NB/SH/T 0885-2014 Determination of total aromatics and total saturates in lubricant base oils by high performance liquid chromatography with refractive index detector

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

• Standard number: NB/SH/T 0885—2014
• Chinese full title: Determination of total aromatics and total saturates in lubricant base oils by high performance liquid chromatography with refractive index detector
• English title: Determination of total aromatics and total saturates in lube base stocks by high performance Liquid chromatography (HPLC) with refractive index detection (in the original English title, "Pertormance" is suspected to be source-text extraction noise, requires confirmation against the source standard (source-text extraction))
• Replacement relationship: not provided in the available source (no replacement of any standard is indicated)
• Adoption relationship: adopted by redrafting method with modification of ASTM D7419-07
• Publication/implementation: published by 2014-06-29, implemented by 2014-11-01; issued by the National Energy Administration
• Drafting organization: Lanzhou Lubricating Oil Research and Development Center of PetroChina Company Limited
• Main drafters: Zhang Dahua, Cheng Wei
• Main technical differences from ASTM D7419-07:
  • The referenced standards were changed to the corresponding national standards and petrochemical industry standards of our country
  • In clause 10.1, the content of a note was added (in ASTM D7419-07 Table 1, the concentration of the calibration standard solution E is relatively high, which may cause integration problems on some instruments; this standard provides supplementary explanation)
  • In clause 11.1, the concentration for sample preparation was modified (in ASTM D7419-07, the required sample concentration is relatively high, which is prone to overloading the chromatographic column and causing incomplete sample separation)

2. Scope of application and non-applicability

• Applicable to the determination of total aromatics and total saturates content in lubricant base oils without additives.
• Applicable to samples with total aromatics content from 0.2% (mass fraction) to 46% (mass fraction).
• If the sample contains polar compounds, the content of polar compounds is counted as total aromatics.
• Not applicable: no explicit non-applicable clause is provided in the available source.

3. Method principle

• 4.1 A certain amount of sample is diluted with mobile phase, and a certain volume is injected into a calibrated high performance liquid chromatography system. The specified chromatographic column has only weak affinity for saturates, but has a certain retention effect on aromatics and polar compounds, thereby separating aromatics and polar compounds from saturates. After the saturates are eluted from the chromatographic column, the chromatographic column is backflushed at a preset time, thereby eluting the aromatics and polar compounds as a narrow peak.
• 4.2 A refractive index detector is connected after the chromatographic column to detect the eluted components, and a data processing system continuously monitors the detector electrical signal. The integrated signals (peak areas) of the saturates and aromatics components in the sample are corrected with pre-measured response factors to obtain the mass fractions of saturates and aromatics plus polar compounds.

4. Reagents and materials (purity / preparation)

• 7.1 n-Heptane: chromatographic grade (HPLC grade); if necessary, it may be dried with molecular sieves and filtered before use. Warning: n-heptane is highly flammable, and inhalation, ingestion, or skin contact is irritating.
• 7.2 Dichloromethane: HPLC grade; if necessary, it may be dried with molecular sieves and filtered before use. Warning: dichloromethane is toxic; use with adequate ventilation.
• 7.3 Octadecylbenzene: purity not less than 97% (mass fraction).
• 7.4 n-Hexadecane: purity not less than 98% (mass fraction).
• 9.3.1 Preparation of system performance test standard (SPS) solution: in a 10 mL volumetric flask, respectively weigh 1.0 g±0.1 g n-hexadecane and 1.0 g±0.1 g octadecylbenzene, then dilute to the mark with n-heptane. n-Heptane from the same manufacturer as the mobile phase should be used; octadecylbenzene should be completely dissolved, and if necessary, an ultrasonic bath may be used to assist dissolution.
• 9.6 Preparation of detection limit test standard: in a sample bottle, weigh 0.01 g octadecylbenzene and add 5.00 g n-hexadecane to obtain a standard with an aromatics content of 0.2% (mass fraction).
• 9.7.1 Preparation of minimum response test mixture: weigh 0.01 g octadecylbenzene and 10 g n-hexadecane to obtain a mixture in which the mass fraction of octadecylbenzene in n-hexadecane is 0.1%.
• 10.1 Calibration standard solutions: prepare five calibration standard solutions (A, B, C, D, E) according to Table 3, weighing accurately to 0.0001 g, and after weighing in a 10 mL volumetric flask, dilute to the mark with n-heptane.

Table 3 Calibration component concentration table

Note: The concentrations recommended in Table 2 cover the range of aromatics content in petroleum fractions within the base oil distillation range, and are also suitable for the requirements of most instruments. However, on some equipment, if peak overloading occurs for calibration standard solution E, the concentrations of n-hexadecane and octadecylbenzene may be appropriately reduced, such as (n-hexadecane concentration of 3 g/10mL, octadecylbenzene concentration of 2 g/10mL), but their relative proportions should be maintained. (The original text here says "the concentrations recommended in Table 2", which is suspected to be an error for "Table 3", requires confirmation against the source standard (source-text extraction))

5. Instruments and equipment

• 6.1 High performance liquid chromatography (HPLC): any high performance liquid chromatography system capable of pumping the mobile phase at a flow rate of 3 mL/min~5 mL/min and with precision better than 0.5% is applicable.
• 6.2 High performance liquid chromatography injection system: in general, the injection system should ensure injection of 10 µL, with repeatability reaching or better than 1%.
• 6.3 Column system: any stainless steel high performance liquid chromatography column packed with a silica stationary phase packing and meeting the resolution and capacity requirements specified in clause 9.3 may be used. A single chromatographic column or two connected together may be used, with a total length of 500 mm, packed with 5 µm particle size stationary phase and an inner diameter of 7.5 mm–10 mm. In addition to the silica column, a chromatographic column packed with a cyano stationary phase as packing also needs to be connected before the silica column; a chromatographic column with a column length of 100 mm–250 mm, a column inner diameter of 7.7 mm–10 mm, and a packing of 5 µm–10 µm particle size stationary phase can basically meet the requirements. Typical chromatographic operating conditions are shown in Table 1, and the chromatographic operating conditions used in the research of this standard are shown in Table 2.
• 6.4 Backflush valve: any manual or automatic (pneumatic or electric) flow path switching valve designed for liquid chromatography systems and capable of withstanding a pressure of 2×10⁴ kPa may be used.
• 6.5 Refractive index detector: any refractive index detector that measures refractive index between 1.3~1.6, meets the sensitivity requirements of the method, has a linear response within the calibration range, and has a suitable output signal to the data system may be used. If the refractive index detector has an independent temperature control device, it is recommended to set this temperature to 5° above the laboratory temperature.
• 6.6 Ultraviolet-visible detector: optional and recommended; the wavelength of this detector is set to 254 nm and it is connected in series with the refractive index detector to help set and monitor the backflush time between saturates and aromatics.
• 6.7 Computer and computing integrator: any data system capable of matching the refractive index detector, with a minimum sampling frequency of 1 Hz, and capable of accurately measuring peak area and retention time may be used. However, this data system should also have at least reanalysis and data reprocessing functions, such as automatic or manual baseline correction and reintegration.
• 6.8 Volumetric flask: B grade or higher, capacity 10 mL.
• 6.9 Autosampler vial: sample vials meeting the equipment manufacturer's requirements and with a capacity greater than 1.5 mL may be used.
• 6.10 Analytical balance: readability of 0.1 mg.

Table 1 Examples of operating conditions used in the cooperative study

Table 2 Chromatographic operating conditions used in this standard study

6. Recommended operating conditions

• Mobile phase: n-heptane
• Flow rate: 3.0 mL/min (9.2.1 adjust the mobile phase flow rate to stabilize it at 3.0 mL/min; 6.1 allow 3 mL/min~5 mL/min)
• Injection volume: 10 µL (9.6.1.1 if sensitivity is insufficient, it may be increased to 20 µL, then all analyses use 20 µL)
• Detector: differential refractive index detector; optional ultraviolet-visible detector (254 nm) in series
• Column system: silica gel column + cyano column (cyano column before silica gel column)
• Backflush mode: used to elute aromatic hydrocarbons and polar compounds
• Typical operating conditions are shown in Table 1, Table 2 (see Section 5)

7. Sampling and sample preparation

• 8 Sampling: The sample used for laboratory analysis must be representative. Unless otherwise specified in the product standard, sampling shall be carried out in accordance with GB/T 4756 or SY/T 5317 to obtain a representative base oil sample. The sample shall be thoroughly mixed before sampling.
• 11.1 Sample preparation: Weigh about 0.5 g of sample into a 1.5 mL sample bottle, then add 1 mL n-heptane, and shake well. If necessary, a larger volume of sample can be prepared, for example, 2 g of sample can be weighed and prepared according to the same sample/solvent ratio. After mixing uniformly, it can be dispensed into autosampler vials or other containers.
• 11.1.1 For samples whose non-aromatic or aromatic content is judged from relative area to exceed the calibration range, correspondingly prepare a more concentrated (0.75 g/0.25 mL) or more dilute (0.25 g/0.75 mL) sample solution.

8. Analysis procedure

• 9.1 Set up the chromatograph, injection system, chromatographic column, differential refractive index detector, computing integrator, and optional column oven and ultraviolet-visible detector according to their respective operating manuals and as shown in Figure 1. Adding a backflush valve to the chromatographic system allows the detector to always remain in the system flow path when the flow direction of the mobile phase in the column is changed (see Figure 1). The injection valve should be at the same temperature as the sample solution, generally room temperature. To reduce signal drift, ensure that room temperature remains relatively stable during analysis and calibration.
• 9.2 New commercial chromatographic columns are generally stored in methanol/water or other polar solvents. Before use, they should first be rinsed with dichloromethane, then with n-heptane; other solvents that can regenerate the column to the required resolution may also be used. If the required resolution cannot be met, the column needs to be regenerated with more dichloromethane; if the required resolution still cannot be achieved, the column needs to be replaced or a similar column from another supplier purchased. Under appropriate conditions, SI60 silica gel chromatographic column can meet the resolution requirements and give good results. When not performing sample analysis, the column can be rinsed at a lower n-heptane flow rate such as 0.1 mL.
• 9.2.1 Adjust the mobile phase flow rate to stabilize it at 3.0 mL/min, and ensure that the reference cell of the differential refractive index detector is filled with mobile phase. Fill the reference cell according to the equipment operating manual.
• 9.2.2 To reduce drift, ensure that the reference cell of the detector is always filled with solvent. There are two suitable methods: (1) immediately rinse the reference cell with mobile phase before analysis (then isolate to prevent solvent evaporation); (2) continuously replenish mobile phase to the reference cell. The replenishment flow rate should be optimized so as to minimize the difference between the analytical cell and the reference cell caused by evaporation, flow rate, temperature, or pressure changes. Typically, the replenishment flow rate is set at about one-tenth of the analytical flow rate.
• 9.3 Column resolution and capacity factor:
  • 9.3.1 Prepare the system performance test standard (SPS) solution (see Section 4).
  • 9.3.2 When the baseline of the differential refractive index detector is observed to be stable, indicating that the operating conditions are ready, inject 10 µL of the system performance test standard solution (see 9.3.1) in forward flush mode (backflush mode off) and record the chromatogram with the data system. Figure 2 gives a typical chromatogram of the system performance test standard solution.
  • 9.3.3 The resolution ≥5 between n-hexadecane and octadecylbenzene, calculated as defined below, shall be ensured. The resolution is calculated using equation (1):
    • R = 2×(t₂ - t₁) / [3×(Y₁ + Y₂)] (the original formula source-text extraction has missing parts, requires confirmation against the source standard (source-text extraction))
    • Where: R— resolution; t₁—retention time of the n-hexadecane chromatographic peak, min; t₂—retention time of the octadecylbenzene chromatographic peak, min; Y₁—half-peak width of the n-hexadecane chromatographic peak, min; Y₂—half-peak width of the octadecylbenzene chromatographic peak, min.
    • If the resolution is less than 5, check whether all parts of the chromatographic system are working properly, and use low dead volume connectors and tubing to minimize the dead volume of the chromatographic system, ensure the use of high-quality mobile phase, and finally, if necessary, regenerate the column or replace the column. Column regeneration can be carried out by first rinsing with dichloromethane, then rinsing with n-heptane until a relatively stable signal is obtained. If after regeneration of the silica gel column the resolution is still less than 5, the column needs to be replaced. Under suitable conditions, Si60 silica gel can achieve satisfactory results. To obtain correct analytical results, the resolution must be at least 5.
  • 9.3.4 Calculate the capacity factor of octadecylbenzene in 9.3.2 using the following equation:
    • K = (t₂ - t₁) / t₁ (the original formula source-text extraction has missing parts, requires confirmation against the source standard (source-text extraction))
    • Where: K— capacity factor; t₁—retention time of the n-hexadecane chromatographic peak, min; t₂—retention time of the octadecylbenzene chromatographic peak, min.
  • 9.3.5 Calculate the approximate backflush time B (min) using the following formula and the retention times of n-hexadecane and octadecylbenzene measured in 9.3.2:
    • The original formula source-text extraction has missing parts, requires confirmation against the source standard (source-text extraction)
    • Where: t₁—retention time of the n-hexadecane chromatographic peak, min; t₂—retention time of the octadecylbenzene chromatographic peak, min.
• 9.4 Once the backflush time is determined, inject 10 µL of the system performance test standard solution again and perform backflush at the determined time. The backflush time can be ensured by observing the signal mark appearing at the bottom of the saturated hydrocarbon chromatographic peak in the chromatogram. Attention should also be paid to observation during actual base oil analysis. If optimization is necessary, the resolution and capacity factor need to be confirmed again and the backflush time checked again. Using an ultraviolet-visible detector can simplify the optimization of the backflush time. Figure 3 gives the chromatogram of the system performance test standard under backflush mode.
• 9.5 Check the system precision in accordance with 12.4. Note: If the repeatability of peak areas is poor, check whether the injection system is operating optimally and whether the baseline is stable and free from noise interference.
• 9.6 Prepare the detection limit test standard (see Section 4).
  • 9.6.1 Inject and analyze the detection limit standard in forward flush mode, ensuring that the signal-to-noise ratio of octadecylbenzene is greater than 8. Figure 4 gives the method for calculating the signal-to-noise ratio.
  • 9.6.1.1 If octadecylbenzene cannot be detected, recheck the instrument to ensure that the differential refractive index detector can achieve the sensitivity specified by the manufacturer. If necessary, increase the injection volume to 20 µL and repeat all operating steps in Chapter 9. If all requirements of this method can be met with an injection volume of 20 µL, then use an injection volume of 20 µL for all analyses. In the reproducibility study of this method, an injection volume of 10 µL was sufficient for all laboratories.
• 9.7 Before performing the following steps, the system should be calibrated in accordance with 10.1. Determine the minimum response obtained at low concentration according to the following steps:
  • 9.7.1 By weighing 0.01 g octadecylbenzene and 10 g n-hexadecane, obtain a mixture with a mass fraction of octadecylbenzene in n-hexadecane of 0.1%, and analyze this mixture as a sample in backflush mode, with the backflush time according to the time measured in 9.3.5.
  • 9.7.2 Figure 5 gives a corresponding example of the chromatogram.
  • 9.7.3 Calculate the mass fraction of octadecylbenzene in accordance with Chapter 13 and ensure that the result obtained is <0.15%.
• 10 Calibration:
  • 10.1 Prepare five calibration standard solutions (A, B, C, D and E) according to the concentrations given in Table 3. Weigh accurately to 0.0001 g. In a 10 mL volumetric flask, weigh the corresponding weights of the various substances and dilute to the mark with n-heptane. (Note see Section 4)
  • 10.2 When a stable baseline is observed, it indicates that the operating conditions have stabilized. Inject 10 µL of calibration standard solution A into the system. For the calibration process, backflush mode should be used, record the chromatogram, and measure the integrated peak areas of n-hexadecane and octadecylbenzene.
  • 10.3 Repeat the procedure of clause 10.2 using calibration standard solutions B, C, D and E. For low-concentration standards, if the baseline drifts, manual integration may be required, see Figure 6 to Figure 8.
  • 10.4 Plot the "g/10 mL" concentration of n-hexadecane and octadecylbenzene against the chromatographic peak integration area; the calibration curve should have good linearity, and when forced through the origin, the correlation coefficient R² should be greater than 0.999; the slope of the calibration curve is used in Chapter 13 to determine the response factor. Computer software such as EXCEL or a data system may also be used to establish the calibration curve. Figure 9 gives an example of the calibration curve. The response factor is required to reach 1.75±0.05.
• 11 Test procedure:
  • 11.1 Sample preparation (see Section 7).
  • 11.1.1 Handling of samples outside the calibration range (see Section 7).
  • 11.1.2 Using the same operating conditions as those used to obtain the calibration data (see clause 10.1), inject the 10 µL sample into the chromatographic system and start data acquisition; at a pre-determined time (see 9.3.5), turn or open the backflush valve so that the aromatic hydrocarbons are eluted as a sharp peak; after the analysis is complete, reverse the direction of the mobile phase (close the switching valve), i.e., return to forward-flush mode, and wait for the baseline to stabilize before performing the analysis of the next sample.
  • 11.1.3 Correctly identify the chromatographic peaks of saturated hydrocarbons and aromatic hydrocarbons. Figure 10 and Figure 11 give typical chromatograms of lubricating base oils.
  • 11.1.4 During sample analysis, an optional UV-visible detector with a detection wavelength of 254 nm may be used to monitor the separation between saturated hydrocarbons and aromatic hydrocarbons; as the aromatic hydrocarbon content in the sample decreases, the separation between saturated hydrocarbons and aromatic hydrocarbons will deteriorate; compared with the aromatic hydrocarbon peak, the signal of the UV-visible detector during the elution of saturated hydrocarbons is very weak. Figure 12 gives an example of the use of a UV-visible detector.
  • 11.1.5 On the chromatogram, by drawing baselines respectively between before the start of the saturated hydrocarbon chromatographic peak and before the start of backflushing, and between before the start of the aromatic hydrocarbon chromatographic peak and after the aromatic hydrocarbon chromatographic peak, the integration areas of saturated hydrocarbons and aromatic hydrocarbons plus polar compounds can be obtained.
• 12 Quality control:
  • 12.1 For each batch of samples, the column resolution should be measured and recorded in accordance with the requirements of 9.3.3; as the column is used, the resolution may decrease, and regeneration of the chromatographic column may be required.
  • 12.2 The calibrated response factor should be ensured to be 1.75±0.05.
  • 12.3 For each batch of samples, a detection limit evaluation should be performed once in accordance with the requirements of clause 9.6.
  • 12.4 When the instrument is first put into use, a short-term precision calculation should be performed in forward-flush mode; in forward-flush mode, the resolution determination standard solution is measured 5 times, and the relative standard deviation of the integration areas of the n-hexadecane and octadecylbenzene components should be ≤0.6%. After the instrument is put into use, select a sample with an aromatic hydrocarbons plus polar compounds content of 8% (mass fraction) to 10% (mass fraction) as a check sample, analyze it multiple times, and establish a statistical quality control SQC monitoring program for interlaboratory precision in accordance with the procedures of NB/SH/T 0843. It should be ensured that the maximum/minimum control limits of this check sample fall within the repeatability and reproducibility requirements specified in this method.
  • 12.5 It is recommended to use other check samples to cover higher and lower ranges of aromatic hydrocarbons plus polar compounds content; for example, if the aromatic hydrocarbons plus polar compounds content of the sample being analyzed is around 45% (mass fraction), then use a check sample with an aromatic hydrocarbons plus polar compounds content of around 45% (mass fraction). For samples with an aromatic hydrocarbons plus polar compounds content below 5% (mass fraction), use a check sample with an aromatic hydrocarbons plus polar compounds content of around 3% (mass fraction) to 5% (mass fraction); if possible, it is recommended to use samples with recognized reference values.

9. Qualitative and Quantitative Determination and Calculation Formulas

• 13.1 Calculation of the aromatic hydrocarbon response factor. Check the calibration curve obtained in clause 10.4, perform a linear regression analysis forced through the origin, and calculate the aromatic hydrocarbon response factor ARF according to formula (4); the value of ARF is required to be 1.75±0.05:
  • ARF = a_na / a_ar (the original formula source-text extraction is missing, requires confirmation against the source standard (source-text extraction))
  • Where: ARF— response factor of the aromatic hydrocarbon component; a_ar— slope of the octadecylbenzene calibration curve; a_na— slope of the n-hexadecane calibration curve.
• 13.2 The contents of saturated hydrocarbons and aromatic hydrocarbons plus polar compounds are calculated according to formula (5) or formula (6):
  • w_na = A_na / (A_na + A_ar/ARF) × 100 (the original formula source-text extraction is missing, requires confirmation against the source standard (source-text extraction))
  • w_ar = (A_ar/ARF) / (A_na + A_ar/ARF) × 100 (the original formula source-text extraction is missing, requires confirmation against the source standard (source-text extraction))
  • Where: w_na— saturated hydrocarbon content, % (mass fraction); w_ar— aromatic hydrocarbons plus polar compounds content, % (mass fraction); A_na— area of the saturated hydrocarbon chromatographic peak; A_ar— area of the aromatic hydrocarbons plus polar compounds chromatographic peak; ARF— response factor of the aromatic hydrocarbon component.
• 14 Report: Report the measurement results in the form of "total saturated hydrocarbons" content and "total aromatic hydrocarbons plus polar compounds" content, accurate to 0.1% (mass fraction).

10. Precision

• 15.1 Precision: The precision of this method was obtained on the basis of 7 laboratories and 22 samples; the base oils include at least 7 different sources, and their total aromatic hydrocarbons plus polar compounds content ranges from 0.2% (mass fraction) to 46% (mass fraction); the polar compounds content in all samples measured by the silica gel solid-phase extraction-gravimetric method is less than 1% (mass fraction).
• 15.1.1 Repeatability r: The difference between two test results obtained by the same operator, in the same laboratory, using the same instrument, according to the same method, on the same test sample in continuous determinations should not be greater than the value specified in formula (7):
  • r = 0.022 × (X + 4)
  • Where: X— average of two replicate test results, % (mass fraction).
• 15.1.2 Reproducibility R: The difference between two single, independent test results obtained by different operators, in different laboratories, using different instruments, on the same test sample should not be greater than the value specified in formula (8):
  • R = 0.11 × (X + 4)
  • Where: X— average of two single, independent test results, % (mass fraction).
• 15.2 An example of precision estimation is given in Table 4.

Table 4 Example of Precision Calculation

• 15.3 Bias: Because no suitable reference material is available for the determination of bias, the bias of this standard has not been determined.

11. Report

• Report the measurement results in the form of "total saturated hydrocarbons" content and "total aromatic hydrocarbons plus polar compounds" content, accurate to 0.1% (mass fraction). (See Chapter 14)

12. Referenced Documents

• GB/T 4756 Petroleum liquids—Manual sampling (GB/T 4756-1998, eqv ISO 3170:1988)
• NB/SH/T 0843 Evaluation of analytical testing systems in the petrochemical industry—Statistical techniques
• SY/T 5317 Petroleum liquids—Automatic pipeline sampling (SY/T 5317—2006 ISO 3171:1988 IDT)
• GB/T Drafted according to the rules given in 1.1—2009 (foreword reference)
• ASTM D7419-07 "Standard Test Method for Determination of Total Aromatics and Total Saturates in Lubricant Base Oils—High Performance Liquid Chromatography with Differential Refractive Index Detector" (modified adoption)

13. Source and verification

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