Principal application and instrumentation of UV visible spectroscopy

1. UV-VISIBLE
SPECTROPHOTOMETER
INSTRUMENTATION

2. Components of UV-Visible
spectrophotometer
• Source
• Filters & Monochromator
• Sample compartment
• Detector
• Recorder

5. LIGHT SOURCES
Various UV radiation sources are as follows
a. Deuterium lamp
b. Hydrogen lamp
c. Tungsten lamp
d. Xenon discharge lamp
e. Mercury arc lamp
Various Visible radiation sources are as follow
a. Tungsten lamp
b. Mercury vapour lamp
c. Carbonone lamp

7. For Visible region
Tungsten filament lamp
• Use for region 350nm to 2000nm.
• These measure most effectively in the
visible region from 320 - 1100 nm
• Instruments that only use Tungsten
halogen lamps as the light source will
only measure in the visible region.

8. For ultra violet region
Hydrogen discharge lamp
• Consist of two electrode contain in Hydrogen filled silica
envelop.
• Gives continuous spectrum in region 185-380nm. above
380nm emission is not continuous

9. Deuterium lamps:
• Deuterium arc lamps measure in the UV region 190 - 370
nm
• As Deuterium lamps operate at high temperatures, normal
glass housings cannot be used for the casing. Instead, a
fused quartz, UV glass, or magnesium fluoride envelope is
used.
• When run continuously typical lamp life for a Deuterium
lamp is approximately 1000 hours, however this can be
extended by up to a factor of three using PTR technology.
• Deuterium lamps are always used with a Tungsten halogen
lamp to allow measurements to be performed in both the UV
and visible regions.

10. TUNGSTEN LAMP DUETERIUM LAMP

11. Filters &Monochromator
The Monochromator/Filter will select a narrow portion of the
spectrum (the band pass) of a given source
• FILTERS ARE OF TWO TYPES:
Absorption Filters
Interference Filters
MONOCHROMATORS ARE OF TWO TYPES:
Refractive type
PRISM TYPE
Reflective type
Diffraction type
GRATING TYPE
Transmission Type

12. FILTERS
ABSORPTION FILTERS
Absorption filters, commonly manufactured from dyed glass or
pigmented gelatin resins
Band widths are extremely large {30 – 250 nm}
Combining two absorbance filters of diffeternt λmax

13. • The most common type of gelatin filter is constructed by
sandwiching a thin layer of dyed gelatin of the desired colour
between two thin glass plates.

14. FILTERS

15. INTERFERENCE FILTERS:
• These are used to select
wavelengths more accurately by
providing a narrow band pass
typically of around 10nm
• These filters rely on optical
interference (destructive wave
addition) to provide narrow
bands of radiation.

17. • Interference filter consists of a dielectric spacer film made up of CaF2,
MgF2 between two parallel reflecting films.
• As light passes from one medium to the other the direction and wavelength
of light can be changed based on the index of refraction of both mediums
involved and the angle of the incident and exiting light (For more look
at Snell’s law).
• Due to this behaviour, constructive and destructive interference can be
controlled by varying the thickness (d) of a transparent dielectric material
between two semi-reflective sheets and the angle the light is shined upon
the surface.
• As light hits the first semi-reflective sheet, a portion is reflected, while the
rest travels through the dielectric to be bent and reflected by the second
semi-reflective sheet.
• If the conditions are correct, the reflected light and the initial incident light
will be in phase and constructive interference occurs for only a particular
wavelength

18. MONOCHROMATORS
• PRISM MONOCHROMATOR:

19. Refractive type prism Monochromator:
• It consists of entrance slit, collimator lens, Prism, Focusing
lens, Exit slit.

20. Reflective type prism Monochromator or Littrow type
mounting :

21. GRATING MONOCHROMATOR
• Gratings are rulings made on glass, Quartz or alkyl
halides
• Depending upon the instrument no. of rulings per
mm defers
• If it is UV-Visible no. of gratings per mm are more
than 3600.

22. DIFRACTION TYPE GRATING:

24. • The mechanism is that diffraction produces reinforcement.
• The rays which are incident on the grating gets reinforced with the
reflected rays and hence resulting radiation has wavelength
governed by equation
m λ =b(sin I + sin r)
m – order
λ – desired wavelength
b – grating spacing
i – angle of incidence
r - angle of diffraction

25. TRANSMISSION TYPE GRATING:
• It is similar to diffraction grating, but refraction produces
instead of reflection
• Refraction produces reinforcement
• The wavelength of radiation produced by transmission grating
can be expressed by
d sin Φ
λ = d = 1/lines per cm
m

26. SAMPLE COMPARTMENT
• Spectroscopy requires all materials in the beam path other than the
analyte should be as transparent to the radiation as possible.
• The geometries of all components in the system should be such as to
maximize the signal and minimize the scattered light.
• The material from which a sample cuvette is fabricated controls the
optical window that can be used.
• Some typical materials are:
• Optical Glass - 335 - 2500 nm
• Special Optical Glass – 320 - 2500 nm
• Quartz (Infrared) – 220 - 3800 nm
• Quartz (Far-UV) – 170 - 2700 nm •

27. DETECTORS
• After the light has passed through the sample, we want to be able to
detect and measure the resulting light.
• These types of detectors come in the form of transducers that are
able to take energy from light and convert it into an electrical signal
that can be recorded, and if necessary, amplified.
• Three common types of detectors are used
 Barrier layer cells
 Photo emissive cell detector
 Photomultiplier

28. BARRIER LAYER CELL or PHOTO VOLTAI CELL
• It cosistitutes
 The steel support plate 'A'
 layer of metallic selenium 'B', which is a few hundredths of a
millimetre in thickness.
 'C' is a thin transparent electrically-conductive layer, applied by
cathodic sputtering.

30. • It consists of a metallic base plate like iron or aluminium
which acts as one electrode.
• On its surface, a thin layer of a semiconductor metal like
selenium is deposited.
• Then the surface of selenium is covered by a very thin layer of
silver or gold which acts as a second collector tube.
• When the radiation is incident upon the surface of selenium,
electrons are generated at the selenium- silver surface and the
electrons are collected by the silver.
• This accumulation at the silver surface creates an electric
voltage difference between the silver surface and the basis of
the cell.

31. Photo emissive cells Detector
• Phototubes are also known as photo emissive cells.
• A phototube consists of an evacuated glass bulb.
• There is light sensitive cathode inside it.
• The inner surface of cathode is coated with light sensitive
layer such as potassium oxide and silver oxide.
• When radiation is incident upon a cathode, photoelectrons are
emitted.
• These are collected by an anode.
• Then these are returned via external circuit. And by this
process current is amplified and recorded.

33. The photomultiplier tube
• The photomultiplier tube is a commonly used detector in UV
spectroscopy.
• It consists of a photo emissive cathode (a cathode which emits
electrons when struck by photons of radiation), several dynodes
(which emit several electrons for each electron striking them) and an
anode.
• A photon of radiation entering the tube strikes the cathode, causing
the emission of several electrons.
• These electrons are accelerated towards the first dynode (which is
90V more positive than the cathode).
• The electrons strike the first dynode, causing the emission of several
electrons for each incident electron.

34. • These electrons are then accelerated towards the second
dynode, to produce more electrons which are accelerated
towards dynode three and so on.
• Eventually, the electrons are collected at the anode.
• By this time, each original photon has produced 106 - 107
electrons.
• The resulting current is amplified and measured.
• Photomultipliers are very sensitive to UV and visible radiation.
• They have fast response times.
• Intense light damages photomultipliers;
• they are limited to measuring low power radiation.

36. Single beam spectrophotometers

37. Double beam spectrophotometers