Figure 2 Absorbance of water in the terahertz domain blue. Sign up to receive regular email alerts from Physical Review E. Journal: Phys.
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X Rev. A Phys. B Phys. C Phys. D Phys. E Phys. Fluids Phys. Theoretical models, principally quantum mechanical models, allow for the absorption spectra of atoms and molecules to be related to other physical properties such as electronic structure , atomic or molecular mass , and molecular geometry.
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Therefore, measurements of the absorption spectrum are used to determine these other properties. Microwave spectroscopy , for example, allows for the determination of bond lengths and angles with high precision. In addition, spectral measurements can be used to determine the accuracy of theoretical predictions.
For example, the Lamb shift measured in the hydrogen atomic absorption spectrum was not expected to exist at the time it was measured. Its discovery spurred and guided the development of quantum electrodynamics , and measurements of the Lamb shift are now used to determine the fine-structure constant.
The most straightforward approach to absorption spectroscopy is to generate radiation with a source, measure a reference spectrum of that radiation with a detector and then re-measure the sample spectrum after placing the material of interest in between the source and detector.
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The two measured spectra can then be combined to determine the material's absorption spectrum. The sample spectrum alone is not sufficient to determine the absorption spectrum because it will be affected by the experimental conditions—the spectrum of the source, the absorption spectra of other materials in between the source and detector and the wavelength dependent characteristics of the detector. The reference spectrum will be affected in the same way, though, by these experimental conditions and therefore the combination yields the absorption spectrum of the material alone.
A wide variety of radiation sources are employed in order to cover the electromagnetic spectrum. For spectroscopy, it is generally desirable for a source to cover a broad swath of wavelengths in order to measure a broad region of the absorption spectrum. Some sources inherently emit a broad spectrum. Examples of these include globars or other black body sources in the infrared, mercury lamps in the visible and ultraviolet and x-ray tubes.
One recently developed, novel source of broad spectrum radiation is synchrotron radiation which covers all of these spectral regions. Other radiation sources generate a narrow spectrum but the emission wavelength can be tuned to cover a spectral range.
Examples of these include klystrons in the microwave region and lasers across the infrared, visible and ultraviolet region though not all lasers have tunable wavelengths. The detector employed to measure the radiation power will also depend on the wavelength range of interest. Most detectors are sensitive to a fairly broad spectral range and the sensor selected will often depend more on the sensitivity and noise requirements of a given measurement.
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Examples of detectors common in spectroscopy include heterodyne receivers in the microwave, bolometers in the millimeter-wave and infrared, mercury cadmium telluride and other cooled semiconductor detectors in the infrared, and photodiodes and photomultiplier tubes in the visible and ultraviolet. If both the source and the detector cover a broad spectral region, then it is also necessary to introduce a means of resolving the wavelength of the radiation in order to determine the spectrum.
Often a spectrograph is used to spatially separate the wavelengths of radiation so that the power at each wavelength can be measured independently. It is also common to employ interferometry to determine the spectrum— Fourier transform infrared spectroscopy is a widely used implementation of this technique.
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Two other issues that must be considered in setting up an absorption spectroscopy experiment include the optics used to direct the radiation and the means of holding or containing the sample material called a cuvette or cell. In both cases, it is important to select materials that have relatively little absorption of their own in the wavelength range of interest. The absorption of other materials could interfere with or mask the absorption from the sample.
For instance, in several wavelength ranges it is necessary to measure the sample under vacuum or in a rare gas environment because gases in the atmosphere have interfering absorption features. From Wikipedia, the free encyclopedia. Fundamentals and Techniques of Biophysics and Molecular biology. New Delhi: Pathfinder publication. Harris, Michael D.
Ingle, Jr. Archived from the original on Retrieved Bibcode : Sci Categories : Scientific techniques Analytical chemistry Spectroscopy Electromagnetic radiation Astrochemistry Radiation. Hidden categories: All articles with unsourced statements Articles with unsourced statements from August Namespaces Article Talk. Views Read Edit View history.
The authors also determined the energetically preferred states of crystal hydration and suggested ways of more closely reconciling theoretical calculations with observed spectra. The extreme sensitivity of THz to solid forms of materials and the presence or absence of water in these materials has obvious applications in process engineering, crystal engineering, IP patrolling, quality control and identification of counterfeit drugs. In PML, this methodology has recently been extended to pharmaceuticals.
This obviously impacts the drug's dosing profile approved by the FDA. The solid form has to be the same not only over the entire production campaign spanning many years but also during storage leading up to consumption. For example, there are many instances in which we are not sure what the spectra are telling us. And very often we find that the theoretical calculations are completely out of agreement with our observations.
Indeed, one of our most important contributions to the field is providing sets of experimental data against which the theory can be tested. The researchers are gradually working their way up to progressively more complicated samples.
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We now have data for non-steroidal anti-inflammatory drugs like Naproxen and Norfloxacin and antibiotics like Amoxicillin and Ampicillin. X-ray crystallography doesn't reveal hydrogen bonding interactions, but THz vibrations do. And knowing the placement and behavior of hydrogen bonds can go a long way toward answering some very fundamental questions, such as how the complex stretches and contorts itself, exactly what happens when the molecules bind, and what constitutes a 'good fit.
Many observers believe that the steep rise in the cost of new drug development has negative consequences for nation's economy, budgetary outlook, and delivery of healthcare to a rapidly aging population. In those circumstances, in silico screening of drug candidates may be an important tool because it provides a cost-efficient way to design and develop drug candidates.