Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 193
CHAPTER V Chemistry and National Well-Being V-A. Better Environment Every society tries to provide itself with adequate food and shelter and a healthful environment. When these elemental needs are assured, attention turns to comfort and convenience. The extent to which all these wishes can be satisfied determines a society's "quality of life." However, choices are usually required because one or another of these needs or wishes is more easily satisfied at the expense of others. Today we find our desires for more abundant consumer goods, energy, and mobility in conflict with maintenance of a healthful envi- ronment. A major concern of our times is the protection of our environment in the face of increasing world population, increasing concentration of population (urbanization), and increasing standards of living. Environmental degradation with its accompanying threats to health and disruption of ecosystems is not a new phenomenon. Human disturbance of the environment has been noted from the earliest recorded history. The problem of sewage disposal began with the birth of cities. Long before the 20th century, London was plagued with air pollution from fires used for heating and cooking. An early example of an industrial hygiene problem was the reduced longevity of chimney sweeps attributed retrospectively to cancer arising from prolonged exposure to soot with its trace carcinogen content (polynuclear aromatic hydrocarbons). There is small consolation, though, in the fact that environmental pollution is not a new invention. The global population burgeons upward, while cities grow even faster. Per capita consumption and energy use continue to increase. Pollution problems are becoming increasingly obvious, and we are recognizing subtle interactions and secondary reverberations that went unnoticed before. A number of environmental disturbances have begun to manifest themselves on a global scale. Occasional industrial accidents, like those at Bhopal and Seveso, remind us that large-scale production of needed consumer products may require handling of large amounts of potentially dangerous precursor substances. 193
OCR for page 194
194 CHEMISTRY AND NATIONAL WELL-BEING On the positive side, the public awareness has been raised about the importance of maintaining environmental quality. In the United States, a large majority of citizens from across the political spectrum have indicated that they are prepared to pay more for "cleaner" products (e.g., lead-free gasoline) and to pay more taxes to improve their environment. These attitudes are spreading abroad, an essential aspect of containment of the problems more global in scope. Elective strategies for safeguarding our surroundings require adequate knowledge and understanding. We must be able to answer the following questions: What potentially undesirable substances are present in our air, water, soil, and food? Where did these substances come from? What options are there- alternative products and processes to alleviate known problems? What is the quantitative degree of hazard as a function of the extent of exposure to a given constituent? How shall we choose among and implement available options that over corrective action? Plainly, chemists play a central role in answering the first three crucial questions. To find out what is around us, we need analytical chemists to apply and develop ever more sensitive and selective analytical techniques. To track pollutants back to their origins, again we look to analytical chemists acting as sleuths, now usually in collaboration with meteorologists, oceanographers, voicanologists, climate dynamicists, biologists, and hydrologists. But finding origins can require detailed chemical understandings of reaction sequences and transformations that intervene between the source and the final noxious or toxic product. Then, developing options calls on the full range of the chemist's arsenal. If the worId's mortality rate from malaria is not to be reduced with DDT because of its environmental persistence, what substances can be synthesized that are as elective as DDT in saving lives and spontaneously degradable as well? If we must use lower grade energy sources to satisfy our society's energy needs, what catalysts and new processes can be developed to avoid exacerbating aIready-existing problems of acid rain and carcinogen release from coal-fired power plants? Thus our society must assure the health of its chemistry enterprise if it wants earlier warning of emerging environmental degradation, better understandings of the origins of that degradation, and access to a full array of economically feasible options from which to choose solutions. Other disciplines make their own particular contributions, but none plays a more central and essential role than chemistry. The fourth question, the quantitative degree of the hazard/exposure equation, is the province of the medical profession, toxicologists, and epidemiologists. These scientific disciplines face serious challenges now that society has recog-
OCR for page 195
V-A. BETTER ENVIRONMENT nized the inverse relationship between what is taken to be a tolerable risk and the cost to society in attaining it. The medical profession must refine its knowledge of risks associated with such substances as lead in the atmosphere, chloroform in drinking water, radiostrontium in milk, benzene in the work- place, and formaldehyde in the home. A qualitative statement that a certain class of substances might be carcinogenic will no longer suffice. We must be able to weigh risks and costs against benefits that would be lost if use of that class of substances were restricted. We must be able to compare these risks to those already present because of natural background levels. More importantly, society cannot afford to pay the exorbitant cost of eliminating all risk, because, as the requested degree of risk approaches zero, the cost escalates toward infinity. Finally, the choice among options and their implementation moves properly into the public arena. Chemists and scientists in the other relevant disciplines have a secondary, but important, informational responsibility here. Every political decision deserves the best and most objective scientific input available. There is nothing more frustrating to our citizens and our government than to befaced with decisions without the benefit of facts and a usefully predictive scientific knowledge base. Scientists, including chemists, must meet their responsibility to provide the public, the media, and the government with an objective picture expressed in language free of technical jargon to help establish the scientific setting for a given decision and the options that lie before us. Turning Detection into Protection All our environmental protection strategies should be founded on realistic hazard thresholds and on our ability to detect a particular offending substance well before its presence reaches that threshold. Chemists must continue to sharpen their analytical skills so that, even at minute concentrations well below the hazard threshold, a given substance can be monitored long before crisis-corrective action is dictated. When this is possible, we see that detection can be equated to protection. Unfortunately, the media, the public, and government agencies have too often equated detection with hazard as a result of the prevalent assumption that a substance that is demonstrably toxic at some particular concentration will be toxic at any concentration. There are innumerable examples to prove that this is not a generally applicable premise. Consider carbon monoxide (CO). This ubiquitous atmospheric constituent becomes dangerously toxic at concentra- tions exceeding 1000 parts per million and is considered to have adverse health effects for prolonged exposure to concentrations exceeding 10 parts per million. We do not, however, leap to the conclusion that CO must be completely removed from the atmosphere. This would be foolish (and impossible) because we live and thrive in a natural atmosphere that always contains easily detectable CO at about 1 part per million. Clearly, our task is to decide where we should place 195
OCR for page 196
196 CHEMISTRY AND NATIONAL WELL-BEING controls between the known toxicity threshold and the known safe range- as EPA has in fact attempted to do. In recent trends, the naive "zero-risk" approach is gradually being supplanted by a more sophisticated risk assessment/risk management rationale. In both the assessment and management phases, a major theme is the crucial importance of being able to analyze complex air, water, soil, and biological systems that may contain hundreds of natural chemical compounds. The roles that chemical analysis and monitoring play in protecting and managing our air and water resources are analogous to the role that intelligence-gathering plays in protect- ing and promoting the nation's public interest in the military, geopolitical, and economic realms. Conclusions regarding causes and erect, sources, movement, and fates of pollutants in crucial issues such as acid deposition, global climatic change, ozone layer destruction, and toxic waste disposal depend upon environ- mental measurements that must be made with sufficient selectivity and sensitivity. Enormously costly decisions about how to protect and enhance the quality of our air, water, and land resources are sometimes based on environ- mental "intelligence" that can be grossly inadequate and inaccurate. Crash projects to remedy crises caused by past indiscretions or ineffective strategies that were based on insufficient knowledge have been expensive. A small fraction of the money spent on such a corrective program, if invested in long-term fundamental environmental science and monitoring techniques, can significantly reduce the need for future expensive remedial programs. THERE ARE 22 DIFFERENT TETRACHLORODIOXINS Increased effectiveness of environmental measure- ments requires improved sur- veilIance tools. The challenge is to measure trace levels of a particular compound present in a complex mixture contain- ing many innocuous com- pounds. The principal objec- tives of research in envir- ct~o~ct Cl C1~0~ 2,3,6,9 C' Ct CO~O~Ct HOW MUCH IS IN THE TOXIC 2,3,7,8 FORM? AN ANALYTICAL CHALLENGE onmental analysis and moni- toring are improved sensitiv- ity, selectivity, separation, sampling, accuracy, speed, and data interpretation. For example, an active research area is connected with sepa- ration techniques to allow rapid and unequivocal analy- sis of complex mixtures of pollutants and pesticides found in toxic wastes, polluted streams and lakes, and biological samples. A success story in analytical selectivity is the development of analytical methods to allow separation and
OCR for page 197
V-A. BETTER ENVIRONMENT quantitative measurement of each of the 22 individual isomers of tetrachIo- rodioxin at the parts-per-trillion level. Research is needed on multi-stage separation and detection methods, such as tandem mass spectrometry. Additional research on developing techniques that exploit the separation powers of selective sorbents, liquid chromatography, supercritical fluid chromatography, field flow fractionation, and parametric pumping will be fruitful. Attention must be given to basic research that will produce selective detectors (based, e.g., on laser-induced fluorescence or on chemiluminescence) that can be used to simplify analysis and minimize inter- ferences that can accompany environmental sampling. Highly reactive species in the atmosphere cannot be sampled and transported to the laboratory for analysis. Such substances pose special challenges in measurement and will require research aimed at remote sensing techniques capable of measuring them in situ. Past successes include the measurement of formaldehyde and nitric acid in the atmosphere of Los Angeles, during severe smog attacks, by Fourier transform infrared spectroscopy in which absorbance due to these pollutants was measured in situ over a 1-kilometer path. With these experiments it was possible to perform a detailed characterization of the simultaneous concentrations of formaldehyde, formic acid, nitric acid, peroxyacetyl nitrate, and ozone in the ambient air at the part-per-billion level at which these substances are contributors to photochemical smog. Notice that 1 part per billion (1 part of a pollutant in 109 parts of air) is a minute concentration, but it is still sufficient to be significant in atmospheric reactions. Differential scanning laser devices based on radar-like technology ("lidar") have been used successfully to measure sulfur dioxide plume profiles downwind of coal-fired power plants at the part-per-million level. Tunable diode lasers are also capable of providing real-time in situ detection of pollutants from internal combustion engines and industrial processes. Several laser techniques, including linear methods (e.g., absorption, fluores- cence), nonlinear methods (e.g., coherent antistokes Raman spectroscopy, opti- cal heterodynes), and double resonance methods (e.g., laser magnetic reso- nance), need to be examined more extensively. Other spectroscopic methods, such as Fourier transform infrared and photoacoustic spectroscopy, are prom- ising and warrant further study. One goal of such research should be better measurements in the stratosphere and troposphere. Rapid, reliable, accurate, and less-expensive methods are needed for measuring concentrations of trace species, such as OH radicals, that play key roles in atmospheric chemistry. At the same time that research aimed at more sophisticated measurement technology is conducted, parallel efforts need to be devoted to simpler, less costly routine monitoring techniques. Research directed at fixing the chemical state of environmental constituents (speciation research) is gaining importance because we now recognize that transport mechanisms and toxicities vary markedly with chemical form. Chro- mium in the hexavalent oxidation state is toxic, while in the trivalent form it is 197
OCR for page 198
198 CHEMISTRY AND NATIONAL WELL-BEING much less so, and, for some living systems, it may be an essential trace element. Arsenic in some forms can move rapidly through aquifers, while other forms are rapidly adsorbed on rock or soil surfaces. Of the 22 distinct structural arrange- ments of tetrachIorodioxin, the most toxic is three orders of magnitude more toxic (to test animals) than the second most toxic. These examples illustrate the importance of analytical methods that allow identification of chemical form as well as quantity of potential pollutants. Electrochemistry, chromatography, and mass spectrometry are among the powerful tools for speciation studies. The complexity of environmental problems requires analysis of massive amounts of data. Research is needed to assist in the interpretation and wise use of the accumulated information. Developments in the field of artificial intelli- gence that use pattern recognition should provide valuable interpretive aid. Recent advances in microprocessors and small computers should be exploited to develop intelligent measuring devices and attention should be given to better handling, archiving, and dissemination of environmental data. Ozone in the Stratosphere The possibility of polluting the stratosphere to the point of partially depleting the protective ozone layer was first raised only about a dozen years ago. This seemingly improbable notion found much scientific support, and it is now one of the best examples of a potentially serious environmental problem of global extent. It is a problem, furthermore, that exemplifies chemistry's central role in its understanding, analysis, and solution. Why do we need to worry about stratospheric chemistry? Ozone in the stratosphere is the natural filter that absorbs and blocks the Sun's short wavelength ultraviolet radiation that is harmful to life. The air in the stratosphere a cloudless dry, cold region at altitudes between about 10 to 50 km mixes slowly in the vertical direction, but rapidly in the horizontal. Consequently, harmful pollutants, once introduced into the stratosphere, might remain there for periods as long as years, and, if so, they will rapidly be distributed around the earth across borders and oceans, making the problem truly global. A large reduction of our ozone shield would result in an increase of potentially dangerous ultraviolet radiation at the earth's surface. To understand how easily the ozone layer might be perturbed, it is useful to recognize that ozone is actually only a trace constituent of the stratosphere; at its maximum concentration ozone makes up only a few parts per million of the air molecules. If the diffuse ozone layer were concentrated into a thin shell of pure ozone gas surrounding the earth at atmospheric pressure, it would measure only about 3 millimeters (his inch) in thickness. Furthermore, ozone destruction mechanisms are based on chain reactions in which one pollutant molecule may destroy many thousands of ozone molecules before being trans- ported to the lower atmosphere, chemically transformed, and removed by rain. Chemistry's crucial role in understanding this problem has emerged through the identification and measurement of several ozone-destroying chain processes.
OCR for page 199
V-A. BETTER ENVIRONMENT Fifty years ago, the formation of an ozone layer in the midstratosphere was qualitatively described in terms of four chemical and photochemical reactions involving pure oxygen species (O. 02, and ON Today, we know that the rates of at least 150 chemical reactions must be considered in order to approach a quantitative model for simulating the present stratosphere and predicting changes resulting from the introduction of various pollutants. The chemistry begins with absorption of solar ultraviolet radiation by O2 molecules in the stratosphere. Chemical bond rupture occurs, and ozone, O3 and oxygen atoms, O. are produced. Then, if nitric oxide, NO, is somehow introduced into the stratosphere, an important chemical chain reaction takes place. UV down . \' STRATO SPHERED D to 18 OZONE ABSORBS UV LIGHT f~ -)N~O+hv~N + NO (33+ NO~N(~2 ~ O2 O ~ NO2 ~ NO ~ O2 NO2+ OH net °+O3 iO2+O2 50 km 280°E 0 7 tort HNO3 UV down ~ N2O ~ JO to 300 nm~ ~ TROPOSPHERE ~- ~ ~ ~~ KA1N 210 1E f I: : . : : 2 : : :! : : : _ ~ an _ _.-: .-:: __:: :-. - :~-:: =_—My_ - ~ ~ 7^ HA__ A. -c_ - ., _-. - .. : . e— -I -.~. -` ~ ~ V W ~ 1 ~ . : :~7~1 : :::::::: .:::::::: -'} I....:::: ' :-~_` _- ..~ .- .-. -' -.- ' ' c.' '-- - ' - ' ' - . ~ ..~ - ~ I ,.,.,., ,., ,. , , ., , , , c .. , ,, ,, . . _ .... 115~ _ ,d~ R;,~~1'~:'~''·'· ~~'"~"--— ˘~ 1O-15km ., 11~ ~ ~., The NO and NO2 reactions together furnish a true catalytic cycle in which NO and NO2 are the catalysts. Neither species is consumed, because each is regenerated in a complete cycle. Each cycle has the net effect of destroying one oxygen atom and one ozone molecule (collectively called "odd oxygen". This catalytic cycle is now believed to be the major mechanism of ozone destruction in the stratosphere. In the natural atmosphere, the oxides of nitrogen are provided by biogenic emissions at the Earth's surface by soil and sea bacteria of nitrous oxide, N2O. This relatively inert molecule slowly mixes into the stratosphere where it can absorb ultraviolet light and then react to form NO and NO2. Of course, oxides of nitrogen directly introduced to the stratosphere are expected to destroy ozone as well, and this was the basis of the first perceived threat to the ozone layer large fleets of supersonic aircraft flying in the _ stratosphere and depositing oxides of nitrogen via their engine exhausts. Nuclear explosions also produce copious quantities of oxides of nitrogen, which are carried into the stratosphere by the buoyancy of the hot fireballs. A significant depletion of the ozone layer in the event of a major nuclear war was 199
OCR for page 200
200 CHEMISTRY AND NATIONAL WELL-BEING forecast in a 1975 study by the National Academy of Sciences, although this environmental erect of nuclear war may pale in comparison with the recently suggested potential of a "nuclear winter." Both effects underscore the delicacy of the atmosphere and its sensitivity to chemical transformations. Then, in 1974, just as the possible impact of stratospheric planes was reaching the analysis stage, concern was raised about other man-made atmospheric pollutants. Halocarbons, such as CFCI3 and CF2CI2 (chIorofluoromethanes, or CFMs), had become popular as spray-can propellants and refrigerant fluids, mainly because of their chemical inertness. The absence of reactivity meant absence of toxicity or other harmful effects on terrestrial life. Ironically, this meant that there was no place for the CFMs to go but up up into the stratosphere where ultraviolet photolysis could occur. Chemists then recognized that if this occurred, the resultant chlorine species, CT and ClO, could enter into their own catalytic cycle, destroying ozone in a manner exactly analogous to the destruction caused by the oxides of nitrogen. , . STR A TO split Fit UV down to 180nm rCF2CD2 + he ) C9 + CF2CD OZONE |F O3 ~ c/ ~ L9U · oz ABSORBS ~ 0 ~ coo ~ c, ~ O2 LIGHT l net ° ~ O3 ~ O2+ O2 ~ UV down ~ CF2C02 relet HC0 TROPO SPHERE I_ ~ - - - Ad. - - ~ . - -. _ 50 Em 280° ~ \0.7 tort CD ~ CHID 10-15km RAIN 210°Z ~ . . . . . . . ~ . . ^ : . . . . arc ~ ~~ ~ ~~ ~~. ~ 70torr A is. ~ . ..... . ~ ~ ~ , ^,~ ~-------·-----~-·~-~~-;- - --.--. - ·-~ Once this possibility had been recognized, analysis of the whole stratospheric ozone chemistry began in earnest. An international committee of scientific experts assembled by the National Academy of Sciences examined in detail the state of our knowledge of every aspect of the problem. It became clear that the additional chemistry introduced to the stratosphere added not just these 2 catalytic chemical reactions to the roster, but a total of about 40 new reactions involving such species as CI, ClO, HCI, HOCI, ClONO2, the halocarbons, and several others. Most of these reactions had never before been studied in the laboratory. I,aboratory kineticists and photochemists responded to the challenge by .
OCR for page 201
V-A. BETTER ENVIRONMENT providing reliable rate con- stants and absorbances for the proposed processes using the growing arsenal of mod- ern experimental methods. Recent progress in the exper- imental accomplishments of this field has been remark- able. It has become possible, a/ CH4 ~ O3 \ HOCl /\ To HCl _ Cl OH `` ClO O1 NO / for example, to generate he \ / NO2 nearly any desired reactive REACTIONS KEY TO \ molecular species in the labo- STRATOSPHERIC \ ~ ratory in a variety of ways, to PROTECTION ClONO2 bring them together with other reactive species, and to measure their rates of reaction under known, controlled conditions. Such direct measurements of these extremely rapid reactions were only a distant goal a decade ago, but they are now a reality. Finally, field measurements of minor atmospheric species have been revolu- tionized by some of the recent advances in analytical chemistry. Methods originally developed for ultra-sensitive detection of extremely reactive species in laboratory studies have been modified and adapted to measure such constit- uents as O. OH, Cl, ClO and others at parts-per-trillion concentrations in the natural stratosphere. This has been accomplished recently in experiments in which a helium-filled balloon carries an elaborate instrument package to the top of the stratosphere where the package is dropped while suspended by a parachute. As the instrument traverses the stratosphere, it measures concen- trations of several important trace chemical species and telemeters the infor- mation to a ground station. Very recently, the first successful reel-down experiment was performed in which the instrument package was lowered 10 to 15 km from a stationary balloon platform and reeled back up again like a giant yo-yo. This method results in a huge increase in the amount of data that can be obtained in a single balloon flight. It will also allow for the first time a study of the time evolution and variability of the stratosphere. Much has been accomplished in the past 10 years. Many of the needed 100 to 150 photochemical and rate processes have been measured in the laboratory, and many of the trace species measured in the atmosphere. Yet, research remains to be done. For example, two of the important chemical species containing chlorine, HOCl and ClONO2, have yet to be measured anywhere in the stratosphere. Refinements in the reaction rates for many of the important processes are still required, and exact product distributions for many of the reactions are still lacking. Nevertheless, the original NAS study, the research 20}
OCR for page 202
202 Natural Sources Marsh— I ndustry—Transportation Ocean Man-made Sources CHEMISTRY AND NATIONAL WELL-BEING programs it spawned, and the subsequent follow-up studies provided a firm and timely basis for legislative decisions about regulation of CFM use. Industrial chemists produced alternative, more readily degradable substances to replace the CFMs in some applications. Monitoring programs are in place so that trends in the stratospheric composition can be watched. The stratospheric ozone issue provides a showcase example of how science can examine, clarify, and point to solutions for a potential environmental disturbance. Premature initiation of regulation was avoided because the problem was recognized early enough to permit deliberate, objective analysis and focused research to narrow the uncertainty ranges. From first recognition on, chemists played a lead role. Reducing Acid Rain Acid rain is one of the more obvious and pressing results of degradation of air quality. Acidic substances and their precursors are formed when fossil fuels are burned to generate power and provide transportation. These substances are principally acids derived from oxides of sulfur and nitrogen. There are some natural sources of these compounds such as lightning, voIcanos, burning biomass, and microbial activity, but, except for rare volcanic eruptions, these are relatively small compared with emissions from power plants, smelters, and vehicles in industrial regions. The effects of acidic rainfall are most evident and highly publicized in Europe and the northeastern United States, but areas at risk include Canada and Prevailing Winds ~ ,.. i' Airy': .' . it, '\ W >'W\ W" , ~ ~ ,' ',, Photochemistry _,~T: ~c~ ~3~ ~ Aquatic Ecosystem ACID RAIN—SOURCES HERE, IMPACT THERE
OCR for page 203
V-A. BETTER ENVIRONMENT perhaps the California Sierras, the Rocky Mountains, and China. In some places precipitation as acidic as vinegar has occasionally been observed. The extent of the erects of acid rain is the subject of continuing controversy. Damage to aquatic life in lakes and streams was the original focus of attention. More recently, damage to buildings, bridges, and equipment has been recognized as another costly consequence of acid rain. The effect of polluted air on human health is the most difficult to quantify. Greatest damage is done to lakes that are poorly buffered. When naturally alkaline buffers are present, the acidic compounds in acid rain, largely sulfuric acid, nitric acid, and smaller amounts of organic acids, are neutralized, at least until this alkalinity is consumed. However, lakes lying on granitic (acidic) strata are susceptible to immediate damage because acids in precipitation can dissolve metal ions, such as aluminum and manganese, causing reduction in biological productivity and, in some lakes, the decline or elimination of fish populations. Damage to plants from pollution ranges from adverse effects on foliage to destruction of fine root systems. In a region such as the northeastern United States the principal candidates for pollutant reduction are the power plants burning coal with high- sulfur content. Chemical scrubbers that prevent the emission of the pollutants offer one of the possible remedies. Catalysts that reduce oxides of nitrogen emissions from both stationary and mobile sources offer yet another example of the role that chemistry can play in improving air quality. The various strategies for reducing acid rain involve possible investments of billions of dollars annually. With the stakes so high, it is imperative that the atmospheric processes determining the transport, chemical transformation, and fate of pollutants be well understood. Acid deposition consists of both "wet" precipitation (as in rain and snow) and dry deposition (in which aero- sols or gaseous acidic com- pounds are deposited on sur- faces such as soil particles, plant leaves, etc.~. What is finally deposited has usually been injected into the atmo- sphere in a quite different chemical form. For example, sulfur in coal is oxidized to sulfur dioxide, the gaseous form in which it is emitted from smokestacks. As it moves through the atmo- lARGE UNCERTAINTIES REMAIN IN THE GLOBAL NOx BUDGET sphere, it is slowly oxidized and reacts with water to form sulfuric acid the form in which it may be deposited hundreds of miles downwind. The pathways by which oxides of nitrogen are formed, undergo chemical 203 TROPOPAUSE -o 5 Z o , _ V) 0 1 ~14 28 En as ~ ~ :, ~ 4-24 ,~ - 1 ° NH3 + OK] ~ NOx 1 1- 10 L I GHTN I NG (PRECIPITATION: ~ —W~ MICROBIAL I ACTIVITY DRY DEPOSITION 12-42 IN SOILS I j ~ 1 4 - 1 6 12-22 TERRE STR 1 A L < 1
OCR for page 268
268 TABLE V-5 ~ ~ ~ . CHEMISTRY AND NATIONAL WELL-BEING laser Raman spectroscopy, second harmonic generation surface spectroscopy, and laser ellipsometry all provide information about the surface chemical bonds of adsorbed atoms and molecules. Solid state nuclear magnetic resonance is specially well suited for determination of the structure of high surface area solids like the molecular sieve materials. Instrument Developments Needed There are many, well-perfected techniques for the study of surfaces under well-controlled conditions. Developments of the next few years will focus more attention on techniques that clarify the molecular structure and behavior of the adsorbate. EELS is such a technique. While great effort and design skill has been invested in achieving the EELS resolution now available, about 40 cm-i, it is only marginally able to provide the structural information desired. A 10-fold improvement is needed, to a spectral resolution of 5 cm-i or better; such a gain would enormously increase the value of this technique. Research systems have been built with capacity to move a sample from an ultrahigh vacuum environment into contact with a gas and then to return to the vacuum situation. This is an important capability and should be made available commercially. Surface samples should be readily held at cryogenic tempera- tures down to 4 K. Laser techniques, too, offer special promise for chemistry- oriented surface studies. Then, to open the door to kinetics of reactions as they take place on surfaces, techniques must be developed for pulsed excitation and subsequent temporal analysis on a short time scale. Costs Table V-5 lists some of the most important instruments from Table V-4 and approximate current costs. It is important to realize that rarely can an investigator ef- fectively address a problem of surface chemistry with only one of these techniques. In- stead, an effective laboratory will need the synergism and flexibility provided by having access to three or four comple- mentary techniques. For ex- ample, the first five entries in Table V-5 are mutually sup- portive. Thus, the research group of a single investigator will require a capital investment exceeding $500K and, of course, a substantial on-going support to ensure cost-e~ective maintenance and operation of these systems. The instruments listed in Table V-4 permit us now to inaugurate a new era Approximate Current (1985) Costs ($) for Surface Science Instruments Electron energy loss spectrometer Low energy electron diffraction Auger electron spectrometer Tunable laser sources and detectors Thermal Resorption mass spectrometry X-ray photoelectron spectrometer TDS XPS _ A A Ion-scattering spectrometer ISS Secondary ion mass spectrometer (static mode) SIMS Secondary ion mass spectrometer (dynamic mode) SIMS Rutherford back-scattering system Laser microprobe mass spectrometer Raman microprobe X-ray absorption fine structure attachment to synchrotron EELS 200-225K LEED 150-175K AES 150-250K 100-150K 150-200K 150-200K 150-175K 170-200K 650-700K 500-600K 300-325K 150-170K 400-500K EXAFS
OCR for page 269
V-E. INSTRUMENTATION of investigation of chemistry in the surface domain. Sections IlI-A, IlI-B, and V-D show that both the economic and intellectual stakes are high. We must increase chemistry funding levels to place these powerful tools in the hands of those chemists working on catalysis. Only then can we expect to maintain a worId-leadership position in this crucial field. The United States cannot afford merely to watch with admiration as the field of catalysis is developed in properly equipped laboratories abroad. Surface Analysis As is always the case, sensitive measurement techniques can be regarded as analytical tools. This is the case in the surface sciences. Every one of the capabilities listed in the last column of Table V-1 can be put to analytical use in the pursuit of questions that may be only remotely connected to the surface sciences. As an example, a state-of-the-art laser microprobe device designed to desorb molecules from a solid surface can be used to detect the presence of a pesticide on the leaf of a plant. Such a capability was quite impossible only 10 years ago; today it permits us to contemplate tracking the amount, stability, weathering, and chemistry of a pesticide in field use. Of course, the analytical technique may, as well, be concerned with monitor- ing or clarifying chemical changes that take place on a surface or with a surface. Many of these analytical studies relate to catalysis. In Section V-D, examples were given of the use of EELS to determine the molecular structures that exist on a catalyst surface as it functions. Similarly, X-ray photoelectron spectroscopy (ESCA) studies of the cobalt molyb~ate catalyst show another facet of the chemical role of a catalyst. This catalyst (with 3 percent cobalt) is used commercially to remove sulfur from petro- leum (to reduce acid rain). In actual use, the catalyst surface is first prepared by chemical exposure to hydro- gen gas at a high temperature to reduce surface oxides. Then, the catalyst is "acti- vated" by exposing it to a hy- drogen sulfide/hydrogen mix- ture. Understanding how the catalyst has been chemically changed in each of these treatments is a crucial part of understanding how the cata- Tyst works. Present-day ESCA measurements clearly reveal the changes in the cobalt atoms as the oxide coating is removed by hydrogen and then as they are converted to the sulfide. 269 14. cat 1970 cat 1980 ~T - ~ -- 1 1 1 1 1 1 1 1 790 786 782 778 774 BINDING ENERGY (eV) /\ J Before _' A ter reduction in H2 After activation .} 'it ~ P with H2S In 1 1 1 1 1 1 1 1—1 790 786 782 778 774 BINDING ENERGY (eV) ESCA SHOWS HOW A COBALT CATALYST CHANGES ON ACTIVATION
OCR for page 270
270 C" a: :) cn CHEMISTRY AND NATIONAL WELL-BEING Fifteen years ago, the best ESCA equipment in existence was unable to distinguish these changes. The myriad of applications that lie somewhere between the two examples mentioned above has been made possible by the array of surface science instruments shown in Table V-4. These applications have given rise to surface analysis, a new subdivision of analytical chemistry. Applicability Surface analysis is quite different from bulk analysis; frequently, factors important for surface analysis are not important to bulk analysis. The most common distinguishing feature is the effective sampling depth of the analytical technique used. For each technique, the sampling depth of that technique defines the exact surface sampled. Sampling depth is important because the measuring technique should be appropriate to the phenomenon under study. For example, bonding to the surface, Nettability, and catalysis involve only a few atomic layers, whereas passivation and surface hardening treatments involve 10 to 1000 atomic layers. Typical sampling depths for the primary surface analytical techniques are one or two atomic layers for low energy in ion scattering, ~ A for static SIMS, 20 ~ for the ESCA and Auger techniques, and 100 ~ for dynamic SIMS. Laser mass spectrometry, the Raman microprobe, and scanning electron microscopy reach into the surface from 1000 to 10,000 ~ (i.e., to 1 micron). Another important matter is the microscopic heterogeneity or microcrystal- linity of the sample. The dis- O ~ OO ~ OOO LATERS tribution of the species across MEL - / a surface and its depth rlistri button inward from the sur- face can determine the behav- ior of the surface and must be known. The shallower the sampling depth of the tech- nique, the more finely it is able to define the depth pro- file of a sample. 8 ~3_~ ~~//////~/~/////////~//////~ WORE FUNCTION /////// __//// OPTICAL ABSORPTION ~ I, . CORROSION I _/////// _ ~ _ ISS SIMS ESCA SIMS LASER SEM (STATIC ) AUGER (DYNAMIC ) MS RAMAN HOW DEEP IS THE SURFACE? Developments Needed A major challenge in the development of surface ana- lytical instrumentation is the reinforcement of its quantita- tive dimension. Most of the examples given have been concerned with what is there. We must also be able to deter-
OCR for page 271
V-E. INSTRUMENTATION mine how much. Relative quantitative surface analysis measures a species of interest against a component already present. To quantify surface species without resorting to such internal standards is a difficult problem whose solution will expand surface analysis to many applications, particularly those . . . . . nvo. ~vlng organic species. Another important problem is the development of microprobes that can provide both chemical and spatial information about surface species. Currently, Auger and ion microprobes are useful in this respect for probing elemental composition, such as the presence and location of the trace contaminants phosphorus and lead in silicon chips. However, they are not able to probe for large organic molecules such as carcinogens or therapeutic drugs. Thus, development of new organic microprobes is an urgent need. Characterization of small particles is another important challenge for surface analysis: this is ~ · ~ ~ · ~ ~ - ~ ~ ~ , particularly important in environmental monitoring where the analysis of carcinogenic hydrocarbons on particulates is a current problem. Finally. devel- ~ ~ ~ · · 1 1 ~ ~ 1 _ _ 1 _ ~ ~ -—.~ 7 -- - -— opment of new hardware IS Important. An example has been mentioned earlier: it is important to interface high-vacuum surface spectroscopic techniques with samples at atmospheric pressure. Development of cells to permit real time examination of surfaces in contact with a reactive gas is a challenging instrumental problem. Costs The cost listed in Table V-5 are applicable in analytical uses as well. Just as for research applications, surface analytical problems are seldom solved by a single, stand-alone technique. Instead, the more interesting problems require a combination of instrumental approaches. Therefore, an effective surface ana- lytical laboratory will have to be equipped with several instruments. A considerable capital investment is implied, again approaching or exceeding $500K. Chromatography Chromatography separates molecules or ions by partitioning species between a moving and stationary phase. The technique exploits small differences in properties such as solubility, adsorbability, volatility, stereochemistry, and ion exchange, so that understanding the fundamental chemistry of these interac- tions is basic to progress in the field. Liquid chromatography has shown an impressive growth since 1970. The current $400M annual sales are mainly by U.S. manufacturers. The growth has come through innovations, such as high pressure and gradient moving phases to give greater speed and resolution, bonded-molecule stationary phases to give greater selectivity and column life, and electrochemical, fluorometric, and mass spectrometric detectors sensitive to as little as 1o-~2 g. Although gas chromatography is a more mature field by perhaps a decade, important advances continue to appear. High-speed separa- tions can now be accomplished in a few tenths of a second; portable instruments 271
OCR for page 272
272 CHEMISTRY AND NATIONAL WELL-BEING the size of a matchbox are in use. A complex mixture can be separated into literally thousands of components using fused silica capillary columns that are a direct spin-off from optical fiber technology for communications. It is even possible to separate compounds that differ only in isotopic composition. Despite this well established record of success, chromatography is still expanding its horizons. High-performance liquid chromatography and capillary column chromatography provide convincing examples of new concepts advanc- ing the field. High-Performance Liquid Chromatography (HPI~CJ During the 1970s, theoretical understandings of the complex flow and mass transfer phenomena involved in chromatographic band dispersion helped opti- mization of column design. During this same period, small diameter (3 to 10 micron) silica particles with controlled porosity were introduced and synthetic advances in silica chemistry led to tailoring of particle diameter, pore diameter, and pore size distribution. Today, 15-cm columns with efficiencies exceeding 10,000 theoretical plates are routine. The instrumentation ancillary to these high-performance columns required development of specialized pumps to drive liquid flows with high precision and low pulsation through the small particle columns. Detector advances occurred as well. First UV and refractive index, then more selective detectors based on fluorescence and amperometry/coulometry, were developed for HPEC. Still another major advance of the 1970s was the introduction of chemically bonded phases in which surfaces of porous silica are functionalized with organosilanes. Especially important is the use of hydrocarbonaceous phases (such as n-octy] and n-octadecyI) in which the mobile liquid phase is typically an organic-aqueous mixture. This is called reversed phase chromatography (RPEC), and it currently provides well over 50 percent of all HPEC separations. It is especially well suited to substances at least partially soluble in water (e drugs, biochemically ::3nd nolvn,~rl~r ~rom~t.ir.~) _' ~ or-- ^ Am. ____ ~ . .. . ~ .. ... . ~ _ ·= 7 1nus no IS a vlorant new wltn new crevelopments continually affecting many disciplines. New small particle supports based on silica and organic polymeric materials have recently been introduced for the ion exchange and reversed phase HPEC separation of biopolymers. Whereas separations previ- ously required days for completion, today it is becoming possible to accomplish 1 1 1 ~ · - · . ~ - ~ · . ~ ~ even netter resolutions within a tew minutes. (column design is improving as well. Instead of the conventional 4- to 5-mm diameters, narrower bore columns from .6-2-mm i.d. are providing routes to sensitive analyses even when the amount of sample is limited. Also, the lower flow rates at the same linear velocity permit coupling of LC to powerful vapor phase detectors, such as mass spectrometry and flame ionization. Open tubular capillary IN columns of 1- to 10-micron inner diameter are being investigated for the potential of generating high resolving power in the separation of extremely complex mixtures (e.g.,
OCR for page 273
V-E. INSTRUMENTATION fossil energy fuels). In all these examples, specially designed instruments are needed to accommodate the small sample sizes. Finally, the microprocessor/computer is playing an increasing role. "Smart" HPEC instruments are under development that use statistical optimization schemes. Semi-empirical solvent and stationary phase characterization schemes have been developed to enhance the power of these approaches. New detectors of greater sensitivity and selectivity are on the horizon. In particular, laser spectroscopy promises to yield highly sensitive devices for subpicogram detection. Because of these performance improvements, HPEC is having a major impact on diverse fields of biochemistry, biomedicine, pharmaceutical development, environmental monitoring, and forensic science. Today, peptide analysis and isolation requires HPEC because of its separating power and speed. Analysis of PTH-amino acids in protein/peptide sequencing is conventionally accomplished by RPEC. In clinical analysis, therapeutic drug monitoring can be accomplished by HPEC. The analysis of catecholamines is typically accomplished by RPEC with electrochemical detection. Isoenzyme analysis, important, for example, in assessment of damage after heart attack, can be rapidly accomplished by HPEC. Analysis of parent drugs and their metabolites in a pharmoco-kinetic study is typically accomplished by HPI,C. The isolation of synthesized drugs in purified form is typically achieved by HPEC. The analysis of polar and high-molecular- weight organic species in waste streams can be performed by EPIC, while the separation and analysis of phenols by RPEC is recommended. Analysis of narcotics, inks, paints, and blood represent only a few of the samples of forensic relevance. Capillary Chromatography This version of chromatography dispenses with the granular materials normally packed into the column. Instead, it uses an open capillary tube with a thin retentive layer on its inner wall. It began with capillary gas chromatog- raphy and now is being transferred to use with liquids. Although many satisfactory GC columns are available today, surface inves- tigations are continuing to improve the general understanding of thin liquid films and related superficial interactions. Glass as an inert material for the preparation of GC capillary columns carried with it a fragility that discouraged many potential users. Now we have flexible, fused-silica capillaries with a polymer overcoat; these columns are a spin-off of fiber-pntics technolo~v. The . . ... . . . . . . ,& ~" advances In capillary column recnno~ogy lea to intensive commercialization during the 1970s. Today's capillary columns exhibit efficiencies between 105 to 106 theoretical plates and are capable of separating literally hundreds of components within a narrow boiling-point range. As the corresponding chro- matographic peaks are separated by seconds or less, the sample input and output measurements must be comparably rapid. Direct introduction of samples at the nanogram levels has been developed, and much effort has been spent on 273
OCR for page 274
274 CHEMISTRY AND NATIONAL WELL-BEING optimization of gas-phase ionization detectors. Among these detectors are some of the most sensitive measurement devices known. Combined advances in the column and detector areas now make feasible trace analytical determinations below 10-~2-gram levels by capillary gas chromatography. The highly sensitive electron capture detector and element-specific detectors, in conjunction with capillary GC, currently find numerous applications in environmental and biomedical research. Of particular note is the combination of capillary GC with powerful identifi- cation methods, such as mass spectrometry and Fourier transform infrared spectroscopy, as mentioned in Section V-D. The combined techniques are now routinely capable of identifying numerous compounds of interest that are present in complex mixtures in only nanogram quantities. They have been used to identify new biologically important molecules, as well as in drug metabolism studies, forensic applications, and identifications of trace environmental pollu- tants. During the last decade, microcolumn high-performance liquid chromatogra- phy (HPEC) has been under intensive development. Two advantages are the reduced consumption of the expensive and often environmentally undesirable mobile phase and the possibility of exploring new detection methods. Open tubular columns and partially packed columns of"capilIary dimensions" have been tried. The open tubular columns for GC typically have 200- to 300-micron inner diameters to obtain optimal solute mass-transfer processes between the two chromatographic phases. Because of radial diffusion rates, liquid chroma- tography in open tubular columns necessitates considerably smaller column dimensions, as small as ~ to 10 microns, and nanoliter volumes are needed for both sample introduction and detection volumes. Packed capillary columns of dimensions ranging from 40 to 300 microns have now been developed that perform satisfactorily: several hundred thousand theoretical plate performance can now be achieved in several hours' time, and resolution of quite complex mixtures has been demonstrated. Microcolumn technology has recently found yet another application in capil- lary high-voltage zone electrophoresis. The small diameters (typically 60 microns) of open tubular columns permit application of voltages in excess of 30,000 volts without overheating. Very efficient separations of certain charged species were already demonstrated, but improvements are still needed. Capillary supercritical fluid chromatography has recently emerged as a promising approach to the analysis of complex nonvolatile mixtures. As the solute diffusion coefficients and viscosities of supercritical fluids are more favorable than those observed in the normal condensed phase, chromatographic performance is substantially enhanced. Furthermore, the relative optical trans- parency of supercritical fluids makes them attractive for certain optical detec- tion techniques. Besides its analytical potential, supercritical fluid chromatog- raphy appears to be an ideal method for measuring physicochemical parameters in the vicinity of the critical point.
OCR for page 275
V-E. INSTRUMENTATION Field-Flow Fractionation (FFF) Chromatography becomes more difficult to apply as molecular size grows and becomes ineffectual in separating macromolecules and colloidal particles in the size range .01 to 1 micron in diameter. A recent innovation, field flow fractionation, may fill this need. In FFF, a liquid sample is injected into a thin (.1-.3 mm), ribbon-like flow channel. A thermal, sedimentation, or electric field gradient is applied through the ribbon. Each constituent in the sample distributes itself in a steady-state concentration gradient that is determined by its response to the gradient and its diffusional properties. Since flow through the channel is fastest near the middle of the ribbon, constituents that are pulled close to the wall move more slowly through the ribbon than constituents that reside near the middle of the flow channel. Separations are thus achieved. A useful aspect of this technique is that the strength of the applied field can be varied in a deliberate and programmed way during the course of the separation. Thermal gradients are effective in separating most synthetic polymers. Sedimentation gradients separate large colloids, and electrical fields are appro- priate for charged species. Because of the range of operating parameters, FFT has proven capable of separating both charged and uncharged species in either chain or globular configuration. The method works both in aqueous and nonaqueous media. The mass range of molecules and particles to which FFF has been applied extends from molecular weights of 1000 up to 10~8, that is, up to particle sizes of about 100-micron diameters FFF appears to be applicable to nearly any complex molecular or particulate material within that vast range. Because the channel geometry and flow are well characterized, the rate of displacement of a given constituent can be related rigorously to such properties as mass, size, diffusivity, density, charge, and thermal diffusion rates. Hence, the above properties can be determined by measuring displacement rates, and FFT becomes an accurate tool for particle characterization. Applications of FFF have so far included macromolecules and particles of biological and biomedical relevance (proteins, viruses, subcellular particles, liposomes, artificial blood, and whole celIs), of industrial importance (both nonpolar and water soluble polymers, lattices, coal liquid residues, emulsions, and colloidal silica), and of environmental significance (waterborne colloids and fly ash). Costs Chromatography is an essential too] to every synthetic chemist and to analytical chemists in a variety of fields. An advanced analytical gas chromato- graph might cost $20K, an HPEC about the same, and a preparative (large volume) chromatograph somewhat more, perhaps $30K. While each item has only a modest cost, three or four such instruments will be needed as dedicated 275
OCR for page 276
276 an a: o CHEMISTRY AND NATIONAL WELL-BEING . instruments by each research group. Thus the total may approach $100K, a capital investment that must be available for effective research. Infrared Spectroscopy The infrared spectral region reveals molecular vibrational motions. Because these motions are sensitive to bond strengths and molecular architecture, infrared spectroscopy has become one of the routine diagnostic tools of chemis- try. A large, research-oriented chemistry department might operate five to ten such instruments with capabilities ranging from rugged, low-resolution instru- ments for instruction in an advanced first-year chemistry course to high- resolution Fourier transform instruments (FTIR) suited to molecular structure determination and specialized research use. Computer-Aided Spectrometers Modern research infrared spectrometers incorporate dedicated computer capability for programmed operation, data collection, and data manipulation. The major impact of computers, however, has been their influence on the accessibility and reliability of Fourier transform interferometers. As mentioned earlier in Section TTT-E, the perfection of the Fourier transform algorithm plus the reduction in accompanying computer costs brought the interferometer from a trouble-plagued, research-only instrument to a routine, high-performance workhorse. Spectral resolutions of .25 cm-i are readily obtained over a long scan range (e.g., 4000 to 400 cm- in 20 or 30 minutes. A notable capability is the ease and accuracy with which difference spectra can be displayed. One important application relates to infrared spectra of biological samples in which evidence of a chemical change associated with a localized biological function can be completely masked by the heavy infrared spectrum of the inactive substrate. The digitized data permit pre- cise spectral subtraction so that the background spectrum can be virtually eliminated to reveal the spectral changes of interest. Another vivid display of the value of the difference capability is provided by photolysis of molecules suspended in a cryogenic solid ("matrix isolations. If the digi- tized spectrum before photolysis is subtracted from the spectrum after photolysis, only the fea- tures that change are seen. Any molecule that is DIFLUOROPROPENE IN SO L I D KRYPTON, 12 4~ cis gauche hL`11 ~ 1 41 I gaucbet | ~rA4, cis1 1 400 1200 1000 V(cm l) s~ BEFORE PBOTOLTSIS (hi) Ha_ ~ DETER (hi)— BEEORE (hi) ~ __0 FTIR DIFFERENCE SPECTROSCOPY SHOWS ROTAMER INTERCONVERSION
OCR for page 277
V-E. INSTRUMENTATION being consumed presents its spectral features downward, while the product spectral features extend upwards. This has been used, for example, to distin- guish the two rotameric forms (cis- and gauche-) of 2,3,-difluoropropene in the cluttered spectrum of a complex mixture. Interconversion is caused by laser irradiation of one of the adsorptions of one rotamer. Applications The coupling of FTIR with gas chromatographic separations in a variety of analytical uses has been discussed in Section V-D. This coupling is facilitated by computerized data collection, and it is made possible by the reduced scan time that accompanies the high-performance characteristics of FTIR instruments. Also as noted in Sections V-A and V-D, infrared spectroscopy is a specially effective method for monitoring and studying atmospheric chemistry. This is because gaseous molecules of Tow molecular weight are important, including formaldehyde, 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 stratospheric disturbance of the ozone layer, and the greenhouse effect. TABLE V-6 Additional Instrumental Techniques in Modern Chemistry Instrument Information Obtained Approximate Cost ($) Ion cyclotron resonance spectrometer Laser magnetic resonance spectrometer Laser-Raman spectrometer Fluorimeter Circular dichroism spectrometer Flow cytometer Protein sequencer Oligonucleotide synthesizer Electron diffraction Scintillation counter Reaction rates of gaseous molecular ions Precise molecular structures of gaseous free radicals Vibrational structure of molecules or of chromophores in complex Energies and lifetimes of electronically excited molecules Stereoconformations of complex molecules Laser-activated cell sorter Automated analysis of protein sequence Automated synthesis of design oligonucle- otides Molecular structures of gaseous molecules Tracking radio-tracers through chemical reactions 125K 75K 60K 40K 50K 150K 120K 40K 150K 50K Costs Fourier transform infrared spectrometers are now sufficiently easy to use- with the high performance described above that they are becoming ubiqui- tous. In 1983, perhaps 200 such instruments were sold by U.S. companies, and foreign instruments are appearing (from West Germany and Canada). Costs 277
OCR for page 278
278 CHEMISTRY AND NATIONAL WELL-BEING currently range from $140K to $200K, depending upon the resolution and scan range. Accessories permit long-wavelength spectroscopy to 10 cm-i and near- infrared spectroscopy reaching into the visible. Other Instrumentation In Sections IlI-E, IV-E, and V-E, there has been explicit discussion of more than a dozen different classes of instrumentation that are important in defining and advancing the current frontiers of chemistry. By no means, however, is the list all-inclusive. Table V-6 lists additional types of equipment, what kinds of chemical information each one provides, and approximate current costs. Plainly, these, too, contribute to the capital investment needed to sustain frontier research in chemistry.
Representative terms from entire chapter: