Lecture 2 - Instruments For Optical Spectros
Lecture 2 - Instruments For Optical Spectros
Lecture 2 - Instruments For Optical Spectros
Sources of Radiation
In order to be suitable for spectroscopic studies, a source
Continuum Sources:
Emit radiation that changes in intensity only slowly as a function
of wavelength.
Widely used in absorption and fluorescence spectroscopy
For the ultraviolet region, the most common source is the
deuterium lamp.
High pressure gas filled arc lamps that contain argon, xenon,
or mercury serve when a particular intense source is required.
For the visible region of the spectrum, the tungsten filament
lamp is used universally.
The common infrared sources are inert solids heated to 1500 to
2000 K.
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Line Sources:
• Sources that emit a few discrete lines find wide use in atomic
absorption spectroscopy, atomic and molecular fluorescence
spectroscopy, and Raman spectroscopy.
Wavelength Selectors
Need to select wavelengths (λ) of
light for optical measurements.
… Wavelength Selectors
There are two types of wavelength selectors:
FILTERS and
MONOCHROMATORS
There are 3 types of filters:
• absorption filters
• interference filters
• cut-off filters (may be considered as absorption filter)
2 types of dispersing elements can be used in a MC
prism
grating
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Filters
Simple, rugged (no moving parts in general)
Relatively inexpensive
Can select some broad range of wavelengths
Most often used in ;
• field instruments
• simpler instruments
• dedicated to monitoring a single wavelength range.
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Drawbacks of filters
Monochromators
For many spectroscopic methods, it is necessary or desirable to be able to continuously
vary the wavelength of radiation over a broad range. This process is called scanning a
spectrum.
Monochromators for ultraviolet, visible, and infrared radiation are all similar in
mechanical construction in the sense that they use slits, lenses, mirrors, windows, and
gratings or prisms.
The materials from which these components are fabricated depend on the wavelength
region of intended use.
Sample Containers
The cells or cuvettes that hold the samples must be made of
material that is transparent to radiation in the spectral
region of interest.
Quartz or fused silica is required for work in the ultraviolet region
(below 350 nm), both of these substances are transparent in the
visible region.
Silicate glasses can be employed in the region between 350 and
2000 nm.
Plastic containers can be used in the visible region.
Crystalline NaCl is the most common cell windows in the IR region.
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Radiation Transducers
• The detectors are devices that convert radiant energy into electrical signal.
• The detectors for early spectroscopic instruments were the human eye or a
photographic plate or film.
• Nowadays more modern detectors are in use that convert radiant energy into
electrical signal.
• A detector should be sensitive, and has a fast response over a considerable range
of wavelengths.
Ideal transducer
High sensitivity
High signal to noise ratio (S/N) i.e., low noise
Constant response over many λ’s (wide range of
wavelength)
Fast response time
S = 0 if no light present (low dark current)
Signal α P (where P = radiant power)
Rugged, cheap and simple
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Some terminologies
Signal-to-noise ratio: The ratio of the signal’s intensity to
the average intensity of the surrounding noise.