Chapter 6.

10

Applicatio n of Atom. Ab .
tc sorpt1on
Spectr~ph otometry (AAS) in S .
Analysis 011

P.C. SRIVASTAVA, S.P. PACHAURIANDV EE

RSINGH

The elemental analysis is undertaken by soil water pl t .

. 'th d'fli . . ' ' an' environmental and food testing
laboratories wi 1 erent obJectives where both classical a d d .
n mo em analytical methods
are used. In the nature,
. some
. elements that occur in traces are ei·the r v1•ta1 "1or the metabolic
.
functions or toxic to all. hve forms. Such elements need to be determme • d accurately for
successful_crop an~ amm~l husbandry. The_estimation of such trace elements by classical
methods like colonmetry is not only long time consuming but is less accurate as well. In
this context, atomic absorption spectrophotometry (AAS) is an indispensible analytical
technique which allows rapid analysis of many elements at an affordable cost. In AAS,
the absorption of electromagnetic radiation is utilized for quantitative determination of
elements. Some advantages of this analytical technique are: (i) high specificity for the
elements in the sample, (ii) freedom from spectral interferences, (iii) few interferences
which can be easily taken care of, (iv) improvised detection limits for many important
elements as compared to flame emission spectroscopy (FES) or flame photometry, and
(v) simple instrumentation and easy handling.

6.10.1. Principle
The principle of this technique was propounded by Bunsen and_Kirchoff ':hile studying
the spectrum of sodium. They observed that every element has its own umque spectrum
·m the vapour phase · the atomic
· st at e can absorb the same wavelength
and a metal m
radiation (resonant wavelength) which it emits.
. 1 th generatec1 i!' a specific
In atomic absorption spectroscopy a beam of resonant wave eng .
. ' h. h absr .,., , } ,_ resonating
light source is passed through the ground state atoms w ic . t·cular
1
· 1 1 t n trans ' :i par
wavelength. This wavelength is specific to a particu are ec ro · · · .' t' width of
element on , , ·· ne
element. In general, each wavelength correspond s to one . ·, ~ 1 x 10-10 cm),
·
an absorption line is of the order of only a few picome ters (1 P1c or •. · -
·
which gives this technique its elemental selectivity·
 NALYSIS
SOIL A
564
. ht at the resonant wavelength Which.
t of 1lg · lS
. b rptmn . measures the arnounms A s the number of .atoms m the light Path
The atomic a so h a cloud of ato · s By measunng the amount of ligh
· sses throug 1·ncrease · t
absorbed as it pa f 1. ht absorbed also nt of interest can be achieved. Th
h mount o ig f the eieme e
increases, t e a . . d termination o . of wavelength allow the specific
absorbed, a quantitative e and careful selecuo~ the presence of others.
use of special light s~urces .ndividual element in .
. .
quantitative determination of an i · absorp 1 t ·on measurements is
.
produced by
1 d require . d 1'"or
'
the atomic pie to d·ssociate
1 the chemical
. compounds
The atom c ou energy to the sarn . flame aligned m the path of light
· ough therma1 · into a
supplymg en . t·ng a sample solution di"tions most of the atoms remain
. f t O ms Aspira i flame con , .
mto ree a _- Under the proper tical wavelength commg from a
beam serves this purpose. b diation at the ana1y
in the ground state an d absor ra

source lamp. b bance is given by Eq. 1:

. . . terms of a sor
The absorption of radiation m
.. . (1)
Absorbance -- log~=
I KLN
1

where,
Io = Intensity of incident light,
I = Intensity of transmitted light,
(

K = Absorption coefficient,
L = Path length of flame, and
N = Number of atoms per cubic cm.
In emission spectroscopy, the atoms of analyte are excited upon absorption of energy
and emit radiation of characteristic wavelength when they come back to the ground state.
The number of excited atoms, which are capable of emitting light/ characteristi
wavelength, is always limited. The population of excited atoms is much less (near 2%;
than the ground state atoms. As the phenomenon of atomic absorption is based on the
ground state atoms whose population is many-fold higher than of excited atoms, th e
AAS is more sensitive and efficient than the flame photometry. The ease and speed al
which precise and accurate determinations can be made with this technique have made
AAS _as one o: th~ mos~ _Popular methods for the determination of many elements .
Atomic abso~t10n is sensitiv~ to measure parts per million (ppm) concentration of many
elem~n~s. With flameless version, the concentration can be measured at the level of parts
per btlhon (ppb).

. 6.10.2. Instrumental Components

Different components of an AAS are:
(1) Atomizer
(2) A light source for resonant w
ave1ength
(3) Monochromator/Spectrometer
 ATOMJc ABSORP
TION SPECTROPHOTOMETRY (AAS)
565
These component
s are described in the .
followmg sections:
(a) Atomizers
For elemental analysis b AA
c ·
1orm m an atomizer • y S, the first st ·
Th ep 1s to convert th .
· e commonl e sample mto atomic vapour
electrothermal atomizers O h Y used atomizers . fl .
. . . t er atomizers 1·k are . ame atomizers and
atonnzat.Ion, or cold-vapour t . . 1 e glow-discharge t · . .
a om1zation are a om1zation, hydride
1
a so used for special purposes.
Flame Atomizers
The most commonly used atomiz •
· ers m AAS are the fl · •
(C,HJ flame with a temperature of b ames , prmc1pally the air-acetylene
- a out 2300 oc and .
flame with a temperature of about
2700 0 C rutrous dioxide (N 20)-acetylene
reducing environment, which is suitabl t · Th~ la~er type of flame also offers a more
to oxygen. e or estimation of elements having high affinity

In a flame atomizer, sample in solution form is as · t db . . .

. . prra e y a pneumatic nebulizer which
sucks the sample and converts 1t mto an aerosol (Fig 1) Th •
. . . . . e aeroso1 enters mto a spray
chamber, where it is IIDXed with the flame gases and only the finest aerosol droplets (<
10 µm) enter the flame. This process allows only about 5% of the aspirated sample
solution to reach the flame and also guarantees a relatively higher freedom from the
interference. The burner made from titanium metal produces a laterally long (5-10 cm)
flame which is only a few mm thick. In the flame , first the solvent is evaporated
(desolvation) and then nano-particles of dry sample are produced which are converted to
gaseous molecules (vaporization phase) and further due to thermal energy these gaseous
molecules are dissociated into fre e atoms. Some alkali- and alkaline earth-metals
(depending on their ionization potential) may ionize to produce ions also. Ionization is
generally undesirable, as it reduces the number of atoms that are available for the

D
absorption of radiation.

..=,,.-➔ Burner bead

Flow spoiler~~-- --- -~

~
for rcn•ova\ ot · •

Impact bt'ad
Nebulizer ""------~,~
....----------- • •:I /
L."
,~- ? Air/ Oxidant supply

Sample feeder capillary )

h..erber 1993
. . bl (Adapted from lk,lt, . n,
Fig. I. A schematic diagram of a nebulizer with burner assem Y
~-------~--
566
SOIL ANA LYS IS
◄
e at its lon ges t axis. . Thef flow -rates of fl ame
The radiation beam passes through this flam . h and th
the hig est con cen tration o free atom s
gas need to be adjusted so as to pro duc e . h d"iati.on b earn o pas s thro ugh the zonee
t
also be adju. sted to per mit t e ra . .. . .
burner height can. . hes t sen siti vity 1s achieved in
clou d den sity m the flam e so tha t the hig
of highest atom ic
analysis by this technique.

Electrothermal Ato miz ers internal

tube hav ing the leng th of 20-25 mm and
In an electrothermal atomizer , a graphite mass
sured vol um e (10-50 µL) or a wei ghe d
diameter of 5-6 mm is used (Fig . 2). A mea e and subjected
(typically around 1 mg) of a solid sample
is introduced into the gra phi te tub st sample
achieving different seq uen tial age s of
to a programmed temperature ram p for of
(for the eva por atio n of the solv ent) , pyr oly sis (rem ova l of the maj orit y
like drying
stitu ents ), atom izat ion (rel ease of ana lyte elem ent into the gas eou s pha se) and
matrix con
ova l of eve ntua l resi due s from the gra phi te tub e at hig h tem per atu re). The
cleaning (rem
s are hea ted thro ugh thei r ohm ic resi stance usin g a low -vo ltag e hig h-c urre nt
graphite tube
The tem pera ture for the ind ivid ual stages can be con trol led ver y closely,
power supply.
ture ram ps betw een the ind ivid ual stag es faci lita te the sep ara tion of sam ple
and tempera por ate
m cou ld be of 30-40 sec at 150 °C to eva
components . Typically, the heating progra cha r the
off any vol atil e org ani c ma teri al and
the solvent, 30 sec at 600 °C to drive for 5_
rate (ca, 1500 °C sec·') to 200 0-2 500 °C
sample to ash , and with a very fast heating . Fin ally ,
(including the ele me nt bei ng ana lyz ed)
10 s~c to vaporize and atomize elements it rea dy
tem per atur e (27 00 °C) clea ns it to ma ke
heatmg the graphite tube to a still hig her
for the nex t sample.
atu re
rsely for a mo re hom oge neo us tem per
~he _gra~hite tubes may be heated transve gra phi te
izat ion of the sam ple occ urs fro m a
distributi~n over ~ei r length. The atom se in the
to dela y ato miz atio n unt il the gas pha
platf~rm mserted mto the graphite tube ol sis
s helps to stab iliz e the ana lyte to a pyr
aton uze r reaches a_ stable temperature . Thi · t
maj orit y of ma trix com pon ent s·, and m egr at1 0n
~
temperature, sufficient forhrem•oving the
of the abso b ng pea k
ien t a:so rpti on sign al i~s tea d of usi
height abso:baa:Ccee f:;; u:n :i;~ ::o ~f ;:;s
lya;;;n~ oadgende~ated tran sien t_te abs orp tion sig nal
is directly proportional to the mass of ana r uce mto the gra phi tub e.

p·
ig. 2. A grap hite tube
 AT 01- nc ABso
RPT ION SPE CTR Qp
HOTOM ETRY (AAS) 567
Th e adv ant age s .
associated With .
Th e sensiti.vity of th·is tech this technique are ·.
• .
ue is 2- 3 t·1 s hig .
con cen tra tio n (µg L·' rangeniq ~ me h than that of flame AAS
er d1
. l sam ple or a typ ica l ple vol , an ow
a typica sam
1
mass of 1
x ma t . mg ) can be ach· ume of 20 µL and. ng g-' range for
e em ent s in com ple ieved for th e determmation of trace
rices
• Sam ple s of very sm a 11 volume· c
an be used .
• Th e pro ble m of inte rfierence is r l . ely negligible
e ativ
·
Glow-Discharge Atomization
. ..
A glo w-d isc har ge device (G D) can s1m I ·
introd uce and ato · h
d • occ urs in u tan eou sly
Th e g1ow isc har ge a 1ow -p ressure ( 1-1 o tor ) . mize t e sample.
ele ctr ode s at a hig h . r 1n arg on (Ar )
pai r of potential (D C voltage of 250-1000 V) . . gas atmosphere. A
ic fiel d, gas ion s are l ioruze Ar gas and under
the electr acc e era ted int0 fac h . the sample
. d . the cathode sur avmg
Th ese ion.s bo mb ard the sam ple an eJect n ra1 atoms from thee sa ( . ·
).
rs pro du ced by th· ct· eut . . mp 1e spu ttermg
Th e ato mi c vap. ou is ischarg s gro und stat e t d
. ms . Wh en th . e con sist of ion ' a om s, an
fract10 n of. exc ite d . ato e excited at
. em itte d. Th e bas · . om s relax back. to their ground state, a
low-m ten sity glo w is . ic requirement .for glow discharge atomizers is that
ctr ica l con duc t Th •.
the sam ple sho uld be ele or· e techmmi que
. can
b
e uttl~zed to analyse liquid
as non -co ndu cti ng materials b
sample s as we ll xmg em with a conductor like
th
y
graphite.

Hydride Ato mi zat ion

.
tec hni que hel ps in the ana lys is of arsenic (As) ' ant'imony (S b), tm (Sn) selenium
This .' .
·
mu th
(B")
i , an
d
lea d (Pb ). Th e hydride atomizat·ion •improves the u detection
(Sn), bis .
. .
tor of 10 to 100 , com par ed to the other alternati·ve meth ods. The hydnde
limits by a fac . . . ple to the
era ted ~y add mg an_ ac1 d1f ied aqueous solution of the sam
vapours are ~en by an inert
_of s_o dm m bo roh yd nd e (1 %) . Th e volatile hydride is carried
aqu~ous sol uti on cess forms
m1 zat 10n cha mb er, wh ere it undergoes decomposition. This pro
gas mt o the ato AAS.
e, wh ich can the n be measured by
an ato mi zed for m of the ana lyt

Cold-Vapour Ato mi zat ion ) as it has a

ur tec hn iqu e is use d for the det erm ina tio n of only mercury (Hg
The col d-v apo version of
ssu re at the am bie nt tem per atu re. Th e method involves the con
large vap ou r pre H 2SO 4, foll owed by a reduction of
2 + ion s by ox ida tio n with HN O 3 and
me rcu ry int o Hg bubbling a
2 wi th SnC1 • Th e Hg is the n sw ept int
o a long-pass abs orptio n tub e by
Hg + tion is det ermined by
rea cti on mi xtu re. Th e concentra
2
str eam of ine rt gas thr ou gh the 1
d giv es imp rov ed detection imits tll
abs orb anc e at 253 . 7 nm . Th is me tho
rec ord ing the
1 ple ).
the tun e of pp b ran ge (ng g- sam
Wa vel en gth
(b) Light Source for Re son an t ~ tlC ' ._, ,\
in a lin~ ):
e res on ant wa ve len gth /lin e for an ele me nt is gen era ted ~1 01 a
Th lam p (l·
) or ele ctr od ele ss dis cha rge
ho llo w cat ho de lam p (H CL
re xe no n sho rt arc lam p).
con tin uu m sou rce (hi gh-pre ssu
 568 SO IL AN AL YS IS

1
Quartz Window <E --Q ua rtz
l W ind ow
Inert Ga s
~- (A r/ N e)
Anode
l
Insulating
disc t La mp wi th
,.____ _ Hollow Cathode Me tal salts
l'- i_. .,,_ _ Ce ram
of Specific ic
metal(s) RF Coil Ho lde r

Hollow Cathode Lamp Co nti nu um So urc e La mp

Fig. 3. A schematic diagra
m me of hollow cathode
lam p an d a co nti nu um
so urc
e lam p
The use of hollow catho
de lamps is very comm
medium-resolution monoc on in m os t of ~he AA
hromator. The hollow ca S w ith on ly a
(made up of glass) in wh thode la m p is like an or
ich neon (Ne) or argon (A di na ry bulb
torr). It has a cathode ma r) gas is filled at a lo w
de up of a hollow cylinde pr es su re {l-5
The material of the anode r of specific el em en t an
is not critical. Thus , for Zn d an an od e.
estimation, Fe-cathode an estimation, Zn -c at ho de
d so on can be used. Whe an d for Fe
at a voltage of 100 or 200 n the hollow ca th od e la
V, the Ar /N e gas presen m p is op er at ed
Ne ions are generated (F t inside th e bulb gets io
ig. 4). They start bomba ni ze d an d A r/
such an intensity that the rding the in ne r wa ll of
y are able to dislodge so ca th od e w ith
hollow cathode. Due to me metal at om s pr es en
the collision among the t in sid e th e
atoms get excited and em metal at om s an d A r/N
it a resonance line/wavele e- io ns , m et al
hollow cathode lamp, a ngth of th at el em en t. A
quartz window is provid t th e to p of
through the hollow catho ed, so th at th e re so na nc
de lamp. e lin e pa ss es
!h e hollow ca ~o de l~m
ps have a shelf-life in m
mcrease~ la ~p mtens1~ illi-ampere ho ur s. In cr
but reduces the lamp-lif ea sin g cu rr en t,
broadenmg, i.e., atoms_m e an d also results in
the h~llow cathode lamp se lf- ab so rp tio n
hollow cathode lamp itself begin to absorb lig ht em
range of calibration curve. . This leads to lower absorbance an itt ed fr om th e
d re d t · · h .
uc m n m t e lm ea r
Another type of resonan
ce line source is electrod
contain a small quantity
of a given analyte metal eles d .
at low pressure. The bu or it s I~charge la m ps
fr lb is inserted into (E ~L ) w hi ch
equency 1eId. This
fi . ·1 s sal~ m a qu ar tz bu lb
results in a low-pre a co1 ge ne ra tin g an w ith A r ga s
T~e e~ission from an ED •d . 1
e ec tro m ag ne ttc.
L is higher th ::u :~ ;~ f:: ra di o
w1d~~ is generally narrowe ~v el y co up le d di sc ha rg
r, bu t EDLs ne d e in th e la m p.
stabilize . an ho llo w ca th od e la m
e a separate po we r su pp p. Th e lin e
ly an d a .
A ~ontinuum radiation so 1on ge r tim e to
urce used for A A .
W ith a co nt in uu m ra .
monochromator. In addi di at io n so u . s_ is a high-pressure xe
no n sh
tion i . rc e, It is ne ce ss ar y to us
least an order of magnitu . or t ar c la m p.
d 11·t is necessary th at
entire wavelength range
of ; 90~t~~ the la m p em ·t
th an th at of a typical ho
nm . 11
e . a. h1 gh ?e so lu tio n
~:rad1hatton of m te ns ity
ca t od e la m p ov er that
e
 AT0~11c ABSon,...
•u-TiON SPEC
TROPHOTOMETRY (AAS) 569

F" 4 A h
Ne• -Ne• +

Gas ionization step

e ____.,
~ M•
SPuttering or Ne•
atoms
metal
M•

_· ._ir_N_e•_J
_ •_.._
+ --::----M
Excitation or metal
~ ¥
E~ M•/ ~~ I
•
.,

ig. • SC ematic represent . atoms upon c0 II' . Emission or resonant

lamp (Adapted from Lev ation of processes . ,s,ons wave length (I,)
L ond on ) enson R (2002) in generation f
' . More Modern Ch ? resonance wave length in hollow cathode
.
em,cal Techn ·,ques, Royal Society of Chemistry ,

(c) Monochromator/Spect
rometer
The spectrometer includes
. a monochromat .
d erector as we11 . Ahne source AAS
. . (LS-AAs or to · t
isolate . wave length of merest an d a
while a continuum source AAS (CS-AA ) u~es a medium-resolution monochromator
LS-AAS, the narrow line emission f S) requires a high-resolution monochromator. I~
h . rom the radiat · .
1
to reso ve t e analytical line from h . . ion source requires a monochromator
. ot er radiations e .tt d b
by allowing a band pass betwee mi e Y the lamp. This is achieved
n O·2 nm and 2 O nm usmg ·
a medium-resolution
monochromator. To make LS-AAS 1 . ·
. e ement-spec1fic the · • • .
with a selective amplifier tuned to th '. pnmary radiation 1s modulated
e same modulation freq Th' ·
of unwanted radiation. Simple h uency · 1s a11ows exclusion
monoc romators of the Littr h c T
design are generally used for LS-AA . . ow or t e zerny- urner
. . . S· A photomult1pher tube or solid state detector
·
havmg better signal to noise ratio is used for reco rd'mg th e mtens1ty • of resonance wave
length.
The CS-AAS essentially requires a high-resolution monochromator. The resolution should
be equal to or better than the half width of an atomic absorption line (about 2 picometer)
to avoid the loss of sensitivity and linearity. This spectrometer uses a compact double
monochromator with a pre-monochromator prism and a high resolution Echelle grating
monochromator for high resolution. A linear charge coupled device (CCD) array with
200 pixels is used as the detector. As the second monochromator does not have an exit
slit, therefore, the spectral environment at both the sides of analytical line becomes
evident at a high resolution. Only 3-5 pixels are used to measure the atomic absorption,
while other pixels are available for correction purpose.

6.10.3. Correction of Background Absorption

The narrow width of atomic absorption lines of different elements reduces the chance of
. h h O f another element. However, the molecular
spectral overlap of one element wit t at . h t ·c ·ne The molecular
h' h overlap wit an a om1 11 •
absorption is much broader W IC may . t ments in the sample or from
. . d lecules of concom1tan e1e
absorption from un-d1ssociate mo . . w·th electothennal atomization ,
. 1 'th t O mic absorption 1ines. i .
flame gases might over ap w1 a . , t d during atomization due to
. . b the particles genera c
the scattering of primary ra d 1at1on Y
. . h olysis stage occurs.
incomplete removal of matrix mt e pyr . , .. cments of tl,t ,1
. . rrected by sequcut ,J mcc1~ur .
In LS-AAS, the background absorptwn is co d b the rw 1.,11re11c11t uf b,tL l
absorption (atomic plus background), followe y surc,u , 11 t:,. ~ives the nd '), •
absorption only. The difference of these two mea '
 ATOMrc
ABSORPTION SP
ECTROPHOTOMETRY (AAS) 571

Q==J-9__
Hollow
cathode
6
~
:-
Mechanical
1
s : Entrance Slit
I

-~:---E~:i-
•
1
--=:::::::::.-_ • a.au

lamp Chopper C2H2-Air/N O Flam

Graphite Fu~ e or Monochromator Readout
ace O etector device
Fig. 5. A schematic sketch of •
Beaty and Kerber 1993) a Single beam atomic ab .
sorpt1on spectrophotometer (Adapted from

for the background correction and - . .

· · h' e1immatton of la ·
noise rat10 1s ac 1eved in this case. mp noise , a much better signal-to-

6.10.4. Types of Configu t·

ra ion of An Atomic Absorption
Spectrophotometer
The configuration of an AAS is generally of two types:
(i) Sino/e beam type: In a single beam ty AAS 1· h · · ·
b . pe , 1g t commg from the hght source is
modulated electromcally or chopped mechanically by a rotating chopper (Fig. 5).
The purpose of mechanical or electronic chopper is to distinguish the radiation
emitted from the sources other than light source (i.e., hollow cathode lamp). A
detector with AC electronics is tuned with mechanical/electronic chopper for the
detection of intensity of light/radiation coming from the light source. Both are
placed functionally in co-ordination. However, if the intensity of radiation from the
light source changes with the passage of time, the measurement will be undesirably
affected.
(ii) Double beam type: In a double beam type AAS, light beam from a light sou_rce is ~plit
into two beams, namely reference beam, and sample beam, by a_ rotatmg nurror
(Fig. 6). The sample beam passes directly through the flame, whil~ the .refere_n_ce
beam passes round the flame · The detector response represents the ratio of mtens1ties
. . .
eams This helps in minimizing the effect of variation m
of sample an. d re fierence b . . . h h to
. . o f 1amp em1·ssion , detector sensitivity and electronic gam by t e P o
mtens1ty
multiplier device.

Mirrors Reference Beam Mirrors

Hollow
cathode
Sample Beam
® \•UP
Detector Readout
lamp Light beam \ Monochromator devic.:
Splitter CH-Air/No 2
Bearn
Flame
2 2
Recomhiner
or Graphite Furnace
.l'
hotometer (Adapted t.u1
. absorption spectrop
Fig. 6. A schematic sketch of a double beam atomic
Kerber 1993)
 SOIL ANALYSI S
572

. At mic Absorp tion Spectr oscopy

5 Interfer ences in o
6. lo · · . .
d to a change in the intensity of the anal
,. h omenon that 1ea s . c Yte
An 'Interference is a p en tral or spectra1. The non-spe ctral mter1ere nces affect th
signal. It could be non-spec . e
1 . t ferences result in higher light absorpti on du
formation of analyte, while the sp~ctra hm erth the analyte. These are describe d in the
b b. g species ot er an e
to the presence of a sor m
following sections.

(a) Non-Spectral Interferences . (i) matrix interfere nce, (ii) chemical

The non-spectral interferences are of three types.
interference, and (iii) ionizatio n interference.

Matrix Interference . be d "f'C: · th

d ds there will 1 1erences m e sample
If a sample is more viscous than the lstan Iatr 'n be minimiz ed by matchin g the matrix
l· · roneous va ues. ca
uptake rate, resu tmg m e~ f Therefor e the standard s need to be prepared
composition of standard with that o samp1e. . ' . f lant samples
in the soil extractant used or acid dige st used m th e procesSm g O p ·

Chemical Interference
If a sample contains a chemical species which forms a thermal ly stable co~pou nd with
the analyte which is not completely decompo sed to release the analyte m the flame,
causes a reduction in the signals due to anlyte. Some element s like titanium (Ti), tungsten
(W), zirconium (Zr), molybde num (Mo) and alumini um (Al) easily combin e with 0 to
2
form thermally stable oxides. In order to reduce such chemica l interfer ences, these
elements need to be analyzed with a nitrous oxide-ac etylene flame which gives much
higher flame, temperatures for the dissociat ion of refracto ry oxides as compar ed to air-
acetylene flame . Similarly, if the refractory compou nds are formed due to the excess of
another element, e.g. reduction in signal of calcium (Ca) in a matrix contain ing phospha te
anion due to the formation of refractory calcium phospha te, then an excess of lanthanu m
is added to precipita te phospha te to check chemica l interfere nce.

Ionization Interference
Th~ el~m~nts having smaller ionizatio n potentia l (alkali and alk I" h
easily wmzed in acetylene-air flame reducin th . a ine eart metals) are
atoms for absorption of resonant wav; le th ~h. e popul~t1 0n of ground state neutral
by adding excess of an element wh1"ch . ng . 1~ ty~e of interfer ence can be eliminated
.
envrronment in the flame to suppress1s. more
. .
easily ton· d t O
ize produce an electron -rich
• 10rnzat1 0n of th
potassmm (K), rubidium (Rb) and
. .
. e ana1yte. The soluble salts of
caesmm (Cs) are g 11 . .
to Suppress 10mzation of analyte ato enera Y used m high concent ration s
ms.

(b) Spectral Interferences

If the atomic absorption line of
of intere t th some element ov 1 •
s ' e spectral interference can ca er aps with spectral line of the element
use erroneo us valu
es . In such a case, it is better
 ATO MIC ABSO RPTI ON
SPEC TRO PHO TOM ETRy (AAS) 573

to opt for som e oth

und isso
. 1 ciat ed molecul
er es
spec
oftral line
part1c es, unv apo uriz . of the ana1yte element S
ed 1 mat nx havi ng broa .
. d b d b . ome ttme s, the presence of
so ve t
over a wid e wavelength re _n d roplets or mol an a sorpt"
ecular s . ,on spectra and tiny solid
interest to result a hi h g1on and may overla a pec1es m the flame scat
corr ecte d by mea su . g back grou nd abso ter light
rpt· p tom1c abso rptio n of the anal yte 0 f
measured absorption.nng and subtracting themn. In such
backgrou nd case
absos,rptio
the nproblemthecantota
from bel

Cal ibra tion

The inst rum ent need s to b
~
dilu ting the stoc solu tio e alwa ys calib rate
back grou nd chemkical en d by usin g a
of the anal yte. Nec .
. v1ro nme nt of h essa ry ca senhes of stan dard s prep ared by
yp1c ally, con cent ratio ns yi ld" re s ould be tak k
1"b · t e stan dard s simi lar to th t Of en to eep the
ca I rati on curv e are used (F'e1g. mg
T O2
7). . to O. 3 absorbance exh'b a
1 •itmg unkn own sa 1
. good lmea
. ritymp in the
es.
0.45 0
0.40 0
y=0. 98x
0.35 0 R 2=0.999
Q.) 0.30 0
u
C
0.25 0
"'
.s:::,
"""
0
Cl) 0.20 0
.s:::,
~
0 .150

D
0.10 0
0.05 0
0.00 0
0.0 1.0 2.0 3.0 4.0 5.0
Zinc conc entra tion (µg Zn mL·1)

Fig. 7. A calib ratio n curve of Zn (C H -air flame) at

2 2 213.7 nm wave len.~t

The cali brat ion curv e help s to provide the

slope facto r in the r ,la , · .,,p between
abso rban ce and con cen trat ion und er a given
set of experimental cc , , , "· In the new
models, the abso rban ce valu es are stored in the
memory of insm rn ·r and can be used
directly by leas t squ are met hod to display the
concentration of aualyte in the unknown
sam ples . In case the unk now n samples are
having absorbancc beyond 0.3 absorbance,
eith er unk now n sam ples sho uld be diluted to
yield absorbance values in the linear range
th
or a fresh cali brat ion curv e sho uld be con
structed using concentrations covering e
range exhibited by the samples. ' . . .t ' of the instrument or
ln atom ic abs orp tion spec trop hoto met ry, the
. term
f SenS
lyte pro duce 101io absorption- for
toittviY
ope rati ng con diti ons refer to the conc
. entrat10hn ana
exa mpl e if the max imu m abso rpuo n of t
°
1 te on the instr. urnent is J.00, the h
e ana Y .
' . 0 0033 bsorption unit wou Id be considered as t '
con cen trat ion cap able of prod ucin g ·
a
sensitivity of the instrument.
 SOIL ANALYSIS
574

The 'Detection limit' of the instrument is the lowest concent:ation of th e analyte Which
can be clearly differentiated from zero. For a given concentration, repeated 1:1easurernents
of that concentration are made for a given standard along with blank readmgs recorded
before and after running the standard. The 'Detection limit' is calculated as per Eq. 2:
Detection limit = (Standard concentration) x 3 (Std. dev.)/Mean ... (2)

Precautions/ Comments/Suggestion s
• All precautions mentioned in the user manual of the instrument muS t be followed to
avoid any mishap or damage to the instrument.
• If there is some salt or carbon deposition in the burner slit, the solid residues must
be cleaned by brass cleaning strips and finally by aspirating 0.5% nitric acid, followed
by detergent solution. The burner needs to be dried before fixing it.
• The plastic capillary tubing should be free of any bends or choking and should fix
tightly to the nebulizer. If there is any blockage in the nebulizer capillary, the
nebu~izer needs to be cleaned using a cleaning wire and sonicated in a 0.5% soap
s~lut10n for about 5-10 min. In case the blockage persists, the nebulizer cleaning by
wire and followed by sonication needs to be repeated again.
• The impact bead or flow spoiler and O-ring should be regularly checked.
• Be~ore shutting off the instrument, about 500 mL of distilled water should be
asprrated for cleaning the nebulizer and burner.

Suggested Readings

Beaty, R.D. and Kerber, J.B. (1993) Concepts Instru .

photometry. Second Edition, The Perkin-Elme ;entatto~ and Techniques in Atomic Absorption Spect
. r orporat10n, Norwalk CT US A ro-
K01rtyohann, S.R. (199l) A . . ' • · · •
103 lA. history of atomic absorption spectrometry. Analyt. l C'h .
tea emistry 63, 1024A-
Levensen, R. (Ed ) (2002) Mi
183. . ore Modem Chemical Techniques: Ro,,al S .
-' ocrety of Chemistry L
L' B , ondon, pp.
vov, . (1990) Recent advances in ab .
etry. Spectrochimica Acta Part B· s~lute analysis by graphite furn .
L'vov, B. V. (2005) Fifty . Atomic Spectroscopy 45, 633-655. ace atomic absorption spectrom-
382-392. years of atomic absorptio
~ n spectrometry. Journal
)koog, D. (2007) Prin . l of Analytical Chemistry 60
crp es ofInstrumental An l . '
1/eJz, B. and Sperling, M. (1999) . ays1s. 6th Edition. Canada: Th
Atomic Absorption S omson Brooks/Cole
,pectrometry. w·1 ·
I ey-VCH, Weinheim, Germany.