Atomic Absorption and Atomic Fluorescence Spectroscopy

8/12/2019 Atomic Absorption and Atomic Fluorescence Spectroscopy

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Chapter 9- Atomic Absorption and Atomic Fluorescence Spectrometry

A: Sample Atomization Techniques

Atomic Spectroscopy

Atomic spectroscopy is used for the qualitative and quantitative determination of perhaps70 elements. Sensitivities of atomic methods lie typically in the parts-per-million to parts-

per-billion range. Additional virtues of these methods are speed convenience unusually

high selectivity and moderate costs.Spectroscopic determination of atomic species can only be performed on a gaseous

medium in !hich the individual atoms are !ell separated from one another.

"onsequently the first step in all atomic spectroscopic procedures is atomization a

process in !hich the sample is volatilized and decomposed in such a !ay as to producean atomic gas. The efficiency and reproducibility of the atomization step in a large

measure determine the method#s sensitivity precision and accuracy so atomization is by

far the most critical step in atomic spectroscopy.

$n AAS and A%S there are t!o factors involved. These are: The intensity of light source

The probability of transition.

$t should be noted that in AAS and A%S the ground state is the starting point but inA&S some higher levels partially populated to start !ith. 'oth AAS and A%S obey the

selection rules but A&S does not because it involves the use of thermal e(citation. AAS

!as founded by Allen )alsch *Australia+ in the ,/s !hile 'unsen founded A&S in the,0/s. $n terms of simplicity AAS is the simplest spectroscopic instrument to use

follo!ed by A%S.

%lame Atomization

$n a flame atomizer a solution of the sample is nebulized by the flo! of gaseous o(idantmi(ed !ith a gaseous fuel and carried into a flame !here atomization occurs.


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The first step is de-solvation in !hich the solvent is evaporated to produce a finely

divided solid molecularaerosol. As a result de-solvation of these molecules then leads to

an atomic gas. Some of the atoms ionize and give cations and electrons. 1ndoubtedlyother molecules and atoms are also produced in the flame as a result of interactions of the

fuel !ith the o(idant and !ith various species in the sample. As sho!n in the diagram

above !e see that the heat of the flame also e(cites a fraction of the molecules atomsand ions thus giving atomic ionic and molecular emission spectra.

Types of Flames

The table belo! lists the common fuels and o(idants employed in flame spectroscopy andthe appro(imate range of temperature realized !ith each of these mi(tures. These

temperatures in the range of ,700-200o" !ere obtained !ith the various fuels !hen air

serves as the o(idant.

Max. flame speed (cm/s Max. temp. (oC

Air-"oal gas ,320

Air-propane 3 ,

Air-hydrogen 40 00

Air-05 o(ygen-acetylene ,60 400


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(ygen-nitrogen-acetylene 620 3,

(ygen-acetylene ,,40 4060

(ygen-cyanogen ,20 2620

8itrous o(ide-acetylene ,30

8itric o(ide-acetylene 0 40

8itrogen dio(yde-hydrogen ,0 660

8itrous o(ide-hydrogen 40 60

The burning velocities listed in the middle column are of considerable importance because flames arestable in certain ranges of gas flo! rates only. $f the gas flo! rate does not e(ceed the burning veloc

the flame propagates itself bac9 into the burner giving flashbac9. As the flo! rate increases the flam

rises until it reaches a point above the burner !here the flo! velocity and the burning velocity are eq

This region is !here the flame is stable.

Flame Structure$n the figure !e see all the importantregions of a flame. $t includes the primary

combustion zone the interzonal region and

the secondary combustion zone. Theappearance and relative size of these regions

vary considerably !ith the fuel-to-o(idant

ratio as !ell as !ith the type of fuel ando(idant. The primary combustion zone in a

hydrocarbon flame is recognizable by its

blue luminescence arising from the band

spectra of " and " and other radicals.Thermal equilibrium is ordinarily not

reached in this region and it is seldom used

for flame spectroscopy. %urthermore theinterzonal area -!hich is narro! in

stoichiometric hydrocarbon flames may

reach several centimeters in height in fuelrich acetylene;o(ygen acetylene;nitrous

o(ide sources. The zone is rich in free atoms and it is the most !idely used part of the flame.

Temperature !rofiles

The ma(imum temperature is located in the flame about ,cm above the primary combustion zone. $timportant- particularly for emission methods- to focus the same part of the flame on the entrance slit

all calibrations and analytical measurements.

The "ffects of Flame Temperature

'oth emission and absorption spectra are affected in a comple( !ay by variations inflame temperature. igher temperatures tend to increase the total atom population of the


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flame and sensitivity. )ith certain elements such as the al9ali metals ho!ever this ii-

increase in atom population is more than offset by the loss of atoms by ionization

Also flame temperature determines the relative number of e(cited and une(cited atomsin a flame. $n an air;acetylene flame for e(ample the ratio of e(cited to une(cited

magnesium atoms can be computed to be about ,0&-3 !hereas in o(ygen acetylene

flame !hich is about 700" hotter this ratio is about ,0&-6. "ontrol of temperature isthus of prime importance in flame emission methods. %or e(ample !ith a 00 #" flame

a temperature increase of $ #" causes the number of sodium atoms in the e(cited 4p state

to increase by about 45. $n contrast the corresponding decreases ii-a the much largernumber of ground state atoms is only about 0.005. So emission methods based as they

are on the population of e(cited atoms. ustment of the position of the flame !ith resp

to the entrance slit is thus critical.

Flame Atomi#ers

A flame atomizer of a pneumatic nebulizer !hich converts the sample solution into amist or aerosol that is then fed into a burner. A common type of nebulizer is the

concentric tube type. $n !hich the liquid sample is suc9ed through a capillary tube by -a

high-pressure stream of a gas flo!ing around the tip of the tube. This process of liquidtransport is called aspiration. The high velocity gas brea9s the liquid into the fine

droplets of various sizes !hich are then carried into the flame. "ross-flo! nebulizers arealso employed in !hich the high-pressure gas flo!s across a capillary tip at right angles.

ften in this type of nebulizer the liquid is pumped through the capillary. $n mostatomizers the high-pressure gas is the o(idant !ith the aerosol containing o(idant being

mi(ed subsequently !ith the fuel.

n this figure !e see a typical commercial laminar flo! burner that employs a concentrictube nebulizer.


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The aerosol is mi(ed !ith fuel and flo!s past a series of baffles that remove all but the

finest droplets. As a result of the baffles the a-ma>ority of the sample collects in the

bottom of the mi(ing chamber !here it is drained to a !aste container. The aerosolo(idant alit fuel are then directed into a slotted burner !hich provides a flame that is

usually or ,0 cm in length.

?aminar flo! burners provide a relatively quiet flame and a long path length. These

properties tend to enhance sensitivity and reproducibility. The mi(ing chamber in thistype of burner contains a potentially e(plosive mi(ture !hich can be ignited by

flashbac9 if the flo! rates are not sufficient.

Fuel $ %xidant &e'ulators

An important variable that requires close control in flame spectroscopy is the flo! rate of

both o(idant and fuel. %uel and o(idant are ordinarily combined in appro(imatelystoichiometric amounts. o!ever a flame that contains an e(cess of fuel may be more

desirable. The most !idely used device for measuring flo! rates is the rotameter !hich

consists of tapered graduated transparent tube that is mounted vertically !ith the smallerend do!n.

!erformance Characteristics of Flame Atomi#ers

%lame atomization appears to be superior to all other methods that have been thus far

developed for liquid sample introduction for atomic absorption and fluorescencespectrometry. o!ever there are t!o reasons that lo!er the sampling efficiency of the

flame. %irst a large portion of the sample flo!s do!n the drain. Second the residencetime of individual atoms in the optical path in the flame is brief.


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Sources of Atomic Spectra

)ithout chemical bonding there can be no vibrational or rotational energy states and

transitions. Therefore atomic emission absorption and fluorescence spectra are made upof a limited number of narro! pea9s or lines.

&lectrothermal Atomization&lectrothermal atomizers !hich first appeared in ,70 generally provide enhanced

sensitivity because the entire sample is atomized in a short period and the average

residence time of the atoms in the optical path is a second or more. They generally havenot been applied for direct production of emission spectra. They are ho!ever beginning

to be used for vaporizing samples for sample introduction in inductively coupled plasma

emission spectroscopy.

"lectrothermal Atomi#ers

@raphite furnace atomic absorption spectrometry *@%AAS+ is also 9no!n by various

other acronyms including electrothermal atomic absorption spectrometry *&TAAS+.

'riefly the technique is based on the fact that free atoms !ill absorb light at frequenciesor !avelengths characteristic of the element of interest *hence the name atomic

absorption spectrometry+. )ithin certain limits the amount of light absorbed can belinearly correlated to the concentration of analyte present. %ree atoms of most elements

can be produced from samples by the application of high temperatures. $n @%AAS

samples are deposited in a small graphite tube !hich can then be heated to vaporize andatomize the analyte.

An ideal graphite furnace should fulfill the follo!ing requirements:

A constant temperature in time and space during the interval in !hich free atoms

are produced uantitative atom formation regardless of the sample composition

Separate control of the volatilization and atomization processes igh sensitivity and good detection limits

A minimum of spectral interferences

%utput Si'nal

At a !avelength at !hich absorbance or fluorescence occurs the transducer output risesto a ma(imum after a fe! seconds of ignition follo!ed by a rapid decay bac9 to zero as

the atomization products escape into the surroundings. The change is rapid enough to

require a high-speed data acquisition system. uantitative analysis is usually based on

pea9 height although pea9 area also has been used.


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!erformance Characteristics of "lectrothermal Atomi#ers

They offer the advantage of unusually high sensitivity for small volumes of sample. The

relative precision of nonflame methods is generally in the range of 5 to ,05 compared!ith the ,5 or better that can be e(pected for flame or plasma atomization. %urthermore

furnace methods are slo!- typically requiring several minutes per element. A final

disadvantage is that the analytical range is lo! being usually less than t!o orders ofmagnitude.

Analysis of Solids ith "lectrothermal Atomi#ers

A !ay of performing such measurements is to !eigh the finely ground sample into a

graphite boat and insert the boat manually in the furnace. Another !ay is to prepare a

BslurryB of the po!dered sample by ultrasonic agitation in an aqueous medium. The

slurry is then pipetted into the furnace for atomization.

Specialized Atomization Techniques

ther atomization methods for sample introduction can be used for atomic absorption

analyses. Three are described in this section.

)lo *ischar'e Atomi#ation

A glo! discharge device produces an atomized vapor that can be s!ept into a cell for

absorption measurements. $n order for this technique to be applicable the sample must

be an electrical conductor such as finely ground graphite or copper. Solution sampleshave also been analyzed by deposition on a graphite aluminum or copper cathode.

Cetection limits !ith this type of device are reported to be in the lo! parts per million for

solid samples.

+ydride Atomi#ation

Atomization of the hydrides requires only that they be heated in a quartz tube.


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Cold-,apor Atomi#ation

This is applicable only to the determination of mercury because it is the only metallic

element that has an appreciable vapor pressure at ambient temperature. Thisdetermination is of vital importance because of the to(icity of various organic mercury

compounds and their !idespread distribution in the environment. Cetection limits in the

parts-per-billion range are realized. Several manufacturers offer automatic instrumentsfor performing this analysis for a variety of types of samples.

': Atomic Absorption $nstrumentation

Flame Atomic Absorption Spectroscopy

%lame atomic absorption spectroscopy is currently the most !idely used of all the atomic

methods. $t is so because of its simplicity effectiveness and relatively lo! cost. Thegeneral use of this technique began in the early ,0s and gre! e(plosively after that.

The reason that atomic absorption methods !ere not !idely used until that time !as

directly related to problems created by the very narro! !idths of atomic absorption lines.

The natural !idth of an atomic absorption or an atomic emission line is on the order of ,0- nm. T!o effects ho!ever cause line !idths to be broadened by a factor of ,00 or

more.


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!ressure roadenin'

Dressure broadening arises from collisions among atoms that cause slight variations in

their ground state energies and slight energy differences bet!een ground and e(citedstates. Dressure broadening becomes greater !ith increases in temperature. So as result

broader absorption and emission pea9s are al!ays encountered at elevated temperatures.

+ollo Cathode amps

The most common source for atomic absorption measurements is the hollo! cathode

lamp. This type of lamp consists of a tungsten anode and a cylindrical cathode sealed in

a glass tube that is filled !ith neon or argon at a pressure of , to torr. The cathode isconstructed of the metal !hose spectrum is desired or serves to support a layer of that

metal. The application of a potential of about 400 E across the electrodes causes

ionization of the argon and generation of a current of to ,0 mA as the argon cations andelectrons migrate to the t!o electrodes. $f the potential is sufficiently large the argon

cations stri9e the cathode !ith sufficient energy to dislodge some of the metal atoms and

thereby produce an atomic cloud- and this process is called sputtering. The sputteredmetal atoms in a lamp eventually diffuse bac9 to the cathode surface of to the !alls of the

lamp and are deposited. The cylindrical configuration of the cathode tends to concentrate

the radiation in a limited region of the metal tube= this design also enhances theprobability that redeposition !ill occur at the cathode rather than on the glass !all.

"lectrodeless *ischar'e amps

&lectrodeless discharge lamps are useful sources of atomic line spectra and provideradiant intensities that are usually one to t!o orders of magnitude greater than their

hollo! cathode counterparts. A typical lamp is constructed from a sealed quartz tube

containing an inert gas such as argon at a pressure of a fe! torr and a small quantity ofthe analyte metal. The lamp contains no electrode but instead is energized by an intense

field of radio frequency or micro!ave radiation. The argon ionizes in this field and the

ions are accelerated by the high frequency component of the field until they gainsufficient energy to e(cite the atoms of the metal !hose spectrum is sought.

Source Modulation

$n the typical atomic absorption instrument it is necessary to eliminate interferencecaused by emission of radiation by the flame. %urthermore most of it is removed by the

monochromator. 8evertheless emitted radiation corresponding in !avelength to the

monochromator setting is inevitably present in the flame due to e(citation and emissionby analyte atoms. $n order to eliminate the effects of flame emission it is necessary to


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modulate the output of the source so that its intensity fluctuates at a constant frequency.

The detector then receives t!o types of signal an alternative one from the source and a

continuous one from the flame. Then these signals converted to the corresponding typesof electrical response.

A simple and entirely satisfactory !ay of modulating the emission from the source is to

interpose a circular metal dis9 or c.2opper in the beam bet!een the source and the flame.Alternate quadrants of this dis9 are removed to permit passage of light.


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Sin'le-eam nstruments

A typical single-beam instrument consists of several hollo! cathode sources a chopperor a pulsed po!er supply an atomizer and a simple grating spectrophotometer !ith a

photomultiplier transducer.

*ouble-eam nstruments

The beam of the hollo! cathode is split by a mirrored chopper one half passing through

the flame and the other half around it. The t!o beams are then recombined by a half-silvered mirror and passed into a "zerney-Turner grating monochromator= a

photomultiplier tube serves as the transducer. The output from the latter is fed to a loc9-

in amplifier that is synchronized !ith the chopper drive. The ratio bet!een the referenceand the sample signal is then amplified and fed to the readout !hich may be a digital

meter or a signal recorder.

": $nterferences in Atomic Absorption Spectroscopy

Spectral $nterferencesSpectral interferencesarise !hen the absorption or emission of an interfering specieseither overlaps or lies so close to the analyte absorption or emission that resolution by the

monochromator becomes impossible.

The To-ine Correction Method

$t employs a line from the source as a reference. The line should lie as close as possible

to the analyte line but must not be absorbed by the analyte. $f the conditions are met it isassumed that any decrease in po!er of the reference line from that observed during

calibration arises from absorption or scattering by the matri( products of the sample.

The Continuum-Source Correction MethodA deuterium lamp provides a source of continuous radiation throughout the ultraviolet

region. The absorbance of the deuterium radiation is then subtracted from that of the

analyte beam. The attenuation of the po!er of the continuum during passage through theatomized sample reflects only the broadband absorption or scattering by the sample

matri( components. A bac9ground correction is thus achieved.


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ac0'round Correction ased on the 1eeman "ffect

)hen an atomic vapor is e(posed to a strong magnetic field a splitting of electronic

energy levels of the atoms ta9es place that leads to formation of several absorption linesfor each electronic transition. This phenomenon !hich is called theZeeman effect is

general for all atomic spectra. Application of the Geeman effect to atomic absorption

instruments is based upon the differing response of the t!o types of absorption pea9s topolarized radiation. Geeman effect instruments provide a more accurate correction for

bac9ground than earlier methods. These instruments are particularly useful for

electrothermal atomizers and permit the direct determination of elements in samples suchas urine and blood. The decomposition of organic material in these samples leads to

large bac9ground corrections and consequent susceptibility to significant error.

ac0'round Correction ased on the Source Self-&e2ersal

The Smith-Hieftje background correction methodis based upon the self-reversal or self-

absorption behavior of radiation emitted from hollo! cathode lamps !hen they are

operated at high currents.

"hemical $nterferences

Chemical interferencesresult from various chemical processes occurring duringatomization that alter the absorption characteristics of the analyte. They are more

common than spectral ones. Their effects can frequently be minimized by a suitable

choice of operating conditions. 'oth theoretical and e(perimental evidence suggest thatmany of the processes occurring in the mantle of a flame are in appro(imate equilibrium.

As a consequence it becomes possible to regard the burned gases as a solvent medium to

!hich thermodynamic calculations can be applied.

The equilibria of principle interest include:

%ormation of "ompounds of ?o! Eolatility

Cissociation


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4umber

total recoverablemetals

400 ground !ater;surface !ater

dissolved metals 400 ground !ater;surface !ater

suspended metals 400 ground !ater;surface !ater

total metals 40,0aqueous samples !astes that contain suspended solids

and mobility-procedure e(tracts

total metals 40,aqueous samples !astes that contain suspended solids


total metals 400aqueous samples !astes that contain suspended solids


total metals 400 sediments sludges and soil samples

total metals 40, sludges sediment soil and oil

rganic Solvents

$t !as recognized that enhanced absorption pea9s could be obtained from solutionscontaining lo!-molecular-!eight alcohols esters or 9etones. The effect of organic

solvents is largely attributable to an increased nebulizer efficiency= the lo!er the surface

tension of such solutions results in smaller drop sizes and a consequent increase in theamount of sample that reaches the flame. A most important analytical application of

organic solvents to flame spectroscopy is the use of immiscible solvents such as methyl

isobutyl 9etone to e(tract chelates of metallic ions. The resulting e(tract is thennebulized directly into the flame. The sensitivity is increased not only by the

enhancement of absorption pea9s due to the solvent but also by the fact that for many

systems only small volumes of the organic liquid are required to remove metal ionsquantitatively from relatively large volumes of aqueous solution.

"alibration "urves

$n theory atomic absorption should follo! 'eer/s la! !ith absorbance being directlyproportional to concentration. o!ever departures from linearity are often encountered.

A calibration curve that covers the range of concentrations found in the sample should be

prepared. Any deviation of the standard from the original calibration curve can then beused to correct the analytical result.


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The idealized calibration or standard curve is stated by 'eer#s la! that the absorbance of

an absorbing analyte is proportional to its concentration.

Standard Addition Fethod$t is !idely used in atomic absorption spectroscopy in order to partially or !holly

counteract the chemical and spectral interferences introduced by the sample matri(.

Techni5ues of nternal Standards

imitations of nternal Standard (S

$S !ill be present in the atomization as the analyte is present and so the $S cannot be present i

the sample to start !ith. $t should be added to the sample *$f present in the original sample $

should have a constant amount.

$n arc and;or spar9 methods the $S should have the same comparable volatilization as the

analyte. $S H analyte should have similar e(citation energy

$SH analyte should have similar $.D.#s.


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Applications of Atomic Absorption Spectrometry

Cetection ?imits

Accuracy

nstruments for the Flame "mission Spectroscopy

$nstruments for flame emission are ire similar in design to flame absorption instruments

e(cept that in the former the flame no! acts as the radiation source a hollo!-cathode

lamp and chopper a(e therefore tin-necessary.

&: Atomic %luorescence Spectroscopy

$nstrumentation

Sources

A continuum source !ould be desirable for atomic fluorescence measurements.o!ever the output po!er of most continuum sources over a region as narro! as an

atomic absorption line is so lo! as to restrict the sensitivity of the method severely.

ollo! "athode ?amp- only observed the fluorescent signal during pulses


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&lectrodeless Cischarge ?amp- produced intensities that e(ceed those of hollo!

cathode lamps= ho!ever it is not available for many elements

?asers- ideal source !ith high intensities and narro! band!idths= ho!ever the

high cost has discouraged their routine application

*ispersi2e nstruments

They are made up of a modulated source an atomizer a monochromator or an

interference filter system a detector and a signal processor and readout.

4ondispersi2e nstrumentsThey ideally are made up of a source an atomizer and a detector.

Advantages:

Simplicity and lo!-cost instrumentation


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Atomic Absorption and Atomic Fluorescence Spectroscopy

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