Introduction to Spectrochemical Methods

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Introduction to Spectrochemical Methods

• Chapter 7

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Introduction

• Spectrochemical methods
• Absorption or emission of light
• More than half or all instrumental methods of
  analysis
• Spectroscopy or spectrometry
• Science that deals with light
• Its absorption and emission by solutions
• Other material substances
• Instrument used
• Spectrometer
• When using a light sensor/phototube
  spectrophotometer
• Spectrochemical analysis
• Degree which light absorbed or the primary light
  emitted
• Related to the amount of analyte present in the
  sample
• Critical measurements!
• Atomic spectroscopy
• Spectral differences between atoms
• Molecular spectroscopy

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Characterizing Light

• Dual nature of light
• Particles
• Photons or quanta
• Particle theory of light
• Waves
• Electromagnetic disturbances or electromagnetic
  waves
• Wave theory of light
• Dual nature not unlike modern description of
  electrons
• Described as particles
• To explain aspects of their behavior
• For more accurate description
• Must be described as entities of energy and NOT
  particles

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Characterizing Light

• Wave theory of light
• Light travels in a fashion similar to that of a
  series of repeating waves of water
• Wave pool at an amusement park
• Electromagnetic waves
• They are wave disturbances that have an
  electrical component and a magnetic component
• Do NOT require matter to exist
• Can travel through a vacuum

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Characterizing Light

• Wavelength, speed, frequency, energy, and
  wavenumber
• Wavelength (?)
• The physical distance from a point on one wave,
  to the same point on the next wave
• Measured in metric units

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Characterizing Light

• Wavelength, speed, frequency, energy, and
  wavenumber
• Speed of light (c)
• Speed in which electromagnetic waves move
• Speed of light in a Vacuum 3.00 x 1010 cm/sec
• Accounts for instantaneous speed that light fills
  a room when switch turned on
• ALL ELECTORMAGNETIC WAVES TRAVEL AT THE SAME
  SPEED IN A VACUUM REGARDLESS OF THEIR WAVELENGTH

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Characterizing Light

• Wavelength, speed, frequency, energy, and
  wavenumber
• Frequency (? nu)
• Number of moving electromagnetic waves past a
  fixed point in 1 second
• Expressed in waves or cycles per second
• hertz (Hz)
• Units sec-1
• Wavelength, speed, frequency can be expressed
  mathematically
• C ?? units are cm x sec-1 or cm/sec
• Wavelength and frequency are inversely
  proportional
• As one increases
• Other decreases

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Characterizing Light

• Wavelength, speed, frequency, energy, and
  wavenumber
• Energy
• B/c light is a form of energy
• Each wavelength or frequency has certain amount
  of energy
• Considered to be the energy associated with a
  single photon of light
• . particle theory and wave theory linked via
  energy
• E h?
• E energy, h proportionality constant called
  Plancks constant
• depends on units used-metric 6.63 x 10-34
  J/sec
• . E hc/?

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Characterizing Light

• Wavelength, speed, frequency, energy, and
  wavenumber
• Wavenumber (?)
• Wavelength expressed in centimeters
• Characterized by the reciprocal of this
  wavelength
• ? 1/ ? (cm) Used in conjunction with infrared
  light.

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The Electromagnetic Spectrum
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The Electromagnetic Spectrum

• Electromagnetic spectrum
• So broad broken down into regions
• Visible light
• That portion of the spectrum we see with our eyes
• 350 nm to 750 nm
• Very narrow region
• UV, infrared, x-ray, radio, and television

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The Electromagnetic Spectrum

• Electromagnetic Spectrum
• UV, visible, and infrared regions
• Mostly ones emphasized
• Nanometer and micrometer units used for
  wavelength
• Something to remember
• Long wavelength low energy
• Infrared region
• Wavelengths extremely short
• Have higher energy than radio or television
• Cause no harm
• Remotes for TVs, VCRs, etc
• UV, x-rays, and gamma rays
• Very short wavelengths
• Very high energy
• Very dangerous!

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Interaction of Light With Matter

• Light striking matter causes different events
• Transmitted
• Pass without interaction through the material
• Light passing through glass
• Reflected
• Changes directions
• Light in a mirror
• Scattered
• Deflected into many different directions
• Occurs when light strikes a substance composed of
  many individual, small particles
• Absorbed
• Light fives up some or all of its energy to the
  material

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Absorption Spectra

• Instruments used to measure absorption
• Some in the UV and visible regions
• Others for the infrared region
• Methods used
• Beam of light formed
• Sample measured contained so that light passes
  through
• Absorption of the wavelengths present in light
  beam measured by a sensor and signal processor

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Absorption Spectra

• An absorption spectrum
• Plot of the amount of light absorbed by a sample
  vs. the wavelength of the light
• Light absorbed called the absorbance (A)
• Obtained by using a spectrometer to
• Scan a particular wavelength region
• To observe amount of light absorbed by the sample
  along the way
• Its a continuous spectrum (fig. 7.13, pg. 189)
• The spectrum is an unbroken pattern
• Does not display breaks or sharp peaks

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Absorption Spectra

• Absorption vs. wavelength
• Can be displayed as a transmission spectrum
• Plotting the amount of light transmitted by a
  sample
• Rather than the light absorbed
• y-axis is transmittance (T) or percent
  transmittance instead of the absorbance (fig.
  7.16, pg. 191)
• High transmittance low absorbance and vice
  versa
• Absorption pattern
• Differs from compound to compound
• molecular fingerprint
• Often useful for identification
• Detecting impurities
• Other sample components

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Absorption Spectra

• Light Emission
• Matter will emit light
• Molecular and ionic analytes
• Useful for qualitative and quantitative analysis
• Called an emission spectrum
• Plot of emission intensity vs. wavelength
• Product of the change in the energy level of an
  electron
• From excited state to ground state (lowest energy
  level)
• Fluorescence
• When molecules/complex ions emit light under
  certain conditions
• When absorption of light in the UV region is
  followed by emission of light in the visible
  region
• Involves the loss in energy from an excited state
  to a lower state

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Absorbance, Transmittance, and Beers Law

• A ebc
• Beers Law
• e extinction coefficient or absorptivity
• Units depends on other parameters
• Absorbance is dimensionless quantity
• b path length
• Distance the light travels through the measured
  solution
• Inside diameter of the sample container
• Usually centimeters or millimeters
• c concentration
• Expressed in any concentration unit
• Usually expressed in molarity, ppm, or
  grams/100ml.
• . c molarity, b cm, then absorptivity L
  mol-1 cm-1

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Absorbance, Transmittance, and Beers Law

• Container to be used
• Varies according to the method
• UV-VIS
• Small test tube or square tube with an inside
  path length of 1 cm
• Called a cuvette
• IR
• Container called the IR liquid sampling cell
• Sample contained in a space between two salt
  plates
• Created with a thin spacer between the plates
• Path length is the thickness of the spacer

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Absorbance, Transmittance, and Beers Law

• Quantitative analyses by Beers law
• Prepared series of standard solutions
• Measure absorbance of each in identical
  containers
• Plotting the measured absorbance vs.
  concentration
• Creates a standard curve
• Absorbance of an unknown solution then measured
  and concentration determined from the standard
  data

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