Opportunities in Chemistry Today and Tomorrow (1987) / Chapter Skim
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V. Instrumentation in Chemistry
V. Instrumentation in Chemistry
Pages 167-202
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From page 167... ...
CHAPTER V Instrumentation in Chemistry All scientific knowledge is rooted in our abilities to observe and measure the world around us. Thus, science benefits enormously when more sensitive measuring techniques come on the scene.
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From page 168... ...
Trying to find out whether there are such reactions led chemists to the discovery of the first chemical laser. O[course, it's hardly news that chemical reactions can emit light.
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We are witnessing a quantum jump ahead in our understanding of the factors that control the rates of chemical reactions. Among the tools responsible for this rapid advance are lasers, computers, molecular beams, synchrotrons, and, on the horizon, free-electron lasers.
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From page 170... ...
PULSE DURATION ~ > SPECTRAL PURITY ,ONE MICROSECOND = .000 OO1 SECOND ONE NANOSECOND S .000 OOO OO 1 SECOND ONE PICOSECOND = .000 OOO OOO OO 1 SECOND ONE FEMTOSECOND - .000 OOO OOO OOO OO 1 SECOND - .ooooo5 cm l .005 cm l < - 5 cm 5OOO cm l PULSE DURATION LIMITS FREQUEN CY ACCURACY AND V ICE VERSA Developments in the Last Decade There were three crucially important developments in laser technology that took place during the 1970s and they are having a great impact on chemistry. First, several types of tunable lasers were developed and became commercially available.
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From page 171... ...
They are most useful in the study of solid matenals, which will usually absorb a wide range of wavelengths of light. For most chemical applications, tunable sources are critically important, and these lasers are often TABLE V-A-1 Some Research Areas Utilizing the Laser Area Research Application Solar energy research, photosynthesis Uranium, plutonium isotope purification Trace element analysis, environmental monitoring Probing flames, explosions Monitoring industrial processes Cell discrimination and separation Photochemistry within biological cells Gas-phase deactivations, Chemical reactions Laser Used Excimer, dye Excimer, dye, TEA CO2 Continuous ion, color center Photochemistry Isotope separation Atomic absorption, fluorescence Combustion diagnostics Atmospheric gas analysis Biological cell sorting Cell bleaching Microsecond kinetics (1-100)
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As the hydrogen atoms at one end rotate past the hydrogen atom at the other end, the energy rises to a maximum. To learn the height of this internal rotation barrier, theoretical calTABLE V-A-2 Relative Computing Speeds of Computer Levels 2000 ,_ 1 500 <5: I: cat z ~ Too lo: o ~ 500 :~ MA I NFRAME COMPUTERS _ _ t ~ 1 1 1 1 1 1 1 1 t I I I 1 960 1 970 1 980 YEAR INDUSTRIAL USE OF LARGE COMPUTERS Computing Superminicomputers Mainframes Supercomputers Example DEC VAX 11/780 IBM 3033 CRAY IS (1)
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Because this allows detection of quite weak signals, this aIgonthm is now routinely used to record ~3C NMR sign~s and to transform infrared inte~erograms. Because of the success of these instruments, the Fourier Transform aigonthm is now being incorporated into ad sorts of equipment: electrochemical, microwave, ion cyclotron resonance, dielectnc, and solid-state NMR instrumentation.
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Such a situation is called a "molecular beam." This provides a special opportunity to study chemical reactions. The most obvious application is to cause two such molecular beams to intersect.
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I 10 100 1000 10,000 PHOTON ENERGY (eV) with velocities near the speed DESIGN GOALS ARE AMBITIOUS - AND PROMISING of light moves through a periodically alternating magnetic field (a wiggler)
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would open the way to many novel applications in chemistry. Examples are vibrational relaxation, multiphoton excitation, nonlinear processes in the infrared region, fast chemical kinetics, infrared study of adsorbed molecules, and light-catalyzed chemical reactions.
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NMR studies have shown that every one of those trees that L~eaicutter dislikes contain compounds with molecular structures like that of carophyllene oxide, the compound that gives Sconce its flavor. They've adso got this notion that it's the fungus that gets sick on those leaves.
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All this is within reach because of diagnostic tools invented by physicists and sharpened by chemists to meet the analytical and structural challenges presented by extremely complex molecules. Foremost among these tools are nuclear magnetic resonance, X-ray diffraction, and mass spectrometry.
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Present-day NMR performance takes advantage of the compactness of superconducting magnets to produce magnetic fields of tens of thousands of gauss (Io-~5 tesla)
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Then, in the penod of 1972-1975, methods were developed in which the tube containing the sample is rapidly spun around an axis tilted relative to the magnetic field. Then, the spectrometer sees a "blur," an average, of the NMR spectra of all of the angles through which the spinning sample moves.
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This angle is ADAMANTANE called '`the magic angle." NMR spectra of solid samples spun at this magic angle provide band sharpening approaching those available for liquids. Today, both organic and inorganic solids can be studied at 0.01 parts per million resolution.
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As chemists are working with more and more complex molecules, better resolution advances research capabilities as soon as it becomes technologically feasible. This can be seen in the steady rise in the magnetic fields available in commercial NMR instruments (as expressed in the proton NMR frequency, usually given in megahertz, MHz)
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From page 183... ...
Thus, the mass spectrum gives far more information than just the molecular weight of the parent molecule. Furthermore, the mass spectrometer can be coupled to other techniques, such as infrared spectroscopy or gas chromatographic fractionation, to add greatly to the significance of the mass spectrum.
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(an active drug from marijuana) can be tracked in blood plasma for over a week down to the 10-~ grams per milliliter level using combined gas chromatography and tandem mass spectrometry.
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From page 185... ...
Costs Just as for NMR, costs of mass spectrometers have increased exponentially over the last few decades but, again, these increasing costs carry with them enormous increases in capability. For example, in 1950, for about $40,000, the best instrument available had a resolution of about one part in 300, and it could be used for molecular weights up to 150.
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From page 186... ...
The atomic spacings are regular, too, in a molecular crystal, like solid naphthalene, C,oHa. But now, there are several types of spacings that will contribute to the diffraction pattern.
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The X-ray diffraction pattern reveals which atoms are bonded to which, the bond lengths and bond angles, and the molecular geometry; and it even indicates how the atoms are moving and how charges are distributed among them! It is as close as we can come to "seeing" the atoms in a molecule.
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From page 188... ...
If available, however, there are two unique advantages of neutrons over X-rays. First, their scattering from protons is of comparable intensity to that from heavier nuclei, so that neutron diffraction gives more precise information on positions and bonding of hydrogen atoms.
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3, 1983. "Mass Spectrometry/Mass Spectrometry" by R.G.
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H—N—~ N - Pt C! H' CQ Cisplatin— The Strong, Silent Type cis H`\ -Pt — N bans Cancer's a tricky dude to fight.
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The techniques of the surface sciences are of dominant importance to the advances being made in catalysis, upon which so many industries depend. Chromatography joins mass spectrometry and laser spectroscopy as an everyday too!
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. Method Electron energy loss EELS spectroscopy Bombard or Irradiate Acronym with: Electrons, 1-10 eV Physical Basis Information Obtained Molecular structure, surface bonding of adsorbed molecules Molecular structure of adsorbed molecules Energy of surface binding Vibrational excitation of surface molecules Infrared spectroscopy IRS Thermal desorption Auger spectroscopy Low-energy electron diffraction Secondary ion mass spectroscopy TDS Auger LEED SIMS Heat Infrared light Vibrational excitation of surface molecules Thermally induced desorpiion of adsorbates Electron emission from surface atoms Back-scattenng, diffraction Ejection of surface Electrons, 2-3 keV Electrons, 10-300 eV Ions, 1-20 keV atoms as ions Surface composition Atomic surface structure Surface composition Electrons are useful surface probes because their energies, and hence, their wavelengths, can be accurately controlled with their accelerating voltage.
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All of these emerging surface science techniques will permit us to watch chemical reactions as they occur on well-charactenzed and clean surfaces. This is an important development in chemistry because surfaces provide the two-dimensional reaction domain that accounts for heterogeneous catalysis.
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Laser mass spectrometry, the Raman microprobe, and scanning electron microscopy (SEM) reach from 1,000 to 10,000 A (i.e.' to one micron)
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Detection also has improved with electrochemical, fluorometric, and mass spectrometric detectors, reaching sensitivities as low as 1o-~2 grams. Although gas chromatography is a more mature field by perhaps a decade, important advances continue to appear.
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Combined advances in the column and detector areas now make trace analytical determinations below 10-~2-gram levels practical by capillary gas chromatography. Of particular note is the combination of capillary GC with powerful identification methods such as mass spectrometry and Fourier Transform infrared spectroscopy, as mentioned in Section IV-C.
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From page 197... ...
Consider, for example, the water molecule. This bent, Resonant Vibrational Motions of H2O it'd \~ /~/ All or ~ '0 'A %~` it\\ 3587 cm~ 1 35(30 cm~ 1 1600 cm~ Vibrational Frequencies Reveal Bond Strengths and Bond Angles 197
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A large, research-oriented chemistry department might operate 5 to 10 infrared spectrometers with capabilities ranging from rugged, low-resolution instrument for instruction in an advanced first-year chemistry course, to high-resolution Fourier Transform Infrared Spectrometers (FTIR) , suited to molecular structure determination and specialized research use.
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From page 199... ...
This is because gaseous molecules of Laser magnetic low molecular weight are importar~t, including formaidehyde, nitric acid, sulfur dioxide, acetaldehyde, ozone, oxides of chlorine and nitrogen, nitrous oxide, carbon dioxide, and the Freons. These substances are influential participants in photochemical smog production, acid rain, stratoTABLE V-C-2 Additional Instrumental Techniques in Modern Chemistry Instrument Information Obtained Reaction rates of gaseous molecular ions Precise molecular structures, gaseous free radicals Vibrational spectrum Lifetimes9 electronically excited molecules Stereo confol~llations Laser-activated cell sorter Automated analysis of protein sequence Automated synthesis of designed DNA segments Molecular structure, gases Tracking radiotracers Ion cyclotron resonance resonance Laser Raman Fluorimeter Circular dichroism Flow cytometer Protein sequencer Oligonucleotide synthesizer Electron diffraction Scintillation counter 199
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24, 1986. "Instrumentation '8~Mass Spectrometry" (C.& E.N.
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CHAPTER VI The Risk/Benefit Equation in Chemistry
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An example is the Fourier Transform Infrared Spectrometer. This sophisticated device can look through a mile or so of city air and identify all the chemical substances present and tell us their concentrations down to the parts-perbillion level.
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