Atomic Absorption and Atomic Fluorescence Spectrometry

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Atomic Absorption and Atomic Fluorescence
Spectrometry

• Wang-yingte
• Department of Chemistry
• 2003.9

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• Sample atomization techniques
• Atomic absorption instrumentation
• Interferences in atomic absorption spectroscopy
• Atomic absorption analytical techniques
• Atomic fluorescence spectroscopy

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A Sample Atomization Techniques

• Flame atomization
• Electrothermal aromizarion
• Specialized atomiztion techniques

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A-1 Flame Atomization

• In a flame atomizer, a solution of the sample is
  nebulized by a flow of gaseous oxidant, mixed
  with a gaseous fuel, and carried into a flame
  where atomization occurs. Undoubtedly other
  molecules and atoms are also produced in the
  flame as a result of interactions of the fuel
  with the oxidant and with the various species in
  the sample.

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A-2 Electrothermal Atomization

• Electrothermal atomizers offer the advantage of
  unusually high sensitivity for small volumes of
  sample. Typically, sample volumes between 0.5 and
  10 µl are used Under these circumstances,
  absolute detection limits typically lie in the
  range of 10-10 to 10-13g of analyte.

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A-3 Specialized Atomization Techniques

• By far, the most common sample introduction and
  atomization technique for atomic absorption
  analyses are flames or electrothemal vaporizers.
  Several other atomization methods find occasional
  use, however. Two of these are described briefly
  in this section. One is cold-vapor atomization
  and the other is hydride atomization.

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B Atomic Absorption Instrumentation

• Radiation sources
• Spectrophotometers

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B-1 Hollow Cathode Lamps

• The most common source for atomic absorption
  measurements is the hollow cathode lamp. The
  efficiency of the hollow cathode lamp depends on
  its geometry and the operating potential. High
  potentials, and thus high currents, lead to
  greater intensities.

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B-2 Single-beam Instruments

• A typical single-beam instrument consists of
  several hollow cathode sources, a chopper or a
  pulsed power supply, an atomizer, and a simple
  grating spectrophotometer with a photomultiplier
  transducer. The 100 T adjustment is then made
  while a blank is aspirated into the flame, or
  ignited in a nonflame atomizer, finally, the
  transmittance is obtained with the sample
  replacing the blank.

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Double-beam Instruments

• The beam from the hollow cathode source is split
  by a mirrored chopper, one half passing through
  the flame and the other half around it. The two
  beams are then recombined by a half-silvered
  mirror and passed into a Czerney-turner grating
  monochromator A photomultiplier tube serves as
  thetransducer.

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C Interferences in Atomic Absorption Spectroscopy

• Spectral interferences
• Chemical interferences

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C-1 Spectral Interferences

• Spectral interferences arise when the absorption
  or emission of an interfering species either
  overlaps or lies so close to the analyte
  absorption or emission that resolution by the
  monochromater becomes impossible.

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C-2 Chemical Interferences

• Chemical interferences are more common than
  spectral ones. Their effects can frequently be
  minimized by a suitable choice of operating
  conditions.

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D Atomic absorption analytical techniques

• Sample preparation
• Organic solvents
• Calibration curves
• Standard addition method
• Applications of atomic absorption spectrometry

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D-1 Sample Preparation

• Many materials of interest, such as soils, animal
  tissues, plants, petroleum products, and minerals
  are not directly soluble in common solvents, and
  extensive preliminary treatment is often required
  to obtain a solution of the analyte in a form
  ready for atomization.

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D-2 Organic Solvents

• Leaner fuel/oxidant ratios must be employed with
  organic solvents in order to offset the presence
  of the added organic material.

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D-3 Calibration Curves

• In theory, atomic absorption should follow beers
  law with absorbance being directly proportional
  to concentration.

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D-4 Standard Addition Method

• The standard addition method is widely used in
  atomic absorption spectroscopy in order to
  partially or wholly counteract the chemical and
  spectral interferences introduced by the sample
  matrix.

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D-5 Applications of Atomic Absorption
Spectrometry

• Atomic absorption spectrometry is a sensitive
  means for the quantitative determination of more
  than 60 metals or metalloid elements. The
  resonance lines for the nonmetallic elements are
  generally located below 200 nm, thus preventing
  their determination by convenient, nonvacuum
  spectrophotometers.

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E Atomic fluorescence spectroscopy

• Instrumentation
• interferences

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E-1 Instrumentation

• Sources
• In the early work on atomic fluorescence,
  conventional hollow cathode lamps often served as
  excitation sources. In order to enhance the
  output intensity without destroying the lamp, it
  was necessary to operate the lamp with short
  pulses of current that were greater than
  detector, of course, was gated to observe the
  fluorescent signal only during pulses.

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• Dispersive instruments
• A dispersive system for atomic fluorescence
  measurements is made up of a modulated source, an
  atomizer, a monochromator or an interference
  filter system, a detector, and a signal processor
  and readout.

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Nondispersive Instruments

• The advantages of such a system are several
• (1) simplicity an low-cost instrumentation,
• (2) ready adaptability to multielement analysis,
• (3) high-energy throughput and thus high
  sensitivity, and
• (4) simultaneous collection of energy from
  multiple lines, also enhancing sensitivity.

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E-2 Interferences

• Interferences encountered in atomic fluorescence
  spectroscopy appear to be of the same type and of
  about the same magnitude as those found in atomic
  absorption spectroscopy.

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E-3 Applications

• Atomic fluorescence methods have been applied to
  the analysis of metals in such materials as
  lubricating oils, seawater, biological
  substances, graphite, and agricultural samples.