Non-Dispersive Ultraviolet Absorption Method (NDUV)

Table of Contents

Measuring Principle

What is Non-Dispersive Ultraviolet (UV) Absorption Method (NDUV)?

Non-dispersive ultraviolet absorption method (NDUV) is a method that uses molecular ultraviolet absorption. In particular, since ozone (O₃) absorbs UV radiation at a specific wavelength, so this method can be used to measure O₃ gas. Since O₃ gas has a maximum absorption band in the ultraviolet region at 253.7 nm, this wavelength is used to measure its concentration. The following section explains the measurement of O₃ in a sample gas by NDUV.

Measuring Principle of Gas Analyzer Using NDUV

Figure 1: Basic structure of a gas analyzer using NDUV.

Ultraviolet light of a specific wavelength (incident light) from the UV light source is absorbed by O₃ in the sample gas introduced into the chamber (gas cell) to be measured. How much is absorbed is obtained by measuring UV selectively transmitted through an optical filter with a photodetector. By Lambert-Beer law for the measurement of the photodetector, the concentration of O₃ in the sample gas is calculated. (Figure 1)

The relationship between UV absorption and gas concentration is determined by Lambert-Beer law.

Lambert-Beer Law

The absorption amount of incident light varies corresponding to the concentration of absorbing molecules. This relationship is expressed by the Lambert-Beer law.

Ι = Ι0exp (-μcd)

Ι: Transmitted light intensity
Ι₀: Incident light intensity
c: Concentration of absorbing molecules (measured component)
μ: Absorption coefficient (constant determined by molecule and wavelength)
d: Thickness of absorbing molecule layer (gas layer)

Structure and Operating Principle of UV Absorption Gas Analyzer

An analyzer using NDUV is called a UV absorption gas analyzer. It is simple in structure, easy to maintain, and has features suitable for continuous measurement, so it is used for measurement of O₃.

Figure 2: Structure and operating principle of UV absorption gas analyzer

Structure and Operating Principle of UV Absorption Gas Analyzer

When UV light (incident light) is continuously irradiated while the sample gas is flowing through the gas cell, the O₃ in the sample gas absorbs UV light in proportion to the gas concentration. As the concentration of O₃ increases, the amount of absorbed UV in the gas cell also increases. The UV light absorbed in the gas cell is selected by an optical filter and transmitted to the detector. This transmitted light is detected by a photo detector (photodiode), and the concentration of O₃in the sample gas is continuously measured by the signal processing for the detection signal.

Cross-modulation method with reference gas, which is generated by removing O₃ in sample gas, improves the noise immunity for the detection signal and reduces the effect of decreasing the light intensity of the UV light source.

Cross-Modulation Method

A Cross-modulation method is a modulation with a periodical switch of gasses for a gas cell by a solenoid valve unit. In the cross-modulation method, a sample gas and a comparison gas are alternately switched by a solenoid valve unit and introduced into the gas cell at regular intervals to modulate the detector signal. By using this modulated detection signal, noise is reduced, and highly accurate continuous gas measurement is possible. The zero point of the analyzer is stable because the reference gas that does not contain the measured component is always measured as a gas with zero concentration.

Structure and Operating Principle of Ozone (O₃) Gas Analyzer

The actual cross-modulation method uses a solenoid valve unit for switching mechanism (modulation mechanism) to alternately flow a sample gas and a reference gas into one gas cell. The reference gas flowing into the gas cell is generated by removing O₃ from the sample gas using an ozone decomposer. (Figure 3)

Figure 3: Structure and operating principle of ozone (O₃) gas analyzer

By NDUV with the cross-modulation, the UV detected by the photodetector is divided into AC signal (modulation signal) and DC signal in the signal processing, and the AC signal is amplified as O₃ concentration and the O₃ concentration is calculated. The use of a modulated signal improves the noise immunity of the detection signal.

The DC signal, which is proportional to the intensity of the UV light source, is used to compensate for the decrease in intensity of the UV light source due to aging. As a result, the zero and span points of the analyzer are stable over a long period of time.
The gas analyzer using NDUV stably and continuously can measure O₃ in the sample gas by these structures and operations.

Related Products

The UV absorption gas analyzer is widely used for continuous measurement and monitoring of O₃ which is the main contributor of photochemical smog. It is also used to monitor O₃ as a contamination in semiconductor clean rooms.