Fluorescence Spectroscopy and Radiation Scattering Photometry Flashcards
State the difference between incandescence and luminescence
Incandescence - heated objects that emit EM radiation.
Luminescence - Light is emitted that is not due to heat. (photoluminescence is a form of luminescence)
Describe photoluminescence at the atomic level
Certain materials are able to absorb EM radiation. This is possible because electrons within that material are able to absorb specific energy levels to bring them from the ground state to an excited state. When light hits the electrons if it is the right energy level (wavelength) it will absorb the light, if it is an energy level that is too high or too low the electron will not absorb the energy.
Electrons are unstable in their excited state, so they will eventually move back to their ground state, by emitting energy in the form of visible light.
Differentiate fluorescence and phosphorescence
Both are examples of photoluminescence.
Fluorescence - Materials that are fluorescent only emit light in the presence of excitation radiation. The absorption and emission of light happen rapidly.
Phosphorescent - Materials that continue to emit light after the excitation radiation is removed (glow in the dark). Compared to fluorescent materials, phosphorescent materials will emit light for a long period of time after the excitation source is removed.
Describe the chemical structure of fluorophores
Fluorophores are substances that exhibit photoluminescence. They typically have aromatic rings, and at least one electron-donating group attached to the aromatic ring.
excitation wavelength
Excitation wavelength: Wavelengths that hit the fluorophore and allow the electrons to move to an excited state.
Emission wavelength
Wavelengths that are emitted from the fluorophore as electrons move back to their ground state. Radiation emitted is proportional to the concentration of the analyte, length of the path, and intensity of the excitation energy.
Define Stokes shift
The energy difference between an excited electron, and an electron at the ground state. Emitted photons have less energy than absorbed photons.
Define Photoluminescence
Luminescence occurs when a substance absorbs EM radiation and re-emits energy in the form of visible light.
List the main components of a fluorometer
Light source - emits the excitation light, therefore must be able to emit high-intensity radiation. Often mercury or xenon arc lamps are used. LASERs (flow cytometry) and LEDs can also be seen.
Wavelength selector - filters (fluorometers) or monochromators (spectrofluorometers) are seen. Two selectors will be used, an excitation (primary) selector before the sample, and an emission (secondary) selector after the sample.
Sample cell - cuvettes are commonly used and must be made of a material that is clear on all sides and does not fluoresce.
Detector - PMTs placed at a 90-degree angle.
List commonly used light sources found in fluorometers
Xenon Arc Lamp
Mercury Lamp
State the function of the excitation/emission filters and describe their placement with respect to the other components of a fluorometer
Primary wavelength selectors are placed in between the light and sample cell. They select the wavelength that excites the fluorophore.
Secondary wavelength selectors are placed between the sample cell and the photon detector. They select the wavelength that is detected.
Discuss the function of the sample cell (cuvette)
To hold the sample - must not interfere with the emitted light.
State the best photon detector for fluorescence instrumentation
PMT (photomultiplier tubes) are used as the light emitted off a sample may be very low, and PMTs are the most sensitive detectors.
Discuss the placement of the photon detector - what angle?
90 degrees - reduces the detection of stray light.
Discuss the following fluorescence sources of error:
Matrix effects
Background fluorescence that is caused by compounds in the sample that fluoresce that are not the analyte being tested.
