Spectrophotometric Determination of Sulfur Content by Ultraviolet Fluorescence Method

Spectrophotometer UV fluorescence method for determining sulfur content is a technique for quantitative analysis based on ultraviolet light excitation to generate fluorescence signals. The core principle is that the sample is burned at high temperature in an oxygen-rich environment, and the sulfur compounds in it are quantitatively converted into sulfur dioxide. When sulfur dioxide molecules are irradiated with UV light at a specific wavelength, they absorb energy and transition to the excited state, and then release fluorescence at characteristic wavelengths when returning to the ground state. The intensity of the fluorescence signal was detected by a high-sensitivity photomultiplier tube, and its signal size was linearly related to the concentration of sulfur in the sample within a certain range, so as to achieve accurate determination of sulfur content. Due to its high sensitivity and good selectivity, this method is widely used in petrochemical, environmental monitoring and materials science.

Principle

The core instrument system required for this method is mainly composed of the following units: injection and combustion unit, reaction gas circuit unit, optical detection unit, and signal processing and display unit. The sample is introduced into a high-temperature combustion tube through an automatic or manual injector, where it burns instantaneously in an oxygen stream. The gas produced by combustion is dried and dehydrated before entering the fluorescence detection tank. The cell contains a UV light source (usually a zinc or xenon lamp) of a specific wavelength, which emits UV light that excites sulfur dioxide molecules to fluoresce. After the fluorescence signal is removed by the filter to remove stray light interference, it is received by the photomultiplier tube and converted into an electrical signal. The electrical signal is amplified by an amplifier, and then the corresponding sulfur content is calculated by the data processing system according to a pre-established standard curve.

The quantitative relationship of this process can be expressed as follows: the fluorescence signal intensity (I) is directly proportional to the sulfur dioxide concentration (c), that is, I = kc, where k is the instrument constant, which is determined by the instrument state and experimental conditions. In the actual analysis, the I-c calibration curve needs to be established through a series of standard samples.

Procedure:

To ensure the accuracy and reliability of the analysis results, the operation process needs to be standardized and rigorous. First of all, it is necessary to fully preheat and stabilize the instrument, and usually require the start-up and warm-up time is not less than the specified time. Second, establish or validate calibration curves using a series of standard samples with known sulfur content, with a concentration range that covers the expected values of the sample to be tested. Before analyzing unknown samples, a systematic blank assay is performed with a suitable solvent or blank matrix to correct for background signals. When injecting, attention should be paid to the representativeness, uniformity of the sample and the accuracy of the injection volume. For solid or high-viscosity samples, ancillary measures may be required to ensure complete combustion. After each sample is analyzed, sufficient purge time is required to prevent cross-contamination. Finally, data processing should be based on a valid calibration curve with necessary corrections for instrument drift.

The precision and accuracy of the analysis results are affected by a combination of experimental conditions. Among them, the temperature and oxygen flow rate of the combustion furnace are the key parameters to ensure the complete combustion of the sample and the quantitative conversion of sulfur to sulfur dioxide. Gas lines must be kept clean and dry, as moisture and dust can quench fluorescence or produce scattering interference. The stability of the UV light source directly affects the excitation efficiency, and its intensity attenuation needs to be monitored regularly. The gain settings and operating temperature of the photomultiplier tubes also need to be optimized to ensure sensitivity and reduce noise. In addition, differences in sample matrix can cause interference, such as the presence of certain metal elements or halogens that may affect the combustion process or fluorescence efficiency, and consider using standard addition methods or necessary pretreatments.

UV fluorescence method shows many characteristics in the determination of sulfur content. It has a low lower limit of detection and can meet the needs of trace sulfur analysis. The method has a wide linear dynamic range and can be used to analyze samples from trace to higher concentrations. Since the detection object is the characteristic fluorescence of sulfur dioxide, the method is relatively less interfered with by other non-sulfur components in the sample, and the selectivity is better. The whole analysis process is highly automated and the analysis speed is fast. However, the method requires high maintenance and operational specifications, and any defects in gas purity, system airtightness, and combustion efficiency may directly affect the measurement results.

Applications:

This method is suitable for the determination of total sulfur content in a variety of sample types.

Notes:

In practical applications, the following points should be noted: Operators should receive professional training and be familiar with instrument operating procedures and safety instructions. High-purity oxygen and carrier gases must be used, and gas purification units must be replaced regularly. Core components such as combustion pipes and quartz windows should be regularly inspected and cleaned to prevent carbon buildup or pollution. The reference material used to establish the calibration curve should match the sample to be tested as closely as possible. For samples that are outside the linear range or have complex substrates, appropriate dilution or pretreatment is required. Laboratories should have a good temperature and humidity control environment and regularly verify instruments with certified reference materials to ensure the continued effectiveness of the measurement system.