SH/T 0657-2007 Determination of trace nitrogen in liquid petroleum hydrocarbons — Oxidative combustion and chemiluminescence method

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SH/T 0657-2007 is applicable to the determination of total nitrogen in naphtha, petroleum fractions and other oils with a boiling range of about 50 °C to 400 °C, a viscosity at room temperature of about 0.2 mm²/s to 10 mm²/s and a total nitrogen content of 0.3 mg/kg to 100 mg/kg.

Evidence note: the content of this page was compiled from the original text of this standard held in the Origin Standard library (entries marked “〔to be verified〕” indicate 〔source text unclear〕); for individual values and the current status of the standard, please refer to the original standard pages and the product certificate.

Scope: Applicable to the determination of total nitrogen in naphtha, petroleum fractions and other oils with a boiling range of about 50 °C to 400 °C, a viscosity at room temperature of about 0.2 mm²/s to 10 mm²/s and a total nitrogen content of 0.3 mg/kg to 100 mg/kg. 1.2 For samples with a total nitrogen content greater than 100 mg/kg, the method of SH/T 0704 is more suitable. Interlaboratory cooperative test studies have shown that this method can also be applied to samples outside the range specified in the method, after the nitrogen content and viscosity range of the sample have been diluted into the range specified in the method by selecting a suitable solvent; however, the operator shall check the solubility of the test portion in the solvent and confirm that, when the diluted sample is injected directly into the furnace with a syringe, pyrolysis of the sample or solvent in the needle does not cause low results. 1.3 SI units are used. 1.4 This standard involves certain hazardous materials, operations and equipment, but does not purport to give advice on all safety problems associated with them; users shall establish appropriate safety and protective measures before use and determine the applicable management rules.

Principle: The liquid petroleum hydrocarbon sample is introduced by syringe or boat inlet system into an inert gas stream (helium or argon); the sample evaporates and is carried into the high-temperature zone containing oxygen, where organic nitrogen is converted to nitric oxide. The nitric oxide contacts ozone and is converted to excited-state nitrogen dioxide; the light emitted when the excited-state nitrogen dioxide returns to the ground state is detected by a photomultiplier tube, and the resulting electrical signal is measured to obtain the nitrogen content of the sample.

Apparatus: Combustion furnace: electronically controlled, maintained at a temperature at which the sample is fully vaporized and cracked and the organic nitrogen in it is oxidized to nitric oxide. The furnace temperature shall be as recommended by the manufacturer (generally about 1000 °C). 5.2 Combustion tube: made to the specifications of the instrument manufacturer. 5.3 Drying tube: water vapour must be removed from the reaction products before they enter the detector. A magnesium perchlorate filter or a membrane drying tube (permeation dryer) may be used. 5.4 Chemiluminescence detector: measures the light emitted by the reaction of nitric oxide with ozone. 5.5 Computing unit: provides variable attenuation and integration of the current given by the chemiluminescence detector. This is done by a built-in microprocessor or a connected computer system. 5.6 Microsyringe: capable of accurately measuring microlitre quantities of sample. The needle shall be long enough to reach the hottest part of the inlet section of the combustion furnace during injection. The syringe may be the automatic sampler and injection device supplied with the instrument. 10 μL, 25 μL, 50 μL or 250 μL. 5.7 Recorder. 5.8 Inlet system: select one of the following injectors. 5.8.1 Manually operated syringe. 5.8.2 Syringe with a constant-speed injection system capable of precisely controlling the injection rate. 5.8.3 Boat inlet system (optional): convenient for the analysis of samples that may react with the syringe or needle. The combustion tube used shall allow the boat to enter fully into the inlet zone of the furnace. The boat inlet system shall be cooled to below room temperature outside the combustion furnace, to help it dissipate the heat carried out of the furnace and to reduce the possibility of the sample burning before it is introduced into the furnace. When the boat inlet system is used to determine volatile samples such as (the source text is 〔source text unclear〕 and breaks off here; to be verified (source text unclear)), the boat inlet system needs to be cooled. 5.9 Quartz combustion tube (optional): packed with copper oxide or another oxidation catalyst recommended by the instrument manufacturer to promote oxidation. Placed at the outlet end of the high-temperature combustion tube. 5.10 Vacuum system (optional): a vacuum system may be fitted to the chemiluminescence monitor to reduce the pressure in the reaction cell (generally increas〔source text unclear〕), which can improve the signal-to-noise ratio of the detector. 5.11 Analytical balance (optional): sensitivity 0.01 mg.

Reagents and reference materials: Reagent purity: all reagents used in the test are of analytical grade; other grades of reagent may also be used if it is established that their purity does not affect the accuracy of the test. 6.2 Magnesium perchlorate: for drying the combustion products (if a permeation dryer is not used). Strong oxidizing agent, irritant. 6.3 Gas: argon or helium, purity not less than 99.9 %. 6.4 Oxygen: purity not less than 99.9 %. Note: combustion support. 6.5 Solvent: isooctane, toluene, acetone and hexadecane (or other solvents similar to the components of the sample to be tested), with low nitrogen content (the source text here is 〔source text unclear〕 given as "$mLo"; the specific limit is to be verified (source text unclear)). 6.6 Nitrogen standard stock solution, concentration 1000 μg/mL: accurately weigh 1.195 g of carbazole (or 0.585 g of pyridine, 1.036 g of 8-hydroxyquinoline) into a 100 mL volumetric flask. When carbazole is used, add 15 mL of acetone to aid dissolution and dilute to the mark with solvent. The stock solution may be diluted to the required nitrogen concentration. Note 1: when pyridine is used to prepare standards, a low-boiling solvent (boiling point below 220 °C) shall be used. Note 2: when carbazole is used, a high-boiling solvent (boiling point above 220 °C) shall be used. Note 3: nitrogen standard solutions shall be prepared periodically in accordance with the regulations and schedule. The usual shelf life of the standard solution is 〔source text unclear〕). Store refrigerated when not in use. 6.7 Copper oxide wire: as recommended by the instrument manufacturer. 6.8 Quartz wool. 6.9 Pyridine. Caution: flammable, irritant. 6.10 Carbazole. 6.11 8-Hydroxyquinoline.

Key conditions: Assemble the instrument according to the instrument instructions. 9.2 Adjust the gas flow and combustion temperature according to the instrument manufacturer's instructions. 10.3 Insert the syringe needle fully into the inlet septum and inject the sample or nitrogen standard solution at a rate of 0.2 μL/s to 1.0 μL/s. The injection rate depends on the sample viscosity, the type of hydrocarbon and the nitrogen content, and the injection shall be continuous and uniform. An automatic syringe can maintain a constant injection rate; if an automatic injection syringe device is not used, determine the injection volume using 10.2.1 or 10.2.3. Note: with a constant-speed injection device or an automatic injection syringe, a constant injection rate and the best analytical results are obtained. Too fast an injection rate may cause combustion (the source text is 〔source text unclear〕 and breaks off here; to be verified (source text unclear)); consult the manufacturer. When injecting samples with a nitrogen content below 5 mg/kg, the needle blank is quite important; inserting the syringe needle into the hot inlet section and allowing the needle to be flushed empty before 〔source text unclear〕) can avoid measurement errors. 10.4 If the boat inlet system is selected, inject the sample into the quartz boat using the methods described in 10.2.1, 10.2.2 and 10.2.3; the quartz boat is 〔source text unclear〕) in the tube. Refer to the instructions for use to select the speed of movement of the quartz boat and the residence time of the boat in the hottest part of the combustion tube. 11.1 Inject 3 μL to 100 μL of sample as required by the instrument. The volume of sample injected shall be similar to the volume of standard injected. 11.2 For samples with a total nitrogen content between 1 mg/kg and 100 mg/kg, the injection volume is generally 10 μL; for samples with a total nitrogen content below 1 mg/kg, the injection volume may be increased to 100 μL. The injection volume is decided according to the sample type, the nitrogen content and the instrument manufacturer's recommendations.

Calculation and results: For samples injected by volume (10.2.1 or 10.2.2), calculate the density using GB/T 1884, SH/T 0604 or another method for measuring density. 12.2 Calculate the nitrogen content N of the sample, in mg/kg, according to equation (1) or equation (2): N = (I - I₀) × K / (S × V × D) …………(1) N = (I - I₀) × K / (S × M) …………(2) where: I — average response signal of the detector, integral value; I₀ — intercept of the calibration curve, integral value; K — dilution factor; S — slope of the calibration curve, integral value/ng N; V — sample volume, μL; D — density of the sample, g/cm³; M — sample mass, mg. 12.3 For analytical instruments equipped with a calibration function, calculate the nitrogen content N of the sample, in mg/kg, according to equation (3) or equation (4): N = (I - B) × K / (V × D) …………(3) N = (I - B) × K / M …………(4) where: I — indicated value of the sample, ng N; B — average indicated value of the blank, ng N; K — dilution factor; V — sample volume, μL; D — density of the sample, g/cm³; M — sample mass, mg.

Quality control: Precision: judge the reliability of test results as follows (95 % confidence level). 13.1.1 Repeatability (r): the difference between two successive test results obtained by the same operator using the same instrument on the same sample shall not exceed the value given by equation (5) or Table 1: r = 0.1825 X^0.5149 …………(5) where: X — arithmetic mean of the repeat determination results. 13.1.2 Reproducibility (R): the difference between two independent test results obtained by different operators in different laboratories on the same sample shall not exceed the value given by equation (6) or Table 1: R = 0.8094 X^0.5149 …………(6) where: X — arithmetic mean of the two determination results. Table 1 Typical values of repeatability and reproducibility | Nitrogen content, mg/kg | Repeatability r | Reproducibility R | | 100 | 2.0 | 8.7 | | 75 | 1.7 | 7.5 | | 50 | 1.4 | 6.1 | | 25 | 1.0 | 4.2 | | 10 | 0.6 | 2.6 | | 1 | 0.18 | 0.81 | | 0.3 | 0.10 | 0.44 | 13.2 Bias: because no liquid petroleum hydrocarbon standard material of known trace-level nitrogen content is available, no bias has been established for this method.

Normative references: GB/T 1884 Crude petroleum and liquid petroleum products — Laboratory determination of density (hydrometer method) (GB/T 1884-2000; eqv ISO 3675:1998) GB/T 4756 Petroleum liquids — Manual sampling (GB/T 4756-1998, eqv ISO 3170:1988) SH/T 0604 Crude petroleum and petroleum products — Determination of density (oscillating U-tube method) (SH/T 0604-2000, eqv ISO 12185:1996) SH/T 0704 Determination of nitrogen content in petroleum and petroleum products (boat inlet chemiluminescence method)

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