In Synch with Advance
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magine a beam of coherent light 1000 times brighter than anything a laboratory source can produce, focused on a 1-mm 2 point positioned 20 m away from the source window. No, it's not a l a s e r — i t ' s an X-ray beam that shines 5 χ 10 1 1 photons/s onto that tiny point, and 70,000 re searchers worldwide make reserva tions as much as a year in advance just to use it for three or four days. The beam is produced by a syn chrotron, a ring in which m a g n e t s accelerate electrons or positrons to produce vacuum-UV radiation and high-intensity X-rays. About 10 fa cilities produce synchrotron radia tion (SR) in the United States—most of them at national laboratories and a few at universities and companies such as IBM—and there are several in Europe and Japan. Synchrotrons got their start in the 1940s as a d a p t a t i o n s of the cyclo trons designed during World War II. Unlike today's facilities, which can reach more than 1 km in circumfer ence, the first s y n c h r o t r o n was a
small affair—about 3 ft in diame t e r — a n d fit on a platform in a crowded laboratory. Back then, few analytical chemists saw the potential of SR as a light source for their methods. X-ray spec troscopists in particular were slow to try it, but as larger and more power ful s y n c h r o t o n s h a v e b e e n b u i l t , spectroscopists have become one of the major groups of SR u s e r s . SR does more than just enhance the ac curacy and speed of conventional an alytical methods; its high spectral
FOCUS brightness makes possible a wide va riety of new techniques that couldn't be dreamed of with standard in-lab radiation sources. Going around the bend A synchrotron accelerates electrons centripe tally to produce high-inten sity radiation with energies ranging
from the visible spectrum through the UV and vacuum-UV to the soft and hard X-ray regions of the spec trum. To do this, electron pulses pro duced by an electron gun are acceler a t e d in a l i n e a r a c c e l e r a t o r to energies of 5 0 - 1 0 0 MeV, t h e n in jected into a synchrotron accelerator and boosted to energies in the GeV range using deflecting or "bending" magnets placed at the corners of the ring. The projected arrangement at the Advanced Light Source (ALS) at Lawrence Berkeley L a b o r a t o r y in California is shown in Figure 1. A bending magnet rotates the elec trons' velocity vector in the ring, and the acceleration causes them to radi ate and lose energy. They are then r e e n e r g i z e d by p a s s i n g t h e b e a m through a radio frequency (rf) cavity in the ring. The magnets and the rf cavity control the electron beam so tightly t h a t it n a r r o w s down to a thin ribbon only a few hundred mi c r o m e t e r s in d i a m e t e r — l e s s t h a n the thickness of a h u m a n hair. The electrons can be maintained at GeV
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993 · 949 A
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- Experiments ^BeamlinesN
Booster synchrotror Storage ^ ring Linear accelerator
S User laboratories and offices
Diagnostic -ψ/Γ sbeamline Insertion d e v i c e β / •^s^^ rf cavity \ _.y^
used to create a beamline determines the characteristics of the light pro duced, and each type has its s t r e n g t h s a n d d i s a d v a n t a g e s , de pending on the energy ranges or in tensities required. For instance, wigglers, which pro duce white light, generally are useful for working in a wide region of the spectrum, and they can extend the energy s p e c t r u m of t h e r a d i a t i o n output to the hard X-ray region. Un dulators, on the other hand, produce the nearly coherent, very b r i g h t beams of light needed for X-ray mi croprobe and microscopy techniques and generally produce light in t h e soft X-ray and vacuum-UV regions. In addition, both wiggler- and undulator-based designs have been adapted to provide circularly and elliptically polarized light for magnetic circular dichroism studies. A new source
Figure 1. Projected floor plan for the Advanced Light Source. (Source: Lawrence Berkeley Laboratory)
energies in the storage ring for lie lifetimes of many hours. Storage rings are run under high vacuum to avoid scattering from par ticles t h a t outgas from the walls of the synchrotron. Many synchrotron facilities allow electrons to circulate for several weeks during s t a r t u p to "scrub" the walls before full opera tion b e g i n s . As t h e p a r t i c l e s a r e pumped out of the ring, the current and the energy of the electron beam can be increased and the beam life time improves. The electrons accelerated by bend ing magnets emit polarized, highly collimated light in a broad, continu ous distribution of photon energies that can be directed away from the storage ring in a "beamline" for ex periments and can be used to provide monochromatic light at wavelengths throughout the spectrum. Older syn chrotrons relied on relatively simple bending-magnet systems to create the SR. In the past 10 y e a r s or so, new magnetic deflection devices called "wigglers" and " u n d u l a t o r s " h a v e been designed to m a n i p u l a t e t h e electron beam in ways that improve on bending-magnet radiation. Wig glers and u n d u l a t o r s — a l s o called "insertion devices" because they are inserted as units into the linear sec
tions of an accelerator ring—consist of groups of dipole magnets with al t e r n a t i n g north and south pole ori e n t a t i o n s t h a t force t h e e l e c t r o n b e a m to o s c i l l a t e s i d e w a y s a s it passes through them. Wigglers bend the electrons through sharper angles than a bend ing magnet can. The radiation emit ted is several times more i n t e n s e , and the spectral peak shifts to higher photon energies, although the spread of energies remains broad. Undula tors, on the other hand, produce ra diation of almost laser-like purity. The spectral peak is much narrower than that produced by either bending magnets or wigglers, and the spec tral brightness is 2 - 3 orders of mag nitude higher as a result of construc t i v e i n t e r f e r e n c e from t h e l i g h t e m i t t e d by each oscillation of t h e electron beam through the a r r a y of m a g n e t i c poles. One a d v a n t a g e of using undulators is t h a t the nearly coherent light can be tuned to a par ticular wavelength by mechanically changing the spacing between t h e undulator poles. Instrumentation for experimental research can be installed at the end of the light beamlines t h a t are di rected out of the storage ring by wig glers, undulators, or bending mag n e t s . The type of i n s e r t i o n device
950 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
T h e f i r s t r e a l i n d i c a t i o n t h a t SR could be used for analytical chemis try came in 1956, when Tomboulian and H a r t m a n of Cornell University published a study of the beryllium K-edge absorption spectrum using SR. However, they had access to the accelerator for only a few weeks. In the 1950s and 1960s, many other ex perimental facilities using SR were built as "parasitic" operations on ac c e l e r a t o r s t h a t had been b u i l t for high-energy physics experiments— high-energy physicists considered the radiation to be lost energy t h a t was useless for their studies. In 1961 the first such parasitic SR facility was built at the National Bu reau of Standards (NBS; now known as the National Institute for S t a n dards and Technology or NIST). Rob e r t M a d d e n , who now directs t h e Synchrotron Ultraviolet Radiation Facility (SURF-II) at NIST, used a small synchrotron t h a t accelerated electrons to 180 MeV. He and Keith Codling made the first systematic use of SR to obtain absorption spec t r a of gases for electron correlation experiments. The first synchrotrons in Europe and J a p a n were also used a t about this time. Technology in these coun tries had lagged behind t h a t in t h e United States, particularly in Japan, because of the conditions for peace at the end of World War II. In 1947 oc cupation troops destroyed four cyclo trons and threw them into the bay at Osaka, thinking that they were be ing used for arms development. By the mid-1960s, however, t h e NBS had made contact with Japanese and European synchrotron facilities, and
in 1976 J a p a n built t h e first syn chrotron facility dedicated as a light source. Tantalus, at the University of Wis consin, was the first dedicated syn chrotron facility built in the United States, and the National Synchro tron Light Source (NSLS) at Brookhaven National Laboratory in New York — w h i c h h a s s e p a r a t e rings for generating vacuum-UV and X-ray synchrotron r a d i a t i o n — w a s the first U.S. facility designed to be a light source. Many existing synchrotrons, in cluding t h e N S L S X - r a y s t o r a g e ring, were upgraded in t h e 1980s. Wigglers and u n d u l a t o r s were in serted in the linear sections of the storage rings to boost the light inten sity in the vacuum-UV and X-ray re gions of the spectrum. In t h e p a s t few years, new synchrotrons, such as the ALS a t Berkeley a n d t h e Ad vanced Photon Source (APS) at Argonne National Laboratory in Illi nois, have been designed with these devices already incorporated. The ALS is complete, and the elec tron beam injected into the ring dur ing s t a r t u p h a s already surpassed the goal of - 400 mA current and has reached 1.4-GeV energy. The facility will produce light in the vacuum-UV and soft X-ray regions of the spec trum for X-ray spectroscopies. Five user beamlines, including two undulator beamlines, have been installed, and there is room for five more in sertion devices on the ring, including an elliptical wiggler. At Argonne, c o n s t r u c t i o n of the APS, which is slated to begin opera tion in 1996 at 7 GeV and will pro duce h a r d X - r a y r a d i a t i o n , is r e ported to be a year and a half ahead of schedule a n d $25 million u n d e r budget. At the end of the first phase of construction, the APS will accom modate 32 user beamlines, many of which will be dedicated for chemical and biochemical spectroscopy, dif fraction, and imaging. The European Synchrotron Radiation Facility in Grenoble, France, with a maximum energy of 6 GeV, has just been com pleted, and the Synchrotron Photon Ring at 8 GeV (SPRing-8) is under construction in Kansai, Japan.
for chemical contrast imaging. All of t h e s e can be e n h a n c e d for speed, sensitivity, or resolution when they are placed on an SR beamline. The SR source allows researchers to perform techniques that just don't work with weaker conventional X-ray sources in a regular labora-
In the 1950s and 1960s, SR facilities were "parasitic" operations at high-energy physics accelerators-the physicists saw the radiation as lost energy that was useless for their studies
SR experiments Analytical methods using SR include X-ray photoemission spectrometry, electron energy loss spectrometry (EELS), X-ray diffraction (XRD), and X-ray microscopies such as those us ing extended X-ray absorption fine structure (EXAFS) or X-ray absorp tion near-edge s t r u c t u r e (XANES)
t o r y . F o r X A N E S , t h e SR X - r a y beam can be focused to an a r e a as small as 0.1 χ 0.1 μπι on a sample, and the energy can be varied to ex p l o i t n i t r o g e n or c a r b o n K - s h e l l near-edge absorption resonances for chemically based contrast imaging between organic compounds. For ex
ample, the slight differences in C=C, C=N, and C=0 content between DNA and proteins, or between dissimilar polymers, p e r m i t imaging of t h e i r spatial distribution. The high spectral brightness of SR provides a good signal-to-noise ratio even for minute samples. At NSLS, the scanning transmission X-ray mi croscope is used to scan X-rays of a specific energy that are diffractively focused to a m i c r o p r o b e . T h e s e X-rays are scanned over a sample in two d i m e n s i o n s to obtain XANES spectral data with 55-nm spatial resolution, and spectra containing 500 data points can be accumulated within 5 s. David Hanson's group at the State U n i v e r s i t y of N e w Y o r k - S t o n y Brook has devised several hyphen a t e d t e c h n i q u e s for photoelectron spectroscopies on an SR beamline. One of them, "energy-resolved, Au ger electron, multiple ion coincidence (ERAEMICO)" spectroscopy, com bines a hemispherical electron en ergy analyzer with a time-of-flight mass spectrometer to study the cor relation between Auger electron final states and the specific fragmentation pattern in a molecule. To accomplish this, the flight tube of the mass spec trometer is adapted at the interface with a small potential g r a d i e n t so t h a t the Auger electrons are avail able to the electron energy analyzer. Edwin Westbrook, director of Argonne's S t r u c t u r a l Biology Center, says t h a t some XRD techniques for protein crystallography simply can not be done with a regular labora tory source, because success in solv ing crystallographic structures depends on counting enough X-ray photons at each point in the Bragg lattice for s t a t i s t i c a l significance. For example, virus crystals generate so many lattice points per unit that the diffraction is very weak at each point. On the other hand, some pro teins only form microcrystals (in the 10 χ 10 x 10 μπι range), which are too small to diffract very strongly. C u r r e n t SR sources can direct a photon flux 6 0 0 - 9 0 0 t i m e s higher t h a n t h a t of a n i n - l a b source on these kinds of samples. The rays are t i g h t l y focused, so t h e diffraction pattern also remains focused, allow ing significant d a t a to be accumu lated in a reasonable amount of time. For microcrystals and for 2D arrays of p r o t e i n s t h a t don't crystallize, s t r u c t u r e s now c a n be solved in 10-20 h of beam time. So far, Westbrook says, the smallest protein crystal successfully analyzed at his NSLS beamline was 20 μπι on a side.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993 · 951 A
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W e s t b r o o k ' s t e a m is u s i n g t h e X-ray storage ring at NSLS as a prototype of the operation they plan to run at the APS when construction is finished. "The machine we're building h e r e a t APS is 1 0 - 1 0 0 t i m e s brighter t h a n the one at NSLS," he says. "That means you can get a protein structure 100 times faster for a regular crystal or you can get a prot e i n s t r u c t u r e w i t h a c r y s t a l 100 times smaller." The t e a m will fit out two b e a m lines a t A P S , one w i t h a b e n d i n g magnet and another with a wiggler and an undulator, for general service to c r y s t a l l o g r a p h e r s . The wiggler will be useful for Laue diffraction experiments, in which a crystal is exposed to the complete energy spect r u m s i m u l t a n e o u s l y to g e n e r a t e - 10,000 spots all at once. "Essentially, you're taking instantaneous snaps h o t s of t h e diffraction p a t t e r n , " Westbrook says. The data are transformed by FT so that the time resolution depends o n l y on t h e b r i g h t n e s s of t h e source—a few years ago, researchers at the Cornell High-Energy Synchrotron Source got a Laue diffraction p a t t e r n for a protein crystal in 100 ps. Eventually, Westbrook says, this method may be used to make "movi e s " of e n z y m e s in a c t i o n , b u t a t p r e s e n t t h e practical problems involved in maintaining a protein crystal lattice while trying to initiate catalysis or other perturbations evenly across it relegate such technology to the future. Getting there
How do you get your project onto one of the synchrotron beamlines? And what do you have to take with you? First, says NIST's Madden, you have to submit your proposal to a suitable synchrotron facility that can provide the photon energy range required by your experiment. A review committee evaluates and prioritizes the proposals, then puts together a schedule. The wait can be a few weeks or more t h a n a year, depending on the quality of the proposal and the popularity of the beamline. Each synchrotron facility has its own policy. In practice, synchrotron facilities are used most by people who work in the same area of the country. Even though, for example, vacuum-UV experiments performed by West Coast u s e r s at the Stanford Synchrotron Radiation Laboratory "could easily be done a t a lower e n e r g y r i n g , " M a d d e n s a y s , " e a c h facility, of course, likes to r u n w i t h as full a house of users as possible."
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952 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
Most of the synchrotrons in this country are funded by the U.S. Department of Energy and the National Science Foundation. Madden says, "Machines supported by these agencies are thought of as national facilities, and the idea is t h a t everyone h a s a r i g h t to p u t in a proposal." Commercial R&D used to be an exception. ' T e a r s ago, things got a little sticky when industrial users app l i e d for t i m e on p r o p r i e t a r y projects." It was decided that industrial users would pay user fees to the facilities for proprietary work. In recent years, policy has shifted in favor of s u p p o r t i n g i n d u s t r i a l r e s e a r c h . About a dozen major pharmaceutical companies have banded together to share the cost of operating dedicated beamlines, and this group will likely p u r c h a s e a t least one beamline a t the APS. However, says Westbrook, "Even though synchrotrons are expensive and big, a lot of small-scale science is done on them, and hundreds of res e a r c h e r s who use t h e b e a m l i n e s come from smaller u n i v e r s i t i e s . A typical research project is supported by a grant of $100,000 a year."
U.S. public synchrotron facilities Advanced Light Source (ALS) Lawrence Berkeley Laboratory 1 Cyclotron Rd. Berkeley, CA 94720 Advanced Photon Source (APS) Argonne National Laboratory 9700 S. Cass Ave. Argonne, IL 60439 Cornell High Energy Synchrotron Source (CHESS) Cornell University Ithaca, NY 14583 The J. Bennett Johnston Sr. Center for Advanced Microstructures and Devices (CAMD) Louisiana State University Baton Rouge, LA 70803 National Synchrotron Light Source (NSLS) Brookhaven National Laboratory Upton, NY 11973 Stanford Synchrotron Radiation Laboratory (SSRL) MS 69, P.O. Box 4349 Stanford, CA 94309 Synchrotron Radiation Center (SRC) University of Wisconsin-Madison 3731 Schneider Dr. Stoughton, Wl 53589 Synchrotron Ultraviolet Radiation Facility (SURF-II) National Institute for Standards and Technology Gaithersburg, MD 20899-0001
Once a project is approved a n d scheduled, the researcher must pre pare samples and a r r a n g e funding for travel. Many beamlines, such as those operated by the Structural Bi ology Center at Argonne, are already outfitted with analytical instrumen tation owned by the facility. Users bring their samples and members of their research t e a m s to the facility for a few days of safety training and orientation followed by a few more days of data gathering; they some times work around the clock to fit in as many samples as possible. Members of the synchrotron facili ty's staff show users how to operate the safety interlocks and help set up the instruments according to the ex perimental r e q u i r e m e n t s . With so much to do and so little time, atten tion focuses almost exclusively on the i n s t r u m e n t s e t u p a n d on t h e project. Although the facility staff is fairly accommodating, t h e s u r r o u n d i n g s tend to be pretty spartan. "The ex perimental stations are cramped, poorly lit, and noisy—most of them were originally built as physics labs and no effort was ever made to make them comfortable," Westbrook says. "Basically, you do an experiment at a synchrotron only when you have to. One of my motivating issues at APS is to make it passably comfortable to use our beamlines." Bringing equipment from home is a lot more complicated. Some beamlines are usually available for use with new equipment brought in by users, but installing it usually means longer setup and optimization times and an additional panel must review the project for the safety of the users, their equipment, and the synchro tron itself. "People bringing in new equipment face the grim reality of t h e people who are there to protect the synchro tron," says Madden. Worst for the safety of the storage ring, he says, is a dirty vacuum system brought in by a user, because hydrocarbon contam inants from the user's equipment can enter the beamline and, once in the system, can a t t e n u a t e the electron beam lifetime and contaminate the system for all the users. Hazardous gases or samples such as highly dan gerous viruses may also be reasons that a project is turned down. Conversely, the high flux and bril liance of the SR beam can damage user equipment, so a researcher may have to redesign the experiment or instrumentation to handle the extra power. "A c r y s t a l m o n o c h r o m a t o r may glow red," Madden says. To pre
vent t h a t , many of the synchrotron facilities have begun to fit out their beamlines with water-cooled crystal and grating monochromators de signed with special copper mount ings for controlled energy dispersal. In a d d i t i o n , t h e i n s t r u m e n t a t i o n must be shielded more than it might be in the laboratory because of the h i g h e r r a d i a t i o n flux. For X - r a y s with 100-200-keV photon energies, which the APS will be able to gener ate, Westbrook says, "We have been persuaded to put neutron absorbers around the equipment."
Not just for research Synchrotron sources can be used for more t h a n e x p e r i m e n t a l research. S U R F - I I at NIST also serves as a n a t i o n a l r a d i o m e t r i c s t a n d a r d for calibrating spectroscopic instrumen t a t i o n from the visible region into the vacuum-UV and soft X-ray r e gions of the spectrum, down to 4-nm wavelengths. With an 84-cm radius a n d a single m a g n e t s y s t e m , t h e storage ring at SURF-II is a minia ture of most synchrotron facilities, but it also maintains the electrons in a perfectly circular orbit with a cross-section of 1500 χ 80 μπι. The
flux at a given distance down a cali bration beamline can be calculated very accurately, says Madden, and SURF-II has been used to calibrate spectrometers for the Space Shuttle. T h e big i n d u s t r i a l use for SR s o u r c e s is X - r a y l i t h o g r a p h y for etching chip circuits. The monochro m a t i c , tightly focused X-ray beam photoetches circuits with greater de t a i l , s h a r p e r c o r n e r s , a n d cleaner lines t h a n those produced by beams from other sources. The higher reso lution allows u s e r s to print circuits with features as small as 0.1 μπι in width, as compared with optical li thography, which can etch features down to 0.25 μπι. This technology m a y e v e n t u a l l y benefit a n a l y t i c a l chemistry as a way to m i n i a t u r i z e instrumentation or to store gigabits of information in chips as small as or smaller than the ones currently stor ing megabits. Deborah Noble
Suggested reading Tomboulian, D. H.; Hartman, P. L. Phys. Rev. 1956, 102, 1423. Synchrotron Radiation Research: Advances in Surface and Interface Science, Vol. 1: Tech niques and Vol. 2: Issues and Technology; Bachrach, R. Z., Ed.; Plenum Press: New York, 1992.
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