Atomic absorption spectroscopy

Atomic Absorption Spectroscopy

1. Inorganic Chemistry
Group D
1
MscChemistry, TU, Nepal
ATOMIC ABSORPTION
SPECTROSCOPY (AAS)
Dr. DamodarKoirala
MscChemistry, TU, Nepal

2. Semester’s Tentative Plan
DayTopic
5Spectroscopy and Optical instruments
6Atomic Spectroscopy Introduction
7Atomic absorption spectroscopy
2
7Atomic absorption spectroscopy
8Atomic emission spectroscopy
9Flame photometry
10Plasma Emission spectroscopy
11Atomic fluorescence spectroscopy
12Molecular luminescence
13Thermal analysis: TGA and DTA

3. Content

Principle of Atomic Absorption Spectroscopy

Sample analysis procedure in AAS

Quantitative analysis using AAS
Components of AAS instrument
3

Components of AAS instrument

Types of spectrophotometer

Significance of AAS

Assignment

4. Terminologies

Vaporization:phasetransitiontovaporform

Decomposition:breakingofcompound

Aerosol:suspensionofsolidparticlesorliquiddropletsinair

Mist:suspensionoffinesolidparticlesorsmallliquiddroplets
inair
4
inair

Sputter:ejectionofmicroscopicsolidparticlesfromitssurface
duetobombardmentwithenergeticparticles.Theprocessis
calledsputtering
Ar
+
M
o

5. Basics of AAS

AllatomsabsorbcharacteristicwavelengthsofEMR

Afterabsorption,electronictransitionoccurs

Theamountoflightabsorbedαconc.ofatoms

Theamountoflightabsorbedαpathlength
5

Theamountoflightabsorbedαpathlength

Instrumentusedisatomicabsorptionspectrometer

Calibrationplotisusedtofindconc.ofatomin
unknown
P
o
P
E
0
E
i

6. Beers-LambartLaw
6

Beer-Lambert law: A = ε.b.c

Ais absorbance

cis the concentration of analyte

bis the path length of EMR
εis absortivityconstant of analyte

εis absortivityconstant of analyte
P
o
P
E
0
E
i
P
0
= power of initial radiation
P= radiation after absorption
T= transmittance

7. Principle of AAS

WhentheatomsingroundstateinteractwithEMR,it
absorbsthecharacteristicwavelengthandreduces
theintensityofthatwavelength

Thecharacteristicwavelengthabsorbedwhen
7

Thecharacteristicwavelengthabsorbedwhen
determinedisusedforqualitativeanalysisofthe
particularatom

Theabsorbancewhencomparedwiththatof
standardsaresimilarexperimentalconditionsis
usedforquantitativeanalysisi.e.todeterminethe
concentrationofgroundstateatoms

8. Basic analytical procedure

Convert the sample into solution

Make up a solution which contains no analyte(blank)

Make up a series of standards

Eg. 1mM, 2mM, 5mM, 10mM, 20mM
8

Eg. 1mM, 2mM, 5mM, 10mM, 20mM

Atomize the blank/standards and measure the response

Plot calibration plot of response vs. concentration of
blank/standard

Atomize the sample solution and measure the response
(absorbance)

Determine the concentration of the sample from plot
Varian AAS manual

9. Quantitative analysis using AAS (Na)
9

The wavelength is fixed at 589nm using the
monochromator

Blank is run and the absorption is set to zero

Run the series of NaClstandards one by one and

Run the series of NaClstandards one by one and
record the absorbance values
Standard
(Labeled)
Concentration
(Prepared)
Absorbance
(Measured)
Blank 0 mM 0
S1 0.1 mM 0.03
S2 0.2 mM 0.07
S3 0.5 mM 0.16
S4 1.0 mM 0.31

10. Quantitative analysis using AAS (Na)
10

Graph absorbance vsconc. as calibration plot

Measure the absorbance of the sample

Determine the concentration of analytein sample
using calibration plot or the equationusing calibration plot or the equation
y = 0.313x
R² = 0.998
0
0.1
0.2
0.3
0.4
00.20.40.60.81
Absorbance
Conc (mM Na)
Unknown
(Labeled)
Absorbance
(Measured)
Concentration
(= Abs/0.313)
U10.210.67mMNa
U20.050.16 mMNa
U30.250.80 mMNa
U40.160.51 mMNa

11. Components of AAS instrument
11

Sourcegenerates EMR of desired wavelength

Atomizercreates a population of free atoms

Wavelength selectorisolates the specific wavelength

Light-sensitive detectormeasure EMR accurately

Signal processor measures response of the detector

12. A spectrophotometer
12
In AAS, sample must be converted into atom.

13. Sample to atom
13
Solution-
Ground
State
M
+
X
-
M
Atomization
Aerosol
Solid
residue
Gas
AtomizerNebulization
M
+
X
-
Solvent
Evaporation
MX
Solid
Vaporization
MX
Atomization

14. Atomization
14

Nebulizer converts liquid solution into fine particles of
aerosols/mist

Solvent gets vaporized leaving behind the solid
residue

The solid residue gets vaporized and the molecule
dissociates into atoms

15. Nebulizer
15

Methods to introduce samples into the atomizer by
creating fine droplets

Needs to avoid the possible formation of oxides

Many oxides are very stable and difficult to reduce

Many oxides are very stable and difficult to reduce

Three major types

Pneumatic Nebulizer

Pneumatic AerosoleGenerator

Ultrasonic Nebulizer

16. Pneumatic nebulizer
16

Basic requirements:

A high velocity gas stream.

A reasonable pressure drop of the liquid injection capillary.

Maximum interaction between the gas and liquid streams
for fine aerosol production.

Freedom from blockage.

17. Pneumatic aerosolegenerator
17

The solution at bottom is pulled up the capillary tube

Jet create a drop in lateral pressure

Droplets smash into baffle to become smaller ones

18. Ultrasonic nebulizer
18

Solution is pumped to vibrating (~1.5
KHz) transducer plate

Dense aerosol is producedDense aerosol is produced

Carried through heater and condenser
to remove as much of the solvent as
possible

For highly volatile solvent such as
petrol, a secondary desolvationstage
is required

19. Atomization
19

Flame:

Nebulizesamplebygaseousoxidantand
gaseousfuel.

Itisthencarriedintoaflamewhere

Itisthencarriedintoaflamewhere
atomizationoccurs.

Electrothermal

Glowdischarge

Hydride

20. Electrothermalatomization
20

Few microliterof sample is evaporated at low temperature

Ash is electrically heated by graphite tube of ~3000C

Absorption is measured in the region above heated surface

21. Glow discharge atomization
21

Sample is placed on a cathode.

Argon gas is ionized by an applied voltage on cell,

The argon ions accelerate to cathode, eject atoms from sample

Absorption is then measured in the cell

22. Hydride atomization
22
NaBH
4
+ 3H
2
O + HCl
➞
H
3
BO
3
+ NaCl + 8H
.
➞
AH
n
+ H
2
(g)
A
m+
ElementHydrideBp(C)
As AsH
3
-55

Avolatilehydridecontainingsampleisgeneratedandcarriedtothe
atomizationchamber(usuallyflameorfurnace)byaninertgas.
As AsH
3
-55
SbSbH
3
-17
SeSeH
2
-42
SnSnH
4
-52
BiBiH
3
-22
TeTeH
2
-4

23. Block diagram of AAS instrument
23
Readout Readout
SourceSource AtomizerAtomizer
Wavelength
selector
Wavelength
selector
DetectorDetector

Source generate stable EMR

Atomizer creates gaseous atoms

There could be one or two wavelength selectors

Detector detects the amount of desired wavelengths

Readout device provide signal the way user understand it
Readout
Device
Readout
Device
SampleSample

24. Source
24

The atomic absorption spectrophotometers
generally use line sources.

Two commonly used line sources are:

Hollow Cathode Lamp

Hollow Cathode Lamp

ElectrodelessDischarge Lamp

25. Hollow cathode lamp (HCL)
25
Anode (strong Nickel
or Tungsten wire)
Hollow cathode (Cd, Cr
or coated with Na, As)
Front :
borosilicate glass
(>400 nm)
UV-transparent glass
(240–400nm)
quartz glass (>160 nm)
Aror Ne at
1-5 torr
Glass or
ceramic shield

26. 
350–500V applied across the electrodes generate inert ions

Ar
+
accelerates toward the hollow cathode and sputter atoms

Sputtered metal excites after collision with energetic Ar+

The metal returns to the ground state by emitting radiation at
characteristic wavelengths.
Hollow cathode lamp
26
characteristic wavelengths.

Metal to be analyzed is used as hollow cathode

The typical half-width of an atomic emission line produced by
a HCL is approximately 0.2 pm
1. Sputtering
Ar
+
M
o
2. Excitation
Ar
+
M
o
M*
3. Emission
M* M
o
+ λ

27. ElectrodelessDischarge Lamp (EDL)
27

Used for elements that cannot be cast into hollow
cathodes, such as mercury, arsenic, antimony etc

Requires a separate power supply for the RF current

28. ElectrodelessDischarge Lamp (EDL)
28

Metal salt is sealed in quartz tube with inert gas

The quartz bulb is located at the center of RF coil.

RF field of ~27MHz is applied, Arionizes releasing electrons

Free atoms collide with electrons, excites and then relax

Eg: Hydrogen/deuterium, He, Ne, Ar, Kr, Xedischarge

29. Wavelength selector
29

Monochromator is used as wavelength selector

30. Detector
30

Photomultiplier tube (PMT)
Photomultipliertube

31. Single beam spectrophotometer
31

Suited for quantitative absorption study at single wavelength.

Instrument is simple, low cost, and ease to maintain

32. Single beam spectrophotometer
32

Shutter controls beam

Collect blank

Blank provides 100% transmission

Insert sample and measure absorbance

33. Double beam spectrophotometer
33

Compensates

most rapid fluctuations in the radiant output of the source

wide variations of source intensity with wavelength

34. Double beam spectrophotometer
34

Light source split

Measure light through flame and light reference cell

Determine %T

Does not consider light absorption in flame

35. Significance of AAS

Can be for analysis of more than 70 elements

Has ability to make ppb determinations on major
components of a sample

Atomic absorption analysis is subject to little
35

Atomic absorption analysis is subject to little
interference.

Most interference that occurs have been well studied
and documented.

Sample preparation is simple (often involving only
dissolution in an acid)

Instrument is easy to tune and operate

36. Assignment

WritetheprincipleofAAS?

DrawwelllabeleddiagramofAASinstrumentand
explainitsmajorcomponents.
ExplainindetailthesignificanceofAAS?
36

ExplainindetailthesignificanceofAAS?

Whataretheadvantagesofdoublebeam
spectrophotometeroversinglebeamone?

WhatistheroleofnebulizerinAAS?Explain

WhatarethepossibleinterferenceinAAS?