Beyond the Molecular Frontier Challenges for Chemistry and Chemical Engineering (2003) / Chapter Skim
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5 Isolating, Identifying, Imaging, and Measuring Substances and Structures
5 Isolating, Identifying, Imaging, and Measuring Substances and Structures
Pages 55-70
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From page 55... ...
Isolating, Identifying, Imaging, and Measuring Substances and Structures GOALS Chemical scientists want to explore the natural world and identify all its chemical components. They also want and need to identify all of the new chemical substances produced directly and indirectly as a result of their synthetic and 55
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From page 56... ...
There is a constant need for better methods of chemical analysis, driven by our need to know "What's in that, how much is present, and how long will it last? " The frontiers in this field lie in improving sensitivity to detect vanishingly small quantities, to separate extremely complex mixtures of chemical substances, and to assess the structures or compositions of components.
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From page 57... ...
PROGRESS TO DATE Structure Determination A vital activity of the chemical sciences is the determination of structure. Detailed molecular structure determinations require identifying the spatial locations of all of the atoms in molecules, that is, the atomic distances and bond angles of a species.
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From page 58... ...
There is much room for further development of theoretical molecular structure calculations, but even so such methods have already become a standard part of molecular structure determinations. The following section presents a variety of instrumental spectroscopic techniques for the determination either of molecular structure or of parameters related to molecular structure.
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From page 59... ...
In some exceptional but important cases, such as the photodetachment of negative ions, the wavelength of light causing photoionization is in the visible or even the near infrared, allowing extremely precise structure determinations.
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From page 60... ...
Diffraction Techniques X-ray, neutron, and electron diffraction techniques are used to determine crystal structures and can thus be used for molecular structure determinations. Because of its high resolution and applicability to small and often weakly diffracting samples, x-ray crystallography and powder diffraction are by far the methods of choice for most structure determinations on crystalline compounds,
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From page 61... ...
Compressed electron pulses can be produced with reasonable intensities and widths of a few picoseconds; these are being used to study relatively simple molecular reactions. X-ray free-electron lasers, based on using high energy linear accelerators providing beams to long undulators, have the promise of easily reaching pulse lengths of only a few hundred femtoseconds and, with additional magnetic and optical compression schemes, likely the regime of only a few femtoseconds.
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From page 62... ...
Moreover, their calculations indicate that sufficiently bright light sources might be capable of imaging ultrasmall samples at sizes approaching that of a single biological molecule. Structure determination has greatly advanced with the invention of new ways to use x-ray crystallography, mainly new mathematical methods that permit the interpretation of the observed patterns of diffraction of x-rays by a crystal, translating it into the molecular structures in the crystal.
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From page 63... ...
Trace-metal analysis has come to be dominated by methods that volatilize the sample and then either measure its spectroscopic emission or absorption, or measure the masses of the gaseous metal ions using mass spectrometry. Volatilization is accomplished by various thermal means that include flames, furnaces, and inductively coupled or microwave plasmas.
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From page 64... ...
Lasers have innumerable uses in measurement science; in the excitation of molecular emission (fluorescence) they have become workhorses of analytical instruments and have produced remarkable levels of analytical sensitivity.
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From page 65... ...
As analytical chemistry has advanced into the world of molecules that biology creates, CE is the method of choice for the hugely complex mixtures of chemicals present in living organisms and their cellular subcomponents. CE has been employed for separation of the contents of individual cells and subcellular compartments (organelles)
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From page 66... ...
In mass spectrometry, there is an ongoing evolution of ways to entice large, thermally fragile biomolecules into the gas phase; the methods of electro spray ionization (ESI) and matrix-induced laser desorption-ionization (MALDI)
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From page 67... ...
DNA chains lying on a surface can be imaged, and even individual molecules can be stretched in order to understand the energy of their folding. Imaging of molecularly soft surfaces such as the surfaces of cells offers an enormous potential for understanding organization of molecules in natural life systems.
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From page 68... ...
The analytical time constants required for real-time measurements would be less than that inherent in the self-assembly, which may have widely varying inherent characteristic times, and are influenced by extensive molecular transport and organization. The close coupling of developments in new on-line measurements with process control theory and practice will be crucial to the fullest possible development of chemical manufacturing.
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From page 69... ...
Moreover, the challenge of solving increasingly complex problems, with the accompanying paradigm shift from hypothesis-driven to informationdriven science, places a premium on rapid, parallel, and inexpensive measurements. These trends are especially evident in the Human Genome Project, in combinatorial chemistry, and in the study of the chemical networks that control cell function.
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From page 70... ...
Separation and analysis of chemical and biological mixtures of extreme complexity and heterogeneity; 5. Determining the structural arrangements of atoms within noncrystalline chemical substances, and resolving how they change as a function of time, on any time scale.
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