1. Introduction to spectroscopy Atomic Absorption Spectroscopy (AAS) Week 12 and 13

2. Topics to be covered Importance of elemental analysis .
 Introduction to spectroscopy.
 Atomic Absorption Spectroscopy (AAS
 Atomic Emission Spectroscopy (AES).

3. Importance of elemental analysis
Monitoring levels of certain elements in samples ( eg. pharmaceutical products and standards) to detect the concentrations of these elements.
homodyalysis
protonialdyalysis
ringer lactate
Monitoring the levels of the toxic elements in samples ( eg. cosmetics, food supplements, entire plant or part of it) to ensure it’s safety.
With these information, we can take steps to approve or withdraw products from the markets.
Widely used in clinical chemistry and environmental laboratories.

4. Elements (Heavy Metals)
The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations.
They have a specific gravity that is at least 5 times the specific gravity of water.
Example: Arsenic 5.7,Cadmium 8.65 , Iron 7.9, Lead Mercury

5. Trace elements :
Heavy metals that are nutritionally essential for a healthy life.
Examples are ( Iron, Manganese, Copper and Zinc).

6. Commonly encountered toxic heavy metals
 Arsenic
 Lead
 Mercury
 Cadmium
 Iron
 Aluminum

7. Types of samples for analysis
 Pharmaceutical
product
 Standards
 Cosmetics
Food
supplements
Entire plant
or part of it
   
Mixture of
known & unknown
herb

8. Spectroscopy
Spectrometric methods are a large group of analytical methods
Spectroscopy is the science that deals with the interactions of radiation with matter (atomic and molecular).
The most widely used spectrometric methods are based on electromagnetic radiation (light, gamma rays, X-rays, UV, microwave, and radio-frequency).

9. Electromagnetic Radiation:
consists of discrete packets of energy, which we call photons
A photon consists of an oscillating electric field component, E, and an oscillating
magnetic field component, M.

10.  The characteristics of these fields are:
Orthogonal ( perpendicular ) to each other
Orthogonal to the direction of propagation of the photon
They flip direction as the photon travels
All photons (in a given, non-absorbing medium) travel at the same
velocity, v.

11. What is Frequency ()?
The number of flips, or oscillations, that occur in one second.
What is A Wavelength ()?
The physical distance in the direction of propagation over which the electric and magnetic fields of a photon make one complete oscillation.
Unit: Angstrom, nm, µm
Velocity Of Light = x m/s
The electromagnetic nature of all photons is the same, but photons can have different frequencies

12. where h is Planck's constant (6.62618x10-34 J·s)
The relationship between the light velocity, wavelength, and frequency is :
The energy, E, of one photon depends on its frequency of oscillation :
where h is Planck's constant ( x10-34 J·s)
v=
E = h = hv / 

13. When light passes through other media, the velocity of light 
The relationship between the speed of light c , wavelength, and frequency is :
When light passes through other media, the velocity of light 
Since the energy of a photon is fixed, the frequency of a photon does not change.
Thus for a given frequency of light, the wavelength must  as the velocity decreases.
The decrease in velocity is quantitated by the refractive index, n, which is the ratio of c to the velocity of light in another medium, v:
C=
n = c / v

14. Electromagnetic Spectrum:

15. 1- Absorption of Radiation
When radiation passes through a layer of solid, liquid, or gas, certain frequencies may be absorbed, a process in which electromagnetic energy is transferred to the sample.

16. Absorption promotes these particles from their ground state to more higher-energy excited state.

17. Tow types of absorption spectra:
Atomic absorption spectrum.
Molecular absorption spectrum.

18. Beer’s Law
Many compounds absorb radiation. The diagram below shows a beam of monochromatic radiation of radiant power P0 directed at a sample solution.
Absorption takes place and the beam of radiation leaving the sample has radiant power P.

20. The amount of radiation absorbed may be measured in a number of ways:
Transmittance, T = P / P0 % Transmittance, %T = 100 T
Absorbance,
A = log10 P0 / P A = log10 1 / T A = log10 100 / %T A = 2 - log10 %T 

21. The last equation, A = 2 - log10 %T , is worth remembering because it allows you to easily calculate absorbance from percentage transmittance data.
The relationship between absorbance and transmittance is illustrated in the following diagram:

22. The equation representing the Beer’s law:
A = ε b c
Where
A is absorbance (no units, A = log10 P0 / P ).
ε is the molar absorbtivity (is a measure of the amount of light absorbed per unit concentration) with units of L mol-1 cm-1.
b is the path length of the sample that is, the path length of the cuvette in which the sample is contained. We will express this measurement in centimeters.
c is the concentration of the compound in solution, expressed in mol L-1.

23. Beer’s law tells us that absorbance depends on the total quantity of the absorbing compound in the light path through the cuvette. If we plot absorbance against concentration, we get a straight line passing through the origin (0,0).

24. The working curves are used to
Determine the concentration of an unknown sample.
To calibrate the linearity of an analytical instrument.

25. What are the Processes by which a molecule can absorb radiation?
1- Rotational transition:
The molecule rotate about various axes, the energy of rotation being at definite energy levels, so the molecule may absorb radiation
and be raised to a higher rotational energy level.

26. 2- Vibrational transition:
The atoms or group of atoms within a molecule vibrate relative to
each other. The molecule may then absorb a discrete amount of energy
and be raised to a higher vibrational energy level..

27. The three types of internal energy are quantized
3- Electronic transition:
The electrons of molecule may be raised to a higher electron Energy.
The three types of internal energy are quantized

28. Rotational transitions: low energy E [long λ (microwave or far-infrared)]
Vibrational transitions: takes place at high energy E [ near, far infrared region]
Electronic transitions: takes place at higher energy E [visible and U.V region]
Type of Radiation
Frequency Range (Hz)
Wavelength Range
Type of Transition
gamma-rays
<1 pm
nuclear
X-rays
1 nm-1 pm
inner electron
Ultraviolet
400 nm-1 nm
outer electron
Visible
4-7.5x1014
750 nm-400 nm
near-infrared
1x1014-4x1014
2.5 µm-750 nm
outer electron molecular vibrations
Infrared
25 µm-2.5 µm
molecular vibrations
Microwaves 3x
1 mm-25 µm
molecular rotations, electron spin flips*
radio waves
<3x1011
>1 mm
nuclear spin flips*

29. Cont. Introduction to Spectroscopy
TRANSITION
SPECTRUM
TECHNIQUE
MAIN USE
Electronic Transitions
UV-vis
UV-vis spectroscopy
Atomic Absorption Spectroscopy
Quantitative Analysis
Vibrational Transitions IR
IR Spectroscopy
Functional groups
Structural Identification
Spin Orientation
Radio
NMR
Structure Identification

30.  Which molecules or atoms can absorb radiation?
For absorption to occur there must be change in the dipole moment (polarity) of the molecule.
i.e polar covalent bond in which a pair of electrons is shared unequally.
eg: of a molecule that can not exhibit a dipole moment.
N  N Can not exhibit a dipole and will not absorb in the I.R region.
eg. of a molecule that can exhibit a dipole moment.
O C O Unsymmetrical diatomic molecule, does have a permanent dipole
and so will absorb light.
OCO Vibration mode  symmetry and no dipole moment
OCO By induced dipole  dipole moment and the molecule can absorb I.R radiation.

31.  Atoms: Incase of atoms only electronic transition occurs.P O N M L K

32. Molecular absorption spectra
Atomic absorption spectra
The outer most electrons occupy ,  or n electronic energy in the ground state.
1- The outer most electrons occupy one of the atomic orbitals and have its energy levels [K, L, M, N,
s, s,p s,p,d s,p,d,f ]
Upon excitation electrons
raised to * or * energy levels
2- Upon excitation electrons are promoted to any permissible higher atomic energy levels
Since there are bonds, there are vibrational and rotational energy levels in both the ground and excited states
3-Since there are no bonds there are no vibrational or rotational energy levels in either the ground or excited state.
The analytical wavelength is the max.
4- The analytical wavelength is the resonance wavelength of the analyte
The spectra are in the form
of bands due to the presence
of very close, superimposed
and unresolved vibrational and
rotational energy levels in the
the excited state.
5- The spectra are line form.

33. Atomic Absorption Spectroscopy (AAS)
AAS was employed in the 1950’s
Used for qualitative and quantitative detection.
It’s used for the determination of the presence and concentrations of metals in liquid samples.
Metals that can be detected include Fe, Cu, Al, Pb, Ca, Zn, Cd and many more.
Concentrations range is in the low mg/L (ppm) range.

34. Elements that are highlighted in pink are detectable by AAS