UV visible spectroscopy principles and instrumentation

UV-VISIBLE SPECTROSCOPY
Dr. Sajjad Ullah
Institute of Chemical Sciences
University of Peshawar

Methods of
Analysis
Classical Instrumental
 Amount of analyte
 Degree of Precision and accuracy
 Sensitivity, selectivity and cost
Gravimetric Volumetric
Acid-base
(neutralization)
complexometric
Redox
• Spectroscopic Techniques
• Electrochemical Techniques
• Chromatographic Techniques
• Radiochemical Techniques
2Dr. Sajjad Ullah, ICS-UoP

Instrumental Methods of Analysis
Spectral
(Absorption, emission,
Scattering of EMR)
Electroanalytica
l
Separative
UV/Vis spectroscopy (molecular)
Luminescence spectroscopy (molecular)
Atomic spectroscopy (AAS, AFS, AES)
NMR spectroscopy (molecular)
R.D. Braun, Introduction to instrumental analysis
3
Others: IR, Microwave spec., Radiochemical analysis, Refractometry, photoacoustic spect., EPR,
XPS, XRF, Raman Spect. (inelastic scattering), turbidimetry and nephelometry (elastic scattering)
Dr. Sajjad Ullah, ICS-UoP

 Instrumental Methods are more sensitive and selective but
less precise (on the order of 1 to 5% or so)
 They are also more expensive
 Amount of analyte
 Degree of Precision and accuracy
 Sensitivity, selectivity and cost

 Spectroscopy is the study of interaction of EMR with matter.
 Spectroscopic methods of analysis use measurements of the amount of
EMR that is absorbed, emitted or scattered by a sample to perform an
assay.
 EMR is a form of energy
 EMR possess the properties of both discrete particles (photons) and
wave (Dual nature, E = hn )

C= Light travelling speed:
in a vacuum: c=2.998 x 108 m s-1 (n=1 exactly, in air n=1.0002926)
in other media: c/n (n = refractive index, generally >1)
Therefore:
Energy is inversely proportional to wavelength
but proportional to frequency or wavenumber
The relationship between energy (E) and frequency (n) :
E = hn = hc/l = hc/nl
h = Planck’s constant (6.626 x 10-34 J s)
n = frequency (most common units = cm-1), n = 1/T
n= refractive index = c/V
Light is energy in the form of electromagnetic field
Properties of light/EMR (photon)
Wavelength (l): Crest-to-crest distance between waves
Frequency (n): Number of complete oscillations that the wave makes each second
units: number of oscillations/sec or s-1 or Hertz |(Hz)
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V = ln = c/n
Where V= velocity

7
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1.54
1.55
1.56
1.57
1.58
1.59
1.60
1.61
1.62
1.63
l m)
n(SiO2
)
Cauchy Equation
SiO2
Dispersion = slope =
Refractive index is different for diffrent Wavelengths

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Spectral Regions

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EMR SPECTRUM
Source: Skoog, West, Holler, Crouch, Fundamentals of analytical Chemistry 9th Edition

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Spectral Regions
R.D. Braun, Introduction to instrumental analysis

11
C. N. Banwell, Fundamental of molecular spectrocopy

12
 Every colour is associated with a wavelength range.
 Colour of an object depends on the region refelcted by it (not absorbed)
 The observed colours depend on a physcial phenonomenon called transitions
b/w electronic levels of a molecule (Δ E = Eexcited - Efundamental)
 Violet: 400 - 420 nm
 Indigo: 420 - 440 nm
 Blue: 440 - 490 nm
 Green: 490 - 570 nm
 Yellow: 570 - 585 nm
 Orange: 585 - 620 nm
 Red: 620 - 780 nm
Visible Light Spectrum

 Refelected colours are complimenatry to those absorbed
When more than one colour
is reflected, we use
subtractive colour system
Dr. Sajjad Ullah, ICS-UoP 13

R.M. Christie, Colour Chemistry, Royal Society of Chemistry, 2001, page. 14

Green
Red
Blue
Colour and Electronic Spectra

E1 : Energy of the fundamental electronic state
E2 : Energy of the excited electronic state
Refrence: C. N. Banwell, the fundamental of molecular spectrocopy, p-13
If the same
molecule both
Emits and absorbs,
how We get signal
of absorption
Only?

17
Pavia, Lampman, Criz, Vyvyan, Introduction to Spectroscopy. p383

18https://www.youtube.com/watch?v=wxrAELeXlekDr. Sajjad Ullah, ICS-UoP

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Quantitaive analysis by absorption spectroscopy
(L.mol-1.cm-1)
Straight Line Equation: Y = mX + C
or X= Y-C/m
X= Concentration
Y= Absorbance
m= Slope
C= intercept

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Electrons involved in electronic Transitions
4- Π-bonding ēs:
ēs involved in Π-bonding, present in unsaturated HCs.
Absorb in near UV or Visible region
1- Closed Shell elctron:
These are inner shell ēs, not inolved in bonding.
Require higher energy X-rays for their excitation
2- Single bonded (σ-electons):
single-bonded ēs presente in satutrated HCs.
Absorb in Far UV region (10-200 nm)
3- non-bonding ēs :
Lone pair of ēs not inolved bonding, usually present
on heteroatoms (Ö, S:, N: and X:).
They absorb in near UV-region, and visble region (if
the heteroatom is presente in unsaturated HCs.
Unfortunately water and most solvents absorb in this region
CH4
NH3

22
Pavia, Lampman, Criz, Vyvyan, Introduction to Spectroscopy. p393
ketone
Methyl amine

23
 n → σ* (l = 150 - 250 nm)
Saturated compounds containing Heteroatos (O, S, N, X),
Examples: CH3OH (183 nm), CH3I (258 nm), CH3NH2 (213 nm)
 σ → σ* (l below 150 nm)
Saturated HCs, High energy required, trasitions occur vacuum UV range
Examples: CH4: l = 125 nm, C2H6 = 135 nm
N2 absorbs below 160 nm
O2 absorbs below 200 nm
  → * (near UV; visible, Generally 160-190 nm)
Unsaturated compounds, double and triple bonds and benzene rings, large
Ɛ values (1000 -15000 L.mol-1cm-1), their lmax depends on substitutents
Examples: Ethylene 171 nm, butadiene (conjugated system) 217 nm
R-Cl (169 nm) <R-Br < R-I (258 nm)
 n → * (near UV and Vis region), ocuur at lower E than  → *
Unsaturated compounds with Heteroatoms, aldehydes, ketones, -C≡N, NO2
Ɛ =10 -100 L.mol-1cm-1
Examples: acetone 277 nm, nitrobutene 665 nmDr. Sajjad Ullah, ICS-UoP

24
n → σ* at 150 nm has
not been shown
Ɛ =15
L.mol/cm
Ɛ =900 L/mol.cm

25
UV spectrum of acetone showing the π → π∗ and n → π ∗ transitions
n → σ* at 150 nm has
not been shown
Ɛ =15 L.mol/cm
Ɛ =900 L/mol.cm

26
Pavia, Lampman, Criz, Vyvyan, Introduction to Spectroscopy. p393Dr. Sajjad Ullah, ICS-UoP

27
Pavia, Lampman, Criz, Vyvyan, Introduction to Spectroscopy. p393Dr. Sajjad Ullah, ICS-UoP

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Selection Rules for Electronic Trnsitions
For any electronic transition to occur, △E= difference b/w HOMO and LUMO
However, even under such conditions, absorption of energy may not be observed
or observed with low intensity. Why?
Because there are certain requirements summarized in
quantum-mechanical selection rules that must be
satisfied if a transition is to occur with higher
probability
Allowed Transtions are the ones which have high
probability to take place
Forbidden Transtions are transitions of low probability
Theoretically all transitions are
possible but practically only
certain transitions are of high
intensity

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Selection Rules for Electronic Trnsitions
1- Spin Multiplicity Rule: △S= 0
2- Laporte Rule: △l= ±1, △m= 0,±1
3- Simultaneous excitation of more than one e is forbidden
Such transition that obey these rules occur with high probability
and are called allowed transition

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1- Spin Multiplicity Rule: △S= 0
“ the promoted electron be promoted without a change in its spin orientation”.
Ground state
(singlet)
(triplet)(singlet)
Two possible excited states
2S+1= 1 (singlet)
2S + 1= 3 (triplet)
(allowed) (forbidden)
Singlet-to-singlet transition are allowed
Singlet-to-triplet transitons are forbidden

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Thus transitions are forbidden for Δl=0
(i.e, between like atomic orbitals such as s-s, p-p, d-d, f-f)
2- Laporte Rule: △l= ±1, △m= 0,±1
“transitions from symmetrical to symmetrical are forbidden while
transitions from symmetrical to asymmetrical are allowed”.
This rule is based on symmetry of the initial and final states. when EMR is
absorbed, electrical in work done and dipole moment changes. If the distribution
of e- before and after absorption is the same, the transition is forbidden. On the
other hand, if the altered electronic distribution is asymmetrical to the original
electron distributions, a change in dipole moment is observed and the Transitions
are allowed.
l = orbital quantum number
m = magnetic quantum number

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Simultaneous excitation rule
“Simultaneous excitation of more than one e is forbidden”.
Ground state
(singlet)
(allowed)(forbidden)
Two possible excited states
Only one e- can be excited at a time!

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Allowed and Forbidden Trasitions
Transition type Approximate ɛ
Spin forbidden, Laporte forbidden 0.1
Spin allowed, Laporte forbidden 10
Spin allowed, Laporte allowed (charge transfer) 10,000
Thus the intensity of absorption can be expresses in terms of molar absorptivity.
A transition of unit probability will give ɛ= 105 (high intensity allowed transition, e.g.; π- π*).
Transitions with ɛ <103 are forbidden (e.g., n-π* have ɛ~100)
Spin forbidden, Laporte allowed 10-5 to 1

Instrumentation of UV-Visible
Spectroscopy
Course Instructor: Dr. Sajjad Ullah
Part-II

Instrumentation of Spectroscopy
Most of the spectroscopic instruments in the
UV/visible and IR regions are made up of five
components,
1. a stable source of radiant energy;
2. a wavelength selector that isolates a limited
region of the spectrum for measurement;
3. one or more sample containers/cells;
4. a radiation detector, which converts radiant
energy to a measurable electrical signal;
5. a signal processing and readout unit.

38
Diagaram of Istrument used for absorption measurements
Recommended: https://www.youtube.com/watch?v=pxC6F7bK8CU
Source: R. D. Braun, Introduction to instrumental analysis, 1987, p 141
A spectrophotometer is an
instrument that resolves
polychromatic radiation into
different λs and measure
absorbance at a specific λ

absorption measurements
fluorescence measurements
emission spectroscopy
excitation λ
emission λ
Douglas A. Skoog, F. James Holler, Stanley R. Crouch Principles of Instrumental Analysis sixth
edition 2006, p-165

https://www.youtube.com/watch?v=wxrAELeXlek

Light Sources
A light source must generate a beam of radiation that is sufficiently powerful for
easy detection and measurement. In addition, its output power should be stable
for reasonable periods to allow measurement of Io and It (T = It/Io) .
Spectroscopic light sources are of two types:
continuum sources, which emit radiation of all λs within a given spectral region
and the intensity changes only slowly from one λ to the other. Example:
Tungsten lamp, deuterium lamp
line sources, which emit a limited number of spectral lines or narrow band of
radiation (<0.01mm) with known λ which are characteristic of the source.
Sources can also be classified as continuous sources, which emit radiation
continuously with time, or pulsed sources, which emit radiation in bursts.

The source of EMR is chosen according to the spectral range to be studied

Douglas A. Skoog, F. James Holler, Stanley R. Crouch Principles of Instrumental Analysis , sixth
edition 2006, p-167
The source of EMR is chosen according to the spectral range to be studied

Continuum Sources in the UV/Visible Region
Tungsten filament lamp:
 Incandescent;
 λ range: 320 to 2500 nm.
 Temperature of operation =2900 K (useful range of λ 350 to 2200 nm.
 The λ of maximum emission is temperature (or filament voltage) dependent: higher
T leads to a shift to shorter λ region (but also shorter life-time due to sublimation
of W from filament).
Tungsten/halogen lamps (quartz/halogen lamps)
contain a small amount of iodine or bromine within the quartz envelope that
houses the filament. Quartz allows the filament to be operated at a temperature
(3500 K), so higher intensities and access to UV region (extended λ range = 240 to
2500 nm). Longer life time as the sublime W reacts with I2 to form WI2 which
redeposit and then decompose on the filaments to leave the W back on the filament

Deuterium/Hydrogen lamps (D2/H2 lamps)
Continuum radiation in the UV region
Make-up: cylindrical tube, containing deuterium at low pressure,
with a quartz window (UV Transparent)
Electrical excitation applying about 40 V between a heated
oxide-coated electrode and a metal electrode.
H2 + Ee H2*
H2*  Ὴ + H` ` + hυ
Ee = EH2* = EῊ + EH` + hυ
The sum of EῊ and EH` can vary from zero to EH2*
and so does the energy of photon (hυ), thus continuous
emission (λ =160 nm to about 375 nm)
Life time: 2000 hours
http://www.photron.com.au/.assets/brochures/deuterium_lamp.pdf

Continuum Sources in the IR region
The continuum sources for IR radiation are
normally heated inert solids. A Globar source
consists of a silicon carbide rod. Infrared
radiation is emitted when the Globar is heated to
about 1500 oC by the passage of electricity.
A Nernst Glower is a cylinder of zirconium and
yttrium oxides which emits IR radiation when
heated to a high temperature by an electric
current. Electrically heated spirals of nichrome
wire also serve as inexpensive IR sources.

Line Sources
Line sources can be used in the UV/visible region. Low-
pressure mercury arc lamps are very common sources
for use in liquid chromatography detectors. The dominant
line emitted by these sources is at 253.7 nm.
Hollow cathode lamps are also common line sources used
specifically for atomic absorption spectroscopy.
Lasers have also been used in molecular and atomic
spectroscopy, both for single wavelength and for scanning
application. Tunable dye laser can be scanned over
wavelength ranges of several hundred nanometers when
more than one dye is used.

High
Pressure
Mercury
Lamp
Mercury vapor lamps
Mercury vapor lamps are probably the
most common and emit intense light
at 253.7 nm (and certain other
wavelengths). Because of the limited
emission spectra of the lamp wavelengths
are not adjustable.
Because of the intensity of the
radiation, fixed wavelength detectors
can be up to 20 times more sensitive
than variable wavelength
detectors.

49
Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis,
Saunders College Publishing, Fort Worth, 1992.
Hollow Cathode Discharge Tube
Apply ~300 V across
electrodes.
Ar+ or Ne+ travel toward the
cathode.
If potential is high enough
cations will sputter metal off
the electrode.
Metal emits photons at
characteristic atomic lines as
the metal returns to the
ground state.

Wavelength Selectors
(Dispersion and Isolation devices)
Spectroscopic instruments in the UV and visible regions are
usually equipped with one or more devices to restrict the radiation
being measured to a narrow band.
These devices enable selection of relatively narrow band of
wavelengths from a broad band of radiation.
 They enhance both the selectivity and the sensitivity of an
instrument.
 Narrow bands of radiation greatly diminish the chance for Beer’s
law deviations due to polychromatic radiation

Effective bandwidth- the width of the band of
transmitted radiation in λ units at half peak
height.
For monochormators EBW = few 10th of a nm
For absorption Filter EBW = 200 nm or more
Transmittance at nominal λ
nominal λ

Wavelength Selectors
Filter (discontinuous) Monochromators (continuous)
Absorption
filter
Interference
filters
1- sharp-cut off
short λ-selectors
(orange filter)
2-sharp-cut off
long λ-selectors
(Blue-green filters)
3- Band pass filters
Prism
Monomoch.
Grating
Monomoch.
Filters are simple, rugged, cheap but can not be
used for λ scanning

http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy%3A_Application
http://micro.magnet.fsu.edu/primer/java/filters/absorption/index.html
Filters
Filters prevent passage of radiation at all λs except in a fixed λ region
Absorption Filters
 Absorption filters limit radiation by absorbing certain portions of the spectra.
 Useful for visible region
 They are made primarily from colored filter glass or dyes suspended in
gelatin and sandwiched between two glass plates
 Effective bandwidth =30‒250 nm
 Advantages: simplicity, ruggedness, low cost.
 Disadvanatage: They can not be used for λ scanning (isolate one band of λs only)
Types:
1- Cut-off filters
2- Band pass filters

Cut-off Filters
These filters have %T of nearly 100% over a portion of the visible spectrum but
then rapidly decreases to almost 0% over the remainder.
1- Sharp-cut off short λ-selector (Orange-red filters):
They transmit light of λ beyond a fixed value and absorbs light of short λ
Maximum Transmittace through these filters is 80-90% of the intensity of
incidente radiation. Transition between 0%T and maximum transmiattance
occurs over a λ range of 40 nm
2- Sharp-cut off long λ-selector (Blue-green filters):
They transmits realtively shorter wavelengths, so also called blue-green
filters

Band Pass or Combination Filters
A band pass filter is constructed by combining spectrally overlaping Green and
Orange cut-off filters.
Radiation is transmitted through the combination (band pass) only in the
spectral region which can be transmitted by both filters
Maximum %T = 25% of incident radiation

Interferance Filters
2d = m λ
2d n = m λ
λ = 2d n/ m
n = ref.index
d =1/2, 1. 3/2 of λ
(m = 1,2,3)
White radiation
The central wavelength of the transmitted band is controlled by
areful construction of the filter with a proper d-value

Comparison of bandwidths of interference and absorption filter

A monochromators consists of entrance/exit slits, a prism or
diffraction grating (dispersive component) and lenses/mirrors (to
collimate/focus the beam).
They generally employ a diffraction grating to disperse the radiation
into its component wavelengths. Older instruments used prisms for
this purpose.
Monochromator
By rotating the grating or Prism, different wavelengths can be made to pass
through an exit slit. The output wavelength of a monochromator is thus
continuously variable over a considerable spectral range
Prism monochromators
Grating
Monochro-
mators

λ2 < λ1
 Refraction occurs at both faces of the prism and radiation is thus dispersed
 Dispersed radiation is focussed onto a curved surface containing exit slit
 Radiation of desired wavelength can be caused to pass the exit slit by rotataion of the prism
 Refractive index (n) of the prism materials depends on λ. As λ increases, n decreases
 Refracation depends on n and n depends on λ; shorter λ, more refraction
 Used in UV, visible and IR region

dQ/dn
Geometric componente
dn/dl
Dispersion
depends on Prism
material,
(See next slide)
depends on Prism
geometry,
e.g., apex angle (α)
and B/b ratio
C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH,
Weinheim, Germany, 1998, vol. 1.

Dispersion = slope = dn/dλ
Prism material
Glass = visible region
Quartz = UV region
NaCl, KBr, CSI = IR region
C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH,
Weinheim, Germany, 1998, vol. 1.
Dispersion: Refractive index-wavelength curves
The dispersion of the prism material
depends on the slope of the refractive
index vs wavelength curve:

Grating monochromator
By rotating the grating, different wavelengths can be made to pass through an exit slit. The output
wavelength of a monochromator is thus continuously variable over a considerable spectral range.
The wavelength range passed by a monochromator, called the spectral bandpass or effective
bandwidth, can be less than 1 nm for moderately expensive instruments to grater than 20 nm for
inexpensive systems.
The output of a grating monochrmomator
λ2 < λ1

Path Diference = CA-BD = nλ
Constructive interferance occurs when the diffrenece in the distance that is
travelled by the diffracted radiation from each surface of the groove to the
wavefront is an integral number of wavelength:
The Grating Equation
R. D. Braun, Introduction to instrumental analysis, 1987, p 166-169
The in-phase radiation can be foucussed at the exit slit using lennese
300-2000 groves/mm (UV-Vis)
10-200 groves/mm (IR)

The Grating Equation
Path Diference = CA-BD = nλ (1)
r +BDC + CDA = 90° (2)
i + r +BDC = 90° (3)
Solving equation 3 for BDC and
substiuting its value in equation 2 gives:
i = CDA (4)
Furthermore, it can be shown that
sin CDA= Sin i = CA/d
CA = d sin i (5)
Similarly,
negative sign is used as r (refelected angle) is
on the opposite side of the normal
Puttin 5 and 6 in equation 1:
d sin i – (-d sin r) = nλ
d (sin i + sin r) = nλ
BD = - d sin r (6)

It is the ability of a monochromator to seprate adjacente wavelengths
For example, If a monochormator seprates two adjacente peaks at 207.3 and 215.1 nm
It will have a resolution:
R = λ/Δλ λ = mean wavelength
R = 211.2/7.8 = 27
What resolution of a monochrmoator is required to seprate Na lines 589.0 and 589.5nm?
R = 589.25/0.5 = 1178.5
Resolution (R) of a monochromator

UV visible spectroscopy principles and instrumentation

Optical Materials or Sample holder
The cells, windows, lenses, mirrors, and
wavelength selecting elements in an optical
spectroscopic instrument must transmit radiation in
the wavelength region being employed. Ordinary
silicate glass is completely adequate for the visible
region and has the considerable advantage of low
cost. In the UV region, at wavelengths shorter than
about 380 nm, glass begins to absorb and fused
silica or must be substituted. Also, glass, quartz,
and fused silica all absorb in the IR region at
wavelengths longer than about 2.5 m. Hence,
optical elements for IR spectrometry are typically
made from halide salts.

Detecting and Measuring Radiant Energy
To obtain spectroscopic information, the radiant power transmitted, fluoresced or
emitted must be detected in some manner and converted into a measurable
quantity.
A detector (also called transducer) is a device that indicates the existence of some
physical phenomenon.
The term transducer is a type of detector that converts various types of physical
and chemical properties (e.g., light intensity, pH, mass, and temperature) into
electrical signals (voltage, charge, current) that can be subsequently amplified,
manipulated, and finally converted into numbers proportional to the magnitude
of the original signal.

Properties of Radiation Transducers
 High Sensitivity: Responds rapidly to low levels of radiant energy over a
broad wavelength range.
 Linear Response: The electrical signal produced by the transducer be
directly proportional to the radiant power P of the beam
G = KP + K’
 Low back ground notice: Produces an electrical signal that is easily amplifies
and has a low electrical noise level (K’~0)
G=electrical response of the detector
in units of current, voltage, or charge.
K= proportionality constant that
measures the sensitivity of the detector
in terms of electrical response per unit
of radiant power input.
K’= A small constant response known
as a dark current, even when no
radiation strikes their surfaces. K’=0
as instrument automatically
compensate by using a counter signal
so that G = KP
P= k G
P0= k G0
(for sample solution)
(for solvent or blank)
A= log P0 / P= log kG/kG0
A= log G0/G
Thus we measure absorbance in terms of
electrical signal

Types of Transducers
Two general types of transducers: one type responds to
photons, the other to heat.
All photon detectors are based on the interaction of radiation
with a reactive surface to produce electrons
(photoemission) or to promote electrons to energy states in
which they can conduct electricity (photoconduction).
Only UV, visible and near-IR radiation possess enough
energy to cause photoemission to occur; thus,
photoemissive detector are limited to wavelengths shorter
than about 2 m (2000 nm).
Photoconductors can be used in the near-, mid-, and far-IR
regions of the spectrum.

…continued…
We detect IR radiation by measuring the
temperature rise of a blackened material located in
the path of the beam or by measuring the increase
in electrical conductivity of a photoconducting
material when it absorbs IR radiation.
Photon Detectors
Widely used types of photon detectors include
phototubes, photomultiplier tubes, silicon
photodiodes, and photodiode arrays.

Photomultiplier Detector
• The type is commonly used especially for low
radiant powers.
• The detector consists of a photoemissive cathode
(coated with cesium oxide) coupled with a series
of electron-multiplying dynode stages.
• The primary electrons ejected from the photo-
cathode are accelerated by an electric field so as to
strike onto the first dynode and then the e emitted
from 1st dynodes are directed onto the 2nd dynodes
and so on.
• Amplification = nd where d is the number of dynodes and n is the
no of electrode emitted per dynode. Usually 106 to107 e are emitted per photon

Photovoltaic or Barrier Layer cell

Barrier Layer/Photovoltaic Detector
• This device measures the intensity of photons by means of the voltage
developed across the semiconductor layer.
• Electrons, ejected by photons from the semiconductor, are collected by
the silver layer.
• The potential depends on the number of photons hitting the detector.
• Mostly used in the visible region with maximum sensitivity at 550 nm
• Advantage: Useful for simple portable low cost filter instruments,
requires no external power supply, provides readily measured response
at high intensity of EMR
• Disadvantages includes difficulty of amplification of output due to
low internal resistance, low sensitivity at low illumination intensity
and fall off of response upon prolong illumination.

Silicon photodiodes
• They consists of reverse biased pn junction formed
on a silicon chip.
• The reverse bias creates a depletion layer that
reduces the conductance of the junction to nearly
zero.
• If radiation is allowed to fall on chip, holes and
electrons are formed braking the junction.
• Produces a current that is proportional to radiant
power
• More sensitive then vacuum tube but less sensitive
than photomultiplier tubes

Heat responding detectors
• In IR region photons are lack the energy to
cause photoemission of electrons. Thus thermal
detector are employed in IR region
• The radiation impinges upon are absorbed by a
blackbody, and rise in temperature is measured.
• These includes
• Thermocouples, Bolometer, Pyroelectric
detectors

Diode Array Detectors
The photo diode array detector passes a wide spectrum of light through
the sample. The spectrum of light is directed to an array of photosensitive
diodes. Each diode can measure a different wavelength which allows for
the monitoring of many wavelengths at once.
- Peak Purity
- Quantify a peak with an
interfering peak
Compound Identification
Monitor compounds with
different UV max.

83
Photoconductivity Detectors
• Most sensitive detector for near IR region. It can be used up to far IR
region by cooling to suppress the noise arising from thermal induced
energies. The resistance decrease when thy absorb radiation.
• A crystalline semi conductor are formed from sulfides some metals
like Pb, Cd, gallium and indium
• Absorption of radiation by this material promotes there bounded
electrons in to an energy state in which they are free to conduct
electrical current. The resulting change in conductivity can then be
measured with a circuit.
• Lead sulfide is the most widely used photoconductive material

Signal Processor and Readout
• The signal processing include amplification of the
electrical signal. Alteration of signal from dc to ac etc.
• They are also called to perform mathematical
operation on signal as differentiation, integration or
conversion to a logarithm.
• Read out unit convert electrical signal to readable
form.
• Read out devices include digital meters, scales of
potentiometers, recorders and cathode ray tubes
• It also include fiber optics or light pipes to flow the
light from one part to another.

Single and Double Beam Spectrometer
• Single-Beam: There is only one light beam or optical path
from the source through to the detector.
• Double-Beam: The light from the source, after passing
through the monochromator, is split into two separate
beams-one for the sample and the other for the reference.

16/9/2006 8:28 Deokate U.A. 87

FIGURE 6-19 Emission spectrum of a brine sample obtained with
an oxyhydrogen flame. The spectrum consists of the supenmposed
line, band, and continuum spectra of the constituents of the sample.
The characteristic wavelengths of the species contributing to the
spectrum are listed beside each feature. (R.Hermann and C. T" J
Alkemade, Chemical Analysis. by Flame photormetry, 2nd ed., p.
484. New York: Interscience, 1979.)
Douglas A. Skoog, F. James Holler, Stanley R. Crouch Principles of Instrumental Analysis sixth
edition 2006, p-150-155

FIGURE 6-21 Energy-level diagrams for (a) a sodium
atom showing the source of a line spectrum and (b) a
simple molecule showing the source of a band spectrum.
Douglas A. Skoog, F. James Holler, Stanley R. Crouch Principles of
Instrumental Analysis sixth edition 2006, p-152

UV visible spectroscopy principles and instrumentation

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