Chemiluminescent luminescence (CLD) is an international standard method for measuring nitrogen oxides (NO ₓ), known for its high sensitivity, selectivity, and rapid response. The core principle is based on the gas-phase chemical reaction between nitric oxide (NO) and ozone (O3).
1、 The detection logic from NO to NO ₂
The detection logic of this instrument follows a sophisticated "step-by-step measurement" design, with the core components being two reaction chambers and a set of switching valve circuits.
Direct measurement of NO:
The gas to be tested first enters the first reaction chamber directly and mixes with the excess O3 generated by the internal ozone generator of the instrument.
Key reaction occurs: NO+O3 → NO ₂ *+O ₂. About 10% of nitrogen dioxide molecules are in the excited state (NO ₂ *).
When NO ₂ * transitions from the excited state back to the ground state, it releases a photon with a wavelength in the range of 600-3000nm. The weak chemiluminescence is detected by a red sensitive photomultiplier tube (PMT) closely attached to the reaction chamber, and its intensity is strictly proportional to the concentration of NO.
Measurement of NO ₓ (total nitrogen oxides) and indirect derivation of NO ₂:
The other gas flows through a molybdenum converter (or high-temperature converter). This device can catalyze the reduction of all nitrogen oxides (including NO ₂) in the sample gas to NO.
The reaction is: NO ₂+Mo → NO+MoO.
Afterwards, this gas, which has been completely converted into NO, enters the second reaction chamber and reacts with O3 to detect its chemiluminescence signal. The concentration corresponding to this signal is NO ₓ (NO+NO ₂).
Finally, by calculating: NO ₂ concentration=NO ₓ concentration - NO concentration, the concentration of nitrogen dioxide can be indirectly obtained.
2、 Key points of error control
Converter efficiency: The attenuation of molybdenum converter efficiency or cross sensitivity to other nitrogen-containing compounds (such as NH3) are the main sources of error. Regular calibration is required (such as verifying efficiency>96% by introducing standard NO ₂ gas), or using more specific photocatalytic conversion methods.
Chemiluminescence interference: Some substances (such as olefins, CO) can also produce weak luminescence when reacting with O3, causing positive errors. By optimizing the reaction chamber pressure (low pressure can effectively quench most of the interference) and using selective filters for control.
Effect: CO ₂, water vapor, etc. in the sample gas will collide and quench the excited state of NO ₂ *, resulting in a decrease in signal and negative error. Desiccant should be used to remove water vapor, and compensation should be made through a calibration curve.
System maintenance: Regularly cleaning the reaction chamber to prevent contamination, checking the stability of the ozone generator, and ensuring that the PMT is at its optimal operating temperature are the basis for ensuring long-term data accuracy.
Through this precise physical and chemical design combined with strict error control measures, the CLD analyzer can achieve accurate and reliable quantitative analysis of NO and NO ₂.