Introduction spectroscopy (ICP-AES) uses an inductively coupled plasma

 

                           Introduction

 

Atomic Emission
Spectroscopy is a method of chemical analysis that uses the intensity of light
emitted from a flame, plasma, arc or spark at a particular wavelength to
determine the quantity of an element in a sample.

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The wavelength of the
atomic spectral line give the identity of the element while the intensity of
the emitted light is proportional to  the
number of atoms of the element.

In atomic emission small
part of sample is vaporized forms free atom that attain energy from excitation
source results in transition from lower to higher energy state on returning
back emit a photon of radiation.

 

 

 

 

Principle
of atomic emission spectroscopy

Principle

Atomic emission spectroscopy is based on the principle
that when a molecule is applied energy in the form of light or heat, the
molecules get excited and move from lower energy level to higher energy level.
At this state the molecules are unstable. Therefore, the excited molecule jumps
from higher energy level to lower energy level, emitting radiation. The
radiations are emitted in the form of photons. The wavelengths of photons
emitted are recorded.

These radiations are recorded in the emission spectrometer. The level of
emissions of a substance is the difference between the substance in excited and
lower state.

Each element has different and unique level of emission which helps the
scientist to detect the elements. The frequencies are recorded as light spectra
in the emission spectrometer.

 

 

 

Instrumentation

 Comprises on:

•     
Source & Sample

•     
Atomizer

•     
Monochromator

•     
Detector & readout device

•     
Schematic
Diagram Of Instrumentation

 

Spectra

Components:
1. Light source

a.    Inductively
coupled plasma (ICP)

b.   Direct
current plasma (DCP)

c.    Flame

d.   Arc
and spark.

e.    Laser
induced breakdown

f.     Laser
induced plasma

g.   Microwave
induced plasma

 

 

Plasma Source:

 ” Plasma is
defined as a neutral gas containing significant number of both positive and
negative ions or free electrons”

 

Mechanism Of Inductively Coupled Plasma

 Inductively
coupled plasma atomic emission spectroscopy (ICP-AES) uses an inductively
coupled plasma to produce excited atoms and ions that emit electromagnetic
radiation at wavelengths characteristic of a particular element.

Advantages of ICP-AES are excellent limit
of detection and linear dynamic range, multi-element capability, low chemical
interference and a stable and reproducible signal. Disadvantages are spectral
interferences (many emission lines), cost and operating expense and the fact
that samples typically must be in a liquid solution.

 

•     
The inductively coupled plasma torch
consist of 3 concentric silica quartz tube.

•     
 Argon stream that carries the sample in the
form of an aerosol, passes through the central tube.

•     
 Plasma is initiated by a spark from a tesla
coil. Argon gas ionized and emission is measured.

Spark and arc atomic emission spectroscopy

 

Spark or arc
atomic emission spectroscopy is used for the analysis of metallic elements in
solid samples. For non-conductive materials, the sample is ground with graphite
powder to make it conductive. In traditional arc spectroscopy methods,
a sample of the solid was commonly ground up and destroyed during analysis. An
electric arc or spark is passed through the sample, heating it to a high
temperature to excite the atoms within it.

The excited analyte atoms
emit light at characteristic wavelengths that can be dispersed with a monochromator
and detected. In the past, the spark or arc conditions were typically not well
controlled, the analysis for the elements in the sample was qualitative.
However, modern spark sources with controlled discharges can be considered
quantitative. Both qualitative and quantitative spark analysis are widely used
for production quality control in foundry and metal casting facilities

 

 

Flame Source:

 

Mechanism:

•     
It consist of total consumption burner in
which sample is drawn through a capillary tube which injected directly into
flame  and flame is high temperature
source that is used to desolvate and vaporize a sample and generate free atom
for spectroscopic study.

 

 

 

Spark source:

 

Mechanism:

•     
It consist of the primary circuit a
voltage of 110-220V is maintained. The high voltage is obtained from setup
transformer which convert the line power to 15000-40000V which then charge the
capacitor. When energy stored in the capacitor, synchronous  trigger the spark between the electrodes.

 

Light source:

1.  
MONOCROMATORS

 

Monochromator

•     
It is a device use to transmit narrow band
of wavelength which is chosen from wavelength of wider range available.

•     
Types of monochromator:

•     
Prism Monochromator

•     
Grating Monochromator

 

 

a. Prism

•     
When the light pass through prism it
emerges out in form of two lines or beam.

•     
To overcome this drawback two half prism
are placed.

•     
When light pass through first prism it
splits into two beams, when it reaches second half prism recombines two beam
into single beam

 

b. Diffraction grating

•     
It gives better result and resolution.

•     
It replaced prism give linear dispersion.

•     
Problem occurred during the identification
of wavelength of emission lines on photographic plate solved through grating
monochromator.

•     
Once we identify known reference line and
other lines identified automatically.

 

 

 

 

 

DETECTORS

 

1.              
Phototube (Photo Electric
Cell)

•     
Glass filled or vacuum tube

•     
Sensitive to light

•     
Depend on frequency and intensity of
incoming photon

•     
Need amplifier. But are replaced by
photomultiplier detectors.

•     
 Vacuum phototubes, are
extremely sensitive detectors of light in the ultra violet, visible,
and near-infrared region ranges of the electromagnetic spectrum.

                    
APPLICATIONS

 

Applications include:

 

1.   It
is used for rapid analysis of  multi-component
pharmaceutical tablet.

2.   It
is used for elemental analysis.

3.   It
is used primarily for the identification and determination of metals in traces
amount.

4.   It
is used for determination of mineral composition of metamorphic rock.

5.   It
is used for routine analysis of wear metals in lubricating oils.

6.   It
is used for the analysis of sodium, potassium and lithium.