GB/T 33648-2017 Identification and determination of typical unconventional additives in motor gasoline — Infrared spectrometry

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Card type: Test method

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

Standard number: GB/T 33648—2017
Chinese full title: Identification and determination of typical unconventional additives in motor gasoline — Infrared spectrometry
English title: Test method for identification and determination of specific non-regular additives in motor gasoline—Infrared spectroscopic method
Replacement relationship: not provided in the available source (the foreword does not list any replaced standard)
Publication/implementation: published on 2017-05-12, implemented on 2017-12-01
Proposed by and under the jurisdiction of: National Technical Committee for Standardization of Petroleum Products and Lubricants (SAC/TC280)
Drafting organization responsible: Sinopec Research Institute of Petroleum Processing
Participating drafting organizations: National Petroleum Product Quality Supervision and Inspection Center, Shenzhen Institute of Metrology and Quality Inspection, Guizhou Provincial Institute of Product Quality Supervision and Inspection, Taicang Entry-Exit Inspection and Quarantine Bureau, Zhejiang Institute of Quality Inspection Science, PetroChina Research Institute of Petroleum and Petrochemicals
Main drafters: Xu Guangtong, Yang Yurui, Zhao Yan, Li Xiaoyun, Song Changsheng, Liao Shangfu, Sun Yuechao, Ji Ming, An Mi
Drafting rules: drafted in accordance with GB/T 1.1—2009

2. Scope of application and non-applicability

Applicable to the rapid identification and determination of typical unconventional additives such as aniline compounds, dimethoxymethane (methylal) and esters in motor gasoline by infrared spectrometry.
Applicable to the determination of specific unconventional additives in motor gasoline and gasoline blending components.
Identification and determination concentration ranges:
- Aniline 3 g/L～35 g/L
- N--methylaniline 4 g/L～35 g/L
- Dimethoxymethane 3 g/L～35 g/L
- sec-Butyl acetate 3 g/L～35 g/L
- Dimethyl carbonate 1.5 g/L～16 g/L
Below the lower limit of the above determination ranges: effective identification or quantitative analysis is not possible.
Above the upper limit of the above determination ranges: the sample may be appropriately diluted before determination.
If the sample contains o-methylaniline and p-methylaniline: identify and quantify according to the test method for aniline.
If the sample contains other ester compounds: identify and quantify according to the test method for sec-butyl acetate; however, in this case the precision of this standard is not applicable.
Not applicable: no other explicit exclusion clauses are provided in the available source.

3. Method principle

The infrared spectral characteristics of hydrocarbon components in motor gasoline are mainly the stretching vibrations and bending vibrations of various C—H groups in saturated hydrocarbons, olefins and aromatics, as well as the spectral characteristics of some specific groups such as double bonds and benzene rings (4.1).
Gasoline containing MTBE and ethanol, in addition to the infrared absorption characteristics of hydrocarbon groups, will have additional C—O bond spectral characteristics; ethanol gasoline also contains —OH spectral characteristics (4.1).
Each of the typical unconventional additives to be identified and determined can be found within the infrared spectral measurement range to have characteristic absorption bands different from those of hydrocarbon groups; these bands are not interfered with by motor gasoline components, and with the aid of these bands the specific unconventional additives can be identified and measured (4.1).
Note: Because different base hydrocarbon components of motor gasoline will carry a certain solvent effect, causing slight changes in the positions of the infrared absorption peaks of specific groups, but within a certain peak window range this does not affect the discrimination of the additives (4.1 note).
By manual or automatic injection, the motor gasoline sample to be tested is injected into a sample cell of a specific path length, and its infrared spectral absorption spectrum is measured with an empty cell as background; qualitative identification is carried out according to the infrared spectral characteristics of the unconventional additives, and their content is determined according to the absorbance values of their characteristic spectra and the established calibration curves (4.2).
If the absorbance of the characteristic spectral band exceeds 2.0 AU, the sample may be diluted by a reasonable multiple with the mixed solvent and then re-measured (4.2).

4. Reagents and materials (purity / preparation)

6.1 Reference materials for calibration: aniline, N--methylaniline, dimethoxymethane, sec-butyl acetate, dimethyl carbonate, all being analytical reagent grade or above.
6.2 Mixed solvent: the dilution solvent is a mixed solvent of petroleum ether 1 (boiling range 60 ℃～90 ℃), petroleum ether 2 (boiling range 90 ℃～120 ℃) and xylene, with analytical reagent purity, in a mixed volume ratio of 35∶35∶30.
6.3 Sample cell rinsing solvent: n-hexane, analytical reagent grade.
Warning: These compounds are all flammable or toxic compounds; if ingested, inhaled or absorbed through the skin, they will cause harm or death to the human body.
Preparation of calibration test solutions (10.2.3, Table 5): In a 25 mL volumetric flask, using the mixed solvent (6.2) as the dilution solvent, accurately weigh each reference material (6.1) to prepare the calibration test solutions shown in Table 5.
- Aniline calibration curve mass concentrations: 5, 10, 20, 30, 35 g/L
- N--methylaniline calibration curve mass concentrations: 5, 10, 20, 30, 35 g/L
- Dimethoxymethane calibration curve mass concentrations: 4, 8, 18, 25, 35 g/L
- sec-Butyl acetate calibration curve mass concentrations: 4, 8, 18, 25, 35 g/L
- Dimethyl carbonate calibration curve mass concentrations: 2, 4, 8, 12, 16 g/L

5. Instruments and equipment

7.1 Fourier transform infrared spectrometer: shall meet the requirements of GB/T 21186. The effective wavenumber measurement range of the spectrum shall be not less than 650 cm⁻¹～4000 cm⁻¹, the spectral resolution shall be better than 2 cm⁻¹ (including 2 cm⁻¹), the signal-to-noise ratio shall be better than 10000∶1, the wavelength accuracy shall be better than 0.01 cm⁻¹, and the data acquisition interval shall be better than 0.5 cm⁻¹ (including 0.5 cm⁻¹).
7.2 Sample cell: an interference-preventing transmission-type sample cell, with a sample cell path length of 0.1 mm. To prevent deliquescence of the sample cell window material from affecting transmittance, the sample cell may use a window material of ZnSe crystal; to obtain a good signal-to-noise ratio and a stable spectral baseline, the sample cell should not produce interference fringes, and an anti-interference sample cell with a wedge-shaped structure may be used. Other window materials may also be used, such as KBr, CaF₂, etc., but attention should be paid to the effect of moisture absorption by the window material on transmittance. Either manual or automatic injection may be used, and there should be no bubbles in the sample cell during measurement.
7.3 Spectral processing software: has functions such as baseline processing and spectral differentiation, so as to measure the peak height of the characteristic spectral peaks of the additives and perform quantitative calculation.
7.4 Volumetric flask: 25 mL.
7.5 Pipette: 10 mL.

6. Recommended operating conditions

Measurement mode: absorption spectrum (9.1).
Wavelength range: 650 cm⁻¹～4000 cm⁻¹ (9.1).
Spectral resolution: 2 cm⁻¹ (9.1).
Spectral acquisition data point interval: 0.5 cm⁻¹ (9.1).
Number of scans: recommended not less than 32 times (9.1).
Background: place the sample cell in position, and measure the background signal of the spectrum with an empty cell as reference (9.1).
Sample injection volume: should not be less than 10 mL, to avoid memory effects (9.1).
Sample cell path length: 0.1 mm (7.2).
Dilution condition: dilute when the absorbance of the quantitative characteristic peak absorption band exceeds 2.0 AU (9.2, 10.2.6).
Dilution operation: use a pipette to transfer 10 mL of the test sample into a 25 mL volumetric flask, and dilute to 25 mL with the mixed solvent (9.2).

7. Sampling and test specimen preparation

8.1 When using this test method to analyze samples, sampling shall be carried out in accordance with the provisions of GB/T 4576.
- Requires confirmation (source-text extraction): Chapter 2, Normative references, lists GB/T 4756 Petroleum liquids — Manual sampling, and at 8.1 the source-text extraction shows “GB/T 4576”, suspected to be source-text extraction noise, and should be GB/T 4756.
8.2 Before sample analysis, the sample should be equilibrated at room temperature 15 ℃～27 ℃.
Test specimen dilution preparation: see 9.2, transfer 10 mL of the test specimen into a 25 mL volumetric flask, and dilute to 25 mL with the mixed solvent.

8. Analysis procedure

9.1 Spectral measurement: set the measurement mode of the instrument spectral measurement to absorption spectrum, the wavelength range to 650 cm⁻¹～4000 cm⁻¹, the spectral resolution to 2 cm⁻¹, and the spectral acquisition data point interval to 0.5 cm⁻¹; to obtain a good signal-to-noise ratio, it is recommended that the number of spectral scans be not less than 32 times. Place the sample cell in position, measure the background signal of the spectrum with an empty cell as reference, inject the test sample or calibration sample into the sample cell, and ensure that the sample cell is filled and no bubbles are present. To avoid memory effects that may be produced by the measurement of the previous sample, the sample injection volume should not be less than 10 mL, and measure the absorption spectrum of the sample.
- Note: During background measurement, it is very important to purge clean the sample or rinsing solvent remaining in the sample cell, so as not to affect the background measurement.
9.2 Dilution of sample: if the absorbance of the quantitative characteristic peak absorption band of the typical unconventional additive exceeds 2.0 AU, the sample may be diluted with the mixed solvent before measurement. The typical operation is to use a pipette to transfer 10 mL of the test specimen into a 25 mL volumetric flask, dilute to 25 mL with the mixed solvent, and measure the spectrum according to the method of 9.1.
10.2.6 Measurement of the sample: Determine the infrared absorption spectrum of the sample to be tested according to the procedure in 9.1. If any additive specified in this standard is found to be present, measure its absorbance value according to the method in 10.2.4, and calculate its content according to formula (2). If the band intensity of the quantitative characteristic peak of the additive is higher than 2.0 AU, then dilute according to the procedure in 9.2 and measure its infrared spectrum.

9. Qualitative and Quantitative Determination and Calculation Formulas

9.1 Qualitative Identification (10.1)

Figures 1 to 5 show the infrared spectral characteristics of five typical non-conventional additives—aniline, N--methylaniline, dimethoxymethane, sec-butyl acetate, and dimethyl carbonate—at different addition levels (g/L) in motor gasoline.
When the measured infrared spectrum of the test sample simultaneously exhibits the specified characteristic absorption of a certain additive, it can be inferred that the sample contains one or several additives.
Note: Aniline, o-methylaniline, and p-methylaniline that may be added to motor gasoline are all identified as aniline and their contents are reported. Other fatty acid monoester compounds that may be added to motor gasoline are identified as sec-butyl acetate and their contents are reported.
When motor gasoline contains o-methylaniline or p-methylaniline, it can be identified within the characteristic band range of aniline. If m-methylaniline is present, the absorption band of the C—N bond shifts to 1293 cm⁻¹.

Table 3 Characteristic bands for identification and quantitative characteristic bands of typical non-conventional additives in motor gasoline

No.	Non-conventional additive	Qualitative identification band and shift range/cm⁻¹	Quantitative band position/cm⁻¹
1	Aniline type	1276±6, 3390±8, 3481±8	3390±8
2	N--methylaniline	1319±4, 3434±15	1319±4
3	Dimethoxymethane	930±4, 1046±4, 1141±4	1141±4
4	Fatty acid monoester type	1243±4, 1743±4	1743±4
5	Dimethyl carbonate	1281±4, 1760±4	1760±4

Table 1 Typical substances and infrared spectral characteristics in motor gasoline (the original table title is as such)

No.	Component type	Absorption characteristic/cm⁻¹	Group assignment
1	Saturated hydrocarbons	2800～3000	—CH₃, —CH₂—, —CH—, C—H stretching vibration
1	Saturated hydrocarbons	1350～1465	—CH₃, —CH₂—, —CH—C—H bending vibration
2	Olefins	3000～3100	＝C—H stretching vibration
2	Olefins	910, 970	＝C—H bending vibration
2	Olefins	1600～1650	C＝C stretching vibration
3	Aromatics	3000～3200	Aromatic ring ＝C—H stretching vibration
3	Aromatics	650～900	Aromatic ring ＝C—H bending vibration
3	Aromatics	1500～1600	Aromatic ring skeleton vibration
4	MTBE	1075, 1208	C—O stretching vibration
5	Ethanol	1050, 1080	C—O stretching vibration
5	Ethanol	3100～3600 broad band absorption	O—H stretching vibration

Table 2 Typical non-conventional additives in motor gasoline and their infrared absorption characteristics

No.	Additive	Absorption characteristic/cm⁻¹	Chemical bond or group assignment
1	Aniline	3390, 3481	—NH₂
1	Aniline	1276	C—N
2	N--methylaniline	3434	—NH—
2	N--methylaniline	1263, 1320	C—N—C
3	Dimethoxymethane	930, 1046, 1141	—O—CH₂—O—
4	sec-Butyl acetate	1743	C＝O
4	sec-Butyl acetate	1243	C—O
5	Dimethyl carbonate	1760	C＝O
5	Dimethyl carbonate	1281	C—O

9.2 Quantitative Analysis (10.2)

10.2.1 According to the Lambert-Beer law, the absorbance of the characteristic absorption band of each additive in motor gasoline has a linear relationship with its content within a certain range. A quantitative calibration curve of the absorbance of the quantitative characteristic band of the additive versus content can be established. Based on the absorbance of the quantitative characteristic band of the additive identified after spectral measurement of the sample, its content can be calculated.
10.2.2 Measurement of quantitative band and absorbance: The quantitative band and baseline positions are given in Table 4.

Table 4 Quantitative characteristic bands and baseline ranges of typical non-conventional additives in motor gasoline

No.	Non-conventional additive	Quantitative characteristic band/cm⁻¹	Baseline range/cm⁻¹
1	Aniline type	3390	3320～3550
2	N--methylaniline	1320	1285～1330
3	Dimethoxymethane	1141	1127～1162
4	Fatty acid monoester type	1743	1685～1770
5	Dimethyl carbonate	1760	1685～1810

Note: The baseline points are the peak valleys near the baseline range.

10.2.3 Preparation of calibration samples: see Table 5 (see Section 4 of this card).
10.2.4 Measurement of absorbance of quantitative characteristic band: For the typical non-conventional additive identified in the calibration sample or test sample, the baseline measurement range of the quantitative band is given in Table 4, and the measurement method for its characteristic absorption peak height is shown in Figures 6 to 10.
10.2.5 Quantitative calibration curve: By measuring the absorbance value Y of the quantitative characteristic band of a certain additive i at different contents, a quantitative calibration curve of the absorbance corresponding to the additive content under the corresponding quantitative characteristic band can be obtained, see formula (1). To ensure the accuracy of quantification, the correlation coefficient (R²) of the quantitative calibration curve should not be less than 0.99.

Formula (1): Yᵢ = aᵢcᵢ + bᵢ

Where:

Yᵢ——the measured absorbance of the quantitative characteristic band of a certain i additive;
cᵢ——the mass concentration of the i additive, in grams per liter (g/L);
aᵢ——the slope of the regression curve of the i additive;
bᵢ——the intercept of the regression curve of the i additive;
i—— a certain additive i.
10.2.6 Measurement of the sample: see Section 8 of this card.

9.3 Calculation of Results (Chapter 11)

Based on the absorbance value of the quantitative characteristic band of the additive measured from the spectrum, calculate the concentration value cᵢ of the additive i in the analytical test sample, see formula (2). If the sample has been diluted, the actual concentration of the additive in the sample should be the concentration value cᵢ calculated according to formula (2) multiplied by the sample dilution factor s.

Formula (2): cᵢ = (Yᵢ − bᵢ)/aᵢ × s

Where:

cᵢ——the mass concentration of the i additive in the measured test sample, in grams per liter (g/L);
Yᵢ——the absorbance value measured from the spectrum of the i additive in the measured test sample;
aᵢ, bᵢ——the parameter values of the calibration curve in formula (1) for the i additive;
s—— the dilution factor of the measured test sample, see 9.2 sample dilution.

9.4 Expression of Test Results (Chapter 12)

If the content of the non-conventional additive in motor gasoline is expressed as mass concentration [in grams per liter (g/L)], then the content of the additive in motor gasoline is the calculated value in Chapter 11, accurate to 0.1 g/L.
If the content of the additive in motor gasoline is required to be reported as mass fraction (%), the conversion method for mass fraction X is given in formula (3):

Formula (3): Xᵢ = cᵢ/(10 × d)

Where:

Xᵢ——the mass fraction of the additive i in the measured sample (%), accurate to 0.01%;
cᵢ——the mass concentration of the additive i in the measured sample, in grams per liter (g/L);
d—— is the density value of the measured motor gasoline sample, in grams per milliliter (g/mL).

10. Precision

13.1 Overview: Determine the precision of the method according to GB/T 6683, and judge the reliability of the test results according to the following provisions (confidence level of 95%).
13.2 Repeatability: For the same operator, using the same instrument, under the same test conditions, and correctly operating according to the standard, the difference between two test results for the same sample shall not exceed the values listed in Table 6.
13.3 Reproducibility: For different operators in different laboratories using different instruments to test the same sample, the difference between two single and independent test results shall not exceed the value listed in Table 6.

Table 6 Repeatability and reproducibility

Serial number	Additive	Repeatability/(g/L)	Reproducibility/(g/L)	Range/(g/L)
1	Aniline	0.11×m^0.58	0.22×m^0.68	3～35
2	N--methylaniline	0.14×m^0.45	0.24×m^0.57	4～35
3	Dimethoxymethane	0.15×m^0.37	0.80×m^0.43	3～35
4	sec-Butyl acetate	0.012×m^0.91	0.45×m^0.43	3～35
5	Dimethyl carbonate	0.07×m^0.42	0.28×m^0.76	1.5～16

Note: m is the arithmetic mean of two results.

11. Report

The test report shall at least give the following contents (Chapter 14):

a) Description of the sample;
b) Standard used;
c) Test results (see Chapter 12);
d) Differences from the specified analytical procedure;
e) Abnormal phenomena observed during the test;
f) Test date.

12. Normative references

GB/T 4756 Petroleum liquids—Manual sampling
GB/T 6683 Petroleum products—Determination of precision data in relation to methods of test
GB/T 21186 Fourier transform infrared spectrometer
Note: At 8.1, source-text extraction is shown as “GB/T 4576”, requires confirmation against the source standard (source-text extraction), suspected to be GB/T 4756.

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.

14. Quick-reference Q&A

Question	Key points of answer
What does this standard measure?	Identification and determination of typical non-conventional additives such as anilines, dimethoxymethane (methylal), and esters in motor gasoline.
What is the method?	Infrared spectroscopy, measuring absorption spectra with an empty cell as background, qualitatively identifying characteristic bands, quantitatively measuring absorbance and substituting into the calibration curve.
Which additives and concentration ranges are applicable?	Aniline 3～35 g/L; N--methylaniline 4～35 g/L; dimethoxymethane 3～35 g/L; sec-butyl acetate 3～35 g/L; dimethyl carbonate 1.5～16 g/L.
What if below the lower limit?	Effective identification or quantitative analysis is not possible.
What if above the upper limit?	The sample may be appropriately diluted before measurement.
How to handle o-methylaniline and p-methylaniline?	Identify and quantify according to the test method for measuring aniline.
How to handle other ester compounds?	Identify and quantify according to the test method for sec-butyl acetate; however, in this case the precision of this standard does not apply.
Characteristics of m-methylaniline?	The C—N bond absorption band shifts to 1293 cm⁻¹.
According to what standard is sampling performed?	Sampling is performed according to GB/T 4756 (in the original text of 8.1, source-text extraction is GB/T 4576, requires confirmation).
Temperature before sample analysis?	Constant at room temperature 15 ℃～27 ℃.
Core instrument requirements?	Complies with GB/T 21186; wavenumber range not less than 650～4000 cm⁻¹; resolution better than 2 cm⁻¹ (inclusive); signal-to-noise ratio better than 10000∶1; wavelength accuracy better than 0.01 cm⁻¹; data acquisition interval better than 0.5 cm⁻¹ (inclusive).
Sample cell path length?	0.1 mm.
Sample cell window material?	ZnSe may be used; KBr, CaF₂, etc. may also be used, but note the influence of moisture.
Measurement mode and parameters?	Absorption spectrum; 650～4000 cm⁻¹; resolution 2 cm⁻¹; data point interval 0.5 cm⁻¹; it is recommended to scan no fewer than 32 times.
Sample injection volume?	Shall not be less than 10 mL.
When to dilute?	When the absorbance of the quantitative characteristic peak absorption band exceeds 2.0 AU.
Dilution operation?	Pipette 10 mL of the test sample into a 25 mL volumetric flask, and dilute to 25 mL with mixed solvent.
Composition of mixed solvent?	Petroleum ether 1 (60～90 ℃), petroleum ether 2 (90～120 ℃), xylene, volume ratio 35∶35∶30, analytical reagent grade.
Rinsing solvent?	n-Hexane, analytical reagent grade.
Requirement for calibration curve correlation coefficient?	R² shall not be less than 0.99.
Quantitative bands and baselines?	Anilines 3390 (3320～3550); N--methylaniline 1320 (1285～1330); dimethoxymethane 1141 (1127～1162); fatty acid monoesters 1743 (1685～1770); dimethyl carbonate 1760 (1685～1810).
Result units and precision?	Mass concentration g/L, accurate to 0.1 g/L; mass fraction %, accurate to 0.01%.
Mass fraction conversion formula?	Xᵢ = cᵢ/(10 × d), where d is the sample density g/mL.
Basis for precision?	Determined according to GB/T 6683, 95% confidence level.
Where are repeatability/reproducibility?	Table 6, formulas and ranges are given separately by additive.
What does the report at least include?	Sample description, standard used, test results, differences from the specified analytical procedure, abnormal phenomena, test date.
What are the interferences?	Hydrocarbon blending components do not interfere; MTBE does not interfere with identification; dimethyl carbonate 1280 cm⁻¹ C—O will affect the identification of anilines at 1276 cm⁻¹ C—N; alcohol —OH 3100～3600 cm⁻¹ broad-band absorption will affect the identification of amines at N—H and the quantification of anilines; the coexistence of sec-butyl acetate with amines or alcohols does not affect identification but affects quantitative accuracy.

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