CHM260 - Spectroscopy Method

LECTURE 1

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The study of the interaction between ELECTROMAGNETIC (EM)

RADIATION and MATTER

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covers

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ATOMIC SPECTROSCOPY

(atomic absorption)

MOLECULAR SPECTROSCOPY

(molecular absorption)

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

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Direction of propagation

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

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

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Period (p) the time required for one cycle to pass a

fixed point in space. Frequency (V @ f )

the number of cycles which pass a fixed point in space per second. Unit in Hz or s-1

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

Radiant Power ( P )The amount of energy reaching a given area per second. Unit in watts (W)

Intensity ( I )The radiant power per unit solid angle.

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c = 3.00 x 108 m/s = 3.00 x 1010 cm/sc = 3.00 x 108 m/s = 3.00 x 1010 cm/s

Speed of light = Wavelength x Frequency

c = V

Where as is the wavelength of the wavesV is the frequency of the wavesc is the speed of light

Speed of light = Wavelength x Frequency

c = V

Where as is the wavelength of the wavesV is the frequency of the wavesc is the speed of light

Wavelength is inversely proportional to frequency

∝ 1/V The Higher the Frequency the Shorter the

Wavelength . The Longer the Wavelength the Lower the Frequency.

Wavelength is inversely proportional to frequency

∝ 1/V The Higher the Frequency the Shorter the

Wavelength . The Longer the Wavelength the Lower the Frequency.

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800 nm

Infrared radiationV = 3.75 x 1014 s-1

Ultraviolet radiationV = 7.50 x 1014 s-1

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EMR is viewed as a stream of discrete particles of energy called photons.We can relate the energy, E of photon to its wavelength, frequency and wavenumber by

E = hV = = hc h = Planck’s constant

h = 6.63 x 10-34 J.s

hc

E = hV = hc

Therefore wavenumber,

= 1/ = V/c

Unit of wavenumber is cm-1

hc

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What is the energy of a 500 nm photon?

V = c/ = (3 x 108 m s-1)/(5.0 x 10-7 m)

V = 6 x 1014 s-1 @ Hz

E = hV = (6.626 x 10-34 J•s)(6 x 1014 s-1) = 4 x 10-19 J

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Region Wavelength Range

UV 180 – 380 nm

Visible 380 – 780 nm

Near-IR 780 – 2500 nm

Mid-IR 2500 – 50000 nm

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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, μm 10-6 m

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Atoms are the basic blocks of matter. They consist of heavy particles (called protons

and neutrons) in the nucleus, surrounded by lighter particles called electrons.

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An electron will interact with a photon. An electron that absorbs a photon will gain

energy. An electron that loses energy must emit a

photon. For absorption to occur, the energy of the

photon must exactly match an energy level in the atom (or molecule) it contacts.◦Ephoton = Eelectronic transition

We distinguish two types of absorption◦Atomic◦Molecular

AbsorptionEMR energy transferred to absorbing molecule (transition from low energy to high energy state).

EmissionEMR energy transferred from emitting molecule to space (transition from high energy to low energy state).

Scatteringredirection of light with no energy transfer.

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Electrons bound to atoms have discrete energies (i.e. not all energies are allowed).

Thus, only photons of certain energy can interact with the electrons in a given atom.

Transitions between electronic levels of the electrons produce line spectra.

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Consider hydrogen, the simplest atom.

Hydrogen has a specific line spectrum.

Each atom has its own specific line spectrum (atomic fingerprint).

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The energy of photon that can promote electrons to excite/jump to a higher energy level depends on the energy difference between the electronic levels.

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Each atom has a specific set of energy levels, and thus a unique set of photon wavelengths with which it can interact.

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Absorption and emission for the sodium atom in the gas phase.

The diagram illustrate the transitions (excitation and emission) of electrons between different energy levels in sodium atom.

ΔEtransition = E1 - E0 = hv hv = hc/hc/

The energy, E, associated with the molecular bands:

Etotal = Eelectronic + Evibrational + Erotational

In general, a molecule may absorb energy in 3 ways:

1.By raising an electron (or electrons) to a higher energy level. (electronic)

2.By increasing the vibration of the constituent nuclei. (vibrational)

3.By increasing the rotation of the molecule about the axis. (rotational)

Eo

h

Absorption

En

Eo

h

Emission

En

h

Rotational absorption

Vibrational absorption

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Absorption spectrum◦A plot of the absorbance as a function of

wavelength or frequency.

Emission spectrum◦A plot of the relative power of the

emitted radiation as a function of wavelength or frequency.

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

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Absorption Spectrum of Na

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Electronic Transition Vibrational Transition Superimposed on the Electronic Transition

Absorption Band – A series of closely shaped peaks

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In solvents the rotational and vibrational transitions are highly restricted resulting in broad band absorption spectra.

Three types of spectra: ◦Lines◦Bands◦Continuum spectra

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Emission spectrum of a brine sample

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Absorption Spectroscopy

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Emission Spectroscopy

1. SourceA stable source of radiant energy

at the desired wavelength (or λ range). 2. Sample Holder

A transparent container used to hold the sample (cells, cuvettes, etc.).

3. Wavelength Selector A device that isolates a

restricted region of the EM spectrum used for measurement (monochromators, prisms, & filters).

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4. Photoelectric Transducer (Detector) Converts the radiant energy into a useable signal (usually electrical).

5. Signal Processor & Readout Amplifies or attenuates the

transduced signal and sends it to a readout device such as a meter, digital readout, chart recorder, computer, etc.

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Generate a beam of radiation that is stable and has sufficient power. A. Continuum Sources

emit radiation over a broad wavelength range and the intensity of the radiation changes slowly as a function of wavelength.

This type of source is commonly used optical instruments.

Deuterium lamp is the most common UV source.

Tungsten lamp is the most common Visible source.

Glowing inert solids are common sources for IR.

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B. Line Sources Emit a limited number lines or bands of radiation at specific wavelengths. Used in atomic absorption spectroscopy.

Types of line sources: 1.Hollow cathode lamps 2.Electrodeless discharge lamps 3.Lasers (Light amplification by stimulated emission of radiation)

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Sample containers usually is called cells or cuvettes, must have side/windows that are transparent in the spectral region of interest.

There are few types of cuvettes1. quartz or fused silica (below 350nm)

required for UV & VIS region

2. silicate glass (350 – 2000nm)cheaper compared to quartz. Used in VIS

3. crystalline sodium chlorideused in IR 45

Wavelength selectors provides a limited, narrow, continuous group of wavelengths called a band.

Two types of wavelength selectors: A) FiltersB) Monochromators

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Early detectors in spectroscopic instruments were the human eye, photographic plates or films. Modern instruments contain devices that convert the radiation to an electrical signal.

Two general types of radiation transducers:a. Photon detectors b. Thermal detectors

Commonly useful in ultraviolet, visible and near infrared instruments.Several types of photon detectors are available:

1. Vacuum phototubes 2. Photomultiplier tubes 3. Photovoltaic cells 4. Silicon photodiodes 5. Diode array transducers 6. Photoconductivity transducers

A. Photon Detectors

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B. Thermal DetectorsUsed for infrared spectroscopy because

photons in the IR region lack the energy to cause photoemission of electrons.

Three types of thermal detectors: 1. Thermocouples2. Bolometers 3. Pyroelectric transducers

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SPECTROMETERis an instrument that provides information

about the intensity of radiation as a function of wavelength or frequency.

SPECTROPHOTOMETERis a spectrometer equipped with one or more

exit slits and photoelectric transducers that permits the determination of the ratio of the radiant power of two beams as a function of wavelength as in absorption spectroscopy. 51

REGION SOURCE SAMPLE HOLDER

DETECTOR

Ultraviolet

Deuterium lamp Quartz /fused silica

Phototube, Photo

Multiplier tube, diode

array

Visible Tungsten lamp Silicate Glass /Quartz

Phototube, Photo

Multiplier tube, diode

array

Infrared Nernst glower (rare earth oxides or silicon carbide

glowers)

Salt crystals (crystalline

sodium chloride)

Thermocouples, bolometers

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