Transcript monochromators - Clayton State University

INSTRUMENTAL ANALYSIS
CHEM 4811
CHAPTER 2
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 2
INTRODUCTION TO SPECTROSCOPY
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)
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
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
ELECTROMAGNETIC RADIATION
Wavelength (m)
Gamma X rays Ultrrays
violet
1020
Visible
10-11
103
Infrared Microwaves
Radio frequency
FM Shortwave AM
Frequency (s-1)
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
104
ELECTROMAGNETIC RADIATION
λ1
node
amplitude
ν1 = 4 cycles/second
λ2
ν2 = 8 cycles/second
peak
λ3
ν3 = 16 cycles/second
trough
one second
ELECTROMAGNETIC RADIATION
Wavelength (λ)
- Distance for a wave to go through a complete cycle
(distance between two consecutive peaks or troughs in a wave)
Frequency (ν)
- 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)
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
ELECTROMAGNETIC RADIATION
- Inverse relationship between wavelength and frequency
λ α 1/ν
c=λν
λ = wavelength (m)
ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz)
c = speed of light (3.00 x 108 m/s)
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
ELECTROMAGNETIC RADIATION
hc
Energy of one photon (E photon)  hν 
 hc~ν
λ
~ν  1  wavenumber (m 1 )
λ
h = Planck’s constant (6.626 x 10-34 J-s)
ν = frequency of the radiation
λ = wavelength of the radiation
E is proportional to ν and inversely proportional to λ
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
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
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
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)
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
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
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
INTERACTIONS WITH MATTER
Ground State: The lowest energy state
Excited state: higher energy state (usually short-lived)
Energy
Excited
state
Absorption
Emission
Ground
state
INTERACTIONS WITH MATTER
- Change in state requires the absorption or emission of energy
Change in energy ( E)  hν 
hc
λ
- 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 (ΔE is positive)
Emission: Energy of species decreases (ΔE is negative)
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 < vibrational < electronic
- The are many associated rotational and vibrational sublevels
for any electronic state (absorption occurs in
closely spaced range of wavelenghts)
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
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
ATOMS AND ATOMIC SPECTROSCOPY
- Emitted energy is equivalent to the absorbed energy (ΔE)
- 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)
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)
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Processes Occurring in Each Region
103
Gamma
UltrX
rays
rays
violet
1020
Visible
10-11
Infrared Microwaves
Radio frequency
FM Shortwave AM
rotation
vibration
Electronic
excitation
Bond breaking
and ionization
104
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
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
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
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
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
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
MOLECULES AND MOLECULAR SPECTROSCOPY
Molecular Spectroscopy Methods
- Molecular absorption spectroscopy
- Molecular emission spectroscopy
- Nuclear Magnetic Resonance (NMR)
- UV-VIS
- IR
- MS
- Molecular Fluorescence Spectroscopy
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
ABSORPTION LAWS
Transmittance (T)
- The fraction of incident light that passes through a sample
T
I
Io
0<T<1
Io = light intensity striking a sample
I = light intensity emerging from sample
Io
I
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)
%T 
I
x 100%
Io
0% < %T < 100%
ABSORPTION LAWS
Absorbance (A)
 I
I 
A  log  o    log     logT
 I
 Io 
- 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
ABSORPTION LAWS
Beer’s Law
A = abc
A = absorbance
a = absorptivity
a = ε [molar absorptivity (M-1cm-1) if C is in units of M (mol/L)]
b = pathlength or length of cell (cm)
c = concentration
ABSORPTION LAWS
Beer’s 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.)
ABSORPTION LAWS
Absorption Spectrum of 0.10 mM Ru(bpy)32+
λmax = 452 nm
ABSORPTION LAWS
Absorption Spectrum of 3.0 mM Cr3+ complex
0.80
0.70
Absorbance
0.60
0.50
0.40
0.30
0.20
0.10
0.00
350
400
450
500
550
Wavelength (nm)
λmax = 540 nm
600
ABSORPTION LAWS
Maximum Response (λmax)
- Wavelength at which the highest absorbance is observed
for a given concentration
- Gives the greatest sensitivity
ABSORPTION LAWS
Deviations from Beer’s 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
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
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
CALIBRATION METHODS
Calibration with External Standards
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
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
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
Concentrat ion of unknown sample 
signal due to untreated sample
slope of calibratio n curve
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
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
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
Concentrat ion ratio (A/S) of unknown sample
signal ratio (A/S) of unknown sample

Concentrat ion ratio (A/S) of standard
signal ratio (A/S) of standard
CALIBRATION METHODS
Internal Standard Calibration
Corrects errors due to
- Voltage fluctuations
- Loss of analyte during sample preparation
- Change in volume due to evaporation
- Interferences
ERRORS ASSOCIATED WITH BEER’S 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
EVALUATION OF ERRORS
Relative error in concentrat ion 
Δc 0.434T

c
TlogT
- ΔT 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 > 0.82) should be
diluted and those with low concentrations (A < 0.19) should
be concentrated
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)
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
OPTICAL SYSTEMS IN SPECTROSCOPY
Main Components of Spectrometers
- Radiation source
- Wavelength selection device
- Sample holder (transparent to radiation)
- Detector
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
COMPONENTS OF THE SPECTROMETER
Absorption (UV-Vis)
b
Light
source
monochromator
(λ selector)
Po
sample
P
detector
readout
COMPONENTS OF THE SPECTROMETER
Absorption (IR)
Light
source
sample
monochromator
(λ selector)
detector
readout
COMPONENTS OF THE SPECTROMETER
Emission
Source
& sample
monochromator
detector
readout
(λ selector)
- Sample is an integral portion of the source
- Used to produce the EM radiation that will be measured
COMPONENTS OF THE SPECTROMETER
Fluorescence
Source
λ selector
sample
monochromator
(λ selector)
detector
readout
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
RADIATION SOURCE
Two types of radiation sources
Continuum Sources
and
Line Sources
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)
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)
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
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
RADIATION SOURCE
Also for IR Region
- NiChrome wire (750 nm to 20000 nm)
- ZrO2 (400 nm to 20000 nm)
RADIATION SOURCE
Laser
- Produce specific spectral lines
- Used when high intensity line source is required
Can be used for
UV
Visible
FTIR
WAVELENGTH SELECTION DEVICES
Two types
Filters
and
Monochromators
FILTERS
- The simplest and most inexpensive
Two major types
Absorption Filters
and
Interference Filters
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)
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
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
MONOCHROMATORS
Dispersion Element
- Disperses (spreads out) the radiation falling on it
according to wavelength
Two main Types
Prisms
and
Gratings
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)
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)
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)
MONOCHROMATORS
Diffraction Gratings
d
Top view
Side view
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
MONOCHROMATORS
Diffraction Gratings
- Constructive interference occurs when
nλ = d(sini ± sinθ)
n = order of diffraction (integer: 1, 2, 3, …)
λ = wavelength of radiation
d = distance between grooves
i = incident angle of a beam of light
θ = angle of dispersion of light
MONOCHROMATORS
Dispersive Resolution
Resolving Power (R):
- Ability to disperse radiation
- Ability to separate adjacent wavelengths from each other
R
λ
δλ
λ = average of the wavelengths of the two lines to be resolved
δλ = difference between the two wavelengths
MONOCHROMATORS
Resolution of a Prism
Rt
dη
dλ
t = thickness of the base of the prism
dη/dλ = rate of change of the refractive index (η) with λ
- Resolving power increases with thickness of the prism and
decreases at longer wavelengths
- Resolution depends on the prism material
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
MONOCHROMATORS
Dispersion of a Grating
dλ
Reciprocal dispersion (D ) 
dy
-1
dλ = change in wavelength
dy = change in distance separating the λs along the dispersion axis
Units: nm/mm
MONOCHROMATORS
Dispersion of a Grating
Spectral bandwidth (bandpass) = sD-1
s = slit width of monochromator
d
D 
nF
-1
d = distance between two adjacent grooves
n = diffraction order
F = focal length of the monochromator system
- D-1 is constant with respect to wavelength
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
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 (θ) is higher
- Results in 10-fold resolution
OPTICAL SLITS
- Slits are used to select radiation from the light source both
before and after dispersion by the λ selector
- Made of metal in the shape of two knife edges
- Movable to set the desired mechanical width
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
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
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
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)
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)
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
SINGLE-BEAM OPTICS
Single-Beam Spectrometer
- Only one beam of light
- First measure reference or blank (only solvent) as Io
b
Light
source
monochromator
(selects λ)
Io
sample
I
detector
computer
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
DOUBLE-BEAM OPTICS
Double-Beam Spectrometer
- Houses both sample cuvet and reference cuvet
b
Light
source
monochromator
(selects λ)
sample
reference
P
Po
detector
computer
SPECTROPHOTOMETERS
Photodiode Array Spectrophotometers
- Records the entire spectrum (all wavelengths) at once
- Makes use of a polychromator
- The polychromator disperses light into component wavelengths
Dispersive Spectrophotometers
- Records one wavelength at a time
- Makes use monochromator to select wavelength
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
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
FOURIER TRANFORM SPECTROPHOTOMETERS
Advantages of FT Systems
- Produce better S/N ratios (throughput or Jacquinot advantage)
- Time for measurement is drastically reduced
(all λs are measured simultaneously)
- Accurate and reproducible wavelength measurements