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An introduction to Ultraviolet/Visible Absorption Spectroscopy. Lecture 24. Instrumentation Light source - selector Sample container Detector Signal processing Light Sources (commercial instruments) D 2 lamp (UV: 160 – 375 nm) W lamp (vis: 350 – 2500 nm).
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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.
(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.
Why add I2 in the lamps?
W + I2 → WI2
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.
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.
spectrophotometer pictured earlier
Double Beam: In - Space
Needs two detectors
Double Beam: In - Time