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Information on the petroleum and petrochemical industry: http://mmbiz.qpic.cn/mmbiz/V30nKrJZmDTLd6vJP5cbSGFqicMvCcKdEef4DEmXrOK4qnuNuWTAcUCwITc3GsGb4E3VRIcy8FZ6RBfoEib6Mr3w/0?wx_fmt=jpeg 1. Introduction: With the establishment of **Sustainable Development Goals and an increasing awareness of environmental protection, higher standards have been set for the quality of gasoline. Currently, the upgrade of gasoline standards from National III to National IV is being implemented nationwide, with some provinces and cities already adopting local National V standards. A comparison of the two versions of the standards shows that the limits on sulfur content are becoming increasingly strict, with the limit dropping from 150 μg/g to 50 μg/g; the sulfur content requirement under the National V standard is 10 μg/g. How to detect the sulfur content in gasoline and various additives quickly and accurately has received widespread attention. As common methods for detecting sulfur content, SH/T 0689—2000 \"Method for determining the total sulfur content in light hydrocarbons, engine fuels, and other oils (ultraviolet fluorescence method)\" and GB/T 11140—2008 \"Determination of sulfur content in petroleum products by wavelength-dispersive X-ray fluorescence spectroscopy\" have different characteristics: the ultraviolet fluorescence method is simple to operate, fast in analysis, and has a wide measurement range, while wavelength-dispersive X-ray fluorescence spectroscopy offers good reproducibility, fast measurement speeds, and high sensitivity. Due to the aforementioned advantages, they are widely used in various refineries, oil sales companies, and oil testing facilities. During the detection process, due to various factors, the measurement results are often inaccurate. In response to this situation, the author conducted numerous testing experiments, thoroughly analyzed various influencing factors, and proposed solutions for the accurate detection of sulfur content in National IV and National V gasoline. 2. Experimental Section 2.1 Experimental Reagents and Instruments The main reagents required for the experiment are sulfur standard samples produced by the Sinopec Research Institute of Petroleum Processing, blank gasoline that meets National IV and National V standards, and isobutylamine. The main instruments required for the experiment are the Jiangyan Gaoke ZDS-2000 ultraviolet fluorescence sulfur analyzer, the American Physic single-wavelength X-ray fluorescence sulfur analyzer, the Thermo Electric TS3000 ultraviolet fluorescence sulfur analyzer, and the MultiTec fully automatic analyzer for total sulfur, nitrogen, halogens, and individual halogens. 2.2 Experimental Principle 2.2.1 Principle of measuring sulfur content by ultraviolet fluorescence method: The sulfides in the sample are converted into sulfur dioxide (SO2) in a high-temperature (around 1050°C) and oxygen-rich environment. The gases generated by the combustion of the sample are subjected to ultraviolet light after water has been removed; sulfur dioxide (SO2) absorbs the energy from this ultraviolet light and is transformed into an excited state, namely SO2*. When the excited state of sulfur dioxide (SO2*) returns to the stable state of sulfur dioxide (SO2), fluorescence is emitted and detected by a photomultiplier tube; the total sulfur content in the sample is calculated from the resulting signal value. 2.2.2 Principle of sulfur content determination by single-wavelength dispersive X-ray fluorescence spectroscopy: Single-wavelength X-ray fluorescence is used to irradiate the sample, causing the K-shell electrons in the sample to escape; the P-shell electrons then transition to the K-shell at a lower energy level, and during this transition, characteristic X-ray fluorescence from sulfur is emitted. For the characteristic X-ray fluorescence of the sample, a spherical crystal is used for spectroscopy; only the characteristic fluorescence representing element S is counted by a proportional counter illuminated at a specific angle. The integrated reading from the proportional counter is converted optoelectronically to obtain the value of total S in the sample. 3. Discussion on the factors affecting sulfur content determination: The following experiments analyzed the impact of various factors on the results of sulfur content measurement by comparing the recovery rates of standard samples or the actual test results of samples under different conditions. The recovery rates of the standard samples obtained were the averages of 5 parallel experiments conducted under the same conditions. 3.1 Effect of pyrolysis temperature on detection results: The recoveries of 25 mg/L and 50 mg/L sulfur standards were determined at different pyrolysis temperatures using a ZDS-2000 ultraviolet fluorescence sulfur analyzer and a single-wavelength X-ray fluorescence sulfur analyzer; the results are shown in Table 1. Table 1 Effect of pyrolysis temperature on measurement results. Pyrolysis temperature/°C; Recovery rate of standard sample by UV fluorescence, %; Recovery rate of standard sample by X-ray fluorescence, %. 25 mg/L sulfur standard sample, 50 mg/L sulfur standard sample, 25 mg/L sulfur standard sample, 50 mg/L sulfur standard sample: 900 – 92.5, 193.77, 94.49, 95.75; 950 – 96.38, 97.06, 95.91, 98.24; 1000 – 99.97, 100.21, 100.02, 100.11; 1050 – 100.08, 101.13, 101.37, 100.92; 1100 – 97.85, 98.52, 97.96, 98.29. As can be seen from Table 1, when the pyrolysis temperature is between 900 and 1000°C, the recovery rate of the standard sample increases as the temperature rises, gradually approaching the true value. This is because at lower temperatures, the organic sulfur in the sample does not burn completely, which hinders the formation of SO2; as a result, the measured values are lower than the actual values. Moreover, the lower the temperature, the greater the deviation from the true value. When the pyrolysis temperature is between 1000 and 1050°C, the recovery rate of the standard sample approaches 100% infinitely closely, meaning that the measurement results come extremely close to the true values. It can be seen that within this temperature range, organic sulfur can burn completely and be entirely converted into SO2. As the pyrolysis temperature is increased further, the recovery rate of the standard sample decreases. This is because at excessively high temperatures, a portion of SO2 is oxidized to SO3, whereas the fluorescence spectroscopy method can only detect the signal of SO2. 3.2 Influence of pyrolysis gas and carrier gas flow rates on detection results: Determining the appropriate flow rates for the pyrolysis oxygen and carrier gas argon is crucial in the process of sulfur content detection; either too high or too low flow rates can cause the detection results to deviate from the true values. An insufficient flow rate of oxygen for pyrolysis hinders the oxidation of the sample, leading to carbon deposition, which causes the test results to be lower than actual values ； When the flow rate is too high, SO3 is likely to be generated, which likewise causes the detection results to be lower. An excessively low flow rate of the carrier gas may cause sulfur to oxidize too rapidly to form SO3, resulting in detection values that are lower than the true values ； Excessive traffic can also affect the displayed values of the detection results. 3.3 Effect of injection volume and injection rate on detection results Under constant conditions of pyrolysis temperature, pyrolysis gas flow rate, and carrier gas flow rate, the sample recovery rates of standard samples at different concentrations were determined using two sulfur detectors at various injection volumes; the results are shown in Table 2. Table 2: Detection results of the ZDS-2000 ultraviolet fluorescence sulfur analyzer. Injection volume/μL, Recovery rate of standard samples by ultraviolet fluorescence method, %; Recovery rate of standard samples by X-ray fluorescence method, %. 25 mg/L sulfur standard sample, 50 mg/L sulfur standard sample, 25 mg/L sulfur standard sample, 50 mg/L sulfur standard sample. For an injection volume of 10: 492.55, 91.32, 94.73, 93.21; for an injection volume of 20: 7.59, 7.39, 97.56, 96.35, 98.36; for an injection volume of 50: 201.00, 32.10, 100.14, 99.95, 100.04; for an injection volume of 100: 509.98, 79.64, 96.37, 99.67, 95.93. It can be seen that for samples of different concentrations, the injection volume has a direct impact on the accuracy of the test results. For samples at 25 mg/L, using a sampling volume of 50 for every 20 units or 100 for every 50 units yields detection results that are close to the true values; if the sampling volume is too low, the detection results are lower than the true values ； For samples at 50 mg/L, a sampling volume of 50 divided by 20 yields results that are closer to the true value; too high or too low a sampling volume reduces the accuracy of the results. Therefore, for samples with low sulfur content, the injection volume can be increased appropriately, while for samples with high sulfur content, the injection volume should be reduced appropriately, so that the samples can burn fully and reduce the formation of carbon deposits. The injection speed also affects the accuracy of the test results; if the injection is too fast, sulfur in the sample does not burn completely, resulting in lower test results ； Too slow injection can cause tailing of the integrated peak, which likewise leads to inaccurate measurement results. 3.4 Influence of the selection of standard curves on detection results: The National IV gasoline standard specifies that the sulfur content should not exceed 50 μg/g; therefore, standard samples with concentrations of 25 mg/L and 40 mg/L were selected. Two types of standard curves, 0–50–100 and 0–30–50, were used for testing, and the results are shown in Table 3. Table 3 Effect of standard curve selection on measurement results: Recovery rate of standards by UV fluorescence method, %; Recovery rate of standards by X-ray fluorescence method, %. 25 mg/L sulfur standard, 40 mg/L sulfur standard, 25 mg/L sulfur standard, 40 mg/L sulfur standard: 0–50–100; 95.75%, 96.28%, 97.28%, 98.53%. 0–30–50: 99.84%, 101.13%, 100.31%, 101.2%. It can be seen that when the sulfur content is below 40 mg/L (approximately 54 μg/g), the curve 0–30–50 should be used, so that the sulfur content of the sample under examination falls within this range without being too wide, which could result in lower measurement values. 3.5 The impact of alkaline nitrogen on test results 3.5.1 The impact of alkaline nitrogen on the sulfur content measurement in National IV gasoline: Due to crude oil extraction and transportation, refining processes, as well as the influence of various additives, trace amounts of alkaline nitrogen and halogen elements are sometimes present in gasoline. To examine the effect of basic nitrogen in the sample on the detection results of sulfur content in National IV gasoline, 2% isobutylamine was added to blank gasoline that met the National IV gasoline standards, and the sulfur content was measured using different testing instruments; the results are shown in Table 4. Table 4 Comparison of sulfur content detection results for samples using different instruments. Sulfur content in μg·g-1; standards used: 2% isobutylamine added to blank gasoline. Instruments: American Phoscora single-wavelength X-ray fluorescence sulfur analyzer – 43.57, 44.02; GB/T 11140—2008; Jiangyan Gaoke ZDS-2000 ultraviolet fluorescence sulfur analyzer – 44.86, 45.27; SH/T 0689—2000; Thermoelectric TS3000 ultraviolet fluorescence sulfur analyzer – 45.96, 52.31; SH/T 0689—2000. As can be seen from Table 4, when using the TS3000 to determine sulfur content, the measurement results are significantly higher than the actual values due to the influence of isobutylamine ； When measured using a ZDS-2000 ultraviolet fluorescence sulfur analyzer and a single-wavelength X-ray fluorescence sulfur analyzer, the sulfur content of gasoline containing isobutylamine was essentially the same as that of blank gasoline. It can be seen that when using GB/T11140—2008 \"Determination of sulfur content in petroleum products — Wavelength-dispersive X-ray fluorescence spectrometry\" to measure sulfur content, the influence of basic nitrogen is not present; whereas under the same testing conditions, SH/T0689—2000 \"Method for determining total sulfur content in light hydrocarbons, engine fuels, and other oils — Ultraviolet fluorescence method\" yields different results depending on the testing instrument used. Upon analysis, the main reason is that the detection sensitivity of the imported device TS3000 is higher than that of the domestic device ZDS-2000; as a result, some organic nitrides were counted as sulfur, leading to higher values in the sulfur content measurements. 3.5.2 Effect of alkaline nitrogen on the sulfur content measurement results of National V gasoline: A MultiTec fully automatic total sulfur, nitrogen, halogens, and individual halogens analyzer was used, and the ASTM D5453–2009 (ultraviolet fluorescence method) standard was applied to determine the sulfur content of the samples under conditions with and without ozone exposure. The addition amounts of isobutylamine were 1% and 2%, respectively. The gasoline used was 92-octane and 95-octane gasoline that meet the National V standard. Five tests were conducted on each batch of gasoline, and the average values of the results were taken. It can be seen that there is a certain difference in the test results before and after introducing ozone: for blank gasoline, the sulfur content measured after ozone introduction decreased by about 0.1 μg/g ； After adding 1% isobutylamine and introducing ozone, the sulfur content decreased by more than 1 μg/g ； After adding 2% isobutylamine and introducing ozone, the sulfur content decreased by more than 2 μg/g, falling from above 10 μg/g to below 10 μg/g. It can be seen that the presence of basic nitrogen in the sample does have a significant impact on the test results, especially when testing National V gasoline. If gasoline contains organic nitrogen compounds, it can turn what would otherwise be a satisfactory test result into an unsatisfactory one, affecting the determination of whether the product meets the standards. Therefore, when selecting a detection method, the influence of nitrogen on the detection results should be avoided. 4. Conclusions 1) The study found that alkaline nitrogen has a significant impact on the results of gasoline sulfur content testing; in particular, when testing gasoline that meets National Standard V, the presence of organic nitrogen compounds affects the accuracy of the measured sulfur content. 2) SH/T 0689—2000 \"Method for determining the total sulfur content in light hydrocarbons, engine fuels, and other oils (ultraviolet fluorescence method)\") is the authoritative method for sulfur content testing. Since factors such as equipment operating conditions – pyrolysis temperature, pyrolysis gas flow rate and carrier gas flow rate, sample volume and injection speed, and the selection of standard curves – have a significant impact on the results of sulfur content determination in gasoline, as oil quality improves, it is necessary to gradually explore operating conditions suitable for instruments from different manufacturers in order to obtain accurate results for gasoline sulfur content. 3) GB/T11140—2008 \"Determination of sulfur content in petroleum products — Wavelength-dispersive X-ray fluorescence spectrometry\" is a method for detecting the sulfur content in oils that is not affected by nitrogen. Given the strict limits on sulfur content specified in the National V gasoline standard, it is recommended to use GB/T11140—2008 as the authoritative method for testing the sulfur content in National IV and National V gasoline, in order to meet the technical requirements for sulfur content testing associated with the improvement of oil quality.