chapter 25

• is satisfactory for the visible region and has the considerable advantage of low cost.

  ordinary silicate glass
• spectroscopic instruments in the UV/visible and IR regions are made up of five components:

  (1) a stable source of radiant energy;

  (2) a wavelength selector to isolate a limited region of the spectrum for measurement;

  (3) one or more sample containers;

  (4) a radiation detector, to convert radiant energy to a measurable electrical signal; and

  (5) a signal-processing and readout unit consisting of electronic hardware and in modern instruments a computer.
• UV region, at wavelengths shorter than about ____, glass begins

  to absorb and fused silica or quartz must be substituted.

  380 nm
• IR region, glass, quartz, and fused silica all absorb at wavelengths longer than about _____.

  2.5 micrometer
• which emit radiation that changes in intensity only slowly as a function of wavelength

  continuum sources
• which emit a limited number of spectral lines, each of which spans a very narrow wavelength range.

  line sources
• which refer to the fact that they emit radiation continuously with time

  continuous sources
• which emit radiation in bursts.

  pulsed sources
• also called quartz/halogen lamps, contain a small amount of iodine within the quartz envelope that houses the filament.

  Tungsten/halogen lamps,
• allows the filament to be operated at a temperature of about 3500 K, leading to higher intensities and extending the range of the lamp well into the UV. 


  Quartz
• are common sources for use in liquid chromatography detectors.

  Low-pressure mercury arc lamps
• are also common line sources that are specifically used for atomic absorption spectroscopy,

  Hollow cathode lamps
• The number of photons per second, called the

  radiant flux
• The resulting amplifier is called a laser, which

  stands for

  light amplification by stimulated emission of radiation.
• The first operating lasers were solid-state lasers in which the active medium was a 


  ruby crystal
• YAG host. 


  yttrium-aluminum-garnet
• are liquid lasers containing a fluorescent dye such as one of the rhodamines, a coumarin, or a fluorescein.

  Dye lasers
• A _____ is a cylinder of zirconium and yttrium oxides that emits IR radiation when heated to a high temperature by an electric current.

  Nernst glower
• consists of a silicon carbide rod.


  Globar source
• A ____ is a device that uses a grating to disperse a spectrum.

  It contains an entrance slit to define the area of the source to be viewed.

  A large opening at its exit allows a range of wavelengths to strike a

  multiwavelength detector.

  spectrograph
• A ______ is a device that contains

  an entrance slit and an exit slit. The

  exit slit is used to isolate a small band

  of wavelengths.

  monochromator
• A _______ contains multiple exit slits so that

  several wavelength bands can be

  isolated simultaneously.

  polychromator
• The __________ of a

  wavelength selector is the width of

  the band of radiation in wavelength

  units at half-peak height.

  effective bandwidth
• are formed from a master grating by a liquid resin casting process


  Replica gratings
• One of the most common types of reflection gratings

  is the

  echellette grating
• permits the design of a monochromator without auxiliary collimating and focusing mirrors or lenses because the concave surface both disperses the radiation and focuses it on the exit slit.

  concave grating
• One of the products that emerged from laser technology

  is an optical (rather than mechanical) technique for forming gratings on plane or concave glass surfaces. They exhibit superior groove shape and flatness and thus produce spectra that are freer from stray radiation and ghosts (double images)

  Holographic Gratings
• operate by blocking or absorbing all but a restricted band of radiation.

  Filters
• are typically used for absorption

  measurements.

  Interference filters
• an interference filter consists of a very thin layer of a transparent

  dielectric material (frequently calcium fluoride or magnesium fluoride) coated on both sides with a film of metal

  transparent

  dielectric material
• A _______ is a nonconducting sub-

  stance or insulator.

  dielectric
• which are generally less expensive and more rugged than interference filters, are limited in use to the visible region.

  Absorption filters
• the constructive and destructive interference of electromagnetic waves are used to obtain spectral information through a technique called 


  Fourier transformation
• A ____ is a device that identifies, records, or indicates a change in one of the variables in its environment such as pressure, temperature, or electromagnetic

  radiation.

  detector
• The _____ is also a detector; it converts visible radiation into an

  electrical signal that is passed to the brain via a chain of neurons in the optic nerve and produces vision.

  human eye
• A_______ converts nonelectrical quantities, such as

  light intensity, pH, mass, and temperature, into electrical signals 


  transducer
• is a current produced by

  a radiation transducer when no light

  strikes the device.

  Dark current
• All photon detectors are based on the interaction of radiation with a reactive surface either to produce electrons

  photoemission
• to promote electrons to energy states in which they can conduct electricity (photoconduction)
• is usually defined as the ratio of the average value

  of the output signal to its standard deviation.

  signal-to-noise ratio