SH/T 0693-2000 Determination of aromatic hydrocarbons in gasoline (gas chromatography)

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Card type: Test method

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

• Standard number: SH/T 0693-2000
• Chinese full name: Determination of aromatic hydrocarbons in gasoline (gas chromatography)
• English name: Standard test method for determination of benzene, toluene, ethylbenzene, p/m-xylene, o-xylene, C9 and heavier aromatics, and total aromatics in finished gasoline by gas chromatography
• Replacement relationship: not provided in the available source (the foreword only states equivalent adoption of ASTM D5580-1995, and does not mention replacement of an old standard)
• Adoption relationship: equivalent adoption of ASTM D5580-1995 "Determination of benzene, toluene, ethylbenzene, p/m-xylene, o-xylene, C9 and aromatics above C9 and total aromatics in finished gasoline by gas chromatography"
• Main differences from ASTM D5580-1995 (foreword):
  1. The standard name is simplified;
  2. The referenced standards adopt the corresponding equivalent national standards and petrochemical industry standards of our country; where our country has no corresponding standard, the substantive content of the reference is written in;
  3. One polar precolumn and one analytical column are added, one optional internal standard substance is added, and the corresponding analysis conditions for the two columns are added;
  4. When converting the mass percentage of aromatic components to volume percentage, the density of the component at 20℃ is used, instead of the relative density at 15.6℃/15.6℃ in ASTM D5580-1995.
• Publication/implementation: published on 2000-09-11, implemented on 2001-01-01; issued by the State Bureau of Petroleum and Chemical Industry
• Secretariat: Sinopec Research Institute of Petroleum Processing
• Drafting organization: Sinopec Research Institute of Petroleum Processing
• Main drafters: Li Changxiu, Jin Ke

2. Scope of application and non-applicability

• 1.1 It is applicable to the determination of benzene, toluene, ethylbenzene, xylene, C9 and aromatics above C9 and total aromatics in finished motor gasoline by gas chromatography.
• 1.2 The separation of aromatics in finished gasoline is not interfered with by non-aromatics. Non-aromatics with boiling points greater than n-dodecane will interfere with the determination of C9 and aromatics above C9. For C8 aromatics, p-xylene and m-xylene elute simultaneously, and ethylbenzene and o-xylene are detected as a single peak. C9 and aromatics above C9 are detected as one group peak.
• 1.3 Determination concentration range: benzene 0.1% (V/V) to 5% (V/V); toluene 1% (V/V) to (the upper concentration limit is missing here in the source text, requires confirmation against the source standard (source-text extraction)); aromatics 10% (V/V) to 80% (the unit is missing in the source text, requires confirmation against the source standard (source-text extraction)).
• 1.4 The determination results are accurate to 0.01% (m/m) or 0.01% (V/V).
• 1.5 Common alcohols or ethers added to gasoline do not interfere with the analysis. Ethers such as MTBE, ETBE, TAME, DIPE, etc. are vented together with non-aromatics from the precolumn; other oxygen-containing compounds including methanol and ethanol elute before benzene and aromatics; 1- methylcyclopentene is also vented from the precolumn and does not interfere with the determination of benzene.
• 1.6 It involves certain safety issues, but does not provide recommendations for all safety issues; users are responsible for establishing appropriate safety and health measures and determining appropriate regulations before use.

3. Method principle

• 4.1 A dual-column gas chromatography system equipped with a switching valve and a flame ionization detector is used. A certain volume of a repeatably measurable sample containing an appropriate internal standard (such as 2- hexanone) is injected into a precolumn containing a polar stationary phase (TCEP). After C6 and non-aromatics lighter than C6 flow out of the precolumn, they are vented through the vent; a thermal conductivity detector may be used to monitor this separation process. Immediately before benzene elutes, the precolumn is placed in backflush state, and the retained components are introduced into an analytical column containing a nonpolar stationary phase (WCOT). Benzene, toluene, and the internal standard elute from the chromatographic column in order of boiling point and are detected with a flame ionization detector. Immediately after the internal standard elutes, the analytical column is backflushed, and the remaining components (C9 and aromatics above C9 and C10 and non-aromatics above C10) are backflushed out of the chromatographic column into the flame ionization detector.
• 4.2 A second analysis is performed so that non-aromatics smaller than C12, benzene, and toluene flow out of the precolumn and are vented, which may be monitored with a thermal conductivity detector. Immediately before ethylbenzene elutes, the precolumn is placed in backflush state, and the retained components are introduced into the analytical column. The internal standard and C8 aromatics elute in order of boiling point and are detected with a flame ionization detector. After o-xylene elutes, the analytical column is immediately backflushed, and C9 and aromatics above C9 are backflushed out of the chromatographic column into the flame ionization detector for detection.
• 4.3 In the first analysis, the peak areas of benzene, toluene, and the internal standard (2- hexanone) are recorded; in the second analysis, the peak areas of ethylbenzene, p/m-xylene, o-xylene, C9 and aromatics above C9, and the internal standard (2- hexanone) are recorded. In the second analysis, the peak backflushed from the analytical column contains only C9 and aromatics above C9.
• 4.4 The flame ionization detector response is proportional to the concentration of each component, and the aromatic component content is calculated with reference to the internal standard.

4. Reagents and materials (purity / preparation)

• 7.1 Carrier gas: use a carrier gas suitable for the detector type, such as helium and hydrogen. Purity not lower than 99.95%. If necessary, additional purification measures may be taken to remove trace oxygen. Warning: helium is usually a high-pressure gas.
• 7.2 Dichloromethane: analytical reagent, no volatile residue, used for column preparation. Warning: harmful if ingested or inhaled at high concentrations.
• 7.3 2,2,4- trimethylpentane (isooctane): analytical reagent, used as solvent when preparing calibration mixtures. Warning: flammable, harmful or fatal if ingested or inhaled.
• 7.4 Qualitative and calibration standard reagents: all components to be determined and the internal standard. Standard reagents are used for qualitative identification by retention time and quantitative calibration, and their purity should be known and they should not contain other components to be analyzed. Warning: these materials are flammable and harmful or fatal if ingested or inhaled.
• Internal standard: 2- hexanone; when a capillary precolumn is used, the internal standard is 4- methyl-2- pentanone (see 11.5.1.1 adoption notes and 12.3).
• Calibration sample components: benzene, toluene, ethylbenzene, o-xylene, and 1,2,4- trimethylbenzene; o-xylene represents xylene, and 1,2,4- trimethylbenzene represents C9 and aromatics above C9 (12.1).
• Table 2 Physical parameters of components (density 20℃, g/cm³):
  • Benzene 0.8789
  • Toluene 0.8670
  • Ethylbenzene 0.8670
  • p/m-Xylene 0.8634 (Note 1: the density of mixed xylene is based on a p-xylene to m-xylene ratio of 1:3)
  • o-Xylene 0.8802
  • 1,2,4- Trimethylbenzene 0.8758
  • C9+ Aromatics 0.8720 (Note 2: based on the average density value of 30 C9 to C10 aromatics)
  • 2- Hexanone 0.8113
  • 4- Methyl-2- pentanone 0.7978
• 12.2 Before preparing standard samples, determine aromatic purity by capillary gas chromatography and correct for impurities; use reagents with purity not lower than 99.9% whenever possible.

5. Instruments and equipment

• 6.1 Chromatographic system (system schematic shown in Figure 1):
  • 6.1.1 Gas chromatograph (GC): capable of operating under the conditions in Table 1, with the column switching and backflush system shown in Figure 1; the carrier gas pressure and flow control device can precisely control the required low column pressure and low flow rate.
  • 6.1.2 Injection system: capable of introducing a representative sample into the chromatographic inlet; a microsyringe or autosampler may be used.
  • 6.1.3 Vaporization system (split type): split injection must be used to ensure that the actual amount of sample entering the chromatographic system meets the requirements for column efficiency and detector linear range.
  • 6.1.3.1 Some gas chromatographs are equipped with an on-column injection system and an autosampler, which can achieve ultra-microliter injection; if column efficiency and detector linear range consistent with a split injection system can be obtained, they may also be used.
  • 6.1.4 Detector: a flame ionization detector (detector A) is used for quantitative detection of components eluting from the analytical column, and it must have sufficient sensitivity and stability to detect aromatic compounds at 0.01% (V/V).
  • 6.1.4.1 It is recommended to install a thermal conductivity detector (detector B) at the outlet of the precolumn to facilitate determination of the valve backflush and reset times and to monitor the separation of components on the precolumn.
  • 6.1.5 Switching backflush valve: located in a temperature-controlled zone, capable of performing the functions described in Chapter 11 and Figure 1; it should have a small internal volume design so as not to significantly reduce chromatographic resolution.
  • 6.1.5.1 It is recommended to use a ten-port valve with fitting outer diameter 1.6 mm; if a chromatographic column with inner diameter 0.32 mm or smaller is used, a ten-port valve with fitting outer diameter 0.8 mm should be used.
  • 6.1.5.2 Some gas chromatographs are equipped with an auxiliary oven that can be used to place the valve body, and the valve body can be maintained at a higher temperature than the precolumn and analytical column to prevent sample condensation and peak broadening; the chromatographic column is placed in the main oven, and the column temperature can be adjusted to obtain optimal aromatic separation.
  • 6.1.5.3 It is recommended to use an automatic valve switching device to ensure repeatability of switching times.
• 6.2 Data acquisition system:
  • 6.2.1 Integrator or computer: capable of recording chromatographic graphics and digital signals in real time; peak areas and retention times can be measured with a computer or electronic integrator.
  • 6.2.1.1 It is recommended that the equipment be capable of multi-level internal standard calibration and can calculate the least-squares correlation coefficient and linear equation of calibration data according to 12.4.
• 6.3 Chromatographic column:
  • 6.3.1 Polar precolumn: achieves preliminary separation of aromatics and non-aromatics in the same boiling point range. Any chromatographic column with column efficiency and selectivity equivalent to or better than the requirements of 6.3.1.1 may be used. Either of the following two precolumns may be selected:
    • 6.3.1.1 TCEP Micro-packed column (column 1): stainless steel tube of length 560 mm, outer diameter 1.6 mm, inner diameter 0.38 mm, packed with 0.14 to 0.15 g of 20% (m/m) TCEP/Chromosorb P (AW) [0.177 to 0.149 mm (80 to 100 mesh)] stationary phase.
    • 6.3.1.2 TCEP Capillary column (column 2): length 20 m, outer diameter 0.6 mm, inner diameter 0.35 mm stainless steel tube, with the inner wall coated with TCEP.
  • 6.3.2 Non-polar analytical column: any chromatographic column having column efficiency and selectivity equivalent to or better than the requirements of 6.3.2.1 may be used.
    • 6.3.2.1 WCOT Methyl silicone column: length 30 m, inner diameter 0.53 mm or 0.32 mm, film thickness 5.0 μm or 2.6 μm cross-linked methyl silicone fused silica capillary WCOT column.

6. Recommended operating conditions

• Table 1 Typical chromatographic operating parameters:
  • Temperature:
    • Inlet (split injector): condition 1 200℃; condition 2 200℃
    • Detector A: condition 1 FID 250℃; condition 2 FID 250℃
    • Detector B: condition 1 TCD 200℃; condition 2 FID (TCD) 250℃ (200℃)
    • Analytical column initial temperature: condition 1 60℃ (6 min); condition 2 40℃ (20 min)
    • Heating rate: condition 1 2℃/min; condition 2 1℃/min
    • Analytical column final temperature: condition 1 115℃ (hold until all components have eluted); condition 2 100℃ (5 min)
    • Precolumn: condition 1 60℃ or the same as the non-polar WCOT column (if the TCEP/WCOT column is in the same heating zone); condition 2 40℃ or the same as the non-polar WCOT column (if the TCEP/WCOT column is in the same heating zone)
    • Valve: condition 1 >115℃ or the same as the non-polar WCOT column (if the valve and the WCOT column are in the same heating zone); condition 2 >115℃ or the same as the non-polar WCOT column (if the valve and the WCOT column are in the same heating zone)
    • The precolumn is held at constant temperature before the backflush time T1, T2, not exceeding the maximum operating temperature.
  • Measurement and conditions:
    • Carrier gas: condition 1 helium or hydrogen; condition 2 nitrogen
    • Precolumn flow (split injector): condition 1 10 mL/min; condition 2 5 mL/min
    • Analytical column flow (makeup flow): condition 1 10 mL/min; condition 2 0 mL/min
    • Split outlet flow: condition 1 100 mL/min; condition 2 50 mL/min
    • Detector gas flow: set as required
    • Split ratio: condition 1 11:1; condition 2 11:1
    • Injection volume: condition 1 1 μL; condition 2 0.4 μL
• 11.2 Initial operating conditions: adjust the operating conditions according to the parameters listed in Table 1; before turning on the detector circuitry, leak-test the system.
• 11.2.1 If a different precolumn and analytical column are used, or a small inner diameter or different film thickness WCOT capillary column is used, or both are used simultaneously, it is necessary to use the corresponding optimum flow and temperature.
• 11.2.2 Table 1 condition 1 applies to the chromatographic column described in 6.3.1.1; condition 2 applies to systems using a TCEP capillary column as the precolumn. If the analytical column used has a different film thickness, the newly selected analytical conditions must be able to completely separate toluene from the internal standard (first analysis) and completely separate ethylbenzene from xylene (second analysis).
• 11.3 Flow (carrier gas) adjustment:
  • 11.3.1 Connect a flow measuring device to the precolumn outlet (or detector B), with the valve in the reset position, adjust the inlet flow to 10 mL/min, measured with a soap film flowmeter; this flow represents the flow through the polar precolumn.
  • 11.3.2 Connect the flow measuring device to the split selector vent, use the flow regulator to adjust the split outlet flow to 100 mL/min; recheck the column outlet flow according to 11.3.1, and if it changes, adjust the flow regulator again; the split ratio is approximately 11:1.
  • 11.3.3 Switch the valve to the backflush position, adjust the damping valve so as to obtain the same precolumn vent flow as set in 11.3.1; this is necessary to minimize flow changes during valve switching.
  • 11.3.4 Switch the valve to the reset position, adjust the auxiliary flow regulator so that the detector A (FID) outlet flow is 10 mL/min.
• 11.4 Detector setup: according to the instrument requirements, adjust the hydrogen, air and makeup gas flows required for FID and ignite; if using TCD (detector B) to monitor the vent outflow when the valve is in the reset state, adjust the reference gas flow and turn on the detector B.
• 11.5 Valve backflush and reset times: the backflush time varies slightly for each column system and must be determined experimentally; the integrator start time and valve timer must be synchronized with the injection.
  • 11.5.1 The valve backflush and reset times are determined as described in 11.5.1.1, 11.5.1.2, 11.5.1.3. The following steps assume that a thermal conductivity detector is installed at the precolumn outlet as detector B; if no detector is available, suitable valve backflush times T1 and T2 must be determined experimentally. If the backflush times T1 and T2 are set incorrectly (switching too late), some benzene and ethylbenzene may be vented.
  • 11.5.1.1 Set the valve to the reset state, inject into the system 1.0 μL of an isooctane solution containing benzene, toluene, ethylbenzene, o-xylene and 2- hexanone (or 4- methyl-2- pentanone) each at 5%. Record the number of seconds from when benzene and ethylbenzene begin to elute from detector B; subtract 6 s from this time to obtain the backflush times T1 and T2. The correct backflush time is the time just before benzene and ethylbenzene elute from the TCEP column. Note: Figure 2 is a schematic chromatogram of the calibration mixture on the polar precolumn, indicating the backflush times T1 and T2; the optimum backflush times T1 and T2 should be selected for each chromatographic system.
  • 11.5.1.2 Re-inject the calibration mixture, place the valve in the backflush state at time T1; when the internal standard peak (2- hexanone) (toluene peak when using a capillary precolumn) returns to baseline, switch the valve to the reset state; this time is called T3.
  • 11.5.1.3 Re-inject the calibration mixture, place the valve in the backflush state at time T2; when the o-xylene peak returns to baseline, switch the valve to the reset state; this time is called T4.
• 11.6 Polar precolumn selectivity check:
  • 11.6.1 The selectivity of the polar precolumn is critical for eliminating non-aromatic interference and thereby accurately determining aromatics of C9 and C9 and above. The precolumn selectivity must be determined so that in the second analysis, backflushing occurs at the appropriate time T2, so that all non-aromatics less than C12 are vented from the precolumn while heavy aromatics are retained on the precolumn.
  • 11.6.1.1 Prepare a solution of approximately 1.5% n-dodecane in 2,2,4- trimethylpentane (isooctane). n-Dodecane represents the high-boiling non-aromatic components in gasoline. Under the conditions described in 11.2 to 11.5, inject 1.0 μL of this mixture, and switch the valve at times T1 (backflush) and T3 (reset). Record the signals of FID (detector A) and TCD (detector B). Confirm that n-dodecane has completely eluted from the polar precolumn before time T1 (backflush). By monitoring with TCD (detector B), the n-dodecane peak returns to baseline before the backflush time T1; otherwise, part of the n-dodecane will be backflushed onto the non-polar WCOT column and detected by FID after the reset time T3. If no TCD is available at the precolumn outlet, the chromatogram obtained from FID can be used to confirm that all n-dodecane is vented from the precolumn; in this case, no significant n-dodecane response is seen on the chromatogram after the reset time T3.
  • 11.6.1.2 If n-dodecane is not completely vented from the precolumn as measured by TCD or FID, recheck the instrument parameters and valve backflush time or replace the polar precolumn. If the valve is installed in a separate thermostatted heating zone, it may be necessary to use a higher temperature to prevent adsorption of a small amount of n-dodecane on the surfaces of the tubing at bends and connections.

7. Sampling and sample preparation

• 10.1 Ensure that the sample taken is representative each time. When sampling from a storage tank or production line, sample according to the method recommended in GB/T 4756.
• 10.2 Appropriate steps should be taken to minimize the loss of light components in the gasoline sample to be analyzed. From the time the sample is received in the laboratory, before and after any subsampling, the sample in the original container should be cooled to 0～5℃ for storage.
• 10.3 If necessary, transfer the cooled sample to a pressure-sealed container and store at 0～5℃ before analysis.
• 13.1 Sample preparation: Using a pipette, transfer 1 mL of the internal standard (W s) into a weighed 10 mL volumetric flask or stoppered vial, record the net mass of the internal standard added, accurate to 0.1 mg. Reweigh the volumetric flask or stoppered vial, add 9 mL of the cooled sample to it, cap, and record the net mass of the sample added (W g). Mix thoroughly. If an autosampler is used, transfer a portion of the solution to a glass vial for gas chromatography and seal with an aluminum cap with a polytetrafluoroethylene liner. If not analyzed immediately, store at 0～5℃.

8. Analytical procedure

• 8 Column preparation:
  • 8.1 TCEP Preparation of micropacked column:
    • 8.1.1 TCEP Preparation of column packing:
      • 8.1.1.1 Any preparation method that can produce a chromatographic column capable of separating aromatic hydrocarbon components from non-aromatic hydrocarbons in the same boiling point range may be used. The following procedure may be used.
      • 8.1.1.2 In a fume hood, completely dissolve 10 g TCEP in 100 mL dichloromethane, then add 40 g 0.177～0.149 mm (80～100 mesh) Chromosorb P (AW). Quickly transfer this mixture to a drying pan; do not scrape any residual packing from the inner wall of the container. Stir the packing continuously and gently until all solvent has evaporated. This packing can be used immediately to pack the TCEP chromatographic column.
    • 8.1.2 TCEP Preparation of micro-packed column:
      • 8.1.2.1 Clean a stainless steel tube of length 560 mm, outer diameter 1.6 mm, inner diameter 0.38 mm with methanol, then purge clean with compressed nitrogen.
      • 8.1.2.2 Insert 6～12 strands of silver wire into one end of the tube, or place a small piece of fine-mesh screen or stainless steel frit as a plug. While drawing a vacuum, slowly add 0.14～0.15 g of packing into the column, and gently vibrate the column so that the packing fills the column uniformly. Leave a space of 6.0 mm at the column head, and use silver wire strands to block the packing in the column.
  • 8.2 TCEP Preparation of capillary column:
    • 8.2.1 Clean a stainless steel tube of length 20 m, inner diameter 0.35 mm with anhydrous ethanol, then purge clean with air.
    • 8.2.2 Coat the inner wall of the prepared stainless steel tube twice in both directions with 5% silica sol, and dry with air. Then connect it to the chromatographic oven, purge with 3 mL/min nitrogen, and simultaneously program the temperature at a rate of 3℃/min up to 370℃ for curing. After cooling, coat the inner wall of the tube twice in both directions with TCEP stationary liquid mixed with less than 0.074 mm (200 mesh) support, and then dry with air.
  • 8.3 WCOT Methyl silicone column: It is recommended to purchase a suitable column directly from the column manufacturer (see 6.3.2.1).
• 9 Column aging: Both the precolumn and the analytical column should be briefly aged before use. Connect the columns to the valve in the chromatographic oven (see Figures 1 and 11.1). Adjust the carrier gas flow rate according to 11.3, and turn the valve to the reset position. After a few minutes, raise the column oven temperature to 120℃ and maintain for 20 min. After cooling the column to below 60℃, turn off the carrier gas.
• 11 Preparation of the apparatus and establishment of conditions:
  • 11.1 Assembly: Use low-volume fittings and narrow-bore tubing to connect the precolumn and analytical column to the valve system (see Figure 1). The volume of the chromatographic system through which the sample passes should be minimized; otherwise, peak broadening will occur.
  • 11.2～11.6 See Section 6.
• 12 Calibration:
  • 12.1 Preparation of calibration samples: Accurately weigh and mix the components in order of increasing volatility of the pure substances to prepare standard samples containing multiple components at the desired concentrations; the components include benzene, toluene, ethylbenzene, o-xylene, and 1,2,4- trimethylbenzene; o-xylene represents xylenes, and 1,2,4- trimethylbenzene represents C9 and aromatics with more than C9. For each aromatic component, use at least 3 calibration points (the original text 'at least use; calibration points' is suspected to be source-text extraction noise, requires confirmation against the source standard (source-text extraction)), and ensure that the concentration of each aromatic component is within the calibration range.
    • The concentration of the benzene calibration sample may be 0.1% (V/V), 0.5% (V/V), 1.0% (V/V), 2.0% (V/V), and 5.0% (V/V).
    • The concentration of the toluene calibration sample may be 1.0% (V/V), 2.5% (V/V), 5.0% (V/V), 10.0% (V/V), and 15.0% (V/V).
    • The concentrations of the ethylbenzene, o-xylene, and 1,2,4- trimethylbenzene calibration samples may be 0.5% (V/V), 1.0% (V/V), 2.5% (V/V), 5.0% (V/V), and 10.0% (V/V).
    • Select the appropriate sample weight of aromatic components according to the densities listed in Table 2 to obtain the desired concentrations.
  • 12.2 Before preparing the standard samples, determine the purity of the aromatic hydrocarbons by capillary gas chromatography and correct for the impurities present. Use reagents of purity not less than 99.9% whenever possible.
  • 12.3 Prepare the standard samples by transferring a certain volume of aromatic hydrocarbons with a pipette, dropper, or syringe into a 100 mL volumetric flask or a 100 mL stoppered vial as follows. Cap the bottle and record the mass of the empty volumetric flask or vial to the nearest 0.1 mg. Open the cap and carefully add the aromatic components to the bottle starting with the least volatile component (1,2,4- trimethylbenzene). Cap the bottle and record the net mass of the added aromatic component (W i) to the nearest 0.1 mg. Repeat this addition and weighing step for each aromatic component. Do not add more than 50% (V/V) of all aromatic components. Similarly, add 10 mL of internal standard (2- hexanone or 4- methyl-2- pentanone), record its net mass (W s) to the nearest 0.1 mg. Dilute each standard sample to the mark with aromatic-free 2,2,4- trimethylpentane (isooctane). When not in use, store the capped calibration standards at 0～5℃.
  • 12.4 Calibration procedure: Start with the valve in the reset position, run each calibration mixture and perform valve switching operations according to 11.5. For the first analysis, perform valve switching operations at times T1 and T3; for the second analysis, perform valve switching operations at times T2 and T4. Note: The first analysis is used to calibrate benzene and toluene; the second analysis is used to calibrate ethylbenzene, xylene (o-xylene), C9, and aromatics with more than C9.
  • 12.4.1 Linearity experiment: Analyze the calibration standards according to the procedure in 12.4. Measure the peak areas of benzene, toluene, and the internal standard from the first analysis; measure the peak areas of the internal standard, ethylbenzene, o-xylene, and 1,2,4- trimethylbenzene from the second analysis. Determine the response ratio (rsp i) and mass ratio (amt i) for each component in each standard according to Equation (2) and Equation (3):
    • rsp i = (A i/A s) … (2)
    • amt i = (W i/W s) … (3)
  • 12.4.1.1 Plot the calibration curve for each aromatic component with the response ratio (rsp i) on the Y axis and the mass ratio (amt i) on the X axis. An example is shown in Figure 3.
  • 12.4.1.2 Calculate the correlation coefficient r value of the calibration curve for each component according to Equation (4), Equation (5), and Equation (6). The r value should be at least not less than 0.990. If the expected r value is not obtained, rerun the calibration procedure or check the instrument parameters and hardware.
  • 12.4.1.3 Table 3 gives an example of calculating the r value based on an ideal data set.
  • 12.4.2 Linear least squares fitting: For each aromatic component i calibration data set, a least squares linear fit equation can be obtained in the following form:
    • rsp i = (m i) (amt i) + b i … (7)
  • 12.4.2.1 Calculate m i and b i values according to Equation (8) and Equation (9) (the original formula content is missing, requires confirmation against the source standard (source-text extraction)).
  • 12.4.2.2 For the example in Table 3: m i = 5/10 = 0.5 … (10); b i = 1.5 - 0.5×3 = 0 … (11) (the original numbering is (10) (11), here according to the original text).
  • 12.4.2.3 The least squares fit equation for the example in Table 3 above is: rsp i = 0.5 (amt i) + 0 … (12). Note: In general, the b i value is not 0, but may be positive or negative. Figure 3 gives an example of the least squares fit equation for benzene obtained according to Equation (7).
  • 12.4.3 Intercept test: To obtain the best calibration, the absolute value of the intercept b i must be at a minimum. In this case, when w i is less than 0.1%, A i approaches 0. In practice, this means that for an aromatic component with zero peak area, the calculated mass percentage should be close to zero. The equation for calculating the mass percentage (w0 i) of aromatic component i simplifies to Equation (13). The b intercept can be calculated using Equation (13):
    • w0 i = (b i/m i) (W s/W g) × 100% … (13)
    • For benzene, w0 i must be less than 0.02% (m/m); for other aromatics, w0 i must be less than 0.2% (m/m). If any w0 i value is greater than the limit value, the calibration process for the aromatic component i must be redone or the instrument parameters and hardware must be checked.
    • Example: For benzene b i=0.0018 and m i=1.41, a typical sample preparation may contain approximately W s=0.8 g (1.0 mL) internal standard and W g=6.75 g (9 mL) gasoline sample. Substituting into equation (13) gives w0 i = (0.0018/1.41)(0.8/6.75) × 100% = 0.01% (m/m) … (14). Since w0 i is less than 0.02% (m/m), for benzene, the axis intercept b i has an acceptable value. Similarly, the w0 i values for all other aromatics can be determined.
• 13 Test procedure:
  • 13.1 Sample preparation: see section 7.
  • 13.2 Chromatographic analysis: Using the same injection technique and injection volume as used for calibration analysis, introduce a representative sample (containing internal standard) into the chromatograph. When the split ratio is 11:1, a suitable injection volume is 1.0 μL. Perform two chromatographic analyses of the sample, and perform valve switching at the times determined according to 11.5. For the first analysis, use times T1 and T3 to backflush and reset the valve; for the second analysis, use times T2 and T4.
  • 13.3 Interpretation of chromatograms: Compare the retention times of components obtained from the sample analysis with those from the calibration analysis to qualitatively identify the aromatic components. From the first analysis, confirm benzene, toluene, and the internal standard; from the second analysis, confirm the internal standard, ethylbenzene, p/m-xylene, o-xylene, and aromatics above C9. Refer to Figures 4 and 5 for the sample chromatograms; for chromatograms when using a capillary precolumn, see Figures 6 and 7.

9. Qualitative and Quantitative Determination and Calculation Formulas

• 14.1 Mass percent concentration of aromatics: After peak identification, measure the peak areas of benzene, toluene, and the internal standard from the first analysis; measure the peak areas of the internal standard, ethylbenzene, p/m-xylene, o-xylene, and aromatics above C9 from the second analysis. According to the slope and y-axis intercept of the equation already determined in 12.4.2 equation (7), use the response ratio of the aromatic component to the internal standard (rsp i) to calculate the mass of each aromatic component in the gasoline sample (W i), with the formula (15) as follows:
  • W i = [(A i/A s - b i)/m i] W s … (15)
  • Where: W i—— mass of aromatic component i, g; A i—— peak area of aromatic component i; A s—— peak area of internal standard; W s—— mass of internal standard added, g.
• 14.1.1 Calculate the mass percent concentration of each aromatic according to equation (16) w i, % (m/m):
  • w i = (W i/W g) × 100 … (16)
  • Where: W g—— mass of gasoline sample, g.
• 14.1.2 Report the mass percent concentrations of benzene, toluene, p/m-xylene, o-xylene, C9, and aromatic components above C9 w i, with results accurate to 0.01% (m/m).
• 14.1.3 The mass percent concentration of total aromatics is obtained by adding the mass percent concentrations of each aromatic w i.
• 14.2 Calculate the volume percent concentration of each aromatic component according to equation (17) v i, % (V/V):
  • v i = (w i × D i)/D s … (17) (original formula content missing, requires confirmation against the source standard (source-text extraction))
  • Where: D i—— density of each aromatic component at 20℃; D s—— density of the test sample at 20℃ determined according to test method GB/T 1884 or SH/T 0604.
• 14.2.1 Report the volume percent concentrations of benzene, toluene, ethylbenzene, p/m-xylene, o-xylene, C9, and aromatic components above C9, with results accurate to 0.01% (V/V).
• 14.2.2 The volume percent concentration of total aromatics is obtained by adding the volume percent concentrations of each aromatic v i.

10. Precision

• 15.1 Precision: The precision of this standard was determined through statistical examination of test results from cooperating laboratories (confidence level of 95%).
• 15.1.1 Repeatability: The difference between two consecutive test results obtained by the same operator using the same equipment under stable operating conditions on the same test sample during normal and correct experimental operation shall not exceed the value in Table 4.
• 15.1.2 Reproducibility: The difference between two single independent test results obtained by different operators in different laboratories on the same test sample shall not exceed the value in Table 5.
• 15.1.3 Bias: Because there is no suitable reference material, no statement regarding bias has been made.
• Table 4 Repeatability (Note: X is the average of two determinations):
  • Benzene: range 0.14%～1.79% (m/m), repeatability 0.0265 (X^0.6) (original exponent source-text extraction unclear, requires confirmation against the source standard (source-text extraction))
  • Toluene: range 2.11%～10.08% (m/m), repeatability 0.0301 (X^0.5) (original exponent source-text extraction unclear, requires confirmation against the source standard (source-text extraction))
  • Ethylbenzene: range 1.57%～2.65% (m/m), repeatability 0.029
  • p/m-Xylene: range 2.06%～9.59% (m/m), repeatability 0.071
  • o-Xylene: range 0.77%～3.92% (m/m), repeatability 0.0296 (X^0) (original exponent source-text extraction unclear, requires confirmation against the source standard (source-text extraction))
  • C9+ Aromatics: range 8.32%～25.05% (m/m), repeatability 0.0145 (X^-5.157) (original exponent source-text extraction