Working and Application of AAS

(Atomic Absorption Spectrometer)

Presented By:

Anuradha Verma

Research Scholar

Atomic absorption spectrometry

Analytical technique that measures the concentrations of

elements.

Sensitive, can measure down to parts per billion of a gram

(μg dm–3) in a sample.

The technique makes use of the wavelengths of light

specifically absorbed by an element. They correspond to the

energies needed to promote electrons from one energy level

to another, higher energy level

Atoms of different elements absorb characteristic

wavelengths of light.

Analyzing a sample to see if it contains a particular

element means using light from that element.

For example with Pb, a lamp containing Pb emits light

from excited lead atoms that produce the right mix of

wavelengths to be absorbed by any lead atoms from

the sample.

Principle

Atomic absorption spectroscopy is based on the same principle as the flame test used in

qualitative analysis

When an alkali metal salt or a calcium, strontium or barium salt is heated strongly in the Bunsen

flame, a characteristic flame colour is observed.

In the flame, the ions are reduced to gaseous metal atoms.

The high temperature of the flame excites a valence electron to a higher-energy orbital. The atom then emits energy in the form of (visible) light as

the electron falls back into the lower energy orbital (ground state).

The ground state atom absorbs light of the same characteristic wavelengths as it emits

when returning from the excited state to the ground state.

The intensity of the absorbed light is

proportional to the concentration of the element in the flame.

Quantitative Analysis

In AAS, the sample is atomised – converted into ground state free atoms in the

vapour state

Beam of electromagnetic radiation emitted from excited lead atoms is passed

through the vaporised sample.

Some of the radiation is absorbed by the atoms in the sample.

The greater the number of atoms in the vapour, the more radiation is absorbed. The

amount of light absorbed is proportional to the number of atoms.

A calibration curve is constructed by running several samples of known

concentration under the same conditions as the unknown.

The amount the standard absorbs is compared with the calibration curve and this

enables the calculation of the lead concentration in the unknown sample.

How it Works

Schematic diagram of an atomic absorption

spectrometer

Components of AAS

• light source

• a sample cell to produce gaseous atoms

• Means of measuring the specific light

absorbed.

Light Source “our e of light is a hollow athode la p .

Contains a W anode and a cylindrical hollow cathode made of the element to be

determined.

These are sealed in a glass tube filled with an inert gas – e.g. Ne or Ar– at a pressure of

between 1 Nm–2 and 5 Nm–2.

The ionisation of some gas atoms occurs by applying a potential

difference of about 300–400 V between the anode and the

cathode.

These gaseous ions bombard the cathode and eject metal atoms

from the cathode in a process called sputtering.

Some sputtered atoms are in excited states and emit radiation

characteristic of the metal as they fall back to the ground state.

Optical System and Detector

Monochromator:

Select the specific wavelength of light (spectral line) 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. This

produces an electrical signal proportional to the light

intensity

A photomultiplier measures the intensity of

the incident light and generates an electrical

signal proportional to the intensity.

Atomisation of the sample

Two systems are commonly used to produce

atoms from the sample.

Aspiration involves sucking a solution of the sample

into a flame.

Electrothermal atomisation : where a drop of

sample is placed into a graphite tube that is then

heated electrically.

Flame aspiration

Ethyne/air (flame temperature of 2200–2400 °C)

Ethyne/N2O (flame temperature of 2600– 2800 °C)

A flexible capillary tube connects the solution to the nebuliser. At

the tip of the apillary, the solutio is nebulised – i.e. broken

into small drops.

The larger drops fall out and drain off while smaller ones

vaporise in the flame.

Only about 1% of the sample is nebulised.

Sample preparation

Chemical form of the element is usually

unimportant. This is because atomization

converts the sample into free atoms irrespective

of its initial state.

The sample is weighed and made into a solution

by suitable dilution.

Background absorption

It is possible that other atoms or molecules apart from those of the element being

determined will absorb or scatter some radiation from the light source.

Includes unvaporised solvent droplets, or compounds of the matrix (chemical species,

such as anions, that tend to accompany the metals being analysed) that are not removed

completely.

This means that there is a background absorption as well as that of the sample.

One way of measuring and correcting this background absorption is to use two light sources, one of which is the hollow cathode lamp appropriate to the element being measured. The second light source

is a deuterium lamp.

Calibration

A calibration curve is used to determine the unknown

concentration of an element

e.g. lead – in a solution.

The instrument is calibrated using several solutions of known

concentrations.

A calibration curve is produced which is continually rescaled as

more concentrated solutions are used – the more

concentrated solutions absorb more radiation up to a

certain absorbance.

The calibration curve shows the concentration against the

amount of radiation absorbed

The sample solution is fed into the instrument and the

unknown concentration of the element – eg lead – is then

displayed on the calibration curve

Application

Determination of Trace Elements in Rice Products

Samples were dried for 12 hours at 85 oC. The samples were cooled and portions of

approximately 0.25 g were accurately weighed and transferred to microwave

digestion vessels.

4 ml nitric acid was added to each vessel and left uncovered for 1 hour. A further 4 ml

nitric acid was added to each vessel, the vessels sealed and samples digested in a high

pressure closed microwave digestion system, by ramping over 20 minutes to 170 oC.

Samples were left to cool before being made up to 250 ml with deionized water for

cadmium analysis. Duplicate samples were prepared for the analysis of copper, lead,

Manganese and zinc (from the same sample) with the digests made up

to 50 ml with deionized water.

Analysis of Zn and B in fertilizer

• Dissolve the mg amount of fertilizer in say in

100mL of water and analyse for Zn, B etc.

ANALYSIS OF COPPER IN VITAMINS

VITAMIN:

Place a mulit-vitamin tablet in a 125mL Erlenmeyer flask and add 25 mL of 6

M HCL.

Pla e this o a hot plate a d ri g it to a oil. “et the hot plate o High.

As soon as the mixture begins to boil, reduce the heat to a setting of about 4

on the hot plate and continue to heat it for 15 additional minutes.

ADD ADDITIONAL HCl if necessary to keep the mixture wet and the volume at

About 25 mL.

Filter the resultant mixture through filter paper into a 25 mL volumetric flask.

Dilute to the mark with distilled water and thoroughly mix.

Presence of chromium in sea water

Linked to skin disorder dermatitis.

• A sample of sea water was analyzed using AA

spectroscopy, along with six standard solutions.

• Can use calibration curve to find the

concentration of the chromium in the sea water.

Application in Various Fields

Clinical analysis

Analysing metals in biological

fluids such as blood and urine.

Environmental analysis

finding out the levels of various

elements in rivers, seawater, drinking water, air, petrol and drinks such as wine, beer and fruit drinks

Pharmaceuticals

minute quantities of a catalyst used in the process (usually a metal) are sometimes present in the final product

Industry

check major elements present, toxic impurities are lower than specified

Mining

metals such as gold in rocks can be determined to see whether it is worth mining the rocks to extract the gold

Advantages

• inexpensive (equipment, day-to-day running)

• high sample throughput

• easy to use

• high precision

Disadvantages of Flame Atomic Absorption

Spectroscopy

• only solutions can be analysed

• relatively large sample quantities required (1 –

2 mL)

• less sensitivity (compared to graphite furnace)

• problems with refractory elements

Thank You