Transcript L24

An introduction to Ultraviolet/Visible
Absorption Spectroscopy
Lecture 24
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Instrumentation
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Light source
 - selector
Sample container
Detector
Signal processing
Light Sources (commercial
instruments)
– D2 lamp (UV: 160 – 375 nm)
– W lamp (vis: 350 – 2500 nm)
Sources
Deuterium and hydrogen lamps (160 – 375 nm)
D2 + Ee → D2* → D’ + D’’ + h
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Deuterium lamp
UV 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) versus
wavelength. Note that the maximum intensity occurs at ~225 m.Typically,
instruments switch from deuterium to tungsten at ~350 nm.
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Visible and near-IR region
(a) A 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.
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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.
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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.
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Tungsten lamps (350-2500 nm)
Why add I2 in the lamps?
W + I2 → WI2
Low limit: 350 nm
1)Low intensity
2)Glass envelope
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3. Xenon Arc Lamps
Passage of current through an atmosphere of
high pressured xenon excites xenon and
produces a continuum in the range from 2001000 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.
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Sample Containers
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.
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Instrumental Components
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Source
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 - selector (monochromators)
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Sample holders
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Cuvettes (b = 1 cm typically)
1. Glass (Vis)
2. Fused silica (UV+Vis)
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Detectors
– Photodiodes
– PMTs
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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.
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1. Single beam
–Place cuvette with blank (i.e.,
solvent) in instrument and take a
reading  100% T
–Replace cuvette with sample and
take reading  % T for analyte
(from which absorbance is calc’d)
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Most common spectrophotometer:
Spectronic 20.
1. On/Off switch and zero
transmission
adjustment knob
2. Wavelength
selector/Readout
3. Sample chamber
4. Blank adjustment knob
5. Absorbance/Transmitta
nce scale
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End view of the exit slit of the Spectronic 20
spectrophotometer pictured earlier
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• Single-Beam Instruments for the Ultraviolet/Visible
Region
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• Single-Beam Computerized
Spectrophotometers
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Inside of a
singlebeam
spectropho
tometer
connected
to a
computer.
2. Double beam (most commercial
instruments)
– Light is split and directed towards both
reference cell (blank) and sample cell
– Two detectors; electronics measure ratio
(i.e., measure/calculate absorbance)
– Advantages:
• Compensates for fluctuations in source
intensity and drift in detector
• Better design for continuous recording of
spectra
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General Instrument Designs
Double Beam: In - Space
Needs two detectors
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General Instrument Designs
Double Beam: In - Time
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Merits of Double Beam Instruments
1.Compensate for all but the most short term
fluctuation in radiant output of the source
2.Compensate drift in transducer and
amplifier
3.Compensate for wide variations in source
intensity with wavelength
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Dual Beam Instruments
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