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Elemental Analysis - Atomic Spectroscopy

Based on the breakdown of a sample into atoms, followed by the measurement of the

atoms absorption or emission of light.

- Deals with absorbance fluorescence or emission (luminescence) of atoms or

elemental ions rather then molecules

- atomization: process of converting sample to gaseous atoms or

elementary ions

- Provides information on elemental composition of sample or compound

- UV/Vis, IR, Raman gives molecular functional group information, but no

elemental information.

- Basic process the same as in UV/Vis, fluorescence etc. for molecules

Eo

E1

h

Absorbance Fluorescence

Types of Atomic Spectroscopy - AAS

Atomic Absorption Spectroscopy

absorption of electromagnetic radiation

promotion of electron to higher energy state

log of transmittance is proportional to concentration

AALamp

Atomizer+Sample MonochromatorDetector

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Types of Atomic Spectroscopy - AES

Atomic Emission Spectroscopy

thermal excitation

M M*

radiative decay to lower energy level

M* M +

emission signal directly proportional to concentration

DetectorMonochromator

AEAtomizer+Sample

Types of Atomic Spectroscopy - AFS

Atomic Fluorescence Spectroscopy

excitation caused by absorption of EMR

followed by radiative decay to lower energy state

fluorescence signal is directly proportional to concentration

AFAtomizer+Sample

Lamp

MonochromatorDetector

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Most Common AA/AE Techniques

Flame AA and AE

Furnace (Electrothermal) AA only

Inductively Coupled Plasma (ICP) AE only

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Flame AA and AE

Well established

Good speed

Moderate sensitivity

Few interferences (well understood)

Low-moderate cost

Operating temperature: 1700-3200C


Furnace (Electrothermal) AA only

Well established

Slow

Single element

Excellent sensitivity

Many interferences

Moderate-high cost


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Inductively Coupled Plasma (ICP) AE only

Established and growing

Fast

Multi-element

Moderate sensitivity

Spectral interferences

High cost


General Uses Quantitative (~70 metals, any sample type)

Low Concentrations (ppb)

Microliter volumes or microgram masses.

Well-established and accepted.

Common Applications Biological, medical, clinical

Environmental

Steel and Metal industry

Pharmaceuticals

Food industry

Atomic Absorption Spectrometry

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Samples Almost any liquid, solid or gaseous sample. Microliter volumes or microgram masses.

Aqueous samples can be analyzed directly.

Complex solutions Diluted

Solvent extraction

Solids can be dissolved/digested

Analysis Time Sample preparation (0 sec to 24 hr)

Analysis after calibration 10 -120 sec.)

Limitations No information on chemical form of metal

Sample prep can be tedious

Limited to metals/metalloids

Destructive

Atomic Absorption Spectrometry II

Accuracy Depends on complexity of matrix.

Homogeneous solution at 10 LOD 1%

Reduced to ~ 3% at LOD.

Linear Range In single metal mode: 103 above LOD

For higher concentrations use dilution.

Detection Limits Flame AA : 0.1 to 1.0 g/mL

Furnace AA : 0.01 to 0.1 g/mL

Atomic Absorption Spectrometry III

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Flame AAS

Flammable gases.

Detection limits.

Disadvantages

Robust.

Easy to use. Fast analysis time.

Modest cost.

Advantages

Furnace AAS

Longer analysis times.

Interferences.

Higher cost.

Lower accuracy (1-5%).

Disadvantages

g masses & L volumes.

LOD 10-100 < flame.

No gases.

Easily automated.

Advantages

Schematic of AA/AE

AA

AE

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Laminar Flow Burner

- adjust fuel/oxidant mixture for optimum excitation of desired compounds

- usually 1:1 fuel/oxidant mix but some metals forming oxides use increase fuel mix

- different mixes give different temperatures.

Laminar nonturbulent streamline flow

Sample, oxidant and fuel are mixed

Only finest solution droplets reach

burner

Most of sample collects in waste

Provides quite flame and a long path

length

Types of Flame/Flame StructureSelection of right region in flame important for optimal performance

- primary combustion zone (blue due to emission from C2, CH & other radicals)

- not in thermal equilibrium and not used

- interconal regioninterconal region

- region of highest temperature (rich in free atoms)- often used in spectroscopy

- can be narrower in some flames (hydrocarbon) tall in others (acetylene)

- outer cone

- cooler region- rich in O2 (due to surrounding air)

- gives metal oxide formation

Flame profile: depends on type of fuel and

oxidant and mixture ration

Temperature varies significantly across flame need to focus on part of the flame

Not in thermal equilibrium and not

used for spectroscopy

Primary region for

spectroscopy

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Maximum Flame Temperature

,

,

,

,,

,

,

,

,

,

,

Consequences:

- Sensitivity varies with element

- must maximize burner position

- multi-element detection difficult

Most sensitive part of flame for AAS varies with analyte

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- For atomic absorbance, fluorescence or emission need to break sample

up into atom to observe atomic spectra

- Basic steps involved in atomization of solution sample

a) nebulization solution sample, get into fine droplets by spraying thru thin

nozzle or passing over vibrating crystal. M+ + A- (solution) M+ + A- (aerosol)

b) desolvation - heat droplets to evaporate off solvent just leaving analyte

and other matrix compounds. M+ + A- (aerosol) MA (solid)

c) vaporization convert solid analyte/matrix into gas phase. MA (solid) MA (gas)

d) atomization break-up molecules into atoms. MA (gas) Mo + Ao (gas)

e) excitation with light, heat, etc. for spectra measurement. Mo M*

f) ionization cause the atoms to become charged. M* M+ + e-

The Flame Process

Hollow Cathode Lamp

Process: uses element to detect element

1. ionizes inert gas to high potential (300V)ArAr+ + e-

2. Ar+ attracted to - cathode & hits surfaces

3. As Ar+ ions hit cathode, some of the deposited element is excited

and dislodged into gas phase (sputtering)

4. excited element relaxes to ground state and emits characteristic radiation

- advantage: sharp lines specific for element of interest

- disadvantage: can be expensive, need to use different lamp for each element tested.

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Inductively Coupled Plasma - ICP

- involves use of high temperature plasma for sample atomization/excitation

- higher fraction of atoms exist in the excited state, giving rise to an increase

in emission signal and allowing more types of atoms to be detected

- plasma electrically conducting gaseous mixture (cations & electrons)

- temperature much higher than flame

- possibility of doing multiple element analysis

~ 40-50 elements in 5 minutes

Advantages

- uniform response

- multi-element analysis, rapid

- precision & accuracy (0.3 3%)

- few inter-element interferences

- can use with gas, liquid or solids sample

Documents

20940269-Lecture-3-30-04-AA-AE