SPECTROPHOTOMETRY

• Spectrochemical methods


  Instrumental methods of analysis that involve the absorption or emission of light

  
• Spectroscopy/spectrometry


  The science that deals with light and its absorption and emission by solutions and other material substances
• Spectrometer


  The broad term for the instrument used
• Spectrophotometer


  A more specific term (when a light sensor known as a phototube is used)

  
• Spectrochemical analysis procedures


  The degree to which light is absorbed, or the intensity of light that is emitted, is related to the amount of an analyte present in the sample tested

  
• Light

  • It has a dual nature - it can be described as consisting of moving particles (photons) or moving electromagnetic disturbances (electromagnetic waves)
• Wavelength


  The length of an electromagnetic wave, symbol: λ

  
• Frequency


  The number of the moving electromagnetic waves that pass a fixed point in 1 sec, symbol: ν

  
• Frequency and energy


  They are directly proportional
• Wavelength


  It is inversely proportional to frequency and energy
• Regions of the electromagnetic spectrum

  • Visible
  • Ultraviolet
  • Infrared
  • X-ray
  • Radio and television waves

  
• Visible region

  • Wavelengths from approximately 350 nm to approximately 750 nm

  
• Radio and television waves


  • Very low energy, on the order of kilometers long
• Microwaves


  • Wavelengths on the order of a centimeter, can be dangerous due to internal heat generation
• Infrared region


  • Wavelengths are extremely short, higher energy than radio/TV waves or microwaves, but cause no harm
• Ultraviolet region


  • Wavelengths are shorter still, higher energy, can cause harm like sunburn
• X-rays


  • Extremely short wavelengths of extremely high energy, penetrate skin and tissue, cause harm
• Gamma rays


  • Wavelengths on the order of atomic diameters, extremely high energy, cause extreme damage to the human body
• Absorption of light


  Objects display a color because some visible light wavelengths are absorbed, while the unabsorbed wavelengths are reflected or transmitted

  
• Absorption spectrum


  A plot of the amount of light absorbed by a sample vs. the wavelength of the light

  
• Transmission spectrum


  A plot of the amount of light transmitted by a sample vs. the wavelength of the light

  
• Transmittance


  The fraction of light transmitted, defined as I/I0

  
• Absorbance


  A parameter that increases linearly with concentration, important for quantitative analysis
• Beer's law


  States that the concentration of the unknown substance is directly proportional to the absorbed light (absorbance) and inversely proportional to the amount of transmitted light (% transmittance)
• Absorptivity


  The inherent ability of a chemical species to absorb light, constant at a given wavelength, pathlength, and concentration

  
• Wavelength of maximum absorbance


  The wavelength at which the absorptivity is a maximum, providing the greatest analytical sensitivity

  
• UV-VIS molecular spectrometry


  Utilizes light in the ultraviolet and visible regions of the electromagnetic spectrum to analyze molecular compounds and complex ions

  
• Tungsten filament lamp


  • A light source used frequently for visible light absorption studies, emits an intense continuous spectrum of light across the visible region

  
• Deuterium lamp


  • A light source used frequently for ultraviolet absorption studies, emits a continuous UV spectrum from 185 nm to about 375 nm

  
• Xenon arc lamp


  • A light source that can be used for both ultraviolet and visible studies
• UV spectrophotometer


  Instrument used when the light source is strictly for ultraviolet work, with only a deuterium lamp present
• UV-VIS spectrophotometer


  Instrument used when both a tungsten filament lamp and a deuterium lamp are present and individually selectable
• Tungsten filament lamp


  • Very bright and emits light over the entire visible region and into the near infrared region
• Deuterium


  Isotope of hydrogen that has one neutron in the atomic nucleus
• Deuterium lamp

  • Contains deuterium at a low pressure, electricity applied to electrodes results in continuous UV emission
  • Wavelength output ranges from 185 nm to about 375 nm, satisfactory for most UV analyses
• Xenon arc lamp


  • Contains xenon at a fairly high pressure, light is formed via a discharge across a pair of electrodes
  • Emits continuous ultraviolet and visible light due to the presence of the xenon
  • In some instruments, the electronic circuitry creates regular pulses of light that are very intense and therefore more useful, resulting in a longer lamp life
• Wavelength selection


  1. Filter out unwanted wavelengths and allow only the wavelength of interest to pass

  2. Measure absorbance at each wavelength to plot absorption spectrum

  3. Carefully select wavelength of maximum absorption for quantitative analysis by Beer's law

  
• Electromagnetic spectrum


  Continuous wavelength band, no sharp delineation between wavelengths

  
• Bandwidth


  Width of the wavelength band that is allowed to pass
• Resolution


  Narrowness of the wavelength band that is allowed to pass