Ion Beam Analysis of Environmental Samples - Advances in Chemistry

27 Ion Beam Analysis of Environmental Samples THOMAS A. CAHILL

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Department of Physics and Crocker Nuclear Laboratory, University of California, Davis,CA95616 Use of energetic (MeV) ions for analytical purposes con tinues to diversify and expand into areas of application ill-served by more conventional techniques. Early programs in Rutherford backscattering (RBS), generally associated with analysis of layered structures in materials science, have been joined by particle induced x-ray emission (PIXE) and charged particle activation analysis (CPAA) based groups. Very active programs have recently evolved around proton microprobes, capable of nondestructive multielemental anal yses to 10 -m dimensions and 10 -kg masses, and radio active dating such as C by accelerators. While all these programs have environmental components, major environ mental activity centers on PIXE programs, often using CPAA and scattering techniques to extend their range into the elements hydrogen through fluorine. -6

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At first glance, the thought of using MeV-energy ion beams from accelerators for analyses of environmental samples seems improbable. At such energies, only elemental determinations are generally accessible, and direct information on chemical species can only be inferred, for example, via elemental ratios, or correlations. The analyses are usually done in vacuum, requiring reductions to solid targets that may pose problems. The radiation and temperature environment of the analyses are severe, and loss of some species has been documented. Finally, access to accelerators and/or cost factors discourage routine use of ion beams for analytical purposes; yet, most environmental problems require large numbers of analyses for statistical reasons. Thus, for these reasons, and others, large-scale analytical use of ion beams appears unlikely. Yet, such large-scale use has become common in the past decade. Despite an almost explosive growth of capability in analytical chemistry, informational needs have grown even more rapidly. One rationale for 0065-2393/81/0197-0511$05.00/0 © 1981 American Chemical Society

Root and Krohn; Short-Lived Radionuclides in Chemistry and Biology Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

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use of ion beams, a n d one that surmounts any barrier stated above, is that no other method is capable of delivering information of prime importance. Here, the nature of the unmet need is the primary factor, not ion beam capabilities. Quasi-random development of analytical capability, no matter how clever, usually fails to achieve any impact as the need can be met b y other methods or the need is not important enough to generate the impulse (fund time) to surmount barriers to ion beam use. There exist other situations i n w h i c h ion beam methodologies may not be uniquely capable but still be widely used. Probably the best example involves most applications of particle induced x-ray emission ( P I X E ) . Circum stances may reduce the importance of the barriers; a relatively inexpensive accelerator may be underused; samples may be enormously chemically complicated, so that less detailed elemental analyses may still be useful; the techniques may be widely known and inexpensive to initiate; person nel may be available, and other such factors. Then, i f there is an important need, ion beam methodologies may become established despite the barriers. This review w i l l examine both types of rationales, using a historical ordering to follow expansion of ion beam techniques into areas of applica tion. T h e first important analytical use of ion beams involves Rutherford backscattering for studies of layered materials.

Ion Beam Methodologies Scattering of Ions. The earliest major use of M e V ions for analyti cal purposes was i n studies of layered materials i n materials sciences v i a Rutherford backscattering ( R B S ) , largely i n the U n i t e d States and Great Britain. I n this technique, ions (generally alpha particles) of a few M e V are scattered at back angles b y the coulomb field of the target nuclei. Separation of elemental species is accomplished b y the kinematic energy lost i n the nuclear recoil, as shown i n Figure 1 from the excellent review by Ziegler ( 1 ) . The method is absolute and nondestructive, w h i c h plays a large part i n its popularity. E v e n more important is that consideration of that energy lost as the incident and scattered ions traverse the layered structures allow accurate measurements of the locations and thicknesses of layers, especially heavy element layers on light substrates such as silicon. (Figure 2 ) . This information is vital to the behavior of these structures. T h e limitations of the method, such as i n distinguishing adjacent elements and lack of trace sensitivity, are not very important i n materials sciences, but they limit use of the method for environmental samples. Nevertheless, persistent efforts have been made to develop alternative scattering techniques, largely because of the inabiUty of x-ray based Root and Krohn; Short-Lived Radionuclides in Chemistry and Biology Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

cAHTJLL Ion Beam Analysis of Environmental Samples

» C

t t Al Fe ENERGY OF BACKSCATTEREO He

tt PbEo

4

Figure 1. Schematic of a RBS measurement (1)


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VERY LIGHT ELEMENT ANALYSIS (18 MeV a's) OXYGEN, CARBON

OXYGEN

V

(inelastics)

HYDROGEN

CARBON

64

128

192 256 CHANNEL NUMBER

320

384

Figure 3. Energy spectra of 18-MeV alpha particles scattered from mylar-based target at 55° methods to detect quantitatively very light elements, hydrogen through fluorine. Since such elements make up about two-thirds of most materials, lack of information on them hampers programs i n atmospheric chemistry and other areas. Thus, for environmental samples, the problem is reversed in that we are looking for very light elements i n the presence of heavier elements, although now considering gross components, not traces. Active programs exist at about a dozen laboratories, mainly those w i t h atmos pheric chemistry programs and P I X E capabilities. A n example of one such analysis is shown in Figure 3. Here, alpha particles are used, but at a forward angle of 55°. Energies are now higher to avoid range strag gling, such as the 18-MeV alphas used i n the Davis program. The use of alphas at forward angles allows detection of the knock-on proton from hydrogen, thus allowing all elements Z = 1 and up to be detected. Be yond sodium, however, separation of adjacent elements becomes more difficult, although the Florida State group has resolved u p to calcium using 16-MeV protons at back angles ( 2 ) . A t such energies, scattering processes now involve nuclear interactions, not coulomb, and inelastic states, reaction products, and variable cross sections make calibrations less obvious than i n R B S . A serious problem i n such studies is that if we have the capability to see a l l elements, w e must handle a severe interference problem i n the sampling substrate. This can be handled via parallel collection devices w i t h different substrates, as w e shall see i n nuclear reaction methods, or b y careful subtraction b y analyzing a blank portion of the substrate, but a more elegant answer has become available through improved filter technology. Recent development of


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very thin, "stretched," Teflon filters allows accurate scattering measure ments to be made from such filters. Then, if we assume negligible fluorine i n the air ( w h i c h could be checked via more conventional filters), we can use the Teflon C / F ratio, stable to ± 0 . 7 % , to ascertain the carbon content of the filter (3,4). This, then, allows excellent subtraction from carbon i n the air, since the stoichiometry of the filters is so good. R a p i d increase i n the use of scattering now appears assured, giving total ele mental range to a combined PIXE-scattering system. Prompt N u c l e a r Reactions. Use of nuclear reactions i n analytical chemistry has a long history, with strong programs i n geological and materials studies especially i n Great Britain, Australia, and South Africa. In many such uses, a single element was studied, or information on very light elements was needed to extend neutron activation analyses. Ener gies were generally below 10 M e V , and a variety of ions were used to enhance the probability for the chosen reaction. F o r instance, common reactions used for light elements include (5): * ! ! ( B , a ) 2 a , H ( H e , p ) , L i (p,a) «, B e (a,u), B e ( d , p ) , and a variety of (p,p'y), ( p , d ) , ( d , p ) , ( d , p y ) , (d,a), (a,p) reactions, giving results such as those shown i n Figure 4, reproduced from (1). The method is absolute and quantitative, but tends to be applied for gross or minor constituents (as opposed to trace). T o achieve such energy resolutions w i t h relatively low energy beams, thin, uniform targets must be prepared. n

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N u c l e a r A c t i v a t i o n . Activation of light elements w i t h ions gen erally results i n positron decays. Often only use of selective ions and energies, maximizing probabilities for the element of choice, and measure ments of half-life allows positive identification to be made. W h i l e heavier elements can be activated by ions to give gamma ray spectra suitable for G e ( L i ) detectors, surmounting the coulomb barrier requires higher energies and larger accelerators, reducing the attractiveness. In addition, well established and successful programs i n neutron activation analysis ( N A A ) compete w i t h such uses. I n terms of environmental samples, several programs exist based upon charged particle activation, generally to detect the very light elements not seen by x-ray methods. A good example is the program i n gamma ray activation for light elements ( G R A L E ) of Macias and coworkers at Washington University, St. Louis (6). Substrate inter ferences are handled for carbon by special quartz filters, and sensitivities are excellent. Generally, the ion beam is optimized for each element under study. Such activation programs are used for environmental pur poses i n about 15 laboratories around the world, according to a recent survey (7). A new approach to charged particle activation has been reported by Schweihert at Texas A & M . I n his technique, relatively thick samples


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CHANNEL NUMBER

Figure 4.

Proton spectra obtained by (a,p) reactions on glass samples (1)

are activated i n high fluxes of l o w energy protons. Detection of the activated materials is made via x-rays rather than the more common gamma rays. A number of counts are made at increasing time intervals, allowing half-lives to be measured. Advantages include easy identification of x-ray peaks, high detector efficiencies, and sensitive detection of intermediate mass elements poorly done b y P I X E . Whether this method w i l l find a permanent place i n ion beam methodologies is yet to be seen, but it illustrates the potentialities.


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Particle Induced X - r a y Emission. The most widely used analytical method based on M e V ions is particle induced x-ray emission ( P I X E ) . In this technique, ions of a few M e V , generally protons, ionize atoms, and the subsequent x-ray emission is detected, generally w i t h Si ( L i ) energy dispersive detectors. It was the development of such detectors around 1970 capable of resolving characteristic x-rays of adjacent elements that lead to the wide application of the method. I n addition, the atomic cross sections are large compared w i t h nuclear cross sections, while the physics of inner-shell ionization allows absolute measurements of high sensitivity and precision. A recent review article (7) lists 83 P I X E labs i n 30 countries, not including the 24 proton microprobes w i t h P I X E detection schemes. The details of P I X E systems can be found i n the extensive literature cited i n this article and Johansson and Johansson (8). Johans son et al. (9) and Johansson et al. (10) are the proceedings of the first and second P I X E conferences i n L u n d , Sweden, and are highly recommended. W h y has P I X E become so widely used? The survey showed that atmospheric particulate matter was the most important application of P I X E . The reasons for this are based both i n the capabilities of P I X E and the nature of fine particles i n the air. The key lies i n the importance of information on both composition and particle size. Particle size governs transport and removal mechanisms, lung capture and light scattering, acidic rainfall, dust soiling, and other effects, as w e l l as serving as an effective way to pinpoint pollution sources ( I I ) . However, simultaneous measurements of size and chemistry require particle collec tion by size through impactors and filters. Such devices, however, can rarely collect more than a few monolayers of particles before sizing errors occur due to clogging or particle bounce. A few monolayers of 1-fi particles only delivers a few hundred micrograms per square centi meter, w h i c h often results i n less than 10 /*g of total mass per size range. This poses serious problems to most chemical methods, and only special methods such as electron beam or neutron activation analyses provide alternatives to P I X E i n such cases. X-ray fluorescence, especially w i t h modern energy dispersive detectors, is a valuable method if larger areas of deposit are available, but this is not often the case for multiple size fractions from impactors. The nature of P I X E also matches the analytical needs of atmospheric particles as well. M a n y elements i n an enormous variety of chemical states are important, so that even elemental analyses are most useful. F o r example, only five elements deliver over 9 9 % of the mass for gaseous pollutants, but 22 elements are required to describe an equivalent fraction of particulate matter (12) (Figure 5 ) . The relatively uniform sensitivity of P I X E from sodium to uranium i n a single, quick analysis delivers the


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Figure 5. Spectrum of a particulate sample analyzed by FIXE. The fit is by the program HEX written by H. Kaufmann (9) required information at low cost, and i n the final accounting, this explains why over 9 0 % of all simultaneous composition-size analyses of three (or more) stage impactors has been done through this method. M a n y other P I X E analyses of aerosols do not have any real advantage over x-ray fluorescence ( X R F ) , but a number of vigorous programs have prospered when access to an accelerator is available. Generally, studies involving only medium and heavy elements collected on filters can be handled well by X R F , and if loadings are reasonable ( > 100 /Dig/cm ) sensitivities via X R F may be superior by a factor of four or so than for P I X E (13). However, most programs i n air chemistry need detailed information on the elements sodium through calcium—elements that make up about one-fourth of all atmospheric particulate mass. These elements, especially sulfur and chlorine, are chemically active and very important i n air chemistry. P I X E does very w e l l for these elements, while X R F must use multiple anodes i n x-ray tubes to get adequate excitation, slowing analysis rates. Thus, there is an extraordinarily good match between P I X E capabilities and the needs of atmospheric aerosol research, which helps explain the heavy use of ion beams i n this work. It might also be mentioned that one of the most attractive features of ion beams is that nuclear processes—scattering and reactions—are avail able that allow detection of any element. Thus, the two-thirds of particu late mass i n the light elements hydrogen through fluorine can be measured in the same laboratory when the need arises. M a n y PIXE-based programs are using such methods more widely, as it improves the quality of the research programs and helps justify the choice of ion-beam based analytical techniques. M o v i n g from atmospheric sciences into other environmental fields, use of ion beam analyses falls off sharply. Analyses of fluids is probably 2


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the next most active area, but the problems of reduction of a sample into an accurate, representative target capable of ion beam analysis are severe (7). Active responses to these problems lie i n a variety of methods for removing the fluid matrix (drying, vapor filtration, lypholizing), and i n bringing the ion beam into air (or helium) and analyzing the fluid i n a cell, dish, or liquid jet (14). The problem of water quality, however, often involves questions about the chemical states of pollutants, w h i c h reduces the attractiveness of elemental methods. In addition, there are many optical methods well suited to samples already present i n fluid form (atomic absorption and emission spectrography, polargraphy), and if only a few elements are of interest, these methods are often simpler and cheaper to use. Nevertheless, for surveys of many metals to p p b levels, P I X E is competitive, and a large survey of U.S. drinking water is handled entirely by P I X E at Purdue University on samples prepared by vapor filtration (15). Very little work has been reported on seawater. Biological tissue presents many of the same problems as fluids, plus some new aspects (16). The major barrier to ion beam analyses i n such samples appears to be the need for information on elemental species at sub-ppm levels. The major ion beam technique—PIXE—does not easily achieve such levels, and thus, we are faced w i t h sample preparation problems to reduce the light element matrix. Brutal methods such as ashing achieve such reductions, but at the expense of loss of volatiles. W e t ashing methods are not very effective, and they add new contami nants i n the process. The best results achieved to date appear from groups using plasma ashing devices, which result i n a reduction of 20 or so i n sample mass w i t h minimal loss of volatiles (17). A n increase i n the use of ion beams is to be expected for biological materials as such units become more common, as multielement surveys are expensive b y alternate techniques. The relative lack of interest i n the elements sodium through chlorine may favor X R F , however, if mass of sample is adequate. The final type of environmental sample considered is dusts, be they soil, fly ash, or industrial waste. I n some ways, multielemental surveys of such dusts appear easy, as samples can be easily prepared that w i l l stand vacuum and irradiation, while a w i d e range of elements are to be expected. Only rather recently has the problem of size-chemistry shifts i n dusts been appreciated, and almost any attempt to prepare a small aliquot of a bulk dust sample for analysis w i l l produce varying results depending on the method used. The solution lies i n separating dusts into size categories, measuring the composition of each category, and then calculating the gross composition (18). E v e n this technique runs into the problems of severe x-ray attenuation corrections for particles above about 15 /*m diameter. Nuclear methods have no such problems, but a different reaction is needed for each element, and surveys are


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difficult. Nevertheless, more and more work is appearing on dusts, often i n response to program i n air chemistry that need source term analyses of soils and such seen i n the air. The Proton Microprobe. N o discussion of ion beam analytical sys tems would be complete without mention of the 'ultimate" i o n beam system—the proton microprobe. As shown i n the excellent review articles by Cookson (5) and Martin and Nobiling (19), i o n beams can be prepared w i t h dimensions of a micron or so. Such finely focussed i o n beams can be supported by the nuclear or atomic analytical methodologies mentioned above, w i t h P I X E being the most widely used. Since the P I X E system's fractional mass sensitivity is not changed b y beam area, one can have a 1-ppm measurement made over an area of 10" c m , yielding mass sensitivities of 10" g or so i n a nondestructive multielemental analysis as shown i n Figure 6. N o other method exists that can obtain such data, so the role of i o n beams is w e l l founded. Explosive growth of these expensive systems has occurred i n the past five years, but the cost has limited applications to most environmental samples. Radioisotope D a t i n g w i t h Accelerators. Another area that has recently gained considerable attention involves use of an accelerator to perform measurements of radioactive atoms (20). I n such cases, the 8

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Minimum Detectoble Limits vs Sensitivity and Beom Diameter

Figure 6. Mass sensitivity achieved by scanning electron microscopes (SEMS), PIXE systems, and PIXE microprobes (7)


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Ion Beam Analysis of Environmental Samples ioV

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MASS SPECTRUM OF CARBON EVENTS r

MASS

Figure 7.

Measurement of C in charcoal and graphite samples by a Tandem van de Graaff accelerator 14

atoms are accelerated to high velocity by a cyclotron or a potential drop accelerator. In the case of the cyclotron, such acceleration yields particles of identical charge-to-mass ratios (such as N and C ) and identical velocities. A relatively simple range chamber w i l l then separate the abundant N from rare C , since the ions are soon stripped to bare nuclei, and the higher nitrogen charge w i l l slow it down faster than carbon. Tandem van de Graaffs can use the fact that nitrogen forms no negative atomic ion while carbon does, allowing separation i n the accele ration process. The energetic ions can then be conclusively identified by standard nuclear A E and E detection schemes (Figure 7 ) . Masses required for detection are very low, and many radioactive species are under study. Other Techniques. There exist other areas of potential application of ion beams. Ion beams can excite transitions i n outer electron shells, with information i n the optical wavelengths. Beam foil spectroscopy has used such methods for years, but no regular analytical use has been made. Nevertheless, samples irradiated under ion bombardment glow w i t h characteristic radiation visible to the eye or television camera. Certain chemical or physical properties could be inferred by analyzing this radiation, perhaps including the physical condition of carbon atoms i n graphitic or organic states. Certainly opportunities exist. 1 4

1 4

3 +

1 4

3 +

1 4

Conclusions Ion beams have become well established i n a number of analytical methodologies, despite the barriers present i n any accelerator-based


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system. I n addition, many opportunities exist for new techniques, assum ing that there is a significant need not well met by other (nonaccelerator) options. These other options, however, have become much more expen sive i n the past decade, so that the cost barrier for both acquisition and operation of an accelerator are not as great a negative factor as i n the past. If, however, the facility already has an accelerator, perhaps associ ated w i t h short-lived isotopes, then accelerator-based techniques may be able to fill a major role with relatively little developmental effort.

Literature Cited 1. Ziegler, J., Ed. "New Uses of Ion Accelerators", Plenum: New York; 1975. 2. Nelson, J. W.; Courtney, W. J., Nucl. Instrum. Methods 1977, 142, 127132. 3. Nelson, J. W.; Hudson, G. M.; Kaufmann, H. C.; Courtney, W. J.; Wil liams, I.; Akselsson, K. R.; Meinert, D.; Winchester, J. W. Technical Report, United States Environmental Protection Agency, Jan., 1978. 4. Andreae, M. O.; Barnard, W. R.; "Light Element Composition of the Atmospheric Aerosol at Cape Grim (Tasmania) and Townsville (Queens land) by PIXE and PESA", Nucl. Instrum. Methods; to be published. 5. Cookson, J. A.;Nucl.Instrum. Methods 1979, 165, 477-508. 6. Macias, E. S.; Radcliffe, C. D.; Lewis, C. W.; Sawicki, C. R. Anal. Chem. 1978, 50, 1120-1124. 7. Cahill, T. A. Annu. Rev. Nucl. Part. Sci. 1980, 30, 211-252. 8. Johansson, S. A. E.; Johansson, T. B. Nucl. Instrum. Methods 1976, 137, 473-516. 9. Johansson, S. A. E. Nucl. Instrum. Methods 1977, 142, 1-338. 10. Johansson, S. A. E. "Proceedings of the Second International Conference on Partical Induced X-Ray Emission and Its Analytical Application", Nucl. Instrum. Methods, to be published. 11. Butcher, S. K.; Charlson, R. J. "Introduction to Air Chemistry", John Wiley & Sons: New York; 1975. 12. Cahill, T. A. In "New Uses of Ion Accelerators", Ziegler, J., Ed.; Plenum: New York, 1975; 1-72. 13. Perry, S. K.; Brady, F. P.; Nucl. Instrum. Methods 1973, 108, 389. 14. Deconninck, G. Nucl. Instrum. Methods 1977, 142, 609-614. 15. Rickey, F. A.; Mueller, K.; Simms, P. C.; Michael, B. D.; In "X-Ray Fluo rescence Analysis of Environmental Samples", Dzubay, T., Ed.; Ann Arbor Sci.: Ann Arbors, Mich., 1977; pp. 135-143. 16. Campbell, J. L. Nucl. Instrum. Methods 1977, 142, 263-273. 17. Mangelson, N. F.; Hill, M. W.; Nielson, K. K.; Eatough, D. J.; Christensen, J. J.; Izatt, R. M.; Richards, D. O.; Anal. Chem. 1979, 51, 133-142. 18. Cahill, T. A.; Ashbaugh, L. L. In "Environmental and Climactic Impact of Coal Utilization", Deepak, A., Ed.; Academic: New York, 1980; pp. 569-572. 19. Martin, B.; Nobiling, R. In "Applied Charged Particle Optics", Septier, A., Ed.; Academic: New York, to be published. 20. Bennet, C. L.; Beukens, R. P.; Clover, M. R.; Elmore, D.; Goue, H . E.; Kilius, L.; Litherland, A. E.; Purser, K. H . Science 1978, 201, 345-347. RECEIVED March 16, 1981.


Ion Beam Analysis of Environmental Samples - Advances in Chemistry

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