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An introduction to Ultraviolet/Visible Absorption Spectroscopy


An introduction to Ultraviolet/Visible Absorption Spectroscopy Lecture 24 Instrumentation Light source - selector Sample container Detector Signal processing Light ... – PowerPoint PPT presentation

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Title: An introduction to Ultraviolet/Visible Absorption Spectroscopy

An introduction to Ultraviolet/Visible Absorption
  • Lecture 24

  • Instrumentation
  • 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
  • D2 Ee ? D2 ? D D h?

Deuterium lamp UV region
  • (a) A deuterium lamp of the type used in
    spectrophotometers and (b)
  • its spectrum. The plot is of irradiance E?
    (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.

Visible and near-IR region
  • 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.

  • 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.

  • 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.

Tungsten lamps (350-2500 nm)
  • Why add I2 in the lamps?
  • W I2 ? WI2
  • Low limit 350 nm
  • Low intensity
  • Glass envelope

  • 3. Xenon Arc Lamps
  • Passage of current through an atmosphere of high
    pressured xenon excites xenon and produces a
    continuum in the range from 200-1000 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.

  • Instrumental Components
  • Source
  • ? - selector (monochromators)
  • Sample holders
  • Cuvettes (b 1 cm typically)
  • Glass (Vis)
  • Fused silica (UVVis)
  • Detectors
  • Photodiodes
  • PMTs

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 calcd)

  • Most common spectrophotometer Spectronic 20.
  1. On/Off switch and zero transmission adjustment
  2. Wavelength selector/Readout
  3. Sample chamber
  4. Blank adjustment knob
  5. Absorbance/Transmittance scale

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  • End view of the exit slit of the Spectronic 20
  • spectrophotometer pictured earlier

  • Single-Beam Instruments for the
    Ultraviolet/Visible Region

  • Single-Beam Computerized Spectrophotometers

Inside of a single-beam spectrophotometer
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

General Instrument Designs Double Beam In - Space
Needs two detectors
General Instrument Designs Double Beam In - Time
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  • Merits of Double Beam Instruments
  • Compensate for all but the most short term
    fluctuation in radiant output of the source
  • Compensate drift in transducer and amplifier
  • Compensate for wide variations in source
    intensity with wavelength

Dual Beam Instruments