Microwave Spectroscopy 

The section of the electromagnetic spectrum extending approximately from 1 mm (300,000 MHz) to 30 cm (1000 MHz) is referred to as the microwave region. Spectroscopic applications of microwaves consist of almost exclusively of absorption work in gaseous samples. With some exceptions, the various types of spectra are distinguished by their energy origins. In the microwave region, transitions between rotational energies of molecules are observed directly as a characteristic of absorbing molecules as a whole with nuclear effects as the first order perturbations.

Microwave spectroscopy instrumentation system consists of three main components: a source of radiation e.g. Klystron, samples cell (typically a waveguide) and a detector (may be silicon crystal, bolometers or other heat-type detectors). Unlike optical spectrometers, the microwave spectrometer is a completely electronic instrument requiring no dispersive components, since the source is monochromatic and any frequency can be selected and measured with very high precision. In addition to the three basic components, a complete spectrometer includes provision for modulation of the absorption spectrum, an AC amplifier for the detector output, final indicator unit such as a strip recorder, a sweep generator to vary synchronously the source frequency, a gas sample handling system and the required power supplies.

Because the lines in a microwave spectrum are usually completely resolved, all you are required to do is to compare these measured frequencies against tables of the frequencies observed for known substances so as to identify molecules. On the other hand, quantitative analysis is rather more complex, but is based on the fact that the integrated intensity and the product of the peak height and half-width of a microwave absorption line can be directly related to the concentration of molecules per unit volume.

Electron Paramagnetic Resonance Spectroscopy: Working Principle and Applications

This is a special part of microwave spectroscopy as it involves the absorption of microwave radiation by paramagnetic substances in a magnetic field. A typical arrangement of this type of spectrometer is shown below:

The electromagnet has a homogeneous gap field H which can be swept continuously from near zero to over 50 microtesla. The sweep generator produces small modulations of the main field H at the center of the air-gap. The sample cavity resonates at the Klystron frequency.

Working Principle

The electron in similar way to the proton is a charged particle; it spins much faster than a proton and so has a much stronger magnetic field. Due to this fact and being lighter than a proton, it precesses much more rapidly in a magnetic field. Hence, when microwaves travel down a waveguide and produce a rotating magnetic field at any fixed point, it can work to flip over electron magnets in matter, just as a rotating field in a coil, flips protons. If a sample is placed on the sidewall of the waveguide and the microwave radiation, applied to the external magnetic field, causes the electrons to precess, then when the precession rate reaches a resonance value and the electron flip, they extract energy from the microwaves, and the reading on the recorder dips accordingly.

If the electron, exhibits not only a magnetic moment along its own spin axis but also one linked with its circulation in an atomic orbit, the electron will possess a total magnetic moment equal to the vector sum of the magnetic moments. The ratio of the total magnetic moment to the spin value is a constant for a given atom in a given environs and is referred to as the gyromagnetic ratio or spectroscopic splitting factor for that specific electron. The verity that these ratios differ for different atoms and environs and that local magnetic fields depends upon the structure of the matter, allows spectral separation and electron paramagnetic resonance (EPR) spectroscopy. Nonetheless, not only all atoms and molecules are susceptible to this technique, in substances in which electrons are paired, magnetism is neutralized. But for unpaired electrons, electronic resonance takes place. This effect is particularly observed in unfilled conduction bands, transition element ions, free radicals and impurities in semiconductors.

Applications

The most common application of this technology is in the magnetic oxygen analyzer in biological field.

This technique is also used to measure the water content in hydrocarbon streams.

Nuclear Magnetic Resonance Spectroscopy

The Concept

When atomic nuclei (the hydrogen proton is the simplest), are placed in a constant magnetic field of high intensity and are subjected to a radio frequency alternating field, a transfer of energy takes place between the high frequency field and the nucleus to produce a phenomenon known as nuclear magnetic resonance.

If a system of nuclei in a magnetic field is exposed to radiation of frequency v such that the energy of a quantum of radiation hv is exactly equal to the energy difference between the two adjacent nuclear energy levels, then the energy transitions may occur in which the nuclei may flip back and forth from one orientation to another. A quantum of energy is equally likely to tip a nucleus in either direction, so that there is a net absorption of energy from the radiation only when the number of nuclei in one energy level exceeds the number in another. Under these conditions a nuclear magnetic resonance spectrum is observed.

Applications

Typical applications of this technique include: issues to do with locating hydrogen atoms in solids, measuring bond lengths, crystal imperfections and the determination of crystalline and amorphous fractions in polymers.

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• Principle and applications of magnetic nuclear resonance
• Radio Frequency (RF) and Microwave Spectral Analysis
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