Atomic Absorption Spectroscopy and Atomic Emission Spectroscopy

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Atomic Spectroscopy

Ashraf M. Mahmoud, Associa



,



Atomic SpectroscopyTo understand the relationship of these techniques to each other, It is

important to understand the atom itself and the atomic process

involved in each technique.

Excited

states

Groundstate

λ3

λ3

λ3

Light energy

Excited state atom

Ground state atom (stable or

normal orbital coni!uration)

Excited

states

Groundstate

Spectral resonance line

(The strongest line



Atomic Spectroscopy!ractically, the ratio of the excited to ground state atoms is extremely small.

Therefore, The a"sorption spectrum is usually only associated #ith transitions

from the ground state to higher energy states

E n e r g y

~100% population

Emission Absorption

~0% population

Ground state atoms

Excited state atoms

$olt%mann equation explain the relatioship "et#een the ground and excited state atoms

&'&) * (g'g)e+E-T

&' &o. of excited atoms &) &o. of ground state atoms E excitation energy

- $olt%mann constant T Temperature in /elvin



Atomic Spectroscopy

The process of excitation and decay to groundstate is involved in the two techniques of atomic

spectroscopy.

We measure the energy absorbed or emitted anduse it for quantification process



Atomic Emission Spectroscopy (AES)

(Flame Photometry)

!rinciple 0lame photometry is "ased upon those particlesthat are electronically excited in the medium.

0lame is the source of excitation energy. (lo# energy source.

1ses0lame photometry is used mainly for the determination of al/ali

metals and easily excited elements (&a, -, Li, 2a, etc. particularly

in "iological fluids and tissues



0lame !hotometry

Ground state atoms

&a)

Solution of metal

salt (&a2l

Solvent evaporation

Solid aerosol of

metal salt (&a2l

3olatili%ation or decomposition

Gaseous metal

salt (&a2l

4tomi%ation 0ree atoms

&a)

Excitation

Excited atoms

&a5

Emission and

return to G)

Events occur in 0ES



Flame Photometry

Inter%onal region

0uel+ oxidant mixture

!rimary

com"ustion %one

Secondary

com"ustion %one

Flame Structure

0lame 6etector

7onochromator

"nstrument

components



Functions o Flame

'. To convert the constituents of liquid sample into the vapor state.

8. To decompose the constituents into atoms or simple molecules

79 9 e+ (from flame 7 9 hν

:. To electronically excite a fraction of the resulting atomic or

molecular species

7 75

Flame Photometry

;xidant + 0uel 7ax. temp. (o2

Air- propane 1!"

Air- acetylene !#$$%xygen- acetylene 31$$

&itrous oxide-acetylene 3$$$

Air-hydrogen !$$$

%xygen-hydrogen !$$

Air ' argon -hydrogen 1"

The flame is composed of

a fuel gas and oxidant gas



Flame Photometry0actors affecting intensity of flame emission

'+ The concentration of the analyte in solution

8+ The rate at #hich excited atoms are formed in the flame.:+ The rate at #hich the sample is introduced into the flame.

<+ Temperature of the flame.

=+ 2omposition of the flame.

>+ The ratio of fuel to oxidant in the flame.?+ Solvent used to dissolve the sample.

The flame temperature is the most important factor. Increase in

flame temperature causes an increase in emission intensity. This is

controlled "y composition of the flame.

@igh temperature flames should not "e used for elements that

ioni%ed easily e.g. &a, -, Li or 2e. @o#ever, high temperature

flames are generally favored for transition elements and al/aline



Effect of the solvent used to dissolve the sampleA if the solvent is

#ater the process is slo# and if it is organic solvent the processis fast and emission intensity is increased.

It is therefore very important that cali"ration curves "e

prepared using the same solvent.

The stochiometric ratio of fuel to oxidant in the flame must "e

used, in #hich "oth fuel to oxidant are totally consumed.

Flame Photometry



Flame Photometry

#he nebuli$er%burner system

&e"uli%er produce an aerosol of the test solution

$urner in #hich the mixing "et#een fuel and oxidant

To convert the test sample into gaseous atoms

#ypes o burner system

1. Pre%mix or laminar lo& burner

4dvantages

6isadvantages

'. @omogenous flame

8. Suita"le for 44S and 4ES as

the path#ay could "e increased

(uel in

%xidant in

)urner

(lame

Aspiratorair

*ixing baffles

+rain

,ample

Suffers from explosion ha%ards



Flame Photometry2. #otal consumption burner

0uel

; x

i d a n t

S a m p l e

;xidant

0uel

: concentric tu"es, the sample, fuel and

oxidant only mix at the tip of "urner

1sed mainly for 0ES (short "ath

4dvantages'. Simple to manufacture

8. 4llo#s a total representative sample

to reach the flame

:. 0ree from explosion ha%ards

6isadvantages

'. 4spiration rate varies #ith different solvents

8. Suita"le only for 4ES



Flame Photometry

'on Flame Atomi$ers0or example @eated Gravite 0urnace

Sample evaporationB time and temp. controlled drying and ashing

4dvantages'. small samples are analysed

8. 'CCC+fold more sensitive than flame

:. ;ven is adapta"le to determination of solid samples

6isadvantages

'. Lo# accuracy 8. Lo# precision

8. 7ore ionic interferences due to very high temp.



Flame Photometry

Monochromators

etectors

4s in 13

0ilms or photomultipliers

Analytical techniue

'. 2hoice of the #avelength of max. sensitivity and min. spectral interferences8. Sample preparation

a. t is very important to obtain the sample in a form of solution where the spectral

and chemical interferences are absent

b. +eminerali/ed dist. Water and very pure reagents are to be used because of thehigh sensitivity of the technique

c. )ecause of the instability of the very dil. ,olution it is advisable to dilute the

soln 0ust before use.

d. ,everal elements can be determined in blood urine cerebrospinal fluid and other

biological fluids by direct aspiration of the sample after dilution with water.



Flame Photometry

2hemical interferences can often "e overcome "y simple

dilution #ith a suita"le reagent solution e.g. serum is diluted "yE6T4 solution for the determination of calcium in order to

prevent interference from phosphate.

:. Standard curves

A b s o r b

a n c e ,

A

Concentration, c

" n t e n

s i t y

6eviations from

linearity may occur



Flame Photometry

:. Dualitative analysis

(lame photometry are useful mostly for the detection of elements in

group I and II of the periodic ta"le. The presence of certain

elements can be detected by the use of a filter or monochromator.

4dvantages and disadvantages

The method is not as relia"le as other atomic emission spectroscopic

methods but it is fast and simple.

<. Duantitative analysis

To perform quantitative analysis the sample is introduced into theflame and the intensity of radiation is measured. The concentration

of the emitting substance is then calculated from a calibration curve

or using standard addition method.



Flame Photometry4pplication of flame photometry in pharmaceutical analysis

'. 7etals are maor constituents of several pharmaceuticals such as dialysissolutions, lithium car"onate ta"lets, antacids and multivitamin + mineral

ta"lets.

8. The elements &a, -, Li, 7g, 2a, 4l and Fn are among the most common

elements su"ected to pharmaceutical analysis using flame emission

technique.

:. Sodium and potassium levels in "iological fluids are difficult to analy%e "y

titrimetric or colorimetric techniques. Their analysis is very important for

control of infusion and dialysis solutions #hich must "e carefully monitored

to maintain proper electrolyte "alance.

4dvantages and disadvantages

'. 0lame emission is the simplest and least expensive technique.

8. The analysis may "e carried out #ithout prior separation as othercomponents such as dextrose, do not interfere.



Atomic Absorption Spectroscopy

6etector

Monochromator

"nstrument components

Source *ampslow-pressure inert gas

nert carrier gas

&e or Ar @ollo# 2athod

lamp

4tomic 4"sorption spectroscopy involves the study of the a"sorption

of radiant energy "y neutral (ground state atoms in the gaseous state.

P P 0

Sample




Electrodless 6ischarge Lamps, E6L

0or easily evapori%ed elements as @g or 4s

1sed for 44S and 4ES

Give much greater radiation intensities than hollo# cathod

There is no electrode, "ut instead , the inert carrier gas is

energi%ed "y an intense field of radiofrequency or micro#ave

radiation B plasma formation #hich cause excitation of the

metal inside




6egree of a"sorption

Total amount of light a"sor"ed * (e8mc8&f Hhere

e * electronic charge, m * mass of electron

c * speed of light, & * total &o. of atoms that can a"sor" light

f * 4"ility of each atom to a"sor" light

, e, m, and c are constants, therefore

Total amount of light a"sor"ed * constant x &f

Since f is also constant for the same su"stance

4 2



4tomic 4"sorption Spectroscopy

At i Ab ti S t




Spectral Interferences'. They arise #hen the a"sorption line of an interfering species

either overlaps or lies so close to the analyte a"sorption line that

resolution "y the monochromator "ecomes impossi"le. Ex. 7g in

presence of 2a.8. They occur from "and or continuous spectra #hich are due to

a"sorption of molecules or complex ions remaining in the flame

:. They arise from flame "ac/ground spectrum.

2orrection'. It may "e useful to shift to another spectral line

8. T#o line correction method (Instrumental correction

It employs a line from the source as a reference. The line should lie as close as

possi"le to the analyte line "ut must not "e a"sor"ed "y the analyte. If the

conditions are met, any decrease in the reference line from that o"serveddurin cali"ration arises from a"sor tion " the matrix of the sam le.

Interferences




2hemical Interferences

occurrs during atomi%ation that prevent the gaseous atomsproduction of the analyte. They are more common than spectral

ones.

Types of chemical interferences

'. 0ormation of sta"le compounds B incomplete dissociation of the sample in flame

8. 0ormation of refractory oxides B #hich fail to dissociate into the constituent atoms

Examples '. 6etn. of 2a in presence of sulphate or phosphate

8. 0ormation of sta"le refractory oxides of Ti;8, 38;= or 4l8;:

reaction #ith ;8 and ;@ species in the flame

;vercome'. Increase in the flame temp. B 0ormation of free gaseous atoms

e.g. 4l8;: is readily dissociated in acetylene+nitrous oxide flame




8. 1se of releasing agents 7+J 9 K B KJ 9 7 ex. 6etn of 2a

in presence of phosphate(2a + phosphate 9 Sr2l8 B Sr+phosphate 9 2a atoms or

(2a phosphate 9 E6T4 B 2a+E6T4 easily dissociated complex .

:. Solvent extraction of the sample or of the interferring elementsIoni%ation Interferences

Ioni%ation of atoms in the flame B decrease the a"sorption or emission

;vercome '. 1se of lo#est possi"le temp #hich is satisfactory forthe sample ex. 4cetylene air must not "e used for easily ionised

elements as &a, -, 2a, $a

8. 4ddition of an ionisation supressant ( soln of cation has a lo#er

ionisation potential than that of the sample, e.g. addition of -+solnto 2a or $a soln. 2a B 2a89 9 8e - B - 9 9 e




!hysical Interferences

'. 3ariation in gas flo# rate

8. 3ariation in sample viscosity

:. 2hange in flame temp.

;vercome '. "y continuous cali"ration

8. 1se of internal standard

4dvantages of 44S 3ery sensitive. 0ast.

6isadvantages of 44S @ollo# cathode lamp for each element.

Expensive element.



elationship )etween Atomic Absorption and (lame

2mission ,pectroscopy

4tomic 4"sorption 0lame Emission

'. 7easures the radiationa"sor"ed "y the unexcited

atoms

'. 7easures the radiationemitted "y the excited atoms

8. 6epends only on thenum"er of unexcited atoms

8. 6epends only on thenum"er of excited atoms

:. 4"sorption intensity is&;T affected "y thetemperature of the flame

:. Emission intensity is greatlyaffected "y the temperaturevariation of the flame



Atomic Emission Spectroscopy

+sin! 'on%Flame excitation sources'. There is no single excitation source can excite all

elements

8. The emitted radiation usually consists of sharp #ell

defined lines, #hich fall in 13 or visi"le region

:. Identification of the λ of these lines permits qualitative

analysis of these elements, #hereas measurements of

their intensities permits quantitative analysis

4dvantages

'. Excellent method for trace element analysis at ppm level

8. 1sed nearly for all elements in periodic ta"le




@igh energy excitation sources

!lasma excitation sources

Laser

4rc and spar/ emission spectrometry (Spectrography

7icro#ave and x+ray

:. 1sed for very small samples, even less than ' mg

<. There is no need for prior separation=. Kelatively rapid technique

6isadvantages '. Expensive

8. Lo# precision and accuracy

:. 6estroying the sample

<. 1sed mainly for metals



'. 4 plasma is a cloud of highly ioni%ed gas containing significant num"ers of

positive and negative ions, free electrons and neutral particles.

8. !lasma sources operate at high temperatures "et#een ?CCC and '=CCC -.

Thus, it produces a greater num"er of excited emitted atoms, especially in

the 13 region, than that produced "y flame.

:. 1sing this technique, excitation operates through a plasma produced

electrically in a carrier gas such as nitrogen or argon.


!lasma excitation sources

:. The main types of argon plasma sources

a. Inductively coupled plasmaA I2!

". 6irect current plasmaA 62!

c. 4 micro#ave+induced plasmas is recently introduced to spectro+chemical

analysis methods.

At i E i i S t



Induction coils

Sample flo#

Duart% tu"es

7agnetic field

!lasma

4rgon

tangential flo#




A i E i i S



It consists of a high+voltage discharge "et#een t#o graphite

electrodes. The recent design employs a third electrode arranged

in an inverted M+shaped #hich improves the sta"ility of discharge.

The sample is ne"ulised at a flo# rate of ' mlmin. 4rgon is used

as carrier gas. The argon ioni%ed "y the high+voltage is a"le to

sustain a current.

62! generally has lo#er detection limits than I2!. @o#ever, 62!is less expensive than I2!.

6irect current, 62!




'+ The sample could "e introduced in solution form through a

ne"uli%er (easy for quantitative analysis.

8+ It is suita"le for quantitative multielement determinations

:+ The high temperature of plasma eleminates many chemical

interferences present in a flame

<+ It is #ell suited for refractory (oxide forming elements e.g.!, 1r and tungeston and for difficult+to+excite elements

such as Fn and 2d.

=+ The emission intensity+versus+cencentration range is linear

over a very #ide dynamic ranges of analytes.

4dvantages of plasma excitation source




Laser "eam is used to vapori%e the sample, #hich is then excited

electrically.

The sample is loaded ust "eneath the t#o electrodes that #ill "eused to generate the electrical discharge.

4 ru"y laser is then focused through a microscope onto the

surface. The energy from the laser causes an intense local hot spot #hich vapori%es a small quantity of sample.

The vapor circuits the electrodes and electrical discharge occurs

#hich excites the metals in vapor. The excited metals emit typical

emission spectra #hich are collected and measured as usual.

Laser excitation source




'. Laser excitation produces a high density plasma and is used

for the spectrochemical analysis of solid materials.

8. The locali%ation effect permits examination of areas as smallas =C Nm in diameter, providing the "iological researcher #ith

a tool capa"le of examining the insides of individual cells

#ithout destruction of organic materials.

:. In laser excitation, the sample needs not to "e electrically

conducting.

4dvantages of laser excitation source




If the composition of sample and matrix is un/no#n. The internal

standard is added to "oth un/no#n and cali"ration standards.

The internal standard should

'. resem"le the element to "e determined in rate of volatili%ationand chemical reactivity.

8. have a measura"le emission line in the same spectral vicinity as

the sample emission line.

:. It must not also present in the original sample.

1se of an inernal standard

Duantitative analysis

Then, "y plotting the ratio of intensities of the element to the

internal+standard element vs. concentration of the element, any

fluctuations should "e compensated for.



Standard 4ddition 7ethod

in order to partially or #holly counteract the chemical and

spectral interferences introduced "y the sample matrix.

4 li ti f 4ES ( i fl it ti



4ES is rapid method for qualitative and quantitative

determination of most metals.

It is superior than flame and atomic a"sorption methods. 0lameemission spectroscopy has the limitations of "eing only good for

fe# elements #hile atomic a"sorption techniques need a separate

source lamp for each element. 4ES methodsA "eing very sensitive,

have numerious applications in analysis of "iological samples.0or examples

'. evaluation of platinum in "ody fluids and tissues after

administration of platinum containing anticancer drugs

8. determination of organic and inorganic Se compounds in"iological fluids and environmental samples

:. determination of trace elements such as 2d, 2o, 2r, 2u, 0e,

@g, 7n, &i and !"

<. Silicon is recogoni%ed as an essential trace element

ti i ti i l " d t " li

4pplications of 4ES (using non+flame excitation sources

Atomic Absorption Spectroscopy and Atomic Emission Spectroscopy

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