x ray fluorescence

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X-ray fluorescence

• 
  By
  Prateek
• 
  Instrumentation 3rd
  year

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Fluorescence

• What is Fluorescence ?
• Many solutions when irradiated with visible or
  ultraviolet light, not only absorb this light ,
  but re-emit light of a different wavelength. This
  effect is known as fluorescence.

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Principle of fluorescence

• The ground state of molecules, molecular energies
  are constant and at minimum values.
• When a quanta of light impinges on a molecule,
  it is absorbed and an electronic transition to a
  higher electronic state take place.
• In a ground state of most molecules, each
  electron in the lower energy levels is paired
  with another electron, whose spin is opposite to
  its own spin. This state is called a singlet
  state.

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Principle of Fluorescence

• When the molecules absorb energy, they are raised
  to an energy level of an upper excited state s1
  or s2 or s3.
• These singlet transition are unstable and the
  molecular energy tends to revert immediately to
  a lower level.
• In going to a lower energy level, some energy
  will be lost responsible for visible
  ultraviolet absorption spectra.

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Principle of Fluorescence

• The excited singlet state persist for a time ,
  which is of order of 10 -8 to 10 -4 s.
• During this time interval, some energy in excess
  of the lowest vibrational energy level is rapidly
  dissipated.
• In case all the excess is not further dissipated
  by collisions with other molecules , the electron
  would return to the ground state with the
  emission of energy. This phenomenon is called
  fluorescence.

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Characteristic of fluorescent molecule

• Fluorescent molecule has two characteristic
  spectra
• The excitation spectrum
• The emission spectrum
• The excitation spectrum shows the relative
  efficiency of different wavelengths of exciting
  radiation emitted to cause fluorescence.
• The shape of the excitation spectrum should be
  identical to that of the absorption spectrum of
  the molecule.

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Characteristic of fluorescent molecule

• The emission spectrum would indicate the relative
  intensity of radiation emitted at various
  wavelengths.
• The shape of emission spectrum is independent of
  the wavelength of exciting radiation.

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Relationship between conc. fluorescence
intensity

• The total fluorescence intensity is equal to the
  rate of light absorption multiplied by the
  quantum efficiency of fluorescence(n),that is
• Fn(Po -P)_________________(
  1)
• Po power of the beam incident upon the
  solution
• Ppower after traversing a length b of the
  medium

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Relationship between conc. fluorescence
intensity

• Acc. To beers law
• P/Po ??
  -?????? _________________(2)
• ???????????? ???????????????????????? of the
  fluorescent molecule
• ?????????????????????????? ??
• (1-P/P0)1- ??
  -??????
• (Po -P)Po (1- ??
  -?????? )_______________(3)
• Substituting eq(3).in eq(1)
• Fn Po (1- ??
  -?????? )
• Fk . n .
  P(2.3??????)
• FK Cm

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Measurement of fluorescence

• Fluorescence measurement are carried out in
  instruments called fluorometers or fluorimeters.
• The various components of these instruments are
  similar to photometers.
• Fluorescence measuring instruments making use of
  monochromators like spectrophotometers are called
  spectrofluorimeters.

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Single-beam filter fluorimeter

• The source of light is a high pressure mercury
  discharge lamp, with glass or fused silica
  envelope.
• Selection of the exciting wavelength is made by
  inserting a primary filter in the incident beam.
• Low pressure lamps, equipped with a silica
  envelope can produce intense radiation at 254um.

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Single-beam filter fluorimeter

• The fluorescence emitted by the sample is
  measured by selecting the wavelength of
  fluorescence radiant energy by a second filter,
  called secondary filter.
• This is placed between the sample and a photo
  detector located at s 90 ?????????? from the
  incident optical path.
• The fluorescent signal is generally of low
  intensity require large amplification for
  carrying out measurement.
• The detector used in fluorescence measurements
  are the sensitive photomultiplier tubes.

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Diagram of single beam fluorimeter
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Double beam filter fluorimeter

• Double-beam instruments are direct reading type
  and are generally preferred because of this
  several advantages.
• In this instruments, the circuit is so arranged
  that the current from the reference detector
  opposes that generated by the measuring
  photocell.
• The reading on the calibrated scale, at which the
  two currents neutralise each other directly gives
  the fluorescence reading.

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Double beam filter fluorimeter

• A mercury lamp is used as a source of radiation.
• A collimated beam is passed through the primary
  filter, which then falls on a cuvette containing
  the sample.
• The measuring photocell is mounted at right
  angles to the excitation beam.
• Fluorescent radiation passes through secondary
  filters, which remove scattered ultraviolet
  radiation, but pass visible light.

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Double beam filter fluorimeter

• The output of the reference the measuring
  photocells is compared and a sensitive
  galvanometer is used to check the null point.
• The circuit is arranged, so that the current from
  the reference cell opposes that generated by the
  measuring photocell, which respond to fluorescent
  radiation.

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Diagram of double beam fluorimeter
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• Thanks