Comprehensive Study Notes on UV-VIS Spectrophotometry

Principles and Introduction to UV-VIS Spectrophotometry

• Radiation Range: Ultraviolet-Visible (UV-Vis) spectrophotometry involves exposing drugs or molecules to radiation within the wavelength range of $200 \times 10^{0}\,\text{nm}$ to $800\,\text{nm}$.

• Energy and Wavelength Relationship: Different wavelengths ($\lambda$) within the UV-Vis range carry different energy levels.

  • Shorter wavelength corresponds to higher energy.

  • Longer wavelength corresponds to lower energy.

• Mechanism of Absorption:

  • Absorbed energy stimulates the valence electrons of a molecule to undergo electronic transitions.

  • Electrons move from lower energy orbitals (ground state) to higher energy orbitals (excited state), a process known as 'excitation'.

  • UV-Vis radiation specifically triggers the temporary excitation of double bonds ($\pi$-electrons) and lone electron pairs ($n$-electrons).

• Quantitative Basis: This phenomenon is utilized for quantitative analyses under the principle of "more drug = more absorption."

Key Terminologies and Molecular Structure

• Conjugated System: These are interlinked unsaturated bonds, typically appearing as alternating saturated and unsaturated bonds. This structure allows for the free movement of "delocalised" $\pi$ and $n$ electrons.

• Auxochromes: Functional groups containing lone electron pairs (EP) that are attached to a conjugated system. They contribute to the absorbance of light and the delocalisation of electrons.

• Chromophore: The structural part of a molecule that absorbs UV-Vis radiation. It comprises the conjugated system and any attached auxochromes.

• Electronic Shifts Based on Chromophore Extension:

  • The more extensive the chromophore system, the less energy is required for electronic excitation.

  • Consequently, the absorbance shifts to a higher wavelength ($\lambda$), representing lower energy.

• Visual Color Perception: If the absorbance shifts from the UV range into the visible (VIS) range, the molecule appears colored. The perceived color is complementary to the color of the absorbed wavelength ($\lambda$).

Examples of Molecular Absorption

• Benzene:

  • Contains $3 \times \text{C=C}$ bonds.

  • Features $6$ delocalised electrons.

  • Absorbs at approximately $\lambda \approx 170\,ns - 250\,\text{nm}$.

• Crotamiton:

  • Contains $2$ conjugated systems and $3$ lone electron pairs.

  • Absorbs at $\lambda = 242\,\text{nm}$.

• Vitamin A:

  • Contains $5 \times \text{C=C}$ bonds.

  • Highly conjugated structure.

  • Absorbs at $\lambda = 326\,\text{nm}$.

• $\beta$-Carotene:

  • Contains $11 \times \text{C=C}$ bonds.

  • Highly conjugated structure.

  • Absorbs at $\lambda = 497\,\text{nm}$.

  • Absorbs mainly blue-green light, resulting in a red-orange appearance.

The Beer-Lambert Law

• Definition: UV-Vis spectrophotometry measures the absorption of radiation by a compound by comparing the amount of radiation entering a solution to the amount of radiation leaving it.

• Key Proportionalities: For dilute solutions, there are clear correlations:

  • Absorbance is proportional to concentration: $A \propto c$ (Beer's Law).

  • Absorbance is proportional to path length: $A \propto l$ (Lambert's Law).

• Mathematical Formulas:

  • $A = A_{1\%}^{1\text{cm}} \times c \times l$

  • $A = \epsilon \times c \times l$

• Variables Defined:

  • $A$: Absorbance (dimensionless unit) at a particular $\lambda$.

  • $A_{1\%}^{1\text{cm}}$: Specific absorbance coefficient.

  • $\epsilon$: Molar absorbance coefficient.

  • $c$: Concentration. Expressed in $g/100\,\text{ml}$ when using $A_{1\%}^{1\text{cm}}$ or in $\text{moles/l}$ when using $\epsilon$.

  • $l$: Path length, measured in $\text{cm}$.

• Standardization: Spectrophotometers typically use standard cuvettes with a pathlength ($l$) of $1\,\text{cm}$, simplifying the law to a linear relationship between absorbance and concentration.

Quantitative Analysis and Calibration

• Beer-Lambert Plot Attributes:

  • The line must pass through the origin (where $c = 0$, $A = 0$).

  • If concentration is in $g/100\,\text{ml}$ ($\% w/v$), the slope of the curve is $A_{1\%}^{1\text{cm}}$.

  • If concentration is in $\text{moles/l}$, the slope is $\epsilon$.

• Linearity Constraints:

  • The Beer-Lambert Law is only valid for dilute solutions.

  • Linearity is lost at high concentrations (c > 0.01\,\text{M}).

  • The recommended "Beer Lambert Range" for absorbance readings is between $0.3$ and $0.8$ to ensure the plot remains linear.

  • Beer-Lambert Law remains valid generally for absorbance values A < 1.5.

• External Standard Method Procedure:

  1. Prepare a stock solution of the drug.

  2. Prepare $3-5$ dilute samples with known concentrations from that stock.

  3. Measure the absorbance of each dilute solution at a specific wavelength.

  4. Plot a calibration curve of concentration ($c$) versus absorbance ($A$).

  5. Measure the absorbance of the drug sample with an unknown concentration (diluting if necessary).

  6. Determine the unknown concentration from the standard curve.

UV-Vis Spectra and Qualitative Analysis

• $\lambda_{\text{max}}$: The wavelength that produces the highest absorbance value in a spectrum. This is commonly used for quantitative analysis.

• Structural Reflection: The UV-Vis spectrum reflects the specific chromophore present in the molecule.

• Sensitivity to Structure: Changes to chemical structure that affect the chromophore will alter the UV-Vis spectrum (e.g., changes in $A_{1\%}^{1\text{cm}}$ or $\lambda_{\text{max}}$).

• pH Dependency: Spectra can be pH-dependent if ionisable auxochromes are present (e.g., phenol or aniline groups). This is used for qualitative identification.

pH-Induced Spectral Shifts

• Phenol Example:

  • Acidic or Neutral Media (Unionised Phenol): $\lambda_{\text{max}} = 270\,\text{nm}$; $A_{1\%}^{1\text{cm}} = 172$.

  • Basic Media (Phenoxide): Auxochrome results in an extended chromophore. $\lambda_{\text{max}} = 287\,\text{nm}$; $A_{1\%}^{1\text{cm}} = 271$.

  • Shifts observed in base:

    • Bathochromic (Red) Shift: Increase in $\lambda_{\text{max}}$.

    • Hyperchromic Shift: Increase in $A_{1\%}^{1\text{cm}}$ or $\epsilon$.

• Aniline Example:

  • Basic or Neutral Media (Unionised Aniline): $\lambda_{\text{max}} = 286\,\text{nm}$; $A_{1\%}^{1\text{cm}} = 179$.

  • Acidic Media (Anilinium): Loss of auxochrome leading to a smaller chromophore. $\lambda_{\text{max}} = 255\,\text{nm}$; $A_{1\%}^{1\text{cm}} = 16$.

  • Shifts observed in acid:

    • Hypsochromic (Blue) Shift: Decrease in $\lambda_{\text{max}}$.

    • Hypochromic Shift: Decrease in $A_{1\%}^{1\text{cm}}$ or $\epsilon$.

Limitations of UV-VIS Spectrophotometry

• Concentration Limits: Precision is lost in non-dilute samples.

  • At high concentrations, aggregation or dimerisation can occur, and aggregates may possess different spectral properties.

  • Detecting the small amount of radiation leaving a highly concentrated sample can be inaccurate or impossible.

• Regulatory Requirements: The British Pharmacopoeia (B.P.) requires absorbance values to range between $0.3$ and $0.8$. Dilution strategies are often necessary to meet this requirement.

• Chemical Stability: It is essential to ensure molecules exist in the same chemical structure. Analysis at extreme pH is sometimes required to ensure fully ionised or unionised states, as this influences lone electron pairs and absorbance.

• Specificity (The Major Limitation):

  • UV-Vis lacks high specificity. All molecules with similar chromophores—including impurities or degradation products—will be measured.

  • Example: Aspirin's degradation product, salicylic acid, will be partially measured, leading to a potential over-estimation of drug content.

  • Interferants must be removed prior to analysis whenever possible.