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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5647_Библиотеки_им_академика_М_И_Перельмана.pdf
210 5 Comprehensive Insights into Atomic Spectroscopy
Keywords
Atomic spectroscopy · Elemental analysis · Atomic absorption spectroscopy ·
Atomic emission spectroscopy · Inductively coupled plasma · Spectral line
broadening
5.1 Introduction
Atomic spectroscopy is a branch of analytical chemistry that focuses on studying the
interaction between light and matter at the atomic level. It is a powerful analytical
technique used to determine the elemental composition of various samples, from
simple solutions to complex materials. Atomic spectroscopy plays a crucial role in
fields such as environmental monitoring, material characterization, pharmaceutical
analysis, and many other areas where precise elemental analysis is required.
In an atom, electrons are arranged in energy levels, organized into subshells
within those energy levels, and further grouped into shells that encircle the nucleus.
Electrons closer to the nucleus occupy lower energy levels, referred to as the ground
state, while those farther away from the nucleus are in higher energy levels, known
as the excited state. Electrons experience a stronger attraction to the nucleus when
they are in closer proximity. The ground state represents the lowest possible energy
configuration of an atom, making it the most stable state. Conversely, the excited
state occurs when an atom’s electrons absorb sufficient energy to transition to higher
energy levels, as illustrated in Fig.
lowest energy level, rendering the atom less stable. Under normal circumstances,
atoms predominantly exist in their ground state. However, when an external source
provides the necessary energy to promote electrons to higher energy levels, the atom
transitions to the excited state, as depicted in Fig.
unstable and tends to spontaneously return to the ground state. This transition from
the excited state to the ground state is accompanied by the release of the absorbed
energy, which is emitted in the form of photons of light.
5.1. In the excited state, electrons are not at their
5.1. The excited state is inherently
5.2 Principle
The principle of atomic spect roscopy is based on the interaction between electromagnetic radiation (light) and atoms. This interaction provides valuable information
about the elemental composition and concentration of a sample. The common
underlying principle include:
5.2.1 Energy Levels and Transitions
At the heart of atomic spectroscopy is the concept that electrons in atoms are
arranged in discrete energy levels or shells. These electrons can move between
5.2 Principle 211
Fig. 5.1 Schematic representation of absorption and excitation of electrons in atomic spectroscopy: The figure illustrates the process of energy absorption and electron excitation in an atom
during atomic spectroscopy. In the ground state, the electron occupies the lowest available energy
level. Upon absorbing light energy (represented by the wavy lines), the electron transitions to a
higher energy level, entering an excited state. This excited state is unstable, and the electron
eventually releases the absorbed energy in the form of photons as it returns to its original ground
state. The lower diagram visually represents the electron’s transition from the ground state to the
excited state, showing the movement of an electron between energy levels (orbitals) after the
absorption of energy
energy levels by absorbing or emitting quantized amounts of energy in the form of
photons (light). When electrons transition from lower to higher energy levels, energy
is absorbed, and when they transition from higher to lower energy levels, energy is
emitted.
5.2.2 Ground State and Excited State
Electrons typically reside in the lowest energy level, known as the ground state.
When atoms or ions are exposed to energy in the form of light or heat, some of the
electrons can be elevated to higher energy levels, resulting in an excited state. The
excited state is less stable, and electrons tend to return to the ground state by emitting
energy.
212 5 Comprehensive Insights into Atomic Spectroscopy
5.2.3 Wavelengths and Spectral Lines
Each element has a unique set of energy levels and, consequently, characteristic
wavelengths of light associated with the transitions of its electrons. These
wavelengths correspond to specific spectral lines, which can be identified and
measured. The arrangement of spectral lines in the electromagnetic spectrum is
called the atomic spectrum, and it serves as a “fingerprint” for each element.
5.2.4 Doppler Broadening
Doppler broadening is a critical phenomenon in atomic spectroscopy that contributes
to the broadening of spectral lines. It occurs due to the relative motion between the
emitting or absorbing atoms and the observer, resulting in a shift in the frequency
(or wavelength) of the light.
In atomic systems, atoms are constantly in motion, either in random thermal
motion or due to external forces. When an atom moves toward the detector
(observer), the wavelength of the light it emits or absorbs is slightly shortened
(blue-shifted). Conversely, when the atom moves away from the observer, the
wavelength is lengthened (red-shifted). This effect is analogous to the Doppler effect
in sound waves, where the frequency of a sound changes with the relative motion of
the source and observer.
The extent of Doppler broadening depends on the temperature of the sample
because temperature influences atomic velocities. Higher temperatures increase
atomic motion, leading to a greater spread of velocities and, consequently, broader
spectral lines. Doppler broadening is described mathematically by the Doppler
profile, a Gaussian distribution that relates the broadening to the atom’s velocity
distribution.
Understanding Doppler broadening is essential in atomic spectroscopy, as it helps
explain how the natural sharpness of spectral lines is altered by the thermal motion of
atoms, impacting the accuracy and resolution of spectroscopic measurements. It is
one of several factors that contribute to the overall spectral line broadening.
5.3 Types of Atomic Spectroscopy
There are several types of atomic spectroscopy techniques, each designed for
specific applications and analytical requirements. The key types of atomic
spectroscopy are:
5.3.1 Atomic Absorption Spectrometry (AAS)
AAS measures the absorption of specific wavelengths of light by ground-state
atoms. The degree of absorption is directly proportional to the concentration of the
5.3 Types of Atomic Spectroscopy 213
analyte in the sample. High-resolution continuum source atomic absorption spectrometry (HR-CS AAS) is an advanced varia nt of AAS. While it shares some
fundamental principles with traditional AAS, HR-CS AAS introduces innovative
features that enhance its analytical capabilities. Here is an overview of HR-CS AAS
and its role within the types of atomic spectroscopy.
5.3.2 Atomic Emission Spectrometry (AES)
AES measures the emissio n of characteristic light by excited atoms when they
transition from higher to lower energy levels. This emitted light is used to identify
and quantify elements in a sample.
5.3.3 Atomic Fluorescence Spectrometry (AFS)
AFS measures the fluorescence emitted by atoms or ions in the analyte, typically
after they have absorbed energy from a primary light source.
5.3.4 Inductively Coupled Plasma-Atomic Emission Spectrometry
(ICP-AES)
ICP-AES combines an inductively coupled plasma as a high-temperature excitation
source with atomic emission spectroscopy. It allows for simultaneous and sensitive
measurement of multiple elements.
5.3.5 Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
ICP-MS couples an inductively coupled plasma with mass spectrometry to determine the isotopic composition of elements, providing high sensitivity and precision.
5.3.6 X-Ray Fluorescence Spectrometry (XRF)
XRF measures characteristic X-rays emitted by atoms when excited by high-energy
X-rays, providing information about the elemental composition of a sample.
5.3.7 Laser-Induced Breakdown Spectroscopy (LIBS)
LIBS uses laser-induced plasma to generate a spark in a sample, followed by the
measurement of emitted light to determine elemental composition.
214 5 Comprehensive Insights into Atomic Spectroscopy
Each type of atomic spectroscopy has its unique strengths and applications,
making them valuable tools for elemental analysis in various scientific and indus trial
contexts. The choice of technique depends on factors such as the elements of interest,
the required sensitivity, and the nature of the sample being analyzed.
5.4 Atomizers Used in Atomic Spectroscopy
In atomic spectroscopy, atomizers are critical components used to convert a sample
(solid, liquid, or gas) into free atoms or ions in the gas phase, where they can interact
with light for absorption or emission measurements. Different types of atomizers are
used depending on the specific spectroscopic technique and the nature of the sample.
The primary types of atomizers in atomic spectroscopy include flame atomizers,
electrothermal (graphite furnace) atomizers, inductively coupled plasma (ICP)
atomizers, hydride generation atomizers, and cold vapor atomizers.
5.4.1 Flame Atomizer
• Technique used: Primarily used in AAS and AES.
• Principle : In flame atomization, a sample is nebulized into fine droplets and
mixed with fuel and an oxidant, then introduced into a flame. The high tempera-
ture of the flame vaporizes the sample, breaking it down into free atoms.
• Advantages: Simple, widely used, and cost-effective for a wide range of metals.
• Disadvantages:
the flame in a free atomic form.
• Common flame types: Air-acetylene and nitrous oxide-acetylene.
Limited
sensitivity, as only a small portion of
the samp
le reaches
5.4.2 Electrothermal (Graphite Furnace) Atomizer
• Technique used: Mainly used in AAS.
• Principle : In this atomizer, the sample is placed inside a small graphite tube or
furnace. The furnace is heated electrically in several stages (drying, ashing,
atomization) to progressively remove the solvent, decompose the sample matrix,
and finally produce free atoms in the gas phase.
• Advantages: High sensi
for trace metal analysis.
• Disadvantages: Slower analysis time compared to flame atomization, more
expensive, and requires more complex operation.
• Applications: Trace
tivity requires
element detection, environmental and biological samples.
a very small sample volume and is suitable
5.4 Atomizers Used in Atomic Spectroscopy 215
5.4.3 ICP Atomizer
• Technique used: ICP-AES and ICP-MS (mass spectrometry).
• Principle : An ICP atomizer uses a high-temperature plasma, typically generated
by inductively coupling an argon gas stream with a radiofrequency field. The
sample is introduced as an aerosol, and the plasma provides enough energy to
atomize and ionize the sample.
• Advantages: Extremely high temperature (~10,000 K) allows complete atomiza-
tion of most elements, excellent sensitivity, and multielement detect
capability.
• Disadvantages:
gas supply (argon) and more complex instrument setup.
• Applicatio
geological, and biological samples.
High cost of operation and maintenance, requires an expensive
ns: Widely
used for the detection of trace elements in environmental,
ion
5.4.4 Hydride Generation Atomizer
• Technique used: AAS and ICP-AES for hydride-forming elements (e.g., arsenic,
selenium, antimony).
• Principle : This technique involves the conversion of specific elements (e.g., As,
Sb, Se) into volatile hydrides by reacting the sample with a reducing agent like
sodium borohydride. The volatile hydrides are then transported into a heated
atomizer (flame or quartz tube), where they decompose into free atoms for
detection.
• Advantages: Increased sensitivity for specific elements that form volatile
hydrides, high detect ion limits for these elements.
• Disadvantages: Limited to specific elements, requires careful ha ndling of
reagents.
• Applicatio
ns: Environ
mental monitoring, water analysis, food safety
testing.
5.4.5 Cold Vapor Atomizer
• Technique used: Primarily for mercury detection in AAS.
• Principle : Mercury is unique because it can be easily reduced to its elemental
form at room temperature. In this method, mercury is reduced chemically to
elemental mercury vapor, which is then transported to an absorption cell for
atomic absorption measurement.
• Advantages: Very high
• Disadvantages:
• Applications: Environmental monitoring (especially water and air analysis), food
safety, industrial applications.
Only applicable
sensitivity
for mercury, simple and effective.
for mercury analysis.
216 5 Comprehensive Insights into Atomic Spectroscopy
5.4.6 Laser Ablation Atomizer
• Technique used: ICP-MS and AES.
• Principle : In laser ablation, a high-energy laser beam is focused on the solid
sample surface, vaporizing a small portion of the sample into an aerosol. This
aerosol is then carried into the ICP for atomization and ionization.
• Advantages: Direct solid sample analysis without complex preparation, suitable
for small and localized areas of a sample.
• Disadvantages:
• Applications: Geological and material science samples, solid-state analysis.
cost requires specialized equipment.
High
5.4.7 Glow Discharge Atomizer
• Technique used: GD-AES and GD-MS.
• Principle : A glow discharge atomizer ionizes a sample using a low-pressure
plasma created by applying a high voltage between two electrodes. The sample
is introduced as a solid, and atoms are sputtered off the sample surface and
ionized for analysis.
• Advantages: Direct analysis of solid samples, minimal sample preparatio
• Disadvantages:
• Applications: Metal coatings, thin films, industrial
Limited to conductive materials, compl
ex instrum
samples.
n.
entation.
5.5 Sample Preparation and Introduction in Atomic
Spectroscopy
In atomic spectroscopy, the accuracy and reliability of the analysis heavily depend
on proper sample preparation and introduction. These essential steps ensure that the
sample is in a suitable form for analysis and is effectively introduced into the
analytical instrument.
5.5.1 Sample Digestion
Sample digestion is the process of breaking down a solid or complex sample into a
form suitable for analysis. This step is crucial for liberating the target analytes and
removing potential interferents. Key points to consider include:
• Digestion methods: Various methods, such as acid digestion, microwave diges-
tion, and fusion, are used to dissolve or break down the sample. The choice of
method depends on the sample matrix a nd the nature of the analytes.
• Acid selection:
Commonly used acids include nitric acid, hydrochloric acid, sulfuric acid, and
perchloric acid.
The selection of appropriate acids and acid mixtures is essential.
5.5 Sample Preparation and Introduction in Atomic Spectroscopy 217
• Temperature and pressure: Digestion conditions, including temperature and
pressure, must be carefully controlled to ensure efficient analyte extraction and
sample decomposition.
5.5.2 Sample Nebulization
Sample nebulization involves converting a liquid sample into a fine aerosol or mist,
which can be introduced into the analytical instrument for further analysis. Key
considerations include:
• Nebulizer types: Various nebulizer types, such as pneumatic, ultrasonic, and
electrothermal nebulizers
ulizer depends on factors like sample type, sensitivity requirements, and the
instrument in use.
• Aerosol formation: Proper aerosol formation is essential for uniform and stable
sample introduction. Factors like nebulizer gas flow rates and sample flow rates
need to be optimized.
• Carrier gas: In some techniques, a carrier gas, such as argon, is used
the
aerosol from the nebulizer to the instrument. The choice of carrier gas can
influence the efficiency of the introduction.
used in atomic spectroscop y. The choice of neb-
, are
to transport
5.5.3 Sample Introduction Systems
Sample introduction systems are integral components of atomic spectroscopy
instruments. These systems are responsible for transferring the sample to the excitation source and ultimately to the detector. Key components include:
• Spray chambers: Spray chambers are used to further condition the aerosol,
ensuring that larger droplets are removed, and a fine aerosol is delivered to the
instrument.
• Nebulizer adapters: These components ensure a secure connection between the
nebulizer and the spray chamber.
• Mixing chambers: In techniques like ICP-AES and ICP-MS, mixing chambers are
used to introduce additional reagents, facilitating the ionization process.
• Sample introduction probes: Sample introduction probes or torches are essential
components that introduce the aerosol or vapor into the excitation source, whether
it is an ICP or a flame.
• Alignment a
introduction system are critical for achieving the best analytical results.
Effective samp
atomic spectroscopy analys es. The methods chosen should be tailored to the specific
requirements of the sample matrix and the analytical technique used. Proper
nd optim
ization: Proper alignment and optimization of the sample
le preparation and introduction are vital for accurate and precise
218 5 Comprehensive Insights into Atomic Spectroscopy
execution of these steps ensures that the sample is appropriately transformed for
analysis and that the instrument receives a representative and stable sample.
5.6 Data Analysis and Interpretation in Atomic Spectroscopy
In atomic spectroscopy, data analysis and interpretation are crucial steps in
extracting meaningful information from the collected spectra. This chapter delves
into the essential aspects of data analysis and interpretation, covering calibration and
standardization, quantification methods, qualitative analysis, and spectral
interpretation.
5.6.1 Calibration and Standardization
Calibration and standardization are vital processes in atomic spectroscopy that
establish a relationship between the instrument response (e.g., signal intensity) and
the concentration of the analyte. Key considerations include:
• Calibration curves: The construction of calibration curves involves analyzing
standard solutions with known analyte concentrations. The resulting data is used
to create a mathematical relationship that relates the instrument response to
analyte concentration.
• Linear range: Determining the linear range
ensure accurate quantification. This range should encompass the expected
concentrations in the sample.
• Internal standards: In some cases, internal standards are used to account for
variations in sample introduction and instrument conditions. Internal standards
are elements of known concentration added to the sample.
of
the calibration curve is important to
5.6.2 Quantification Methods
Quantification in atomic spectroscopy is a critical aspect, especially for determining
the concentration of analytes in a sample. Various quantification methods are
employed:
• Standard addition:
analyte to the sample, creating a calibration curve that accounts for matrix effects.
This is useful when sample matrices are complex.
• External calibration: External calibration uses a calibration curve constructed
from separate standard solutions. The instrument response of the sample is
compared to this curve for quantification.
Standard addition
involves adding known amounts of the
5.6 Data Analysis and Interpretation in Atomic Spectroscopy 219
• Isotope dilution: Isotope dilution is employed in mass spectrometry to determine
the concentration of an element. A known quantity of a stable isotope of the
element is added to the sample, and the isotope ratio is measured.
5.6.3 Qualitative Analysis
Qualitative analysis in atomic spectroscopy focuses on identifying the presence of
elements or speci fic spectral lines in a sample. Key points include:
• Element identification: Spectral lines or wavelengths are compared to reference
data
to identify the elements present in the sample.
• Pattern matching: Qualitative analysis may involve matching observed spectral
patterns with know n reference patterns for confirmation.
• Interference assessment: Qualitative analysis also includes assessing spectral
interfere
affect the accuracy of the analysis.
• Spectral interpretation: Spectral interpretation is an essential aspect of atomic
spectroscop
spectra:
– Line positions: Identifying the positions of spectral lines in the spectrum,
– Line intensity: Line intensities provide
– Spectral patterns: Recognizing spectral patterns and resolving closely spaced
– Peak integration: In some techniques, such as UV-VIS absorption spectros-
nces, which are spectral lines from other elements or sources that can
y that involves understanding the information conveyed by the
which
are specific to each element, is essential for both qualitative and
quantitative analysis.
information about the concentration of
the element in the sample, and they are used in quantitative analysis.
spect
ral lines is crucial for accurate analysis, especially when dealing with
complex samples or multielement determinations.
peak integration may be necessary for quantification, involving the
copy,
calculation of the area under the absorption curve.
Proper data
results obtained from atomic spectroscopy. Whether quantifying analytes,
identifying elements qualitatively, or interpreting complex spectra, a thorough
understanding of these principles is essential for successful analytical outcomes.
analysis and interpretation ensure the accuracy and reliability of
5.6.4 Sensitivity and Detection Limits
The sensitivity and determination of detection limits are critical aspects of atomic
spectroscopy, as they define the analytical instrument’s ability to detect and quantify
analytes accurately. Overall, sensitivity and detection limits in atomic spectroscopy
are pivotal for accurate and reliable analytical results. Factors influencing sensitivity,
the signal-to-noise ratio, and the deter mination of limits of detection and