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INTRODUCTION TO OPTICAL ATOMIC SPECTROSCOPY (Chapter 8)

• Atomic spectroscopy techniques:Optical spectrometryMass spectrometryX-Ray spectrometry

• Optical spectrometry:Elements in the sample are atomizedbefore analysis.

• Atomization:Elements present in the sample areconverted to gaseous atoms or elementaryions. It occurs in the atomizer (see Table 8-1).

• Optical spectroscopy techniques:Atomic Absorption Spectroscopy (AAS)Atomic Emission spectroscopy (AES)Atomic Fluorescence Spectroscopy (AFS)

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Optical Atomic Spectra• Figure 8-1a shows the energy level diagram for sodium.• A value of zero electron volts (eV) is arbitrarily assigned to

orbital 3s.• The scale extends up to 5.14eV, the energy required to

remove the single 3s electron to produce a sodium ion.5.14eV is the ionization energy.

• A horizontal line represents the energy of and atomic orbital.• “p” orbitals are split into two levels which differ slightly in

energy:3s → 3p: = 5896Å or 5890Å3s → 4p: = 3303Å or 3302Å3s → 5p: = 2853.0Å or 2852.8Å

• There are similar differences in the d and f orbitals, but theirmagnitudes are usually so small that are undetectable, thusonly a single level is shown for orbitals d.

Spin-orbit coupling

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Multiplicity: number of possible orientations ofthe resultant spin angular momentum = 2S +1

98

Atomic Line Widths

• Widths of atomic lines are quite important inatomic spectroscopy.

• Narrow lines in atomic and emission spectrareduce the possibility of interference due tooverlapping lines.

• Atomic absorption and emission linesconsists of a symmetric distribution ofwavelengths that centers on a meanwavelength (0) which is the wavelength ofmaximum absorption or maximum intensityfor emitted radiation.

• The energy associated with 0 is equal tothe exact energy difference between twoquantum states responsible for absorption oremission.

• A transition between two discrete, single-valued energy states should be a line withline-width equal to zero.

• However, several phenomena cause linebroadening in such a way that all atomiclines have finite widths.

• Line width or effective line width (1/2) of anatomic absorption or emission line is definedas its width in wavelength units whenmeasured at one half the maximum signal.

Sources of broadening:(1) Uncertainty effect(2) Doppler effect(3) Pressure effects due to collisions(4) Electric and magnetic field effects

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Uncertainty Effect

• It results from the uncertainty principlepostulated in 1927 by Werner Heisenberg.

• One of several ways of formulating theHeisenberg uncertainty principle is shown inthe following equation:

t x E = h

• The meaning in words of this equation is asfollows: if the energy E of a particle or systemof particles – photons, electrons, neutrons orprotons – is measured for an exactly knownperiod of time t, then this energy isuncertain by at least h/ t.

• Therefore, the energy of a particle can beknown with zero uncertainty only if it isobserved for an infinite period of time.

• For finite periods, the energy measurementcan never be more precise then h/ t.

• The lifetime of a ground state is typically long,but the lifetimes of excited states aregenerally short, typically 10-7 to 10-8 seconds.

• Line widths due to uncertainty broadening arecalled natural line widths and are generally10-5nm or 10-4Å.

Note: = 1/2

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Doppler Effect

• In a collection of atoms in a hot environment,such as an atomizer, atomic motions occur inevery direction.

• The magnitude of the Doppler shift increaseswith the velocity at which the emitting orabsorbing species approaches or recedes thedetector.

• For relatively low velocities, the relationshipbetween the Doppler shift ( and the velocity(v) of an approaching or receding atom isgiven by:

0 = v / cWhere 0 is the wavelength of an un-shiftedline of a sample of an element at rest relativeto the transducer, and c is the speed of light.

• Emitting atom moving: (a) towards a photondetector, the detector sees wave crests more oftenand detect radiation of higher frequency; (b) awayfrom the detector, the detector sees wave crestsless frequently and detects radiation at lowerfrequency.

• The result is an statistical distribution of frequenciesand thus a broadening of spectral lines.

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Pressure Effects Due to Collisions

• Pressure or collisional broadening iscaused by collisions of the emitting orabsorbing species with other atoms orions in the heated medium.

• These collisions produce small changesin energy levels and hence a range ofabsorbed or emitted wavelengths.

• These collisions produce broadening thatis two to three orders of magnitude graterthan the natural line widths.

• Example: Hollow-cathode lamps (HCL):• Pressure in these lamps is kept really low

to minimize collisional broadening.• Glass tube is filled with neon or argon at

a pressure of 1 to 5 torr.

’ ’

Ene

rgy

(eV

)

Atom 1 Atom 2

E1

E2

E1

E2

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The Effect of Temperature on Atomic Spectra

• Temperature in the atomizer has a profound effect on theratio between the number of excited an unexcited atomicparticles.

• The magnitude of this effect is calculated with theBoltzmann distribution equation:

Nj / N0 = (gj / g0) x [exp(-Ej/kT)]where:- Nj and N0 are the number of atoms in the excited stateand ground state, respectively- k is the Boltzmann’s constant- T is absolute temperature (K)- Ej is the energy difference between the excited and theground state.- gi and g0 are statistical factors called statistical weightsdetermined by the number of states having equal energyat each quantum level.

• Example shows that a temperature fluctuation of only 10Kresults in a 4% increase in the number of excited sodiumatoms.

• A corresponding increase in emitted power by the twolines would result.

• An analytical method based on the measurement ofemission requires close control of atomizationtemperature.

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Band and Continuum Spectra Associated with Atomic Spectra

• When atomic line spectra are generated, bothband and continuum radiation are usuallyproduced as well.

• Molecular bands often appear as a result ofmolecular species in the atomizer. Molecularspecies can be associated or not to the elementof interest. For instance, the molecular bandsshown in the figure can be used to determine Ca.

• Continuum radiation appears as a result ofthermal radiation from hot particulate matter inthe atomization medium.

• Molecular bands and continuumradiation are a potentialsource of interferencethat must be minimizedby proper choice ofwavelength, by backgroundcorrection, or by change inatomization conditions.

Molecular flame emission and flame absorption spectra for CaOH

Background emission spectra from an ICP. The upper recording was taken favoringcontinuum and band emission while the lower recording was taken under conditionsminimizing continuum and band emission.

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Sample Introduction Methods

• Achilles’ heel of Atomic Spectroscopybecause in many cases limits the accuracy,the precision and the limits of detection ofanalytical method.

• Primary purpose is to transfer a reproducibleand representative portion of a sample intoone of the atomizers presented in Table 8-1.

• Table 8-2 lists the common sampleintroduction methods for AtomicSpectroscopy and the type of samples towhich each method is applicable.

• Atomizers “fit” into two classes: continuousand discrete atomizers.

• Continuous atomizers: flames and plasmas.Samples are introduced in a steady manner.

• Discrete Atomizers: electro-thermalatomizers. Sample introduction isdiscontinuous and made with a syringe or anauto-sampler.

Discontinuous

Con

tinuo

us

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Nebulizers

• Direct nebulization is the most common method ofsample introduction with continuous atomizers. Thesolution is converted into a spray by the nebulizer.

• Types of nebulizers:(a) Concentric tube: most common nebulizer. It

consists of a concentric-tube in which the liquidsample is drawn through a capillary tube by a high-pressure stream of gas flowing around the tip of thetube.

(b) Cross-flow: the high pressure gas flows across acapillary tip at right angles. It provides independentcontrol of gas and sample flows.

(c) Fritted disk: the sample solution is pumped onto afritted surface through which a carrier gas flows. Itprovides a much finer aerosol than a and b.

(d) Babington: it consists of a hollow sphere in which ahigh pressure gas is pumped through a small orificein the sphere’s surface of the sphere. It is lesssubject to clogging than a-c. It is useful for samplesthat have a high salt content or for slurries with asignificant particulate content.

ContinuousAtomizer

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Electro-thermal VaporizersDiscreteAtomizer

• Sample introduction in discrete atomizers istypically made manually with the aid of asyringe.

• The steps that convert the liquid solution into avapor of free atoms are the same as those incontinuous atomizers.

• The most common type of discrete atomizer isthe electro-thermal atomizer.

• An electro-thermal atomizer is a small furnacetube heated by passing a current through itfrom a programmable power supply.

• The furnace is heated in stages. The dry andash step removes water and organic or volatileinorganic matter, respectively.

• The atomization step produces a pulse ofatomic vapor that is probed by the radiationbeam from the hollow-cathode lamp (HCL).

Free-Atom Formation in

Atomizers

• Desolvation and Volatilization : desolvation leaves a dry aerosol of molten or solid particles. The solidor molten particle remaining after desolvation is volatilized (vaporized) to obtain free atoms. The efficiencyof desolvation and volatilization depends on a number of factors: atomizer temperature, composition ofanalytical sample (nature and concentration of analyte, solvent and concomitants) and size distribution. Inthe case of nebulizers, it also depends on the nebulizer design, aerosol trajectories and resident times ofthe particles.

• Dissociation and Ionization: in the vapor phase, the analyte can exist as free atoms, molecules or ions.In localized regions of the atomizer, molecules, free atoms and ions co-exist in equilibrium.

• Dissociation of molecular species: molecular formation reduces the concentration of free atoms andthus degrades the detection limits. The dissociation constant for a molecular species (MX) into itscomponents (MX <=> M + X) can be written as: Kd = nM.nX / nMX, where n is the number density (numberof species per cm3). For a diatomic molecule:

logKd = 20.274 + 3/2log MMMX/MMX + log ZMZX/ZMX + 3/2(logT) – 5040Ed/Twhere Mi is the molecular or atomic weight of species i, Zi is the partition function of species i, Ed is thedissociation energy in eV, and T is the temperature in K. Note: The final term in this equation describesmost of the temperature dependence: small values of Ed and high temperatures lead to large values of Kdand thus high degrees of dissociation.

• Ionization: it can also be consider an equilibrium process: M <=> M+ + e-. The ionization constant can bewritten as: Ki = n M+ .ne / nM, where ne is the number density of free electrons. The ionization constant canbe obtained from:

logKi = 15.684 + logZM+ /ZM + 3/2(logT) – 5040Eion/Twhere Eion is the ionization energy in eV. Note: The final term in this equation describes most of thetemperature dependence: small values of Eion and high temperatures favor the formation of ions.

ContinuousAtomizer

DiscreteAtomizer

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108

Atomic Absorption (AAS) and Atomic Fluorescence (AFS) Spectrometry (Chapter 9)

• The two most common methods of sampleatomization encountered in AAS and AFS are flameand electro-thermal atomization.

• Flame atomization: A solution of the sample isnebulized by a flow of gaseous oxidant, mixed with agaseous fuel and carried into the flame whereatomization occurs.

Oxidant (g)

Fuel (g)

Carrier (g)

Note:• If the gas flow rate does not exceed the burningvelocity, the flame propagates back into theburner, giving flashback.• The flame is stable where the flow velocity andthe burning velocity are equal.• Higher flow rates than the maximum burningvelocity cause the flame to blow off.

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Flame Structure

• Primary combustion zone: Thermal equilibrium is not achieved inthis zone and thus it is rarely used for flame spectroscopy.

• Inter-zonal area: Free atoms are prevalent in this area. It is the mostwidely used part of the flame for spectroscopy.

• Secondary combustion zone: products of the inner core areconverted to stable molecular oxides that are then dispersed to thesurroundings.

• Maximum temperature: it is located in the flame about 2.5cm abovethe primary combustion zone.

• Note: It is important to focus the same part of the flame on theoptical beam for all calibrations and analytical measurements.

• Optimization: of optical beam position within the flame prior toanalysis provides the best signal – to – noise ratio (S/N). It dependson element and it is critical for limits of detection.

Increased number of Mg atoms produced

by the longer exposure to the heat

of the flame.

Secondary combustion zone: oxidation of Mg

occurs. Oxide particles do not absorb at the

observation wavelength.

Ag is not easily oxidized so a continuous increase

in absorbance is observed.

Cr forms very stable oxides that do not absorb

at this observation wavelength.

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AASpectrometer

Commercial Flame Atomizers

• Typical commercial laminar-flow burner.• The aerosol formed by the flow of oxidant is

mixed with fuel and passes a series ofbaffles that remove all but the finest solutiondroplets.

• The baffles cause most of the sample tocollect in the bottom of the mixing where itdrains to waste container.

• The aerosol, oxidant, and fuel are thenburned in a slotted burner to provide a 5- to10-cm high flame.

• The quiet flame and relatively long-pathlength minimizes noise and maximizesabsorption. These features result inreproducibility and sensitivity improvementsfor AAS.

Excitation source

Flame

Monochromator

Detector

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Commercial Electro-thermal AtomizersCylindrical graphite tube

where atomization occurs. Dimensions:

about 5cm long and 1cm internal diameter.

This tube is interchangeable.

Cylindrical graphite electrical contacts. These

contacts are held in a water-cooled metal

housing.

L’vov platform. Made of graphite, sample is evaporated

and ashed on this platform. Temperature on the platform does not change as fast as it changes in the walls of the

furnace. Atomization occurs in an environment where

temperature does not change so fast, which improves

reproducibility of measurements.

Facilitates furnace cleaning, which reduces memory effects.

Longitudinal (b) and transversal (c) furnace heating. Transversal mode is preferred because it provides a uniform

temperature profile along the entire length of the tube and optical path.

Output signal

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Flame Atomizers versus Electro-thermal Atomizers

• Advantages of Flame Atomizers• Better reproducibility of measurements

RSD:Flame ≈ 1%Electro-thermal ≈ 5% - 10%

• Much faster analysis times than electro-thermal atomizers

• Wider linear dynamic ranges, up to 2 ordersof magnitude wider than electro-thermalatomizers.

• Advantages of Electro-thermal Atomizers• Smaller sample volumes (0.5L to 10L of

sample) than flame atomizers.• Better absolute limits of detection (ALOD ≈

10-11 to 10-13 g of analyte) than flameatomizersNote: ALOD = [Sample Volume] x LOD

Electro-thermal atomization is the method of choice when flame atomization provides inadequate limits of detection or sample availability is limited.

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Specialized Atomization Techniques

• Glow-Discharge Atomization• Cold-Vapor Atomization• Hydride Atomization

• Hydride Atomization• It provides a method for introducing samples

containing arsenic (As), antimony (Sb), tin (Sn),bismuth (Bi) and lead (Pb) into the atomizer as agas.

• This procedure improves limits of detection 10 to100x.

• Their determination at low levels is very importantbecause of their high toxicity.

• Volatile hydrides are generated by adding an acidifiedaqueous solution of the sample to a small volume of a1% aqueous solution of sodium borohydride containedin a glass vessel. A typical reaction is:3BH4

-(aq) + 3H+(aq) + 4H3AsO4(aq) →3H3BO3(aq) + 4AsH3(g) + 3H2O (l)

• The volatile hydride is swept into the atomizationchamber by an inert gas.

• The chamber is usually a silica tube heated to severalhundred degrees in a furnace or in a flame whereatomization takes place.

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Atomic Absorption Instrumentation

• Main components:(1) Radiation source(2) Sample holder = atomizer(3) Wavelength selector(4) Detector(5) Signal processor and readout

• Radiation source:• Why not using a broadband source with a

monochromator for excitation?• The emission profile of the source should have a

narrower effective bandwidth than the absorptionline of the element of interest.

• HCL and electrodeless discharge lamps (EDL)satisfy this condition.

• Disadvantage over broadbandsource/monochromator: One source per element.

HCL EDL

EDL provide intensities one to two orders of magnitudebetter than HCL. EDL are only available for ≈ fifteenelements. Particularly useful for Se, As, Cd and Sbbecause it provides better LOD.

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Source Modulation

• The output of the source is modulated so itsintensity fluctuates at a constant frequency.

• The detector receives two types of signal, analternating signal from the source and a continuoussignal from the flame.

• A high-pass RC filter is then used to remove thecontinuous signal and pass the alternating signal foramplification.

• Source modulation can be done with a chopper orrotating disk or a power supply.

Emission from the sample + emission from the flameMonochromator is able to eliminate flame interference based on wavelength separation. However, when the wavelength of

interference is the same as the analyte wavelength the monochromator is unable to eliminate interference.

H2 - O2

C2H2-N2O

C2H2 – O2

High-pass filter

Choppers

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Spectrophotometers for AAS

• Single beam and double beam instruments.• Double beam instruments provide the advantage

of correcting for source fluctuations. In thisarrangement, however, the reference beam doesnot pass through the flame and, therefore, it doesnot correct for loss of radiant power due toabsorption or scattering by the flame itself.

• Loss of radiant power due to absorption orscattering in the flame could have differentsources:(a) fuel and oxidant mixture alone(b) concomitants in sample matrix(c) all of the above

• When the source of loss of radiant power is onlydue to the fuel and the oxidant of the flame thesolution is simple: make blank measurementsand correct analytical data. This type ofcorrection should be done with both types ofspectrophotometers, i.e. single and double beam.

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Loss of radiant power due to absorption or scattering from concomitants in the sample matrix

• Example of spectral matrix interference dueto absorption: presence of CaOH in theanalysis of Barium.Solution: raise the temperature of the flame.Higher temperatures will decompose CaOHand remove its potential interference.

• Example of spectral interference due toscattering by products of atomization: mostoften encountered when particles withdiameters greater than the absorptionwavelength are formed in the flame:

a) Concentrated solutions containing Ti, Zr, andW. These elements form refractory oxides.

b) Organic species or when organic solvents areused to dissolve the sample. Incompletecombustion of the organic matter leavescarbonaceous particles.

• Methods for correcting spectral interference:(1) The two-line correction method(2) The continuum source correction method(3) Zeeman background correction method

HCL

> 0

Scattering

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Zeeman Effect

• It takes place when an atomic vapor isexposed to a strong magnetic field(≈10KG).

• It consists of a splitting of electronicenergy levels, which leads to formationof several absorption lines for eachelectronic transition.

• The simplest splitting pattern isobserved with singlet transitions, whichleads to a central ) line and twoequally spaced () satellite lines (≈0.01nm).

• The central line corresponds to theoriginal wavelength. It has anabsorbance that is twice theabsorbance of the satellite lines. Both lines have the same intensity.

No Yes

Ene

rgy

NoA

YesA

119

Background Correction Based on the Zeeman Effect

• The behavior of and lines is different with respect to plane polarized radiation: lines absorb plane polarized radiation parallel to the external magnetic field (II). lines absorb plane polarized radiation perpendicular (90°) to the external magnetic field.

120

Ionization Equilibria• Ionization of atoms and molecules in atomizers

can be represented by the equilibrium:M <=> M+ + e-

• The equilibrium constant for this reaction is:K = [M+] [e-] / [M]

• The ionization of a metal will be stronglyinfluenced by the presence of other ionizablemetals in the flame:

B <=> B+ + e-

• The ionization of M will be decreased by themass-action effect of the electrons formed fromB.

• B can then act as an ionization suppressor.• Ionization suppressors are often added to the

flame to improve sensitivity of analysis.• Ionization suppressors are commonly used with

higher temperature flames such as N2O-acetylene.

• The concentration of ionization suppressorneeds to be controlled to avoid primary innerfilter effects, i.e. absorption of excitationradiation.

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Atomic Fluorescence Spectroscopy (AFS)

• There are five basic types of fluorescence: resonance fluorescence, direct-line fluorescence,stepwise-line fluorescence, sensitized fluorescence and multi-photon fluorescence. The figureshows an energetic diagram level for resonance fluorescence.

• In all cases, the basic instrumentation is the same. EDL are the best excitation sources for AFS.• The advantage of AFS over AAS is that it provides better limits of detection for several elements.

Ene

rgy

122

Atomic Emission Spectrometry (Chapter 10)

• The figure shows the typical configuration of a flame orplasma emission spectrometer.

• There are three primary types of high temperatureplasmas:ICP = inductively coupled plasmaDCP = direct current plasmaMIP = microwave induced plasma

• ICP and DCP are commercially available.• Both types of plasmas sustain temperatures as high as

10,000K.

ICP:• It consists of three concentric quartz tubes through which streams of argonflows. The diameter of the largest tube is 2.5cm.• Surrounding the top of the largest tube is a water-cooled induction coil thatis powered by a radio-frequency generator, which radiates 0.5 to 20KW ofpower at 27.12MHz or 40.68MHz. This coil produces a fluctuating magneticfield (H).• Ionization of the flowing argon is initiated by a spark from a Tesla coil.The interaction of the resulting ions, and their associated electrons, with Hmakes the charges to flow in closed annular paths.• The resistance of ions and electrons towards the flow of charges causesohmic heating of the plasma.• The tangential flow of argon cools the internal walls of the ICP.• Spectral observations are generally maed at a height of 15 to 20mm abovethe induction coil, where the temperature is 6,000-6,500K and the region is“optically transparent” (low background).

123

DCP• It consists of three electrodes configured in an inverted Y. A

graphite anode is located at each arm of the Y and a tungstencathode at the inverted base.

• Argon flows from the two anode blocks toward the cathode.• The plasma is formed by bringing the cathode into momentary

contact with the anodes.• Ionization of the argon occurs an a current develops that

generates additional ions to sustain the current indefinitely.• The temperature at the arc core is between 5,000K and 8,000K.

ICP versus DCP:• DCP present lower background than ICP.• ICP is more sensitive than DCP; LOD in ICP are approximately one order of magnitude better.• DCP and ICP have similar reproducibility of measurements.• DCP requires less argon usage.• ICP is easier to align because the optical window of a DCP is relatively small.• Graphite electrodes must be replaced every few hours, whereas the ICP requires little maintenance.

124

Instrumentation

• The main two types of instruments for AESfit into two general categories: sequential ormulti-channel spectrometers.

• Sequential instruments are designed to readone line per element at the time.

• Multi-channel instruments are designed tomeasure simultaneously the intensities ofemission lines for a large number ofelements (50 or 60 elements).

• Both instruments require wavelengthselectors with high spectral resolution.

Sequential instrumentation:• It uses a slew-scanning monochromator.• Hg lamp is used for calibration.• Two PMT, one is used for the UV and

the other for the VIS.• A flipping mirror selects the exit slit and

the PMT.

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Scanning Echelle instrumentation• It can be used either as a single channel or as a

“simultaneous multi-channel spectrometer”.• Scanning is accomplished by moving the PMT in both x and

y directions to scan an aperture plate located on the focalplane of the monochromator.

• The plate contains as many as 300 slits. The time it takes tomove the PMT from one slit to another is approximately 1s.

• This arrangement can be converted to a multi-channelsystem by placing small PMT behind several exit slits.

Polychromators• The entrance slit, the exit slits, and the grating

surface are located along the circumference of aRowland circle.

• The curvature of a Rowland circle corresponds tothe focal plane of a monochromator.

• Each exit slit is factory configured to transmit linesfor selected elements.

• The entrance slit can be moved tangentially to theRowland circle to provide scanning.

126

Charge-Coupled Device Instrumentation• Typically incorporate two CCD, one for the

UV (165nm – 375nm) and one for the VIS(375nm – 782nm).

• The Schmidt cross-disperser separates theUV from the VIS radiation and the orders ateach emission wavelength.

Elements Determined

Tl and Nb curves are not linear probably because of incorrect background subtraction. Self-absorption is another cause of non-linearity. It occurs at

high concentrations where the non-excited atoms absorb emitted radiation.

127