Atomic Emission Spectrometry

Flame Emission SpectroscopyMeasure the intensity of emitted radiation

Excited State

Ground State

Emits SpecialElectromagnetic Radiation

InstrumentationConsists of:

1. Nebulizer2. Burner3. Monochromator4. Detector5. Readout device / computer

IntroductionBasic Schematic

Scanning instruments can detect multiple elements.

Many lines detected so sometimes it is a quantitatively difficult method.

Source can be flame, but more commonly plasma because it is much hotter.

Atomizer Waveleng

thSelector Detect

or

In flame emission spectroscopy

Each element emit its own characteristics line spectrum

Quantitative analysis can be performed here by observing what are emitted & comparing these with various standards.

Detector permits qualitative as well as quantitative analysis

Wavelength of emitted radiation indicates what element is present and the radiation intensity indicates how much of the element is present

In flame emission spectroscopy

Intensity of the emitted light increase with concentration

Relationship between intensity and concentration is usually linear

I = kcUnknown concentration can be detected by comparison with one or a series of standards in the same manner for the molecular techniques

I

c

Types of Atomizer

FlamePlasmaArc and spark

Flame Atomization

Emitted

Flame Lens Filterer

Detector

Slit Slit

ProcessSample is sprayed by the nebulizer into the burner.

Carried into the flame

Atomized & excited

The emission from the excited atoms passes into the monochromator where the selected wavelength is passed through for measurement.

Intensity of the emitted wavelength is measured by the detection system & indicated on the readout/computer.

Relationship Between Atomic Absorption and Flame Emission Spectrosopy

Flame Emission it measures the radiation emitted by the excited atoms that is related to concentration.

Atomic Absorption it measures the radiation absorbed by the unexcited atoms that are determined.

Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame.

The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations.

Flame Emission SpectroscopyFlame Emission Spectroscopy is based upon those

particles that are electronically excited in the medium.

The Function of Flame1. To convert the constituents of liquid sample into the

vapor state.2. To decompose the constituents into atoms or simple

molecules:M+ + e- (from flame) M + hv

3. To electronically excite a fraction of the resulting atomic or molecular speciesM M*

INTERFERENCES

Spectral interferenc

e

Chemical interferenc

e

NOTE: same interference which occur in AAS

Comparison btw AAS & AES(Based on Flame) Flame Atomic Absorption

Flame Atomic Emission

Process measured

Absorption (light absorbed by unexcited atoms in flame)

Emission (light emitted by excited atoms in a flame)

Use of flame Atomization Atomization & excitation

Instrumentation

Light source No light source

Beer’s Law Applicable Not applicable

Data obtained A vs c I vs c

2. PlasmaPlasma – highly ionized, electrically neutral

gaseous mixture of cations and electrons that approaches temperature 10, 000 K.

There are three types of plasma sources:a) Inductively coupled plasma (ICP)b) Direct current plasma (DCP)c) Microwave induced plasma (MIP)

ICP is the most common plasma source.

Inductively Coupled Plasma (ICP)Constructed of three concentric

quartz tube.

RF current passes through the water-cooled Cu coil, which induces a magnetic field.

A spark generates argon ions which are held in the magnetic field and collide with other argon atoms to produce more ions.

Argon in outer tube swirls to keep plasma above the tube.

The heat is produced due to the formation of argon ions.

Inductively Coupled Plasma (ICP)Plasma Appearancea. Excitation RegionThe bright, white, donut

shapedregion at the top of the torch.

Radiation from this region is a continuum with the argon line spectrum superimposed.

Temperature: 8000 – 10 000 K

b. Observation RegionThe flame shaped region

above the torch with temperatures 1000 – 8000 K.

The spectrum consists of emission lines from the analyte along with many lines from ions in the torch.

Inductively Coupled Plasma (ICP)1. Sample Introductiona. Liquid Sample

- Nebulizer similar to FAAS - Sample nebulized in a stream of argon with a flow rate of 0.3 – 1.5 L/min. - Sample aerosol enters the plasma at the base through the central tube.

b. Solid Samples - Sample atomized by electrothermal atomization a and carried into the plasma by a flow of argon gas.

Advantages of ICP-AES over flame AES:

a) Temperature is two to three times higher than in a flame or furnace, which results in higher atomization and excitation efficiencies.

b) There is little chemical interference.

c) Atomization in the inert (argon) atmosphere minimizes oxidation of the analyte.

d) Short optical path length minimizes the probability of self-absorption by argon atoms in the plasma.

e) Linear calibration curves can cover up to five orders of magnitude.

ICP-AES over Flame AESMuch lower detection limit because:

Higher temperature with the plasma will increase the population of excited state atoms.

The plasma environment is relatively chemically inert due to the higher population of electrons which will minimize the interference of ionization.

AAS and AESBoth methods use atomization of a sample and

therefore determine the concentrations of elements.

For AAS, absorption of radiation of a defined wavelength is passed through a sample and the absorption of the radiation is determined. The absorption is defined by the electronic transition for a given element and is specific for a given element. The concentration is proportional to the absorbed radiation.

In AES, the element is excited. A rapid relaxation is accompanied by emission of UV or visible radiation is used to identify the element. The intensity of the emitted photon is proportional to element concentration.