Atomic-absorption spectrophotometry

1. Lecture 3. Atomic-absorption
spectrophotometry in the
infrared region.
Lecturer: Lyazzat Sagyndykova,
master degree of medicine science

2. Outlist
1. Apparatus.
2. Sample preparation.
3. Identification using standard samples and spectra.
4. Impurities in gases.

3. Atomic spectroscopy is a method for determining the elemental
composition of a sample based on its electromagnetic or isotopic
spectrum.
Spectrum is the set of all values of a particular physical quantity.
Atomic absorption spectrum is a
set of wavelengths (or frequencies)
of electromagnetic radiation that an
atom can absorb.
Atomic emission spectrum is a set
of wavelengths (or frequencies) of
electromagnetic radiation that an
atom can emit.

4. Methods of atomic spectroscopy belong to the elemental level of
analysis.
1. Elemental – determination of the elemental composition of a
sample;
2. Functional – identification of functional groups in chemical
compounds;
3. Molecular – detection and quantitative analysis of the
molecular composition;
4. Phase – analysis of the presence and quantitative content of
individual phases;
5. Determination of the physicochemical characteristics of the
sample.

5. 1. Absorption of energy causes
an electron to move to a higher
energy level (E2)  AA
2. The excited electron will
eventually drop back to the ground
state and emit light
at a particular wavelength
(emission)  MP-AES, ICP-OES
3. If there is enough energy, the
electron will leave the atom
completely and leave behind
a positively charged ion
(ionization)  ICP-MS

6. Absorption methods:
• Atomic Absorption Spectrometry (AAS/FAAS);
Emission methods:
• Flame Atomic Emission Spectrometry (FAES);
• Spark Optical Emission Spectrometry (OES);
• Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-
AES);
• Microwave Plasma Atomic Emission Spectrometry (MP-AES).
Mass spectrometric methods (emission):
• Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

7. Introduction
Timeline of Early Developments
Greenfield
used the
ICP as an
analytical
tool
1964
First
commerci
al
AAS
1962
Reed
first major
application
ICP for
growing
crystals at high
temperature
1961
Walsh
explores
potential
of atomic
absorption
1952
Babat
experiments
with RF-
ICP
1941
Lundgardh
develops
Flame
Emission
technique
1930’s
Hittorf
researches low
pressure,
electrode-less
ring discharges
1884
First
commercial
ICP-MS
1983
Houk
demonstrated
the
possibilities
offered by the
ICP-MS
technique
1980
Fassel & Gray
experimented with
inductively coupled
argon plasmas
coupled to
mass spectrometer
1978
Gray
coupled a
capillary direct
current arc
plasma to a
quadrupole mass
spectrometer
1975
First
commercial
ICP-OES
1973
Wendt and
Fassel used
the ICP as a
Spectroscopic
source
1965

8. AAS (Atomic Absorption Spectrometry) is a
method for quantitative and, less frequently, qualitative
elemental analysis based on atomic absorption spectra.
The method allows for the determination of more than 70
elements. Primarily, these are metals such as Al, Ba, Be, V, Bi,
W, Fe, Ca, Cd, Co, Si, Mg, Mn, Cu, Mo, Ni, Sn, Pb, Ti, Cr, Zn,
Hg. However, the method can also be applied for the
determination of certain non-metals, including As, B, I, P, Se,
Si, Te, with a sufficiently high level of sensitivity.
Quantitative analysis is based on the Beer-Lambert-Bouguer
law.

9. Atomic absorption spectra began to be applied for analytical purposes in the
1930s, used for identifying certain elements in the atmospheres of stars and for
practical applications on Earth, such as determining the mercury content in
various samples and the atmosphere of laboratory spaces. However, the
widespread adoption of the method as a laboratory technique was limited.
The direct correlation between the
spectra of atomic absorption and
emission and the chemical composition
of a heated gas was established in the
works of Bunsen and Kirchhoff (1859-
1861).

10. This seemingly straightforward solution, which forms the
basis of the analytical method known as atomic
absorption spectrometry, laid the groundwork for the
rapid development of the method. Alan Walsh is
recognized as the developer of the laboratory method of
atomic absorption.
In 1955, Australian scientist
Alan Walsh proposed a
simple and practically
feasible method for the
quantitative determination of
the elemental content in
solutions, which were
aerosolized in an acetylene-
air flame, based on the
absorption of radiation from
atomic lines emitted by
special selective lamps.

11. Through a layer of atomic vapors obtained with an
atomizer, the sample is exposed to mono- or
polychromatic radiation in the optical range (190-
900 nm).
As a result of the absorption of quanta, atoms
(valence electrons of atoms) transition to excited
energy states.
These transitions in atomic spectra correspond to
resonant lines that are characteristic of a particular
element.

12. Upon irradiation of an atom (e.g., sodium) with light, the absorption of
emitted photons occurs, causing the outer electrons to transition to
higher energy levels. In the case of Atomic Absorption Spectrometry
(AAS) for sodium, when irradiated with a wavelength of 589.59 nm, the
outer 3s electron transitions to the 3p level. During this process, only the
absorption of the photon occurs; the electron transitions back to 3s
without emission.
External
energy level

13. To obtain an atomic spectrum, it is necessary to atomize the
sample, i.e., convert it into atomic vapors. This is achieved
by either spraying its solution into a flame or evaporating the
dry residue of the solution in an electric furnace within the
temperature range of 2000-3000 °C.
In this temperature range, over 90% of the atoms are in an
unexcited state, and the surrounding atoms and molecules
cannot alter it, thus cannot influence the magnitude of atomic
absorption. This, along with the low number of absorption
lines, accounts for the high selectivity of the Atomic
Absorption Spectrometry (AAS) method.

14. The scheme of an Atomic Absorption Spectrometer (AAS)

15. A specific type of lamp used in atomic absorption spectrometry as a radiation
source with a line (narrow) spectrum.
These lamps are glass or quartz vessels filled with an inert gas (Ar or Ne) at
low pressure (1-5 mm Hg). Inside the vessel, there are two electrodes - a
cathode and an anode. The cathode has a concave shape and is made of pure
metal. Applying voltage to the electrodes generates a glow discharge with the
formation of positively charged ions of the buffer gas.
Ions of the buffer gas bombard the surface of the cathode, ejecting metal atoms
into the gaseous phase. Through collisions with other atoms, they transition to
an excited state. During relaxation, the process of light emission occurs with a
wavelength characteristic of the corresponding electron transitions of the metal
atom. Thus, the emission spectrum of a hollow cathode lamp is the atomic
spectrum of the cathode material, which includes lines emitted by the excited
atoms of the filling gas.
Hollow cathode lamp

16. The main drawback of such lamps is the ability to detect only one element.
Cathode
Anode
Getter spot
Pyrex
envelope
Electrical
contacts

17. Pyrex* shell (Пирексная оболочка)
Cathode (Катод)
Gas absorber (Газопоглотитель)
Noble gas (Благородный газ)
Anode (Анод)
Contacts (Контакты)
*Pirex – heat-resistant borosilicate glass.

18. Non-electrode lamps
Inside the casing, a strong alternating electromagnetic field is created
using a coil through which a high-frequency current passes. A quartz
cuvette containing approximately 10 mg of a volatile compound (the
element to be determined) is placed within the coil. The lamp is filled
with inert gas at a pressure of 2-3 mm Hg. When the coil is activated,
the high-frequency field ionizes the inert gas (Ar). Argon ions,
accelerated by the electromagnetic field, atomize the volatile
compounds and excite the atoms of the specified element. The emission
resulting from the return of these atoms to the ground state is emitted
from the lamp through a window transparent to this radiation and is
focused on the atomizer by an optical system. This radiation is partially
absorbed by the atoms of the element present in the analyzed sample.

19. Currently, non-electrode gas discharge lamps have been developed
for almost all elements, but the best characteristics in terms of
operational stability and intensity of emitted radiation are found in
lamps designed for volatile elements (Cs, Rb, Hg, P, Te). The
sensitivity for detecting certain elements (P, As, Se, Sb) is 1.5-3.0
times higher than when using hollow cathode lamps.
Advantages of non-electrode
lamps:
• Long-lived;
• Homogeneous beam;
• Easy to use;
• For several elements, the best
detection limits.
Drawback: The requirement for a
high-frequency generator (large
size, high cost), and these lamps
only start providing a stable
emission after 30-40 minutes of
being turned on (long warm-up
time).

21. The Walsh Conditions
To measure the magnitude of atomic absorption (A), two conditions
formulated by Walsh must be met:
1. The wavelength corresponding to the maximum absorption of atomic
vapors (A) must be equal to the wavelength of the maximum intensity of the
source's radiation (E):
𝝀𝑬𝒎𝒂𝒙 = 𝝀𝑨𝒎𝒂𝒙
2. The half-width of the absorption line of atomic vapors (δA) must be at
least twice the half-width of the emission line of the source (δE):
𝜹𝑨 ≥ 𝟐𝜹𝑬

22. 1. 𝝀𝑬𝒎𝒂𝒙 = 𝝀𝑨𝒎𝒂𝒙
𝟐. 𝜹𝑨 ≥ 𝟐𝜹𝑬
1. 𝝀𝑬𝒎𝒂𝒙 = 𝝀𝑨𝒎𝒂𝒙

23. If the first condition is not met, atomic absorption does not occur at
all. If the second condition is not met, only a small portion of the
source radiation is absorbed by the atoms because the emission line
contour is broader than the absorption line contour. This leads to a
significant deterioration in the sensitivity of atomic absorption
determination.
If the half-width of the atomic absorption line is around 0.01 nm,
then the half-width of the corresponding emission line should not
exceed 0.005 nm.
Typically, absorption bands are broader than emission bands due to
natural reasons. The natural broadening of spectral lines, caused by
the Heisenberg uncertainty principle, is on the order of 10-5 nm.
Additional broadening is caused by the Doppler effect (2nd
order). The line width also depends on the pressure in the atomizer
(Lorentzian broadening) – collisional broadening (due to atomic
collisions – 2-3 orders of magnitude larger than the natural width).

24. Sample preparation
Sample preparation involves the process of dry or wet
mineralization.
1. Dry mineralization involves heating the sample in air to a
temperature of 450-550 °C in a muffle furnace. Ashing
occurs due to oxidation by oxygen. The ash is then
transferred into a solution using acids (nitric).
2. Wet mineralization entails using reagents (mixtures of
strong inorganic acids), often under microwave irradiation
conditions. It is faster, prevents the loss of certain sample
components, but requires exceptionally pure reagents.

25. Wet mineralization
More often, mixtures are used for wet mineralization, such as:
• HNO3 – HCl – H2SO4;
• HNO3 – HClO4;
• H2SO4 – HClO4;
• HNO3 – H2O2.
The temperature regime is 250-400 °C in a muffle or
microwave furnace. The wet mineralization time is 4-12 hours,
compared to 3-40 hours for dry mineralization.

26. Atomizer
Atomization is the process that converts
a liquid sample into free atoms.
The diagram shows the different steps
that occur during atomization, starting with
the element being prepared as a solution.
Element M undergoes different stages:
• Solution: MAliquid (compound)
• Nebulization: MAliquid (compound)
• Desolvation: MAsolid (A = solution anion)
• Vaporization: MAgas
• Atomization: M0
• Excitation: M*
• Ionization: M+

27. In the flame, processes of ionization/recombination and association/
compete with each other.
Ions Atoms Molecules
Recombination Association
Ionization Dissociation
Atomization methods
1. Atomization in flame;
2. Electrothermal atomization;
3. Preparation of volatile hydrides;
4. Cold steam method

28. Atomic Absorption Spectroscopy
Flame AAS Atomizer
• In flame AAS (FAAS) the sample is prepared
as a liquid and nebulized into the flame.
• The fundamental characteristic of this
technique is the atomization that happens in
the flame.
Schematic diagram of flame or graphite furnace atomic
absorption spectrometer system
Flame AAS
Advantages
•Short analysis time possible
•Good precision
•Easy to use
•Cheap
Limitations
•Sensitivity
•Dynamic range
•Requires flammable gases
•Unattended operation is not
possible
because of flammable gases
•Must not contain excessive
amounts of dissolved solids

29. Flame Atomization
The atomizer consists of a burner where the flame takes the form of a
narrow elongated slit. Such a flame shape allows for a thicker absorbing
layer to be obtained.
The sample is prepared in liquid form and sprayed into the flame.
Combustible gas mixtures of various compositions are used to create the
flame, for example, acetylene-air, acetylene-oxygen, propane-air, etc.
Typically, hydrocarbons with various oxidizers (air, oxygen, N2O, etc.)
are used for flame combustion.
The sample is introduced into the flame in the form of an aerosol
generated by a pneumatic nebulizer. The flame should ideally have low
emission in the range of 190-850 nm.

30. Electrothermal Atomization
In 1959, Boris Lvov proposed the use of a graphite tube furnace in
atomic absorption spectroscopy. In the modern version of the graphite
furnace, the sample is evaporated and simultaneously atomized in a
pulsed mode.
Around the graphite tube, an inert gas such as Ar or N2 is continuously
passed at a constant rate. Additionally, an inert gas is passed into the
internal space of the tube and exits through the sample hole. It protects
the heated parts of the graphite furnace from atmospheric oxygen and
facilitates the removal of the evaporated and atomized sample.
To heat the graphite furnace, large currents are required, so a cooling
system for the graphite contacts, known as contact coolers, is necessary.

31. The temperature program typically goes through the following stages:
1. Stage 1 - evaporation of the solvent (vaporization);
2. Stage 2 - ashing, combustion of the sample, removal of matrix
components;
3. Stage 3 - atomization (at the temperature of decomposition of the
compound of the determined element);
4. Stage 4 - intermediate;
5. Stage 5 - annealing to eliminate "memory" effects.
After annealing, the analyzed sample is dosed only after the furnace has
cooled down to 45-500 °C. As evident, this type of atomization requires
much more time compared to flame atomization.

32. In atomic absorption spectroscopy, atomization in a graphite furnace
occurs in three stages:
1. Drying
2. Ashing
3. Atomization
The method employing a graphite furnace is supplementary to traditional
flame AAS and introduces some advantages to the analysis.
A sample in the volume of 5-50 (sometimes up to 1000 µL) is introduced
using a micropipette or automatically (autosampler).
Hf, Nb, Ta, W, and Zr, which form refractory carbides with carbon,
cannot be determined by AAS using this type of atomization.

33. Graphite furnace
Advantages
•High sensitivity due to
−entire sample is atomized at one time
−free atoms remain in the optical path longer
•Reduced sample volume
•Ultra trace analysis possible
•Can run unattended, even overnight
Limitations
•Very slow
•Fewer elements can be analyzed
•Poorer precision
•More chemical interferences (compared to flame AA)
•Method development requires skill
•Standard additions calibration required more frequently (compared to flame AA)
•Expensive consumables (graphite tubes)

34. Graphite Furnace AAS Atomizer
The graphite tube sits in
this apparatus which
supplies an inert gas and a
powerful to heat the tube,
which then desolvates and
atomizes the sample.

35. Atomic Absorption Spectroscopy
Elemental Coverage in AAS
H Flame Only He
Li Be Flame & Furnace B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn SB Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra Ac
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu AM Cm Bk Cf Es Fm Mo No Lr

36. Other Atomizers
• Hydride generation technique
Suitable for elements forming volatile
hydrides (As, Sn, Bi, Sb, Te, Ge and Se) when
reacted with a reducing agent, such as sodium
borohydride.
Advantages
• Separation of specific elements as hydrides which can
eliminate matrix interference
• Good sensitivity due to 100% sampling efficiency
• Good precision
• Faster than graphite furnace AA
Limitations
• Limited to specific elements
• Some chemical interferences
• Requires specific sample preparation (analyte
must be converted to a specific oxidation state)
• Cold vapor technique
Used specifically for mercury (has a large
enough vapor pressure at ambient tempera-
ture) which can be reduced to atomic state
by a strong reducing agent, such as sodium
borohydride, tin (II) chloride).
Advantages
• Eliminates many matrix interferences
• Good sensitivity due to 100% sampling efficiency
• Good precision
• Faster than graphite furnace AA
Limitations
• Limited to mercury only
• Mercury must be stabilized in solution

37. Detectors
1. Photomultiplier tubes (PMT)
2. Semiconductors (SSD) and diodes.
The presence of other elements in the sample complicates the
identification of the target component.
The overall scattering of light also reduces the sensitivity of
the method.
To eliminate matrix effects and overall scattering, various
correction methods are required: Deuterium, Zeeman, Smith,
Hiftju.

38. Features of AAS (Atomic Absorption Spectroscopy)
Method:
1. Relatively long sample preparation.
2.Translation of the analyte into solution is required.
3. High sensitivity (1-100 µg/L - flame, 1-100 ng/L - graphite furnace).
4. High selectivity.
5. Interfering influences are well studied.
6. It is not possible to determine multiple elements in one run (usually
one element per run).

39. Interferences in AAS
• Chemical Interferences
Any formation of a compound hindering the quantitative atomization of the
determined element is considered a chemical interference.
• Physical Interferences
The concept of "physical interferences" encompasses all interferences
affecting the total number of formed atoms based on different physical
properties of the sample solution (density, viscosity, and surface tension). The
cause of these interferences is almost always the pneumatic nebulizer of the
atomizer.
• Ionization Interferences
Many metals, especially in a hot flame, undergo ionization to varying
degrees.