Something you must know – electrons are

found in increasing energy levels around the

nucleus.

#The electrons in the atom became excited i.e.

move to higher energy level (shell) when energy

is supplied

#The electron then return from an excited level

to a more stable energy level closer to the

nucleus.

#In making this transition between levels, the electron

released a FIXED quantity of energy (Quantum Energy) in

the form of electromagnetic radiation (light rays) of a fixed

wavelength.

The level of absorption of radiation

of a specific wavelength by a

sample is called its absorbance

*Absorbance is directly

proportional to the concentration

-The fraction of light of a given

wavelength absorbed will depend on

the concentration of the species in

solution.

-The higher the concentration, the

greater the amount of light absorbed

-This means that we can use the

absorbance of light for both qualitative

and quantitative analysis

• The corresponding spectrum may exhibit a continuum, or may have

superposed on the continuum bright lines (an emission spectrum) or dark

lines (an absorption spectrum), as illustrated in the following figure.

Origin of Continuum, Emission, and Absorption Spectra

• emission spectra are produced by thin gases in which the atoms do not

experience many collisions (because of the low density). The emission

lines correspond to photons of discrete energies that are emitted when

excited atomic states in the gas make transitions back to lower-lying

levels.

• A continuum spectrum results when the gas pressures are higher.

Generally, solids, liquids, or dense gases emit light at all wavelengths

when heated.

• An absorption spectrum occurs when light passes through a cold, dilute

gas and atoms in the gas absorb at characteristic frequencies; since the

re-emitted light is unlikely to be emitted in the same direction as the

absorbed photon, this gives rise to dark lines (absence of light) in the

spectrum.

Identifying an unknown compound

-The absorbance spectrum of a

particular compound is unique.

-A data bank of the absorbance spectra

for different compounds will allow the

analyst to identify an unknown

compound

Determining the concentration of a compound

in a mixture

-A series of standard solution is prepared

-The instrument is adjusted so that only light of

the wavelength of maximum absorbance is

passed through the test solution (sample) and

the standard solution (reference)

-By measuring the intensity of the light transmitted through

the sample, the instrument can provide a reading of its

absorbance

The readouts of absorbances for the standard

solution (reference) are use to produce a calibration

curve

• It identifies the element and determines the

concentration of the element in materials

•One of the most widely used of modern

instrumental technique

•Very versatile, being capable of detecting 68

elements

•Extremely sensitive, detecting concentrations of

element at parts per million (ppm) levels , in some

cases parts per billion (ppb)

•Used to determined the concentration of metal ions

in aqueous solution (for low concentration only,

accurate dilution is needed if the sample is too

concentrated)

•e.g. Drinking water, biological fluids (likes milk,

blood, urine), wine and effluent from industrial plant.

•If sample in solid form, it need to dissolve first.

•e.g alloys, hair, finger nails, foodstuffs, soil.

•If sample in gas form, it need to dissolve first also (pass the gaseous

sample through a solution that absorbs any constituents under

investigation) e.g cigarette smoke, exhaust gases from combustion of

fuels, air in indoor workplaces..

The practical requirements for atomic

absorption spectroscopy are as follows:

1.A source of radiation (Hollow Cathode

Lamp)

2.A system for placing the sample in the path

of the incident ray from the source

3.A monochromator which selects one

frequency of transmitted radiation to be

passed on to the detector

4.A detector to measure the intensity of the

transmitted radiation

Spectrophotometer

http://www.umd.umich.edu/casl/natsci/slc/slconline/ADVAA/AdvAA.swf

Schematic of a hollow-cathode lamp

As a source to produce a sharp, intense and constant

light beam with specific wavelengths, i.e. to produce

electromagnetic radiation having energies characteristic

of the metal (characteristic/specific/unique excitation

wavelength)

* Exp. Magnesium hollow cathode lamp use to detect

magnesium only.

-To vapourise or spray the sample (from

solution to gas) and mixed with the fuel-air

mixture

Flame mixture Exp:

air-acetylene (max temperature : 2250 C)

Nitrous oxide –acetylene (max temp :

2955°C)

- An “Atomic Cloud” is created, where

metallic compounds (in ion form) are

decomposed and the metal ions are

reduced to the elemental state by pick

up the free electrons in the flame.

Aspirator-burner + Flame == act as atomiser

To select one of the key absorption frequencies/

wavelengths for the metal under investigation.

In doing this, only a narrow spectral line impinges

on the detector.

Exp: Calcium- 422.7nm ; Iron- 248.3nm

The selected radiation is sent to a detector which

measures it intensity

1. The atoms absorb unique wavelength of light to excite

their electrons to higher energy levels

2. Only atoms of the same element as the material in the

cathode of the hollow cathode lamp will absorb the

radiation, WHY??

3. atoms/metal ions require different energies and

therefore will absorb different wavelengths. There

would be no interference from other atoms/ions

present

4. The amount of light being absorbed is

directly proportional to the number of the metal

atoms in the flame/i.e. the amount of light

absorbed measures the number of atoms

present (concentration)

-To do quantitative analysis by AAS, a calibration

curve must be obtained first

-Calibration data is obtained by measuring the

absorbance of a series of standard solutions of the

interested element

-A calibration curve can be obtained by plotting a

graph absorbance versus concentration

-The unknown concentration of the same element

can then be read from the curve by interpolation

AAS UV-Vis

deals with metallic atoms, it

excites valence electrons

deal with molecules, it excites

electrons in molecular orbitals

(i.e. electrons in bonds

, AAS operates in the visible

spectrum.

operates over UV and visible

part of the spectrum

a metal cathode lamp (matching

the sample)

uses a lamp that can generate a

wide spectrum

a monochromator to select one

of these wavelengths

uses a monochromator

to select the wavelength

required for the analysis

analyses vapourised metal

atoms

analyses solutions

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