SO₂ and Sulfur Measurement by UV Fluorescence

How does the UV fluorescence method work?

Sulfur dioxide (SO₂) molecules become excited when they absorb light at specific wavelengths in the ultraviolet region. As they return to the ground state, they emit fluorescent light at a longer wavelength. The intensity of the emitted light is proportional to the SO₂ concentration and is measured by a photomultiplier tube. In analyzers using a pulsed UV source, stability is increased by separating the excitation light from the fluorescence signal.

For H₂S measurement, the sample is first passed through a heated converter that turns it into SO₂, and the result is again read by fluorescence. When a selective SO₂ scrubber is used, the H₂S value alone can be obtained. In total sulfur analyzers, the sample is burned at high temperature, all sulfur compounds convert to SO₂, and the resulting SO₂ is measured by fluorescence.

What UV fluorescence measures and where it is used

The main areas of use for the method are:

• SO₂ and H₂S monitoring stations in ambient air
• H₂S monitoring around refineries, at petrochemical plants, and in wastewater and biogas applications
• SO₂ measurement in flue and process gases
• Total sulfur control in refinery streams, fuel, LPG and gas lines
• Sulfur measurement in petroleum products (SO₂ fluorescence after combustion)

Where UV fluorescence works well and where it struggles

UV fluorescence provides high sensitivity and a fast response at low SO₂ concentrations; because it has few moving parts, its maintenance requirement is limited. It is widely preferred for ambient air measurement.

Some hydrocarbons and aromatic compounds can interfere with the measurement; analyzers use a hydrocarbon remover (kicker) to address this effect. Moisture and particles in the sample can affect the optical path, so filtration and correct sample conditioning are important. H₂S and total sulfur measurements require regular checks of the converter or furnace.

Related standards

The UV fluorescence method for measuring sulfur dioxide in ambient air is defined in EN 14212. Type approvals are assessed under the EN 15267 certification system.

Choosing an instrument for UV fluorescence

The target component (SO₂, H₂S or total sulfur), measurement range and sample properties are the deciding factors. Internal calibration options such as a permeation oven are considered for trace-level ambient measurements, and ATEX-compliant versions for process applications.

Tetra and UV fluorescence analyzers

Tetra supplies, installs, commissions, calibrates and services UV fluorescence analyzers in Türkiye. The product range includes Thermo Scientific 43i and 43iQ SO₂ analyzers, 450i and 450iQ H₂S-SO₂ analyzers and the SOLA iQ total sulfur analyzer for process streams. PAC NSure measures sulfur and nitrogen in petroleum products. Tetra also prepares H₂S and sulfur analyzer systems that combine SOLA iQ with sample conditioning and an ATEX enclosure.