Apparatus and Instruments used for Uv and Vis measurements

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Apparatus and Instruments used for Uv and Vis
measurements
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EMR
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Deuterium Lamps



Used to obtain ultraviolet absorption spectra.
Above 350 nm its output is too weak for accurate
measurements of absorption. The irradiance is
too low for fluorescence measurements. This is a
low pressure discharge lamp that produces light
by the reaction, D2 e? ? D2 ? 2D hn

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Quartz Halogen Lamps
Used to obtain visible absorption spectra. Its
output below 300 nm is too weak for accurate
measurements of absorption. The irradiance is
too low for most fluorescence measurements.
Power supply 12 V dc at 4 A, and highly
regulated.
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Xenon Arc Lamps
it is difficult to obtain high quality spectra in
those regions. This is a high Used to obtain
fluorescence excitation, emission spectra, and
make quantitative measurements. Below 300 nm the
output is too weak to make accurate measurements.
The lines between 450 and 500 nm and above 650
nm make pressure discharge lamp.
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Mercury Arc Lamps
Used to perform quantitative fluorescence
measurements, or obtain emission spectra. The
line output precludes obtaining excitation
spectra. Note how the 2537 ? line is inverted!
The UV, blue and green lines emit 10-100 times
more light than a xenon arc lamp. This is a high
pressure discharge lamp.
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Calibration Pen Lamps
Pen lamps can be purchased with a wide variety of
gases that emit sharp line spectra. The line
output is commonly used to calibrate wavelength
separators, such as grating monochromators. A
mercury pen lamp emits intense lines in the
ultraviolet through the blue at 2537, 3130, 3650,
4047, and 4358 ?. To calibrate in the red use a
neon lamp.
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Absorption Filters
The figure shows the transmission curve for three
bandpass filters. 51715 passes everything except
the deep UV. It would be used to reject the
mercury 253.7 nm line. 51670 passes a band from
the near UV to the green, while 51660 passes a
band from 280 to 400 nm. These would be used
with fluorescence excitation.
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Absorption filters
The figure shows the transmission curve for two
high pass filters. 51294 rejects wavelengths
below 500 nm and passes those above. 5131
performs the same function with a cutoff
wavelength of 580 nm. Note that it is nearly
impossible to make a low pass colored glass
filter.
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Interference Filter
An interference filter is created when light is
multiply reflected between two parallel surfaces.
The beams exiting the filter must be in phase.
Only specific wavelength can pass through
The pass band varies with the order, N. To
restrict transmission to only one order, the exit
surface is most often a colored glass filter.
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Interference Filter
Center wavelengths can be from the UV to the
infrared. Pass bands are narrower than absorption
filters, but broader than grating monochromators.
The filter above is centered at 415 nm with a 10
nm FWHM and a transmission 33. As the
bandwidth narrows the transmission drops.
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Prisms
Because of a complicated mathematical
relationship between wavelength and bend angle,
prisms are seldom used in modern instruments.
For maximum dispersion (angular resolution),
visible light is separated with a glass prism and
ultraviolet is separated with a quartz prism.
One common design is the constant deviation, or
Pellen-Broca, prism shown in the figure.
Constructed from one piece of material, it can be
viewed as three separate prisms. For a given
orientation, only one wavelength exits at a 90?
deviation. To select a different wavelength the
prism is rotated about the juncture of the dotted
lines, P.
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Diffraction Grating
A diffraction grating is created by a spatial
modulation in phase or amplitude of an incoming
plane wave. When exiting the grating, the
input is separated into several plane waves
traveling at angle to the original direction of
propagation - these are called diffraction
orders. The number and intensity of the orders
depends upon the functional form of the
modulation. The figure at the top is a
transmission grating, while that at the bottom is
a reflection grating. The diffraction angle
depends upon order, m, wavelength, l, and spatial
modulation period, a.
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Blazed Gratings
By forming the bottom of the grooves into a saw
tooth shape, the angle of reflection can be made
to correspond to one of the diffraction orders.
One order then blazes, that is, it has most of
the incoming optical power. Show below are
three gratings blazed at 300, 400 and 500 nm.
Note how the efficiency drops on either side of
the blaze wavelength.
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Grating Resolution
The angular distribution of a monochromatic beam
of light reflecting off a diffraction grating is
a sinc function (sinx/x). The node spacing of
the sinc function is called the angular
resolution, Dq, where N is the number of
grating grooves and a is the groove spacing. The
goal is to minimize Dq, which is accomplished by
maximizing the grating width, Na. You can also
increase the order, which increases qm.
Grating spectral resolution is defined as,
where Dl is the range of wavelengths
occurring over Dq, and l is the center
wavelength. High resolution means small Dl.
The angle at which light leaves a grating leads
to an ambiguous wavelength, e.g. 600 nm, m 1
300 nm, m 2 200 nm, m 3.
7.3 9
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Commercially Available Gratings
The vast majority of commercially available
gratings are plastic replicas. The master
grating is coated with an epoxy resin to form a
negative image. A thin layer of plastic coats
the negative and cured. When removed from the
negative, the replica is mounted on glass for
rigidity and coated with aluminum. Ruled
gratings are made with a spacing from 25 mm-1 to
1,800 mm-1. The upper size is 15?15 cm, with 5?5
cm being typical. Concave ruled gratings can be
manufactured with great difficulty. Ruled
gratings have a scattered light figure of 10-3.
Holographic gratings are made by coating the
surface of glass with a photoresist. Two laser
beams irradiate the surface at an angle to each
other creating interference fringes. Bright
fringes polymerize the photoresist. The surface
is washed with an organic solvent to dissolve
remaining monomer, then coated with aluminum.
Spacings up to 2,400 mm-1 are possible, as well
as concave gratings. Holographic gratings have a
scattered light figure of 10-4.
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• Reverse bias creates a depletion
• layer that reduces the
• conductance of the junction
• nearly to zero
• When radiation impinges on the
• chip holes and electrons are
• formed in the depletion layer and
• those provide a current that is
• proportional to radiant power

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IC Integrated Circuit
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General Instrument Designs
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Spectronic 20 optical diagram
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1. Double- beam-in-space configuration This
requires two detectors that must be perfectly
matched
2. Double- beam-in-time configuration Sample and
reference measurements are separated in time.
Rapidly rotated mirror or chopper is used. Only
one detector is used
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General instrument designs double beam-in-space
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General instrument designs duoble beam-in-time
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Multichannel photon detector
It consists of an array of tiny photosensitive
detectors that are arranged in a pattern that all
elements of a beam of radiation that has been
dispersed by a grating can be measured
simultaneously
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Multichannel diode array spectrometer(Multichanne
l photon detector)
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Characteristics of UV/Vis Methods Wide
applicability to organic and inorganic
systems Sensitivities to 10-4 to 10-7
M Moderate to high selectivity Good accuracy,
about 1-3 relative uncertainty Ease and
convenient data acquisition
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Derivative and Dual wavelength Spectrohotometery

• The use of dual dispersing systems are arranged
  in
• such a way that two beams of slightly different
  
• wavelengths (typically 1 or 2 nm) fall
  alternatively
• onto a sample cell and its detector no
  reference
• beam is used.
• The ordinate parameter is the difference
  between the
• alternate signals, which provides a good
• approximation of the derivative of
  absorbances as a
• function of wavelength (?A/ ??).

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• Dual Wavelength spectrophotometer
• Two beams of differing wavelengths having same
• intensity are passed alternatively through a
  single
• sample cell (previous types used single-
  wavelength)
• Two monochromators are used. One monochromator
• may be used but light from the monochromator is
  
• chopped and the monochromator is shifted
  between
• the two wavelengths
• A single covette is used thus, scattering and
• instrumental stray light are cancelled

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Dual-Wavelength Spectrophotometer using two
monochromators
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Derivative Spectroscopy
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Photoacoustic Spectroscopy

• Photoacaustic spectroscopy is based on light
  absorption effect.
• This effect is observed when a gas in a closed
  cell is irradiated with a chopped beam of
  radiation of a wavelength that is absorbed by the
  gas.
• The absorbed radiation causes periodic heating of
  the gas, which in turn results in regular
  pressure fluctuations within the chamber.
• If the chopping rate lies in the acoustical
  frequency range, these pulses of pressure can be
  detected by a sensitive microphone.
• Photoacoustic or optoacoustic spectroscopy, which
  was developed in the early 1970s, provides a
  means for obtaining ultraviolet and visible
  absorption spectra of solids, semisolids, or
  turbid liquids.
• Acquisition of spectra for these kinds of samples
  by ordinary methods is usually difficult at best
  and often impossible because of light scattering
  and reflection.

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• In photoacoustic studies of solids, the sample
  
• is placed in a closed cell containing air or
  
• some other nonabsorbing gas and a sensitive
• microphone.
• The solid is then irradiated with a chopped
• beam from a monochromator.
• The photoacoustic effect is observed provided
• the radiation is absorbed by the solid the
• power of the resulting sound is directly
  related
• to the extent of absorption.
• Radiation reflected or scattered by the sample
• has no effect on the microphone and thus does
• not interfere. This latter property is perhaps
  the
• most important characteristic of the method.

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Photoacoustic Spectrometer

• Light source is modulated at an audio frequency
• The sample absorbs the radiation and becomes a
• heat source producing alternating regions of
  
• compressions and refractions in the enclosed
  gas
• that is acoustic or sound wave
• The acoustic signal is converted into electrical
  signal by microphone
• Solid sample is directly irradiated with the
  modulated source resulting in production of
• acoustic waves in the surrounding gas

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