Electrothermal AA Sources

1

• Electrothermal AA Sources
• The atom reservoir is a heated carbon tube in
  which a liquid sample is desolvated, vaporized
  and atomized
• In flames, the atom residence times are 10-4 s,
  whereas in the electrothermal sources,
  residence times of atoms are 1 s
• In flame systems, much of the analytical sample
  is lost, whereas in the electrothermal source,
  all the sample is observed
• These two characteristics produce a dramatic
  improvement in sensitivity and detection limits
  in electrothermal sources compared to flames
• The device consists of a graphite tube in which a
  few microliters of analytical solution is
  introduced
• Electricity is passed through the tube to raise
  its temperature to cause desolvation
• An additional increase in temperature causes
  ashing of the sample
• Finally, an increase in temperature to 3,000 oC
  over a few ms causes vaporization and atom
  production
• Light from a hollow cathode lamp or electrodeless
  discharge lamp passes down the hollow axes of
  the tube for an absorption measurement

2

• Electrothermal AA Sources
• Instrument designs - see Figures in SHN
• The designs incorporate one or two inert gas flow
  systems
• An external inert gas flow covers the outside of
  the graphite tube to prevent its incineration
• Some atomizers pass inert gas through the ends of
  the tube out the central port to carry away
  vapors produced during desolvation and ashing
• The introduction of the Lvov platform gives
  higher reproducibility
• This is a flat carbon plate inserted into the
  tube
• Atomization is delayed because the sample is not
  in direct contact with the carbon tube
• The temperature is not changing rapidly during
  the atomization step
• The graphite tube is porous and to prevent
  absorption of analyte in the graphite and
  carryover from sample to sample, tubes are
  layered with pyrolytic carbon
• Pass a mixture of methane and inert gas through
  the tube while it is held at a constant, high
  temperature
• Absolute detection limits 10-13-10-10 g of
  analyte on 0.5-10 mL of sample

3

• Electrothermal AA Sources
• Background correction is necessary when using
  electrothermal atomizers because a great deal
  of smoke is produced during the vaporization
  and atomization process
• The smoke introduces large spectral interferences
  that must be taken into account
• The usual way of background correction is the use
  of an auxiliary continuum source
• Light from the atomic source and continuum
  sources through the atomizer is alternated
  either mechanically of electrically
• The difference in radiant power from the
  alternating sources passing through the flame
  is electronically subtracted to give absorbance
  due only to the analyte atoms

4

• Inductively Coupled Argon Plasma AE Sources
• A plasma is an electricically conducting gas
  atoms are ionized to cations and electrons
• The ICAP consists of three concentric quartz
  tubes and a water cooled Cu induction coil
  through which Ar flows at a total flow rate of
• 11-17 L/min
• Cooling Ar is passes tangentially through the
  anular space between the outer most and next
  outer most tubes
• Ar that produces the plasma is passed in laminar
  flow through the annular space between the
  innermost and next outer tubes
• Ar carrying sample aerosol is passed through the
  innermost tube
• The induction coil is connected to a 2 kW 27 mHz
  RF power supply
• A Tesla coil produced spark produces sufficient
  ions to create a self sustaining plasma when RF
  power is applied
• The ions and electrons move in a circular path in
  the oscillating RF magnetic field
• Heating is produced due to Ohmic resistance to
  charge transport

5

• Inductively Coupled Argon Plasma AE Sources
• Sample introduction
• Most samples are in the form of aqueous solutions
  and are introduced by a nebulizer system similar
  to that used in flame systems
• Ultrasonic nebulizers are also available for
  liquids and solids
• Samples can also be introduced by deposition on a
  Ta strip and evaporated into an Ar stream by
  electrical heating
• Plasma appearance and spectra
• The plasma consists of a brilliant, white opaque
  region within the volume of the induction coil
  extending 2 mm above its top
• This region consists of a continuum spectrum
  superimposed on the line spectrum of Ar
• Between 10-30 mm above the brilliant core is an
  optically transparent, continuum free region
  with little emission from Ar
• Analytical observations are made in the region
  15-20 mm above the core
• Some elements are in ionized form in this region
  (Ca, Ca2, Cd, Cr2, Mn2)
• Atomization and Ionization
• Before reaching the observation point, atoms will
  have been at 6,000 K- 8,000 K for 2 ms each 2-3
  x that for a flame
• Vaporization and atomization much more complete
  than in flames resulting in less interference
• Ionization interference tends to be small because
  of high e- concentration

6

• Inductively Coupled Argon Plasma AE Sources
• Atomization and Ionization
• Atomization takes place in an inert environment
  preventing oxide formation
• Self absorption is reduced because the
  temperature profile is very constant across the
  volume of the plasma
• Calibration curves - see Figure in SHN
• Detection limits and sensitivities - see table in
  SHN and handouts
• Instrumentation
• Simultaneous instruments based on Rowland circle
• Sequential instruments based on Czerny-Turner
  mounts