INSTRUMENTAL ANALYSIS CHEM 4811

1
INSTRUMENTAL ANALYSIS CHEM 4811

• CHAPTER 2

DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
2
CHAPTER 2 INTRODUCTION TO SPECTROSCOPY
3
DEFINITIONS
Spectroscopy - The study of the interactions of
electromagnetic radiation (radiant energy) and
matter (molecules, atoms, or ions) Spectrometry -
Quantitative measurement of the intensity of one
or more wavelengths of radiant
energy Spectrophotometry - The use of
electromagnetic radiation to measure chemical
concentrations (used for absorption measurements)
4
DEFINITIONS
Spectrophotometer - Instrument used for
absorption measurements Optical Spectrometer -
Instrument that consists of prism or grating
dispersion devise, slits, and a photoelectric
detector Photometer - Instrument that uses a
filter for wavelength selection instead of a
dispersion device
5
ELECTROMAGNETIC RADIATION
- Also known as radiant heat or radiant energy -
One of the ways by which energy travels through
space - Consists of perpendicular electric and
magnetic fields that are also perpendicular to
direction of propagation Examples heat energy in
microwaves light from the sun X-ray radio waves
6
ELECTROMAGNETIC RADIATION
Wavelength (m)
10-11
103
Radio frequency FM Shortwave AM
Gamma rays
Ultr- violet
Infrared
Microwaves
Visible
X rays
Frequency (s-1)
104
1020
Visible Light VIBGYOR Violet, Indigo, Blue,
Green, Yellow, Orange, Red 400 750 nm - White
light is a blend of all visible wavelengths -
Can be separated using a prism
7
ELECTROMAGNETIC RADIATION
?1
node
amplitude
?1 4 cycles/second
?2
?2 8 cycles/second
peak
?3
?3 16 cycles/second
trough
one second
8
ELECTROMAGNETIC RADIATION
Wavelength (?) - Distance for a wave to go
through a complete cycle (distance between two
consecutive peaks or troughs in a wave)
Frequency (?) - The number of waves (cycles)
passing a given point in space per
second Cycle - Crest-to-crest or
trough-to-trough Speed (c) - All waves travel at
the speed of light in vacuum (3.00 x 108 m/s)
9
ELECTROMAGNETIC RADIATION
Plane Polarized Light - Light wave propagating
along only one axis (confined to one
plane) Monochromatic Light - Light of only one
wavelength Polychromatic Light - Consists of
more than one wavelength (white light) Visible
light - The small portion of electromagnetic
radiation to which the human eye responds
10
ELECTROMAGNETIC RADIATION
- Inverse relationship between wavelength and
frequency ? a 1/? c ? ? ? wavelength
(m) ? frequency (cycles/second 1/s s-1
hertz Hz) c speed of light (3.00 x 108 m/s)
11
ELECTROMAGNETIC RADIATION
- Light appears to behave as waves and also
considered as stream of particles (the dual
nature of light) - Is sinusoidal in shape -
Light is quantized Photons - Particles of light
12
ELECTROMAGNETIC RADIATION
h Plancks constant (6.626 x 10-34 J-s) ?
frequency of the radiation ? wavelength of the
radiation E is proportional to ? and inversely
proportional to ?
13
INTERACTIONS WITH MATTER
- Takes place in many ways - Takes place over a
wide range of radiant energies - Is not visible
to the human eye - Light is absorbed or
emitted - Follows well-ordered rules - Can be
measured with suitable instruments
14
INTERACTIONS WITH MATTER
- Atoms, molecules, and ions are in constant
motion Solids - Atoms or molecules are arranged
in a highly ordered array (crystals) or arranged
randomly (amorphous) Liquids - Atoms or
molecules are not as closely packed as in
solids Gases - Atoms or molecules are widely
separated from each other
15
INTERACTIONS WITH MATTER
Molecules Many types of motion are involved -
Rotation - Vibration - Translation (move from
place to place) - These motions are affected
when molecules interact with radiant energy -
Molecules vibrate with greater energy amplitude
when they absorb radiant energy
16
INTERACTIONS WITH MATTER
Molecules - Bonding electrons move to higher
energy levels when molecules interact with
visible or UV light - Changes in motion or
electron energy levels result in changes in
energy of molecules Transition - Change in
energy of molecules (vibrational transitions,
rotational transitions, electronic transitions)
17
INTERACTIONS WITH MATTER
Atoms or Ions - Move between energy levels or in
space but cannot rotate or vibrate The type of
interactions of materials with radiant energy
are affected by - Physical state - Composition
(chemical nature) - Arrangement of atoms or
molecules
18
INTERACTIONS WITH MATTER
Light striking a sample of matter may be -
Absorbed by the sample - Transmitted through the
sample - Reflected off the surface of the
sample - Scattered by the sample - Samples can
also emit light after absorption
(luminescence) - Species (atoms, ions, or
molecules) can exist in certain discrete states
with specific energies
19
INTERACTIONS WITH MATTER
Transmission - Light passes through matter
without interaction Absorption - Matter absorbs
light energy and moves to a higher energy state
Emission - Matter releases energy and moves to
a lower energy state Luminescence - Emission
following excitation of molecules or atoms by
absorption of electromagnetic radiation
20
INTERACTIONS WITH MATTER
Ground State The lowest energy state Excited
state higher energy state (usually short-lived)
Excited state
Energy
Ground state
Absorption
Emission
21
INTERACTIONS WITH MATTER
- Change in state requires the absorption or
emission of energy
- Matter can only absorb specific wavelengths or
frequencies - These correspond to the exact
differences in energy between the two states
involved Absorption Energy of species increases
(?E is positive) Emission Energy of species
decreases (?E is negative)
22
INTERACTIONS WITH MATTER
- Frequencies and the extent of absorption or
emission of species are unique - Specific atoms
or molecules absorb or emit specific frequencies
- This is the basis of identification of
species by spectroscopy Relative energy of
transition in a molecule Rotational lt vibrational
lt electronic - The are many associated
rotational and vibrational sublevels for any
electronic state (absorption occurs in closely
spaced range of wavelenghts)
23
INTERACTIONS WITH MATTER
Absorption Spectrum - A graph of intensity of
light absorbed versus frequency or wavelength -
Emission spectrum is obtained when molecules emit
energy by returning to the ground state after
excitation Excitation may include - Absorption
of radiant energy - Transfer of energy due to
collisions between atoms or molecules - Addition
of thermal energy - Addition of energy from
electrical charges
24
ATOMS AND ATOMIC SPECTROSCOPY
- The electronic state of atoms are quantized -
Elements have unique atomic numbers (numbers of
protons and electrons) - Electrons in orbitals
are associated with various energy levels - An
atom absorbs energy of specific magnitude and a
valence electron moves to the excited state -
The electron returns spontaneously to the ground
state and emits energy
25
ATOMS AND ATOMIC SPECTROSCOPY
- Emitted energy is equivalent to the absorbed
energy (?E) - Each atom has a unique set of
permitted electronic energy levels (due to unique
electronic structure) - The wavelength of light
absorbed or emitted are characteristic of a
specific element - The absorption wavelength
range is narrow due to the absence of rotational
and vibrational energies - The wavelength range
falls within the ultraviolet and visible regions
of the spectrum (UV-VIS)
26
ATOMS AND ATOMIC SPECTROSCOPY
- Wavelengths of absorption or emission are used
for qualitative identification of elements in a
sample - The intensity of light absorbed or
emitted at a given wavelength is used for the
quantitative analysis Atomic Spectroscopy
Methods - Absortion spectroscopy - Emission
spectroscopy - Fluorescence spectroscopy - X-ray
spectroscopy (makes use of core electrons)
27
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Processes Occurring in Each Region
10-11
103
Gamma rays
Ultr- violet
Radio frequency FM Shortwave AM
X rays
Infrared
Microwaves
Visible
1020
104
Electronic excitation
rotation
vibration
Bond breaking and ionization
28
MOLECULES AND MOLECULAR SPECTROSCOPY
- Energy states are quantized Rotational
Transitions - Molecules rotate in space and
rotational energy is associated - Absorption of
the correct energy causes transition to a higher
energy rotational state - Molecules rotate
faster in a higher energy rotational state -
Rotational spectra are usually complex
29
MOLECULES AND MOLECULAR SPECTROSCOPY
Rotational Transitions - Rotational energy of a
molecule depends on shape, angular velocity, and
weight distribution - Shape and weight
distribution change with bond angle - Molecules
with more than two atoms have many possible
shapes - Change in shape is therefore restricted
to diatomic molecules - Associated energies are
in the radio and microwave regions
30
MOLECULES AND MOLECULAR SPECTROSCOPY
Vibrational Transitions - Atoms in a molecule
can vibrate toward or away from each other at
different angles to each other - Each vibration
has characteristic energy associated with it -
Vibrational energy is associated with absorption
in the infrared (IR region) Increase in
rotational energy usually accompanies increase
in vibrational energy
31
MOLECULES AND MOLECULAR SPECTROSCOPY
Vibrational Transitions - IR absorption
corresponds to changes in both rotational and
vibrational energies in molecules - IR
absorption spectroscopy is used to deduce the
structure of molecules - Used for both
qualitative and quantitative analysis
32
MOLECULES AND MOLECULAR SPECTROSCOPY
Electronic Transitions - Molecular orbitals are
formed when atomic orbitals combine to form
molecules - Absorption of the correct radiant
energy causes an outer electron to move to an
excited state - Excited electron spontaneously
returns to the ground state (relax) emitting UV
or visible energy - Excitation in molecules
causes changes in the rotational and vibrational
energies
33
MOLECULES AND MOLECULAR SPECTROSCOPY
Electronic Transitions - The total energy is the
sum of all rotational, vibrational, and
electronic energy changes - Associated with
wide range of wavelengths (called absorption
band) - UV-VIS absorption bands are simpler
than IR spectra
34
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Spectroscopy Methods - Molecular
absorption spectroscopy - Molecular emission
spectroscopy - Nuclear Magnetic Resonance (NMR) -
UV-VIS - IR - MS - Molecular Fluorescence
Spectroscopy
35
ABSORPTION LAWS
Radiant Power (P) - Energy per second per unit
area of a beam of light - Decreases when light
transmits through a sample (due to absorption of
light by the sample) Intensity (I) - Power per
unit solid angle - Light intensity decreases as
light passes through an absorbing material
36
ABSORPTION LAWS
Transmittance (T) - The fraction of incident
light that passes through a sample
0 lt T lt 1 Io light intensity striking a
sample I light intensity emerging from sample
Io
I
37
ABSORPTION LAWS
Transmittance (T) - T is independent of Io - No
light absorbed I Io and T 1 - All light
absorbed I 0 and T 0 Percent Transmitance
(T)
0 lt T lt 100
38
ABSORPTION LAWS
Absorbance (A)
- No light absorbed I Io and A 0 Percent
Absorbance (A) 100 - T - 1 light absorbed
implies 99 light transmitted - Higher
absorbance implies less light transmitted
39
ABSORPTION LAWS
Beers Law A abc A absorbance a
absorptivity a e molar absorptivity (M-1cm-1)
if C is in units of M (mol/L) b pathlength or
length of cell (cm) c concentration
40
ABSORPTION LAWS
Beers Law - I or T decreases exponentially with
increasing pathlength - A increases linearly
with increasing pathlength - A increases
linearly with increasing concentration - More
intense color implies greater absorbance - Basis
of quantitative measurements (UV-VIS, IR, AAS
etc.)
41
ABSORPTION LAWS
Absorption Spectrum of 0.10 mM Ru(bpy)32
?max 452 nm
42
ABSORPTION LAWS
Absorption Spectrum of 3.0 mM Cr3 complex
?max 540 nm
43
ABSORPTION LAWS
Maximum Response (?max) - Wavelength at which
the highest absorbance is observed for a given
concentration - Gives the greatest sensitivity
44
ABSORPTION LAWS
Deviations from Beers Law - Deviations from
linearity at high concentrations - Usually used
for concentrations below 0.01 M - Deviations
occur if sample scatters incident radiation -
Error increases as A increases (law generally
obeyed when A 1.0
45
CALIBRATION METHODS
Calibration - The relationship between the
measured signal (absorbance in this case) and
known concentrations of analyte - Concentration
of an unknown analyte can then be calculated
using the established relationship and its
measured signal
46
CALIBRATION METHODS
Calibration with External Standards - Solutions
containing known concentrations of analyte are
called standard solutions - Standard solutions
containing appropriate concentration range are
carefully prepared and measured - Reagent blank
is used for instrumental baseline - A plot of
absorbance (y-axis) vs concentration (x-axis) is
made
47
CALIBRATION METHODS
Calibration with External Standards
48
CALIBRATION METHODS
Calibration with External Standards - Equation
of a straight line in the form y mx z is
established m slope ab z intercept on the
absorbance axis - Concentration of unknown
analyte should be within working range (do not
extrapolate) - Must measure at least three
replicates and report uncertainty
49
CALIBRATION METHODS
Method of Standard Additions (MSA) - Known
amounts of analyte are added directly to the
unknown sample - The increase in signal due to
the added analyte is used to establish the
concentration of unknown - Relationship between
signal and concentration of analyte must be
linear - Analytes are added such that change in
volume is negligible
50
CALIBRATION METHODS
Method of Standard Additions (MSA) - Different
concentrations of analyte are added to different
aliquots of sample - Nothing is added to the
first aliquot (untreated) - Concentrations in
increments of 1.00 is usually used for
simplicity - Plot of signal vs concentration of
analyte is made
51
CALIBRATION METHODS
Method of Standard Additions (MSA) Useful - In
emergency situations - When information about
the sample matrix is unknown - For elimination
of certain interferences in the matrix
52
CALIBRATION METHODS
Internal Standard Calibration - Signal from
internal standard is used to correct for
interferences in an analyte - The selected
internal standard must not be already present in
all samples, blanks, and standard solutions -
Internal standard must not interact with
analyte Internal Standard - Known amount of a
nonanalyte species that is added to all samples,
blanks, and standards
53
CALIBRATION METHODS
Internal Standard Calibration - For an analyte
(A) and internal standard (S) Signal ratio (A/S)
is plotted against concentration ration
(A/S) Concentration ratio (A/S) of unknown is
obtained from the linear equation
54
CALIBRATION METHODS
Internal Standard Calibration Corrects errors
due to - Voltage fluctuations - Loss of analyte
during sample preparation - Change in volume due
to evaporation - Interferences
55
ERRORS ASSOCIATED WITH BEERS LAW
- Indeterminate (random) errors are associated
with all spectroscopic methods Examples - Noise
due to instability of light source - Detector
instability - Variation in placement of cell in
light path - Finger prints on cells
56
EVALUATION OF ERRORS
- ?T is the error in transmittance measurement -
The relative error is high when T is very high or
very low - For greatest accuracy, measurements
should be within 15 - 65 T or 0.19 - 0.82 A -
Samples with high concentration (A gt 0.82) should
be diluted and those with low concentrations (A lt
0.19) should be concentrated
57
EVALUATION OF ERRORS
Ringbom Method (100 T) is plotted against
log(c) - The result is an s-shaped curve
(Ringbom plot) - The nearly linear portion of
the curve (the steepest portion) is the working
range where error is minimized
(100-T)
Log(c)
58
OPTICAL SYSTEMS IN SPECTROSCOPY
Fundamental Concepts of Optical Measurements -
Measurement of absorption or emission of
radiation - Providing information about the
wavelength of absorption or emission -
Providing information about the intensity or
absorbance at the wavelength
59
OPTICAL SYSTEMS IN SPECTROSCOPY
Main Components of Spectrometers - Radiation
source - Wavelength selection device - Sample
holder (transparent to radiation) - Detector
60
OPTICAL SYSTEMS IN SPECTROSCOPY
- FT spectrometers do not require wavelength
selector - Radiation source is the sample if
emission is being measured - External radiation
source is required if absorption is being
measured - Sample holder is placed after
wavelength selector for UV-VIS absorption
spectrometry so that monochromatic light falls
on the sample - Sample holder is placed before
the wavelength selector for IR, fluorescence,
and AA spectroscopy
61
COMPONENTS OF THE SPECTROMETER
Absorption (UV-Vis)
b
Po
P
Light source
monochromator (? selector)
sample
readout
detector
62
COMPONENTS OF THE SPECTROMETER
Absorption (IR)
Light source
monochromator (? selector)
readout
detector
sample
63
COMPONENTS OF THE SPECTROMETER
Emission
Source sample
monochromator (? selector)
readout
detector
- Sample is an integral portion of the source -
Used to produce the EM radiation that will be
measured
64
COMPONENTS OF THE SPECTROMETER
Fluorescence
Source ? selector sample
monochromator (? selector)
readout
detector
65
RADIATION SOURCE
- Must emit radiation over the entire wavelength
range being studied - Intensity of radiation of
the wavelength range should be high - A reliable
and steady power supply is essential to provide
constant signal - Intensity should not
fluctuate over long time intervals - Intensity
should not fluctuate over short time
intervals Flicker short time fluctuation in
source intensity
66
RADIATION SOURCE
Two types of radiation sources Continuum Sources
and Line Sources
67
RADIATION SOURCE
Continuum Sources - Emit radiation over a wide
range of wavelengths - Intensity of emission
varies slowly as a function of wavelength - Used
for most molecular absorption and fluorescence
spectrometric instruments Examples - Tungsten
filament lamp (visible radiation) - Deuterium
lamp (UV radiation) - High pressure Hg lamp (UV
radiation) - Xenon arc lamp (UV-VIS region) -
Heated solid ceramics (IR region) - Heated wires
(IR region)
68
RADIATION SOURCE
Line Sources - Emit only a few discrete
wavelengths of light - Intensity is a function of
wavelength - Used for molecular, atomic, and
Raman spectroscopy Examples - Hollow cathode
lamp (UV-VIS region) - Electrodeless discharge
lamp (UV-VIS region) - Sodium vapor lamp (UV-VIS
region) - Mercury vapor lamp (UV-VIS region) -
Lasers (UV-VIS and IR regions)
69
RADIATION SOURCE
Tungsten Filament Lamp - Glows at a temperature
near 3000 K - Produces radiation at wavelengths
from 320 to 2500 nm - Visible and near IR
regions Dueterium (D2) Arc Lamp - D2 molecules
are electrically dissociated - Produces radiation
at wavelengths from 200 to 400 nm - UV region
70
RADIATION SOURCE
Mercury and Xenon Arc Lamps - Electric discharge
lamps - Produce radiation at wavelengths from 200
to 800 nm - UV and Visible regions Silicon
Carbide (SiC) Rod - Also called globar -
Electrically heated to about 1500 K - Produces
radiation at wavelengths from 1200 to 40000 nm -
IR region
71
RADIATION SOURCE
Also for IR Region - NiChrome wire (750 nm to
20000 nm) - ZrO2 (400 nm to 20000 nm)
72
RADIATION SOURCE
Laser - Produce specific spectral lines - Used
when high intensity line source is required Can
be used for UV Visible FTIR
73
WAVELENGTH SELECTION DEVICES
Two types Filters and Monochromators
74
FILTERS
- The simplest and most inexpensive Two major
types Absorption Filters and Interference
Filters
75
FILTERS
Absorption Filters - A piece of colored glass -
Stable, simple and cheap - Suitable for
spectrometers designed to be carried to the
field Disadvantage - Range of wavelengths
transmitted is very broad (50 300 nm)
76
FILTERS
Interference Filters - Made up of multiple
layers of materials - The thickness and the
refractive index of the center layer of the
material control the wavelengths transmitted -
Range of wavelengths transmitted are much smaller
(1 10 nm) - Amount of light transmitted is
generally higher - Transmits light in the IR,
VIS, and UV regions
77
MONOCHROMATORS
- Disperse a beam of light into its component
wavelengths - Allow only a narrow band of
wavelengths to pass - Block all other
wavelengths Components - Dispersion element -
Two slits (entrance and exit) - Lenses and
concave mirrors
78
MONOCHROMATORS
Dispersion Element - Disperses (spreads out) the
radiation falling on it according to
wavelength Two main Types Prisms and Gratings
79
MONOCHROMATORS
Prisms - Used to disperse IR, VIS, and UV
radiations - Widely used is the Cornu prism
(60o-60o-60o triangle) Examples Quartz
(UV) Silicate glass (VIS or near IR) NaCl or KBr
(IR)
80
MONOCHROMATORS
Prisms - Refraction or bending of incident light
occurs when a polychromatic light hits the
surface of the prism - Refractive index of prism
material varies with wavelength - Various
wavelengths are separated spatially as they are
bent at different degrees - Shorter wavelengths
(higher energy) are bent more than longer
wavelengths (lower energy)
81
MONOCHROMATORS
Diffraction Gratings - Consists of a series of
closely spaced parallel grooves cut (or ruled)
into a hard glass, metallic or ceramic surface -
The surface may be flat or concave - Reflective
coating (e.g. Al) is usually on the ruled
surface - Used for UV-VIS radiation (500 5000
grooves/mm) and IR radiation (50 200
grooves/mm)
82
MONOCHROMATORS
Diffraction Gratings
d
Side view
Top view
83
MONOCHROMATORS
Diffraction Gratings - Size ranges between 25 x
25 mm to 110 x 110 mm - Light is dispersed by
diffraction due to constructive interference
between reflected light waves - Separation of
light occurs due to different wavelengths being
dispersed (diffracted) at different angles
84
MONOCHROMATORS
Diffraction Gratings - Constructive interference
occurs when n? d(sini sin?) n order of
diffraction (integer 1, 2, 3, ) ? wavelength
of radiation d distance between grooves i
incident angle of a beam of light ? angle of
dispersion of light
85
MONOCHROMATORS
Dispersive Resolution Resolving Power (R) -
Ability to disperse radiation - Ability to
separate adjacent wavelengths from each other
? average of the wavelengths of the two lines
to be resolved d? difference between the two
wavelengths
86
MONOCHROMATORS
Resolution of a Prism
t thickness of the base of the prism d?/d?
rate of change of the refractive index (?) with
? - Resolving power increases with thickness of
the prism and decreases at longer wavelengths -
Resolution depends on the prism material
87
MONOCHROMATORS
Resolution of a Grating R nN n the order N
total number of grooves in the grating that are
illuminated by light from the entrance slit
(whole number) Increased resolution results from
- Longer gratings - Smaller groove spacing -
Higher order
88
MONOCHROMATORS
Dispersion of a Grating
d? change in wavelength dy change in
distance separating the ?s along the dispersion
axis Units nm/mm
89
MONOCHROMATORS
Dispersion of a Grating Spectral bandwidth
(bandpass) sD-1 s slit width of monochromator
d distance between two adjacent grooves n
diffraction order F focal length of the
monochromator system - D-1 is constant with
respect to wavelength
90
ECHELLE MONOCHROMATOR
Echellette Grating - Grooved or blazed such that
it has relatively broad faces from which
reflection occurs - Has narrow unused faces -
Provides highly efficient diffraction grating
91
ECHELLE MONOCHROMATOR
- Contains two dispersion elements arranged in
series - The first is known as echelle
grating - The second (called cross-dispersion)
is a low-dispersion prism or a grating Echelle
grating - Greater blaze angle - The short side of
the blaze is used rather than the long side -
Relatively coarse grating - Angle of dispersion
(?) is higher - Results in 10-fold resolution
92
OPTICAL SLITS
- Slits are used to select radiation from the
light source both before and after dispersion by
the ? selector - Made of metal in the shape of
two knife edges - Movable to set the desired
mechanical width
93
OPTICAL SLITS
Entrance Slit - Allows a beam of light
(polychromatic) from source to fall on the
dispersion element - Radiation is collimated
into a parallel beam with lenses or front-faced
mirrors - One (selected) wavelength of light
(monochromatic) is focused on the exit slit
after dispersion
94
OPTICAL SLITS
Exit Slit - Allows only a very narrow band of
light to pass through sample and detector -
The dispersed light falls on the exit slit - The
light is redirected and focused onto the detector
for intensity measurements - Slits are kept as
close as possible to ensure resolution
95
CUVET (SAMPLE CELL)

• - Cell used for spectrometry
• Identical or Optically Matched Cells
• - Cells that are identical in their absorbance or
  transmittance of light
• Fused silica Cells (SiO2)
• Transmits visible and UV radiation
• Plastic and Glass Cells
• - Only good for visible wavelengths
• NaCl and KBr Crystals
• - IR wavelengths

96
DETECTORS
- Used to measure the intensity of radiation
coming out of the exit slit - Produces an
electric signal proportional to the radiation
intensity - Signal is amplified and made
available for direct display - A sensitivity
control amplifies the signal - Noisy signal is
observed when amplification is too much - May be
controlled manually or by a microprocessor (the
use of dynodes)
97
DETECTORS
Examples - Phototube (UV) - Photomultiplier tube
(UV-VIS) - Thermocouple (IR) - Thermister (IR) -
Silicon photodiode - Photovoltaic cell - Charge
Transfer Devices (UV-VIS and IR) Charge-coupled
devices (CCDs) Charge injector devices (CIDs)
98
SINGLE-BEAM OPTICS
- Usually used for all emission methods where
sample is at the location of the source Drift -
Slow variation in signal with time - Can cause
errors in single-beam methods Sources of Drift -
Changes in Voltage which changes source
intensity - Warming up of source with time -
Deterioration of source or detector with time
99
SINGLE-BEAM OPTICS
Single-Beam Spectrometer - Only one beam of
light - First measure reference or blank (only
solvent) as Io
b
Io
I
Light source
monochromator (selects ?)
sample
computer
detector
100
DOUBLE-BEAM OPTICS
- Widely used - Beam splitter is used to split
radiation into two approximately equal beams
(reference and sample beams) - Radiation may
also alternate between sample and reference with
the aid of mirrors (rotating beam chopper) -
Other variations are available - The reference
cell may be empty or containing the blank - More
accurate since it eliminates drift errors
101
DOUBLE-BEAM OPTICS
Double-Beam Spectrometer - Houses both sample
cuvet and reference cuvet
b
P
Light source
monochromator (selects ?)
sample
computer
detector
Po
reference
102
SPECTROPHOTOMETERS
Photodiode Array Spectrophotometers - Records the
entire spectrum (all wavelengths) at once -
Makes use of a polychromator - The polychromator
disperses light into component wavelengths Dispe
rsive Spectrophotometers - Records one wavelength
at a time - Makes use monochromator to select
wavelength
103
FOURIER TRANFORM SPECTROPHOTOMETERS
- Have no slits and fewer optical
elements Multiplex - Instrument that uses
mathematical methods to interpret and present
spectrum without dispersion devices -
Wavelengths of interest are collected at a time
without dispersion - The wavelengths and their
corresponding intensities overlap - The
overlapping information is sorted out in order to
plot a spectrum
104
FOURIER TRANFORM SPECTROPHOTOMETERS
- Sorting out or deconvoluting the overlapping
signals of varying wavelengths (or frequencies)
is a mathematical procedure called Fourier
Analysis - Fourier Analysis expresses complex
spectrum as a sum of sine and cosine waves
varying with time - Data acquired is Fourier
Transformed into the spectrum curve - The
process is computerized and the instruments
employing this approach are called FT
spectrometers
105
FOURIER TRANFORM SPECTROPHOTOMETERS
Advantages of FT Systems - Produce better S/N
ratios (throughput or Jacquinot advantage) -
Time for measurement is drastically reduced (all
?s are measured simultaneously) - Accurate and
reproducible wavelength measurements