Ultraviolet Fluorescence Method (UVF)

Table of Contents

Measuring Principle

What is Ultraviolet Fluorescence Method (UVF)?

Fluorescence is the emission of light by a substance that has absorbed light energy, such as ultraviolet light. When a molecule absorbs the energy of light, it becomes an unstable, high-energy state (excited state), and after emitting light, it returns to the original energy state (ground state).

For example, if the measured component is sulfur dioxide (SO₂) in the sample gas, irradiating the sample gas with ultraviolet radiation of a specific wavelength (190nm-230nm) will cause fluorescence. When SO₂ molecules are irradiated with UV radiation, they absorb the UV radiation and transition to an excited state (SO₂*) at a certain rate, and the SO₂* returns to the ground state by emitting UV radiation with a wavelength longer than the wavelength of the absorbed UV radiation. (Equation 1)

Equation 1: Fluorescence reaction (for SO₂)

(1) shows that SO₂ absorbs the energy hv1 of UV radiation to become an excited state, and
(2) shows that the energy hv2 of UV radiation is emitted when the SO₂* in the excited state returns to the ground state.

Since the SO₂ concentration is proportional to the intensity of fluorescence (240-420nm) caused by SO₂*, the SO₂ concentration is measured by detecting the intensity of fluorescence.

This principle is used for the continuous concentration measurement of sulfur dioxide (SO₂) in a sample gas.
The following sections describe the measurement of SO₂ concentration in a sample gas using UVF.

Structure and Operating Principles of a Gas Analyzer Using UVF

The gas analyzer using UVF flows a sample gas into a fluorescence cell, and the UV fluorescence (Equation 1) caused by the UV radiation and the SO₂ in the fluorescence cell is selectively transmitted through an optical filter and detected by a photodetector to measure the gas concentration (Figure 1).

Figure 1: Structure and operating principles of a gas analyzer using UVF

Structure and Operating Principle of a Sulfur Dioxide (SO₂) Gas Analyzer

This section explains the construction and operating principles of a gas analyzer that continuously measures SO₂ in ambient air.

An example of the overall structure of the analyzer is shown in Figure 2. A xenon flash lamp is used as the UV light source. In order to continuously measure SO₂ concentration with high accuracy, the analyzer incorporates various mechanisms to reduce error factors for measurement , including a detector to compensate for changes in light intensity from the xenon flash lamp.

Figure 2: Structure and operating principle of the UV fluorescence gas analyzer

The operating Principle is as Follows.

The sample gas is irradiated with ultraviolet light of a specific wavelength while flowing through a fluorescence cell. Depending on the SO₂ concentration in the sample gas, the UV light absorption changes, and the intensity of light from fluorescence also changes. The fluorescence enters the detector as transmitted light selected by an optical filter and is detected by a photodetector (photomultiplier tube). This detected signal is processed to calculate SO₂ concentration, thereby continuously measuring SO₂ concentration in the sample gas.

Reduction of Factors Influencing the Measurement (Figure 3)

The measurement of SO₂ concentration with UVF is influenced by the reduced light intensity of the UV light source and specific aromatic hydrocarbons in the sample gas. The analyzer using UVF reduces these influencing factors by various mechanisms and functions.

Figure 3: Reduction of factors influencing the measurement.

Reduction of Influence by Ultraviolet Light Sources

To cause sufficient fluorescence even when the concentration of the measured gas is low, a xenon flash lamp with high luminance is used as the UV light source. Xenon flash lamp emits light at various wavelengths, so if the light transmits the fluorescence cell as it is as excitation light, the fluorescence other than SO₂ will also transmit the detector and influence the measurement value. As a countermeasure against this influence, HORIBA uses several reflective optical filters to select the necessary wavelengths for the excitation light.Since the amount of UV light of the xenon flash lamp is reduced by long-term lighting, changes in the light intensity are measured with a compensation detector for the light intensity, and SO₂ concentration is corrected corresponding to the detected change.

Reduction of Influence by Stray Light Incident for Photodetector

Optical design to reduce stray light in the fluorescent cell reduces the noise in fluorescence measurement. The inner wall of the fluorescent cell is specially coated to reduce reflection of the excitation light. These measures reduce stray light incident on the photodetector. In addition, the special coating on the inner wall of the fluorescent cell prevents SO₂ absorption.

Reduction of Influence by Gas Components

Aromatic hydrocarbons such as toluene and xylene, which absorb UV and emit fluorescence, influence SO₂ measurement. To reduce this influence, these gases are removed by an aromatic hydrocarbon remover unit before the sample gas flows to the fluorescent cell. In addition, an optical filter that selectively transmits SO₂ fluorescence is used to reduce the influence of fluorescence caused by other gases.

Comparison with Non-Dispersive Infrared Absorption Method (NDIR)

HORIBA uses either the ultraviolet fluorescence method (UVF) or the non-dispersive infrared absorption method (NDIR) as the measurement method of the analyzer to measure SO₂. HORIBA provides the optimum analyzers for the purpose of use and the operating environment by taking advantage of the features of each method. This section summarizes the features of both methods. (Table 1)

For more information about non-dispersive infrared absorption (NDIR), click here.

This table is a comparison of our products.

Table 1: Comparison of the Ultraviolet Fluorescence method and the Non-Dispersive Infrared Absorption method (SO₂)

The optimum method is mainly selected based on the SO₂ measurement concentration range. For example, when measuring SO₂ in the ambient air, a UVF analyzer is used, and when measuring SO₂ in exhaust gases, a NDIR analyzer is used.

Related Products

The Ultraviolet Fluorescence method (UVF) analyzer is widely used for continuous measurement and monitoring of sulfur dioxide (SO₂), one of the pollutants in ambient air. It is also used to monitor sulfur dioxide (SO₂), a pollutant in semiconductor clean rooms.