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Plasma Emission Spectroscopy
INTRODUCTION
The plasma is used as an atomisation source for emission spectroscopic method is known as plasma emission spectroscopy. Plasma is defined as a cloud of highly ionised gas which is composed of electrons. In plasma emission spectroscopy, the gas usually used is argon which is ionised by the influence of a strong electrical field either by a direct current or by radio frequency.
Based on the current applied, plasma emission spectroscopy is classified as the following:
- Direct current plasma emission spectroscopy.
- Inductively coupled plasma emission spectroscopy.
Plasma was first identified by Sir William Crookes in 1879 in a Crookes tube.
PRINCIPLE
The main principle involved here is atomisation process. It is carried out by the following steps:
- An aqueous solution in a flame gets desolvated and converted into a solid form.
- Then it is vaporised.
- This obtained gas is subsequently heated for atomisation.
- And this element undergoes ionisation.
- On excitation, an electron moves from ground state to excited state which is having higher energy.
- This excited state is not a stable one so the electron returns to the ground state by emitting the energy.
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| plasma electron spectroscopy |
Diagram for the plasma formation
THEORY
A transition is given by the following equation:
E = hn
where E is the energy; h is Plank's constant; v is the frequency.
In plasma emission spectroscopy, there are no of transitions which can be depicted in the energy level diagrams.
Energy transitions in the ESR
Direct current plasma: The direct plasma source consists of high voltage discharge between the two electrodes. The recent developments are observed by incorporating the third electrode which is in inverted Y shape. The incorporation of the third electrode increases the stability of the discharge. The sample is carried the argon carrier gas. The argon is ionised before mixing with the sample solution. The argon gas is ionised by the high voltage discharge.
Diagram of the direct current plasma source
Advantages of Direct Current Plasma
- Less expensive
- Simple to handle
Disadvantages of Direct Current Plasma
- Low detection limits.
- Replacement of the electrodes is necessary after every use.
Inductively coupled plasma: This consists of the three concentric silica quartz tubes with opening at the top. The sample is mixed with the argon gas and is allowed for the formation of aerosol. This aerosol is again mixed with the argon gas and passed into the tubes from the bottom. The gas is excited by the radio frequency power. The plasma gas flows as the helical pattern which provides the stability of the plasma. Then the plasma is initiated by the spark from a tesla probe.
Diagram for the ICP source
INSTRUMENTATION
The instrument of the plasma emission spectroscopy contains the following components:
- Nebulisers
- Pumps
- Spray chamber
- Sample injector
- Torch
- RF generator
- Detector
Instrumentation of ESR
Nebuliser: These are used for the conversion of the liquid into aerosol. This aerosol is carried out into the plasma which is an excitation source. The commonly employed nebulisers are the following:
- Pneumatic nebulisers: The principle involved in this is the sample is subjected to the high-speed gas flow to form an aerosol.
- Pneumatic nebuliser
- Concentric nebulisers: The main principle involved in this is the sample is introduced through the capillary tube to a low-pressure region which produces the aerosol.
- Concentric nebuliser
- Babington nebuliser: The principle involved in the liquid to flow over a smooth surface with a small hole in it. High-speed argon gas from the hole shears the sheet of liquid into small drops. This is very sensitive nebuliser.
- Babington nebuliser
- Pumps: These are mainly used to introduce the samples into the nebuliser. Commonly the peristaltic pumps are employed. These are composed of the rollers which pushes the sample solution through the tube. This process is known as the peristalsis.
- Peristaltic pump diagram
- Spray chambers: The spray chamber is used to transport the aerosol into plasma which is placed between the nebuliser and torch. This also helps in the removal of the large droplets. These must be made up of corrosion-resistant material.
- Spray chamber diagram
- Drains: This is mainly used to drain the sample from the spray chamber to waste container.
- Torch: This is mainly used to produce the plasma. This contains the three concentric tubes for argon gas flow and the sample aerosol injection. The gas flow carries the sample aerosol injected into the plasma through the sample injector tube.
- Torch or plasma source diagram
- RF generator: This is mainly used for the generation and sustainment of the plasma discharge. It ranges from the 700 to 1500 watts. The commonly used RF generators are the following:
- Crystal controlled generators: This consists of the piezoelectric quartz crystal to produce an RF oscillating signal.
- Free running generators: This operates at an oscillation frequency.
- Detectors: The photomultiplier tubes are generally employed as the detectors. The principle involved in the detector is the secondary emission of the electrons when the light falls on the cathode and the replication of the anodes. The secondary emission of the electrons produces the current signals.
- Photomultiplier tube detector diagram
ADVANTAGES
- High resolution
- Low stray light
- Wide dynamic range
- High accuracy
- High precision
- Highly reproducible
LIMITATIONS
- Time consuming
- Temperature dependent
- Less stability
APPLICATIONS
- Used in the trace metal analysis.
- Example: Copper, iron, manganese, magnesium and calcium
- Used in the estimation of the aluminium in blood.
- Example: Estimation of traces of the aluminium in blood
- Used in the estimation of the Cu in brain tissue.
- Example: Used in the scanning of the brain tissue incase of brain tumor
- Used in the estimation of Na in the breast milk.
- Example: Salts estimation in the breast milk
- Used in the analysis of agricultural products.
- Example: Pesticidal residues analysis
- Used in the study of the earth sciences.
- Used in the steel analysis.
- Example: Hardness of the steel
- Used in the alloys analysis.
REVIEW QUESTIONS
- What is plasma?
- What are the different types of sources used in the PES?
- What is the theory involved in the PES?
- What are the different steps involved in the PES process?
- What are the different instrument components of PES?
- What are the advantages of the PES?
- What are the different applications of the PES?
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