CM3007

1
Lecture 6 CM3007
2
Spectral Interferences

• Emission Interference is controlled by
• Correct alignment of the furnace
• Cleanliness of the furnace and spectrometer
  sample compartment windows.
• avoidance of extreme high atomisation
  temperatures
• Background Absorption
• Attenuation of the incident line source radiation
  due to the light scattering by smoke particles or
  molecular absorption by matrix components in the
  graphite furnace, can cause false analytical
  signal unless it is corrected.

3
Spectral Interferences

• Background reduction techniques
• Effective use of Pyrolysis step permits removal
  of many matrix components.
• However, it is not always possible to remove all
  of the matrix at this stage due to the relative
  volatilities of the analyte and matrix.
• It is possible to control the relative
  volatilities by use of chemical modifiers.
• A Chemical Modifier is a reagent which is added
  to a sample to
• (a) increase the volatility of the matrix or
• (b) decrease the volatility of the analyte

4
Spectral Interferences

• A wide range of modifiers have been used
• (a) Most samples are made up in a dilute
  solutions of nitric acid.
• This promotes the formation of volatile HCl
• (b) One of the more problematic matrices is NaCl.
  This is a non-volatile compound, to remove it
  requires pyrolysis temperatures that would result
  in the loss of many analytes.
• By adding ammonium nitrate the sample matrix is
  converted to a more volatile form
• Reaction NaCl NH4NO3 ? NaNO3 NH4Cl

5
Spectral Interferences

• Other classical modifiers convert the analyte to
  a less volatile form, these modifiers include Ni
  and Pd.
• The effect of Ni as a modifier in the
  dtermination of Se is illustrated below.

6
Spectral Interferences

• Palladium retains analyte when in zero valence
  state, reduced by graphite, H2 or ascorbic acid.
• Pd can also be used in a mixed modifier with
  Magnesium nitrate.
• Using reduced Pd, char temperatures can be
  increased by an average of 400K.
• Mechanism may be the formation of a
  stoichiometric complex of Pd-analyte.
• The physical occlusion of the analyte in the Pd
  melt has also been postulated

7
Spectral Interferences

• Several other techniques can be used to minimise
  background interferences
• use of smaller sample volumes
• modification of the atomisation temperature
• use of alternative wavelengths.
• It is usually not possible to completely
  eliminate background, especially when a complex
  matrix is present
• To eliminate background interference is is
  necessary to measure background alone as well as
  background plus analyte signal

8
Non-Spectral Interferences

• The basis of AA is that free analyte atoms are
  pesent in the light path.
• Non-spectral interferences occur when matrix
  components interfere with the formation of free
  atoms.
• Depletion of free atom concentration is due to
  the following mechanisms
• Loss of volatile species during the dry and char
  stages.
• Occlusion of the analyte in a refractory matrix.
• Suppression of analyte chloride dissociation in
  the gas phase.
• Expulsion of the analyte by rapid evolution of
  vapourised matrix.
• Formation of stable analyte compounds.

9
Control of Non-Spectral Interferences

• Standard Addition Add known quantity of analyte
  to boost the signal.
• Only viable if the matrix component affecting
  formation of atoms from the analyte originally
  present affects the added analyte in exactly the
  same way.
• Graphite Tube Surface The tube surface itself
  can lead to certain non-spectral interferences,
  especially carbide formation.
• To reduce this, pyrolytically coated gaphite
  tubes are used. The denser surface on these tubes
  reduce analyte soakage reducing analyte
  interaction with the carbon and therefore carbide
  formation.
• Chemical Modifiers Modifiers delay of analyte
  atoms. This delay allows additional time for the
  furnace to reach constant temperature before
  atoms are produced

10
The Lvov Platform

• To ensure freedom from non-spectral
  interferences, it is desirable to delay the
  appearance time of the analyte until the furnace
  has reached the designated atomisation step.
• This favours free atom formation, maximises
  sensitivity and produces constant sensitivity
  regardless of matrix.
• When the sample is placed directly onto the
  surface wall, it is heated quickly as the wall
  temperature increases.
• The temp. of the gas inside the furnace lags
  behind that of the wall and the gas is cool
  relative to the wall.
• When the wall reaches the temp. at which atoms
  are produced, atoms are released from the hot
  tube surface into the cooler gas phase.
• This sudden cooling inhibits atomisation of the
  vapourised molecular species and non-spectral
  interference results

11
The Lvov Platform

• The Lvov platform is a small piece of pyrolytic
  graphite which is place in the bottom of the
  graphite tube.
• The sample is pipetted into a shallow depression
  in the bottom of the tube.

12
The Lvov Platform

• By contrast to wall atomisation, when the sample
  is placed on the platform it follows the temp. of
  the platform and not the wall.
• The platform is in poor physical contact with the
  wall and is heated by radiative rather than
  conductive energy.
• Therefore the platform temp. is far closer to
  that of the gas phase.
• When atoms are produced from the platform, they
  experience a hotter, more isothermal gas phase.

13
Tube Wall and Platform Temp. Profiles
14
Hydride Generation and Cold Vapour Techniques

• Alternative to flame atomisation for group IVb,
  Vb and VIb elements that form volatile hydrides
  namely
• Ge As Se
• Sn Sb Te
• Pb Bi
• These elements have resonance wavelengths from
  193 - 283nm and show poor atomisation efficiences
  - detection limits obtained are poor.

15
Hydride Generation and Cold Vapour Techniques

• Hydride formation removes analyte from matrix and
  hydrides decompose at 1000oC thus improving
  atomisation efficiency.
• Samples are reacted in an external system with
  reducing agent (usually sodium borohydride)
• Gaseous reaction products are carried to sampling
  cell in light path of the AA spectrometer.
• Reaction products are volatile
• GeH4 AsH3 SeH2
• SnH4 SbH3 TeH2
• PbH4 BiH3

16
Reduction Process

• Efficient hydride formation requires analyte in
  correct oxidation state, e.g., As can exist as As
  (III) or as As (V).
• Hydride formation is better for As (III). A
  preliminary reduction step is required often by
  addition of KI solution.
• Hydride formation results on addition of NaBH4
  solution.
• Need to optimise acid concentration (use HCl, a
  non-oxidising acid) and NaBH4 concentration.
• Use 1 w/v NaBH4 in 1M NaOH (for stability) is
  sufficient for most elements.

17
Hydride Generation Apparatus

• 1 Reduction Cell
• 2 Collection System
• 3 Atomisation Cell
• 1 Reduction Cell
• Usually glass cell of 50ml volume, typically
  sample is 10ml. Place sample in cell first, add
  HCl and thenNaBH4.
• Rapid generation of hydrides can be brought about
  by complete mixing in the reduction cell
• Hydride and H2 vapours evolved. For some
  elements, however, the kinetics are less
  favourable and a collection system is required

18
Collection System

• Gas is flushed from the reaction chamber using
  inert gas into a cold trap in liquid nitrogen.
• Trap is PTFE or silanised glass loop with packing
  material to absorb hydride. H2 passes through.
• Heating trap with water bath results in rapid
  hydride release.
• Removal of H2 has conflicting effects, H2 is
  produced erratically therefore the H2/air flame
  can become erratic.
• However H2 is necessary for atom formation e.g.,
• AsH3 H. ? AsH2 . H2
• AsH2 . H. ? AsH H2
• AsH H. ??? As H2
• If there is no H2, dimers form, not atoms, so
  steady stream of H2 added to purge gas

19
Atomisation Cell

• Once evolved from trap, hydride passes into
  atomisation cell. Variety of cell options
• 1 H2 / air flame
• 2 Quartz tube heated in flame
• 3 Graphite Furnace
• 4 Electrically heated quartz tube

20
Disregard the next 2 slides
21
Spectral Interferences (Contd.)

• The timing of the background correction is
  illustrated below.
• The background and analyte absorbance and the
  background only (BG) are measured alternately.
• Therefore, the background is not measured at
  exactly the same time as the analyte absorbance

22
Spectral Interferences (Contd.)

• To compensate for this, an interpolation
  technique exists.