atomic absorption spectroscopy and mass spectroscopy
DESCRIPTION
basic principle and working model of atomic absorption spectroscopy and mass spectroscopyTRANSCRIPT
ATOMIC ABSORPTION AND MASS SPECTROSOPY
G.Vanitha
ATOMIC ABSORPTION SPECTROSOPY
INTRODUTION
Atomic Absorption Spectroscopy is a very
common technique for detecting metals and
metalloids in samples.
It is very reliable and simple to use.
It can analyze over 62 elements.
It also measures the concentration of metals in
the sample.
HISTORYThe first atomic absorption spectroscopy was built
by CSIRO scientist Alan Walsh in 1954.
The first commercial atomic absorption
spectroscopy was introduced in 1959.
Elements detectable by atomic absorption are highlighted in pink
in this periodic table.
PRINCIPLE
The technique uses basically the principle that free atoms
(gas) generated in an atomizer can absorb radiation at
specific frequency.
Atomic Absorption spectroscopy quantifies the absorption
of ground state atoms in the gaseous state.
The atoms absorb ultraviolet or visible light and make
transitions to higher electronic energy levels. The analyte
concentration is determined from the amount of absorption.
Concentration measurements are usually determined from a
working curve after calibrating the instrument with
standards of known concentration.
INSTRUMENTATION
LIGHT SOURCEHollow Cathode Lamp are the most common radiation
source in AAS.
It contains a tungsten anode and a hollow cylindrical
cathode.
These are sealed in a glass tube filled with an inert gas
(neon or argon ) .
Each element has its
own unique lamp which
must be used for that
analysis .
Hollow Cathode Lamp for
Aluminum (Al)
NEBULIZER
Suck up liquid samples at controlled rate.
Create a fine aerosol spray for introduction into
flame.
Mix the aerosol and fuel and oxidant thoroughly
for introduction into flame.
ATOMIZER
Elements to be analysed needs to in atomic state.
Atomization is separation of particles into
individual molecules and breaking molecules into
atoms. This is done by exposing the analyte to high
temperature in a flame or graphite furnace.
Sample Atomization Technique
Flame Atomization
Electro thermal Atomization
Hydride Atomization
Cold-Vapor Atomization
Flame Atomization
Nebulizer suck up liquid samples at controlled
rate.
Create a fine aerosol spray for introduction into
flame.
Mix the aerosol and oxidant thoroughly
for introduction into flame.
An aerosol is a colloid of fine solid particles or
liquid droplets, in air or another gas.
Flame Atomization
sample mistSolid/gas aerosol
Gaseous molecules
Atoms
nebulization volatilizationdesolvationdissociation
Disadvantages of Flame
Atomization
Only 5-15% of the nebulized sample reaches the
flame.
A minimum sample volume of 0.5-1.0 ml is
needed to give a reliable reading.
Samples which are viscous require dilution with
a solvent.
Electro Thermal Atomization
Uses a graphite coated furnace to vaporize the
sample.
samples are deposited in a small graphite coated
tube which can then heated to vaporize and
atomize the analyte.
The graphite tubes are heated using a high
current power supply.
Advantages
Small sample size
Very little or no sample preparation is needed
Sensitivity is enhanced
Direct analysis of solid samples
Analyte may be lost at the ashing stage
The sample may not be completely atomized
Analytical range is relatively low
Disadvantages
MONOCHROMATOR
This is very important part in an AAS.
It is used to separate out all of the thousands of
lines.
A monochromator is used to select the specific
wavelength of light which is absorbed by the
sample, and to exclude other wavelengths.
The selection of the specific light allows the
determination of the selected element in the
presence of others.
DETECTOR
The light selected by the monochromator is
directed onto a detector that is typically a
photomultiplier tube, whose function is convert
the light signal into an electrical signal
proportional to the intensity.
The processing of electrical signal is fulfilled by
a signal amplifier.
The signal could be displayed for readout, or
further fed into a data station for printout by the
requested format
Calibration CurveA calibration curve is used to determine the
unknown concentration of an element in a
solution.
The instrument is calibrated using several
solutions of known concentrations.
The absorbance of each known solution is
measured and then a calibration curve of
concentration vs absorbance is plotted.
The sample solution is fed into the instrument, and
the absorbance of the element in this solution is
measured.
The unknown concentration of the element is then
calculated from the calibration curve
ApplicationsDetermination of even small amounts of metals
(lead, mercury, calcium, magnesium, etc.)
Environmental studies: drinking water, ocean
water, soil.
Food industry.
Pharmaceutical industry.
Presence of metals as an impurity or in alloys
could be done easily
Level of metals could be detected in tissue
samples like Aluminum in blood and Copper in
brain tissues
MASS SPECTROSOPY
IntroductionMass Spectroscopic method is one of the most popular
molecular analysis methods today.
Mass Spectroscopy is an analytical spectroscopic tool
primarily concerned with the separation of molecular
(and atomic) species according to their mass.
It is a microanalytical technique requiring only a few
nanomoles of the sample to obtain characteristic
information pertaining to the structure and molecular
weight of analyte.
It is not concerned with non- destructive interaction
between molecules and electromagnetic radiation.
Principle
Mass spectroscopy is the most accurate method
for determining the molecular mass of the
compound and its elemental composition.
In this technique, molecules are bombarded with
a beam of energetic electrons.
The molecules are ionised and broken up into
many fragments, some of which are positive
ions.
Mass spectra is used in two general ways:
To prove the identity of two compounds.
To establish the structure of a new a compound.
The mass spectrum of a compound helps to
establish the structure of a new compound in
several different ways:
It can give the exact molecular mass.
It can give a molecular formula or it can reveal
the presence of certain structural units in a
molecule.
TYPICAL DIAGRAM OF MS
METHODOLOGY
Gaseous or liquid substances that vaporize under
vacuum are admitted to a mass spectroscopy.
The gas is diluted by being partially pumped down to
a low pressure (molecular flow range) in a vacuum
chamber and ionized through electron bombardment.
The ions thus generated are introduced to a mass
filter and separated on the basis of their charge to
mass ratio.
IONISATION
The atom is ionised by knocking one or more
electrons off to give a positive ion. (Mass
spectrometers always work with positive ions).
The particles in the sample (atoms or molecules) are
bombarded with a stream of electrons to knock one or
more electrons out of the sample particles to make
positive ions.
ACCELERATION
The ions are accelerated so that they all have the same
kinetic energy.
The positive ions are repelled away from the
positive ionization chamber and pass through three
slits with voltage in the decreasing order.
The middle slit carries some intermediate voltage
and the final at ‘0’ volts.
All the ions are accelerated into a finely focused
beam.
DEFLECTION
The ions are then deflected by a magnetic field
according to their masses. The lighter they are, the more
they are deflected.
The amount of deflection also depends on the number
of positive charges on the ion -The more the ion is
charged, the more it gets deflected.
Different ions are deflected by the magnetic
field by different amounts. The amount of
deflection depends on:
The mass of the ion: Lighter ions are deflected
more than heavier ones.
The charge on the ion: Ions with 2 (or more)
positive charges are deflected more than ones
with only 1 positive charge.
DETECTION
The beam of ions passing through the machine is
detected electrically.
When an ion hits the metal box, its charge is
neutralized by an electron jumping from the metal on
to the ion.
That leaves a space among the electrons in the
metal, and the electrons in the wire shuffle along
to fill it.
A flow of electrons in the wire is detected as an
electric current which can be amplified and
recorded. The more ions arriving, the greater the
current.
APPLICATIONS
Pharmaceutical analysis
Bioavailability studies
Drug metabolism studies, pharmacokinetics
Characterization of potential drugs
Drug degradation product analysis
Screening of drug candidates
Identifying drug targets
Biomolecule characterization
Proteins and peptides
Oligonucleotides
Environmental analysis
Pesticides on foods
Soil and groundwater contamination
Forensic analysis/clinical
Reference
Mass Spectroscopy Amruta S. Sambarekar
Mass Spectroscopy - An Overview Dr. M. Vairamani, IPFT
http://www.uga.edu/~sisbl/aaspec.html
http://www.clu-in.org/char/technologies/graphite.cfm
B. Welz, M. Sperling, Atomic Absorption Spectrometry, Wiley-VCH, Weinheim, Germany, ISBN 3-527-28571-7.
Skoog, Douglas (2007). Principles of Instrumental Analysis (6th ed.). Canada: Thomson Brooks/Cole. ISBN 0-495-01201-7.
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