Atomic Absorption Spectroscopy

Ahmad Aqel Ifseisi

Assistant Professor of Analytical Chemistry
College of Science, Department of Chemistry
King Saud University
P.O. Box 2455 Riyadh 11451 Saudi Arabia
Building: 05, Office: 2A/149 & AA/53
Tel. 014674198, Fax: 014675992
Web site: http://fac.ksu.edu.sa/aifseisi
E-mail: ahmad3qel@yahoo.com
aifseisi@ksu.edu.sa
Atomic spectroscopy
Is the study of the electromagnetic radiation absorbed and emitted by
atoms.

Since unique elements have characteristic (signature) spectra, atomic

spectroscopy, specifically the electromagnetic spectrum or mass
spectrum, is applied for determination of elemental compositions.

The science of atomic spectroscopy has yielded three techniques for

analytical use:
- Atomic Absorption.
- Atomic Emission.
- Atomic Fluorescence.
The Emission Spectra of H, He and Hg.
Atomic absorption spectroscopy (AAS) is a spectroanalytical
procedure for the qualitative and quantitative determination of chemical
elements using the absorption of optical radiation (light) by free atoms in
the gaseous state.

Typically, these methods can detect parts-per-million to parts-per-billion

amounts, and in some cases, even smaller concentrations. Atomic
spectroscopic methods are also rapid, convenient, and usually of high
selectivity.

Flame atomic absorption spectroscopy is currently the most widely used

of all the atomic methods because of its simplicity, effectiveness, and
relatively low cost.
The most important problem in atomic absorption methods was directly related to
problems created by the very narrow widths of atomic absorption lines. The
widths of atomic absorption lines are much less than the effective bandwidths of
most monochromators.

No ordinary monochromator is
capable of yielding a band of
radiation as narrow as the width of
an atomic absorption line (0.002 to
0.005 nm). As a result, the use of
radiation that has been isolated
from a continuum source by a
monochromator inevitably causes
instrumental departures from
Beer’s law.

The problem created by narrow absorption lines was surmounted by using

radiation from a source that emits not only a line of the same wavelength as the
one selected for absorption measurements but also one that is narrower. For
example, Hollow-cathode lamps.
AAS block diagram
Spectroscopic determination of atomic species can only be performed on
a gaseous medium in which the individual atoms or elementary ions, such
as Fe+, Mg+, or Al+, are well separated from one another.

Consequently, the first step in all atomic spectroscopic procedures is

atomization, a process in which a sample is volatilized and decomposed
in such a way as to produce gas-phase atoms and ions.

The efficiency and reproducibility of the atomization step can have a large
influence on the sensitivity, precision, and accuracy of the method. In
short, atomization is a critical step in atomic spectroscopy.
 Atomization Process

Processes leading to atoms, molecules, and ions with continuous sample

introduction into a plasma or flame. The solution sample is converted into a spray
by the nebulizer. The high temperature of the flame or plasma causes the solvent
to evaporate leaving dry aerosol particles. Further heating volatilizes the particles
producing atomic, molecular, and ionic species. These species are often in
equilibrium at least in localized regions.
Several methods that are used to atomize samples for atomic spectroscopy.
Inductively coupled plasmas, flames and electrothermal atomizers are the most
widely used atomization methods. Flames and electrothermal atomizers are found
in atomic absorption spectrometry, while the inductively coupled plasma is used in
optical emission and in atomic mass spectrometry.

Classification of Atomic Spectroscopic Methods

Atomization Method Typical Atomization Types of

Temperature, oC Spectroscopy
Flame 1700–3150 -Absorption
-Emission
-Fluorescence
Electrothermal 1200–3000 -Absorption
-Fluorescence
Inductively coupled 6000–8000 -Emission
plasma -Mass
Direct-current plasma 5000–10000 -Emission
Electric arc 3000–8000 -Emission
Electric spark Varies with time and position -Emission
Flame Atomizers

A laminar flow burner used in flame

atomic absorption spectroscopy
Properties of Flames

When a nebulized sample is carried into a

flame, the droplets are desolvated in the
primary combustion zone.

The resulting finely divided solid particles

are carried to a region in the center of the
flame called the inner cone. Here, in this
hottest part of the flame, the particles are
vaporized and converted to gaseous atoms,
elementary ions, and molecular species.

Finally, the atoms, molecules, and ions are

carried to the outer cone, before the
atomization products disperse into the
atmosphere.
Regions of a flame
Types of Flames Used in Atomic Spectroscopy

When the oxidant is air, temperatures are in the range of 1700 to 2400°C. At these
temperatures, only easily excitable species, such as the alkali and alkaline earth
metals, produce usable emission spectra. For heavy-metal species, which are not
so easily excited, oxygen or nitrous oxide must be used as the oxidant. These
oxidants produce temperatures of 2500 to 3100°C with common fuels.

Fuel Oxidant Temperature, oC

Natural gas Air 1700–1900
Natural gas O2 2700–2800
H2 Air 2000–2100
H2 O2 2500–2700
C2H2 Air 2100–2400
C2H2 O2 3050–3150
C2H2 N2O 2600–2800
Effects of Flame Temperature
Both emission and absorption spectra are affected by variations in flame
temperature. Higher temperatures increase the total atom population of the flame
and thus the sensitivity. With elements such as the alkali metals, however, this
increase in atom population is more than offset by the loss of atoms by ionization.

Flame temperature determines to a large extent the efficiency of atomization, which

is the fraction of the analyte that is desolvated, vaporized, and converted to free
atoms and/or ions.

The flame temperature also determines the relative number of excited and
unexcited atoms in a flame.
-For example, in an air/acetylene flame, the ratio of excited to unexcited
magnesium atoms is about 10-8, whereas in an oxygen/acetylene flame, which is
about 700°C hotter, this ratio is about 10-6 (100 times higher).
-For example, with a 2500°C flame, a temperature increase of 10°C causes the
number of sodium atoms in the excited 3p state to increase by about 3%.
Region of the Flame for Quantitative Measurements

The optimum region of a flame

used in a determination must
change from element to element
and that the position of the flame
with respect to the source must
be reproduced closely during
calibration and measurement.

Height profiles for three elements in AAS.

The plot shows absorbance versus height
above the burner for Mg, Ag, and Cr.
Instrumentation
The instrumentation for AA can be fairly simple.

single-beam AA spectrometer

Block diagram of a single-beam atomic absorption spectrometer. Radiation from a line source is focused on
the atomic vapor in a flame or electrothermal atomizer. The attenuated source radiation then enters a
monochromator that isolates the line of interest. Next, the radiant power from the source, attenuated by
absorption, is converted into an electrical signal by the photomultiplier tube (PMT). The signal is then
processed and directed to a computer system for output.
Line Sources
The most useful radiation source for atomic absorption spectroscopy is the hollow-
cathode lamp (HCL). It consists of a tungsten anode and a cylindrical cathode sealed in
a glass tube containing an inert gas, such as argon, at a pressure of 1 to 5 torr.
The cathode either is fabricated from the analyte metal or else serves as a support for a
coating of that metal. If a potential difference of about 300 V is applied across the
electrodes, the argon ionizes, and as the argon cations and electrons migrate to the two
electrodes, a current of 5 to 10 mA is generated.

The cations strike the

cathode with sufficient
energy to dislodge some of
the metal atoms and to
produce an atomic cloud.
This process is called
sputtering. Some of the
sputtered metal atoms are
in an excited state and
emit their characteristic
wavelengths as they return
Diagram of a hollow cathode lamp to the ground state.
Hollow-cathode lamps for about 70 elements are available from
commercial sources. For some elements, high-intensity lamps are
available that provide about an order of magnitude higher intensity than
normal lamps.

Because a different hollow-cathode lamp is required for each element, only

a single element can be determined at a time, and this is the major
drawback of atomic absorption.

However, some hollow cathode lamps have a cathode containing more

than one element and thus provide spectral lines for the determination of
several species.
 UV/Vis Spectra for Atoms

When a beam of UV or Vis radiation passes

through a medium containing atoms, only a
few frequencies are attenuated by absorption,
and when recorded, the spectrum consists of
a number of very narrow absorption lines (no
vibrational and rotational motions for atoms).

The energy of UV and Vis electromagnetic

radiation is sufficient to cause a change in an
atom’s valence electron configuration.

Sodium, e.g., with a valence shell electron

configuration of
1s22s22p63s1
has a single valence electron in its 3s atomic
orbital. Unoccupied, higher energy atomic
orbitals also exist. The Figure shows a partial
energy level diagram for sodium’s occupied
and unoccupied valence shell atomic orbitals. Valence shell energy diagram for sodium
The most obvious feature of this spectrum is that it consists of a few, discrete
absorption lines corresponding to transitions between the ground state (the 3s
atomic orbital) and the 3p and 4p atomic orbitals. Absorption from excited states,
such as that from the 3p atomic orbital to the 4s or 3d atomic orbital, which are
included in the energy level diagram in Figure, are too weak to detect.

Since the lifetime of an excited state

is short, typically 10–7–10–8 s, an
atom in the excited state is likely to
return to the ground state before it
has an opportunity to absorb a
photon.

Atomic absorption spectrum for sodium

Example

The energy difference between the 3p and the 3s

orbitals in Sodium is 2.107 eV. Calculate the
wavelength of radiation that would be absorbed in
exciting the 3s electron to the 3p state
(1 eV = 1.60 x 10-19J).

Solution

Partial energy level diagram for sodium,

showing the transitions resulting from
absorption at 590, 330, and 285 nm.