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240 5 Comprehensive Insights into Atomic Spectroscopy
7. Explain the concept of spectral interference in atomic spectroscopy.
8. What is the primary function of internal standards in atomic spectroscopy, and
how do they work?
9. How does miniaturized and portable atomic spectrometry benefit various
industries?
10. What emerging technology is expected to enhance the customization of instrument components in atomic spectroscopy, and how can it impact the field?
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analytical science. 3rd ed. Oxford: Academic Press; 2019. p. 137–43.
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Pharmaceutical analysis E-book: a textbook for pharmacy students and pharmaceutical
Comprehensive Insights into Atomic Absorption Spectroscopy
Abstract
Atomic absorption spectroscopy (AAS) is a fundamental analytical technique that
plays a pivotal role in elemental analysis across diverse fields. AAS relies on the
principle of measuring the absorption of specific wavelengths of light by free
metal atoms, offering exceptional sensitivity and selectivity. In this method, a
hollow cathode lamp is employed to generate free metal atoms within a flame or
graphite furnace. Subsequent analysis of the absorbed light enables the quantifi-
cation of targe t elements down to parts per billion levels. AAS is widely
employed in pharmaceutical analysis for drug purity assessment, quality control,
and elemental impurity testing. It finds applications in clinical and biological
studies to monitor metal levels in biological samples, pharmacokinetics, and
toxicology investigations. Additionally, AAS is pivotal in environmental analy-
sis, geological exploration, food and beverage safety, and materials science.
Recent innovations include miniaturized and portable AAS instruments,
hyphenated techniques, and advancements in laser-induced breakdown spectros-
copy (LIBS), expanding the scope of this technique. As AAS continues to evolve,
it remains an indispensable tool in analytical chemistry, aiding in vital research,
quality control, and safety assessments.
6
Keywords
Atomic absorption spectroscopy · Instrumentation · Analytical techniques ·
Calibration methods · Background correction · Trace element analysis
241
242 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.1 Introduction
Atomic absorption spectroscopy (AAS) is a widely employed technique for elemental analysis. It operates on the principle of energy absorption by atoms in their
ground state in the gaseous phase. The amount of light absorbed by these atoms in
the ground state is used to determine the concentration of a specific metal in a
sample.
In 1952, Australian scientist Alan Walsh, while gardening at his Melbourne
home, conceived the idea of examining the ligh t absorbed by atoms rather than the
light they emit. This revelation led to the development of AAS, which not only has
the potential to save lives but also demonstrated that atoms selectively absorb light
with precise energy levels required to excite their electrons.
AAS involves the absorption of radiation by non-excited atoms in vaporized form
and is employed for quantifying various metals (e.g., Cu, Fe, Zn, Mg) in diverse
matrices such as soil, blood, urine, air, water, and food. It primarily utilizes the
visible region of the electromagnetic spectrum to identify the presence of metals in a
sample. The primary energy sources in AAS are hollow cathode lamps and
electrodeless discharge lamps. AAS is particularly useful for trace metal analysis,
regardless of the molecular form of the metal in the sample. It can determine the total
metal content in a sample, whether the metal is present as a salt (e.g., chloride or
sulfate) or in other chemical forms.
6.2 Principle
When a sample containing metallic species is introduced into a flame, it vaporizes
the metallic elements. This process occurs because atoms of a particular element, in
their ground state, selectively absorb light radiation at a specific wavelength. This
absorption of light with the precise wavelength required by the metal causes the
electrons in the sample to transition from a lower energy level (ground state) to a
higher energy level (excited state), as illustrated in Fig.
In AAS, two critical processes occur:
Fig. 6.1 Schematic representation of atomic excitation via light absorption. The diagram illustrates
the excitation process of an atom when it absorbs light energy. Initially, the atom exists in its ground
state. Upon absorption of light energy at a specific wavelength, the atom transitions to an excited
state. This fundamental principle underlies various atomic absorption spectroscopy techniques,
where the extent of absorption is directly related to the concentration of elements in the sample
6.1.
6.3 Components of AAS 243
• When the metal sample absorbs light energy, it generates free atoms without
ionization.
• These free atoms, once produced, further absorb radiation from an external
source.
This sequence of events begins with the absorption of radiation, causing the
atoms in the sample to transition from their ground state (with low energy levels) to
an excited state (with higher energy levels). The amount of light absorbed by a
specific element in the sample is directly related to the density of atoms present in the
flame. AAS quantifies the absorbed light, facilitating the estimation of the metal
concentration in the sample. The total amount of light at a specific wavelength
absorbed by the sample solution is calculated using the equation:
2
Total light absorbed by the metal in the sample = π e
=mc Nf
Where:
e = charge of the electron
c = speed of light
N = total number of atoms
f = oscillator strength
6.3 Components of AAS
A schematic diagram of AAS has been shown in Fig. 6.2. The components of AAS
include:
Fig. 6.2 Schematic representation of components of AAS: This diagram illustrates the key
components involved in AAS. The radiation source emits light, which is focused using lenses
and directed into the atomizer, where the sample is introduced and atoms are generated. The light
interacts with these atoms, and the resulting spectrum passes through a monochromator to isolate
specific wavelengths. A detector measures the absorption, and the signal is amplified and processed
for analysis, allowing for quantification of the element present in the sample
244 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.3.1 Radiation Source
In AAS, radiation sources are crucial for providing the light required to excite atoms
in the sample. This provides the primary source of energy. Common sources include
hollow cathode lamps or electrodeless discharge lamps, which emit light at specific
wavelengths corresponding to the elements of interest.
• Hollow cathode lamp: In a hollow cathode lamp (HCL), a hollow cup serves as
the cathode, holding the sample of interest. The anode, made of tungsten wire,
separates them in an inert gas atmosphere. The lamp’s window, typically quartz,
silica, or glass, permits observation. Applying voltage initiates ionization of the
gas, vaporizing metal atoms in the cathode, generating the metal’s spectrum.
Lamp pressure should be 1–5 torr. The emitted spectral lines identify the cathode
metal, for example, a copper cathode lamp produces a copper spectrum (Fig.
– Principle: HCL operates by passing an electric current through a sealed glass
tube filled with an inert gas (e.g., neon or argon) and a metal cathode made
from the element being analyzed. The current ionizes the gas, causing
collisions that sputter atoms from the metal cathode. These metal atoms are
excited and emit light at specific wavelengths characteristic of the element.
– Advantages: HCLs provi
de highl
y specific and stable emission lines, ensuring
accurate measurements for individual elements.
– Applications:
Widely used
for single-element analysis in AAS due to their
precision in generating element-specific wavelengths.
6.3).
Fig. 6.3 Schematic representation of a hollow cathode lamp: This schematic illustrates the basic
structure of a hollow cathode lamp, commonly used in AAS for element-specific analysis. The lamp
contains a glass shield and a quartz or Pyrex window, providing isolation and transparency for light
transmission. Inside the lamp, a hollow cathode, coated with the element to be analyzed, is
positioned opposite the anode. A noble gas, typically neon (Ne) or argon (Ar), fills the lamp at a
pressure of 1–5 torr. The electrical discharge between the anode and cathode ionizes the gas,
exciting the coated material, which then emits characteristic spectral lines that are used in analytical
measurements
6.3 Components of AAS 245
Fig. 6.4 Schematic representation of an electrodeless discharge lamp (EDL): This schematic
depicts the structure of an EDL, used AAS for enhanced elemental analysis. The lamp is encased
in a ceramic holder, with a quartz window at the front to allow emission of light. The radio
frequency coil surrounds the lamp, generating the electromagnetic field needed to ionize the gas
inside. This ionization excites the atoms of the element being analyzed, which then emit characteristic spectral lines through the quartz window for detection. The absence of electrodes prolongs the
lamp’s operational life and improves the stability of the emitted light
– Limitations: The y are element-specific, so a separate HCL is needed for each
element being analyzed, and intensity may decline with long-term use.
• Electrodeless discharge lamp: A limitation of HCL is the difficulty in
constructing it using volatile elements like germanium and arsenic. An alterna-
tive, the electrode discharge lamp (EDL), addresses this issue. The EDL is an
evacuated tube (see Fig.
6.4) in which the metal of interest is sealed. The tube
contains low-pressure argon and is sealed. Placing this sealed tube in a microwave
discharge cavity transforms the argon into a plasma state, exciting the sealed
metal and producing its spectrum.
– Principle: EDLs consist of a quartz tube containing the element of interest in a
gaseous form. A radiofrequency (RF) or microwave field is applied to excite
the atoms without using electrodes. This method results in the emission of
characteristic wavelengths of light.
– Advantages: EDLs produce more intense radiation than HCLs, maki ng them
suitable for elements that are difficult to detect due to low sensitivity, such as
arsenic, selenium, and antimony.
– Applications:
Often used
when higher sensitivity is needed or when HCLs do
not provide adequate light intensity for certain elements.
– Limitations: More
complex and expensive compared to HCLs, and not as
widely available for all elements.
246 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.3.2 Chopper
The chopper , resembling a rotating wheel, is positioned between the radiation source
and the flame (Fig.
6.5). Its purpose is to convert the steady light from the lamp into
pulsating light, generating a pulsating current in the photocell. The light emitted by
the flame results in a steady current. While both a steady and pulsating current are
present, only the pulsating current is amplified and recorded by the readout device.
6.3.3 Atomizers
Atomizers facilitate atomization, which is the process of separating molecules in a
sample into individual atoms, particularly those of the metal whose concentration is
being determined. This occurs by subjecting the sample, in a flame at high
temperatures, to generate free atoms of the metal. There are two types of atomizers
in AAS.
6.3.3.1 Flame Atomizers
This method commonly employs a flame to convert a liquid sample containing the
metal of unknown or determinable concentration into a gaseous state. The flame also
serves to transform the metal from its molecular form into its atomic form, which
exists in a vapor state. Flame atomizers typically utilize two types of burners,
detailed in the following subsections:
Fig. 6.5 Schematic representation of the working of a chopper in AAS: This schematic
demonstrates the working principle of a rotating chopper in AAS. The analyte-specific hollow
cathode lamp and the deuterium lamp provide light sources that are alternately interrupted by the
rotating chopper. The chopper modulates the light beams from both sources, allowing them to pass
sequentially toward the electrothermal chopper. After modulation, the light beams are directed to a
monochromator, which isolates the specific wavelengths needed for absorption measurements. This
setup enables background correction by comparing signals from the analyte hollow cathode lamp
and the deuterium lamp, enhancing the accuracy of trace element detection
6.3 Components of AAS 247
Fig. 6.6 Schematic representation of a total combustion burner and its combustion zones: (a)
Schematic representation of a total combustion burner illustrating the flow of fuel, oxidant, and
sample solution. The fuel and oxidant combine to create the flame, where combustion takes place,
while the sample solution is introduced and dispersed in the flame for analysis. (b) Diagram
showing the key regions of the burner flame: the primary combustion zone where the initial fueloxidant mixture ignites, and the secondary combustion zone where further reactions occur. The
interzonal region is identified as an area where chemical species interact, critical for sample
atomization and emission in analytical applications
• Total consumption burner: In this flame atomizer type, the sample solution with
the metal of unknown concentration, oxidizing gases, and fuel travel through
distinct passages, converging at the flame’s base (Fig.
6.6a). An intense, hot flame
is generated using a mixture of hydrogen or acetylene with oxygen. The liquid
sample is atomized by the flame, forming droplets that subsequently evaporate
and combust. This combustion leaves behind residues, reducing them into atoms
of the metal with unknown concentration from the sample solution. Following are
the three main regions of the flame in total combustion burner (Fig.
6.6b).
• Primary combustion zone: This region, locat ed at the flame’s tip or opening, lacks
thermal equilibrium and is rarely used for spectroscopic analysis due to its
instability.
• Interzonal combustion
region: Found between the primary and secondary com-
bustion zones, this narrow region is rich in free metal atoms. It is the hottest part
of the flame and is extensively employed for spectroscopic analysis of metals.
248 6 Comprehensive Insights into Atomic Absorption Spectroscopy
• Secondary combustion zone: In this region, metal species transform into stable
molecular oxides, dispersing into the flame’s surroundings. This area is infrequently used for spectroscopic analysis of metals.
6.3.3.2 Premixed Burner
Fuel gas and oxidants are aspirated into a large pressurized chamber. Fine droplets of
the premixed fuel, oxidants, and larger drops of the sample, containing the metal of
unknown concentration, are collected at the chamber’s outlet and introduced into the
flame. Excess material is subsequently drained.
6.3.4 Nebulization
Nebulization involves transforming a liquid sample into fine droplets before entering
the burner. It is commonly achieved using high-velocity gas, known as pneumatic
nebulization. This method, illustrated in Fig.
6.7, is for forming small, fine droplets.
6.3.5 Monochromators
Monochromator, a critical component of AAS, separates numerous spectral lines
into individual ones. Inadequate monochromators can severely compromise the
detection limit for spectral lines of metallic species, impacting AAS efficiency.
Monochromators select light of a specific wavelength for the metal of interest. The
sample with the metal of unknown concentration absorbs this specific wavelength,
Fig. 6.7 Schematic representation of nebulization and sample atomization in AAS: Schematic
representation of the nebulization phenomenon in atomic absorption spectroscopy. The solution
sample is converted into a fine aerosol spray by a nebulizer. Following this, desolvation occurs
where the solvent evaporates, resulting in dry aerosol particles. These particles undergo volatilization in the flame, producing free atoms, molecules, and ions. The flame facilitates the transformation
of the sample into these species, which are essential for the subsequent absorption of light and
spectrometric analysis
6.4 Working of AAS 249
excluding others. This allows the detection of the target metal’s concentration even
in the presence of other elements.
6.3.6 Detectors
Detectors employed in AAS include:
• Photomultiplier tubes (PMTs): PMTs are sensitive devices that amplify the signal
produced when atoms absorb light. They are commonly used in AAS for their
excellent sensitivity and wide spectral range.
• Photodiodes: These solid-state detectors are efficient for measuring light
at specific wavelengths. They are compact and have fast response times.
tion
• Charge-c oupled devices (CCDs): CCD detectors are versatile and can capture an
entire spectrum simultaneously. They are particularly useful for multi-element
analysis.
• Flame ionization detectors (FIDs): Typically used in flame AAS, FIDs detect
changes in the flame’s ionization due to the absorption of light by metal atoms.
• Solid-state detectors: These detectors include
nium and are suited for specific wavelength ranges.
materials
like silicon and germa-
absorp-
6.3.7 Amplifier
The amplifier amplifies the signal from the detector, making it more suitable for
further processing.
6.3.8 Readout Device
In AAS, the readout device is the component responsible for displaying or recording
the results of the analysis. It provides a way to interpret the information obtained
from the detector.
6.4 Working of AAS
Before using a spectrometer, calibration with a standard solution of known solute
concentration is essential. Cuvettes are filled with this standard solution and placed
in the spectrometer’s sample holder. Specific wavelength light passes through a
series of prism, diffraction grati ng, and mirrors. The prism separates light into
different wavelengths, the mirror guides the light, and the diffraction grating selects
the required wavelength directed at the cuvette. Reflected light is analyzed and
compared to predetermined standards. Some light is absorbed by the solution, and
the rest reaches the detector, which converts it into electrical signals. These signals