physical analysis 2010 - introduction to spectrochemistry ... analysis 2010 - introduction to...

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Spectrochemical methods

G. Galbács

The interactions of radiations and matter are the subject ofspectroscopy or spectrochemical methods (also called

Introduction to spectrochemical methods

p py p (spectrometry). Spectrochemical methods usually measure theelectromagnetic radiation produced (emitted) or absorbed bymolecular or atomic species of interest. It has to be added thoughthat spectroscopy nowadays includes some methods that do notinvolve EM radiation, such as acoustic and mass spectroscopy.

A spectrum always refers to a graph which shows the intensity ofspec u a ay o a g ap o y oradiation or the count of particles as a function of energy.

Spectroscopic methods are among the most selective andsensitive analytical methods, and therefore are the second mostwidely used methods (after chromatography). Spectroscopy alsoplayed an important role in the development of atomic theory.

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Introduction to spectrochemical methodsProperties of electromagnetic radiation

EM radiation (e.g. light) is a form of energy that is transported throughspace at very high velocities EM radiation has a double naturespace at very high velocities. EM radiation has a double nature.

On one hand, it can be described as a wave with properties ofwavelength, frequency, velocity and amplitude. It also showswave phenomena such as refraction, reflection, interference,diffraction. But in contrast to e.g. acoustic waves, EM waves require nosupporting medium for transmission (it propagates also in vacuum).


The frequency of EM radiation is determined by its source and remainsconstant regardless of the medium traversed. In constrast, thevelocity of propagation depends both on the wavelength and themedium. The governing equation is v= ν·λ where v is the velocity. Inair, v is very close to c (speed of light in vacuum).

Wavenumber (k, ) is defined as the reciprocate of lambda in cm.ν

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On the other hand, EM radiation can also be described asconsisting of photons or quanta (particles). It is neccessary, asthe wave model fails to account for the absorption and emissionprocesses. For these processes, EM radiation is best treated asdiscrete packets of energy. The energy of a photon can be related toits wavelength, frequency or wavenumber

ν⋅⋅=λ⋅

=ν⋅= chvh

hE

where h is the Planck constant (6.63·10-34 J·s).

Radiant power (P, sometimes also called intensity) in watts is theenergy of a beam that reaches a given area per unit time. The powerof radiation is directly proportional to the number of photons persecond.

Introduction to spectrochemical methodsSome calculational examples – part 1.

Calculate the wavenumber of a beam of EM radiation that has:

a) a wavelength of 5 µmb) a frequency of 100 GHz

The correct answers are:

a) 2000 cm-1

b) 3.33 cm-1

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Calculate the energy in joules for one photon of the radiation that has:

a) a wavelength of 1 µmb) a frequency of 2 THz

Remember that the value of the Planck constant is 6.63·10-34 J·s.


a) 1.98 · 10-19 Jb) 1.32 · 10-21 J


Calculate the power in watts for a laser light pulse that has a durationof 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 ithas a wavelength of 1 µm.

The correct answer is : 55 · 1016

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Introduction to spectrochemical methodsThe electromagnetic spectrum

Classification of spectrochemical methods can given in terms of thespecies they deal with (atomic or molecular) or the radiationprocesses involved (e.g. absorption, emission). But most often it isdone in terms of the energy. See the electromagnetic spectrum:

Introduction to spectrochemical methodsThe 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) andIR (infrared) range too (see the visible spectrum below). Light isassociated with electronic transitions in atoms and molecules. Notethat spectrochemical methods working in the UV, visible and IR rangeare often called optical methods.

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Introduction to spectrochemical methodsSpectroscopic measurement modes

In spectroscopy, the sample is stimulated in some way by applyingenergy in the form of heat, electrical energy, light, particles orh i l i P i h i l h l i d i lchemical reaction. Prior to the stimulus, the analyte is predominantly

in its lowest energy state or ground state. The stimulus then causessome of the analyte to undergo a transition to a higher energy state,or excited state. In most cases, we acquire information about theanalyte either by measuring the EM radiation emitted as the analytereturns to ground state or by measuring the amount of EM radiationabsorbed 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 methodsEmission spectroscopy

In emission spectroscopy, the analyte is stimulated by electrical,thermal or chemical energy. We record the emission spectrum,gy p ,which is the intensity (power) of emitted radiation as a function ofwavelength or frequency.

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Introduction to spectrochemical methodsEmission spectroscopy – schematic of the instrument

In emission spectroscopy, the analyte is stimulated by electrical,thermal or chemical energy. We record the emission spectrum,gy p ,which is the intensity (power) of emitted radiation as a function ofwavelength or frequency.

Introduction to spectrochemical methodsAbsorption spectroscopy

In absorption spectroscopy, the analyte is stimulated by EMradiation from an external source and some of this incidentradiation is absorbed be the analyte. We record the absorptionspectrum, which is the amount of absorbed intensity (power) ofincident radiation as a function of wavelength or frequency.

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Introduction to spectrochemical methodsAbsorption spectroscopy – schematic of the instrument

In absorption spectroscopy, the analyte is stimulated by EMradiation from an external source and some of this incidentradiation is absorbed be the analyte. We record the absorptionspectrum, which is the amount of absorbed intensity (power) ofincident radiation as a function of wavelength or frequency.

Introduction to spectrochemical methodsPhotoluminescence spectroscopy

In photoluminescence spectroscopy, the analyte is also stimulatedby EM radiation from an external source Some of this incidentby EM radiation from an external source. Some of this incidentradiation is absorbed by the analyte and we record the emissionspectrum. In other words, we do emission spectroscopy afterexcitation by EM radiation. Two subclasses of this spectroscopy are thefluorescence and phosphorescence spectroscopies.

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Introduction to spectrochemical methodsPhotoluminescence spectroscopy – schematic

In photoluminescence spectroscopy, the analyte is also stimulatedby EM radiation from an external source. Some of this incidentradiation is absorbed by the analyte and we record the emissionspectrum. In other words, we do emission spectroscopy afterexcitation by EM radiation. Two subclasses of this spectroscopy are thefluorescence and phosphorescence spectroscopies.

Introduction to spectrochemical methodsMass spectrometry

In mass spectrometry, the sampleass spec o e y, a pcomponents are ionized usingchemical, thermal or electric energyand then the ions are be separatedaccording to their m/z (mass overcharge) ratio by a mass analyzer. Thenumber of charged particles for eachm/z ratio reaching the detector is

N2+

O2+

Ar+N+m/z ratio reaching the detector iscounted. This principle of operationneeds no EM radiation to be applied ormeasured, hence its instrumentationis unique. The example here showsthe mass spectrum of air.

Ar

CO2+O+

H2O+

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Introduction to spectrochemical methodsInstrumentation - optical materials

Sample holders, windows, lenses and wavelength selecting elementsall must transmit radiation in the wavelength region of interest. Thislimits the choice of optical materials. For example, simple glass is finefor the VIS range, but fused silica or quartz is needed in the UV. In theIR range, halide salts are often used.

Introduction to spectrochemical methodsInstrumentation – sample holders in UV/Vis range

cuvettesfor liquids for liquids (and gases)

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Introduction to spectrochemical methodsInstrumentation – radiation sources

Absorption and luminescencespectroscopies need a stable radiationsource that emits a radiation of highintensity. In terms of wavelength,spectroscopic radiation sources fallinto two categories: continuumsources and line sources.Continuum sources emit radiation in abroad spectral range (intensity

continuum spectrum

changes slowly as a function ofwavelength). Line sources emitradiation only at specific wavelengths.Sources can also be classified ascontinuous or pulsed, according totheir operation as a function of time.

line spectrum

Introduction to spectrochemical methodsContinuum radiation sources in the UV/Vis range

As continuum radiation sources inthe UV typically deuterium orthe UV, typically deuterium orhydrogen lamps (on the left) andin the VIS range, tungsten lamps(on the right) are used.Tungsten lamps are ordinaryfilament lamps. Deuterium (orhydrogen) lamps are filled with low

H th t i it d bpressure H2 gas, that is excited byelectrical energy. This excitedspecies then dissociates to two Hatoms and a UV photon. The energyof the photon can vary within arange of energies.

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Introduction to spectrochemical methodsContinuum radiation sources in the IR range

IR continuum radiation sourcesIR continuum radiation sourcesare normally inert heated solids.

Examples include SiC rods(Globar lamp) and cylinders ofa composite made of 85% ZrO2

and 15% YO2 (Nenrst glower).

Introduction to spectrochemical methodsLine 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 betweenThe high voltage (several hundreds of volts) connected to betweenthe cathode and anode will produce electrons that collide with andionize 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 methodsLine sources in the UV/Vis range - lasers

Laser operation is based on the stimulated emission of radiationinitiated by population inversion in an active medium byl i l/ i l ielectrical/optical pumping.

Laser beams are:• highly collimated• possessing very high intensity• highly monochromatic• pulsed or continuous• coherent (interference capable)

Introduction to spectrochemical methodsInstrumentation – monochromators

Czerny-Turner

Bunsen

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Introduction to spectrochemical methodsInstrumentation – photon detectors (ca. 200-900 nm)

Alk li l l id (C Sb)

Phototube Photoelectron multiplier (PMT)

Alkali metal or metal oxide (Cs-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 methodsInstrumentation – 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 hole-electron pairs thus extra carriers The created current will be

For IR ranges, an InGaAs semiconductor is needed

electron pairs, thus extra carriers. The created current will be proportional to the radiant power.

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Introduction to spectrochemical methodsInstrumentation –array/multichannel detectors

Photodiode arrays (PDA)

Charge coupled detectors (CCD)

Linear PDA or CCD array

Charge coupled detectors (CCD)

Introduction to spectrochemical methodsInstrumentation – thermal detectors (IR range)

Thermal detectors, that sense the heating effect of photons, areneeded 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 onthe fact that the resistance of metals depend on T.

Thermopiles contain thermocouples (two metals or alloys connectedto each other) that respond to temperature.

Semiconductor detectors, such as InGaAs.

Golay cell is a pneumatic cell, in whichXe gas is expanding/contracting in responseto the IR radiation and this causesan internal light beam to be deflectedfrom a thin mirror (T).

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Introduction to spectrochemical methodsRadiation absorption – the Lambert-Beer law

If a parallel beam of monochromatic radiation passes through anabsorbing sample, then the intensity of the beam will be attenuated.Transmittance (T) and absorbance (A) can be defined as:

The Lambert-Beer law declares that A is linearly proportional to thethickness of the sample and its concentration.

0II

T =T1

lgTlgII

lgA 0 =−==

where ε is the molar absorptivity, c is the molar concentration and l isthe thickness in cm. Note, that A refers to absorbance at a givenwavelength, and ε is also a function of wavelength.

lcA ⋅⋅ε=

Introduction to spectrochemical methodsRadiation absorption – the absorption spectrum

The absorption spectrum shows the absorbance as the function ofwavelength. As it is recorded in one setting, with the othermeasurement 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 methodsRadiation absorption – multicomponent samples

Absorption at a wavelength is an additive feature. This means that ifthere are more than one species in the sample, the absorbance will be

the sum of absorbances for each compounds at the given wavelength.

)c(l)lc(AA iiiiitotal ⋅εΣ⋅=⋅⋅εΣ=Σ=

Colored curves are individual spectra of components; dotted curve is the spectrum of the mixture

abso

rban

ce

nm


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 thecuvette length (for the measurement of the same solution):

a) absorbanceb) transmittance


a) A2 = 2 · A1

b) T2= T1 · T1

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Calculate the molar absorptivity for a 0.008M solution in a 15 mmcuvette, 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 a0.15M 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 methodsDerivative spectroscopy

Similarly to other analytical methods, it is also beneficial inspectroscopy to plot the first derivative of the spectrum curve asp py p pit can reveal very fine details. See the examples below.

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Introduction to spectrochemical methodsRadiation absorption – deviations from Beer’s law

There are deviations from the linear law. Some of these are real andsome 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 theanalyte species.

Chemical deviations. When theabsorbing species in the sample undergoassociation dissociation or reaction with A-

HA

association, dissociation or reaction withthe solvent, the resulting species willabsorb differently. This example showsthe case of an unbuffered solution of HA.At higher concentrations, dissociationdiminishes.


Instrumental deviations. The law is only valid if the radiation ismonochromatic and no stray light reaches the detector. Realspectrometers always have some flaws/limited bandpass, etc.

Effect of polychromatic radiation Effect of stray light

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Non-specific radiation losses (e.g. scattering, reflection) anddifferences between sample holders (cuvettes) can also result inp ( )deviations from the law. Losses due to reflection at cellinterfaces can amount to as much as 8-10%, which iscomparable to the true light attenuation, therefore needs to becorrected for...

Introduction to spectrochemical methodsBlank correction

This can be done by blank correction. First, a blank (only solventcontaining) solution is measured in an identical (or the same)cuvette and the resulting signal is used for the correction. Theattenuation of light in the blank solution is due to the lossesassociated with the cuvette and the absorption of the solvent.

true

0

III

II

lgA

−=

=

blank0sampletrue

blank0truesample

blank0losses

lossestruesample

losses0blank

IIII

IIII

III

III

III

−+=

+−=

−=

−=

=

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Introduction to spectrochemical methodsSingle/double beam spectrophotometers

The blank correction has to be done manually (sequentially) with asingle beam spectrophotometer. With a double beamspectrophotometer, it is easier because it has two cuvette holders:Isample and Iblank are measured in parallel. Of course, the cuvettesneed to be matched (very similar) in this case. See the schematicsbelow.

single beam spectrophotometer double beam spectrophotometer

Molecular (UV/Vis) absorption spectrometry

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UV/Vis molecular absorption spectroscopyAnalytical information in the UV/Vis range

Light absorption of molecules in the UV and Vis range is mainly dueto electronic transitions between molecular orbitals, and d-d orbitalsi l i ( l h ll l i i i h i )in metal ions (valence shell electronic transitions that is).

As in molecules usually many electrons are residing at very similarenergy levels, therefore UV/Vis absorption features in the spectrumappear as „bands”, or wide peaks (50-200 nm breadth).

Useful spectral range of UV/Vis spectrophotometry is the ca. 190 to800 nm range, limited mainly by the optical materials used to makesample holders (cuvettes) and the absorption of common solvents.

UV/Vis molecular absorption spectroscopyAbsorption by organic molecules

In the case of organic molecules, light absorption in this rangecomes mainly from n π* and π π* transitions. Other transitions(such as σ σ* and n σ*) usually are not excited in this range,so single bonds and isolated (non conjugated) double bonds do nottypically 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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Chromophores are the parts of molecules that make them colorful,that is absorb light in the UV/Vis range. For organic molecules, these

i h j d d bl b d i l b d b d i hare parts with conjugated double bonds, triple bonds, or bonds withheteroatoms (in the latter case, for the nonbonding electrons).


It should be noted however, that absorption wavelengths (peakmaxima) for the various bonds or atomic environments are noth l fi d b d d l i i d h lsharply fixed, but depends on structural variations and the solvent.

Solvent effects arise from theinteraction of either the ground orexcited state of the molecule, thuschanging the wavelength orabsorption by ca. 10-20 nm (blueor red shift) Also a polar solventor red shift). Also, a polar solventfor a polar solute means moreinteraction, thereby smearing overabsorption, leading to the loss offine absorption features in hespectra. See the example of phenolon the right (polar solute case).

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UV/Vis molecular absorption spectroscopyAbsorption by inorganic species

The ions and complexes ofmany transition elements aremany transition elements arecolored due to absorptioninvolving transitions fromfilled to unfilled d-orbitals.(for rare earth ions thetransitions are for 4f and 5felectrons).The energy differencesThe energy differencesbetween these orbitalsdepend on oxidation state andthe ligand the ion is bondedto.

UV/Vis molecular absorption spectroscopyQuantitative applications

Quantitative applications involve the use of Lambert-Beer law to thedetermination of the concentration of an analyte. For increased

i i i l hl h ll / b dsensitivity, long pathlength cells/cuvettes can be used.

Direct applications, that is when a molecule strongly absorb UV/Vislight (e.g. KMnO4) are quite straightforward, but relatively rare.

It is more common that a non-absorbing (e.g. colorless) analytespecies is quantitatively converted into another chemical form thatabsorbs much strongly (chemical conversion). Here, the generalconcept is that we either attach a chromophore to the analyte speciesor oxidise/reduce it to produce a chromofore. Possibilities are wide, asthis approach can be used not only on molecules but also on metalions (e.g. complex formation) and inorganic ions. An importantadvantage of this approach is that the chemical reaction can provideextra selectivity to the determination.

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UV/Vis molecular absorption spectroscopyQuantitative applications

iodate determination

conversion of metal ions into colorful complexes:

2

2

reagent: diethyl-dithiocarbamate

reagent: diphenyl-tiocarbazone

UV/Vis molecular absorption spectroscopySelected quantitative applications - environmental

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UV/Vis molecular absorption spectroscopySelected quantitative applications – clinical samples

o-toluidine

Green color, absorption at 630 nm

UV/Vis molecular absorption spectroscopyQualitative applications

In solution, the qualitative applications of UV/Vis spectrophotometry islimited. It is due to the fact that absorption bands are very wide and

h i i ( h i l h i i fl d b fnot too characteristic (their wavelength is influenced by many factors,e.g. association/dissociation, pH). Chemical conversion is usuallyneeded to provide selectivity.

Visible absorption spectrum of benzene in various phases/media

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UV/Vis molecular absorption spectroscopyQualitative applications – effect of pH (isosbestos point)

The protonated and deprotonated form of many molecules havedifferent light absorption, hence the pH has a strong effect on the

Th f i b i i h ispectrum. The presence of an isosbestos point in the spectrum is anevidence that only two principal species are present.

benzeneazodiphenylaminephenol red

UV/Vis molecular absorption spectroscopyCharacterization applications – equilibrium constant (K)

If we know the characteristic wavelengths (and ε) for some PX and Xcompounds, that are in equilibrium with each other according to

then the equilibrium constant for the reaction can be determinedspectrophotometrically (Scatchard plot). Consider a series ofsolutions, in which increments of X are added to a constant amount ofP0. Substituting [P]= P0 - [PX]into K’s expression, we get

]X][P[]PX[

K =PXXP ↔+

p , g

meaning that a plot of [PX]/[X] yieldsa straight line with a slope of - K.[PX] and [X] are determined photometrically.

])PX[P(K]X[]PX[

0 −⋅=

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UV/Vis molecular absorption spectroscopyCharacterization applications – photometric titrations

Spectrophotometry can also be used as an end-point detectionmethod for titrations, assuming that the titration reaction consumesor generates a chromophore at a given monitoring wavelength.Examples include acid-base titrations in the presence of an indicatordye or complexometric titrations. Precipitation titrations canobviously 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

physical analysis 2010 - introduction to spectrochemistry ... analysis 2010 - introduction to...

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