Unit - 2

Spectroscopic techniques and applications

Q1) What do you mean by rotational spectroscopy?

A1) It is concerned with the measurement of the energies of transitions between quantized rotational states of molecules in gaseous form. The spectra of polar molecules can be measured in absorption or emission or IR Spectroscopy. the non-polar molecules can be measured by Raman spectroscopy. Homonuclear diatomic molecules such as molecular hydrogen nitrogen, and oxygen have no net dipole moment. when the molecule stretches and compresses itself in its one vibrational mode, the equilibrium is unchanged, and at no time does a dipole moment exist. On the other hand, heteronuclear diatomic such as HCl and CO have permanent dipole moment, which changes in magnitude as the molecule vibrates. Hence, HCl and CO have electric dipole-allowed transitions and therefore show infrared absorptions corresponding to vibrational excitation.

Q2) Briefly explain electronic spectroscopy?

A2) Electronic spectroscopy depends on the quantized nature of energy states, an c electron can be excited from its original ground state or initial excited state (hot band) and briefly exist in a higher energy excited state. Electronic transitions involve exciting an electron from one principle quantum state to another without incentive, an electron will not transition to a higher level only by absorbing energy, can an electron be excited.

In general, an excitation source such as X-rays, electrons or synchrotron radiation will eject from an inner-shell orbital of an atom.

Q3) Explain UV-VIS spectroscopy with an example?

A3) An absorption spectrometer measures the amount of light is absorbed by a compound varies across the UV and visible spectrum. We need a light source which gives the entire visible spectrum together with the near ultra-violet so that you are covering the range from about 200 nm to about 800 nm. (This extends slightly into the near infra-red as well.)

This range of wavelengths is not obtained from a single lamp, and so a combination of two is used - a deuterium lamp for the UV part of the spectrum, and a tungsten / halogen lamp for the visible part. The combined output of these two bulbs is focused on to a diffraction grating.

Q4) Where is UV-VIS spectroscopy widely used?

A4) UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules. Spectroscopic analysis is commonly carried out in solutions but solids and gases may also be studied. A wide array of different spectroscopic techniques can be applied in virtually every domain of scientific research - from environmental analysis and biomedical sciences to space exploration endeavours.

Q5) Define Fluorescence?

A5) Fluorescence, is an emission of electromagnetic radiation, usually visible light, caused by excitation of atoms in a material, which then reemit almost immediately (within about 10−8 seconds). The initial excitation is usually caused by absorption of energy from incident radiation or particles, such as X-rays or electrons. Because reemission occurs so quickly, the fluorescence ceases as soon as the exciting source is removed. A fluorescent light bulb is coated on the inside with a powder and contains a gas; electricity causes the gas to emit ultra violet radiation, which then stimulates the tube coating to emit light. The pixels of a television or computer screen fluoresce when electrons from an electron gun strike them. Fluorescence is often used to analyse molecules, and the addition of a fluorescing agent with emissions in the blue region of the spectrum to detergents causes fabrics to appear whiter in sunlight. X-ray fluorescence is used to analyse minerals.

Q6) How is fluorescence useful in medical field?

A6) It is used in various fields including Cell and molecular biology, Automated sequencing of DNA by the chain termination method, Biotechnology, Bone-margin detection: Alizarin-stained specimens and certain fossils can be illuminated by fluorescent lights to inspect anatomical structures, including bone margins. DNA detection: the compound Ethidium bromide, in aqueous solution, has very little fluorescence, as it is quenched by water etc.

Q7) Explain the principle of NMR?

A7) NMR is an analytical technique used to identify compounds. It is based on the principle that major nuclei of atoms possess magnetic moments and angular momentum and when external magnetic field is applied to them, they arrange themselves in the direction of the magnetic field, is based upon the spin of nuclei in an external magnetic field. In absence of magnetic field, the nuclear spins are oriented randomly. Once a strong magnetic field is applied, they reorient their spins i.e. aligned with the field or against the field. Orientation parallel to alignment of applied force is lower in energy. When nuclei are irradiated with RF radiation the lower energy nuclei flip to high state and nuclei said to be in resonance, hence the term nuclear magnetic resonance.

Q8) List two areas where NMR is applicable?

A8) NMR is used in biology to study the biofluids, cells, perfused organs and biomacromolecules, such as Nucleic acids (DNA, RNA) Carbohydrates, proteins, peptides and also labelling studies in biochemistry

NMR is used in food science.

NMR is used in Physical chemistry, Physics to study high pressure, diffusion, liquid crystals, liquid crystal solutions, rigid solids and membranes.

Q9) Briefly explain MRI?

A9) It is a non-invasive imaging technology that produces three dimensional detailed anatomical images, it is often used for disease detection, diagnosis and treatment monitoring; the technology involves excitation and detection of the changes in the direction of the rotational axis of proton found in water that makes up the living tissues. MRI employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field, when a radiofrequency current is pulsed through the patient, the protons are stimulated and spin out of equilibrium. The brain, spinal cord and nerves as well as muscles, ligaments and tendons are seen more clearly wit MRI than with regular X-ray and CT scans.

Q10) Suggest why MRI better than X-ray and CT scan?

A10) MRI is a vital imaging modality because of the soft tissue contrast it can provide. Depending on the type of scan, MRI can also provide an impressive level of spatial resolution. The combination of high soft tissue contrast and high resolution can enable the visualization of detailed structures not seen even in x-ray imaging and scans.