LIF Principle

Laser-Induced Fluorescence (LIF) is a spectroscopic technique used to study and analyse the properties of molecules by exciting them with a laser and measuring the subsequent emission of light as they return to their ground state.

When it comes to measuring nitrogen dioxide (NO₂) using LIF, the process involves the following steps:

1. Excitation: A laser beam, typically in the ultraviolet (UV) or visible range, is directed onto the sample containing the molecules of interest. The energy from the laser is absorbed by the NO₂ molecules, causing them to become electronically excited to higher energy levels.

2. Relaxation and Emission: After absorbing the laser energy and transitioning to higher energy states, the excited NO₂ molecules quickly relax back to their lower energy states. During this relaxation process, they emit fluorescent light as they return to their ground state. The emitted light is at longer wavelengths (lower energy) than the excitation wavelength.

3. Detection: The emitted fluorescent light passes through filters to separate the emitted light from the excitation light, before falling on a sensitive detector.

4. Quantification: The intensity of the emitted fluorescent light is proportional to the concentration of the target molecule (NO₂) in the sample. By comparing the emitted light intensity to a known calibration curve or standard, the concentration of NO₂ in the sample can be determined.

Advantages

• LIF is a highly sensitive method, capable of detecting very small concentrations of NO₂.

• LIF is a highly selective technique as it specifically targets the fluorescence of molecules of interest, reducing cross sensitivity to other molecules present.

• LIF provides rapid real time measurements making it an effective technique for dynamic environments.

• LIF provides a direct measurement of NO₂ concentration without the need for traditional NOx converters.