NB/SH/T 0864-2023 Determination of elements in grease by inductively coupled plasma optical emission spectrometry

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Card type: Test method

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

• Standard number: NB/SH/T 0864—2023
• Chinese full title: Determination of elements in grease by inductively coupled plasma optical emission spectrometry
• English title: Standard test method for determination of elements in lubricating greases―Inductively coupled plasma optical emission spectrometry
• Replacement relationship: replaces NB/SH/T 0864—2013 "Determination of metal elements in grease by inductively coupled plasma optical emission spectrometry"
• Publication/implementation: published on 2023-12-28, implemented on 2024-06-28
• Previous versions: first published in 2013 as NB/SH/T 0864—2013; this is the first revision
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants, Subcommittee on Petroleum Fuels and Lubricants (SAC/TC280/SC1)
• Proposed by: China Petrochemical Corporation

2. Scope of application and non-applicability

• Applicable to the determination of the content of multiple elements in unused grease, using the inductively coupled plasma optical emission spectrometry (ICP-OES) method.
• Applicable metal elements: aluminum, antimony, barium, calcium, iron, lithium, magnesium, molybdenum, sodium, zinc.
• Applicable non-metal elements: boron, phosphorus, silicon, sulfur.
• The determination range and recommended wavelengths for each element are given in Table 1. For elements with content below the range in Table 1, the method of increasing the sample amount may be used for determination; for elements with content above this range, the method of reducing the sample amount or increasing the dilution factor after sample dissolution may be used for determination; however, the precision under these conditions has not been established.
• It is also applicable to the determination of other metals, such as bismuth, cadmium, chromium, copper, lead, manganese, potassium, titanium, etc., but their precision has not been established; these metals may be considered impurities or additive elements in grease.
• Not applicable to the determination of element content in grease samples that produce visible insoluble matter after sample preparation (assuming that the insoluble matter contains part of the elements to be determined).

Table 1 Determination range and recommended wavelengths of the elements to be determined

Note: When using wavelengths below 190nm to determine elements such as phosphorus and sulfur, vacuum or inert gas purging of the optical path is required.

3. Method principle

Accurately weigh the grease sample, decompose the sample by sulfate ash digestion or closed-vessel microwave digestion, and make up to volume with acid solution. Introduce the acid-soluble standard solution and sample solution into the ICP instrument by siphon or peristaltic pump for measurement, and calculate the content of the elements to be determined in the sample solution by comparing the emission intensities of the elements in the acid-soluble standard solution and the sample solution. (See Chapter 4 Method summary)

4. Reagents and materials (purity / preparation)

All reagents shall use the specified purity. If verified in advance, other grades of reagents that can achieve sufficient purity without affecting the accuracy of the determination may also be used.

• 7.1 Sulfuric acid: concentrated sulfuric acid, analytical reagent, ≥98% (mass fraction). Warning: can cause severe burns, corrosive.
• 7.2 Nitric acid: concentrated nitric acid, analytical reagent, 65%～68% (mass fraction). Warning: can cause severe burns, corrosive.
• 7.3 Hydrochloric acid: concentrated hydrochloric acid, analytical reagent, 36%～38% (mass fraction). Warning: can cause severe burns, corrosive.
• 7.4 Hydrofluoric acid: concentrated hydrofluoric acid, analytical reagent, ≥40% (mass fraction). Warning: can cause severe burns, corrosive.
• 7.5 Water-soluble standard solution: single-element water-soluble standard solution with mass concentration of 100mg/L. It can be prepared according to GB/T 602, or purchased directly. Multi-element water-soluble standard solutions are more convenient to use.
• 7.6 Internal standard: water-soluble cobalt, indium, scandium, yttrium or other single-element standard solution, with mass fraction of 500mg/kg, and the grease sample or calibration standard solution does not contain this component.
• 7.7 Water: meets the requirements for grade two water in GB/T 6682—2008.
• 7.8 Quality control (QC) sample: one or more stable and representative grease samples.

5. Instruments and equipment

• 6.1 Balance: sensitivity of 0.001g or 0.0001g, with a range that can meet the needs of preparing solutions and weighing samples.
• 6.2 Inductively coupled plasma optical emission spectrometer: a ICP instrument with a quartz torch and a spectrometer capable of forming plasma with a high-frequency generator (whether sequential scanning type or simultaneous determination type) may be used.
• 6.3 Peristaltic pump: a peristaltic pump should be used to provide a stable solution flow rate, with a pump flow range of 0.5mL/min～3mL/min. The pump tubing should be able to withstand at least 6h in the diluent.
• 6.4 Microwave digestion instrument
  • 6.4.1 A microwave digestion instrument with a temperature sensor and a pressure sensor should be used to ensure the safety and accuracy of sample preparation.
  • 6.4.2 Microwave oven: a commercial laboratory microwave digestion oven with sufficient power (at least 1000W) should be used, equipped with a microwave digestion tray or similar accessories. Warning: take necessary measures to prevent high-frequency radiation.
  • 6.4.3 Microwave digestion vessel: composed of microwave-transparent, solvent-resistant inert polymer materials (such as tetrafluoroethylene-perfluoroalkoxy vinyl ether copolymer PFA and modified polytetrafluoroethylene TFM). For higher pressures, the microwave digestion vessel may use a multi-layer, microwave-transparent liner composed of different materials to ensure strength, resistance and safety. The inner vessel volume is at least 45mL, and it can withstand at least 3040kPa pressure and can control pressure release. It is preferable to use a silicon-free polytetrafluoroethylene (PTFE) closed vessel that can withstand the pressure generated by digesting 0.2g sample (a 100mL vessel, the pressure generated by 0.2g sample may exceed 690kPa).
  • 6.4.4 Microwave digestion instrument temperature controller: provides the main feedback control device for the digestion method. During digestion, a temperature sensor should be installed to implement temperature control (one vessel or multiple vessels). The microwave digestion system should be able to sensitively detect temperature changes within ±2.5℃ and allow adjustment of the microwave system power output within 2s.
  • 6.4.5 Rotating disk: rotation speed at least 3r/min, which can ensure uniform distribution of microwave radiation in most systems.
• 6.5 Crucible: porcelain crucible, quartz crucible or platinum crucible, 50mL.
• 6.6 Volumetric flask: glass volumetric flask or volumetric flask made of polypropylene or similar material, 25mL and 50mL.
• 6.7 Muffle furnace: temperature maintained at 525℃±25℃, can accommodate several (not less than 3) 50mL crucibles at the same time, and a muffle furnace with oxygen supply capability may be used. Warning: take necessary measures to prevent contact with overheated surfaces.
• 6.8 Electric furnace: 1kW (adjustable).

6. Recommended operating conditions

• 10.1 Equipment: due to differences among various instrument designs, ICP excitation sources, and differences in the analytical wavelengths selected by various spectrophotometers, detailed (uniform and fixed) instrument operating conditions cannot be given. Instrument parameters may be set by referring to the operating conditions in the manuals of each instrument.
• 10.2 Peristaltic pump: if a peristaltic pump is needed, check the pump sample introduction tubing before each start, and replace it if necessary. Check the solution uptake rate and adjust it to the required value.
• 10.3 ICP Excitation source: before sample analysis, 30min the plasma should be ignited, and during this warm-up period, deionized water or dilute acid (5%) solution is aspirated for nebulization. Note: some instrument manufacturers recommend a longer warm-up period to minimize changes in the slope of the calibration curve.
• 10.4 Wavelength profile scan: perform the wavelength profile scan according to the instrument operating instructions.
• 10.5 Selection of operating parameters: establish appropriate instrument operating conditions so that the desired elements can be measured. These parameters include: element, wavelength, background correction point (optional), inter-element interference coefficient (see 5.1) and integration time. Each measurement requires multiple integrations (usually 3 times).
• 12.1 Before analysis, the analytical linear range should be determined for the specific instrument. By measuring the blank solution and a series of calibration standard solutions (3～5 concentration points are recommended), establish the calibration curve for each element, with the calibration curve correlation coefficient R² value ≥0.9999. The measured concentration of the sample solution must fall within the linear range of each calibration curve.
• 12.2 Before starting each group of sample tests, perform a two-point calibration with the blank solution and calibration standard solution. Use a check standard solution to confirm whether each element is in the calibration state. When the analysis results of all elements in the check standard solution are within the expected concentration 5% error range, sample analysis may proceed; otherwise, the instrument needs to be adjusted and recalibrated.
• 12.3 Calculate the correction factor K from the intensity ratio. The instrument may also be calibrated through the computer program provided by the instrument manufacturer.
• 13.2 Analyze the sample solution in the same manner under the conditions of the calibration standard solutions [i.e., the same integration time, background correction point (optional), plasma maintenance conditions, etc.] to obtain the concentration of the element to be determined in the solution. Between two samples, at least rinse with water spray for 60s. If the sample contains boron, between sample determinations, run (rinse) with water for 60s or longer.
• 13.3 If the concentration of the analyte exceeds the linear range of the calibration curve, the sample solution should be diluted to within the linear range of the calibration curve and then reanalyzed.
• 13.4 After every 5 samples analyzed, test the check standard solution once. If the obtained result exceeds 5% of the expected concentration, the instrument needs to be recalibrated, and the test samples reanalyzed.

7. Sampling and Sample Preparation

• Sampling: According to the requirements of the method in SH/T 0229, obtain a representative test sample. (See Chapter 8)
• General rules for sample preparation: Digest the sample according to the requirements of 9.2 or 9.3. (See 9.1)
• 9.2 Sulfated Ash Digestion Method
  • Note 1: Adding sulfuric acid will interfere with the determination of sulfur, and sulfur will also volatilize and be lost during sample combustion and ashing; therefore, this method is not suitable for the determination of sulfur.
  • Note 2: Adding sulfuric acid will interfere with the determination of barium; this method is not suitable for the determination of barium.
  • Note 3: This method is not suitable for the determination of boron.
  • 9.2.1 Accurately weigh 1g～2g of sample into a 50mL crucible. Carbonize the sample on an electric furnace until the sample amount is reduced to about 0.5g. Note: Some soap-based greases are alkaline and will corrode porcelain crucibles and quartz crucibles; therefore, when handling alkaline soap-based greases, a platinum crucible should be used.
  • 9.2.2 After the carbonized residue has cooled, add 1mL～2mL sulfuric acid, heat slowly on an electric furnace, taking care to avoid splashing, and heat until no more fumes are emitted.
  • 9.2.3 Place the carbonized sample in a muffle furnace at a temperature of 525℃±25℃ until the carbon is completely oxidized, which generally requires 2h.
  • 9.2.4 If black residue is present, it indicates incomplete ashing; steps 9.2.2 and 9.2.3 should be repeated to achieve complete sulfation.
  • 9.2.5 Add approximately 5mL nitric acid, hydrochloric acid, or another suitable inorganic acid to the residue, and heat slowly to dissolve the remaining solid residue. If perchloric acid is used, the sample should be fully acidified with nitric acid before adding it.
  • 9.2.6 Dilute the solution with water and make up to volume in a 25mL or 50mL volumetric flask. Note: Dilution may be based on mass or volume.
• 9.3 Closed Vessel Microwave Digestion Method
  • 9.3.1 In a PTFE microwave digestion vessel with a pressure relief device, accurately weigh 0.1g～1g of sample, and add 4mL nitric acid, hydrochloric acid, or another suitable inorganic acid. Note 1: From a safety perspective, if the digestion sample contains volatile or easily oxidizable organic components, the initial sample weight should not exceed 0.1g, and observe the sample reaction before capping. If a violent reaction occurs, wait until the reaction stops before capping. If no obvious reaction occurs, the maximum sample weight can be up to 1g. Note 2: Some models of microwave ovens can process multiple microwave digestion vessels simultaneously. Warning: Analysts should be aware of potential violent reactions. If a violent reaction occurs immediately upon adding reagents or if the sample is suspected to contain easily oxidizable substances, the sample may be pre-digested in an uncapped microwave digestion vessel. For safety reasons (e.g., rapid release of carbon dioxide from carbonates, rapid oxidation of easily oxidizable organics, etc.), heating may be applied at this step; once the initial reaction stops, the sample can continue digestion according to the digestion procedure.
  • 9.3.2 According to the instrument's operating requirements, seal the microwave digestion vessel and place it correctly in the digestion furnace, and connect the temperature sensor and pressure sensor properly.
  • 9.3.3 If the instrument operating conditions are determined by temperature control, pressure control suitable for a specific matrix applies. Because different matrix samples have different reaction characteristics, for samples of various special matrix types, temperature control should be used to determine suitable conditions before using pressure control. After digestion is complete, allow the microwave digestion vessel to cool in the digestion furnace for at least 5min before removing it.
  • 9.3.4 After the heating cycle is complete, place the hot microwave digestion vessel in an ice bath to cool for at least 1h, then transfer the entire digested sample to a 25mL or 50mL volumetric flask and make up to the mark with water (see Note 9.2.6). Note 1: Exercise extra caution when opening the microwave digestion vessel to prevent liquid from splashing out. Note 2: If the grease sample contains difficult-to-dissolve elements such as silicon, molybdenum, barium, etc., some insoluble residue may appear in the solution during the final dissolution stage. In this case, hydrofluoric acid may be added to dissolve these substances, and hydrofluoric acid-resistant vessels should be used during the volume make-up process. Similarly, if quantitative analysis of silicon content is required, a hydrofluoric acid-resistant ICP sample introduction system should be used, which can reduce the potential damage of hydrofluoric acid to the glass components of the ICP sample introduction system. Some digested organic residues also remain in the microwave digestion vessel and lid. They may be considered as floating immiscible organic residues, or as a "ring" on the liquid surface inside the microwave digestion vessel, and are best filtered out of the analyte, but without causing it to contain any elements.
  • 9.3.5 After testing, all microwave digestion vessels and containers should be carefully soaked in acid solution. Then rinse with water. When alternately processing high-concentration samples and low-concentration samples, the PTFE liner of the microwave digestion vessel should be soaked in hot hydrochloric acid (hydrochloric acid volume fraction about 50%, temperature greater than 80℃, but below boiling point) for at least 2h, then soaked in hot nitric acid (nitric acid volume fraction about 50%, temperature greater than 80℃, but below boiling point) for at least 2h, then rinsed with water and dried in a clean environment. If it is unclear whether the microwave digestion vessel has been used before or if cross-contamination is suspected, this cleaning procedure also applies. Volumetric and storage containers made of polymeric materials or glass should be soaked in a more dilute acid (volume fraction about 10%), then rinsed with water and dried in a clean environment. Other cleaning methods may also be used if they meet the requirements.
  • 9.3.6 It is difficult for this document to formulate safety recommendations for specific models of microwave digestion systems and manufacturers, but users of this document are advised to carefully read the equipment manual, consult the manufacturer, and refer to other literature on the correct use of microwave digestion systems. Operators are advised to note that using this document to digest samples may rapidly generate high pressures exceeding the capacity of the microwave digestion vessel, causing vessel rupture and damage to the digestion system. Warning: Take special care when handling heated microwave digestion vessels, as they may have high internal pressure. Warning: The outer vessel of the microwave digestion vessel is not as resistant to acids and solvents as the liner material, and should not be subjected to chemical or physical damage to ensure performance and safety requirements. Routine inspection of the vessel material is required to ensure safe use. Warning: Attention should be paid to the safety of sealed microwave digestion vessels without pressure relief devices. Temperature is a very important variable parameter for controlling the reaction. Within a safe range, temperature can be increased by increasing pressure. Some microwave digestion vessels will rupture, break, or even explode under certain pressures. Only microwave digestion vessels made of suitable inert polymeric materials with pressure relief devices (such as PFA or TFM and others), or microwave digestion vessels with a suitable inert polymeric liner and pressure relief device, may be used as microwave digestion vessels in this method. It is recommended not to use household microwave ovens or microwave digestion vessels without pressure relief devices for sample digestion. Warning: Household microwave ovens should not be used in the laboratory for this experiment. The main reasons are as follows: In a closed vessel microwave digestion unit, when acid (such as nitric acid) is used to achieve sample digestion, the released acid vapor may corrode the system safety device (which automatically disconnects the microwave oven magnetron control device when the door is opened), causing the operator to be exposed to the microwave field. An isolation device and corrosion-resistant digestion system can be used to avoid such risks. Warning: During the digestion reaction, toxic gases such as nitrogen oxides, hydrogen fluoride, and chlorine (from added hydrochloric acid) are usually generated. Therefore, the microwave digestion vessel must be opened in a well-ventilated fume hood. Warning: Analysts should wear protective gloves and face protection; under no circumstances should acid-containing solutions contact the skin or be inhaled into the lungs.

8. Analytical Procedure

• 11.1 Blank solution: Prepare a blank solution using the same concentration of acid as used for dissolving the sample, e.g., for the closed vessel microwave digestion method, add the same amount of inorganic acid as used for the grease sample to a volumetric flask and dilute to the mark with water (usually nitric acid with a volume fraction of 5%), mainly used to confirm the zero-point intensity of the calibration curve.
• 11.2 Calibration standard solutions: Dilute water-soluble standard solutions (see 7.5) to appropriate concentrations; the solutions usually contain nitric acid 1mg/L～10mg/L.
• 11.3 Check standard solution: Prepare an instrument check standard solution using the same method as for the calibration standard solutions; the concentration of elements in the check standard solution should be close to the concentration of elements in the sample solution.
• 11.4 Internal standard stock solution (optional)
  • 11.4.1 Select a suitable single-element internal standard based on the performance of the ICP-OES instrument (wavelength selectivity, sensitivity). The grease sample or calibration standard solution composition should not contain the selected internal standard element. Table 2 lists recommended common internal standard elements, suggested wavelengths, and their approximate concentrations when used.
  • 11.4.2 Preparation of internal standard stock solution: Weigh an appropriate amount of 500mg/kg internal standard and dilute with dilute acid to a suitable concentration. Prepare an internal standard stock solution in which the internal standard element concentration is 50 times that of the grease sample and calibration standard solutions. Prepare a fresh internal standard stock solution weekly.
• 13.1 (Optional) Determine the ICP detection limit for all elements to be measured as follows: Prepare a dilute acid blank solution containing the internal standard (optional): pipette 1000μL of the prepared internal standard stock solution into a 50mL volumetric flask, dilute to the mark with dilute acid, cap and mix well. Perform 10 consecutive analyses of all elements to be measured in this solution under the same conditions or parameters as the two-point calibration standard solutions, and calculate the standard deviation. The detection limit for each element is three times its standard deviation. The detection limit should be determined after instrument calibration.
• 13.2 Analyze the sample solution in the same manner under the conditions of the calibration standard solutions [i.e., the same integration time, background correction point (optional), plasma maintenance conditions, etc.] to obtain the concentration of the element to be determined in the solution. Between two samples, at least rinse with water spray for 60s. If the sample contains boron, between sample determinations, run (rinse) with water for 60s or longer.
• 13.3 If the concentration of the analyte exceeds the linear range of the calibration curve, the sample solution should be diluted to within the linear range of the calibration curve and then reanalyzed.
• 13.4 After every 5 samples analyzed, test the check standard solution once. If the obtained result exceeds 5% of the expected concentration, the instrument needs to be recalibrated, and the test samples reanalyzed.

Table 2 Concentration and recommended wavelength for internal standard use

Note: Other internal standards, wavelengths, and concentrations may also be used.

9. Qualitative and Quantitative Analysis and Calculation Formulas

Calculate the mass fraction of the element to be determined in the test sample according to formula (1).

ω = (ρ × V) / m ····················································· (1)

Where:

• ω — mass fraction of the element to be determined in the test sample, in milligrams per kilogram (mg/kg);
• m — sample mass, in grams (g);
• ρ — mass concentration of the element to be determined in the test solution, in milligrams per liter (mg/L);
• V — constant volume, in milliliters (mL).

Calculate the mass concentration of the element to be determined in the test solution ρ according to formula (2).

ρ = K (Ie − IBe) ·················································· (2)

Where:

• Ie — intensity of element e;
• IBe — blank intensity of element e;
• K — correction factor, the correction factor is obtained by linear regression analysis of a series of calibration standard solutions and their corresponding intensities.

(Note: The original formula (1) source-text extraction is displayed in the form “ω = ρV/m”, the formula symbol typesetting has noise, requires confirmation against the source standard (source-text extraction).)

10. Precision

• 17.1 The precision of this document is determined according to GB/T 6683.1, through statistical analysis of interlaboratory analysis results. The precision of boron element is obtained through analysis, statistical analysis and calculation of 10 grease samples of different types and different boron content ranges by 8 cooperative laboratories. The precision of other elements is determined through statistical analysis of duplicate analyses of 9 grease samples by 10 participating laboratories. Judge the reliability of test results according to the following provisions (95% confidence level).
• 17.2 Repeatability (r): The difference between two consecutive 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 shall not exceed the requirements of Table 3.
• 17.3 Reproducibility (R): The difference between two single, independent test results obtained by different operators, in different laboratories, using different instruments, according to the same method, on the same test sample shall not exceed the requirements of Table 3.

Table 3 Precision of elemental analysis of grease, in milligrams per kilogram

Note: X₁—average of two replicate test results/(mg/kg); X₂—average of two single, independent test results/(mg/kg).

(Note: The original text of the iron element reproducibility index is “0.7457”, which is inconsistent with the repeatability index “0.7475”, requires confirmation against the source standard (source-text extraction). The boron element reproducibility coefficient “0.1927” is obviously smaller than the repeatability coefficient “0.9727”, requires confirmation against the source standard (source-text extraction).)

11. Report

When the element content is less than 1000mg/kg, report the result to three significant figures, in mg/kg; when the element content is greater than or equal to 1000mg/kg, report the result to two significant figures, in mass fraction (%); when the element content is greater than or equal to 10000mg/kg, report the result to three significant figures, in mass fraction (%). (See Chapter 15)

12. Normative References

Normative references:

• GB/T 602 Chemical reagents — Preparation of standard solutions for impurity determination
• GB/T 6682—2008 Water for analytical laboratory use — Specification and test methods
• GB/T 17476 Determination of multiple elements in lubricating oils and base oils — Inductively coupled plasma optical emission spectrometry
• SH/T 0229 Sampling method for solid and semi-solid petroleum products

Bibliography:

• [1] GB/T 17989.2 Control charts — Part 2: Conventional control charts
• [2] GB/T 6683.1 Petroleum and related products — Determination of precision of measurement methods and results — Part 1: Determination of precision data of test methods
• [3] NB/SH/T 0843 Statistical techniques for evaluation of testing systems in the petrochemical industry
• [4] ASTM D6792 Standard operating procedures for quality management systems of testing laboratories for petroleum products, liquid fuels and lubricating oils

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

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