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

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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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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!

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• 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