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An introduction to Ultraviolet/Visible Absorption Spectroscopy
D2 + Ee → D2* → D’ + D’’ + h
(a) A deuterium lamp of the type used in spectrophotometers and (b)
its spectrum. The plot is of irradiance Eλ (proportional to radiant power) versus
wavelength. Note that the maximum intensity occurs at ~225 m.Typically,
instruments switch from deuterium to tungsten at ~350 nm.
Visible and near-IR region
(b). Intensity of the tungsten source is usually quite low at wavelengths shorter than about 350 nm. Note that the intensity reaches a maximum in the near-IR region of the spectrum.
The tungsten lamp is by far the most common source in the visible and near IR region with a continuum output wavelength in the range from 350-2500 nm. The lamp is formed from a tungsten filament heated to about 3000 oC housed in a glass envelope. The output of the lamp approaches a black body radiation where it is observed that the energy of a tungsten lamp varies as the fourth power of the operating voltage.
Tungsten halogen lamps are currently more popular than just tungsten lamps since they have longer lifetime. Tungsten halogen lamps contain small quantities of iodine in a quartz envelope. The quartz envelope is necessary due to the higher temperature of the tungsten halogen lamps (3500 oC). The longer lifetime of tungsten halogen lamps stems from the fact that sublimed tungsten forms volatile WI2 which redeposits on the filament thus increasing its lifetime. The output of tungsten halogen lamps are more efficient and extend well into the UV.
Why add I2 in the lamps?
W + I2 → WI2
3. Xenon Arc Lamps
Passage of current through an atmosphere of high pressured xenon excites xenon and produces a continuum in the range from 200-1000 nm with maximum output at about 500 nm. Although the output of the xenon arc lamp covers the whole UV and visible regions, it is seldom used as a conventional source in the UV-Vis. The radiant power of the lamp is very high as to preclude the use of the lamp in UV-Vis instruments. However, an important application of this source will be discussed in luminescence spectroscopy which will be discussed later.
Sample containers are called cells or cuvettes and are made of either glass or quartz depending on the region of the electromagnetic spectrum. The path length of the cell varies between 0.1 and 10 cm but the most common path length is 1.0 cm. Rectangular cells or cylindrical cells are routinely used. In addition, disposable polypropylene cells are used in the visible region. The quality of the absorbance signal is dependent on the quality of the cells used in terms of matching, cleaning as well as freedom from scratches.
Types of Instruments
Instrumental designs for UV-visible photometers
or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector.
1. Single beam
Most common spectrophotometer: Spectronic 20.
End view of the exit slit of the Spectronic 20
spectrophotometer pictured earlier
Inside of a single-beam spectrophotometer connected to a computer.
2. Double beam (most commercial instruments)
General Instrument Designs
Double Beam: In - Space
Needs two detectors
General Instrument Designs
Double Beam: In - Time