UV Visible spectroscopy Instrumentation by Dr. Monika Singh part-2 as per PCI syllabus

Dr. MONIKA SINGH 22-09-2020
BP701T INSTRUMENTAL METHODS
OF ANALYSIS 1
by
Dr. MONIKA SINGH
(M.Pharm, PhD)
ULTRA VIOLET
SPECTROSCOPY
(Instrumentation)
BP701T-Instrumental Method of
Analysis
1
Electromagnetic Spectrum.
Wavelength, , cm
frequency, , (cycles/sec)
-ray
-ray
ultraviolet
visible
infrared
microwave
radio
Power
violet
blue
green
yellow
orange
red
visible region
400 500 600 700 800
1020
1018
1016
1014
1012
1010
108
106
104
102
10-10
10-8
10-6
10-4
10-2
1 102
104
106
108
Wavelength, , nm
Analysis
2

OF ANALYSIS 2
T=I/Io
A= - log T = -log (I/Io)
Calibration curve
Analysis
3
Ultra Violet Spectrometry
The absorption of ultraviolet radiation by
molecules is dependent upon the electronic
structure of the molecule. So the ultraviolet
spectrum is called electronic spectrum.
Analysis
4

OF ANALYSIS 3
Analysis
5
Instrumentation
Four parts in a
spectroscopic
instrument.
• Light sources
• Monochromators
• Sample cell
• Detectors
There are many variations and hybrids of spectroscopic
techniques, but all follow the same basic theories.
Analysis
6

OF ANALYSIS 4
Instrumentation
• Light sources - Hydrogen Discharge lamp
- Duterium Discharge lamp
- Xenon arc
• Wavelength selectors -Filters
Absorption Filters
Interference Filters
- Monochromators
Prism
Grating
• Sample cell
• Detectors - Photo tube
- Photovoltaic cell
- Photomultiplier tube
Analysis
7
General optical spectrometer
Analysis
8

OF ANALYSIS 5
UV Radiation sources
• Deuterium and Hydrogen lamps
• Tungsten filament lamps / Incandescent
Lamps
• Xenon Arc lamps
• Hollow cathode lamps
• Mercury Arc Lamp
Analysis
9
1. Deuterium & Hydrogen Lamps (UV range)
D2 + Eelect  D*
2  D’ + D’’ + h (light produced)
Excited state
- continuous source, broad
range of frequencies
-based on electric excitation
of H2 or D2 at Low pressure
-Mechanical slit is present to
Narrow the discharge
- Radiation ball is formed
towards Cathode.
In presence of arc, some of the
electrical energy is absorbed by
D2 (or H2) which results in the
disassociation of the gas and
release of light
Analysis
10

OF ANALYSIS 6
- h will vary continuously from ~ 160nm up to 375nm (UV range) due
to different frequencies going into D’ and D’’
- need to make cell from quartz since glass absorbs light at  # 350 nm
- cost ~ $350-$500
Intensity Spectrum Of Deuterium Arc LampBP701T-Instrumental Method of
Analysis
11
2. Tungsten Filament Lamp (Vis – Near IR)
or Incandescent Lamp
- continuous source, broad range of frequencies
- based on black body radiation:
heat solid filament to glowing, light emitted will be
characteristic of temperature more than nature of
solid filament
Low pressure (vacuum)
Tungsten Filament
Analysis
12

OF ANALYSIS 7
Halogen Lamp
Analysis
13
- need high temperatures to get high light intensity (power) and low max
- Typical Tungsten lamp T ~ 2870K
-  range: 350 – 2500 nm
- cost ~ $10-15
Temperature Dependence of 
Analysis
14

OF ANALYSIS 8
3. Xenon Arc Lamps (UV – Vis Range)
- Continuous source, broad range of frequencies
-  range: 250 – 600 nm
- works by passage of current through Xe, causes thermal excitation
- Blackbody emission
- Gives Very intense radiation over frequency range.
(developed for search lights during WW-II)
- problems: - higher heat
- more stray light
- higher cost
- shorter lifetimes
4. Mercury Arc Lamps
Analysis
15
Analysis
16

OF ANALYSIS 9
Wavelength selectors
• Wavelength filters
• Absorption
• Interference
• Monochromators
• Prism
• Grating
Analysis
17
Why separate wavelengths?
• Each compound absorbs different colors
(energies) with different probabilities
(absorbtivity)
• Selectivity
• Quantitative adherence to Beer’s Law
A = abc
• Improves sensitivity
Analysis
18

OF ANALYSIS 10
Wavelength Dispersion
• Prisms (nonlinear, range
depends on refractive
index)
• Gratings (linear, Bragg’s
Law, depends on spacing
of scratches, overlapping
orders interfere)
• Interference filters
(inexpensive)
Analysis
19
Filters
(inexpensive alternative)
• Filters are wavelength selectors that allow
narrow bandwidths of radiation to pass through.
They can be divided into four main categories:
absorption filters, cut-off filters,
interference filters, and interference wedges.
• Absorption type
- glass with dyes to adsorb chosen colors
• Interference filters
- multiple reflections between 2 parallel
reflective surfaces - only certain wavelengths
have positive interferences - temperature effects
spacing between surfacesBP701T-Instrumental Method of
Analysis
20

OF ANALYSIS 11
- material remove undesired ’s by absorbing them.
- typically made from colored glass or dye suspended in gelatin
between glass plates.
- fixed , much energy lost due to absorption.
- cheap
 wide range of  allowed through.
 can combine filters with different  range.
 typical bandpass (30-250 nm).
Effective bandwidth for two types of
filters and the result of combining
filters.
Absorption Filters
bandwidth
A band-pass filter or bandpass filter
(BPF) is a device
that passes frequencies within a
certain range and rejects (attenuates)
frequencies outside that range.
Analysis
21
Analysis
22

OF ANALYSIS 12
Analysis
23
- made up of thin layers of metal and dielectric (eq. CaF2 ,MgF2 , SiO)
material sandwiched between glass plates, partially reflecting metal films
- dielectric material is of uniform, known thickness.
- metal acts as partial mirror
Interference Filters
Analysis
24

OF ANALYSIS 13
As light enters, some goes through but some is
reflected. The distance the light travels before
it exits generates constructive and destructive
interference on the other side of the filter.
’s transmitted through filter:
N = 2dh
h – refractive index
d – thickness of dielectric
N – integer
 - wavelength
Bandpass can be 1-20nm. (narrow), but filter is fixed at given value as
much intensity is lost due to reflection
Analysis
25
Figure Effective bandwidths for two types of filters.
Analysis
26

OF ANALYSIS 14
Wedge filter
• It consist of wedge shaped
slab of dielectric depositing
between semi reflecting
metallic layers.
• Peak transmission =50-60%
• Drawback: Incident radiation
are highly convergent or
divergent.
Analysis
27
Monochromator
- separates frequencies () from polychromatic light in time
or space.
- allows only certain ’s to be selected and used.
Dispersing Monochromator:
Prism: based on refraction of
light and fact that different ’s
have different values of
refraction index (hi) in a medium.
Grating Monochromator: based on
diffraction of light (constructive and
deconstructive interference)
a) Transmission Grating: groves or
slits placed or made on a
transparent material.
b) Reflection Grating: most
commonly used
Refraction is the change in direction of waves that occurs when waves travel from one medium to another. Refraction is always
accompanied by a wavelength and speed change.
Diffraction is the bending of waves around obstacles and openings. The amount of diffraction increases with increasing wavelength.
Analysis
28

OF ANALYSIS 15
Monochromator
• Entrance slit - provides narrow optical image
• Collimator - makes light hit dispersive
element at same angle
• Dispersing element – Prism/Grating
• Collimator/Focusing element - image on slit
• Exit slit - isolates desired color to exit
Analysis
29
Grating
prism
Analysis
30

OF ANALYSIS 16
Dispersing Monochromator:
Prism: based on refraction of light and fact that different ’s
have different values of refraction index (hi) in a medium.
Analysis
31
Analysis
32

OF ANALYSIS 17
Dispersion Curve: change in hi as a function of 
Normally want to work in areas of normal dispersions for prisms.
Anomalous dispersion occurs near where substance itself absorbs light.
Analysis
33
h1sinq1 = h2sinq2Recall Snell’s Law of Refraction
Also, remember that no refraction occurs if light at normal or q1 = 0
So, light must hit prism at an angle.
Most common is a 60o prism
(glass or quartz).
Analysis
34

OF ANALYSIS 18
Figure Dispersion by a prism: (a) Quartz Cornu types
(b) Littrow type.
Analysis
35
Bunsen Prism:
Important Components:
i) Entrance slit
ii) Collimating lens or mirror – makes radiation parallel before hitting
dispersing element
iii) Grating or Prism
iv) Focusing Lens or mirror – to focus light of desired  on exit slit.
Analysis
36

OF ANALYSIS 19
Figure Echelle grating: i = angle of incidence; r = angle
of reflection; d = groove spacing.
Grating
Analysis
37
Analysis
38

OF ANALYSIS 20
Transmission Grating:
Order of Interference (n): n = d sinq
Different ’s will have constructive
interference at different points.
Can select desired  by letting light
at different points into instrument.
Groves or slits placed or made on a transparent material.
Analysis
39
Reflection Grating:
Now, spacing of slits (d) is distance from one groove to next.
Typically have 300-2000 grooves/mm.
Constructive and deconstructive interference occurs because
light travels different distances when reflected from each grating
Angle at which constructive interference occurs is now given by:
n = d(sin i+ sin r)
i : incident angle
r : diffraction angle
most commonly used
- grooved surface with
reflective coating (Al, Au,
Pt)
Analysis
40

OF ANALYSIS 21
Czerney-Turner Grating:
Important Components:
i) Entrance slit
ii) Collimating lens or mirror – makes radiation parallel before hitting
dispersing element
iii) Grating or Prism
iv) Focusing Lens or mirror – to focus light of desired  on exit slit.BP701T-Instrumental Method of
Analysis
41
Resolving Power of Grating

R = ----- = n N
d
where N => number of lines
illuminated by the radiation from
entrance slit
Analysis
42

OF ANALYSIS 22
Resolving Power of Prism
R => resolving power
 dn
R = ------ = b  -----
d d
where b=> length of prism base
Analysis
43
Analysis
Gratings vs. Prisms
Advantages
Gratings
– dispersion nearly constant w/ wavelength
– simplier monochromator
– better dispersion for same size
Prisms
– cheaper in the past
44

OF ANALYSIS 23
Analysis
Gratings vs. Prisms
Disadvantages
Gratings
– stray radiation
– higher order spectra
Prisms
– larger
– may be unstable to atmosphere of lab
45
Analysis
46

OF ANALYSIS 24
Cuvette:
Ordinary glass is partially transparent to UV but is opaque
to shorter wavelengths while Silica or quartz glass,
depending on quality, can be transparent even to vacuum
UV wavelengths. Ordinary window glass passes about 90%
of the light above 350 nm, but blocks over 90% of the light
below 300 nm.
1. Use quartz cuvettes for UV experiments.
2. Use quartz, glass, or plastic for visible wavelengths.
SAMPLE CELL
Analysis
47
DETECTORS
Analysis
48

OF ANALYSIS 25
Detectors
For sensitive detector we need a small work
function - alkali metals are best
• Phototube
• Photovoltaic cell
• Photomultiplier tube - amplification to improve
sensitivity (10 million)
Analysis
49
Photo Tube
Analysis
50

OF ANALYSIS 26
Process:
- photoemission material (Cs2O) ejects an e- when “hit” with a photon
(photoelectric effect).
- potential of 90V across cathode(-) and anode(+). As light “hits” cathode, e-
are emitted from cathode and attracted to anode. Produces current that can be
measured.
- current % number of photons.
- smaller current then photovoltaic cell, but can be amplified because of larger
resistance.
- 90V difference sufficient to collect all e- produced (working at saturation).
- various photoemission material (sensitive to certain photon ’s)
Advantages: sensitive, signal easily amplified.
Disadvantages: some dark current (from thermal e- emission & natural decay of 40K in glass
housing
Analysis
51
Photodiodes
also c/d Photovoltaic cell (Barrier-Layer Cell)
• It is semiconductor that conducts in one
direction only when light is present
• Rugged and small
• Photodiode arrays - allows observation of a
number of different locations (wavelengths)
simultaneously
• Somewhat less sensitive than PMT
In-Ga-As photodiode
Analysis
52

OF ANALYSIS 27
Process:
light of sufficiently high energy passes through the thin
transparent silver layer and hits selenium causing electrons to
be released which move across barrier toward silver layer
(electropositive) and collected at iron layer to neutralize
selenium layer.
- Current produced is proportional to photons hitting surface
- Maximum response at 550 nm (10% at 350-750 nm ~ same as
human eye).
Advantage: cheap, rugged, no external power source, good for portable instruments.
Disadvantage: not very sensitive, shows fatigue (decrease in response with continued
illumination), difficult to amplify signal-small resistance (Ohm’s law: I=(V/R)).
Analysis
53
BP701T-Instrumental Method of Analysis
54

OF ANALYSIS 28
n+
R
Cathode
Guard ring
Anode
Depletion region
P-type Si bulk
n+
AR coating
IpVo
V B
PIN Photodiode with Guard Ring
to Reduce Dark Current
hh
P i
p+ diffusion
Analysis
55
Reverse-bias: no current flows
Semiconductor material – conducts current only under certain conditions
free electrons inside the N-type material need some
extra energy to overcome the repulsion of the P-type's
acceptor atoms.
- Light shining on the silicon diode provides the energy needed for the
electrons to travel into the P region.
- Flow of current is related to intensity of light.
Silicon Diode or Photodiode detectors
56

OF ANALYSIS 29
S i3 N 4
n G a 0 .4 7 In 0 .5 3 A s
P h o to c o n d u c to r
In P
S e m i-in s u la tin g s u b s tra te
P i (W /c m 2
)
A GaInAs Photoconductor for  = 1.3 m
Analysis
57
Spectral sensitivity is a function
of photocathode material• Ag-O-Cs mixture gives broader range but less efficiency
• Na2KSb (trace of Cs)has better response over narrow range
• Max. response is 10% of one per photon (quantum efficiency)
58

OF ANALYSIS 30
Dynodes – all covered with photoemissive material
Photo-multiplier Tube (PMT)
This is similar to the photo-emissive cell but has a tube containing positive
electrodes coated with an electron emitting material.
59
Photomultiplier
dynodes of
CuO.BeO.CsSb
or GaP.Cs
Analysis
60

OF ANALYSIS 31
Process:
a) light hits cathode and e- emitted.
b) an emitted e- is attracted to electrode #1
(dynode 1), which is 90V more positive.
Causes several more e- to be emitted.
c) these e- are attracted to dynode 2, which is
90V more positive then dynode 1, emitting
more e-.
d) process continues until e- are collected at
anode after amplification at 9 dynodes.
e) overall voltage between anode and cathode
is 900V.
f) one photon produces 106 – 107 electrons.
g) current is amplified and measured
Advantages: very sensitive to low intensity, very fast response.
Disadvantages: need high voltage power supply, intense light damages
61
Analysis
62

OF ANALYSIS 32
Analysis
63
Wavelength dependence in
spectrometer
• Source
• Monochromator
• Sample
• Detector
Analysis
64

OF ANALYSIS 33
Common UV-vis instuments
cuvette
Tungsten
Filament (vis)
slit
Photomultiplier
tube
monochromator
Deuterium lamp
Filament (UV)
slit
Scanning Instrument
Analysis
65
HPLC-UV
Mobile
phase
HPLC
Pump
syringe
6-port
valve
Sample
loop
HPLC
column
UV
detector
Solvent
wasteBP701T-Instrumental Method of
Analysis
66

OF ANALYSIS 34
Instrumentation
Analysis
67
Applications of UV- Visible
Spectroscopy
• 1. Detection of Impurities
UV absorption spectroscopy is one of the best
methods for determination of impurities in organic
molecules. Additional peaks can be observed due
to impurities in the sample and it can be compared
with that of standard raw material. By also
measuring the absorbance at specific wavelength,
the impurities can be detected.
Benzene appears as a common impurity in
cyclohexane. Its presence can be easily detected by
its absorption at 255 nm.
Analysis
68

OF ANALYSIS 35
• 2. Structure elucidation of organic compounds.
UV spectroscopy is useful in the structure elucidation of
organic molecules, the presence or absence of unsaturation,
the presence of hetero atoms.
From the location of peaks and combination of peaks, it can
be concluded that whether the compound is saturated or
unsaturated, hetero atoms are present or not etc.
• 3. Quantitative analysis
UV absorption spectroscopy can be used for the quantitative
determination of compounds that absorb UV radiation. This
determination is based on Beer’s law which is as follows.
A = log I0 / It = log 1/ T = – log T = abc = εbc
Where ε is extinction co-efficient, c is concentration, and b is
the length of the cell that is used in UV spectrophotometer.
Other methods for quantitative analysis are as follows.
a. calibration curve method
b. simultaneous multicomponent method
c. difference spectrophotometric method
d. derivative spectrophotometric method
Analysis
69
• 4. Qualitative analysis
UVabsorption spectroscopy can characterize those types of
compounds which absorbs UV radiation. Identification is done
by comparing the absorption spectrum with the spectra of
known compounds.
UV absorption spectroscopy is generally used for
characterizing aromatic compounds and aromatic olefins.
• 5. Dissociation constants of acids and bases.
PH = PKa + log [A-] / [HA]
From the above equation, the PKa value can be calculated if
the ratio of [A-] / [HA] is known at a particular PH. and the ratio
of [A-] / [HA] can be determined spectrophotometrically from
the graph plotted between absorbance and wavelength at
different PH values.
Analysis
70

OF ANALYSIS 36
• 6. Chemical kinetics
Kinetics of reaction can also be studied using UV
spectroscopy. The UV radiation is passed through the
reaction cell and the absorbance changes can be
observed.
• 7. Quantitative analysis of pharmaceutical substances
Many drugs are either in the form of raw material or in the
form of formulation. They can be assayed by making a
suitable solution of the drug in a solvent and measuring
the absorbance at specific wavelength.
Diazepam tablet can be analyzed by 0.5% H2SO4 in
methanol at the wavelength 284 nm.
Analysis
71
8. Molecular weight determination
Molecular weights of compounds can be measured
spectrophotometrically by preparing the suitable derivatives
of these compounds.
For example, if we want to determine the molecular weight
of amine then it is converted in to amine picrate. Then
known concentration of amine picrate is dissolved in a litre
of solution and its optical density is measured at λmax 380
nm. After this the concentration of the solution in gm moles
per litre can be calculated by using the following formula.
"c" can be calculated using above equation, the weight
"w" of amine picrate is known. From "c" and "w",
molecular weight of amine picrate can be calculated. And
the molecular weight of picrate can be calculated using the
molecular weight of amine picrate.
Analysis
72

OF ANALYSIS 37
• 9. As HPLC detector
A UV/Vis spectrophotometer may be used as a
detector for HPLC. The presence of an analyte
gives a response which can be assumed to be
proportional to the concentration. For more
accurate results, the instrument's response to
the analyte in the unknown should be compared
with the response to a standard; as in the case
of calibration curve.
Analysis
73
74
References
• William kemp , Organic Spectroscopy ,1998, 37,
2854
• Silversteen, Organic Spectroscopy ,1998, 37, 2854
• Skoog Douglas A., West Donald M., Holler F.
James, Fundamentals of Analytical Chemistry,
Suanders College Pub,1995. 7th Edition (January 1,
1995)
• Willard, H.H.; Merritt, L.L. Jr.; Dean, J.A.; Settle,
F.A. Jr. Instrumental Methods of Analysis, 7th
edition (1988) United States: Wadsworth Publishing
Company.
Analysis

OF ANALYSIS 38
Analysis
75
Analysis
76

UV Visible spectroscopy Instrumentation by Dr. Monika Singh part-2 as per PCI syllabus

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