Atomic Spectroscopy

1
Atomic Spectroscopy

• Chapter 9

2
Review and Comparisons

• Atomic spectroscopy
• Absorption and emission of UV-VIS light
• Atoms and monoatomic ions
• Conceptually similar to absorption and emission
  of UV-VIS light by molecules

3
Review and Comparisons

• Atomic spectroscopy
• Differences from UV-VIS
• Limited to the elements
• METALS
• Most analysis is for metals!
• Sample preparation
• Place metals in water solution
• Metals present as ions in water
• Must have a means for converting metal ions into
  free gas phase ground state atoms to be measured
• Called atomization
• Uses large amount of thermal energy

4
Review and Comparisons

• ATOMIC SPECTROSCOPY
• Differences from UV-VIS
• SAMPLE CONTAINER
• SOURCE OF THE THERMAL ENERGY NEEDED FOR THE
  CONVERSION OF IONS IN SOLUTION TO ATOMS IN THE
  GAS PHASE
• Atomizer
• Does NOT resemble a cuvette
• Has a flame container

5
Review and Comparisons

• ATOMIC SPECTROSCOPY
• Differences from UV-VIS
• Various types of atomizer and instrument designs
• Based on the same theory
• Spectral line sources used as light sources
• Instead of continuums sources in UV-VIS
• Several require NO light source at all!
• Limited types of analytes
• . Quantitation is well known
• Using elements that are well characterized
• Look up spectra in most reference text

6
Summary of Techniques and Instrument Designs

• Most important aspect thermal energy
• Flame atomic absorption (flame AA)
• Graphite furnace atomic absorption (graphite
  furnace AA)
• Inductively coupled plasma atomic emission (ICP)
• Less important
• Flame emission and atomic fluorescence
• Two that do not require thermal energy/minimal
  thermal energy
• Cold vapor mercury system
• Metal hydride generation
• One that requires electrical energy
• Arc and spark emission

7
Summary of Techniques and Instrument Designs

• Flame AA
• Large flame as the atomizer
• Sample-solution-drawn into the flame by a vacuum
  mechanism
• Atomization occurs immediately
• Light beam for the absorption measurements
  directed through width of the flame

8
Summary of Techniques and Instrument Designs

• Graphite furnace
• Actually a small graphite tube
• Quickly electrically heated to a very high
  temperature
• Small volume of sample solution placed in tube
• Manually with a micropipette
• Drawn with a vacuum
• Electrically brought to high temperature to
  atomize sample
• Light beam directed through the tube and measured
• There is a cloud of atoms

9
(No Transcript)
10
Summary of Techniques and Instrument Designs

• ICP
• Emission technique
• Does not use a light source
• Light measured is light emitted by the
  atoms/monoatomic ions in the atomizer
• Atomizer
• Extremely hot plasma
• High-temperature ionized gas composed of
  electrons and positive ions
• Confined by a magnetic field
• Extremely high temperatures
• Atoms and monoatomic ions undergo sufficient
  excitation
• Relatively intense emission spectra result
• Sample drawn with vacuum
• Intensity of an emission line is measured and
  related to concentration

11
(No Transcript)
12
Flame Atomic Absorption

• Flames and Flame Processes
• After metal ions introduced into flame, several
  processes occur in rapid order (fig. 9.4, pg.
  248)
• Solvent evaporates
• Leaves behind formula units
• Dissociation of salt into atoms
• Metal ions atomize/transformed into atoms
• Atoms raised to excited states by thermal energy
  of the flame
• A resonance process occurs
• Atoms resonate back and forth between ground
  state and excited statte

13
Flame Atomic Absorption

• Flames and Flame Processes
• Only small of atoms- state at any moment
• Atoms drop back to ground state
• Emission spectrum emitted
• Atoms in the excited state
• Emit light in the visible region of spectrum
• Entire flame in element takes on color
  characteristic of the element that is in the
  flame
• Each element has a characteristic color
• It an atomic fingerprint
• Possible to quantitate these elements using flame
  emission

14
Flames and Flame Processes

• Flames and Flame Processes
• Unexcited atoms in the flame- 99.9
• Available to be excited by a light beam
• Light source used
• Light beam directed through the flame
• It is a Beers law experiment
• Width of the flame being the pathlength
• Flame temperature important both for the
  atomization and excitation process

15
Flames and Flame Processes

• Flame Atomic Absorption
• Flame requirements
• Fuel and an oxidant
• Natural gas and air
• Max temperature 1800K
• Does not sufficiently atomize most metal ions
• Does not excite a sufficient of atoms for
  quantitation
• . need something hotter!
• Acetylene as fuel and air is the oxidant
• Max temperature 2300K

16
Spectral Line Sources

• Light sources emit spectral lines
• Lines in the line spectrum of the analyte being
  measured
• Preferred b/c they represent the precise
  wavelengths needed for the absorption in the
  flame
• Flame contains this particular analyte
• Emitted b/c they contain the analyte to be
  measured
• When lamp is on
• Internal atoms are raised to the excited state
• Emit their line spectrum when they return to the
  ground state
• This is the light directed through the flame

17
Hollow Cathode Lamp

• Hollow Cathode Lamp
• Most widely used spectral line source
• Cathode
• Negative electrode
• Contains the internal atoms
• Hollowed cup
• Internal excitation and emission process occurs
  inside this cup when lamp is on
• Anode
• Positive electrode
• Connected with cathode to a high voltage
• Light emitted

18
Hollow Cathode Lamp

• Hollow Cathode Lamp
• Sealed glass tube
• Filled with inert gas at low pressure
• Neon or argon
• How it works (fig.9.6, pg 251)
• Lamp turned on and argon atoms ionize
• Positively charged argon ions then crash into the
  negatively charged cathode
• Causes sputtering
• Transfer of surface atoms in the solid phase to
  the gas phase due to the collisions
• More collisions of argon ions with metal atoms
  cause metal atoms to be raised to the excited
  state
• Light emitted with they drop back to the ground
  state

19
(No Transcript)
20
Hollow Cathode Lamp

• Hollow Cathode Lamp
• Must contain the element being measured
• Usually have number of different lamps in stock
• Interchanged in the instrument
• Some are multi-elemental
• Several different specific atoms present in the
  lamp
• Separated by a monochromator after the flame to
  isolate the specific spectral line of the analyte

21
Premix Burner

• Premix burner
• Burner for flame AA
• All components-fuel, oxidant, and sample
  solution-are premixed
• Take common path to the flame
• Fuel and oxidant
• Originate from pressurized sources
• Compressed gas cylinders
• Flow is controlled for optimum rate

22
Premix Burner

• Sample solution
• Aspirated by vacuum
• Converted to aerosol/fine mist before mixing
• Accomplished with a nebulizer at head of mixing
  chamber
• Resembles nozzle to create a water spray (fig.
  9.7, pg. 252)
• Connects to the sample tube
• Pulls sample into the mixing chamber
• Produces aerosol spray

23
(No Transcript)
24
Premix Burner

• Aerosol spray
• Emerges from nebulizer
• Contains variable-sized solution droplets
• Also mixed with oxidant and fuel
• Contains impact device
• Baffles or glass bead near tip of nozzle
• Separates larger particles (fall to bottom of
  chamber)
• 90 of sample never reaches flame

25
Optical Path

• Arranged in this order
• Light source, flame (sample container),
  monochromator, and detector
• Flame
• Positioned in open area
• Light can leak from room light and flame
• Monochromator located between flame and detector
• Detector
• Receives alternating light signals
• Source light and flame emissions
• Flame emissions only
• Detector able to eliminate flame emissions by
  subtraction

26
Optical Path

• Either single-beam or double beam
• Single-beam
• Fewer problems than in UV-VIS
• Seldom measure absorption spectra
• Wavelength seldom changed
• No need for re-calibration with blank as in
  UV-VIS
• Source drift and fluctuations still exist
• Minimized with improved electronics

27
Optical Path

• Either single-beam or double beam
• Double beam
• Uses beam splitter
• Diverts light from source around the flame
• Two beams joined again before entering the
  monochromator
• Eliminates problems due to source drift and noise
• Source warm-up time eliminated since changes in
  intensity compensated fro
• Rapid changeover of lamps possible

28
Practical Matters and Applications

• Slits and Spectral Lines
• More than one spectral line for an element
• .more than one line to choose from for setting
  the monochromator
• One line gives the optimum absorptivity
• Pick that one!!!!
• Found on the HCL
• Called the primary line
• Monochromator usually set at that wavelength
• Others called the secondary lines
• May be chosen if 1O is inappropriate
• If another element is the sample is similar to 1O
• Automated equipment usually set to primary line

29
Practical Matters and Applications

• Slits and Spectral Lines
• Slit control
• Helps correct problem of close lines
• Wider the slit
• Greater the bandpass
• More incidental/close spectral lines allowed to
  be captured
• Usually choose between 0.2 and 2.0 nm
• Value represents the bandpass for both entrance
  and exits slits
• If interfering line at the optimum setting
• Slit is narrowed
• Or 2O line chosen
• Both result in less desirable sensitivity

30
Practical Matters and Applications

• Hollow Cathode Lamp Current
• Current is adjustable
• Optimum setting represents most intense light
  without shortening the life of the lamp (VERY
  EXPENSIVE!)
• Lamp Alignment
• Must have proper alignment for optimum intensity
  through the optical path
• May need adjustment when changing lamps

31
Practical Matters and Applications

• Interferences
• Causes
• Chemical sources
• Spectral sources

32
Practical Matters and Applications

• Chemical interferences
• Result of problems with sample matrix
• Viscosity/surface
• May affect aspiration rate
• Nebulized droplet size
• Standard additions method
• Certain volume of the sample solution present in
  same proportion in all standard solutions
• Equivalent to adding standard amounts of analyte
  to the sample solution
• Solves interference problem
• Sample matrix always present in same concentration

33
Practical Matters and Applications

• Standard additions method
• Prepare standards in usual way
• Add a volume of the sample solution to each
  before diluting to the mark with solvent
• Gives a series of standards which the
  concentration of analyte added known
• Standard curve
• Its a plot of absorbance vs. concentration added
  instead of just concentration
• Y-axis is not true position
• Offset to the right by the concentration of the
  zero-added concentration, which is the sample
  solution
• Concentration of THIS solution is the
  concentration sought
• To show on graph
• Curve extrapolated to intersect with the x-axis
  (y 0)
• Represents the concentration of the amount added
• Precise concentration in the zero-added solution
  found using the equation of the straight line
• YmX b

34
Spectral Interferences

• Spectral Interferences
• Caused by substances in the flame
• Absorb same wavelength as analyte
• Causes absorbance measurement to be high
• Rarely an element
• If suspected, switch to 2O wavelength
• More often caused by presence of light-absorbing
  molecules in the flame and light dimming due to
  small particles
• Called background absorption
• Fix by background corrections
• Subtract background interference
• Its the purpose of the deuterium lamp

35
Safety and Maintenance

• Safety issues with the AA
• Acetylene, flame, and contamination of lab air
  with combustion products
• Acetylene
• Compressed gas cylinders must be secured to
  immovable object-the wall
• Approved pressure regulators in place
• Tubing free of leaks
• Must have independently operated vent hood over
  flame
• Removes excess solvent fumes
• No volatile fumes near the flame!

36
Safety and Maintenance

• Safety issues with the AA
• Flashbacks
• From improperly mixed fuel and air
• When flow regulators ore improperly set
• When air is drawn back through drain line of
  premix burner
• Cleaning
• Burner head and nebulizer
• Ensures minimal noise level from impurities in
  flame
• Carbon deposits in slit
• Scrape with sharp knife or razor blade

37
Summary

• Sensitivity
• Concentration of an element that will produce and
  absorption of 1
• Smallest concentration that can be determined
  with a reasonable degree of precision
• Detection limit
• Concentration that gives a readout level that is
  double the electrical noise level inherent in the
  baseline.
• Qualitative parameter in that it is the minimum
  concentration that can be detected
• Not precisely determined
• Would tell the analyst that the element is
  present
• Not at a precisely determinable concentration
  level
• Table 9.2 and 9.3, pg. 267