UV-Visible Spectroscopy
From The Science of Spectroscopy
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When a sample of an unknown compound is exposed to light, certain functional groups within the molecule absorb light of different wavelengths UV Absorption Table. In UV/Visible Spectroscopy, the term chromophore is used to indicate a functional group that absorbs electromagnetic radiation, usually in the UV or visible region. The type of functional groups that absorb ultraviolet light can be conjugated species, such as alkenes, aromatics, etc, making UV/Visible spectroscopy useful for distinguishing conjugated dienes from conjugated trienes, and so forth. Also many metal-ligand complexes also absorb UV/visible light. Its important to remember that UV/visible EM radiation causes electronic transitions within a molecule, promoting bonding and non-bonding electrons to higher, less stable antibonding orbitals. The molecule then loses this excess energy by rotation and vibrational relaxation, but some compounds can lose their energy by emission processes such as fluorescence.
Ultraviolet spectrometers consist of a light source, reference and sample beams, a monochromator and a detector. The ultraviolet spectrum for a compound is obtained by exposing a sample of the compound to ultraviolet light from a light source, such as a Xenon lamp.
The reference beam in the spectrometer travels from the light source to the detector without interacting with the sample. The sample beam interacts with the sample exposing it to ultraviolet light of continuously changing wavelength. When the emitted wavelength corresponds to the energy level which promotes an electron to a higher molecular orbital, energy is absorbed. The detector records the ratio between reference and sample beam intensities (Io/I). At the wavelength where the sample absorbs a large amount of light, the detector receives a very weak sample beam. Once intensity data has been collected by the spectrometer, it is sent to the computer as a ratio of reference beam and sample beam intensities. The computer determines at what wavelength the sample absorbed a large amount of ultraviolet light by scanning for the largest gap between the two beams.
When a large gap between intensities is found, where the sample beam intensity is significantly weaker than the reference beam, the computer plots this wavelength as having the highest ultraviolet light absorbance when it prepares the ultraviolet absorbance spectrum.
What does the chopper do?
The chopper is a simple device that consists of a circular disk with one third of the disk opaque, one third mirrored, and one third cut-away to allow light to pass through. Essentially, the chopper separates the light from the reference beam from the sample beam, allowing only one to reach the photomultiplier at any given time.
The instrument is synchronized so that the electronics know which beam is reaching the detector.
