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Introduction to Spectrochemical Analysis

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Title: Introduction to Spectrochemical Analysis


1
  • Introduction to Spectrochemical Analysis
  • One of the earliest means of characterizing
    matter was based on the observation of color
  • The explanation of the origin of color - and the
    way light interacts with matter - has given
    more information about the basic structure of
    matter than probably any other kind of experiment
  • The color of matter results from the absorption,
    luminescence and emission of light by by matter
  • Since a fundamental property of matter is how it
    interacts with light, it is a simple extension
    to see how the analysis of chemical species can
    be accomplished by the examination of how light
    interacts with matter
  • An important subdiscipline of analytical
    chemistry is spectrochemical analysis
  • Spectrochemical analysis can be defined as the
    use of the spectrum of electromagnetic radiation
    in qualitative and quantitative analysis
  • A spectrum is a plot of some function of the
    frequency of electromagnetic radiation vs...
    the frequency of the radiation

2
Introduction to Spectrochemical Analysis A
spectrum is a plot of some function of the
frequency of electromagnetic radiation vs.. the
frequency of the radiation
  • Qualitative information is obtained from the
    shape of the spectrum
  • Quantitative information is obtained from the
    magnitude of f(n) a particular frequency

f(n)
l n
  • Consider why these kinds of information can
    obtained from a spectrum
  • Electromagnetic radiation is made up of packets
    containing a fixed amount of energy
  • These energy packets can be absorbed by matter to
    cause transitions between states of matter
    having fixed energy differences equal to the
    energy of the light packets
  • Light packets can be produced when matter
    undergoes transitions from a higher energy state
    to a lower energy state
  • Different species have unique sets of energy
    levels producing unique absorption or emission
    spectra
  • The spectrum becomes a map of the energy states
    of a sample of matter giving qualitative
    information

3
  • Introduction to Spectrochemical Analysis
  • Consider why
  • Furthermore, the number of times each transition
    occurs per unit time is related to the
    transition probability for each transition which
    is both of qualitative and quantitative utility
  • f(n) can be related to the quantity of matter
    present in a sample
  • Examine a figure representing the various parts
    of the electromagnetic spectrum
  • Frequencies range from the audio - 10-3 Hz - to
    g-rays - 1022 Hz
  • Different frequency ranges cause different kinds
    of transitions in matter
  • Microwaves cause changes in rotational energy
    levels for molecules having permanent dipole
    moments
  • Infrared radiation causes changes in the
    vibrational states of molecules when the
    vibration causes a change in dipole moment
  • UV-Visible-Near IR cause transitions between the
    electronic states of molecules
  • X-rays cause the ionization of matter
  • g-rays involve nuclear energy transitions
  • When matter is placed in a magnetic field
  • Radio waves can cause nuclear spin transitions -
    NMR
  • Microwaves can cause electron spin transitions -
    ESR

4
  • Introduction to Spectrochemical Analysis
  • Examine a figure representing
  • Different parts of the electromagnetic spectrum
    make use of different kinds of instrumentation
    to produce and examine radiation
  • Defining terms for electromagnetic radiation
  • Wave Nature of Light
  • One viewpoint about light is that it has a wave
    nature
  • There is an oscillating electric field
    perpendicular to an oscillating magnetic field
  • Both fields oscillate perpendicular to the
    direction of travel
  • The wave travels through a vacuum at velocity c
    3.00 x 108 m/s
  • The wavelength, l,is the distance between two
    adjacent amplitude maxima
  • The frequency, n, of light is the number of
    oscillations the wave makes in 1 s.
  • nl c
  • The period is the time it takes a full
    wavelength to pass a given point in space

5
  • Introduction to Spectrochemical Analysis
  • Defining terms for electromagnetic radiation
  • The wavelength, l, is expressed in units
    appropriate to the region of the electromagnetic
    spectrum under examination
  • meters for radio waves
  • centimeters for microwaves
  • micrometers for infrared
  • nanometers for UV-Visible-Near Infrared
  • nanometers and picometers for x-ray
  • The frequency is usually expressed as
  • In the IR, frequency is usually expressed as
  • Spectral purity is the width of a band of
    electromagnetic radiation, usually measured as
    frequency or wavelength at the half- amplitude
  • The radiant power (P) of electromagnetic
    radiation is a measured quantity which is
    proportional to the square of the amplitude
  • This is the amount of energy transmitted by a
    beam of light per unit time

6
  • Introduction to Spectrochemical Analysis
  • Defining terms for electromagnetic radiation
  • Sometimes the amplitude is called intensity
  • Rigorously, intensity is defined as the radiant
    power from a point source per unit solid angle
    and has units of watt/sterradian
  • A beam of radiation can have intensity if it
    originates from a point source
  • Cannot talk about the intensity of a columnated
    beam of radiation which is the usual form of
    radiation used in spectroscopy
  • One classification of the kinds of interactions
    light has with matter
  • Absorption occurs if a beam of radiation is
    attenuated in radiant power if the photons in
    the beam have energies that match energy level
    differences of states in the matter
  • The mechanism of the interaction depends on the
    nature of the phenomenon responsible for the
    absorption
  • The attenuation is related to the number of
    absorbing species and the photon flux

7
  • Introduction to Spectrochemical Analysis
  • One classification of the kinds of interactions
    light has with matter
  • Luminescence is the production of light by a
    sample of matter that as absorbed light
  • When the sample of matter absorbs light it is
    placed in a higher energy state
  • This energy will be lost by the sample, usually
    in the form of heat
  • Often the energy is lost by production of photons
    whose energy matches the difference in energy
    between the excited state and the lower state of
    involved in the transition
  • Fluorescence involves an electronic transition in
    which the electron spin is not changed
  • If the fluorescent photons have the same energy
    as the exciting photons the luminescence is
    called resonance fluorescence
  • Phosphorescence involves an electronic transition
    in which the spin multiplicity changes for the
    transition
  • Luminescence is isotropic whereas absorption is
    directional
  • Luminescence is useful for both qualitative and
    quantitative analysis

8
  • Introduction to Spectrochemical Analysis
  • One classification of the kinds of interactions
    light has with matter
  • Emission results in the production of light by a
    sample of matter that has been excited to a
    higher energy state by a means other than the
    absorption of light
  • The excitation can come from thermal or
    electrical sources or by particle collision
  • Atoms, Molecules or ions can be caused to emit
    electromagnetic radiation
  • The emitted radiation provides light sources for
    general lighting but also as sources of
    radiation for analytical chemistry
  • The light emitted by the sample is characteristic
    of the sample and so can be used to characterize
    the sample qualitatively and quantitatively

9
  • Introduction to Spectrochemical Analysis
  • One classification of the kinds of interactions
    light has with matter
  • Scattering of light does not involve the
    absorption of light by transitions between
    quantized energy states
  • Appears to be a randomization in the direction of
    the beam of radiation passing through a sample
    of matter
  • Rayleigh scattering involves scattering of light
    by particles small in dimensions compared to
    the wavelength of incident light
  • As the light beam passes near small particles an
    intense disturbance in the particle is caused by
    the oscillating electro-magnetic fields of the
    radiation
  • If the particle is polarizable
  • The oscillating fields of the light can perturb
    the electron cloud of the particle causing a
    charge separation
  • The individual charges will oscillate as the
    fields of the radiation change polarity
  • The oscillating dipoles in the particle produce
    oscillating fields at the same frequency as the
    light
  • The particle thus becomes a source of light which
    is isotropic
  • The intensity is
  • Explains why the sky is blue!

10
  • Introduction to Spectrochemical Analysis
  • One classification of the kinds of interactions
    light has with matter
  • Mie scattering involves particles that are large
    compared to the wavelength of incident
    radiation
  • Only part of the particle is set into oscillation
  • Produces light having the same wavelength as the
    incident radiation
  • The scattered light has a non-uniform angular
    distribution in radiant power
  • The angular distribution of the scattered
    radiation can be used to obtain the size and
    shape of the particle causing the scattering
  • Mie scattering is important in turbidimetry and
    nephelometry
  • Raman and Brillouin scattering produce a shift in
    the frequency of the scattered radiation
    relative to the frequency of the incident light
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