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120 3 Comprehensive Insights into UV-VIS Spectrophotometry
Fig. 3.7 A schematic diagram of a UV-visible spectrophotometer. The system consists of a light
source emitting a broad spectrum of light, which passes through a wavelength selector (monochromator) to isolate the desired wavelength. The selected light (P
reference solution, where part of the light is absorbed, and the remaining light (P) is transmitted to
the light detector. The detector measures the transmitted light, and the results are displayed on an
LCD screen, providing information about the sample’s absorbance or transmittance
Fig. 3.8 Schematic representation of a UV-visible spectrophotometer. The system consists of a
light source that emits radiation, which passes through a monochromator to isolate a specific
wavelength. The radiation is split to pass through a sample cell and a reference cell. The transmitted
light is detected, and the resulting signal is displayed on an LCD for analysis
) passes through the sample or
0
absorbance value is subtracted from sample measurements to eliminate solvent and
cell effect s. It is typically used in the wave length range of 190–1100 nm for
analyzing samples like nucleic acids, proteins, and organic molecules. In the visible
region (340–750 nm), it is ideal for analyzing colored samples.
3.8.2 Double-Beam UV-VIS Spectrophotometer
Double-beam spectrophotometers have the capability to simultaneously measure the
sample and reference beams (Fig.
3.8), providing enhanced accuracy and stability.
These instruments are commonly used for quantitative analysis and are capable of
compensating for changes in light source intensity or drift.
3.8.3 Split-Beam UV-VIS Spectrophotometer
Split-beam spectrophotometers are a variation of the double-beam design. They split
the light beam into two separate paths, one for the sample and the other for the
reference. This design is particularly useful for monitoring samples with unstable or
changing baseline absorbance.
3.8 Types of UV-VIS Spectrophotometer 121
3.8.4 Scanning UV-VIS Spectrophotometer
Scanning spectrophotometers are equipped with a monochromator that can scan a
range of wavelengths. They are used for obtaining complete absorption spectra,
making them suitable for applications like identifying unknown compounds or
monitoring complex reactions.
3.8.5 Array UV-VIS Spectrophotometer (PDA—Photodiode Array
Detector)
Photodiode array spectrophotometers use an array of photodiodes to simultaneously
measure a range of wavelengths. This allows for rapid spectral scanning and the
collection of full UV-VIS spectra in a short time. They are valuable for multicomponent analysis a nd kinetic studies.
3.8.6 Fixed-Wavelength UV-VIS Spectrophotometer
Some UV-VIS spectrophotometers are designed for specific applications, with fixed
wavelengths optimized for certain analyses. These instruments offer simplicit y and
cost-effectiveness but are limited in versatility.
3.8.7 Microvolume UV-VIS Spectrophotometer
Microvolume spectrophotometers are designed for analyzing small sample volumes,
typically in the microliter range. They are suitable for applications where sample
volume is limited or precious, such as DNA, RNA, or protein quantification.
3.8.8 Nanodrop UV-VIS Spectrophotometer
Nanodrop spectrophotometers are specialized instruments for ultra-small sample
volumes, often in the nanoliter range. They are commonly used in genomics and
proteomics for highly concentrated samples.
The choice
requirements, the range of wavelengths needed, the accuracy required, and the
nature of the samples to be measured. Each type of spectrophotometer has its own
advantages and limitations, making it essential to select the most suitable instrument
for a given application.
of UV-VIS spectrophotometer depends on the specific analytical
122 3 Comprehensive Insights into UV-VIS Spectrophotometry
3.9 Sample Preparation Techniques for UV-VIS Spectroscopy
Sample preparation for UV-VIS spectroscopy is a critical step to ensure accurate and
reliable measurements. Proper samp le handling and preparation help eliminate
interferences, ensure the sample’s homogeneity, and reduce errors in the analysis.
Here are some common sample preparation techniques for UV-VIS spectroscopy:
3.9.1 Sample Stability
Perform measurements promptly after sample preparation to minimize potential
changes in the sample due to chemical reactions or degradation.
3.9.2 Dilution
Dilution may be necessary to ensure that the sample concentration is within the
linear range of the instrument. When diluting, accurately measure the volume of the
sample and the diluent. Mix thoroughly to ensure homogeneity.
3.9.3 Filtration
Filtration is essential to remove particulate matter and ensure the clarity of the
sample. Use syringe filters or membrane filters with appropriate pore sizes to prevent
clogging. It is particularly important when working with samples containing
suspended solids.
3.9.4 Extraction
In cases where the analyte is not soluble in the chosen solvent, extraction techniques,
such as liquid-liquid extraction or solid-phase extraction, may be required to transfer
the analyte into a suitable solvent for UV-VIS analysis. Proper extraction procedures
are crucial to obtaining accurate results.
3.9.5 Selection of Solvent
Choose a suitable solvent that is transparent in the UV-VIS range and does not
interfere with the analyte’s absorption. Common solvents include water, methanol,
ethanol, and acetonitrile.
3.9 Sample Preparation Techniques for UV-VIS Spectroscopy 123
3.9.6 Dissolution
Ensure that the analyte is completely dissolved in the chosen solvent. Use gentle
heating, sonication, or agitation to aid dissolution if necessary.
3.9.7 Cuvettes
UV-VIS spectroscopy typically uses cuvettes to hold the sample. Ensure that the
cuvettes are clean and free from scratches, as these can affect the measurements.
Rinse cuvettes with the solvent or sample solution to avoid contamination.
3.9.8 Blank Solution
Prepare a blank solution containing the solvent used for sample preparation. This
serves as a reference for baseline measurements and helps correct for solvent
absorbance.
3.9.9 Homogenization
Ensure that the sample is thoroughly mixed and homogeneous. Agitate or stir the
sample to prevent settling of particles.
Sample temperature
UV-VIS measurements can be temperature-sensitive. Maintain the sample at a
constant and controlled temperature, especially when analyzing temperaturedependent reactions.
3.9.10 Handling Light-Sensitive Compounds
If your sample is light-sensitive, perform the analysis in a dark room or use amber or
opaque cuvettes to protect the sample from light during measurement.
3.9.11 Sample Volume
Fill the cuvette to an appropriate level to ensure consistent and reproducible
measurements. Avoid overfilling or underfilling, which can lead to inaccurate
results.
124 3 Comprehensive Insights into UV-VIS Spectrophotometry
3.9.12 Background Correction
Take baseline measurements with the blank solution or the solvent used for sample
preparation. This helps correct for any absorbance due to the solvent itself.
3.9.13 Solid Sample Analysis
For solid samples, especially when analyzing powders or solids, it is necessary to
convert the sample into a solution or a form suitable for UV-VIS measurement. This
can involve techniques like dissolution, digestion, or digestion and dilution. The
sample preparation steps must ensure complete dissolution and homogeneity.
3.9.14 Calibration Standards
Calibration standards are essential for generating a calibration curve and quantifying
the analyte concentration. Prepare standards of known concentrations by accurately
measuring and diluting stock solutions. Use the same solvent and cuvettes as for the
sample. It is important to include calibration standards at different concentrations to
cover the expected range of sample concentrations.
3.9.15 Temperature Control
Temperature can affect the meas urements, especially in temperature-sensitive
reactions. If temperature control is critical, use a water bath, temperature-controlled
cuvette holder, or other suitable equipment to maintain a constant temperature during
analysis.
3.9.16 Sample Stability
Be aware of the stabili ty of your samples. Some samples may degrade or change
over time. Perform measurements promptly after sample preparation to minimize
potential changes in the sample. For long-term storage, consider appropriate
conditions (e.g., refrigeration or protection from light) to maintain sample stability.
3.9.17 Record Sample Information
Document relevant details such as sample concentration, solvent used, preparation
date, and any specific conditions.
3.10 Absorbance Laws 125
Proper sample preparation is crucial for obtaining accurate and reproducible
results in UV-VIS spectroscopy. Following these techniques will help ensure the
reliability of your measurements and the quality of your analytical data.
3.10 Absorbance Laws
There are basically three laws for spectroscopy that describe the absorbance of light
through a material. The details of these laws have been described below.
3.10.1 Beer’s Law
It can be described as the beam intensity of the monochromatic light is decreased
exponentially when the concentration of analyte increases arithmetically (Fig.
In quantitative analysis, primarily concerned with solutions, the effect of concentration of the colored constituent in solution depends upon the light absorption or
transmission. It can be expressed as:
-klc
I = Ie
3.9).
3.10.1.1 Beer Derivation
In 1852, Beer described the following relationship between the concentration of a
solution and absorbance of light:
0
0
c
k
0
=
r
2:303
OR A =
log
I
I
The Beer derivation
Where, c is the concentration of analyte in sample solution, k
0
0
k
c
2:303
n
is a proportionality
constant, and A is the absorbance of light.
Fig. 3.9 Relationship between percent transmittance and absorbance. The diagram illustrates the
inverse logarithmic relationship between percent transmittance (%T ) and absorbance (A). As %T
increases from 0 to 100, absorbance decreases from 2.0 to 0. This relationship follows the eq. A =-
log(T ), where T is the transmittance fraction. This concept is fundamental in spectrophotometry for
determining the concentration of a substance in a solution based on its light absorption properties
126 3 Comprehensive Insights into UV-VIS Spectrophotometry
The amount of light absorbed can be measured by multiple ways:
T = P=Po
%T = 100T
And the absorbance (A) can be calculated as:
A = log 10
A = log 10
A = log 10
Po
P
1
T
100
%T
A = 2 - log 10%T
The last eq. (A = 2 - log
%T) allows to calculate the absorbance of light from %
10
transmittance data.
3.10.2 Lambert’s Law
The rate of decrease in the intensity of incident light with the thickness of the
medium is directly proportional to the intensity of incident light. It can also be stated
that the intensity of emitted light is decreased exponentially as the thickness of
absorbing medium is increased. It is expressed as:
-k2l
I = Ie
3.10.3 Beer–Lambert Law
The Beer–Lambert law describes the relationship between the absorbance of light by
a sample and its concentration. Mathematically, the law is expressed as:
A = ε · c · l
Where:
• A is the absorbance
• ε is the mol
ar absorptivity (or molar extinction coefficient) in L mol
(dimensionles
s).
-1 cm-1
which reflects how strongly a chemical species absorbs light at a particular
wavelength.
,
3.10 Absorbance Laws 127
• c is the concentration of the analyte in mol L
-1
.
• l is the path length of the cuvette in cm.
3.10.3.1 HOMO and LUMO Conceptual Integration
The underlying principle of UV-VIS spectroscopy involves the electronic transitions
between molecular orbitals, particularly between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). When a
molecule absorbs light in the UV-VIS region, energy is absorbed that promotes
electrons from the HOMO to the LUMO.
• HOMO to LUMO transitions: Molecules with conjugated systems or unsaturated
bonds often exhibit electronic transitions such as π → π* or n → π*. These
transitions are critical for absorbing energy in the UV-VIS range. In simple
molecules like alkenes, π → π * transitions (involving the promotion of an
electron from the bonding π orbital to the antibonding π* orbital) occur, and
the absorbance in UV-VIS depends on the energy gap between HOMO
and LUMO.
• Absorbance and wavelength: The energy difference between HOMO and LUMO
determines the wavelength of light absorbed by the molecule. A larger energy gap
results in the absorption of higher energy (shorter wavelength) light, while a
smaller energy gap leads to lower energy (longer wavelength) absorption. For
example, molecules with extensive conjugation, which lower the HOMO-LUMO
gap, tend to absorb at longer wavelengths (visible region), resulting in more
intense colorations.
• Effect of solvents and auxochromes: Polar solvents and auxochromic groups
like –OH or – NH₂ can stabilize the LUMO or HOMO, effectively reducing the
energy gap between HOMO and LUMO, causing a bathochromic shift (redshift),
where the absorption moves to a longer wavelength.
• Beer–Lambert law and absorbance: The Beer–Lambert law is predicated on the
linearity of absorbance with concentration, which works best when the absorbing
species maintains independent be havior and the transition from HOMO to LUMO
remains constant across concentrations. However, deviations may occur at higher
concentrations when molecular interacti ons (like dimerization or aggregation)
alter the energy levels or when saturation effects reduce the effective path length
or change the extinction coefficient.
• HOMO-LUMO gap variations: If molecular interactions at higher concentrations
change the electronic environment of the chromophore, the HOMO-LUMO gap
may alter, leading to changes in absorption properties and deviations from the
Beer–Lambert law.
• Nonlinearity: The
Beer–Lambert law assumes that the molar absorptivity ε is
constant at a given wavelength. However, if the HOMO-LUMO gap changes due
to concentration effects (like changes in the molecular environment or
interactions), this can cause nonlinearity between absorbance and concentration.
128 3 Comprehensive Insights into UV-VIS Spectrophotometry
By incorporating the concepts of HOMO-LUMO transitions and molecular
orbital theory into the explanation, we gain a deeper understanding of the
mechanisms driving UV-VIS absorbance, as well as potential deviations from the
Beer–Lambert law due to molecular interactions and structural effects.
3.10.3.2 Deviations from Beer–Lambert Law
Ideally, Beer’s law is most accurately applied with truly monochromatic radiation.
According to Beer’s law, a linear calibration curve should result, with the absorbance of monochromatic light plotted against the analyte concentration in a series of
standard solutions. This ideal curve should have a zero intercept. In practice,
however, calibration curves are not always perfectly linear (Fig.
3.10).
If the incident radiation consists of just two wavelengths λ' and λ ", with powers
' and P0", considering that A =-log (P/P0), then the power of the radiation to
P
0
come out from (P) the cell for each wavelength would be:
0
0
= P
P
P} = P
-ε0bC
10
0
}
-ε}bC
10
0
Where ε' and ε" are the molar absorptivities of each wavelength. Therefore, the
measured absorbance A
may be
m
Fig. 3.10 Deviations from Beer–Lambert law. The graph illustrates the relationship between
absorbance and concentration under different conditions. The ideal condition (black line) follows
the Beer–Lambert law, where absorbance is directly proportional to concentration. Positive deviation (red curve) occurs when absorbance increases more than expected, possibly due to molecular
interactions or saturation effects. Negative deviation (green curve) results from factors such as
scattering, refractive index changes, or chemical equilibria affecting light absorption. These
deviations highlight limitations in spectrophotometric analysis at high concentrations
3.10 Absorbance Laws 129
Fig. 3.11 Effect of wavelength selection on absorbance–concentration relationship. The left graph
shows the absorbance spectrum of a sample, with two selected wavelengths (1 and 2) highlighted in
yellow. Wavelength 1 corresponds to the peak absorbance, while wavelength 2 is at a lower
absorbance region. The right graph illustrates the relationship between absorbance and concentration for these wavelengths. At wavelength 1, the absorbance increases linearly with concentration
(Beer–Lambert law), whereas at wavelength 2, the deviation from linearity is more pronounced.
This emphasizes the importance of selecting the appropriate wavelength for accurate spectrophotometric analysis
0
}
þ P
P
-ε0bC
0
0
P
0
P
0
þ P
þ P
þ P
}
0
}
0
10
}
0
-ε}bC
=
Am =-log
log
0
P
0
10
The last equation indicates a nonlinear relation between Am and C. The
proportionality between A
and C is restored only if ε' = ε". The same situation
m
occurs when a radiation consists of many wavelengths. The situation is illustrated in
Fig.
3.11.
When employin
g polychromatic radiation, it is preferred to position the radiation
beam at the center of a relatively wide absorption peak (as shown in position 1 in
3.11). In this scenario, the proportionality between absorbance (A) and concen-
Fig.
tration (C) is maintained, as molar absorptivities are relatively consistent across all
wavelengths. Conversely, significant deviations from Beer’s law are expected when
the radiation beam is situated in spectral regions such as the sides of absorption
where a broad range of molar absorptivity values can
peaks (position 2 in Fig.
3.11),
be encountered. It has been observed that deviations from Beer’s law are negligible
when measuring absorbance at the peak maximum of narrow absorption peaks,
provided that the effective bandwidth of the incident beam is less than 1/10 of the
width of the absorption peak at half height.