-tF 0 m
I
of coherent light 1000 times brighter than anything a laboratory source can produce, focused on a 1-mm2 point positioned m the source window. laser-it’s an X-ray beam that shines 5 x 10l1 photons/s onto that tiny point, and 70,000 researchers worldwide make reservations as much as a year in advance just to use it for three or four days. The beam is produced by a synchrotron, a ring in which magnets accelerate electrons or positrons to produce vacuum-UV radiation and high-intensity X-rays. About 10 facilities produce synchrotron radiation (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 adaptations of the cyclotrons designed during World War 11. Unlike today’s facilities, which can reach more than 1km in circumference, the first synchrotron was a
saw the potential of SR as a light source for their methods. X-ray spectroscopists in particular were slow to
the major groups of SR users. SR does more than just enhance the accuracy and speed of conventional analytical methods; its high spectral
> brightness makes possible a wide variety of new techniques that couldn’t be dreamed of with standard in-lab radiation sources. Going around the bend A synchrotron accelerates electrons centripetally to produce high-intensity radiation with energies ranging
energies of 50-100 MeV, then i jected into a synchrotron accelerato 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)a t Lawrence Berkeley Laboratory i California is shown in Figure 1. A bending magnet rotates the el trons’ velocity vector in the ring the acceleration causes them to ate and lose energy. They ar reenergized by passing t h e through a radio frequency (rf) in the ring. The magnets and cavity control the electron beam s tightly t h a t it narrows down to thin ribbon only a few hundred micrometers in diameter-less than the thickness of a human hair. The electrons can be m
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21 , NOVEMBER 1,1993
949 A
FUCUS
used to create a beamline determines the characteristics of the light produced, a n d e a c h t y p e h a s i t s strengths and disadvantages, depending on the energy ranges or intensities required. For instance, wigglers, which produce white light, generally are useful for working in a wide region of the spectrum, and they can extend the energy spectrum of t h e radiation output to the hard X-ray region. Undulators, on the other hand, produce t h e nearly coherent, very bright beams of light needed for X-ray microprobe and microscopy techniques and generally produce light in the soft X-ray and vacuum-W regions. In addition, both wiggler- and undul a t o r - b a s e d designs have been adapted to provide circularly and elliptically polarized light for magnetic circular dichroism studies.
Figure 1. Projected floor plan for the Advanced Light Source. (Source: Lawrence Berkeley Laboratory)
energies in the storage ring for lle lifetimes of many hours. Storage rings are run under high vacuum to avoid scattering from particles that outgas from the walls of the synchrotron. Many synchrotron facilities allow electrons to circulate for several weeks during startup to “scrub” the walls before full operation begins. As t h e particles a r e pumped out of the ring, the current and the energy of the electron beam can be increased and the beam lifetime improves. The electrons accelerated by bending magnets emit polarized, highly collimated light in a broad, continuous distribution of photon energies that can be directed away from the storage ring in a “beamline” for experiments and can be used to provide monochromatic light at wavelengths throughout the spectrum. Older synchrotrons relied on relatively simple bending-magnet systems to create the SR. In the past 10 years or so, new magnetic deflection devices called “wigglers” and “undulators” have been designed to manipulate t h e electron beam in ways that improve on bending-magnet radiation. Wigglers and undulators-also called “insertion devices’’ because they are inserted as units into the linear sec950 A
tions of an accelerator ring-consist of groups of dipole magnets with alternating north and south pole orientations t h a t force the electron beam to oscillate sideways a s it passes through them. Wigglers bend t h e electrons through sharper angles than a bending magnet can. The radiation emitted is several times more intense, and the spectral peak shifts to higher photon energies, although the spread of energies remains broad. Undulators, on the other hand, produce radiation of almost laser-like purity. The spectral peak is much narrower than that produced by either bending magnets or wigglers, and the spectral brightness is 2-3 orders of magnitude higher as a result of constructive interference from t h e light emitted by each oscillation of the electron beam through the array of magnetic poles. One advantage of using undulators is that the nearly coherent light can be tuned to a particular wavelength by mechanically changing the spacing between the undulator poles. Instrumentation for experimental research can be installed a t the end of the light beamlines that are directed out of the storage ring by wigglers, undulators, or bending magnets. The type of insertion device
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1,1993
A new source The first real indication that SR could be used for analytical chemistry came in 1956,when Tomboulian and Hartman of Cornel1 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 experimental facilities using SR were built as “parasitic” operations on accelerators that had been built for high -energy physics experimentshigh- energy physicists considered the radiation to be lost energy that was useless for their studies. In 1961 the first such parasitic SR facility was built at the National Bureau of Standards (NBS; now known as the National Institute for Standards and Technology or NIST). Robe r t Madden, who now directs t h e Synchrotron Ultraviolet Radiation Facility (SURF-11) a t NIST, used a small synchrotron that accelerated electrons to 180 MeV. He and Keith Codling made the first systematic use of SR to obtain absorption spectra of gases for electron correlation experiments. The first synchrotrons in Europe and Japan were hlso used at about this time. Technology in these countries had lagged behind that in the United States, particularly in Japan, because of the conditions for peace at the end of World War 11. In 1947 occupation troops destroyed four cyclotrons and threw them into the bay at Osaka, thinking that they were being used for arms development. By the mid- 1960s,however, the NBS had made contact with Japanese and European synchrotron facilities, and
.,
in 1976 Japan built the first synchrotron facility dedicated as a light source. Tantalus, at the University of Wisconsin, was the first dedicated synchrotron facility built in the United States, and the National Synchrot r o n Light S o u r c e ( N S L S ) a t Brookhaven National Laboratory in New York-which h a s s e p a r a t e rings for generating vacuum-W and X-ray synchrotron radiation-was the first U.S.facility designed to be a light source. Many existing synchrotrons, including the NSLS X-ray storage ring, were upgraded in the 1980s. Wigglers and undulators were inserted in the linear sections of the storage rings to boost the light intensity in the vacuum-W and X-ray regions of the spectrum. In the past few years, new synchrotrons, such as the ALS at Berkeley and the Advanced Photon Source (APS)at Argonne National Laboratory in Illinois, have been designed with these devices already incorporated. The ALS is complete, and the electron beam injected into the ring during startup has already surpassed the goal of 400 mA current and has reached 1.4-GeV energy. The facility will produce light in the vacuum-W and soft X-ray regions of the spectrum for X-ray spectroscopies. Five user beamlines, including two undulator beamlines, have been installed, and there is room for five more insertion devices on the ring, including an elliptical wiggler. At Argonne, construction of the APS, which is slated to begin operation in 1996 a t 7 GeV and will produce hard X-ray radiation, is reported to be a year and a half ahead of schedule and $25 million under budget. At the end of the first phase of construction, the APS will accommodate 32 user beamlines, many of which will be dedicated for chemical and biochemical spectroscopy, diffraction, and imaging. The European Synchrotron Radiation Facility in Grenoble, France, with a maximum energy of 6 GeV, has just been completed, and the Synchrotron Photon Ring a t 8 GeV (SPRing-8) is under construction in Kansai, Japan.
,,’
,
.
3
for chemical contrast imaging. All of these can be enhanced 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-
-
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 using extended X-ray absorption fine structure (EXAFS) or X-ray absorption near-edge structure (XANES)
tory. For XANES, t h e SR X-ray beam can be focused to an area as small as 0.1 x 0.1 pm on a sample, and the energy can be varied to exploit nitrogen or carbon K-shell 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=O content between DNA and proteins, or between dissimilar polymers, permit imaging of their 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 microscope is used to scan X-rays of a specific energy that are diffractively focused to a microprobe. These X-rays are scanned over a sample in two dimensions 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 University of New York-Stony Brook has devised several hyphenated techniques for photoelectron spectroscopies on an SR beamline. One of them, “energy-resolved, Auger electron, multiple ion coincidence (ERAEMICO)” spectroscopy, com bines a hemispherical electron energy analyzer with a time-of-flight mass spectrometer to study the correlation between Auger electron final states and the specific fragmentation pattern in a molecule. To accomplish this, the flight tube of the mass spectrometer is adapted at the interface with a small potential gradient so that the Auger electrons are available to the electron energy analyzer. Edwin Westbrook, director of Argonne’s Structural Biology Center, says that some XRD techniques for protein crystallography simply cannot be done with a regular laboratory source, because success in solvi n g crystallographic s t r u c t u r e s depends on counting enough X-ray photons a t each point in the Bragg lattice for statistical significance. For example, virus crystals generate so many lattice points per unit that the diffraction is very weak a t each point. On the other hand, some proteins only form microcrystals (in the 10 x 10 x 10 pm range), which are too small to diffract very strongly. Current SR sources can direct a photon flux 600-900 times 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 tightly focused, so t h e diffraction pattern also remains focused, allowing significant data to be accumulated in a reasonable amount of time. For microcrystals and for 2D arrays of proteins that don’t crystallize, s t r u c t u r e s now can be solved in 10-20 h of beam time. So far, Westbrook says, t h e smallest protein crystal successfully analyzed a t his NSLS beamline was 20 pm on a side.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
951 A
lines at APS, one with
the future.
sieves in addition to the original full-height 6-sieve model. This extends the range of GRADEX ... providing more separations for a given application, or handling several applications without sieve this in addition to the b advantages ... Guarantees more ac sieve analysis results Frees technicians for 0th Provides for more tests manpower In-plant use enables line perform sieve analysis For details, request Catalog 22 541-1236 or Fax 513-541-4888. ROTEX INC., 1230 Knowlton St., Cincin 5223-1845 U.S.A.
KIIEVSCREENERS
MEGATEX*SCREENERS G W 0 A
CIRCLE 74 ON READER SERVICE CARD
posals, then puts ule. The wait can more than a year, quality of the prop larity of the beamline. tron facility has its own PO In practice, synchrotron
up
, IYT
I IYI
Stanford Synchrotr Laboratory (SSRL) MS 69, P.O. Box 4349 Stanford, CA 9430s Synchrotron Radiation Center (SRC University of Wisconsin-Madiso 3731 Schneider Dr. Stoughton, WI 53589
signed with special copper mountings for controlled energy dispersal. I n addition, the instrumentation must be shielded more than it might be in the laboratory because of the higher radiation flux. For X-rays
as many samples Members of the
persuaded to put neutron absorbers around the equipment.” Not just for research
national radiometric standard for tion focuses almost
accommodating, the surroundings tend to be pretty spartan. “The experimental stations are cramped, poorly lit, and noisy-most of them were originally built as physics labs and no effort was ever made to make
wavelengths. With an 84-cm radius and a single magnet system, t h e storage ring a t SURF-I1 is a miniature of most synchrotron facilities, but it also maintains the electrons in a perfectly circular orbit with a
bration beamline can be calculated very accurately, says Madden, and SURF-I1 has been used to calibrate spectrometers for the Space Shuttle. The big industrial use for SR sources is X-ray lithography for etching chip circuits. The monochromatic, tightly focused X-ray beam photoetches circuits with greater de tail, sharper corners, and cleaner lines than those produced by beams from other sources. The higher resolution allows users to print circuits with features as small as 0.1 pm in width, as compared with optical lithography, which can etch features down to 0.25 pm. This technology may eventually benefit analytical chemistry as a way to miniaturize instrumentation or to store gigabits of information in chips as small as or smaller than the ones currently ing megabits. Suggested reading Tomboulian, D. H.; Hart
Reu. 1956, 102, 1423. Synchrotron Radiation Research: Advances in Sutjiace and Inter/ace Science, Vol. 1: Techniques and VoZ. 2: Issues and Technology; Bachrach, R . Z., Ed.; Plenum Press: New York, 1992.
system, can attenuate the electron beam lifetime and contaminate the system for all the users. Hazardous gases or samples such as highly dangerous viruses may also be reasons that a project is turned down. Conversely, the high flux and brilliance of the SR beam can damage instrumentation to handle the extra
7425 N. Oak Park Av For your FREE cataio
ANALYTICAL CHEMISTRY,
iles, IL Gft4 I
BER 1,1993
953 A
