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Atomic Emmision Spectroscopy


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Analytical chemistry is the science of accurateand precise qualitative and quantitativemeasurements involving chemical systems.

Spectroscopy is defined as the interaction of

light and matter and has both physical andanalytical applications.

Physical spectroscopists use emitted light,

absorbed light, or scattered light in order tounderstand the mechanics of a chemicalsystem.


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Atomic (or optical) emission spectrometry

(AES, OES) is an important technique for themulti element analysis of a wide range ofmaterials.

AES used for measurement of trace elementsin rocks, water, soil, manufactured goods, andbiological specimens.

The technique is used to monitor the levels of

different chemicals and trace elements in theenvironment and to determine thecompositions of solids, liquids, and gases.


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Atomic spectra are derived from the transition of electrons from one

discrete electron orbital in an atom to another.

In atomic emission spectrometry (AES) a reproducible and representative

amount of the sample is introduced into an atomization-excitation source

where it is converted into atomic vapours of the analyte in excited state.

This is accompanied by the emission of electromagnetic radiation,

normally in the form of light in the UV-VIS region.

This radiation is characteristic of the constituents of the sample. Due

resolution of the emitted radiation generates an emission spectrum,whose detailed analysis can be exploited to obtain qualitative as well as

quantitative details of the analyte.


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An ideal atomization-excitation source should have the followingcharacteristics :

completely separate the analyte from its original matrix so as to minimizeinterferences.

have appropriate energy to ensure complete atomization but keepionization to a minimum.

provide an inert environment, so as to keep the undesirable molecularspecies formation to a minimum.

negligible background radiation.

provide for the analysis of samples in all possible forms like, solids,liquids, gases or slurries.

inexpensive, need minimal maintenance and be easy to operate.


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Advantages of AES

increased atomization/excitation

wider range of elements

emission from multiple species simultaneously wide dynamic range

These extend from dc arc, ac spark and

universal arc-spark, to dc plasmas, microwaveplasmas, glow discharge lamps, lasers and

inductively coupled plasma (ICP).


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Instrument of ICP-AES

The essential components of a plasma basedatomic emission spectrometer are as givenbelow.

1. Plasma source: an atomisation-excitation

device

2. Nebuliser : a sample introduction device

3. Monochromator: a dispersion device

4. Detector: a radiation detection device

5. Processing and read out device


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Example


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Types of AES

Inductively coupled plasma (ICP)

Direct current plasma (DCP)

Microwave induced plasma (MIP)


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1. Inductively Coupled Plasma(ICP)

ICP is plasma induced by radiofrequency. Theenergy transfer is mediated by an induction in

that produces a magnetic field which helps in

establishing and sustaining the plasma. The energy of a high frequency generator is

transferred to a gas, generally argon, flowing

at atmospheric pressure, a typical inductively

coupled plasma source is called a torch.


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Fig. 1: A schematic section diagram of an ICP torch


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ICP TORCH


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Quartz tube:

- The body of the torch consists of three concentric quartztubes. The gas traveling up the central channel is called

auxiliary gas and also has a tangential flow pattern. The

innermost third inlet is connected to the nebuliser from which

the gas enters the plasma along with the sample in a laminar

flow.

Work coil:

- consists of a water-cooled induction coil that is powered by a

radio frequency generator.

Argon gas supply :

- conventional argon plasma torch uses between 13 and 20

liters of argon per minute


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2. Direct Current Plasma(DCP)

DCP is created by an electrical discharge betweentwo electrodes. A plasma support gas is necessary,

and Ar is common.

Samples can be deposited on one of the electrodes,

or if conducting can make up one electrode.Insulating solid samples are placed near the

discharge so that ionized gas atoms sputter the

sample into the gas phase where the analyte atoms

are excited. This sputtering process is often referredto as glow-discharge excitation.


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3. Microwave Induced Plasma

In a microwave induced plasma (MIP) source frequencies in

the microwave region are used as external energy source. A microwave radiation of a frequency of 2450 MHz is

commonly used.

A microwave induced plasma is obtained when the ultrahigh

frequency ac power is capacitatively coupled into a stream ofnoble gas (helium or argon) or nitrogen at about 3 dm3 /min.

in a resonant cavity.


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Low power MIP (50 to 100W) cannot accept solution aerosols

without prior desolvation, high power MIP (800W) are more

robust and can vaporise and atomise aerosols withoutdesolvation.

The MIP is also useful for the excitation of volatile hydride

forming elements after stripping the hydrides from the excess

of hydrogen.

A conical hollow nozzle acts as the tip of the torch through

which the plasma gas argon flows. As there is only one flow

channel, the sample is also injected through the same.

The small size of microwave sources is an advantage over theICP because it makes it more flexible and less expensive.

However, the MIP is found to be inferior to the ICP in respect

of detection limits, sensitivity, reproducibility, ionization

interference and etc.


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Fig.3: A schematic diagram showing the generation of a microwave

induced plasma


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Application of AESAs many as 60 elements can be determined by ICP-AES in a wide range ofanalyte samples such as rocks, minerals, soil , air, water, agriculture,

forestry ecology, food analysis, etc.

1. Agricultural science

Analysis of agricultural products and foods besides soil analysis.

2. Health sciences Determination of Al in blood, Cu in brain tissue, Se in liver, Na in breast

milk.

Direct determination of Ca, Fe, Cu, Mg, Na and K in serum samples.

3. Geological sciences

presence of lanthanides and other elements in rock samples.

4. Forensic Sciences

crime scene soil analysis.


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5. Metallurgy

analysis of trace elements in stainless steel.

6. Environmental science

waste water analysis, determination of pollutant metals in

variety of matrices.

7. Industry

presence of metals like Cu, Fe, Ni, and Si in lubricating oils

or gasoline at tracer concentration. traces of metals like Ca, Cu, Fe, Mn, Mg, P, K and Zn in beer or wine;

determination of trace elements in polymers, evaluation ofcatalysts, and so on.

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