# Spectroscopy Methods

Urban and Civil

Nov 16, 2013 (4 years and 7 months ago)

93 views

INTRODUCTION TO
SPECTROSCOPIC METHODS
OF ANALYSIS

LECTURE 1

1

WHAT IS SPECTROSCOPY?

The

study
of the interaction between
MATTER

2

SPECTROSCOPIC ANALYSIS

covers

ATOMIC
SPECTROSCOPY

MOLECULAR
SPECTROSCOPY

3

T
O

U
NDERSTAND

S
PECTROSCOPY

W
E

M
UST

U
NDERSTAND

E
LECTROMAGNETIC

R

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

For example: Ultraviolet, visible, infrared,

4

WAVE PROPERTIES

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

x

y

z

Electric Field

Magnetic Field

Direction of
propagation

5

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.

6

W
AVE

PARAMETERS

We Use Symbols to Designate the Various
Properties of Waves

is the wavelength of the waves

V

is the frequency of the waves

c is the speed of light

Time or Distance

-

+

Electric Field

0

Amplitude (A)

Wavelength (

)

7

D
EFINITIONS
:

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.

Amplitude

(A)

The maximum length of the electric vector in the
wave (Maximum height of a wave).

Wavelength

(

)

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

Wavenumber

(

)

-

The number of waves per cm in units of cm
-
1
.

8

(
P
)

-

The amount of energy reaching a given area
per second.

Unit in watts (W)

Intensity

( I
)

-

The radiant power per unit solid angle.

9

D
EFINITIONS
:

R
ELATIONSHIP

B
ETWEEN

T
HESE

V
ARIABLES

Speed of light = Wavelength x Frequency

c =

V

=
c/
V

V

=
c/

For Electromagnetic Waves the Speed (
c) is a Constant

c = 3.00 x 10
8

m/sec = 3.00 x 10
10

cm/sec

10

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.

11

T
HE

R
ELATIONSHIP

B
ETWEEN

F
REQUENCY

AND

W
AVELENGTH

Wavelength is inversely proportional to frequency

12

800 nm

V = 3.75 x 10
14

s
-
1

V = 7.50 x 10
14

s
-
1

P
ARTICLE

PROPERTIES

OF

L
IGHT
:
P
HOTONS

Wave theory failed to explain phenomena associated with the

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

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

E

=
hV

V
-

frequency

= h c

-

wavelength

υ

-

wavenumber

=
hc
υ

h

Planck’s constant
=6.63x10
-
34

J∙s

13

T
HE

E
LECTROMAGNETIC

S
PECTRUM

14

R
EGIONS

OF

THE

UV, V
ISIBLE

AND

IR S
PECTRUM

Region

Wavelength Range

UV

180

380 nm

Visible

380

780 nm

Near
-
IR

0.78

2.5
μ
m

Mid
-
IR

2.5

50
μ
m

15

P
REFIXES

FOR

U
NITS

Prefix Symbols Multiplier

giga
-

G 10
9

mega
-

M 10
6

kilo
-

k 10
3

deci
-

d 10
-
1

centi
-

c 10
-
2

milli
-

m 10
-
3

micro
-

µ 10
-
6

nano
-

n 10
-
9

pico
-

p 10
-
12

femto
-

f 10
-
15

atto
-

a 10
-
18

16

W
AVELENGTH

U
NITS

FOR

V
ARIOUS

S
PECTRAL

R
EGION

Region

Unit

Definition (m)

X
-
ray

Angstrom unit,
Å

10
-
10

m

Ultraviolet/visible

Nanometer, nm

10
-
9

m

Infrared

Micrometer,
μ
m

10
-
6

m

17

INTERACTION OF ELECTROMAGNETIC

Infrared primarily acts to set molecules into vibration.

UV and visible light primarily acts to elevate electrons to higher energy levels.

18

INTERACTION OF ELECTROMAGNETIC

The interaction of radiation with matter can cause redirection
of the radiation and/or transitions between the
energy levels

of the atoms or molecules.

1.
A transition from a lower level to a higher level with transfer
of energy from the radiation field to the atom or molecule is
called
absorption
.

2.
A transition from a higher level to a lower level is called
emission

if energy is transferred to the radiation field, or

decay if no radiation is emitted.

3.
Redirection of light due to its interaction with matter is called
scattering
, and may or may not occur with transfer of energy,
i.e., the scattered radiation has a slightly different or the same
wavelength.

19

T
YPES

OF

SPECTRA

1.

Absorption spectrum

2.

Emission spectrum

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.

20

ATOMIC
vs

MOLECULAR TRANSITIONS

21

A
TOMIC

TRANSITION

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

Only electronic transition is quantized.

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

E = E
1

E
0

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

E

Transitions between electronic levels produce
line
spectra
.

22

A
TOMIC

TRANSITION

E
0

lowest energy electronic level or ground state

E
1
, E
2

higher
-
energy electronic levels

23

M
OLECULAR

TRANSITION

In molecules the electronic states are subdivided
into vibrational states.

The energy of a band in a molecular absorption
spectrum is the sum of three different energy
components.

E = E
electronic

+ E
vibrational

+ E
rotational

Transitions between electronic
-
vibrational
-
rotational states give rise to spectra that appear
to have bands.

24

25

Vibrational
energy level

M
OLECULAR

TRANSITION

Energy

A
TOMIC

ABSORPTION

SPECTRUM

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

26

Absorption Spectrum of Na

M
OLECULAR

ABSORPTION

SPECTRA

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.

27

E
MISSION

SPECTRUM

Three types
of spectra:

Lines

Bands

Continuum
spectra

28

Emission spectrum of a brine sample

COMPONENTS OF INSTRUMENTS
FOR OPTICAL SPECTROSCOPY

29

G
ENERAL

D
ESIGN

OF

O
PTICAL

I
NSTRUMENTS

Absorption

Emission

30

F
IVE

B
ASIC

O
PTICAL

I
NSTRUMENT

C
OMPONENTS

1)
Source

-

A stable source of radiant energy at the desired wavelength (or

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).

4)
Photoelectric Transducer

-

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

5)

-

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

31

I.

S
OURCES

OF

R

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

A. Continuum Sources

-

of the radiation changes slowly as a function of wavelength.

This type of source is commonly used in UV, visible and IR
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 instruments
.

32

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

-

L
ight
a
mplification by
s
timulated
e
mission of
r

33

II.

W
AVELENGTH

S
ELECTORS

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

Two types

of wavelength selectors:

A)

Filters

B)

Monochromators

34

A.

F
ILTERS

Two types of filters:

1)

Interference filters

2)

Absorption Filters

B.

Monochromators

Wavelength selector that can continuously scan a broad range of
wavelengths

Used in most scanning spectrometers including UV, visible, and IR
instruments.

35

III.

R

T
RANSDUCERS

(D
ETECTORS
)

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

36

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

Commonly useful in ultraviolet, visible, and near infrared instruments.

37

Three types of thermal detectors :

1.

Thermocouples

2.

Bolometers

3.

Pyroelectric transducers

B.

Thermal Detectors

Used for infrared spectroscopy because photons in the IR region lack
the energy to cause photoemission of electrons.

38

Sample containers, usually called
cells

or
cuvettes

must have
windows that are transparent in the spectral region of interest.

There are few types of
cuvettes
:

-

quartz or fused silica

-

silicate glass

-

crystalline sodium chloride

QUARTZ

OR

FUSED

SILICA

-

REQUIRED

FOR

UV
AND

MAY

BE

USED

IN

VISIBLE

REGION

SILICATE

GLASS

-

CHEAPER

COMPARED

TO

QUARTZ
. U
SED

IN

UV

CRYSTALLINE

SODIUM

CHLORIDE

-

USED

IN

IR

IV.

S
AMPLE

H
OLDER

(C
ONTAINER
)

cuvette

39

SPECTROMETER

-

is an instrument that provides information about the intensity of
radiation as a function of wavelength or frequency

SPECTROPHOTOMETER

-

is 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.

40

REGION

SOURCE

SAMPLE
HOLDER

DETECTOR

Ultraviolet

Deuterium lamp

Quartz/fused
silica

Phototube, PM
tube, diode array

Visible

Tungsten lamp

Glass/quartz

Phototube, PM
tube, diode array

Infrared

Nernst glower (rare earth
oxides or silicon carbide
glowers)

Salt crystals e.g.
crystalline
sodium chloride

Thermocouples,
bolometers

Types of source, sample holder and detector
for various EM region

SUMMARY

41