Spectrochemical methods
G. Galbács
Introduction to spectrochemical methods
The interactions of radiations and matter are the subject of
spectroscopy
p
py or spectrochemical
p
methods ((also called
spectrometry). Spectrochemical methods usually measure the
electromagnetic radiation produced (emitted) or absorbed by
molecular or atomic species of interest. It has to be added though
that spectroscopy nowadays includes some methods that do not
involve EM radiation, such as acoustic and mass spectroscopy.
A spec
spectrum
u a
always
ay refers to
oag
graph
ap which shows
o
the intensity
yo
of
radiation or the count of particles as a function of energy.
Spectroscopic methods are among the most selective and
sensitive analytical methods, and therefore are the second most
widely used methods (after chromatography). Spectroscopy also
played an important role in the development of atomic theory.
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Introduction to spectrochemical methods
Properties of electromagnetic radiation
EM radiation (e.g. light) is a form of energy that is transported through
space at very high velocities.
velocities EM radiation has a double nature.
nature
On one hand, it can be described as a wave with properties of
wavelength, frequency, velocity and amplitude. It also shows
wave phenomena such as refraction, reflection, interference,
diffraction. But in contrast to e.g. acoustic waves, EM waves require no
supporting medium for transmission (it propagates also in vacuum).
Introduction to spectrochemical methods
Properties of electromagnetic radiation
The frequency of EM radiation is determined by its source and remains
constant regardless of the medium traversed. In constrast, the
velocity of propagation depends both on the wavelength and the
medium. The governing equation is v= ν·λ where v is the velocity. In
air, v is very close to c (speed of light in vacuum).
Wavenumber (k, ν ) is defined as the reciprocate of lambda in cm.
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Introduction to spectrochemical methods
Properties of electromagnetic radiation
On the other hand, EM radiation can also be described as
consisting of photons or quanta (particles). It is neccessary, as
the wave model fails to account for the absorption and emission
processes. For these processes, EM radiation is best treated as
discrete packets of energy. The energy of a photon can be related to
its wavelength, frequency or wavenumber
E = h⋅ν =
h⋅v
= h⋅c⋅ ν
λ
where h is the Planck constant (6.63·10-34 J·s).
Radiant power (P, sometimes also called intensity) in watts is the
energy of a beam that reaches a given area per unit time. The power
of radiation is directly proportional to the number of photons per
second.
Introduction to spectrochemical methods
Some calculational examples – part 1.
Calculate the wavenumber of a beam of EM radiation that has:
a) a wavelength of 5 µm
b) a frequency of 100 GHz
The correct answers are:
a) 2000 cm-1
b) 3.33 cm-1
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Introduction to spectrochemical methods
Some calculational examples – part 2.
Calculate the energy in joules for one photon of the radiation that has:
a) a wavelength of 1 µm
b) a frequency of 2 THz
Remember that the value of the Planck constant is 6.63·10-34 J·s.
The correct answers are:
a) 1.98 · 10-19 J
b) 1.32 · 10-21 J
Introduction to spectrochemical methods
Some calculational examples – part 3.
Calculate the power in watts for a laser light pulse that has a duration
of 10 ns and a pulse energy of 110 mJ.
The correct answer is : 1.1 · 106 W
Calculate how many photons the above laser light pulse contains, if it
has a wavelength of 1 µm.
The correct answer is : 55 · 1016
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Introduction to spectrochemical methods
The electromagnetic spectrum
Classification of spectrochemical methods can given in terms of the
species they deal with (atomic or molecular) or the radiation
processes involved (e.g. absorption, emission). But most often it is
done in terms of the energy. See the electromagnetic spectrum:
Introduction to spectrochemical methods
The electromagnetic spectrum
Strictly speaking, light only refers to EM radiation in the visible range
(ca. 380 to 800 nm), but it is often used in the UV (ultra violet) and
IR (infrared) range too (see the visible spectrum below). Light is
associated with electronic transitions in atoms and molecules. Note
that spectrochemical methods working in the UV, visible and IR range
are often called optical methods.
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Introduction to spectrochemical methods
Spectroscopic measurement modes
In spectroscopy, the sample is stimulated in some way by applying
energy in the form of heat, electrical energy, light, particles or
chemical
h
i l reaction.
i
P i to the
Prior
h stimulus,
i
l
the
h analyte
l
i predominantly
is
d
i
l
in its lowest energy state or ground state. The stimulus then causes
some of the analyte to undergo a transition to a higher energy state,
or excited state. In most cases, we acquire information about the
analyte either by measuring the EM radiation emitted as the analyte
returns to ground state or by measuring the amount of EM radiation
absorbed by the analyte during excitation.
In fact, four different measurement modes can be identified:
•
•
•
•
emission spectroscopy
absorption spectroscopy
photoluminescence spectroscopy
mass spectrometry
Introduction to spectrochemical methods
Emission spectroscopy
In emission spectroscopy, the analyte is stimulated by electrical,
gy We record the emission spectrum,
p
,
thermal or chemical energy.
which is the intensity (power) of emitted radiation as a function of
wavelength or frequency.
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Introduction to spectrochemical methods
Emission spectroscopy – schematic of the instrument
In emission spectroscopy, the analyte is stimulated by electrical,
thermal or chemical energy.
gy We record the emission spectrum,
p
,
which is the intensity (power) of emitted radiation as a function of
wavelength or frequency.
Introduction to spectrochemical methods
Absorption spectroscopy
In absorption spectroscopy, the analyte is stimulated by EM
radiation from an external source and some of this incident
radiation is absorbed be the analyte. We record the absorption
spectrum, which is the amount of absorbed intensity (power) of
incident radiation as a function of wavelength or frequency.
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Introduction to spectrochemical methods
Absorption spectroscopy – schematic of the instrument
In absorption spectroscopy, the analyte is stimulated by EM
radiation from an external source and some of this incident
radiation is absorbed be the analyte. We record the absorption
spectrum, which is the amount of absorbed intensity (power) of
incident radiation as a function of wavelength or frequency.
Introduction to spectrochemical methods
Photoluminescence spectroscopy
In photoluminescence spectroscopy, the analyte is also stimulated
by EM radiation from an external source.
source Some of this incident
radiation is absorbed by the analyte and we record the emission
spectrum. In other words, we do emission spectroscopy after
excitation by EM radiation. Two subclasses of this spectroscopy are the
fluorescence and phosphorescence spectroscopies.
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Introduction to spectrochemical methods
Photoluminescence spectroscopy – schematic
In photoluminescence spectroscopy, the analyte is also stimulated
by EM radiation from an external source. Some of this incident
radiation is absorbed by the analyte and we record the emission
spectrum. In other words, we do emission spectroscopy after
excitation by EM radiation. Two subclasses of this spectroscopy are the
fluorescence and phosphorescence spectroscopies.
Introduction to spectrochemical methods
Mass spectrometry
In mass
ass spec
spectrometry,
o e y, the sample
a p
components
are
ionized
using
chemical, thermal or electric energy
and then the ions are be separated
according to their m/z (mass over
charge) ratio by a mass analyzer. The
number of charged particles for each
m/z ratio reaching the detector is
counted. This principle of operation
needs no EM radiation to be applied or
measured, hence its instrumentation
is unique. The example here shows
the mass spectrum of air.
N2+
N+
O+
O2+
H2O+
Ar+
CO2+
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Introduction to spectrochemical methods
Instrumentation - optical materials
Sample holders, windows, lenses and wavelength selecting elements
all must transmit radiation in the wavelength region of interest. This
limits the choice of optical materials. For example, simple glass is fine
for the VIS range, but fused silica or quartz is needed in the UV. In the
IR range, halide salts are often used.
Introduction to spectrochemical methods
Instrumentation – sample holders in UV/Vis range
cuvettes
for liquids
(and gases)
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Introduction to spectrochemical methods
Instrumentation – radiation sources
Absorption
and
luminescence
spectroscopies need a stable radiation
source that emits a radiation of high
intensity. In terms of wavelength,
spectroscopic radiation sources fall
into two categories: continuum
sources
and
line
sources.
Continuum sources emit radiation in a
broad
spectral
range
(intensity
changes slowly as a function of
wavelength).
Line
sources
emit
radiation only at specific wavelengths.
Sources can also be classified as
continuous or pulsed, according to
their operation as a function of time.
continuum spectrum
line spectrum
Introduction to spectrochemical methods
Continuum radiation sources in the UV/Vis range
As continuum radiation sources in
the UV,
UV typically deuterium or
hydrogen lamps (on the left) and
in the VIS range, tungsten lamps
(on the right) are used.
Tungsten
lamps
are
ordinary
filament
lamps.
Deuterium
(or
hydrogen) lamps are filled with low
pressure H2 gas, that
th t is
i excited
it d by
b
electrical
energy.
This
excited
species then dissociates to two H
atoms and a UV photon. The energy
of the photon can vary within a
range of energies.
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Introduction to spectrochemical methods
Continuum radiation sources in the IR range
IR continuum radiation sources
are normally inert heated solids.
Examples include SiC rods
(Globar lamp) and cylinders of
a composite made of 85% ZrO2
and 15% YO2 (Nenrst glower).
Introduction to spectrochemical methods
Line sources in the UV/Vis range – hollow cathode lamps
Hollow cathodes are made of the metal of interest (or its compound).
The high voltage (several hundreds of volts) connected to between
the cathode and anode will produce electrons that collide with and
ionize inert gas atoms, which in turn will sputter the cathode (-)
material. The sputtered atoms will be excitated by further collisions.
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Introduction to spectrochemical methods
Line sources in the UV/Vis range - lasers
Laser operation is based on the stimulated emission of radiation
initiated by population inversion in an active medium by
electrical/optical
l
i l/
i l pumping.
i
Laser beams are:
• highly collimated
• possessing very high intensity
• highly monochromatic
• pulsed or continuous
• coherent (interference capable)
Introduction to spectrochemical methods
Instrumentation – monochromators
Czerny-Turner
Bunsen
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Introduction to spectrochemical methods
Instrumentation – photon detectors (ca. 200-900 nm)
Phototube
Photoelectron multiplier (PMT)
Alk li metall or metall oxide
Alkali
id (C
(Cs-Sb)
Sb)
90 V or more
Every next dynode is held on a ca. 100 V more positive potential
Dynodes produce secondary electrons
PMTs are very sensitive and fast detectors with very wide linear dynamic range
(e.g. amplification is 109, dynamic range is 9-10 orders, etc.)
Introduction to spectrochemical methods
Instrumentation – semiconductor photon detectors
Doped Si semiconductors
Photodiode
Without light, conductance is very low (nA-µA) under reverse
bias. When photons strike the p-n junction, they create holeelectron pairs,
pairs thus extra carriers
carriers. The created current will be
proportional to the radiant power.
For IR ranges, an InGaAs
semiconductor is needed
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Introduction to spectrochemical methods
Instrumentation –array/multichannel detectors
Photodiode arrays (PDA)
Linear PDA or CCD array
Charge coupled detectors (CCD)
Introduction to spectrochemical methods
Instrumentation – thermal detectors (IR range)
Thermal detectors, that sense the heating effect of photons, are
needed in the IR range. The four common classes are the following:
Bolometers are very thin, black metallic conductor layers (e.g. Pt
„soot”, Sb, etc.) that have very little reflectivity. Operation is based on
the fact that the resistance of metals depend on T.
Thermopiles contain thermocouples (two metals or alloys connected
to each other) that respond to temperature.
Semiconductor detectors, such as InGaAs.
Golay cell is a pneumatic cell, in which
Xe gas is expanding/contracting in response
to the IR radiation and this causes
an internal light beam to be deflected
from a thin mirror (T).
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Introduction to spectrochemical methods
Radiation absorption – the Lambert-Beer law
If a parallel beam of monochromatic radiation passes through an
absorbing sample, then the intensity of the beam will be attenuated.
Transmittance (T) and absorbance (A) can be defined as:
T=
I
I0
A = lg
I0
1
= − lg T = lg
I
T
The Lambert-Beer law declares that A is linearly proportional to the
thickness of the sample and its concentration.
A = ε ⋅c ⋅l
where ε is the molar absorptivity, c is the molar concentration and l is
the thickness in cm. Note, that A refers to absorbance at a given
wavelength, and ε is also a function of wavelength.
Introduction to spectrochemical methods
Radiation absorption – the absorption spectrum
The absorption spectrum shows the absorbance as the function of
wavelength. As it is recorded in one setting, with the other
measurement conditions fixed (c and l is constant), it practically
depicts
ε(λ).
Visible absorption spectra of KMnO4 solutions
of various concentrations
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Introduction to spectrochemical methods
Radiation absorption – multicomponent samples
Absorption at a wavelength is an additive feature. This means that if
there are more than one species in the sample, the absorbance will be
the sum of absorbances for each compounds at the given wavelength.
A total = ΣAi = Σ(εi ⋅ ci ⋅ l) = l ⋅ Σ(εi ⋅ ci )
absorbance
Colored curves are individual spectra
of components; dotted curve is the
spectrum of the mixture
nm
Introduction to spectrochemical methods
Some calculational examples – part 4.
Calculate the transmittance for a solution whose absorbance is 0.245.
The correct answer is: T= 0.568
Explain, how the following quantities change when we double the
cuvette length (for the measurement of the same solution):
a) absorbance
b) transmittance
The correct answers are:
a) A2 = 2 · A1
b) T2= T1 · T1
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Introduction to spectrochemical methods
Some calculational examples – part 5.
Calculate the molar absorptivity for a 0.008M solution in a 15 mm
cuvette, whose absorbance is 0.760.
The correct answer is: ε= 63.33 dm3/mol⋅cm-1
Calculate the absorbance of a sample prepared by mixing 10 mL of a
0.15M solution and 20 mL of a 0.35M solution of the same compound
(ε= 4.5 dm3/mol⋅cm-1, cuvette length is 1 cm).
The correct answer is: 1.26
Introduction to spectrochemical methods
Derivative spectroscopy
Similarly to other analytical methods, it is also beneficial in
spectroscopy
p
py to p
plot the first derivative of the spectrum
p
curve as
it can reveal very fine details. See the examples below.
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Introduction to spectrochemical methods
Radiation absorption – deviations from Beer’s law
There are deviations from the linear law. Some of these are real and
some are due to chemical changes, or instrumental imperfections.
Concentration limitation. Validity is limited to diluted solutions
(concentrations well below 0.01 M) due to interactions between the
analyte species.
Chemical
deviations.
When
the
absorbing species in the sample undergo
association dissociation or reaction with
association,
the solvent, the resulting species will
absorb differently. This example shows
the case of an unbuffered solution of HA.
At higher concentrations, dissociation
diminishes.
A-
HA
Introduction to spectrochemical methods
Radiation absorption – deviations from Beer’s law
Instrumental deviations. The law is only valid if the radiation is
monochromatic and no stray light reaches the detector. Real
spectrometers always have some flaws/limited bandpass, etc.
Effect of polychromatic radiation
Effect of stray light
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Introduction to spectrochemical methods
Radiation absorption – deviations from Beer’s law
Non-specific radiation losses (e.g. scattering, reflection) and
differences between sample
p holders ((cuvettes)) can also result in
deviations from the law. Losses due to reflection at cell
interfaces can amount to as much as 8-10%, which is
comparable to the true light attenuation, therefore needs to be
corrected for...
Introduction to spectrochemical methods
Blank correction
This can be done by blank correction. First, a blank (only solvent
containing) solution is measured in an identical (or the same)
cuvette and the resulting signal is used for the correction. The
attenuation of light in the blank solution is due to the losses
associated with the cuvette and the absorption of the solvent.
A = lg
I0
Itrue
Iblank = I0 − Ilosses
Isample = Itrue − Ilosses
Ilosses = I0 − Iblank
Isample = Itrue − I0 + Iblank
Itrue = Isample + I0 − Iblank
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Introduction to spectrochemical methods
Single/double beam spectrophotometers
The blank correction has to be done manually (sequentially) with a
single
beam
spectrophotometer.
With
a
double
beam
spectrophotometer, it is easier because it has two cuvette holders:
Isample and Iblank are measured in parallel. Of course, the cuvettes
need to be matched (very similar) in this case. See the schematics
below.
single beam spectrophotometer
double beam spectrophotometer
Molecular (UV/Vis) absorption spectrometry
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UV/Vis molecular absorption spectroscopy
Analytical information in the UV/Vis range
Light absorption of molecules in the UV and Vis range is mainly due
to electronic transitions between molecular orbitals, and d-d orbitals
i metall ions
in
i
( l
(valence
shell
h ll electronic
l
i transitions
ii
that
h is).
i )
As in molecules usually many electrons are residing at very similar
energy levels, therefore UV/Vis absorption features in the spectrum
appear as „bands”, or wide peaks (50-200 nm breadth).
Useful spectral range of UV/Vis spectrophotometry is the ca. 190 to
800 nm range, limited mainly by the optical materials used to make
sample holders (cuvettes) and the absorption of common solvents.
UV/Vis molecular absorption spectroscopy
Absorption by organic molecules
In the case of organic molecules, light absorption in this range
comes mainly from n Æ π* and π Æ π* transitions. Other transitions
(such as σ Æ σ* and n Æ σ*) usually are not excited in this range,
so single bonds and isolated (non conjugated) double bonds do not
typically give rise to UV/Vis absorption.
wavelengths of these transitions are too low, so we
don’t see them in UV/Vis spectra
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UV/Vis molecular absorption spectroscopy
Absorption by organic molecules
Chromophores are the parts of molecules that make them colorful,
that is absorb light in the UV/Vis range. For organic molecules, these
are parts with
i h conjugated
j
d double
d bl bonds,
b d triple
i l bonds,
b d or bonds
b d with
i h
heteroatoms (in the latter case, for the nonbonding electrons).
UV/Vis molecular absorption spectroscopy
Absorption by organic molecules
It should be noted however, that absorption wavelengths (peak
maxima) for the various bonds or atomic environments are not
sharply
h
l fixed,
fi d but
b depends
d
d on structurall variations
i i
and
d the
h solvent.
l
Solvent effects arise from the
interaction of either the ground or
excited state of the molecule, thus
changing
the
wavelength
or
absorption by ca. 10-20 nm (blue
or red shift).
shift) Also,
Also a polar solvent
for a polar solute means more
interaction, thereby smearing over
absorption, leading to the loss of
fine absorption features in he
spectra. See the example of phenol
on the right (polar solute case).
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UV/Vis molecular absorption spectroscopy
Absorption by inorganic species
The ions and complexes of
many transition elements are
colored due to absorption
involving
transitions
from
filled to unfilled d-orbitals.
(for rare earth ions the
transitions are for 4f and 5f
electrons).
The
energy
differences
between
these
orbitals
depend on oxidation state and
the ligand the ion is bonded
to.
UV/Vis molecular absorption spectroscopy
Quantitative applications
Quantitative applications involve the use of Lambert-Beer law to the
determination of the concentration of an analyte. For increased
sensitivity,
ii i
l
long
pathlength
hl
h cells/cuvettes
ll /
can be
b used.
d
Direct applications, that is when a molecule strongly absorb UV/Vis
light (e.g. KMnO4) are quite straightforward, but relatively rare.
It is more common that a non-absorbing (e.g. colorless) analyte
species is quantitatively converted into another chemical form that
absorbs much strongly (chemical conversion). Here, the general
concept is that we either attach a chromophore to the analyte species
or oxidise/reduce it to produce a chromofore. Possibilities are wide, as
this approach can be used not only on molecules but also on metal
ions (e.g. complex formation) and inorganic ions. An important
advantage of this approach is that the chemical reaction can provide
extra selectivity to the determination.
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UV/Vis molecular absorption spectroscopy
Quantitative applications
iodate determination
conversion of metal ions into colorful complexes:
2
2
reagent: diethyl-dithiocarbamate
reagent: diphenyl-tiocarbazone
UV/Vis molecular absorption spectroscopy
Selected quantitative applications - environmental
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UV/Vis molecular absorption spectroscopy
Selected quantitative applications – clinical samples
Green color,
absorption at 630 nm
o-toluidine
UV/Vis molecular absorption spectroscopy
Qualitative applications
In solution, the qualitative applications of UV/Vis spectrophotometry is
limited. It is due to the fact that absorption bands are very wide and
not too characteristic
h
i i (their
( h i wavelength
l
h is
i influenced
i fl
d by
b many factors,
f
e.g. association/dissociation, pH). Chemical conversion is usually
needed to provide selectivity.
Visible absorption spectrum of benzene
in various phases/media
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UV/Vis molecular absorption spectroscopy
Qualitative applications – effect of pH (isosbestos point)
The protonated and deprotonated form of many molecules have
different light absorption, hence the pH has a strong effect on the
spectrum. The
Th presence off an isosbestos
i
b
point
i
i the
in
h spectrum is
i an
evidence that only two principal species are present.
phenol red
benzeneazodiphenylamine
UV/Vis molecular absorption spectroscopy
Characterization applications – equilibrium constant (K)
If we know the characteristic wavelengths (and ε) for some PX and X
compounds, that are in equilibrium with each other according to
P + X ↔ PX
K=
[PX]
[P][X]
then the equilibrium constant for the reaction can be determined
spectrophotometrically (Scatchard plot). Consider a series of
solutions, in which increments of X are added to a constant amount of
P0. Substituting [P]= P0 - [PX]
into K’s expression,
p
, we g
get
[PX]
= K ⋅ (P0 − [PX])
[X]
meaning that a plot of [PX]/[X] yields
a straight line with a slope of - K.
[PX] and [X] are determined photometrically.
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UV/Vis molecular absorption spectroscopy
Characterization applications – photometric titrations
Spectrophotometry can also be used as an end-point detection
method for titrations, assuming that the titration reaction consumes
or generates a chromophore at a given monitoring wavelength.
Examples include acid-base titrations in the presence of an indicator
dye or complexometric titrations. Precipitation titrations can
obviously not be generally followed spectrophotometrically.
Example: determination of a bismuth/copper mixture.
At 745 nm, Cu-EDTA absorbs strongly, but neither does EDTA nor Bi-EDTA
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