Study Notes on UV-Visible Spectrophotometry from PHARMACEUTICAL ANALYSIS II by Feruza Ahmed

PHARMACEUTICAL ANALYSIS II

UV-Visible Spectrophotometry - Feruza Ahmed (B.pharm, MsC)

Introduction to Spectroscopy

• Definition: Spectroscopy is the study of the interaction between electromagnetic radiation (EMR) and matter.   - EMR is a form of energy characterized by both wave and particle properties.   - It consists of oscillating electric and magnetic fields propagating through space along a linear path.

Characteristics of Electromagnetic Waves

1. Wavelength (λ): The distance between two successive peaks or troughs.    - Units for wavelength include Angstroms, centimeters, micrometers, and nanometers.    - Conversion:
        - $1 ext{ m} = 10^2 ext{ cm} = 10^3 ext{ mm} = 10^6 ext{ µm} = 10^9 ext{ nm} = 10^{10} ext{ Å}$
        - $1 ext{ Å} = 10^{-1} ext{ nm} = 10^{-4} ext{ µm} = 10^{-7} ext{ mm} = 10^{-8} ext{ cm} = 10^{-10} ext{ m}$

2. Wavenumber (v'): Number of waves per centimeter. Calculated as ( \bar{v} = \frac{1}{\lambda} )

3. Frequency (v): The number of waves per second.

4. Amplitude (A): The height from the midline of a wave to its peak or trough.

5. Velocity (c): The product of wavelength and frequency.

Nature of Electromagnetic Radiation (EMR)

• Wave-Particle Duality: The wave model is inadequate for explaining phenomena related to photon absorption and emission.   - EMR can also be treated as discrete packets of energy known as photons or quanta.   - The energy of a photon is directly proportional to its frequency defined by:
       $E = hv = \frac{hc}{\lambda} = hc\bar{v}$
    - Where
       - (h) is Planck's constant (6.626 x 10^{-34} J·s)
       - (c) is the speed of light in vacuum (2.9979 x 10^8 m/s).

Electromagnetic Spectrum

• Range of Energies: The electromagnetic spectrum encompasses a wide range of energies (frequencies) and wavelengths, representing various atomic and molecular transitions.   - Boundaries between spectral regions are not fixed, with overlaps being common.   - Increasing wavelengths (from gamma rays to radio waves) indicate decreasing frequencies and increasing energy respectively.

Interaction of EMR with Matter

1. Absorption: EM energy transfers to an absorbing molecule, exciting it from a lower to a higher energy state.

2. Emission: Energy is transferred from an excited molecule back to space, moving it from a high energy state to a lower energy state.

3. Scattering: Involves the redirection of light without energy transfer.   - Key effects:     - Absorption affects electron transitions, while the absorption of infrared radiation impacts vibrational and rotational energies.

Excitation Process

• Excitation can occur to various energy levels, with the following key points:   - Low energy excitations often involve outermost electrons first.   - When an electron becomes excited, it is unstable and returns to its ground state in steps, potentially involving multiple energy levels.

Excited Molecular States

• An excited molecular species is short-lived, with deactivation occurring via:   1. Radiative Decay: Excited molecules emit energy as photons when transitioning down to lower energy states.      - This emission can manifest as visible light (fluorescence) or invisible light (phosphorescence).   2. Non-Radiative Processes: Includes mechanisms such as:      - Vibrational Relaxation: Energy from photons is distributed to vibrational modes within the molecule, occurring in 10^-14 to 10^-11 seconds.      - Internal Conversion: If vibrational energy levels overlap with electronic energy levels, electrons may transition from one vibrational level in an electronic state to another vibrational level in a lower electronic state.      - Collisional Quenching: Excited species interact with other molecules (solvent, gas), transferring energy to their surroundings.      - Chemical Reactions: Excited species may engage in reactions.

Energy States

• Energy levels within a molecule are quantized, requiring specific energy to transition from ground state (S0) to various excited states (S1, S2, etc.).   - The energy difference can be described as:
       $E_s = E_{0v} - E_{1v}$

UV/Visible Spectroscopy

• Technique Overview: Utilizes the UV/Visible spectrum (200-800 nm) for analysis; particularly focused on electronic transitions.   - Regions:     - Near UV (200-400 nm)     - Visible (400-800 nm)

Electronic Transitions in Organic Molecules

• Electrons in organic molecules can exist in various energy levels (σ-, π-, n-electrons):   1. σ-Electrons: Lowest energy bonding electrons in valence bonds.   2. π-Electrons: Higher energy bonding electrons involved in pi-bonds.   3. n-Electrons: Non-bonding electrons in atomic orbitals of heteroatoms (e.g., N, O).

Types of Electronic Transitions

1. σ → σ: Requires significant energy to break σ bonds, typically seen in short wavelengths.    - Example: Methane exhibit absorbance at 125 nm for σ → σ transitions.

2. n → σ*: Needs less energy and occurs in saturated compounds with lone pairs; absorbs between 150-250 nm.

3. n → π* and π → π*: Most common in organic compound spectroscopy, falling within 200-700 nm; requires unsaturated groups for π electrons.

Absorption Characteristics of Chromophores

• Chromophores: Functional groups that absorb UV/Visible light, often containing unsaturated bonds (e.g., C=C, C=O).

• Auxochromes: Affect the intensity of chromophores, leading to bathochromic shifts (red shifts) by increasing absorption intensity.

Effects of pH on Absorption Spectra

• Changes in pH can affect molecules with ionizable functional groups (e.g., Carboxylic acids, Amines, Phenols), altering their spectra characteristics:   - Bathochromic shifts are common in alkaline media, while Hypsochromic shifts occur in acidic conditions.

Effects of Solvent on Absorption Spectra

• Solvents can induce shifts in absorption bands based on their polarity and interaction with solutes:   - Bathochromic shifts: Occur due to stabilization of excited states.   - Hypsochromic shifts: Happening in solvent interactions that increase energy gaps.

Requirements for Solvents in UV/Visible Spectroscopy

1. Transparency: Should not absorb light in the UV range.

2. Dissolution capability: Must dissolve sufficient amounts of analyte.

3. Chemical Inertness: Not reactive with the sample.

4. Low Volatility: To minimize concentration changes during measurement.

5. Suitable Refractive Index: To reduce reflection losses in quantitative analysis.

Woodward-Fieser Rules for λmax Calculation

• Base value for parent compounds determines the λmax, with contributions from substituents.   - Proposed formula:      $\lambda_{max} = \text{Base Value} + \sum \text{(substituent contributions)} + \sum \text{(other contributions)}$

Quantitative Analysis with UV-Visible Spectrophotometry

• Centered around Beer-Lambert law.   - Beer’s Law: Relates absorbance to concentration:      $A = \epsilon b C$   - Important considerations include reflection, absorption by the sample container, and background scattering effects.

Applications of UV-Visible Spectrophotometry

1. Qualitative Analysis: Identification of chromophores and functional groups.

2. Quantitative Analysis: Accurate concentration measurements based on absorbance.

3. Differential Spectrophotometry: For analyzing mixtures by measuring absorbance differences under controlled conditions.

4. Spectrophotometric Titrations: Using absorbance changes to find endpoints, advantageous because of their sensitivity and accuracy.

Measurement Techniques

• Single Beam vs. Double Beam Spectrophotometers:   - Single Beam: Cheaper; requires blank adjustments before every measurement.   - Double Beam: More complex; allows simultaneous measurement.

Conclusion

• UV-Visible spectrophotometry is a fundamental technique in pharmaceutical analysis, capable of providing qualitative and quantitative insights crucial for research and development in drug formulation and delivery. It provides essential data for determining compound structures and behaviors relevant to pharmacological applications.