Question

There are several distinct differences between IR and UV-Vis spectroscopy. Discuss the following as they relate to:
(a)   The types of interactions responsible for the signals generated
(b)   Source of radiant energy
(c)   Signal detection
(d)   Sample cells
(e)   Sample preparation

Answer 1

Ans:

S. No.	Discussion Points	IR Spectroscopy	UV-Vis Spectroscopy
(a)	The types of interactions responsible for the signals generated	The IR region is from 2 -15 µm in the electromagnetic spectrum. It’s a molecular spectroscopy. The IR radiations interacts or absorbed by different types of the vibration of bonds or functional groups present in the molecule. Various types of molecular vibrations like, stretching, bending, scissoring, wagging, twisting interacts with the IR rays and absorbs them. The molecule should have net dipole moment to interact with the IR rays.	The UV-Vis region is from 180 – 1100 nm in the electromagnetic spectrum. It’s an atomic spectroscopy. It corresponds to the excitation of outer electrons of the molecules. There are three types of electronic transition: 1. Transitions involving p, s, and n electrons 2. Transitions involving charge-transfer electrons 3. Transitions involving d and f electrons
(b)	Source of radiant energy	The Nernst glower - Made up of rare earth oxides. The Globar source - Made up of silicon carbide. The incandescent wire source – Made up of Nichrome wire.	Tungsten filament (300 - 2500 nm) or a deuterium arc lamp (190 - 400 nm) or a Xenon arc lamp which is continuous from 160 - 2,000 nm
(c)	Signal detection	Thermocouples - Consist of a pair of junctions of different metals. Pyroelectric detectors - Single crystalline wafer of a pyroelectric material, such as triglycerine sulphate. Photoelectric detectors - Mercury cadmium telluride, comprise a film of semiconducting material deposited on a glass surface. Commonly used in the modern FTIRs	A photomultiplier tube or photodiode, PDA (photodiode array) or a CCD (charge-coupled device).The signal is measured as peaks.
(d)	Sample cells	Potassium bromide windows for liquid films, Pellets made by mixing and pressing sample with KBr, ZnSe Discs for liquid films, NaCl cells for quantitation of the samples in solution form. Direct sample powder/liquid by ATR accessory.	Quartz Cuvettes of different path length, e.g. 1 cm or 5 cm or 10 cm with lid, to hold the solution between the source and detector. The beam of EM radiation should pass through the sample to get the spectra.
(e)	Sample preparation	As such in ATR accessory. Liquid film in KBr/NaCl/ZnSe windows. Solutions in NaCl cells using spacers for quantitative analysis.	Generally samples are prepared in solvent in which the sample or compound is fully soluble. It can be organic solvents or also the water. The same solvent is to be used as blank measurement. The concentration is generally in 1 – 100 mg/Kg range to get the signal in the range of 0 – 1 AU.