Atomic Absorption Spectroscopy

Sangam B. KanthaleSCHOOL OF PHARMACY

SWAMI RAMANAND TEERTH MARATHWADA UNIVERSITY, NANDED

Spectroscopy : Study of interaction of electromagnetic radiation

with matter.

Electromagnetic Radiation:These are the waves which is a form of alternating current of high frequency having the same speed as that of light.

Atomic Spectroscopy:This deals with the interaction of electromagnetic radiation with atoms which are most commonly in their lowest energy state called ground state

Electromagnetic Spectrum:

Absorption & Emission Spectra: Defn of spectroscopy. After interaction there is variation in intensity of

electromagnetic radiation.

Spectrometer:Instrument which records this variation in intensity.

Sample itself emits radiation- Emmision spectroscopy

Sample itself absorbs radiation- Absorption spectroscopy

Introduction to AAS:AAS is the most powerful instrumental method

for quantitative determination of trace metals in liquids.

Gives total metal content of the sample & it is independent on molecular form of the metal.

Determine concentration of-- 60-70 metals- metals- non-metals indirect method

Periodic table:

Principle:• AAS based on the absorption of energy by

ground state atoms in the gaseous state.

Total amount of light absorbed shown by equation-

v light absorbed = πe2 Nf

mc

Where,

e charge on electron of mass m

c the speed of light

N total number of atoms that may absorb

Advantages of atomic absorption spectroscopy over emission: AAS is independent on flame temperature.

i.e. variation in flame temperatures represents relatively less effects in AAS than in FES (flame emission spectroscopy) in which the smaller number of atoms are giving an emission signal.

In terms of detection limits AAS & FES are quite similar

It is specific since atoms of a particular elements can only absorbed radiation of their own characteristic wavelength.

Difference between AAS & FES:In FES the emission intensity is dependant upon the

number of exciting atoms & greatly influenced by temperature variation whereas in AAS depends upon number of unexcited atoms and absorption intensity independent on temperature.

Analytical signal results from sum of all energies emitted as excited atoms and in AAS signal is achieved by difference in intensities in presence and absence of liquid sample containing metals.

In AAS Beer’s law is obeyed i.e. absorption is directly proportional to concentration. This is not true in case of FES

Schematic Diagram of an Atomic Absorption Spectrometer

Light source(hollow cathode Lamp )

atomizer monochromat

or

Detector and readout device

Instrumentation:

The instrument consists of:1. Light source 2. atomizer3. Monochromator4. Detector

13

Light SourceDetector

SampleCompartment

Light Source

The light source is usually a hollow cathode lamp of the element that is being measured.

It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined.

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

cathode

Anode

Quartz window

Pyrex body

Anode

Cathode

From bottom to top, the lamps are for -Mg-Ca-K-combination of Fe, Co, Ni, Mn, Cu, and Cr. Each element uses a specific wavelength of light.

Atomizer:

Elements to be analyzed needs to be 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 temperatures in a flame.

The role of the atom cell is to primarily dissolvate a liquid sample and then the solid particles are vaporized into their free gaseous ground state form . In this form atoms will be available to absorb radiation emitted from the light source and thus generate a measurable signal proportional to concentration.

Flame :

Flame AA can only analyze solutions , where it uses a slot type burner to increase the path length, and therefore to increase the total

absorbance . Sample solutions are usually introduced into a

nebuliser by being sucked up a capillary tube .In the nebuliser the sample is dispersed into tiny droplets , which can be readily broken down in the flame.

Total Consumption Burner:

Premix Burner:

Types of Flames Used in Atomic Spectroscopy

Monochromators This is a very important part in an AA

spectrometer. It is used to separate out all of the thousands of lines. Without a good monochromator, detection limits are severely compromised.

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 to convert the light signal into an electrical signal proportional to the light 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.

Photomultiplier tube:

Applications:

1) Quantitative & Qualitative analysis2) Determination of metallic elements in biological

materials3) Simultaneous multicomponent analysis4) Determination of lead in petrol5) Determination of metallic elements in food

industry6) Determination of calcium, magnesium,sodium

and potassium in blood serum