JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy IV Year INSTRUMENTAL METHODS OF ANALYSIS Previous Question Paper (February-2022 - R17)

i) Auxochromes: These are functional groups attached to a chromophore that do not produce color themselves but modify the ability of the chromophore to absorb light, typically by shifting the absorption maximum to longer wavelengths and increasing the intensity of absorption. Examples include -OH, -NH2, and -OCH3 groups.

ii) Chromophores: These are unsaturated functional groups or covalent bonded structures responsible for the absorption of light in the ultraviolet or visible region. They contain electrons capable of undergoing electronic transitions (e.g., π → π* or n → π*). Examples include C=C, C=O, and -N=N- groups.

iii) Spectral shifts: These refer to changes in the position of the absorption maximum (λmax) due to structural changes or solvent effects. Key types include:
Bathochromic shift (red shift): Shift to a longer wavelength.
Hypsochromic shift (blue shift): Shift to a shorter wavelength.
Hyperchromic effect: Increase in molar absorptivity (intensity).
Hypochromic effect: Decrease in molar absorptivity.

i) Photomultiplier tube (PMT): A highly sensitive detector that converts light into an electrical signal. It consists of a photoemissive cathode followed by a series of dynodes at increasing potentials. When a photon hits the cathode, it releases electrons which are accelerated through the dynodes, triggering secondary electron emission at each stage, resulting in massive signal amplification.

ii) Silicon photodiode: A semiconductor-based detector that functions by generating electron-hole pairs when photons strike the p-n junction. It is compact, robust, and offers a wide linear dynamic range. It is commonly used in modern spectrophotometers due to its high quantum efficiency and stability compared to vacuum-based tubes.

iii) Sample cells (Cuvettes): These are containers used to hold the liquid sample in the light path of the instrument. They must be transparent to the radiation being measured. Glass or plastic cuvettes are used for the visible region, whereas quartz or fused silica cells are required for the ultraviolet region because glass absorbs UV light.