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Prepared by:Wan Elina Faradilla Wan KhalidFaculty of Applied ScienceUiTM Negeri Sembilan

Properties of electromagnetic radiation

Electromangnetic spectrumTransmittanceAbsorbanceBeer’s LawLimitations of Beer’s Law

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The study of the interaction between ELECTROMAGNETIC (EM) RADIATION and MATTER

Measuring amount of radiation produced/absorbed by molecular/atomic species

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ATOMIC SPECTROSCOPY

MOLECULAR SPECTROSCOPY

covers

What is Electromagnetic Radiation?

is a form of energy that has both Wave and Particle Properties.

For example: Ultraviolet, visible, infrared, microwave, radio wave.

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Wave-like propertiesWave-like properties

Particle-like properties (photon or quanta)

Particle-like properties (photon or quanta)

5Photon: unit of energy equal to hv (h is planck constant & f is frequency)

Energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field

Result of the acceleration of a charged particle

Does not require a material medium and can travel through a vacuum

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EM radiation is conveniently modeled as waves consisting of perpendicularly oscillating electric and magnetic fields, as shown in the diagram.

At 90° to the direction of propagation is an oscillation in the ELECTRIC FIELD.

At 90° to the direction of propagation and 90° from the electric field oscillation (orthagonal) is MAGNETIC FIELD oscillation.

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Period (p) – the time required for one cycle to pass a fixed point in space.

Frequency (v) – the number of cycles which pass a fixed point in space per second.

v = 1/p (s-1 = Hz)

v depends on the source, but is independent of the propagating (transmitting) material.

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Amplitude (A) – The maximum length of the electric vector in the wave (maximum height of a wave).

Wavelength (λ) – The distance between two identical adjacent points in a wave (usually maxima or minima)

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Velocity of propagation (vi)The rate at which a wave front moves

through a propagating medium in meters per second (ms-1). vi = vλi

Velocity of light- is given special symbol, c = 3.00 108 ms-1 = 3.00

1010 cms-1

C = vλ = 3.00 108 ms-1

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Wavenumber (v) – The number of waves per cm in units of cm-1 .

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Speed of light = Wavelength x Frequency

c = V = c/V V = c/

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For Electromagnetic Waves the Speed (c) is a Constant

c = 3.00 x 108 m/sec = 3.00 x 1010 cm/secc = 3.00 x 108 m/sec = 3.00 x 1010 cm/sec

This Constant Speed Means a Direct, Inverse Relationship Between Wavelength and Frequency

∝ 1/V

The Higher the Frequency the Shorter the Wavelength . The Longer the Wavelength the Lower the Frequency.14

Wavelength is inversely proportional to frequencyWavelength is inversely proportional to frequency

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800 nm

Infrared radiationV = 3.75 x 1014 s-1

Ultraviolet radiationV = 7.50 x 1014 s-1

Pic 1. Pic 2

Wave theory failed to explain phenomena associated with the absorption and emission of radiation of radiant energy.

Thus, EM is viewed as a stream of discrete particles of energy called photons.

We can relate the energy of photon to its wavelength, frequency and wavenumber by

E = hv = hc = hcv λ h – Planck’s constant = 6.63 10-34 J.s

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IBM scientists have developed a prototype computer chip that operates at a frequency of 350 GHz. Calculate the energy in joules.

Solution:

E = hv = 6.63 x 10-34 J.s x 350 x 109 Hz = 6.63 x 10-34 J.s x 350 x 109 s-1 = 2.32 x 10-22 J

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For electromagnetic radiation with a wavelength of 562 nm ;

 i) Calculate the frequency in Hz.ii)Calculate the energy of this

radiation.

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1 nm = 1x 10-9 m

The electromagnetic spectrum covers a wide range of wavelengths and frequencies.

The divisions are based on the methods used to produce and observe the various types of radiation.

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Region Wavelength Range

UV 180 – 380 nm

Visible 380 – 780 nm

Near-IR 0.78 – 2.5 m

Mid-IR 2.5 – 50 m

Regions of the UV, Visible and IR Spectrum

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Region Unit Definition (m)

X-ray Angstrom unit, Å

10-10 m

Ultraviolet/visible

Nanometer, nm

10-9 m

Infrared Micrometer, m

10-6 m

Wavelength Units for Various Spectral Region

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Types of spectroscopic methods based on EM radiation

1 Å = 10-10 m = 10-8 cm1 nm = 10-9 m = 10-7 cm1 m = 10-6 m = 10-4 cm

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You have to

remember!!

Name four (4) types of electromagnetic radiation

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Spectroscopist use the interaction of radiation with matter to obtain information about a sample.

Absorption of Radiation: Absorption occurs only if :- There is an interaction between the

electromagnetic radiation and the material. Sample usually stimulated by applying energy in

the form of heat, electrical energy, light or a chemical reaction.

The stimulus then causes some of the analyte species to undergo a transition to a higher energy state from ground state

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Measure the amount of EM radiation emitted as it returns to the ground state/ measuring the amount absorbed as the result of the excitation

Io Sample I

I < Io (absorption depends on concentration of absorbing molecule and path length)

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Quantitative absorption methods require two power measurements:

one before a beam has passed through the medium that contains the analyte (Po) and the other after the sample (P).

Two terms, which are widely used in absorption spectrometry and are related to the ratio of Po and P, are transmittance and absorbance.

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T = P/ Po (value from 0 to 1)

Transmittance: The transmittance T of the medium is the fraction of incident radiation transmitted by the medium.

where, Po is the incident power of the beam and P is the power of the beam after absorbed by the sample.

Transmittance is often expressed as a percentage,%T = P/ Po x 100 % (value from 0 to 100)

Absorbance: The absorbance A of a medium is defined by the equation,A = -log10T = -log P/ Po = log Po/ P = log 1/T

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remember

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1. Convert the following percent transmittance data into absorbance:

i) 33.6 ii) 92.1 iii) 1.75

Answer: i) 0.474ii) 0.0357iii) 1.76

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i) %T = 33.6T = 33.6 / 100

= 0.336

A = - log T = - log 0.336 = 0.474

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2. Convert the following absorbance data into percent transmittance:

i) 0.375 ii) 1.325 iii) 0.012

Answer: i) 42.2% ii) 4.73% iii) 97.3%

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How to solve??

Antilog -

Beer’s Lambert Law shows linear relationship between

absorbance, concentration of the species measured, sample path length and the absorptivity of the species.

= molar absorptivity, liter mol-1 cm-1

b = sample path length, cmc = concentration, mol per liter

A = bc

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A = abc

Absorbance

Absorptivity(Lg-1cm-1)

Pathlength(cm)

Concentration(g/liter)

- The term “a” is a proportionality constant called absorptivity.

- Absorptivity is a constant for a given chemical species at a specific wavelength.

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E.g. 1. What is the concentration of an absorbing

species if its molar absorptivity is 1500 L/mol.cm and the measured absorbance in a 1.00 cm cuvette is 0.742?

Answer:A = bc c = A / b

c = 0.742 = 4.95 10-4 M(1 cm)(1500 L/mol.cm)

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E.g:2. The measured absorbance of a sample in

a 1.00 cm cuvette is 0.544. If the concentration is 1.40 10-3 M, what is the molar absorptivity for the species?

Answer:A = bc = A / bc

c = 0.544 = 389 L/mol.cm (1 cm)(1.40 10-3 mol/L)

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E.g.:3 A sample in a 1.0 cm cell is determined with a

spectrometer to transmit 80% light at a certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, what is the concentration of the substance?

Answer: The percent transmittance is 80%. So, T=0.80. Value

for A = 0.0969 A = abc - log T = 2.0 L/g.cm 1 cm c - log 0.80 = 2.0 L/g c c = 0.0969 = 0.0485 g/L

2.0 L/g37

A 25.6 mg sample of a compound with a molecular weight of 317.17 g/mol was dissolved with solvent in a 300 mL volumetric flask. The absorbance of this solution at 238 nm was 0.624 in a 1.00-cm cuvette. Calculate the molar absorptivity of this compound

M = mol/L

Ans: 2.32 x 103 L mol-1 cm-1

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Need to find the

concentration (molarity) of the sample

first

A = abc

ConcentrationWidth of cuvetteInherent ability for absorbing species to absorb light

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Inherent ability for the absorbing species to absorb light

Chemical species vary with respect to this inherent ability since absorption depends on individual electronic and vibrational transitions available in a given species.

Width of cuvette

Wider cuvette more absorbing species present in the path of the light, hence absorbance is greater

b

b

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Applying Beer’s Law to Mixtures

- Beer’s law also applies to solutions containing more than one kind of absorbing substance, provided there is no interaction among the various species.

- Total absorbance for a multicomponent system at a single wavelength is the sum of the individual absorbances.

Atotal = A1 + A2 + …………+ An = 1bc1 + 2bc2 + ……… + nbcn

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Deviations are frequently observed from the direct proportionality between absorbance, A and concentration, c when pathlength, b is constant.

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Deviations may be due to:

1. Fundamental/Real deviations2. Instrumental deviations3. Chemical deviations

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1. Fundamental deviations- Real limitation to the law- At high concentration (> 0.01M) each

particle affects the charge distribution of its neighbours.

- Therefore, this interaction alter the ability of analyte species to absorb a given wavelength of radiation.

Causing deviation from the linear relationship between absorbance and concentration.

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2. Instrumental deviationsdue to polychromatic radiation

Beer’s Law strictly applies when measurements are made with monochromatic source radiation.

In practice, polychromatic sources that have a continuous distribution of wavelengths are being used.

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2. Instrumental deviationsdue to presence of stray radiationDue to instrument imperfections.This stray radiation is the result of

scattering and reflection off the surfaces of gratings, lenses or mirrors, filters and windows.

The wavelength of stray radiation differs greatly from the principal radiation & may not have passed thru’ the sample.

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3. Chemical deviations

- Occur when the analyte undergo dissociation, association or reaction with the solvent to give products that absorb differently than the analyte.

Eg: aqueous solutions of acid-base indicators

HIn H+ + In-

color 1 color 247

1. Absorption spectrum2. Emission spectrum

Absorption spectrum A plot of absorbance versus wavelength

Emission spectrum A plot of the relative power of the

emitted radiation versus wavelength.

When radiation passes through a sample certain frequency may be selectively removed by absorption, a process in which electromagnetic energy is transferred to the atoms, ions or molecules. Absorption promotes these particles from ground state to one or more higher energy states. The energy of the exciting photon must exactly match the energy difference between the ground state and one of the excited states of the absorbing species. Since these energy differences are unique for each species, a study of the frequencies of absorbed radiation provides a means of characterizing the constituents of a sample of matter.

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Atomic Absorption: The passage of polychromatic ultraviolet or visible radiation through a medium that consists of monoatomic particles results the absorption of a few well-defined frequency. Such spectra is very simple due to the small number of possible energy states for the absorbing particles.

Molecular Absorption: Absorption spectra for polyatomic molecules are considerably more complex than atomic spectra because the number of energy states of molecules is generally enormous when compared with the number of energy states for isolated atoms. The energy E of a molecule is made up of three components,E = Eelectronic + Evibrational + Erotational 50

The two peaks arise from the promotion of a 3s electron to the two 3p states

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Absorption Spectrum of Na

The sharpness of molecular absorption spectra also depends on the state of the sample.

Figure (b) shows an absorption band due to transitions between electronic-vibrational-rotational states

Figure (d) shows a continuous spectra due to the sample is in the condensed state. In condensed states the spectra broaden due to molecular collisions. 52

Atomic transitions are usually very discrete changes of electrons from one quantum state to another energy level (shells, spins, etc)

Only electronic transition is quantized. No vibrational or rotational transition.

E0 – lowest energy electronic level or ground stateE1, E2 – higher-energy electronic levels

• When an atom changes energy state, it absorbs or emits energy equal to the energy difference

∆E = E1 – E0

•The wavelength or frequency of radiation absorbed or emitted during a transition proportional to ∆E

∆E = hv = hc

produce line spectra 53

Molecular transition consists of 3 processes:

- Rotational transition- Vibrational transition- Electronic transition

∆E = ∆Eelectronic + ∆Evibrational + ∆E rotational

Due to vibrational and rotational transitions - produce band spectra

Energy

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Rotational Transition- Molecule rotates about various axes- Erotation being at definite Elevels

- Therefore, the molecule absorb radiation and be raised to a higher rotational energy level.

Vibrational Transition- Atoms or group of atoms within the molecules vibrate relative to

each other- Evibration occur at quantized levels- Therefore, the molecule absorb radiation and be raised to a higher

vibrational energy level.

Electronic Transition- Electron of a molecule may be raised to a higher electron energy .

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Emission of Radiation: Electromagnetic radiation is produced when excited particle (atoms, ions, or molecules) relax to lower energy levels by giving up their excess energy as photons. Radiation from an excited source is characterized by means of an emission spectrum. X X* X + h

Excitation can be done by –1. Bombardment with electrons2. Electric current ac spark3. Heat of a flame4. An arc or a furnace (produce uv, vis or ir radiation)5. Beam of electromagnetic radiation

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Three types of emission spectra:

1. Line Spectra2. Band Spectra3. Continuum Spectra

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58Emission spectrum of a brine sample