15.3 Emission spectroscopy and fluorescence spectroscopy
3 min read•Last Updated on July 22, 2024
Emission and fluorescence spectroscopy are powerful tools for analyzing materials. These techniques use light to excite atoms or molecules, which then emit characteristic wavelengths. This allows scientists to identify and study various substances, from simple elements to complex biological molecules.
The methods rely on specialized equipment like light sources, monochromators, and detectors. Factors such as temperature and sample structure affect the results. These techniques find applications in diverse fields, including environmental monitoring, biochemistry, and nanomaterial research.
Emission and Fluorescence Spectroscopy
Principles of emission and fluorescence spectroscopy
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Emission spectroscopy involves exciting atoms or molecules to higher energy states by absorbing energy from a heat source (flame) or electrical discharge (plasma)
Excited species relax back to lower energy states, emitting photons with characteristic wavelengths specific to the element or molecule
Emitted light is dispersed by a monochromator (diffraction grating or prism) and detected, creating an emission spectrum unique to the sample
Fluorescence spectroscopy involves molecules absorbing photons and being excited to higher electronic states
Vibrational relaxation occurs, bringing the molecule to the lowest vibrational level of the excited electronic state
Molecule returns to the ground electronic state, emitting a photon with a longer wavelength than the absorbed photon (Stokes shift)
Emitted fluorescence is detected at a right angle to the excitation source to minimize interference from the incident light
Instrumentation for spectroscopic techniques
Light sources vary depending on the spectroscopic technique
Emission spectroscopy uses hollow cathode lamps (element-specific), inductively coupled plasma (ICP), or electrical discharges (arc or spark)
Fluorescence spectroscopy employs xenon arc lamps (broad spectrum), lasers (monochromatic), or light-emitting diodes (LEDs) for excitation
Monochromators disperse light into individual wavelengths using diffraction gratings or prisms
Select specific wavelengths for excitation (fluorescence) or emission (both techniques) to improve signal-to-noise ratio and specificity
Detectors convert the optical signal into an electrical signal for quantification and analysis
Photomultiplier tubes (PMTs) offer high sensitivity and fast response time
Charge-coupled devices (CCDs) allow simultaneous detection of multiple wavelengths
Avalanche photodiodes (APDs) provide high sensitivity and fast response time, suitable for single-photon counting experiments
Factors affecting spectral output
Electronic structure of the sample determines the energy levels and transitions, which dictate the wavelengths of the emitted or fluoresced light
Molecular structure, bonding, and functional groups (aromatic rings, conjugated systems) affect the electronic transitions and spectral features
Temperature influences the population of higher vibrational and rotational levels, leading to broadening of spectral lines at higher temperatures
Non-radiative relaxation processes (internal conversion, intersystem crossing) become more likely at higher temperatures, reducing emission or fluorescence intensity
Quenching effects decrease the emission or fluorescence intensity through various mechanisms
Collisional quenching: Deactivation of excited states through collisions with other molecules (oxygen, heavy atoms)
Static quenching: Formation of non-fluorescent complexes between the fluorophore and quencher (metal ions, aromatic compounds)
Applications in sample analysis
Atomic emission spectroscopy (AES) identifies and quantifies elements in a sample based on their characteristic emission lines
Applications in metallurgy (alloy composition), environmental monitoring (water and soil analysis), and geochemistry (mineral identification)
Molecular fluorescence spectroscopy studies the structure, dynamics, and interactions of molecules
Applications in biochemistry (protein folding, enzyme kinetics), material science (polymer characterization), and environmental analysis (pollutant detection)
Specific examples include monitoring DNA-ligand interactions, detecting pesticides in water samples, and studying the photophysics of organic semiconductors
Nanomaterial characterization investigates the optical properties, size distribution, and surface characteristics of nanomaterials
Quantum dots and fluorescent nanoparticles exhibit unique emission and fluorescence properties due to quantum confinement effects
Applications in bioimaging (cell labeling), sensing (chemical and biological sensors), and optoelectronics (light-emitting diodes, solar cells)