Absorption Spectrometry

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Absorption Spectrometry
Particularly UV-Visible
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• One difference between certain compounds is their
  colour.
• Quinone is yellow
• Chlorophyll is green
• 2,4-dinitrophenylhydrazone derivatives of
  aldehydes and ketones range in colour from bright
  yellow to deep red, depending on double bond
  conjugation
• Aspirin is colourless.

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• The human eye is functioning as a spectrometer
  analyzing the light reflected from the surface of
  a solid or passing through a liquid.
• We see sunlight (or white light) as uniform or
  homogeneous in color, it is actually composed of
  a broad range of radiation wavelengths in the
  ultraviolet (UV),
• visible and infrared (IR) portions
• of the spectrum.

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Violet   400 - 420 nm Indigo   420 - 440 nm Blue   440 - 490 nm Green   490 - 570 nm Yellow   570 - 585 nm Orange   585 - 620 nm Red   620 - 780 nm
Violet   400 - 420 nm Indigo   420 - 440 nm Blue   440 - 490 nm Green   490 - 570 nm Yellow   570 - 585 nm Orange   585 - 620 nm Red   620 - 780 nm                                                                                                 
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Absorption of 420-430 nm light renders a
substance yellow, and absorption of 500-520 nm
light makes it red.
Green is unique in that it can be created by
absorption close to 400 nm as well as absorption
near 800 nm.
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• Wavelength is the distance between adjacent peaks
  (or troughs), in meters, centimeters or
  nanometers (10-9 meters).
• Frequency is the number of wave cycles that
  travel past a fixed point per unit of time, and
  is usually in cycles per second, or hertz (Hz).
• Visible wavelengths range from 400 to 800 nm.
  The longest visible wavelength is red and the
  shortest is violet. Other common colors of the
  spectrum, in order of decreasing wavelength, may
  be remembered by the mnemonic ROY G BIV.

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The energy associated with a given segment of the
spectrum is proportional to its frequency. The
energy carried by a photon of a given wavelength
of radiation                                  
                                                  
                                                  
                                                  
                                                  
          

• ?? h?
• h Plancks constant 6.6x10-34 J.sec
• ? c/? ? frequency, ? wavelength,
• c speed of light 3 x 108 m/sec
• Frequency remains constant
• Wavelength and the speed of light change with the
  medium

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Refractive index c/v

• medium n
• air 1.0003
• Water 1.333
• 50 sucrose in water 1.420
• carbon disulfide 1.628
• crystalline quartz 1.544 (no)
• 1.553 (ne)
• diamond 2.417

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Polarized electromagnetic wave
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Carvone
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• Each photon of light has a distinct energy
• E h?
• Causes transitions between quantized energy
  states in atoms, molecules etc.
• Absorption
• Emission
• Scattering

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Absorption
Only if energy states differ by h? Other
frequencies pass through Measure decrease in P at
each frequency
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Emission

• Chemical species can be excited by
• Thermal
• Chemical
• Electrical energy.
• If the subsequent relaxation to the ground state
  results in the release of light
• This is emission

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Luminescence

• When energy is absorbed the chemical species are
  excited.
• The excited species will have a limited lifetime
• They will relax lose the excess energy- and
  return to the ground state.
• If the excitation is by light and light is
  emitted upon relaxation you have luminescence
  fluorescence or phosphorescence.
• Incoming beam is unidirectional, luminescence is
  emitted in all directions

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Why is the sky blue?
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Light passes more molecules when coming from the
horizon so some is scattered away and sky is
very pale blue
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WHY IS THE SUNSET RED?

• As the sun sets, light must travel farther
  through the atmosphere before it gets to you.
• More of the light is reflected and scattered.
• As less reaches you directly, the sun appears
  less bright. The color of the sun appears to
  change, first to orange and then to red. This is
  because even more of the short wavelength blues
  and greens are now scattered. Only the longer
  wavelengths are left in the direct beam that
  reaches your eyes.

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Rayleigh Scattering
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Mie Scattering
For particle sizes larger than a wavelength, Mie
scattering predominates. Mie scattering is not
strongly wavelength dependent and produces the
almost white glare around the sun when a lot of
particulate material is present in the air. It
also gives us the white light from mist and fog.
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Tyndall effect
When a very dilute dispersion of small
particles or droplets is viewed directly against
an illuminating light source it may appear to be
transparent. In contrast, when the same
dispersion is viewed from the side (at a right
angle to the illuminating beam), and against a
dark background, the dispersion may appear turbid
and blue-white. The scattered light is due to
Tyndall scattering and the optical effect is
referred to as the Tyndall effect.
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Raman Scattering

• Like Rayleigh scattering, Raman scattering
  depends upon the polarizability of molecules.
• The incident photon can excite vibrational
  modes of the molecules, yielding scattered
  photons which are diminished in energy by the
  amount of the vibrational transition energies.
• Thus the scattered light is at lower energy than
  the incoming light.
• Occurs with particles much smaller than
  wavelength of light

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An application of Raman

• The scattering produced by a laser beam directed
  on the plume from an industrial smokestack can be
  used to monitor the effluent for molecules which
  will produce recognizable Raman lines.
• We will see some Raman scattering as an
  interference in our fluorescence spectra.

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• Turbidity is a critical water quality parameter
• many applications, from drinking water to
  ultrapure processes.

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Turbidimetry
Standards formazin (insoluble polymer)
Turbidimeter- measures the amount of radiation
that passes forward Nephelometer measures
scattered radiation (good if low turbidity) Some
instruments use the ratio of these two
measurements
Turbidity is a critical water quality parameter
in many applications, from drinking water to
ultrapure processes.
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Electronic transitions- generally from HOMO to
LUMO
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• Unsaturated functional groups that absorb UV or
  visible light are called chromophores
• Single C-C bonds hold their electrons too tightly
  for transitions to occur.
• But not so C-S, C-Br, C-I
• The energy absorbed from the UV is comparable to
  some bond energies some bonds can be broken
  called photolysis

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was used to color the robes of the royal and
wealthy
widely distributed in plants, but is not
sufficiently stable to be used as permanent
pigment, other than for food coloring
A common feature of all these colored compounds,
displayed below, is a system of extensively
conjugated pi-electrons.
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To Longer Wavelength Bathochromic
To Shorter Wavelength Hypsochromic
To Greater Absorbance Hyperchromic
To Lower Absorbance Hypochromic
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Absorption Analysis

• Usually done on liquids, but gases can also be
  analyzed

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1,2,4,5-tetrazine
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Why are the bands so broad?

• Within each electronic state there are numerous
  vibrational states. At room temperature, Molecule
  is in lowest vibrational state.
• But it can excite into a variety of vibrational
  levels (and rotational levels within each)
• In liquids additional broadening occurs because
  of collisions with the solvent which further
  reduce the lifetime of the excited state. (Short
  lifetime, broad peak Heizenberg uncertainty
  principle)

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Advantages of UV-visible absorption

• Moderately low LOD
• 10-7 to 10-6 M
• Two types of applications
• Trace
• Micro
• Compared to gravimetric-rapid and convenient
• Low cost
• Can automate (increases instrument cost)

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Absorption
Only if energy states differ by h? Other
frequencies pass through Measure decrease in P at
each frequency
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Beer-Lambert law

• Extent of absorption depends on number of
  encounters between photons and absorbing species.
• P0 power of incident radiation
• b pathlength of cell

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Percent Transmittance

• A log P0/P -log T
• A log 1/T
• T 100 T
• Ranges from 0 to 100
• A log 100/T

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Deviations from Beers law

• Instrumental
• Non-monochromatic light
• negative deviation at high conc.
  
• Wide slits give lower A values
• Stray light
• a) Reflections
• b) Higher orders
• c) slit diffraction
• All cause negative deviation

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Real Deviations

• Refractive index increases with concentration
  at high concentrations there is a negative
  deviation
• At concentrations gt 0.01 M, each molecule affects
  the charge distribution of its neighbour
• This can alter the ability of the molecules to
  absorb light.
• Can also occur with high concs of surrounding
  electrolyte

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Chemical Deviations Equilibria - acid base pH
control Activity coef. Temperature Solvent
effects
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Equilibrium
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Analysis of two component sample
If the components have absorptions that do not
overlap, then measurement of each can be done
independently of the other.
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Analysis of two-component sample

• Beers law is additive
• The total absorbance will equal the sum of the
  absorbances
• If two compounds, x and y, absorb at different
  enough wavelengths
• At ??1, A1 (Ax)1 (Ay)1
• ?x1 x ?y1y (assuming
  b1cm)
• At ??2, A2 (Ax)2 (Ay)2
• ?x2 x ?y2 y

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• Determine the molar absorptivities (?) for x and
  y for each wavelength, ??1 and ??2
• Then solve the simultaneous equations

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If there is a lot of overlap between spectra

• Am ?xbx ?yby at any wavelength
• Axs ?xbxs
• Ays ?ybys

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The absorbances of the standards and mixtures are
measured at a variety of wavelengths. The data
at a particular wavelength will give one point on
the graph. Thus measurements need to be done at
at least 5 wavelengths
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