LAB - UV-VIS Spectroscopy

• Spectroscope - is an instrument for visually identifying the elements in a

  sample that have been excited in a flame or other hot medium.
• Colorimeter - an instrument for absorption measurements with the human

  eye as detector; a set of standards are required for comparison.
• Photometer - a photoelectric instrument used for absorption, emission or fluorescence measurements in UV, VI or IR radiation; it uses absorption or

  interference filters.
• Fluorometer - a photometer used for fluorescence; it measures the luminous intensity of electromagnetic radiation
• Spectrograph - records spectra on a photographic plate or film; used primarily for qualitative elemental analysis based on emission
• Spectrometer - a monochromator equipped with a fixed slit at the focal plane used for absorption, emission and fluorescence measurements; used for

  measuring wavelengths
• Spectrophotometer - is a spectrometer equipped with a phototransducer at the exit slit
• Photransducer - converts light energy to electrical energy
• Emission spectroscopy - the analyte is stimulated by

  applying heat or electrical energy
• Absorption spectroscopy - an external source stimulates the analyte and the amount of light absorbed is measured
• Photoluminescence Spectroscopy - emission is measured following absorption
• Parts of the typical optical instruments: Absorption Spectroscopy

  A) Source

  B) Wavelength selector

  C) Sample

  D) Detector

  E) Signal processor and readout

  
• Fluorescence Spectroscopy:

  A) Source

  B) Wavelength selector

  C) Wavelength selector

  D) Detector

  E) Signal processor and readout

  
• Emission Spectroscopy:

  
• Tungsten Filament Lamp

  • Wavelength range: 320 to 2500 nm
  • Filament operating temperature: 3000 K
• Tungsten-halogen lamp - contain a small amount of iodine within a quartz case that extends the lifetime of the lamp. operates up to 3500 K leading to higher

  intensities and extends UV range
• Deuterium Lamp

  
• Low-Pressure Mercury Arc Lamps

  
• Hollow Cathode Lamps

  
• Globar - consists of a silicon carbide rod heated to

  1500 °C by electricity
• Nernst Glower - cylinder of zirconium and yttrium oxides heated to high temperature by electricity
• Monochromator - a device that contains an entrance slit and an exit slit to isolate a small band

  of wavelengths, one band at a time
• Polychromator - contains multiple exit slits so that several wavelengths can be isolated

  simultaneously
• Spectral bandpass or effective bandwidth

  • the variable output wavelength of a monochromator
  • can be less than 1 nm for moderately expensive instruments to greater than 20 nm for less expensive systems
• Holographic grating - use laser technique for forming gratings on a plane or concave glass surface exhibit superior groove shape and flatness
• Interference Filters - relies on optical interference to provide a narrow band of radiation

  • transmit a bandwidth of 5 to 20 nm
  • radiation outside the transmitted bandpass is removed by destructive interference
• Absorption filters - colored glass plate that absorbs part of the incident radiation and transmit the desired band

  • effective bandwidth: 30 to 250 nm
• Absorption filters

  • Advantage: less expensive and more rugged than interference filters
  • Disadvantage: only one band of wavelength can be isolated, a new filter is needed for a different band
• Interferometer - use constructive and destructive interference of electromagnetic waves to

  obtain spectral information through a technique called Fourier transformation
• Detector - a device that identifies, records or indicates a change in one of the variables in

  its environment
• Transducer - a type of detector that converts various types of chemical and physical quantities (such as light intensity, pH, mass and temperature) into electrical signals
• Photoemission - responds to radiation
• Photoconduction - responds to heat
• Phototube - a layer of photo emissive material emits electrons when irradiated with light of

  appropriate energy
• Photoconductive cells
• Photoconductive cells - transducers that consist of a thin film of a semiconductor material, such as

  PbS, mercury cadmium telluride (MCT) or indium antimonide deposited on a non-conducting glass surface sealed in an evacuated surface
• Silicon photodiodes and photodiode arrays - based on silicon semiconduction
• Thermal detectors - consist of a tiny blackened surface that increase in temperature as IR

  radiation is absorbed
• Spectrometer - a spectroscopic instrument that employs a monochromator or polychromator

  in conjunction with a transducer to convert radiant intensities into electrical

  signals
• Absorption Law - Beer's Law; determines quantitatively how the amount of attenuation depends on the concentration of the absorbing molecules and the pathlength over which

  absorption occurs