UV instrumentation

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UV instrumentation
Abu Yousuf , PhD  Associate Professor
Department of Chemical Engineering Polymer
Science Shahjalal University of Science
Technology Sylhet-3114, Bangladesh ayousufcep_at_yah
oo.com
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• Component of spectrophotometer
• Source
• Monochromator
• Sample compartment
• Detector
• Recorder

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Radiation Sources

• Continuum Sources
• -Hydrogen and Deuterium lamp UV (high pressure
  argon, xenon or mercury gas arc lamp)
• Tungsten filament lamp VIS (350-2000nm)
• - Tungsten halogen lamp
• Line Sources
• Source emitting discrete linesmercury or
  sodium vapor lamp for UV/VIS
• Hollow cathode and discharge lamps for atomic
  absorption and fluorescence

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Types of lamp

• Tungsten filament lamp
• Advantages- inexpensive, reliable and stable
  sources of visible radiation (350-8nm),
• longer life
• Limitations- Evaporating tunhsten on the glass
  envelope reducing the output.

b) Tungsten halogen lamp Is same as tungsten
filament lamp but the filament in a halogen
fitted quartz envelope. Advantages- prevents
tungsten evaporation but more expensive than
filament lamp.
Tungsten filament lamp
Tungsten halogen lamp
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• Deuterium lamp
• A deuterium arc lamp (or simply deuterium lamp)
  is a low-pressure
• gas-discharge light source often used in
  spectroscopy when a continuous spectrum
• in the ultraviolet region is needed.
• Electrical excitation at low pressure (lt0.5 torr)
  , low voltage
• (40V DC)
• Forms molecular excited state that undergoes
  dissociation and
• Photoemission
• Provides continuum from 160-380 nm
• it requires separate power supply
• It has shorter life time and expensive to replace

Lamp operational circuit
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Line Sources in the UV and Vis
Hollow Cathode Lamp Cathode is coated with atom
of interest Tube is filled with Ar or Ne High
voltage ionizes gas, charged ions are accelerated
toward electrodes Produces sputtering of atoms
(ground and excited) Excited atoms emit light
at atomic lines Design of HCL results in
redepostion of metal atoms onto electrodes -
recycling Need to avoid excessively high
potentials Line broadening (Doppler)
Self-absorption Need separate lamp for each
element
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Wavelength Dispersion and Selection
Most instruments use a monochromator to separate
light form the source into discrete wavelength
segments Components Entrance slit
Collimating/focusing device - mirror or lens,
nonideal Dispersing device -filter, grating or
prism Collimating/focusing device - mirror or
lens Exit slit
A monochromator is an optical device that
transmits a mechanically selectable narrow band
of wavelengths of light or other radiation chosen
from a wider range of wavelengths available at
the input.
Collimate To make parallel line up.
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Diffraction grating
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Diffraction grating
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Spectroscopy Slit

• Narrow slit have to be applied to improve
  quantification and improve absorbance
• Slit below 0.14 mm is not affecting absorbance
• More important than narrow slit is reproducible
  slit settings
• Decreasing slit width causes reduction of
  radiation energy to the power of second order
• Slit have to be set up to the narrowest value
  when good resolution of spectrum is achieved.

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Sample Considerations
Several possible fates for photon Reflection
Scattering AbsorptionAbsorption Choose cell
and sample composition carefully. Match
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Detectors for UV-VIS
Photon Transducers Convert photon energy to
electrical signal (current, voltage,
etc.) Detectors based on photoelectric effect
Detectors based on photoelectric
effect Phototubes, Photomultiplier tubes
Phototube Incident photon causes release of an
electron Photocurrent a P Not best for
low-light scenarios
A photoemissive cell, commonly known as a
phototube, makes use of the photoelectric effect,
the phenomenon whereby light-sensitive surfaces
give off electrons when struck by light.
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Detectors for UV-VIS
Photomultiplier Ejected photoelectron strikes
dynode, secondary strikes dynode, secondary e-
released Voltage accelerates e- to next dynode
and so on big voltage divider Result is
large charge packet hitting anode High Gain
Dynode an electrode onto which a beam of
electrons can fall, causing the emission of a
greater number of electrons by secondary
emission. They are used in photomultipliers to
amplify the signal.
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Detectors for UV-VIS
Semiconductor-based detectors Photodiodes,
Photodiode arrays, CCD, CID A photodiode is a
type of photodetector capable of converting light
into either current or voltage, depending upon
the mode of operation
semiconductor detector
Photodiodes and Photodiode Arrays -Reverse
biased junction Photons produce e- - hole pairs
? current Current a Plight less sensitive
than PMTs but they are small and robust
When a photon strikes a semiconductor, it can
promote an electron from the valence band (filled
orbitals) to the conduction band (unfilled
orbitals) creating an electron(-) - hole() pair.
The concentration of these electron-hole pairs is
dependent on the amount of light striking the
semiconductor, making the semiconductor suitable
as an optical detector.
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Detectors for UV-VIS
Reverse Biased Junction
Forward Biased Junction
When the battery is connected as shown, the
positive terminal of the battery attracts
negative electrons away from the barrier. The
negative terminal attracts holes away from the
barrier. The insulating barrier widens and no
current flows. The junction is REVERSED
BIASED. If the reverse voltage is made high
enough, then the junction will break down and
electron current will flow from anode to cathode
(under normal conditions, current flows from
cathode to anode, when forward biased).
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Photodiode Arrays PDA Assembly of individual
photodiodes on a chip Each diode can be
addressed individually Experiment is set up so
that monochromator disperses light across PDA,
With a small of diodes per wave lengtht
allow simultaneous collection of all wavelengths
A photodiode array (PDA) is a linear array of
discrete photodiodes on an integrated circuit
(IC) chip. For spectroscopy it is placed at the
image plane of a spectrometer to allow a range of
wavelengths to be detected simultaneously. In
this regard it can be thought of as an electronic
version of photographic film. Array detectors are
especially useful for recording the full uv-vis
absorption spectra of samples that are rapidly
passing through a sample flow cell, such as in an
HPLC detector.
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Instrument Assemblies
Single Beam Instruments
The wavelength selector is a filter and the
detector is usually a photovoltaic cell or a
vacuum phototube. The source is a tungsten
halogen lamp.
Two sources are used, a tungsten halogen and a
deuterium lamp where a mechanism for source
selection should be available. The wavelength
selector is a grating or prism onochromators and
the detector is usually a vacuum phototube or a
photomultiplier tube in higher cost instruments.
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Instrument Assemblies

• UV-Vis Double Beam Spectrophotometer (in space)
•  

Two sources are existent in a double beam UV-Vis
configuration namely a tungsten halogen lamp and
a deuterium lamp and a lamp selection mechanism
is present. The beam from the source is split
into two beams by a beam splitter which is a
semipermeable mirror. The detector is usually a
pair of photomultiplier tubes connected to a
difference amplifier and the wavelength selector
is a grating or prism onochromators
UV-Vis Double Beam Spectrophotometer (in time)
Two sources are existent in a double beam UV-Vis
configuration namely a tungsten halogen lamp and
a deuterium lamp and a lamp selection mechanism
is present. The beam from the source is split
into two beams by a chopper and the transmitted
beam is passed to the detector through a beam
splitter which is a semipermeable mirror. The
detector is usually a photomultiplier tube and
the wavelength selector is a grating or prism
onochromators.
Time separated double beam
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Thanks to all