Atomic Absorption Spectroscopy

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Atomic Absorption Spectroscopy

• Atomic Emission Spectroscopy
• Lecture 18

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Detection Limits

•  Usually, atomic absorption based on
  electrothermal atomization has better
  sensitivities and detection limits than methods
  based on flames. In general, flame methods have
  detection limits in the range from 1-20 ppm while
  electrothermal methods have detection limits in
  the range from 1-20 ppb.

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• This range can significantly change for specific
  elements where not all elements have the same
  detection limits. For example, detection limits
  fro mercury and magnesium using electrothermal
  atomization are 100 and 0.02 ppb while the
  detection limits for the same elements using
  flame methods are 500 and 0.1 ppm, respectively.

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Accuracy

• Flame methods are superior to electrothermal
  methods in terms of accuracy. The relative error
  in flame method can be less than 1 while that
  for electrothermal method occurs in the range
  from 5-10. Also, electrothermal methods are more
  susceptible to molecular interferences from the
  matrix components. Therefore, unless a good
  background correction method is used, large
  errors can be encountered in electrothermal
  methods depending on the nature of sample
  analyzed.

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Flame Photometry

• The technique referred to as flame photometry is
  a flame emission technique. We introduce it here
  because we will not be back to flame methods in
  later chapters. The basics of the technique are
  extremely simple where a sample is nebulized into
  a flame. Atomization occurs due to high flame
  temperatures and also excitation of easily
  excitable atoms can occur.

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• Emission of excited atoms is proportional to
  concentration of analyte. Flame emission is good
  for such atoms that do not require high
  temperatures for atomization and excitation, like
  Na, K, Li, Ca, and Mg. The instrument is very
  simple and excludes the need for a source lamp.
  The filter is exchangeable in order to determine
  the analyte of interest and, in most cases, a
  photomultiplier tube is used as the detector.

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Atomic Emission Spectroscopy
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Atomic Emission Spectroscopy

• Atomic emission spectroscopy (AES), in contrast
  to AAS, uses the very high temperatures of
  atomization sources to excite atoms, thus
  excluding the need for lamp sources. Emission
  sources, which are routinely used in AES, include
  plasma, arcs and sparks, as well as flames. We
  will study the different types of emission
  sources, their operational principles, features,
  and operational characteristics. Finally,
  instrumental designs and applications of emission
  methods will be discussed.

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Plasma Sources

• The term plasma is defined as a homogeneous
  mixture of gaseous atoms, ions and electrons at
  very high temperatures. Two types of plasma
  atomic emission sources are frequently used
• Inductively coupled plasma
• Direct current plasma

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Inductively Coupled Plasma (ICP)
A typical ICP consists of three concentric quartz
tubes through which streams of argon gas flow at
a rate in the range from 5-20 L/min. The outer
tube is about 2.5 cm in diameter and the top of
this tube is surrounded by a radiofrequency
powered induction coil producing a power of about
2 kW at a frequency in the range from 27-41 MHz.
This coil produces a strong magnetic field as
well.
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• Ionization of flowing argon is achieved by a
  spark where ionized argon interacts with the
  strong magnetic field and is thus forced to move
  within the vicinity of the induction coil at a
  very high speed. A very high temperature is
  obtained as a result of the very high resistance
  experienced by circulating argon (ohmic heating).

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• The top of the quartz tube will experience very
  high temperatures and should, therefore, be
  isolated and cooled.
• This can be accomplished by passing argon
  tangentially around the walls of the tube. A
  schematic of an ICP (usually called a torch
  plasma) is shown below

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• The torch is formed as a result of the argon
  emission at the very high temperature of the
  plasma. The temperature gradients in the ICP
  torch can be pictured in the following graphics

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Plasma Appearance and Spectra

• A plasma torch looks very much like a flame but
  with a very intense nontransparent brilliant
  white color at the core (less than 1 cm above the
  top). In the region from 1-3 cm above the top of
  the tube, the plasma becomes transparent. The
  temperatures used are at least two to three
  orders of magnitude higher than that achieved by
  flames which may suggest efficient atomization
  and fewer chemical interferences.

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• The viewing region used in elemental analysis is
  usually about 6000 oC, which is about 1.5-2.5 cm
  above the top of the tube. It should also be
  indicated that argon consumption is relatively
  high which makes the running cost of the ICP
  torch high as well. Argon is a unique inert gas
  for plasma torches since it has few emission
  lines. This decreases possibility of
  interferences with other analyte lines.

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• Ionization in plasma may be thought to be a
  problem due to the very high temperatures, but
  fortunately the large electron flux from the
  ionization of argon will suppress ionization of
  all species.

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The Direct Current Plasma (DCP)

• The DCP is composed of three electrodes arranged
  in an inverted Y configuration. A tungsten
  cathode resides at the top arm of the inverted Y
  while the lower two arms are occupied by two
  graphite anodes. Argon flows from the two anode
  blocks and plasma is obtained by momentarily
  bringing the cathode in contact with the anodes.
  Argon ionizes and a high current passes through
  the cathode and anodes.

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• It is this current which ionizes more argon and
  sustains the current indefinitely. Samples are
  aspirated into the vicinity of the electrodes (at
  the center of the inverted Y) where the
  temperature is about 5000 oC. DCP sources usually
  have fewer lines than ICP sources, require less
  argon/hour, and have lower sensitivities than ICP
  sources. In addition, the graphite electrodes
  tend to decay with continuous use and should thus
  be frequently exchanged. A schematic of a DCP
  source is shown below

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• A DCP has the advantage of less argon
  consumption, simpler instrumental requirements,
  and less spectral line interference. However, ICP
  sources are more convenient to work with, free
  from frequent consumables (like the anodes in
  DCPs which need to be frequently changed), and
  are more sensitive than DCP sources.