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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