Atomic Absorption Spectrometry
Atomic Absorption Spectrometry
Atomic Absorption Spectrometry
Atomic absorption
Types of atomic
spectroscopy:
• fluorescence following
absorption of laser radiation
An important difference between atomic and molecular spectroscopy is the width
of absorption or emission bands.
Absorption and emission spectra of liquids and solids typically have bandwidths of
10 to 100 nm,.
In contrast, spectra of gaseous atoms consist of sharp lines with widths of 0.001
nm
Atomic absorption spectrophotometer is used to measure concentration of trace
metals by detecting absorption of electromagnetic radiation by atoms rather than by
molecules.
Atomic emission spectroscopy first used and the underlying principles were
established in the 2nd half of the 19th century by Robert Wilhelm Bunsen and
Gustav Robert Kirchhoff, both professors at the University of Heidelberg, Germany
Modern form of AAS was developed in 1950s by a team of Australian chemists led
by Sir Alan Walsh at the Commonwealth Scientific and Industrial Research
Organisation (CSIRO), Division of Chemical Physics, in Melbourne, Australia
Atoms absorb light in the ground state and make transition to higher energy levels.
Light from cathode lamp on entering the flame some of it is absorbed from the
ground state atom resulting in net decrease of intensity of beam. This process is
called as atomic absorption.
Instrumentation
Components
Hollow Cathode Lamp is made of metal of the substance to be analysed
Current is applied to the cathode and metal atoms collide with neon or argon gas .
They loose their energy on collision and emit their characteristic radiation.
In an ideal interference free system only the same sample metal atoms absorb
light from cathode when it passes through the flame.
Light originating from the Cathode Lamp is pulsed while that from flame is
continuous
Detector detects all light but amplifier is tuned to pulsed signals and can subtract
the background light helping to discriminate between flame background emission
and sample atomic absorption
Nebulisation
Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant
(usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it
leaves the capillary. The spray is directed against a glass bead, upon which the
droplets break into smaller particles. The formation of small droplets is termed
nebulization.
Excess liquid collects at the bottom of the spray chamber and flows out to a drain.
Aerosol reaching the flame contains only about 5% of the initial sample.
Premix burner
The sample is aspirated volatilise and burned to form atoms of metal in the gas
phase.
The advantage of this laminar flow premix burner is that large droplets go to waste
while the fine mist enters the flame.
Most flame
spectrometers
use a premix
burner, such as
that in Figure, in
which
fuel,oxidant, and
sample are mixed
before
introduction into
the flame.
Atomiser
Atomisation is separation of particles and breaking molecules into atoms.
Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant
(usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it
leaves the capillary. The spray is directed against a glass bead, upon which the
droplets break into smaller particles. The formation of small droplets is termed
nebulization.
Excess liquid collects at the bottom of the spray chamber and flows out to a drain.
Aerosol reaching the flame contains only about 5% of the initial sample.
Furnaces
Furnaces: An electrically heated graphite furnace is more sensitive than a flame and
requires less sample. From 1 to 100 mL of sample are injected into the furnace tube
through the hole at the center.
Temperature of rod is raised in steps to dry, char (remove organic material) and
finally atomizes the sample into gas phase.
This technique is more sensitive and permits determination of trace metals in small
samples
Prism
Diffraction Grating
Detectors
It is a photomultiplier tube whose function is to convert light signal into an electrical
signal proportional to the intensity
Zeeman effect
Chopper open: Flame + Cathode lamp
light reaches detector
Light from the hollow-cathode lamp is absorbed by analyte and absorbed and
scattered by background.
The difference between absorbance measured with the hollow-cathode lamp and
absorbance measured with the D2 lamp is the absorbance of analyte.
Zeeman effect
When a magnetic field is applied parallel to the light path through a furnace, the
absorption (or emission) line of analyte atoms is split into three components. Two
are shifted to slightly lower and higher wavelengths and one component is
unshifted.
Strong magnetic field is pulsed on and off. Sample and background are observed
when the field is off. Background alone is observed when the field is on. The
difference is the corrected signal.
Advantage of Zeeman
background correction is that it
operates at the analytical
wavelength. In contrast, D2
background correction is made
over a broad band
Interference in Atomic absorption Spectometry
Interference is any effect that changes the signal while analyte concentration
remains unchanged. It is of two types: Spectral and non spectral
Spectral Interference:
Solute volatilisation : Contaminant forms non volatile species with the analyte
ICP MS can measure multiple metals simultaneously and with greater sensitivity