ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 20

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment 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 byemission or absorption radiation

- Concentrations are measured at the ppm level

Lightsource

monochromator

(λ selector) Flame readout detector

Sample

ATOMIC SPECTROSCOPY

PPo

Atomization- The process of breaking analyte into gaseous atoms- The process of breaking analyte into gaseous atoms

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

HgCuAgZnK

Emission Line (Ǻ)

25373248328133453447

ATOMIC EMISSION SPECTROSCOPY

Quantitative Analysis

- Techniques rely on intensity of emission lines

I = kPoc

k is a proportionality constantPo 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 muchhigher 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 absorptionof 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

EEFECT OF TEMPERATURE

T1T2 T3

T1 < T2 < T3

Energy

num

ber - More atoms are excited as temperature increases

- However, most are still in the atomic state

Minimumenergy forionization

- 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 stateat a given energy level

- Number of states is referred to as degeneracies

EEFECT OF TEMPERATURE

Degeneracy at Eo = go

Degeneracy at E* = g*

Absorption Emission ΔE

E*, g*

Eo, go

EEFECT OF TEMPERATURE

Boltzmann Distribution

- Describes relative populations of different statesat thermal equilibrium

ΔE/kT

oo

eg

g

N

N

- 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

EEFECT OF TEMPERATURE

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

- 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 atomizationof 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 decreasethe 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

INTERFERENCE

- 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+

INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY(ICP-MS)

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

SUMMARY