioning condition 1 in Table 3.
  • Conditioning condition 2: Connect the inlet of column A to the vaporization chamber, adjust the helium or nitrogen flow rate to about 40mL/min, and condition column A according to the time schedule in Table 3; then connect column A and column B, adjust the helium or nitrogen flow rate to about 40mL/min, and condition column B according to the time schedule in Table 3.
  • Table 3 Column conditioning time schedule:

    Conditioning condition 1 (column A and column B) temperature/℃	Time/h	Conditioning condition 2 (column A) temperature/℃	Time/h	Conditioning condition 2 (column B) temperature/℃	Time/h
    50	0.5	50	0.5	50	0.5
    100	0.5	100	0.5	100	4
    150	1	150	1
    170	3	170	3

  • Footnote 1): Because the maximum operating temperature of the domestic TCEP chromatographic stationary phase is 100℃, the two columns use different aging temperatures.
• Column installation (11.2): Connect the outlet of column B to the detector; adjust the operating conditions according to Table 1 or Table 2, but do not turn on the detection system; check for gas tightness.
• System setup and flow adjustment (11.3):
  • System 1 (11.3.1): Connect the columns according to Figure 1; use a Deans switch center-cut system with microfluidic flow control to switch components between two different types of columns; the inlet of column A is connected to the split injection port, and the outlet is connected to column B through a Deans switch connection assembly, and the outlet of column B is connected to the detector; the damping column is a section of empty quartz capillary column whose resistance matches that of column B; by changing the open/closed position of the solenoid valve, the flow direction of the carrier gas exiting column A is changed; at the start of injection the solenoid valve is set to "open", and the components exiting column A enter column B in sequence for further separation; at the set time the solenoid valve is switched to "closed", changing the carrier gas flow direction so that the components exiting column A pass through the damping column and enter detector 1 for detection. It is also possible to connect two capillary columns through a zero-dead-volume tee or cross, and by changing the pressure of the two columns, backflush the heavy components out of the chromatographic system through the split outlet; the different connection modes that can be used are given in Appendix A.
  • System 2 and system 3 pressure backflush system (11.3.2): Connect the columns according to Figure 2; open switch A and switch B, close switch C, so that the flow through the column system reaches the recommended value in Table 2 (equivalent to a gauge pressure of about 205kPa); measure the flow at the sample side of the detector outlet, and observe the pressure of pressure gauge Gc; close switch A, open switch B and switch C, the pressure of gauge GA should rapidly drop to zero; otherwise open the needle valve until the pressure drops to zero; close switch B, adjust the second-stage pressure regulating valve until the reading of gauge Gc is higher than the value observed in 11.3.2.1 by 3.5kPa～7kPa; open switch B, adjust the needle valve at the backflush outlet until the reading of gauge GA is about 14kPa～28kPa; forward-flow state: open switch A and switch C, close switch B; backflush state: close switch A, open switch B, and observe the baseline; there should be no baseline change when switching from the forward-flow state to the backflush state; if there is a baseline change, slightly increase the second-stage pressure.
  • System 2 and system 3 valve switching system (11.3.3): Connect the columns according to Figure 3; adjust the valve to the forward-flow position, adjust flow control valve A so that the flow reaches the recommended value (Table 2); measure the flow at the sample side of the detector outlet; adjust the valve to the backflush position, measure the flow at the sample side of the detector outlet; if the flow has changed, adjust flow control valve B to obtain the correct value (the flow change should be within ±1mL/min); switch the valve from the forward-flow to the backflush position several times and observe the baseline; the pressure change initially caused by valve switching should not produce baseline changes or drift; if the baseline changes, slightly adjust flow control valve B to stabilize the baseline (continuous drift indicates a leak somewhere in the system).
  • System 4 (11.3.4): Open the needle valve at the split outlet, adjust flow-stabilizing valve A to obtain the required pre-cut column flow; this flow can be measured from vent 2 (split outlet); adjust the needle valve to obtain the required split ratio and column flow; the required flow can be measured through vent 2 and the capillary column outlet; place the valve in the backflush state, adjust flow-stabilizing valve B to obtain the required backflush flow; this flow can be measured through vent 1.
• Determination of valve switching time or backflush time (11.4):
  • Overview: The backflush time or valve switching time is different for each column system, and the appropriate value should be determined by the following method.
  • Determination of valve switching time for system 1 (11.4.2): For a dual capillary column system, when columns are connected using a Deans switch center-cut system, set the chromatographic temperature and pressure conditions according to Table 1 and 11.3.1; place the solenoid valve in the "closed" state, use the injection method described in clause 12.4 to inject the standard sample No. 1 in Table 4 of 0.5μL or the resolution confirmation mixed solution (see 11.5.2, 11.5.3) into the chromatographic system, and start the chromatographic run program; record the chromatogram obtained through detector 1 until toluene elutes and the recorder pen returns to the baseline; the time for the toluene chromatographic peak on the chromatogram to completely return to the baseline plus 0.02min is the appropriate solenoid valve switching time; the chromatograms obtained by passing the mixed solutions described in 11.5.2 and 11.5.3 through detector 1 are shown in Figure 5 and Figure 6; re-analyze the aforementioned mixed solution, setting the solenoid valve to the "open" state at the start of injection, and at the selected time, switch the solenoid valve to "closed"; all components in the solution with boiling points not greater than toluene will be further separated on column B and then detected through detector 2; components with boiling points greater than toluene will be prevented from entering column B and will pass through the damping column into detector 1 for detection; the chromatograms obtained by passing the mixed solutions described in 11.5.2 and 11.5.3 through detector 2 are shown in Figure 7 and Figure 8; when two capillary columns are connected through a zero-dead-volume tee or cross, the determination of the backflush time is given in Appendix A.
  • Determination of backflush time for system 2, system 3 and system 4 (11.4.3): Prepare an n-nonane solution containing isooctane at a volume fraction of 5%, and use the injection method described in clause 12.4 to inject 1μL of this solution into the system in the forward-flow state; 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 isooctane and n-nonane peaks, in seconds; at this time, isooctane has completely eluted, while n-nonane has not; half of this measured time can be approximately taken as the "backflush time", and should be between 30s～60s; repeat the above operations including injection, and switch to the backflush state according to the "backflush time" determined above; in the chromatogram of isooctane thus obtained, the n-nonane peak should be very small or not visible; if necessary, further tests can be done to adjust the "backflush time" until all isooctane gives a peak and there is little or no n-nonane peak; the actual "backflush time" of valve operation thus obtained must be used in all subsequent calibration and analysis. Note: When using system 4, the allowable variation range of the suitable backflush time is relatively small, between 5s～10s, and must be selected carefully.
  • Re-determination of backflush time (11.4.4): After long-term use of the column, the retention times of components will shorten; when the requirements of the quality control check cannot be met, or when the pre-cut column, analytical column or damping column has been replaced, the appropriate valve switching or backflush time should be re-determined according to 11.4.2 or 11.4.3.
• Evaluation of separation performance (system 1, 11.5):
  • Calculation of resolution when determining alcohol-containing gasoline (11.5.1): When used for the determination of alcohol-containing gasoline, it should be confirmed that the alcohol chromatographic peak does not interfere with the determination; the separation effect of the chromatographic column can be evaluated by calculating the resolution of the chromatographic peaks; calculate the resolution Rp according to formula (1): Rp = 2(Tp2 − Tp1) / [1.699(Wh1/2 + Wh2/2)] (Formula 1) Where: Rp— resolution of the two chromatographic peaks; Tp1, Tp2— retention times of chromatographic peak 1 and chromatographic peak 2 (Tp2＞Tp1), in minutes (min); Wh1/2, Wh2/2— half-height widths of chromatographic peak 1 and chromatographic peak 2, in minutes (min).
  • Evaluation of resolution when determining gasoline mixtures containing ethanol (11.5.2): Prepare an isooctane solution containing ethanol at a volume fraction of 20%, benzene at 1%, 4% MIBK and toluene at 20%, inject and analyze according to the determined sample analysis conditions and record the chromatogram, see Figure 7; this mixed solution can also be used to determine the backflush or valve switching time; calculate the resolution Rp of the chromatographic peaks, which should satisfy: the resolution between ethanol and benzene is greater than 2; the resolution between MIBK and adjacent chromatographic peaks is greater than 1.5.
  • Evaluation of resolution when determining gasoline mixtures containing C4 alcohol (11.5.3): Prepare an isooctane solution containing ethanol at a volume fraction of 20%, benzene at 1%, sec-butanol at 4%, n-propanol at 0.1%, 4% MIBK, toluene at 20%, isobutanol at 20%, n-butanol at 5%, inject and analyze according to the determined sample analysis conditions and record the chromatogram, see Figure 8; this mixed solution can also be used to determine the backflush or valve switching time; calculate the resolution Rp of the chromatographic peaks, which should satisfy: the resolution between ethanol and benzene is greater than 2; the resolution between MIBK and adjacent chromatographic peaks is greater than 1.5; the resolution between toluene and adjacent chromatographic peaks is greater than 0.6. Note: For the separation of other alcohol-ether compounds that may be contained in motor gasoline and certain unconventional added oxygen-containing compounds such as methylal, sec-butyl acetate, dimethyl carbonate, etc. from benzene, toluene and the internal standard, see Appendix B.
  • Determination of analysis time when determining gasoline mixtures containing C4 alcohol (11.5.4): Since n-butanol is a possible C4 alcohol isomer, the analysis time for gasoline samples containing C4 alcohol should be maintained until n-butanol elutes from the chromatographic column; the analysis time can be determined by analyzing the mixture prepared in 11.5.3, see Figure 8; when necessary, the column temperature can be appropriately increased or the run time extended to ensure that n-butanol completely elutes from the analytical column.
  • Determination of benzene detection limit (11.5.5): Analyze standard sample No. 7 in Table 4; the signal-to-noise ratio of the benzene chromatographic peak should be not less than 50; if this value cannot be reached, obtain the required value by reducing the split ratio or increasing the injection volume; the injection volume should not exceed 0.5μL.

7. Sampling and sample preparation

• Sampling (Chapter 8): Obtain a representative sample according to the procedure given in GB/T 4756 or GB/T 27867.
• Sample preparation (13.1): Using a pipette, accurately measure 1.0mL MIBK (System 1 and System 4) or MEK (System 2 and System 3), add to a 25mL volumetric flask, and add the sample to be tested to the mark, and mix thoroughly.
• Inject sample (12.4):
  • It is recommended to use an automatic liquid sampler; if manual injection is used, the injection technique in 12.4.2 should be used to obtain sharp and symmetrical chromatographic peaks.
  • Rinse the 5μL microsyringe with the calibration mixture sample at least three times, then draw approximately 3μL of sample (avoid any air bubbles); slowly push the syringe until 0.2μL～2.0μL of sample remains in the syringe; wipe the needle with filter paper, then pull back the syringe plunger to draw in 1μL～2μL of air; insert the syringe needle into the septum cap of the chromatograph until the syringe barrel rests on the septum cap; push the syringe plunger all the way down and quickly withdraw the syringe; this injection technique is necessary to obtain sharp and symmetrical chromatographic peaks.

8. Analysis Procedure

• Calibration (Chapter 12):
  • Standard samples (12.1): Prepare seven standard samples, including benzene at a volume fraction of 0%～5% and toluene at 0%～20%; according to Table 4, use a pipette to measure certain volumes of benzene and toluene respectively into 100mL volumetric flasks, and dilute to the mark with isooctane; all mixtures and glassware are maintained at room temperature.
  • Calibration mixture (12.2): Using a pipette, accurately measure 1.0mL MIBK (System 1 and System 4) or MEK (System 2 and System 3) into a 25mL volumetric flask, and add standard sample No. 1 to the mark; prepare the remaining calibration mixtures according to this procedure.
  • Chromatographic analysis (12.3): Perform chromatographic analysis on each calibration mixture according to the conditions established in clause 11.4 and the procedure in clause 12.4.
  • Inject sample (12.4): See Section 7.
  • Calibration (12.5): Measure the peak areas of the aromatic hydrocarbons and the internal standard according to clause 13.4; calculate the ratio of the peak area of benzene to the peak area of MEK (or MIBK); plot the relationship curve between benzene concentration and ratio, see Figure 9; perform the same calculation and curve for toluene; optionally, standard samples at concentrations 5～7 in Table 4 may be selected to establish a calibration curve; the lowest and highest concentrations of the calibration curve should cover the concentration range of the sample to be tested; this calibration process should be carried out to ensure that the entire chromatographic system operates normally and that the concentration of any component does not exceed the linear range of the response of each part, including the chromatographic column, detector, integrator, and other parts; the calibration curve should be linear, and the correlation coefficient r² should be no less than 0.99. Note: If calibration has been shown to be linear, the calibration factor may be calculated by the least squares method; the precision described in Chapter 17 is derived from the data of the calibration curve; if the calibration factor is used, this precision may not apply.
• Test procedure (Chapter 13):
  • Sample preparation (13.1): See Section 7.
  • Chromatographic analysis (13.2): Perform chromatographic analysis on the sample according to the "backflush time" in clause 11.4 and the injection technique in clause 12.4; at the backflush or switching time determined in clause 11.4, change the pressure or switch the six-way valve to the backflush state to prevent unnecessary components from entering the analytical column; typical chromatograms are shown in Figure 10～Figure 13.
  • Interpretation of chromatogram (13.3): Determine the chromatographic peaks of benzene, toluene, and the internal standard according to the retention times of the standard samples. Note: When using the described polydimethylsiloxane column and TCEP (or polyethylene glycol) chromatographic column, the elution order is non-aromatic hydrocarbons, benzene, MEK (or MIBK), and toluene.
  • Peak area measurement (13.4): Measure the peak areas of aromatic hydrocarbons and MEK (or MIBK) by conventional methods. Note: The precision described in Chapter 17 is derived from data obtained by an electronic integrator or chromatographic workstation; if other integration methods or peak area measurement methods are used, the statements regarding precision may not apply.
• Quality control check (Chapter 15):
  • System stability and repeatability test (15.1): At the beginning of method establishment, or when the main operating conditions of the method change (such as replacement of the chromatographic column, change of column flow rate, etc.), take standard sample No. 4 (see Table 4) and perform continuous injection analysis 6 times, calculate the standard deviation of the measurement results; the value obtained by multiplying the standard deviation by 2.77 should not exceed the repeatability allowance stated in clause 17.1; then a quality control sample with a composition close to the sample to be tested may be selected, measured after each batch of analysis, and a statistical quality control chart (SQC) prepared.
  • Standard substance test (15.2): At the beginning of method establishment or when operating conditions change (such as replacement of the chromatographic column, recalibration, adjustment of backflush switching time, etc.), analyze one or more independent certified standard substances with known reference values; in routine analysis, analyze such standard substances at least 1 times per quarter; at a confidence probability of 95%, the difference between the measured value and the reference value should not exceed the square root of the method reproducibility value; prepare a quality control chart (SQC) for the standard substance.

9. Qualitative and Quantitative Analysis and Calculation Formulas

• Qualitative: Determine the chromatographic peaks of benzene, toluene, and the internal standard according to the retention times of the standard samples (13.3).
• Quantitative (14.1): Calculate the ratio of the peak areas of benzene and toluene to the peak area of MEK (or MIBK); read the liquid volume fraction of benzene and toluene corresponding to this ratio from the appropriate calibration curve.
• Mass fraction conversion (14.2):
  • ωbenzene = (φbenzene / ρ) × 878.9 (Formula 2) Where: ωbenzene—mass fraction of benzene, %; φbenzene—volume fraction of benzene, %; ρ—density of the sample at 20℃, in kilograms per cubic meter (kg/m³); 878.9—density of benzene at 20℃, in kilograms per cubic meter (kg/m³).
  • ωtoluene = (φtoluene / ρ) × 867.0 (Formula 3) Where: ωtoluene—mass fraction of toluene, %; φtoluene—volume of toluene