3Sources Deuterium and hydrogen lamps (160 – 375 nm) D2 + Ee → D2* → D’ + D’’ + h
4Deuterium lampUV region(a) A deuterium lamp of the type used in spectrophotometers and (b)its spectrum. The plot is of irradiance Eλ (proportional to radiant power) versuswavelength. Note that the maximum intensity occurs at ~225 m.Typically,instruments switch from deuterium to tungsten at ~350 nm.
5A tungsten lamp of the type used in spectroscopy and its spectrum Visible and near-IR regionA tungsten lamp of the type used in spectroscopy and its spectrum(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.
6The 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 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.
7Tungsten 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.
8Tungsten lamps (350-2500 nm) Why add I2 in the lamps? W + I2 → WI2 Low limit: 350 nmLow intensityGlass envelope
93. Xenon Arc LampsPassage of current through an atmosphere of high pressured xenon excites xenon and produces a continuum in the range from 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.
11Sample ContainersSample 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.
13Types of InstrumentsInstrumental designs for UV-visible photometersor 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.
18End view of the exit slit of the Spectronic 20 spectrophotometer pictured earlier
19Single-Beam Instruments for the Ultraviolet/Visible Region
20Single-Beam Computerized Spectrophotometers Inside of a single-beam spectrophotometer connected to a computer.
212. Double beam (most commercial instruments) Light is split and directed towards both reference cell (blank) and sample cellTwo detectors; electronics measure ratio (i.e., measure/calculate absorbance)Advantages:Compensates for fluctuations in source intensity and drift in detectorBetter design for continuous recording of spectra
22General Instrument Designs Double Beam: In - SpaceNeeds two detectors22
23General Instrument Designs Double Beam: In - Time
25Merits of Double Beam Instruments Compensate for all but the most short term fluctuation in radiant output of the sourceCompensate drift in transducer and amplifierCompensate for wide variations in source intensity with wavelength