SH/T 0806-2008 Determination of aromatic hydrocarbon content in middle distillates by high performance liquid chromatography with differential refractive index detector

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

• Standard number: SH/T 0806—2008
• Chinese name: Determination of aromatic hydrocarbon content in middle distillates by high performance liquid chromatography with differential refractive index detector
• English name: Determination of aromatic hydrocarbon types in middle distillates—High Performance liquid chromatography method with refractive index detection
• Replacement relationship: not provided in the available source (the foreword only states modified adoption of ASTM D6591-06, and does not mention replacement of an old version standard)
• Adoption relationship: modified adoption of ASTM D6591-06 "Determination of aromatic hydrocarbon content in middle distillates—high performance liquid chromatography with differential refractive index detector", redrafted according to ASTM D6591-06
• Main differences from ASTM D6591-06 (foreword):
  • The referenced standards of ASTM D6591-06 are replaced by corresponding national standards and industry standards of our country
  • The report information of ASTM D6591-06 12.2 is deleted
  • The wording of repeatability and reproducibility is modified according to the practice of our country
  • The keywords clause of ASTM D6591-06 is deleted
• Appendix: Appendix A is an informative appendix
• Publication/implementation: published on 2008-04-23, implemented on 2008-10-01
• Publishing unit: National Development and Reform Commission of the People's Republic of China
• Responsible organization: Sinopec Research Institute of Petroleum Processing
• Drafting units: Engineering Research Institute of Luoyang Petrochemical Engineering Corporation of Sinopec Group, Shanghai Gaoqiao Branch of China Petroleum & Chemical Corporation, Lanzhou Lubricating Oil Research and Development Center of PetroChina Company Limited
• Main drafters: Lin Yu, Bai Zhengwei, Tao Wensheng, Zhang Dahua
• ICS 75.160.20; E 31

2. Scope of application and non-applicability

• 1.1 is applicable to the determination of monocyclic aromatic hydrocarbons, dicyclic aromatic hydrocarbons, tricyclic* aromatic hydrocarbons and polycyclic aromatic hydrocarbons in diesel and petroleum fractions with a distillation range of 150℃~400℃ by high performance liquid chromatography. The total aromatic hydrocarbon content is obtained by summing the contents of each type of aromatic hydrocarbon.
• Not applicable: not applicable to the determination of aviation fuel and petroleum fractions with a distillation range of 50℃–300℃ (the original text source-text extraction is "504–300 generation", requires confirmation against the source standard (source-text extraction)). The aromatic hydrocarbon content of aviation fuel and petroleum fractions with a distillation range of 50℃~300℃ may be determined by ASTM D6379 or other methods.
• 1.2 precision applicable range (mass fraction): monocyclic aromatic hydrocarbons 4%~40%; dicyclic aromatic hydrocarbons 0%~20%; tricyclic* aromatic hydrocarbons 0%~6%; polycyclic aromatic hydrocarbons 0%~26%; total aromatic hydrocarbons 4%~65%. Determined using diesel and its blending components.
• 1.3 Sulfur-, nitrogen- and oxygen-containing compounds in the sample may affect the determination results; monoolefins have no effect on the determination results; if conjugated dienes and conjugated polyenes are contained, they may affect the determination results.
• 1.4 The aromatic hydrocarbon types are defined according to their elution properties on a specific liquid chromatography column compared with model compounds; quantification is by working curves, using a separate aromatic hydrocarbon compound for each type, and these compounds may represent (or may not represent) the aromatic hydrocarbons present in diesel. Other methods may define and quantify each aromatic hydrocarbon type differently from this method.
• 1.5 If the diesel contains fatty acid methyl esters (FAME), it will interfere with the determination of tricyclic* aromatic hydrocarbons; when used for diesel containing FAME, the tricyclic* aromatic hydrocarbon content will be high.
• 1.6 This standard is not intended to address all safety issues involved. Before use, the user must establish appropriate safety and protective measures and determine that applicable management systems exist.

3. Method principle

A certain amount of sample is diluted with mobile phase, and a certain amount of sample solution is injected into a high performance liquid chromatography system equipped with a polar column. The polar column has almost no affinity for non-aromatic hydrocarbons but has good selectivity for aromatic hydrocarbons, so aromatic hydrocarbons are separated from non-aromatic hydrocarbons and are separated into monocyclic aromatic hydrocarbons, dicyclic aromatic hydrocarbons and tricyclic* aromatic hydrocarbons according to the ring structure. After the dicyclic aromatic hydrocarbons elute, at a pre-determined time point, the column is backflushed to elute the tricyclic* aromatic hydrocarbons as a sharp peak. The column is connected to a differential refractive index detector, and detection is performed when the components are eluted. The electrical signal generated by the detector is continuously monitored by a data processor. The signal (peak area) generated by the aromatic hydrocarbons in the sample solution is compared with the signal of a pre-determined standard solution to calculate the contents of monocyclic aromatic hydrocarbons, dicyclic aromatic hydrocarbons and tricyclic* aromatic hydrocarbons; the polycyclic aromatic hydrocarbon content is obtained by summing the dicyclic aromatic hydrocarbon and tricyclic* aromatic hydrocarbon contents; the total aromatic hydrocarbon content is obtained by summing the contents of each type of aromatic hydrocarbon.

4. Reagents and materials (purity / preparation)

• 7.1 n-Heptane: HPLC grade. n-Heptane may contain benzene impurities; batch-to-batch moisture in the mobile phase can affect the mobile phase, and dehydration and filtration of the mobile phase help reduce contamination. Degassing the mobile phase is recommended; online degassing purge, vacuum degassing or ultrasonic oscillation may be used. Poor degassing may cause negative peaks. Warning: n-heptane is irritating when formulated, ingested or in contact with skin.
• 7.2 Mobile phase: n-heptane (the original text 7.2 title source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)). Changes in viscosity, refractive index and temperature, etc. may cause unpredictable changes in column performance.
• 7.3 Cyclohexane: purity not less than 98%.
• 7.4 o-Xylene: purity not less than 99%.
• 7.5 1- methylnaphthalene: purity not less than 98%.
• 7.6 Dibenzothiophene: purity not less than 99%.
• 7.7 9- methylanthracene: purity not less than 95%.
• Note: Purity is determined by gas chromatography with a hydrogen flame ionization detector or other equivalent methods. Requirements for aromatic hydrocarbons (reference materials) (the original text source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)). Wear plastic gloves.
• Reference materials: reference materials of known purity.
• Preparation of system performance verification standard solution (SPS) (9.3): weigh 1.0g±0.1g cyclohexane, 0.5g±0.05g o-xylene, 0.05g±0.005g dibenzothiophene, 0.05g±0.005g 9- methylanthracene, place in a 100mL volumetric flask, and dilute to the mark with n-heptane. Before adding n-heptane, ensure that dibenzothiophene and 9- methylanthracene are dissolved in cyclohexane and o-xylene (ultrasonication or other methods may be used). Note: If the bottle is well sealed and stored in the dark at 5℃~25℃, it can still be used after storage for SPS one year.
• Standard solution concentrations (Table 1, unit g/100mL):

(Note: The values source-text extraction in the "Phenanthrene" row of Table 1 are misaligned, requires confirmation against the source standard (source-text extraction))

5. Instruments and equipment

• 6.1 High performance liquid chromatograph: any high performance liquid chromatograph in which the mobile phase can enter the system at a flow rate of 0.5mL/min~1.5mL/min, with precision better than 0.5% under the conditions specified in Chapter 9, and with a wavefront less than 1% of full scale, may be used.
• 6.2 Injection system: any injection system capable of injecting 10μL (nominal) sample solution with repeatability better than 1.0% may be used.
  • 6.2.1 The injection volumes of the sample solution and the standard solution shall be the same and fixed. Manual and automatic injection systems (partially filled injection loop or fully filled injection loop) that meet the repeatability requirement of 6.2 may be used. For the partial-fill injection mode, it is recommended that the injection volume be less than half the loop volume; for the full-fill injection mode, the injection loop shall be filled at least 6 times to obtain good results.
  • Note: The repeatability of the injection system can be obtained by injecting the system performance verification standard solution at least 4 times (see 9.3) and then comparing their peak areas.
  • 6.2.2 The injection volume may be other than 10μL (generally 3μL~20μL), but it must meet the injection repeatability requirement (see 6.2), the sensitivity of the differential refractive index detector (see 9.4.2 and 10.1.5) and the column resolution requirement (see 9.4.3).
• 6.3 Sample filter: use a microfilter with a pore size not greater than 0.45μm to remove particles from the sample solution; the microfilter is required to be resistant to hydrocarbons (the original text is truncated, requires confirmation against the source standard (source-text extraction)).
• 6.4 Column system: any high performance liquid chromatography stainless steel column packed with an amino-bonded (or polar amino/cyano-bonded) silica stationary phase that meets the resolution requirement specified in 9.4.3 may be used. Refer to Appendix A for selecting a suitable column.
• 6.5 High performance liquid chromatography column oven: any column oven capable of maintaining a constant temperature within the range of 39℃~40℃ (±1℃) may be used.
  • Note 1: The differential refractive index detector is sensitive to sudden or gradual changes in mobile phase temperature, so necessary measures shall be taken to keep the temperature of the high performance liquid chromatography system constant.
  • Note 2: Other temperature control methods may also be used, such as laboratory constant temperature (the original text source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)).
• 6.6 Backflush valve: any manual or automatic flow direction switching valve (pneumatic or electric) designed for high performance liquid chromatography that can operate normally under a pressure of 2×10⁷Pa may be used.
• 6.7 Differential refractive index detector: refractive index detection range (the original text source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)), capable of outputting a suitable signal to the data system, meeting the sensitivity requirement of 9.4.2 and within the measurement range. The differential refractive index detector has a separate temperature control device at the same temperature as the column oven.
• 6.8 Data system or integrator: capable of peak detection and identification, capable of measuring peak area and retention time, meeting the sensitivity requirement of 9.4.2, within the measurement range, and capable of basic post-processing functions such as calculations. Recommended (the original text source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)) (this is not mandatory).
• 6.9 Volumetric flask: capacity 100mL (the original text source-text extraction is "1006=4", requires confirmation against the source standard (source-text extraction)).
• 6.10 Analytical balance: sensitivity 0.0001g (the source text source-text extraction is “sensitivity large 40.44”, requires confirmation against the source standard (source-text extraction)).

6. Recommended operating conditions

• Mobile phase flow rate: 1.0mL/min±0.2mL/min (9.2); the optimum value is usually in the range 0.8mL/min~1.2mL/min (note in 9.2)
• Column oven temperature: 39℃~40℃ constant temperature (6.5)
• Injection volume: 10μL (nominal) (6.2); may be 3μL~20μL (6.2.2)
• Detector: differential refractive index detector, temperature same as the column oven (6.7)
• Backflush time B is calculated according to formula (2): B = t(9-methylanthracene) + 0.4 × (t(9-methylanthracene) − t(dibenzothiophene)) (the original formula source-text extraction is unclear, requires confirmation against the source standard (source-text extraction))
• Baseline drift: during the analysis period, it should not exceed 0.5% of the cyclohexane peak height (9.4)
• The instrument requires about 1h to stabilize (note in 9.4)
• Column resolution: the resolution of cyclohexane and o-xylene is not less than 5.0 (9.4.3)
• The signal-to-noise ratio of the peak areas of dibenzothiophene and 9-methylanthracene is at least 3:1 (note in 9.4.2)

7. Sampling and test sample preparation

• 8.1 The test sample used for laboratory analysis shall be representative; unless otherwise specified in the product standard, the test sample shall be obtained according to GB/T 4756 or an equivalent method.
• 10.2 Analytical test sample: weigh 0.9g~1.1g (accurate to 0.0001g) of the test sample, place it in a 10mL volumetric flask, and dilute to the mark with n-heptane. Shake vigorously to mix the test sample solution uniformly, then let stand for 10min; if necessary, filter to remove particulate matter from the test sample solution.
• 10.2.1 For some test samples, the aromatic concentration exceeds the working curve range; depending on the situation, prepare a more concentrated (e.g. 7g/10mL) or more dilute (e.g. 0.5g/10mL) test sample. Note: If a dilution factor different from the above is used, the retention time and the calculated amount may change.

8. Analytical procedure

• 9.1 According to the corresponding instrument manual, connect the chromatograph, injection system, chromatographic column, backflush valve, column oven, differential refractive index detector, and integrator. The chromatographic column and backflush valve are installed in the column oven. The backflush valve shall be installed so that the differential refractive index detector is independent of the flow direction of the liquid in the chromatographic column (see Figure 1). The injection valve shall be at the same temperature as the test sample solution, usually room temperature.
  • Note 1: If other methods, such as a constant-temperature laboratory, can keep the ambient temperature constant (see 6.5), the column oven may be omitted. It is recommended to install the backflush valve in the column oven, and the instrument shall be kept away from air currents (not near an air conditioner or fume hood). Pipelines and valves that cannot be heated separately shall be insulated.
  • Note 2: To ensure system stability, it is important to perform regular maintenance on the liquid chromatograph and its components. Leaks or partial blockages in filters, filter plates, syringe needles, and injection valves can cause unstable mobile phase flow rate or poor injection volume repeatability of the injection system.
• 9.2 Adjust the mobile phase flow rate to 1.0mL/min±0.2mL/min, and ensure that the reference cell of the differential refractive index detector is filled with mobile phase. Keep the column oven temperature stable (if the differential refractive index detector has a separate temperature control device, also keep its temperature stable).
  • 9.2.1 To minimize drift, ensure that the reference cell is filled with mobile phase. Two suitable methods: a) before starting the analysis, pass the mobile phase through the reference cell (then isolate it to prevent volatilization); b) continuously replenish the mobile phase to the reference cell. The replenishment flow rate shall be optimized so as to minimize the difference between the reference cell and the analytical cell caused by liquid volatilization, temperature, and pressure changes. Usually, the replenishment flow rate can be set to one tenth of the analytical flow rate.
• 9.3 Prepare the system performance verification standard solution (SPS) (see Section 4).
• 9.4 After the operating conditions are stable (after the baseline is level), inject 10μL standard solution SPS (see 9.3), record the spectrum, and ensure that during the analysis period, the baseline drift does not exceed 0.5% of the cyclohexane peak height.
  • 9.4.1 Ensure that the 4 components of standard solution SPS achieve baseline separation (see Figure 2).
  • 9.4.2 Ensure that the data system can accurately measure the peak areas of dibenzothiophene and 9-methylanthracene.
  • 9.4.3 Ensure that the resolution of cyclohexane and o-xylene is not less than 5.0.
  • 9.4.3.1 Column resolution: the resolution of cyclohexane and o-xylene is calculated by formula (1): resolution = 1.699 × 2×(t₂−t₁)/(Y₁+Y₂) (the original formula source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)). Where: t₁—retention time of cyclohexane, s; t₂—retention time of o-xylene, s; Y₁—half-peak width of cyclohexane, s; Y₂—half-peak width of o-xylene, s.
  • If the resolution is less than 5.0, check whether all parts of the system are operating normally and whether the dead volume of the chromatograph has been minimized. Adjust the flow rate to see whether the resolution improves; otherwise, check whether the mobile phase quality meets the requirements. If the problem remains unresolved, regenerate the chromatographic column.
• 9.5 Use the data processing system to determine the retention times of dibenzothiophene and 9-methylanthracene.
• 9.6 Calculate the backflush time B by formula (2), s (see Section 6 for the formula). Note: The backflush time is the time after injection when the backflush valve backflushes so as to elute the tricyclic* aromatics as a sharp peak.
• 9.7 After the operating conditions are stable (after the baseline is level), inject 10μL standard solution SPS (see 9.3), and record the spectrum. At the pre-determined time point, make the backflush valve backflush so as to elute the tricyclic* aromatics as a sharp, narrow peak (see Figure 3). After the analysis is completed, change the flow direction of the mobile phase (that is, return to the forward flow direction), and stabilize the baseline before the next analysis.
• 9.8 Repeat 9.7 to ensure that the repeatability of the peak areas of cyclohexane, o-xylene, dibenzothiophene, and 9-methylanthracene is within the range specified by the method.
• 10.1 Prepare the working curve:
  • 10.1.1 According to the concentrations in Table 1, weigh the standard substances (accurate to 0.0001g), place them in a 100mL volumetric flask, and dilute to the mark with n-heptane. Note 1: Table 1 covers most test samples in the diesel distillation range. As long as the method requirements are met (such as linearity, detector sensitivity, column resolution), standard solutions of other concentrations may be used. Note 2: Store the standard solution in a well-sealed bottle (such as a 10mL volumetric flask), and store it in the dark at 5℃~25℃. Under these conditions, the standard solution can still be used after storage for half a year.
  • 10.1.2 After the operating conditions are stable (see 9.4), inject 10μL standard solution A, record the spectrum, and measure the peak area of each standard substance (see Figure 3). At the pre-determined time point, make the backflush valve backflush so as to elute the tricyclic* aromatics as a sharp, narrow peak. After the analysis is completed, change the flow direction of the mobile phase (that is, return to the forward flow direction), and stabilize the baseline before the next analysis.
  • 10.1.3 Repeat 10.1.2 with standard solutions B, C, D.
  • 10.1.4 If the phenanthrene area in standard solution D is too small to be measured accurately, prepare a new standard solution D containing a higher concentration of phenanthrene (e.g. 0.02g/100mL) according to 10.1.1.
  • 10.1.5 Plot the concentration (g/100mL) of each aromatic standard substance (o-xylene, 1-methylnaphthalene, phenanthrene) against the peak area. The working curve should be a straight line, the correlation coefficient shall be greater than 0.999, and the intercept shall be less than 0.01g/100mL. A computer or data processing system may be used to prepare the working curve. In routine analysis, only the differential refractive index detector needs to be calibrated. Note: It is recommended that after every five analyses, a reference diesel test sample or a standard solution be used to check the stability of the instrument.
• 10.2.2 When the operating conditions are stable (see 9.4.2) and are the same as those when the working curve was prepared (see 10.1), inject 10μL test sample solution (see 10.2.1). At the pre-determined time point (see 9.6), make the backflush valve backflush so as to elute the tricyclic* aromatics as a sharp, narrow peak. After the analysis is completed, change the flow direction of the mobile phase (that is, return to the forward flow direction), and stabilize the baseline before the next analysis.
• 10.2.3 Referring to Figure 5, accurately identify the monocyclic aromatic peak, dicyclic aromatic peak, and tricyclic* aromatic peak. Figure 5 is a typical spectrum of a diesel test sample.
• 10.2.4 Draw the baseline from the beginning of the non-aromatic peak (A in Figure 5) to the point where the backflush valve is about to start operating (D in Figure 5).
• 10.2.5 Draw a vertical line from the valley between the non-aromatic peak and the monocyclic aromatic peak (E in Figure 5).
• 10.2.6 Draw a vertical line from the valley between the monocyclic aromatic peak and the dicyclic aromatic peak (C in Figure 5).
• 10.2.7 Draw the baseline from the beginning of the tricyclic* aromatic peak (B in Figure 5) to the end of the tricyclic* aromatic peak (F in Figure 5). It can be expected that the baseline will fluctuate as the backflush valve operates; therefore, after the backflush valve operates, wait for the baseline to stabilize before drawing the baseline.
• 10.2.8 From point B to (the source text is truncated, requires confirmation against the source standard (source-text extraction))... this is monocyclic aromatics (Figure 5).
• 10.2.9 From point C to (the source text is truncated, requires confirmation against the source standard (source-text extraction))... this is dicyclic aromatics (Figure 5).
• 10.2.10 From point E to (the source text is truncated, requires confirmation against the source standard (source-text extraction))... this is tricyclic* aromatics (Figure 5).
• Note: If chromatographic data processing is performed automatically, check whether peak identification and peak area integration are correct.

9. Qualitative and quantitative determination and calculation formulas

• 11.1 Content of each type of aromatic: calculate the content of monocyclic aromatics, dicyclic aromatics, and tricyclic* aromatics Xᵢ [% (mass fraction)] by formula (3): Xᵢ = [ (A × S) + I ] × V / m × 100 (the original formula source-text extraction is unclear, requires confirmation against the source standard (source-text extraction)) Where: A— peak area of monocyclic aromatic hydrocarbons or dicyclic aromatic hydrocarbons or tricyclic* aromatic hydrocarbons; S— slope of the working curve of monocyclic aromatic hydrocarbons or dicyclic aromatic hydrocarbons or tricyclic* aromatic hydrocarbons (%m/V versus peak area); I— intercept of the working curve of monocyclic aromatic hydrocarbons or dicyclic aromatic hydrocarbons or tricyclic* aromatic hydrocarbons; m— sample amount, g; V— total volume, mL. Note: This calculation process can also be completed by a data processing system.
• 11.2 Content of polycyclic aromatic hydrocarbons: Calculate the content of polycyclic aromatic hydrocarbons Xₚ [% (mass fraction)] by formula (4): Xₚ = Xdicyclic aromatic hydrocarbons + Xtricyclic* aromatic hydrocarbons
• 11.3 Content of total aromatic hydrocarbons: obtained by summing the contents of various aromatic hydrocarbons (monocyclic aromatic hydrocarbons + dicyclic aromatic hydrocarbons + tricyclic* aromatic hydrocarbons).

10. Precision

• 13.1 Precision: Use the following provisions to judge the reliability of test results (95% confidence level).
• 13.1.1 Repeatability: The difference between two results of repeated determination of the same sample by the same operator using the same instrument shall not be greater than the value in Table 2.
• 13.1.2 Reproducibility: For determinations of the same sample by different operators in different laboratories, the difference between two independent results obtained shall not be greater than the value in Table 2.
• 13.2 Bias: Because no suitable reference material is available for the determination of bias, the bias of this standard cannot be determined.
• Table 2 Precision (X is the average of the two results being compared, % (mass fraction)):

(Note: In Table 2, the repeatability source-text extraction for dicyclic aromatic hydrocarbons is "0.10 (X+2.6", the parenthesis is missing, requires confirmation against the source standard (source-text extraction))

11. Report

• 12 Report: Report monocyclic aromatic hydrocarbons [% (mass fraction)], dicyclic aromatic hydrocarbons [% (mass fraction)], tricyclic* aromatic hydrocarbons [% (mass fraction)], polycyclic aromatic hydrocarbons [% (mass fraction)] and total aromatic hydrocarbons [% (mass fraction)], accurate to 0.1 [% (mass fraction)].
• Note: The foreword states "the report information of ASTM D6591-06 12.2 was deleted".

12. Normative references

• GB/T 4756 Petroleum liquids — Manual sampling (GB/T 4756—1998, eqv ISO 3170:1988)
• GB/T 11132 Determination of hydrocarbon types in liquid petroleum products — Fluorescent indicator adsorption method
• SH/T 0606 Determination of hydrocarbon types in middle distillates — Mass spectrometry
• ASTM D5186 Determination of aromatic and polycyclic aromatic hydrocarbon contents in diesel fuels and aviation fuels — Supercritical fluid chromatography
• ASTM D6379 Determination of aromatic hydrocarbon types in aviation fuels and petroleum fractions — High performance liquid chromatography with refractive index detector

13. Source and verification

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