ANALYTICAL CHEMISTRY
CHEM 3811
CHAPTER 20
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 20
ATOMIC SPECTROSCOPY
ATOMIC SPECTROSCOPY
- Used for elemental analysis
- Deals with the absorption and emission of radiation by atoms
- Deals with free atoms
- Line spectra are observed
- Can be used for both qualitative and quantitative analysis
ATOMIC SPECTROSCOPY
- Atomic spectra have narrow lines (~ 10-4 nm)
Two Major effects That Cause Line Broadening
(yield linewidths of ~ 10-3 to 10-2 nm)
Doppler Broadening
- Species may move towards or away from detector
- Result in doppler shift and broadening of spectral lines
Pressure Broadening
- Species of interest may collide with other species
and exchange energy
- Increase in temperature results in greater effect
ATOMIC SPECTROSCOPY
- Liquid sample is sucked
- Sample passes through a plastic tube into a flame
- Flame breaks molecules into atoms (atomization)
- Monochromator selects wavelength that reaches the detector
- The concentration of elements is measured by
emission or absorption radiation
- Concentrations are measured at the ppm level
ATOMIC SPECTROSCOPY
Atomization
- The process of breaking analyte into gaseous atoms
Light
source
Po
P
Flame
monochromator
(λ selector)
Sample
detector
readout
ATOMIC SPECTROSCOPY
Source
- Line source is required to reduce interference from other elements
Hollow Cathode Lamp (HC)
- Produces emission lines specific for the element used
to construct the cathode
- Cathode is made from the element of interest
- Cathode must conduct current
ATOMIC SPECTROSCOPY
Electrodeless Discharge Lamp
- A salt of the metal of interest is sealed in a
quartz tube along with an inert gas
- A radio frequency (RF) field excites the inert gas
- Excited gas ionizes metal
- Light intensity is about 100 times greater than that of HC
- Less stable than HC
ATOMIC EMISSION SPECTROSCOPY
- Does not require light source
- Excited atoms in the flame emit light that reaches the detector
(luminescence)
Techniques Based on Excitation Source
- Flame Photometry
- Furnace (Electrical Excitation)
- Inductively Coupled Plasma
ATOMIC EMISSION SPECTROSCOPY
Qualitative Analysis
- Techniques rely on specific emission lines
Element
Emission Line (Ǻ)
Hg
Cu
Ag
Zn
K
2537
3248
3281
3345
3447
ATOMIC EMISSION SPECTROSCOPY
Quantitative Analysis
- Techniques rely on intensity of emission lines
I = kPoc
k is a proportionality constant
Po is the incident radiant power
c is the concentration of emitting species
ATOMIC EMISSION SPECTROSCOPY
Flame Photometry
- For liquids and gases
- Most flame spectrometers use premix burner
(sample, fuel, and oxidant are mixed before reaching the flamw)
- Flame decomposes sample into metal atoms (M)
- Oxides (MO) and hydroxides (MOH) may also form
ATOMIC EMISSION SPECTROSCOPY
Flame Photometry
- Flame may be rich (rich in fuel) or lean
- Rich flame reduces MO and MOH formation
(excess carbon reduces MO and MOH to M)
- Lean flame has excess oxidant and is hotter
- Good for Groups 1A and 2A elements (easier to ionize)
ATOMIC EMISSION SPECTROSCOPY
Furnace (Electrical Excitation)
- For liquids and solids
- More sensitive than flame
- Lower detection limits than flame (~ 100 times)
- Requires less sample than flame
- Graphite furnace is highly sensitive
- Operates at a maximum temperature of 2550 oC
ATOMIC EMISSION SPECTROSCOPY
Inductively Coupled Plasma (ICP)
- Makes use of plasma (partially ionized gas)
- Similar to flame photometry but reaches much
higher temperatures (greater than 10000 K)
- More sensitive
- A radio frequency (RF) is used to excite an inert gas (Ar)
- Excited gas ionizes the sample
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
- Atoms absorb light from the source
- Unabsorbed light reaches the detector
- Quantitative analysis is based on the absorption
of light by free atoms
- Makes use of Beer’s Law
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
Drawback
Flame Photometry
- Most atoms remain in the unexcited state
Furnace (Electrical Excitation)
- Most atoms remain in the unexcited state
Inductively Coupled Plasma (ICP)
- Problem of atoms remaining in the unexcited state is minimal
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
Compared to Emission
Advantages
- Less dependent on temperature
- Fewer interferences
- Better sensitivity
Disadvantage
- Quantitative analysis only
- Only used for metals since most nonmetals form oxides
number
EEFECT OF TEMPERATURE
- More atoms are excited as temperature increases
- However, most are still in the atomic state
T1
T2
T3
Minimum
energy for
ionization
T1 < T2 < T3
Energy
EEFECT OF TEMPERATURE
- For a molecule with two energy levels Eo and E*
- Ground state energy level = Eo
- Excited state energy level = E*
E* - Eo = ΔE
- At atom (or molecule) may exist in more than one state
at a given energy level
- Number of states is referred to as degeneracies
EEFECT OF TEMPERATURE
Degeneracy at Eo = go
Degeneracy at E* = g*
E*, g*
Absorption
ΔE
Emission
Eo, go
EEFECT OF TEMPERATURE
Boltzmann Distribution
- Describes relative populations of different states
at thermal equilibrium
N   g   ΔE/kT
  e
No
 go 
- N*/No is the relative population at equilibrium
- T is he temperature (K)
- k is the Boltzmann’s constant (1.381 x 10-23 J/K)
EEFECT OF TEMPERATURE
The Excited State Population
- Increase in temperature has very little effect on the
ground state population
(though an increase in population occurs)
- Has no noticeable effect on the signal in atomic absorption
- Increase in temperature increases the excited
state population (however small)
- Rise in emission intensity is observed
EEFECT OF TEMPERATURE
Atomic Absorption
- Not sensitive to temperature variation
Atomic Emission
- Sensitive to temperature variation
ICP is mostly used for emission
BACKGROUND CORRECTION
- Backgorund emission or absorption should be accounted for
Two Common Approaches
D2 Correction
- Light from source and D2 lamp pass through sample alternately
- D2 output is not very good at wavelengths greater than 350 nm
Zeeman Correction
- Atomic vapor is exposed to a strong magnetic field
- Splitting of the atoms electronic energy level occurs
- Background absorption can then be directly measured
INTERFERENCE
- Result of change in signal when analyte
concentration is unchanged
Spectral Interference
- Overlap of analyte signal by other signals from other
species or flame or furnace
- Commonly caused by stable oxides
Chemical Interference
- Chemical reactions of other species with analyte
- Caused by substances that decrease the extent atomization
of analyte
- Minimized by high flame temperatures
INTERFERENCE
Ionization Interference
- Ionization decreases the concentration of neutral atoms
- Prevalent in analysis of metals with low ionization energies
(alkali metals)
- Ionization suppressor may be added to decrease
the ionization of analyte
(CsCl is used for K analysis)
- The method of standard addition eliminates interference
- Known amounts of analyte are added to unknown
- Standard addition curve is plotted
INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY
(ICP-MS)
- Very sensitive and good for trace analysis
- Plasma produces analyte ions
- Ions are directed to a mass spectrometer
- Ions are separated on the basis of their mass-to-charge ratio
- A very sensitive detector measures ions
- Very low detection limits
INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY
(ICP-MS)
Drawback
Isobaric Interference
- Cannot distinguish ions of similar mass-to-charge ratio
- HCl and H2SO4 create isobaric interferences
so are avoided
- 138Ba2+ interferes with 69Ga+
SUMMARY
Flame Absorption
- Low cost
- Different lamp required for each element
- Poor sensitivity
Furnace Absorption
- High cost
- Different lamp required for each element
- High background signals
- High sensitivity
SUMMARY
Inductively Coupled Plasma Emission
- High cost
- No lamp required
- Low background signals
- Low interference
- Moderate sensitivity
Inductively Coupled Plasma-Mass Spectrometry
- Very high cost
- No lamp required
- Least background signals
- Least interference
- Very high sensitivity