JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D III Year PHARMACEUTICAL ANALYSIS Previous Question Paper (April/May - 2019 - R08)

A double-beam UV-visible spectrophotometer is designed to automatically compensate for fluctuations in light source intensity and detector sensitivity by splitting the monochromatic light beam into two paths: the sample beam and the reference beam.

The construction consists of the following key components:

• Light Sources: Deuterium lamp for UV range (190-350 nm) and Tungsten filament lamp for visible range (350-800 nm).
• Monochromator: Comprises an entrance slit, a dispersing element (prism or diffraction grating), and an exit slit to isolate a specific wavelength.
• Beam Splitter: A rotating sector mirror or chopper that directs the beam alternately between the sample and reference cells.
• Sample and Reference Cells: Quartz cuvettes containing the analyte solution and the solvent blank, respectively.
• Detector: Photomultiplier tubes or photodiodes that convert light energy into electrical signals.
• Amplifier and Recorder: Processes the ratio of the two signals to produce a stable absorbance reading.

The working principle involves the light source emitting radiation, which is dispersed by the monochromator. The beam is split; one part passes through the sample, and the other through the reference. The detector measures the intensity of both beams. The instrument calculates the ratio of the intensities, effectively canceling out variations in source intensity or detector drift, providing a direct measurement of absorbance.

Applications in pharmacy include:

• Quantitative Analysis: Determination of drug concentration in formulations using Beer-Lambert's Law.
• Identification: Comparison of UV spectra of a drug sample with a standard reference spectrum.
• Purity Determination: Detection of impurities by identifying extraneous peaks or shifts in absorbance.
• Dissolution Studies: Monitoring the release rate of drugs from dosage forms over time.
• Kinetic Studies: Determining the rate of degradation or stability of pharmaceuticals.

Ion exchange chromatography separates charged molecules based on their affinity for an ion-exchanger. The stationary phase consists of an insoluble matrix, typically cross-linked polystyrene or cellulose, covalently bonded to functional groups that carry a fixed charge.

Stationary phases are classified as:

• Cation exchangers: Contain negatively charged groups such as sulfonic acid (-SO₃⁻) for strong acid cation exchangers or carboxylic acid (-COO⁻) for weak acid cation exchangers. These attract positively charged analytes (cations).
• Anion exchangers: Contain positively charged groups such as quaternary ammonium (-N⁺(CH₃)₃) for strong base anion exchangers or primary/secondary amines (-NH₃⁺) for weak base anion exchangers. These attract negatively charged analytes (anions).

The mobile phase is usually an aqueous buffer solution. It serves to transport the sample through the column and provides the counter-ions necessary to compete with the analyte for binding sites on the stationary phase. The composition, pH, and ionic strength of the buffer are critical for controlling elution.

Factors affecting ion exchange chromatography include:

• pH of the mobile phase: This is the most critical factor, as it determines the ionization state of both the analyte and the functional groups on the stationary phase.
• Ionic strength: Increasing the concentration of competing ions in the mobile phase reduces the retention of the analyte by displacing it from the stationary phase.
• Nature of the analyte: The charge density and size of the analyte influence its affinity for the stationary phase; higher charge density generally leads to stronger binding.
• Temperature: Changes in temperature can affect the kinetics of ion exchange and the selectivity of the separation.
• Flow rate: Affects the residence time and the efficiency of the ion exchange process; improper flow rates can lead to band broadening.