Laser-induced fluorescence (LIF) is a powerful technique that uses laser light to detect and analyze molecules. It works by exciting molecules with a specific wavelength of laser light, causing them to absorb the energy and jump to a higher energy state. This excited state is unstable, and the molecules quickly return to their original state, releasing the absorbed energy as light of a longer wavelength—this is fluorescence. The emitted fluorescence is then detected, providing information about the molecule's properties and concentration.
The Process Explained:
- Excitation: A laser beam, precisely tuned to a specific wavelength, illuminates the sample containing the target molecules. This wavelength is chosen to match the absorption spectrum of the molecule of interest, ensuring efficient excitation.
- Absorption: The molecules absorb the laser light's energy, causing their electrons to transition to a higher energy level.
- Fluorescence Emission: The excited molecules are unstable and rapidly return to their ground state, releasing the absorbed energy as photons (light). This emitted light is typically at a longer wavelength (lower energy) than the excitation light—a phenomenon known as Stokes shift.
- Detection: Specialized detectors, such as photomultiplier tubes (PMTs), measure the intensity and wavelength of the emitted fluorescence. The signal strength is directly proportional to the concentration of the target molecules in the sample.
Advantages of Laser Fluorescence:
- High Sensitivity: LIF offers excellent sensitivity because the fluorescence signal is measured against a dark background, making it ideal for detecting trace amounts of molecules.
- Specificity: The precise wavelength selectivity of lasers allows for the targeted excitation of specific molecules, minimizing interference from other species in the sample.
- Versatility: LIF is applicable to a wide range of molecules and applications, including environmental monitoring, biomedical imaging, and chemical analysis.
Example Application:
LIF is widely used in environmental monitoring to detect pollutants in air and water. For instance, detecting specific trace amounts of pollutants in air samples by exciting these pollutants with a targeted laser and measuring the subsequent fluorescence. The intensity of the fluorescence is directly related to the concentration of pollutants.
As stated in the reference provided: "Laser-induced fluorescence (LIF) is a spectroscopic technique that involves the excitation of a molecular target by a beam of laser radiation followed by the detection of the subsequent emission or radiation from the target." This perfectly summarizes the core principle of LIF.
