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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5405_Библиотеки_им_академика_М_И_Перельмана.pdf
260 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.9 Methods for Quantitative Analysis in AAS
Both methods are critical in atomic absorption spectroscopy, ensuring reliable and
accurate quantification of elements in diverse sample types:
6.9.1 Calibration Curve Method
The calibration curve method is a fundamental technique used in AAS to determine
the concentration of an element in a sample. Here is a more detailed look at how it
works:
• Preparation of standards: A series of standard solutions with known
concentrations
used to construct a calibration curve.
• Measurement of absorbance: The absorbance of each standard solution is
measured using the AAS instrument. The instrument is set to the wavelength
specific to the element being analyzed.
• Plotting the curve: A graph of absorbance versus concentration is plot
relationship
bance increases proportionally with the concentration.
• Sample analysis: The unknown sample’s absorbance is measured in the same
way. The concentration of the element in the sample is then determined by
comparing its absorbance to the calibration curve.
• Advantages: This method is simple and effective when matrix effects (interfer-
ence from other substances in the sample) are minimal. It is widely used for
routine analysis in fields like environmental monitoring, food safety, and clinical
diagnostics.
• Limitations: If the sample matrix differs significantly from the standards, the
curve may not provide accurate results. In such cases, the standard addition
method (explained next) is preferred.
is typically linear for low concentrations, meaning that the absor-
element to be analyzed a re prepared. These solutions are
of the
ted. The
6.9.2 Standard Addition Technique
The standard addition technique is used when matrix effects or interferences from
other components in the sample are suspected, which can distort the accuracy of a
calibration curve. Here is a more detailed explanation:
• Principle : In the standard addition method, known amounts of the analyte (the
element to be measured) are added directly to the sample, and the change in
absorbance is measured. The assumption is that any interference from the sample
matrix affects both the sample and the added standard equally, allowing for
correction.
• Procedure :
6.10 Interferences of AAS 261
– Step 1: The absorbance of the original sample (without any added standard) is
measured.
– Step 2: Known quantities of a standard solution of the analyte are then added to
the sample in increments. After each addition, the absorbance is measured.
– Step 3: A plot of absorbance versus the concentration of the added standard is
created. The line is then extrapolated to the x-axis, where the intercept
indicates the concentration of the analyte in the original sample.
• Advantages: This method compensates for mat rix effects, making
complex samples where interferences are likely, such as biological fluids or
environmental samples.
• Limitations: While effective for compensating matrix effects, the method is time-
consuming and requires the preparation of multiple solutions. It also requires
careful handling to avoid introducing additional variables that could skew the
results.
it ideal
for
6.9.3 Choosing Between the Two Methods
• Calibration curve: Preferred for simple samples with minimal matrix interfer-
ence. It is faster and more straightforward.
• Standard addition: Chosen when matrix effects are suspected or in complex
sample matrices. While more time-consuming, it offers greater accuracy in
these cases.
6.10 Interferences of AAS
Interferences in AAS refer to changes in light absorption caused by physical or
chemical factors. These effects are typically observed when analyzing elements in
aqueous solutions. The presence of elements other than the one of interest can lead to
the absorption of radiation at the specific wavelength used for the target element.
Commonly encountered interferences in AAS include:
6.10.1 Ionization Interference
Ionization interference in AAS results from ions in the sample affecting measurement accuracy. It can lead to inaccuracies, especially when ions disrupt the formation of required free atoms for precise absorption measurements. Ionization occurs
when sample atoms gain or lose electrons, forming ions. AAS aims to measure light
absorption by free ground-state atoms, but ionization can h inder this by introducing
ions that absorb less light.
262 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.10.2 Background Absorption of Source Radiation Interference
Incomplete atomization leads to background source radiation absorption by
particles. This can be resolved by raising the flame temperature.
6.10.3 Transport of Sample Interferences
Transport interference in AAS can result from factors affecting sample transportation and atomization, potentially causing measurement inaccuracies. These factors
include:
• Nebulization rate: The speed of converting the liquid sample into fine droplets, or
nebulization, can affect atomization efficiency, leading to measurement errors.
• Viscosity: Sample thickness influences flame atomization, with highly viscous
samples potentially reducing AAS accuracy.
• Density: Sample density affects atomization behavior, resulting in variations in
free atom formation and measurement precision.
• Vapor pressure: Sample vapor pressure impacts vaporization efficiency, with low
vapor pressure samples potentially causing measurement interferences.
• Surface Tension: Surface tension plays a role in droplet formation during nebuli-
zation, affecting atomization efficiency and AAS accuracy.
• Sample aspiration rate: The rate at which the sample is draw n into the atomiza-
tion process can impact atomization efficiency, potentially leading to measure-
ment inaccuracies.
6.10.4 Cation–Cation Interference
In certain cases, the signal intensity of the target element in the sample exhibits
irregular decreases. These decreases are neither of ionic nor spectral nature, and their
underlying interaction mechanisms remain unknown.
6.10.5 Anion–Cation Interference
The presence of specific anions in the sample solution can significantly affect the
intensity of radiation emitted by an element, potentially resulting in a significant
analytical error.
6.10.6 Oxide Formation Interference
When oxygen is present in the flame, it can lead to the formation of stable metal
oxides, causing interference. This interference occurs as a large percentage of free
6.10 Interferences of AAS 263
metal atoms is removed from the flame, resulting in a decrease in the intensity of
emitted radiation. Alkaline earth metals’ oxides are particularly susceptible to this
type of interference.
6.10.7 Spectral Interferences
Spectral interferences in AAS arise from the presence of other substances, affecting
the element of interest’s light absorption and causing measurement inaccuracies.
These interferences include:
• Spectral overlap: Absorption lines of the target element overlap with those of
other substances in the sample, making differentiation challenging.
• Molecular absorption: Some sample molecules absorb radiation at AAS analysis
wavelengths, leading to inaccurate target element concentration measurements.
• Light scattering: Light scattering by particles or molecules in the sample reduces
absorbed light intensity, impacting AAS measurement accuracy.
Examples of spectral interferences for certain metals are provided in Table 6.2.
6.10.8 Chemical Interferences
Chemical interferences in AAS result from reactions in the flame or atomization,
affecting element measurement accuracy. They can impact free atom formation and
light absorption. Common types include:
• Oxide formation: Oxygen forms stable metal oxides, absorbing radiation at the
target element’s wavelengths, reducing signal intensity.
Table 6.2 Examples of spectral interferences of interfering elements with that of target elements
Target element Spectral line (nm) Interfering element Spectral line (nm)
Aluminum (Al) 308.215 Vanadium (V) 308.211
Calcium (Ca) 422.673 Ge 422.657
Cadmium (Cd) 228.802 Arsenic (As) 228.812
Cobalt (Co) 252.136 Indium (In) 252.137
Copper (Cu) 324.754 Eu 324.753
Iron (Fe) 271.903 Platinum (Pt) 271.904
Ga 403.298 Mn 403.307
Mercury (Hg) 253.652 Cobalt (Co) 253.649
Mn 403.307 Ga 403.298
Sb 217.023 Lead (Pb) 216.999
Si 250.690 Vanadium (V) 250.690
Zn 213.856 Iron (
Fe)
213.859
264 6 Comprehensive Insights into Atomic Absorption Spectroscopy
• Hydride formation: Some elements react with hydride-forming reagents, creating
metal hydrides that absorb radiation, interfering with target element
determination.
• Complex formation: Stable complexes reduce atomization ef ficiency, affecting
absorption measurements.
• Flame reactions: Flame chemical reactions change element oxidation states,
impacting light absorption.
• Sample matrix effects: Chemical composition, including other elements, can
cause interactions and interference.
6.10.9 Physical Interferences
Physical interferences in AAS affect measurement accuracy by altering sample or
instrument physical characteristics. These nonchemical interferences can lead to
inaccuracies. Common issues include:
• Sample matrix effects: Sample matrix properties like viscosity, density, surface
tension, and vapor pressure impact atomization and vaporization, affecting free
atom formation and measurements.
• Particle size and aggregation: Large particles or aggregates in the sample can
obstruct light paths and hamper atomization, reducing AAS accuracy.
• Resonance line broadening: Pressure and temperature conditions during atomi-
zation can broaden absorption lines, potentially causing spectral interferences and
impacting element measurements.
6.10.10 Vaporization Interferences
Vaporization interferences in AAS affect sample vaporization, leading to measurement inaccuracies. Common issues include:
• Solvent effects: Solvent choice
boiling or viscous solvents hinder vapor generation, reducing AAS accuracy.
• Sample matrix:
residues in the atomizer, obstructing free atom release.
• Vaporization temperature:
incomplete vaporization, while high temps lead to thermal interferences.
Sample compo
in samp
sition, for example, salts or organics, can create
Correct temperature is crucial. Low temps cause
le prep can impact vaporization. High-
6.11 Strategies for Overcoming and Controlling Interferences in AAS 265
6.11 Strategies for Overcoming and Controlling Interferences
in AAS
Overcoming and controlling interferences in AAS is crucial for obtaining accurate
results. Here are some methods and strategies to address and minimize various types
of interferences:
6.11.1 Ionization Suppression
To mitigate ionization interference, ionization suppressors can be added to the
sample to prevent the formation of ions during combustion. Common suppressors
include high concentrations of easily ionized elements like cesium or potassium.
6.11.2 Flame Reactions
Selecting an appropriate flame type and adjusting the fuel-to-oxidant ratio can help
reduce interference caused by flame reactions. Optimization of flame conditions
minimizes the formation of stable molecular oxides.
6.11.3 Use of Chemical Modifiers
For some specific interferences, chemical modifiers can be employed. These
modifiers react with interfering components, forming stable compounds and
preventing interference with the analyte absorption.
6.11.4 Matrix Matching
In the case of sample matrix effects, creating calibration standards with a similar
matrix to the sample can help account for these effects and improve accuracy.
6.11.5 Background Correction
To address background absorption interference, techniques like Zeeman background
correction or Deuterium bac kground correction can be used to compensate for
source radiation absorption.
6.11.5.1 Smith-Hieftje Method
The Smith-Hieftje background correction is a technique used to differentiate
between atomic absorption signals and background absorption caused by molecular
species, scattering, or other interference. It relies on modulating the intensity of the
266 6 Comprehensive Insights into Atomic Absorption Spectroscopy
radiation source, typically a hollow cathode lamp, by alternating between high and
low current pulses.
• How it works.
– At low current, the lamp emits a narrow spectral line that is absorbed by the
target atoms, generating an absorption signal.
– At high current, the lamp produces a broadened emission spectrum due to self-
absorption and self-reversal. In this broadened state, the emission intensity at
the specific wavelength of interest is reduced, minimizing atomic absorption
but still detecting background absorption.
– The instrument alternates between these two states and subtracts the back-
ground signal obtained during the high-current pulse from the total absorption
during the low-current pulse. This provides a corrected atomic absorption
signal, compensating for background interference.
• Advantages: Smith-Hieftje is effective in reducing background interference,
particula
rly for
line-rich spectra and molecular interferences.
6.11.5.2 Zeeman Effect Background Correction
The Zeeman background correction utilizes the Zeeman effect, where the application
of a magnetic field causes the splitting of atomic absorption lines into several
components. This effect is used to distinguish between the atomic absorption signal
and background absorption.
• How it works.
– The atomic absorption line is split into two polarized components (σ + and σ-)
when a magnetic field is applied, while the background signal remains
unchange d.
– The instrument alternates between applying the magnetic field and turning it
off. When the magnetic field is on, only the split polarized components are
detected, which correspond to the atomic absorption signal. When the field is
off, both atomic and background signals are recorded.
– The difference between these two readings (with and without the magnetic
field) isolates the atomic absorption signal, effectively correcting for background noise or scattering.
• Advantages: The Zeeman effect is highly effective for correcting background in
samples with significant scattering or molecular absorption, and is particularly
suited for high-temperature atomization techniques like GFAAS.
6.11.6 Wavelength Selection
Careful selection of the absorption wavelength can help avoid spectral overlap
interference. Choosing a wavelength where the target element has a unique and
strong absorption line can minimize interference.
6.11 Strategies for Overcoming and Controlling Interferences in AAS 267
6.11.7 Sample Dilution
In cases of high sample matrix effects, diluting the sample with a suitable solvent can
reduce interference and improve accuracy.
6.11.8 Temperature and Atomization Control
Maintaining consistent flame temperature and atomization conditions helps minimize thermal and physical interferences.
6.11.9 Use of Standard Addition
In complex matrices, standard addition can be employed to directly measure the
interference and subtract it from the total signal.
6.11.10 Routine Calibration
Regularly calibrating the instrument with standard solutions and running blanks to
correct for background absorption is essential.
6.11.11 Reference Standards
Employing reference standards and certified reference materials can aid in
identifying and addressing interferences.
6.11.12 Method Validation
Validating the method for accuracy and precision, and assessing potential
interferences, is crucial to ensuring reliable results.
6.11.13 Instrument Maintenance
Regular maintenance of the AAS instrument, such as cleaning the optical path and
ensuring the stability of the light source, is essential for minimizing interference.
268 6 Comprehensive Insights into Atomic Absorption Spectroscopy
6.12 Sample Preparation for AAS
Sample preparation is a crucial step in AAS that involves several techniques to
ensure accurate and reliable results. Proper sample preparation is essential to remove
interferences and make the analyte of interest available for AAS analysis. Sample
digestion is essential because many real-world samples, such as biological tissues,
environmental samples, and food products, contain analytes in forms that are not
readily atomized and absorbed in AAS.
• Remove interferences: Many substances can interfere with AAS measurements,
so sample preparation is essential to eliminate or minimize these interferences.
• Dissolve the analyte: In some cases, the analyte might be in solid or insoluble
forms, requiring dissolution to facilitate atomization and absorption.
Sample preparation for AAS typically involves the following steps:
6.12.1 Sample Collection
Collect a representative sample from the source, ensuring it accurately reflects the
material or medium being studied.
6.12.2 Sample Digestion
When dealing with solid samples or complex matrices, digestion techniques are used
to break down the sample and release the analyte. Common digestion methods
include wet digestion, dry ashing, microwave digestion, pressure digestion, and
fusion digestion. These techniques are employed to break down complex sample
matrices, enabling the extraction of the target analyte for accurate AAS analysis. The
process helps release the analyte from the matrix and convert it into a form suitable
for AAS analysis. Several digestion methods are used in AAS sample preparation:
• Wet digestion: This involves dissolving the sample in a suitable solvent or acid.
Commonly used acids include HNO
wide range of sample types and is well-suited for the dissolution of metals.
• Dry ashing: Dry ashin
g employs
the sample matrix, leaving behind the residue of interest. This method is often
used for samples with organic components.
• Microwave diges
tion: Microw
decomposition using microwave energy. It is particularly useful for labs requiring
high-throughput sample preparation.
• Pressure digestion:
In pressure digestion, samples are subjected to high pressures
and temperatures within a closed vessel. This technique can achieve complete
breakdown of complex matrices.
and HCl. Wet digestion is effective for a
3
high-temperature heating to ash or decompose
ave digestion offers rapid and efficient sample
6.12 Sample Preparation for AAS 269
• Fusion digestion: Fusion digestion involves heating the sample with a flux,
usually a mixture of sodium carbonate (soda ash) and other reagents, to form a
fused bead. This method is commonly used for refractory samples.
• Alternative digestion techniques: Depending on the sample type and lab
requirements, alternative digestion techniques may be employed. These could
include enzyme digestion for biological samples or specialized digestion
procedures for unique matrices.
6.12.3 Dilution
In some cases, samples may be too concentrated for AAS analysis. Dilution with an
appropriate solvent is employed to ensure the analyte concentration is within the
instrument’s detection range.
6.12.4 Filtration
To remove solid particulates and debris from the sample solution, filtration is used. It
ensures a clean sample for AAS analysis.
6.12.5 Homogenization
Achieving sample uniformity is vital to ensure representative analysis. Homogenization techniques are used to mix and distribute the analyte evenly.
6.12.6 Standard Solutions
Preparing standard solutions of known analyte concentrations is essential for calibration and quantification in AAS.
6.12.7 Matrix-Matching
When the sample matrix differs significantly from the standard solutions, matrixmatching techniques may be applied to improve accuracy.