Chemiluminescence Method

Table of Contents

• What is the Chemiluminescence Method?
• Structure and Operating Principles of a Gas Analyzer Using Chemiluminescence Method
• Structure and Operating Principle of a Nitrogen Oxide (NOx) Gas Analyzer
• Reduction of Factors Influencing the Measurement
• Comparison with Non-Dispersive Infrared Absorption Method (NDIR)
• Measurement of Ammonia (NH₃) Gas
• Related Products

Chemiluminescence is a phenomenon in which molecules excited by a chemical reaction emit excitation energy as light when they return to their ground state.

For example, when nitrogen monoxide (NO) reacts with ozone (O₃) and is oxidized, NO changes to nitrogen dioxide (NO₂). The resulting NO₂ promotes to an excited state (NO₂) at a fixed rate and emits at a specific wavelength when it returns to the ground state of NO₂. (Equation 1)

Equation 1: Chemiluminescence with a Chemical Reaction

This principle is used for continuous concentration measurement of nitrogen oxides (NOx: NO + NO₂), NO, NO₂ and ammonia (NH₃) in sample gases.

In a gas analyzer using the chemiluminescence method, the sample gas and O₃ flow into a reaction cell, and the light emitted from NO₂* caused by the reaction  (Equation 1) between NO in sample gas and O₃ is transmitted through the optical filter is detected with a photodetector to measure NO concentration. (Figure 1)

Figure 1: Structure and operating principle of a gas analyzer using the chemiluminescence method.

This section explains the operating principles of the chemiluminescence method with a focus on the flow of chemical reactions in the reaction cell for measuring the NO concentration in sample gas as an example.

The sample gas and O₃ flow into the reaction cell and are mixed. (Figure 2-1)

A part of the gas component in the oxidized sample gas emits light of a specific wavelength for each gas component corresponding to the gas concentration by the chemical reaction shown in Equation 1. (Figure 2-2)
The optical filter selects light by the chemiluminescence for the measured component (NO), which enters the photodetector and is detected. The NO concentration of the measured component in the sample gas can be continuously measured with the processing photodetector signal.

After emitting light, all NO in the sample gas is in its ground state NO2 (Figure 2-3)
In the chemiluminescence method, O₃ must be supplied at a sufficient concentration with an appropriate flow rate to completely oxidize NO to the measured component.

Figures 2-1, 2-2, and 2-3 Operating principle of an NO gas analyzer using the chemiluminescence method.

HORIBA provides gas analyzers that use the chemiluminescence method to continuously measure concentrations of nitrogen oxides (NOx: NO + NO₂), NO, NO₂, and ammonia (NH₃), which are pollutants in ambient air and exhaust gas. This section explains the structure and operating principles of the analyzer, which continuously measures NOx, NO, NO₂ in ambient air.

Structure and Operating Principle of a Nitrogen Oxide (NOx) Gas Analyzer

The overall structure of the analyzer is shown in Figure 3. NO₂ does not emit light by the chemiluminescence method, so NO₂ in the sample gas is converted to NO through a converter using a catalyst for measurement. (Figure 3)

　　　　　　　　　　　　　　　　Line A: for NOx measurement Line B: for NO measurement Line C: for zero-point comparison (*)
　　　　　　　　　　　　　　　　The line switching sequence is A, B, C, and so on.

Figure 3: NOx analyzer using chemiluminescence method (3-components analyzer for NOx, NO₂, and NO)

NOx (NO + NO₂) and NO concentrations are measured respectively by alternately flowing the gas through a converter (line A) and the sample gas (line B) into the reaction cell by switching the lines with solenoid valves. NO₂ is calculated from the difference between these two concentrations. (Figure 3, Lines A and B)

To ensure stable and reliable measurement over the long term, a zero-point comparison gas with NO-free content is periodically flowed into the reaction cell, and the zero point is constantly monitored to reduce zero-point drift (*). The zero-point comparison gas uses an exhaust gas without NO from the reactor cell, so there is no need for specialized equipment for this gas generation.

In addition, these gases are flowed into the same reaction cell and detected with the same photodetector by the solenoid valve switching function. This means that changes in sensitivity of the reaction cell and the photodetector over time,  etc. are equally reflected in the detection of these gases, finally minimizing the difference in NO and NOx sensitivity.

*)The zero-point drift is that an analyzer's zero-point gradually shifts in one direction due to temperature, aging, or other factors. The monitoring with zero-point comparison gas for a deviation of the zero point can reduce the influence of zero-point drift.

The chemiluminescence method is influenced by the luminescence of gases other than the measured component and quenching by moisture or carbon dioxide (CO₂) in the sample gas. Measures to reduce these influences are as follows.

Reduction of influences of chemiluminescence by other gas components

When a sample gas contains gas components such as H₂S that emit by chemiluminescence with O₃, an optical filter is used to transmit only specific wavelength light of chemiluminescence by NO with O₃, thereby reducing the influence of other gases for the chemiluminescence. (Figure 1)

Reduction of moisture and CO₂ quenching influences

When moisture is contained in the sample gas or O₃, NO₂ excited in the reaction cell collides with moisture and loses excitation energy of NO, a phenomenon called quenching occurs, which influences the measurement. Moisture and CO₂ generally cause quenching. If sample gas or O₃ contains a large amount of moisture, a dehumidifier is used to remove it. If a high concentration of CO₂ exists in the sample gas, the sample gas is diluted and flowed into the analyzer to reduce the quenching influence.

HORIBA uses either the chemiluminescence method or the non-dispersive infrared absorption method (NDIR) as the measurement method of the analyzer to measure NOx, NO, and NO₂. HORIBA provides optimal analyzers to cater to specific needs and operating conditions, capitalizing on the distinctive attributes of each analytical 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 chemiluminescence and non-dispersive infrared absorption methods (NOx, NO, and NO₂)

Measurement of ammonia (NH₃) gas