JNTU B.Pharmacy IV INSTRUMENTAL METHODS OF ANALYSIS Previous Question Paper (July/August-2022 - R17)

Lambert-Beer's law states that when a beam of monochromatic light passes through a homogeneous absorbing medium, the rate of decrease in intensity of radiation with respect to the thickness of the medium is directly proportional to the intensity of the incident radiation and the concentration of the absorbing species. Mathematically, it is expressed as:

Where A is absorbance, I₀ is the intensity of incident light, Iₜ is the intensity of transmitted light, ε is the molar absorptivity, b is the path length of the cell, and c is the concentration of the solution. The importance of this law lies in its application for the quantitative determination of unknown concentrations of analytes in a solution using spectrophotometry. It provides the fundamental basis for calibration curves, allowing for the precise measurement of substances in pharmaceutical quality control, clinical analysis, and environmental monitoring.

Fluorescence intensity is influenced by several chemical and physical factors:

• Quantum Yield: The ratio of the number of photons emitted to the number of photons absorbed. Higher quantum yields result in greater fluorescence intensity.
• Concentration: At low concentrations, fluorescence intensity is directly proportional to concentration. At high concentrations, the inner filter effect occurs, where the solution absorbs its own emitted light, leading to a decrease in intensity.
• Temperature: Increasing temperature generally decreases fluorescence intensity because it increases the probability of non-radiative deactivation processes, such as molecular collisions.
• pH: The fluorescence of many organic compounds is pH-dependent. Changes in pH can alter the ionization state of the molecule, which significantly affects its electronic structure and fluorescence properties.
• Quenching: The presence of foreign substances (quenchers) like dissolved oxygen, halides, or heavy metal ions can reduce fluorescence intensity by promoting non-radiative transitions from the excited state.
• Solvent Effects: The polarity and viscosity of the solvent can influence the stability of the excited state and the rate of non-radiative transitions, thereby affecting the emission intensity.