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Trace Element , Determinations
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Keith W. Jones and Barry M.
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Brookhaven National Laboratoty UOlOn. NY 11973
X-ray fluorescence(XRF) analysis has played an important role in elemental chemical analysis for many years. The synchrotron radiation (SR) X-ray source that has been developed in the last 1&20 years represents a revolutionary improvement in X-ray production technology. By using SR for XRF analysis, researchers can make measurements with improved sensitivity and spatial resolution. Results obtained with SR show that measurementswith a spatial resolution of less than 100 ,” and a minimum detection limit (MDL) approaching 1fg can he made in just 300 s. The XRF method traditionally has been used hecause of its multielement detection capability, minimal sample preparation, minimal damage to the sample, and ability to operate a t atmospheric pressure with solid, liquid, or gaseous samples. The substitution of the SR source preserves all these attributes and opens new opportunities for exploiting the XRF method. For example, a trace element X-ray microscope can be used to OOO3-2700/89/036 1-34 1A/$01.50/0
@ 1989 American Chemical Society
give elemental maps a t concentration levels helow 1ppm by weight. Synchrotron radiation-induced Xray emission (SRIXE),which combines XRF and the new SR X-ray source, is about to become an important weapon in the arsenal of the analytical chemist for analyzing many types of materials. Familiarity with the new approach to XRF is rapidly becoming necessary for researchers seeking solutions to difficult analytical problems. This article provides a survey of the basic principles of the SRIXE method and the
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The SR 8ouce The production of X-rays in electron or positron storage rings has been discussed extensively in the literature (I). A few of the most important concepts are summarized here for those unfamiliar with the technology. Electrons or positrons a t energies of several GeV are confined in a path comprising circular and straight sections by a set of magnets. Radiation is emitted by the acceleration of the beam as it moves in a circular path in
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1 ways in which it is being employed. Only three laboratories in the United States can make SRIXE measurements using X-rays in the keV energy range: the National Synchrotron Light Source (NSLS) a t Brookhaven Nationa1 Laboratory (BNL), the Cornell High-Energy Synchrotron Source (CHESS) at Cornell University, and the Stanford Synchrotron Radiation Laboratory (SSRL) a t Stanford University. The NSLS X-26 beam lines are the only ones dedicated to SRIXE measurements in the United States.
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the bending magnet sections. The characteristic energy of the radiation (in keV), defined as the median of the power spectrum, is given by 2.218 P / R , where E is the energy of the beam in GeV and R is the radius of curvature of the beam orbit in meters. The characteristics of the radiation emitted can he changed by the use of so-called insertion devices that are placed in the straight sections of the storage ring. Two types exist: wigglers and undulators. Wigglers are magnetic structures that create multiple oscilla-
ANALYTICAL CHEMISTRY, VOL 61, NO 5. MARCH I, 1989 * 341 A
tions around the beam path with small curvatures and hence increase both the energy of the emitted radiation and its intensity. Undulators cause smaller and more frequent wcillatory deflections of the beam. Through interference effects, they create coherent radiation that is concentrated a t specific energies as compared with the continuous spectrum obtained from bending magnets and wigglers. Stored beams last for several hours, so the SR can easily be used for experimental purposes. When applied to XRF or X-ray microscopy (XRM) measurements, three important properties of SR must be considered. First, there is a wide and continuous spectral range of very high intensity. At the NSLS, it extends beyond 30 keV a t a bending magnet and
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100 keV for a superconducting wiggler. The photon fluxes produced by the NSLS bending magnet and superconducting wiggler sources are shown in Figure 1. Second, there is a high degree of polarization in the plane of the electron orbit. This situation reduces background scattering into a detector positioned a t 90’ to the sample in the plane of the ring. The relative amount of vertical polarization, which contributes greatly to scattering backgrounds in the plane of the ring, increases with increasing vertical divergence angle. The fraction of vertical polarization decreases with increasing X-ray energy. Third, there is a natural collimation that permits the focusing of the X-rays with mirrors positioned at small glancing angles. At the NSLS electron energy of 2.5 GeV, the vertical angular di-
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Flgure 1. The photon fluxes produced at the NSLS by a bending magnet, by a sixpole superconductor wiggler, and by a rare-earth cobalt permanent magnet wiggler are shown as a function of photon energy. Tlm aIWal e m ~ yLC (median of the power apemum) hn ea& CIRCLE 65 ON READER SERVICE CAR0
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ANALYTICAL CHEMISTRY, VOL. 61. NO. 5. MARCH 1, 1989
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INSIRUMENTATION vergence at the higher energies to be used for fluorescence excitation is 0.2 mrad. Types ol XRF X-ray mlcmscopes The XRM can be assembled in different ways from a number of components. The beam from the SR source is processed with mechanical collimators, monochromators, and focusing mirrors. The sample is moved past the beam on a motor-driven stage. The fluorescent X-rays and the scattered beam are detected with energy-dispersive spectrometers (EDS)and/or wavelength-dispersive spectrometer (WDS) systems. The simplest approach to the XRM is to use the continuous radiation (white heam) directly from the synchrotron. A mechanical aperture sets the size of the beam, and beam fiiters can be used to give a crude shaping to the energy spectrum. The flux incident on the sample can he increased with an appropriately shaped mirror to produce a focused image of the X-ray source at the target position. The flux of photons is increased because it is possible to collect radiation emitted over an angular range of several mrad in the horizontal direction and over the entire vertical range in the vertical direction. Losses occur because of imperfections in the mirror contours and coatings, whicproduce excessive scattering. Whe high spatial resolution is needed, a second stage of demagnificationcan he included at a cost of increased angular divergence. This result is in accordance with Liouville's theorem, which states that the emittance (product of source size and angular divergence) of a beam a t any point cannot he less than that of the source. The use of monoenergetic exciting X-rays is helpful in many situations. A monochromator on the incident beam can he incorporated with emphasis on high throughput of the X-rays or on high energy resolution. The use of any or all of these elements results in a loss of photons. Several factors must be balanced when deciding which performance factors are most important and how the needed specifications can be achieved. The XRM demands a source capable of placing a high number of photons in a given area per unit time. A dedicated storage ring that has a high brilliance (photonshource area-solid angle) is beat suited for this work. The next generation of machine promises to give better performance than that obtained with existing storage rings. The schematics in Figure 2 illustrate the approaches taken by several different groups. A tabulation of photon fluxes and spatial resolutions for sever344).
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Flgure 2. Schematics showing different approaches used for a high-resolution X-ray microscope: (a)ellipsoidal mirror at NSLS (3, (b) LBL Kirkpatrick-Baez system using spherical multilayer surfaces and tested at NSLS (3).(c)Wolter focusing mirror
system wlth combined ellipsoidal and hyperboloidal surfacesof revolution used at Photon Factory (4).and (d) bent crystal system used at Daresbury (5).
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
al systems is given in Table I. There seems to be merit in choosing the simplest approach; it seems to give the highest flux values economically. The NSLS X-26 XRM The NSLS X-ray microprobe is a fully dedicated facility and currently represents the best opportunity for use by the outside research community. Until 1987 the X-26C beam line was a collimated XRM (CXRM) that used an unfocused "white" beam in which the ex-
Typical X-ray fluxes produced using synchrotron radiatior
Table 1.
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citation spectrum was hardened by A1 absorbers. Beams as small as 20 pm could be obtained by collimation with variable slits. A photograph of the CXRM apparatus is shown in Figure 3. Samples are mounted at 45" to the beam on a stepping-motor-driven translation and rotation stage with 100 mm of travel in the scanning directions and 25 mm in the focusing direction. The rotation staging is used to orient crystalline materials to eliminate Bragg diffraction peaks. The
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Flgure 3. photograph of the collimated X-ray microprobe used on the NSLS X-26 beam line. 346A
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
stages have 1-pm step sizes. The sample is observed with a Nikon SMZ-10 binocular zoom microscope equipped with a high-resolution DAGEiMTI TV camera. The field of view on the TV monitor is 2-10 mm, and the optical resolution is about 10 pm. Lawrence Berkeley Laboratory (LBL), in collaboration with BNL, performed studies a t the NSLS X-26C beam line using a Kirkpatrick-Baez optical system made at LBL and produced a focused XRM (FXRM). The system consisted of mutually perpendicular spherical mirrors coated with W-C multilayers. The source for the mirrors was a 0.5-mm2 collimating slit 300 cm upstream, and the focus was about 6 cm downstream of the first mirror. This provided a 50-fold demagnification in each direction for a 10-pm2 beam spot. The system was designed for operation of a 10-keVenergy with a 10% bandwidth. The photon fluence wan measured to be 3 X 109 photons/s. A diagram of the mirror arrangement is shown in Figure 2b. -lngt-= The XRM can collect data in a variety of modes. The simplest, and the one for which the calculated and experimentally observed sensitivities are quoted above, analyzes a single small area on a target by collecting an entire fluorescence spectrum for a time period on the order of minutes. The spectra are analyzed an discussed below for peak areas and the concentrations determined, usually by comparison with appropriate standards. A spectrum obtained from a sample of a National Institute of Science and Technology (NIST) Standard Reference Material (SRM) 1571, Orchard Leaves, is shown in Figure 4. A number of scanning modes are used on the X-26 microprobe to sample one- or two-dimensional arrays of picture elements (pixels). In all scanning modes the sample stage must be scanned in front of the beam because the beam cannot be rastered, as can charged particle beams. In the fmst scanning mode the entire fluorescence spectrum for each pixel is stored. This procedure requires a large amount of storage for a high-resolution scan as well as an inordinate amount of processing to extract the desired data. This mode can perhaps best be thought of as an automated procedure for point analysis. The second mode of scanning limits the amount of information collected by defining regions of interest (ROI), each of which is a range of channels including the fluorescence peak for the desired element. At each pixel the computer program determines the hack-
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Flgure 4. Typical SR-induced X-ray fluorescence spectrum produced by irradiation of NlST SRM 1571, Orchard Leaves.
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ANALYTICAL CHEMISTRY, VOL. 61. NO. 5, MARCH 1, 1989
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ground-corrected peak areas for each ROI on line and stores only these values. The data are normalized using the integrated ion chamber current at each pixel. As many as 32 ROIs can be defined for each spectrum. Because of the time required to move the stage and procesa the data, the minimum practical time per pixel is about 1-2 s. This mode is illustrated in a series of images in Figure 5, a scan of a rock section of an ore deposit. The upper left image is a backscattered electron image made with a scanning electron microscope and shows the major minerals pyrrhotite (Fe,-,S), chalcopyrite (CuFeSz), and pentlandite (Fer 5N45 s ~ )The . elements Fe, Cu, Ni, Pd, and Se were measured in a scan of 40 X 40 pixels with a collection time of 2 s/pixel. The color intensity scale corresponds to a thermal temperature scale that for each element progresses from black (zero counts) through dark red, orange, and yellow to white (highest count of that element in the scan). The Pd image corresponds to the pentlandite and may indicate pentlandite grains under the surface of the chalcopyrite. The use of white radiation and the higher Pd K a fluorescence energy samples the target much deeper for Pd than for the lighter elements. In a third scanning mode the output of the Si(Li) detector is fed into one or more single-channel analyzers (SCA), and this output is then fed into a multichannel scaler (MCS). The window of the SCA is set to the energy range of the fluorescence peak of interest. The scanning is continuous, and the channel advance signal to the MCS is derived from the stepper motor pulses on the scanning stage. The advantage of this scanning mode is speed; the scan rate is limited only by the counting statistics and not the time required for step-scanning the stage. However, extensive hardware is required to analyze a number of elements simultaneously.
Typkal rmulis I ” the CXRM and FXRM Illustrative spectra obtained with the X-26 CXRM and FXRM show the differences produced by the use of white and monoenergetic exciting radiation. The spectrum obtained from a 20-rm thick section of gelatin that contained several elements at aconcentration level of 10 ppm is shown in Figure 6 for the CXRM and in Figure 7 for the FXRM. Because the spatial resolutions used in obtaining the spectra are different, care must be taken when makine comparisons. The number of counts in the ueaks representing the characteristic fliores-
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Figure 5. Scan of a rock section of an ore deposit. Upper left image: SEM backscatter& electron image of an ore deposit section (512 X 512 pixels of 25-pm resolution); other images: 40 X 40 pixel scan of 30-pm resolution and 2 slplxel. The images represnt tha net counts p s Ka Iluwescenw peak lor Fe, Cu, Ni, W. and Se. respectively. The wIw scale is a mermal temperature scale.
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Figure 6. X-ray fluorescence spectrum produced from prepared trace elemeni 20-pm thick section of gelatin using the collimated X-ray microprobe (9).
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5. MARCH 1, 1989
cent X-rays are extracted using various fitting techniques. BNL has employed an adaptation of the HEX program that was originally used to fit the spectra from proton-induced X-ray emission (12) and a program by Kajfosz and Kwiatek (13)that uses a set of parabolas to fit the background. Petersen et al. (14) have described another program, XSPEK, which is also designed for use with white beams.
Callbratian Or the technhe In principle, XRF analysis is an absolute method in which concentrations are determined through knowledge of photoelectric and scattering cross sections and accurate determination of the experimental parameters. Using SR, Tolkiehn and Petersen (15) determined the mass of a sample from knowledge of the matrix composition and the appropriate scattering cross sections. In general usage, however, yields are compared with known trace element standards in other matrices. The weight concentration, C,, of an element Z of atomic weight A in a target of thickness T em can he expressed as
where K is a constant composed of Avogadro's numher, the atomic weight of the analyte,the fluorescenceproduction cross section, and the detector efficiency including geometry; and ( p / ~ ) are the mass absorption coefficients for the excitation and fluorescence energies in the matrix. The beam and detector each look at the surface of the target at 4 5 O to preserve the beam-detector angle of 90'. The quantity R is the count rate of the characteristic X-ray of the analyte. I,, is the rate a t which excitation photons impinge on the target and is proportional to the ionization chamber count. The thin target approximation, in which one takes the linear term in the expansion of the exponential argument, yields
C, = 0.7 K R / I , ( p T ) where pT is the target thickness in g/ cm? The constant K is evaluated using an appropriate thin standard of the type discussed below. The thick target approximation assumes that the exponential term is zero and results in the relationship
Figure 7. X-ray fluorescence spectrum produced from prepared trace element 20-f.m thick section of gelatin uslng the focused X-ray microprobe. The peah aT 10.5 keV is scatterea I "lead in Uw hut& in10 an mhielded dewclor (9,.
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The comparison with an infinitely thick standard is straightforward if the two materials are of the same matrix; that is, the absorption coefficients are the same. This situation usually is not the case. Another drawback in using thick target standards for microbeams is that the powdered material generally is certified for a minimum sue quantity that is much greater than the interaction volume. Corrections for differences in matrix ,materials ~ and target thickness can he made using programs adapted from those used in electron microprobe work. Lu e t al. (16) adapted the NRLXRF program (17) for use with a white SR beam. Good results were ohtaindd in a comparison of concentrations determined using electron and X-ray probes to investigate samples of feldspars and silicate glass. Ideally, standards for trace element microprobe analysis should have uniformity on a micrometer scale. NIST recently issued two such standards (18)with a total of 13 elements sputtered from a glass matrix onto a polycarbonate film. These standards were applied to produce a calibration curve for a 100-pm2beam spot a t the X-26C line using the LBL Kirkpatrick-Baez optics. The uniformity of standards produced hy MicroMatter Go. was investigated by Tbemner et al. (19)using a 200-pm2 photon-induced X-ray emission beam spot. These foils were evaporated layers of elements or binary compounds on polycarhonate films a t loadings of 50 pg/cm2. A uniformity of *1-2% was found for a series of
12 elements. Gordon (20) produced a multielemental standard to simulate hiomedical thin tissues by depositing a mixed solution of five elements on a Millipore filter of 1.2-pm pore size. The loadings were 5 3 pg/cm2 in the filter of surface density 5 mg/cm2. For a 75-pmz beam spot, the uniformity was better than 2%, whereas a beam spot of 10 pm2 yielded a Mn/Cu ratio of better than 1%for seven measured points >1 cm2. In this latter run, the 10-keV monochromatic beam was below the ahsorption edge of the other elements present. The center portion of the filter was punched out and analyzed to determine the concentrations of the elements by an appropriate independent analytical technique such as neutron activation analysis or a standard procedure on an extracted solution of the filter.
Sensllvity and mlnlmm delectlon
limb Two parameters should be considered in the evaluation of the XRM. The sensitivity of the instrument in terms of the data acquisition rates (numher of counts/s/ppm/sample area) is important in determining bow the instrument can he applied. The related parameter, the MDL, is useful in the design of experiments. The XRM can he set up in different ways to optimize these factors for particular applications. The sensitivity for detection of a specific element is set by the photon flux of appropriate energy that can he deliv-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
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ered to the target and by the efficiency and solid angle of the X-ray detection system. Most experience to date with the XRM has been achieved with energy-dispersive Si X-ray detectors. In an arrangement used for the CXRM and FXRM instruments, the detector bad an area of 30 mm2 and was placed 2 4 cm from the beam spot for a solid angle of 0.019-0.075 81. Detection efficiency using this method is about 1W for energies above 3-4 keV. Given this information, the sensitivity can be readily evaluated. Consider the detection of iron at the 1-ppm level in l-pm3volume of material with a density of 1 g/nn3. The number of iron atoms in that volume is only 104. Using the white beam flux value of 3 x 10' photons/pm2/s, as discussed above, a numerical integration gives a sensitivity of 0.6 counts/s/ppm. To achieve rates above 1Hz and to make the measurement possible in a finite time, it will be necessary to increase the number of photons by focusing, increase the size of the beam spot, and increase the sample volume probed. The MDL is related to the sensitivity and to the magnitude of the background. For the synchrotron source, the backgrounds are dominated hy Compton or Rayleigh scattering of the beam and by imperfect operation of EDS detectors because of incomplete charge collection. Gordon and Jones (2) and Grodzins (21) presented extensive calculations of the sensitivities and MDLs that can be achieved with the XRM. MDL results using the NSLS with an 81 ellipsoidal focusing mirror calculated by Gordon and Jones were obtained for EDS and WDS detection systems using the approach in Figure 2a. Values around 10 ppb for transition elements can he obtained with a spatial resolution of 30 pm and a counting time of 60 8. A comparison of the MDL values obtained using CXRM and FXRM was made hy Jones et al. (9).A 20-pm thick section of gelatin that contained added trace amounts of several elements was used as the standard of comparison (see Figures 6 and 7). The same section was measured with both instruments, and the results were normalized to a common value of 10-pm resolution, storage ring current of 67 mA, and acquisition time of 100 s. Results obtained with the two instruments were comparable. For example, the MDL value obtained with the CXRM for iron was 9 fg; with the FXRM, the value was 3 fg. The CXRM could detect Sr at 28 fg; the FXRM could not because its energy was lower than the Sr absorption edge. The similarities in performance can be antici-
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pated, because the actual photon flux delivered to the sample is about the same with the two methods. The differences result from the differences in the spectral distributions of the two beams that are used. The measured values in either system can be compared with the calculations of MDL. If the MDL of 90ppb for iron calculated by Gordon and Jones (2) for a sample of 2 mg/cm2 thickness with an organic matrix is converted to the experimental conditions used for the CXRM and FXRM measurements, a value of -2 fg of iron is found. The measured and theoretical results are thus of the same magnitude.
Spatial resolullon ol the XRM The spatial resolution of the FXRM was measured by direct observation of the fluorescence produced by the beam when it struck a 10-pm gold wire. The width of the fluorescent spot was determined by calibrating the microscopy/ TV system by moving the sample stage a known distance. The spatial resolution of the FXRM was found to be 10'9 photons/cm2/s. The photon beam was a filtered white beam a t the Cornel1 CHESS facility and had an average energy of 15 keV. Measurements on a renal tissue section showed that it was more resistant to the radiation than was the blood sample. Differences in the rate of damage were also found for components of the blood sample. In the measurements described above, the photon flux was roughly 10'5 photons/cm2/s, and run times of 300 s were used for a fluence of 3 X 10'' photonslcm2. The mean energy of the beam was roughly 6 keV, which would imply less damage than that caused in the Slatkin experiments. It is therefore plausible to expect that there would be little effect from radiation damage for the measurements made to date. This is also consistent with the measurement of the time dependence of the yield from biological materials. The use of more effective focusing devices or more powerful synchrotron sources will presumably change this situation. A more detailed study of the changes in comphsition of biological and other materials bombarded by photon beams will be important for the
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future development of the field of X-ray microscopy.
Appllcatkns of the X-ray microprobe The synchrotron microprobe has been employed for experiments in a number of different fields using the point, line, and map imaging modes. This can be illustrated by examples chosen from several different scientific areas. Chevallier et al. (30) point out that the XRM is well suited to the study of extraterrestrial materials, sucb as micrometeorites from deep sea sediments, Greenland melt lakes, or possible c o m e b y debris collected from the stratosphere. The trace element compositions of these particles and partitioning between phases helps to determine the conditions that existed when the particle was formed. Analysis of these small particles by measuring the whole particle composition or by averaging several points is easily done with the XRM in the point imaging mode. Analysis of the particles cannot be done with the electron pmhe because of its lack of sensitivity a t the required ppm level. Use of other, more sensitive methods'sucb as SIMS is ruled out by the rarity of the material, which makes a nondestructive analvsis method imperative. There are many similar uroblems in terrestrial geochemistry.Rbck samples generally are heterogeneous, and a probe with great sensitivity and good resolution is required. The XRM fits these needs and is easier to calibrate for quantitative measurements than other methods. The nondestructive attribute will also be valuable in many e m s . Its use for the study of trace elements in coal has been reported by White et al. (31, 3% who measured the trace element contents of iron sulfides in English East Midland coals. The samples were grains of sulfides with thicknesses of 20-200 pm that were polished on both sides and mounted on trace-element-free silica disks. Data were obtained for 13 elements. One particularly interesting result was t he discovery of a strong correlation between the arsenic and selenium concentrations. The reasons for using the XRM in the biomedical area are similar to those for using the technique with geological materials: the need for high sensitivity and reasonable spatial resolution as well as minimal radiation damage effects compared with other methods. Trace elements play an important role in biological processes because they are essential for biological reactions, but they can also produce toxic effects. Bockman et al. (33) used the BNL XRM to study gallium deposition in rat bone after gallium nitrate was ad954A
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Flgure 0. Relative concentrations of gallium observed in scan across a thin section of rat tibia diaphysis and growth plate. Time p r point was 900 s (33.
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ministered to the animal. Gallium nitrate is a drug now used for treatment of excess bone calcium loss in some cancer patients. Gallium is effective in halting the loss, but the hiological proceases are not yet fully known. In this experiment the XRM was used to produce both line s~ and maps of the gallium distributions. Figure 8 shows a line scan across the diaphysis, and Figure 9 shows a map of the gallium distribution. Maximum concentrations occur in regions where new bone is being formed at the highest rates. Extensive measurements of the bone concentrations at different doses may make it possible to better understand the pharmacodynamics of the drug in humans. Several groups have reported measurements on tissue sections. Kwiatek et al. (34)and Pounds et al. (35)measured trace element concentrations in the brain and liver of rats and mice for normal animals and for ones that had ingested varying amounts of lead. The work was done with spatial resolutions that were generally about 50 x 50 Wm and had MDLs of 100 ppb or less. The results help to give some idea of baseline concentrations of trace elements in the normal animals and of the effects of lead on the normal trace element levels. Some preliminary hiological experiments have been carried out a t the Daresbury laboratory by Prins e t al. (36).One experiment measured cadmium concentrations in rat placenta and in rat liver. Researchers found that prolonged bombardment results in cadmium loss in the placenta tissue but not in the liver tissue. Differences in cadmium concentrations in various structures in the placenta were also found. Improved operation of the probe is expected when modifications to the Daresbury synchrotron are complete. Beeuwkes et al. (37)measured trace element concentrations in cat cardiac myocytes contained in a simple wet cell continuously fed by a nutrient solution. It was found that measurements could be made while maintaining the ceIl in vivo. The ewe with which specimens can be studied in a wet environment at atmospheric pressure is a singular advantage of the XRM for application to biomedical experiments. The high-intensity microbeams described here can also be used for computed microtomography experiments to produce maps of the linear attenuation coefficients or trace element concentrations of a material with high spatial resolution. A tomogram (38) of a freeze-dried caterpillar head is shown in Figure 10. The tomogram dimensions are 177 X 177 pixels of 30-pm spatial resolution. The density scale is
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la; head. Pixel size I8 30 pm 8qwe; the tomogram is a 177 X 177 m i x . The color scale is a thermal temperatwe scab, and white indicates highest density matorlai.
a thermal temperature scale. These examples show the usefulness of the XRM. They also illustrate how its use complements that of the electron microprobe, not only because of the higher sensitivity and reasonably good spatial resolution of the XRM, hut a h because of the nondestructive nature of the analysis.
txld" Synchrotron X-ray microscopy is an analytical instrument with unique capabilities that can be applied effectively to many different types of experiments. Today's first-generation instruments by no means represent the ultimate in XRM performance. Possible modifications in almost all aspects of the instrument should result in orders of magnitude improvements in sensitivity and resolution over the next five to 10 years. The hasis of the instrument, the synchrotron storage ring, will undergo marked improvements in this time period. The introduction of new types of high-energyundulators at the Stanford facility and, in the future, a t the Advanced Photon Facility a t Argonne National Laboratory as well as the European Synchrotron Facility in Grenoble, France, will improve the photon fluxa t the sample by a factor of 100 to lo00 because of the increased brilliance of the source. The X-ray optics used to focus the
incident beam should improve because of improved multilayers and better techniques for producing grazing incidence mirrors. It is hard to estimate the magnitude of change to he expected, but it could he a factor of 10 or more. Application of different imaging techniques using coded apertures and other methods will make it possible to use the photon beams more efficiently (39).Gains in data-taking rates of 10to 100 should he possible here also. In most cases the detectors used in the XRM have been relatively unsophisticated. More widespread application of image plates, charge-coupled devices, and other methods that give energy. and position-sensitive counters will have a large impact on the field. Many of these detectors now exist, or are being developed,for use in astronomy or plasma research. When these methods diffuse into the XRM research field, more sophisticated types of XRM will result. Conservatively,it will be possible to approach a goal of parts-per-million sensitivity for a volume element of 1 pms in the very near future. The scientific impact of the first-generation instruments will be substantial, and prospects for imaging experiments are very bright. The U.S. Department of Energy, Office of Baaic Energy Science, Division of Chemical Science, supportsresearch on the development of new mal y t i d Uchniques using synchmtmn radiation un-
ANALYTICAL CHEMISTRY, VOL. 61. NO. 5. MARCH 1, 1989
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der Contract No. DE-AC02-76CHW016. The National InatitUtes of Health, Diviaion of Research Reaourcss, aupports use of the x-Ray Micmscopy FaeiLty for biomedical eiperimenta under Grant No. P41RR01838. The National Sdence Foundation supports use of the X-Ray Mier-py Fdi@for measluementaongeolagiealpmb1emaund.r Grant No. EAR-8618346. The National Aemnautics and Space Adminiatretion supports use of the X-Ray Microscopy Facility for measurements on atratenestrial materials under Grant No. NAGQ.,M "
We thank our colleegues. k L. Hanwon. J B Haatinp, L.Grodrine. J. G. Pounds, M. L. Rivers, J. V. Smith, S.R. Sultan. A. C. Thomoion. R. 0. Gisuque, J. H. Underwoal, and P. Sp&, for many helpful dianuuiona and the use of unpublished data 88 examples for this article. Their contributions have been essential in developing the
X-rw microscopy project at the Brookhaven NSLS X-26 beam line. For infomation on t h e w of the NSLS laboratory. contact Susan White-DePace,NSIS, Building 725B. Brookhaven National Laboratory, Upton, NY 11973: (516) 282-7114. For -I information st the CHESS laboratory, contact Penny Ellison,CHESS. Wilson Laboratory,Cornell Universitv.Ithe- NY 14853:(Bo?)255-7163. Contact Katherine Cantwell, Uaer Research Administrstion, SSRL,P.O. Box 4349, Bin 69, Stanford. CA 94305; (416) 8543300,a t 2874, for idomation about the S K U .
(2) Gordon, B. M.; Jones, K. W. Nucl. Instrum. Methods 1985, BlOl11,293.
(3) Thom son, A. C.; Underwood, J. H.;
Wu,Y.;&auque,R. D.;Jones,K. W.;Riv-
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(6) Cornell High Energy Synchrotron Source Newsletter, March 1988. (7) Stephenson, G. B. Nucl. Instrum. Methods 198S.AZSS, 447. (8) Habenschuss. A.; Ice, G.E.;Sparks, C. J.; Neiser, R. A. Nucl. Instrum. Methods 1988, A266 215. (9) Jones. K. W.; Kwiatek, W. M.; Gordon, B. M.; Hanson, A L.;Pounds, J. G.; Rivers, M. L.; Sutton, S.R.; Thom son, A. C.; Underwood, J. H.; Giauque, E.D.;
Wu, Y. In Advances in X-ray Analysis: Barren, C. S.; Gilfrich, J. V.; Jenkins, R.; RUBS,J. C.; Richardson, Jr., J. W.; Predecki, P. K., Eds.; Planum Press: New York, 1988; VoL 31, p 5M8. (10) Giauque, R.D.; Ahompson, A C.; Wu, Y.; Jones, K.W.; Rivers, M.L. A d . Chem. 1988,60,855.
Gohahi, Y.; Aoki, S.; Iida, A,; Hayakawa, S.; Yamaji, H.; Sakurai, K. Aduances
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J. G. N. Proc. ZV. Intern. Symp. on the Medical Applications ojCyelotrons, Turku,Finland, in press.
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Britain has long been a prime source of innovative, high quality products. Today more than ever. When you are looking for profitable new sources of supply in Britain, or wish to find UK companies to represent in the US as an agent or distributor, make British Commercial Services in the USA your first source. We will immediately route your inquiry to British companies eager to supply the products or services you've asked about. We'll maintain records, to steer other UK companies your way in the future if you wish. You can have access to our complete library of trade directories. The cost to you? Nothing. It's all part of the service. Call or write to: British Commercial Services in the USA, British Consulate-General, 845 Third Avenue, New York, NY 10022. Tel: (212) 593-2258. Or call your nearest British Consulate, as listed below. Atlanta (404)5248823 Boston (617) 437-7lW Chicago(312)346-1810
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Visit the British exhibitors at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, at the Georgia World Congress Center, Atlanta, March 6-10,1969. Booths: 5103, 5104, 5106, 5107, 5109-12, 5115-2l, 5123, 5124
INSIRUMENIAIION (29) Slatkin, D. N.; Hanson, A. L.; Jones, K.W.;Kraner,H.W.;Warren,J.B.;Finkel, G. C. Nucl. Instrum. Methods 1984, 227,378. (30)Chevdier, P.; Jehanno, C.; Maurette, M.; Sutton, S. R.; Wang, J. J. Geophys. Res. 1987,92(B4), W 9 . (31) White, R.N.; Spears, D.A.; Smith, J. V. Abstracts of Pa ers, 1987 GSA Meeting, Phoenix. A 2 io. 122607. (32) White, R.N.; Spears, D.k; Smith, J. V.,unpublished results. (33) Bockman, R.; +PO, M.; Warrell, R.; Pounds, J. G.; Kwiatek, W. M.; Lon G. J.; Schidlowky,G.; Jones, K. W. In Ray Microscopy II; Sayre, D.; Howeb, M.; Kirz, J.; Rarback, H., Eds.;SpringerVerlag: Berlin,1988; ,391. (34) Kwiatek, W. M.; &ng, G. J.; R e d , K. R.; Hanson, A.L.; Jones, K. W.; Pounds, J. G. Toxicologist 1987, 7(1), 30%. (35) Pounds, J. G.; Long, G. J.; Kwiatek, W. M.; Jones, K.W.; Gordon, B. M.; Haneon, A. L. In X-Ray MicroscopyII; Sayre, D.; HoweUs, M.; Kirz, J.; Rarback, H., Eds.; Springer-Verlag: Berlin, 1988; p.
eluded kinetic studies of electron transfer reactions in aqueous solutions and nuclear reaction studies. His work in the field of proton-induced X ray fluorescence led to the application of synchrotron radiation to chemical analysis.
2;
A%
(36) Prim, M.; Dries, W.; Lenglet, W.; Da-
vies, S.T.;Bowen, K. Nucl. Instrum. Methods 1985.B10111.299. (37) Beeuwkes, hI, R., personal communication. (38) Spanne, P.; Rivers, M. L. Nuel. Instrum. Methods 1987,B24/25,1063. (39) Bavdas, M.; Knachel, A,; Ketelaen, P.; Petersen, W.; Gurker, N.; Salehi, M. H.; Dietrich, T. Nucl. Instrum. Methods 1988.A266,308.
Keith W . Jones (left)is aseniorphysicist and head of the Diuision of Atomic and Applied Physics in the Department of Applied Science at BNL. He receiued an AB. degree from Princeton Uniuersityand an M.S. degree and Ph.D. in nuclear physics from the Uniuersity of Wisconsin at Madison. His research work has been in nuclear, atomic, and applied physics using ion and photon beams. Barry M. Gordon (right) is a chemist in the Department of Applied Science at BNL. He receiued a B.S. degree from the University of California,Los Angeles, and a Ph.D. in physical chemistry from Washington Uniuersit y in St. Louis. His interests haue in-
CORRECTION NMR Imaging of Materials John M. Listerud, Steven W. Sinton, and Gary P. Drobny ( A w l . Chem. 1989,61,23 A 4 1 A) This article contains an error in the caption for Figure 6. In addition, equation l a b was inadvertently omitted. Corrected versions appear below: Figure 6. Selective sequences consisting of amplitude-modulated rf pulses and slice gradients. (a) Selective excitation scheme used to convert m, to my within a slice and (b) selective refocusing pulse that converts (m,,m,) to (mz,-my)within a slice.
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