Trace Element Determinations with SynchrotronInduced X-Ray Emission
Keith W. Jones and Barry M. Gordon Brookhaven National Laboratory Upton, 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 10-20 years represents a revolu tionary improvement in X-ray produc tion technology. By using SR for XRF analysis, researchers can make mea surements with improved sensitivity and spatial resolution. Results obtained with SR show that measurements with a spatial resolution of less than 100 μηι2 and a minimum detection limit (MDL) approaching 1 fg can be made in just 300 s. The XRF method traditionally has been used be cause of its multielement detection ca pability, minimal sample preparation, minimal damage to the sample, and ability to operate at atmospheric pres sure with solid, liquid, or gaseous sam ples. The substitution of the SR source preserves all these attributes and opens new opportunities for exploiting the XRF method. For example, a trace ele ment X-ray microscope can be used to 0003-2700/89/0361-341 A/$01.50/0 © 1989 American Chemical Society
give elemental maps at concentration levels below 1 ppm 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 diffi cult analytical problems. This article provides a survey of the basic princi ples of the SRIXE method and the
The SR source
The production of X-rays in electron or positron storage rings has been dis cussed extensively in the literature (2). A few of the most important concepts are summarized here for those unfamil iar with the technology. Electrons or positrons at energies of several GeV are confined in a path comprising circular and straight sec tions by a set of magnets. Radiation is emitted by the acceleration of the beam as it moves in a circular path in
INSTRUMENTATION ways in which it is being employed. Only three laboratories in the United States can make SRIXE measure ments using X-rays in the keV energy range: the National Synchrotron Light Source (NSLS) at Brookhaven Nation al Laboratory (BNL), the Cornell High-Energy Synchrotron Source (CHESS) at Cornell University, and the Stanford Synchrotron Radiation Laboratory (SSRL) at Stanford Uni versity. The NSLS X-26 beam lines are the only ones dedicated to SRIXE measurements in the United States.
the bending magnet sections. The char acteristic energy of the radiation (in keV), defined as the median of the power spectrum, is given by 2.218 E3/R, where Ε 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 be 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 1, 1989 · 341 A
INSTRUMENTATION 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 oscillatory deflections of the beam. Through interference effects, they create coherent radiation that is concentrated at 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 at a bending magnet and
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 at 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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Figure 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. The critical energy ec (median of the power spectrum) for each is indicated.
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INSTRUMENTATION vergence at the higher energies to be used for fluorescence excitation is 0.2 mrad. Types of XRF X-ray microscopes
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 beam) directly from the synchrotron. A mechanical aperture sets the size of the beam, and beam filters can be used to give a crude shaping to the energy spectrum. The flux incident on the sample can be 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, which produce excessive scattering. When high spatial resolution is needed, a second stage of demagnification can be 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 at any point cannot be 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 be 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 (photons/s-source area-solid angle) is best 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 sever-
Figure 2. Schematics showing different approaches used for a high-resolution X-ray microscope: (a) ellipsoidal mirror at NSLS (2), (b) LBL Kirkpatrick-Baez system using spherical multilayer surfaces and tested at NSLS (3), (c) Wolter focusing mirror system with combined ellipsoidal and hyperboloidal surfaces of revolution used at Photon Factory (4), and (d) bent crystal system used at Daresbury (5).
344 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
INSTRUMENTATION al systems is given in Table I. There seems to be merit in choosing the sim plest 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 repre sents the best opportunity for use by the outside research community. Until 1987 the X-26C beam line was a colli mated XRM (CXRM) that used an un focused "white" beam in which the ex
Table 1.
citation spectrum was hardened by Al absorbers. Beams as small as 20 μπι 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 direc tions and 25 mm in the focusing direc tion. The rotation staging is used to orient crystalline materials to elimi nate Bragg diffraction peaks. The
Typical X-ray fluxes produced using synchrotron radiation
CHESS A1 NSLS X-20C NSLS X-14
Bandwidth
10 7
8.3 1:1 4mr
6.0
HI
Optics
II!
Source
1.1%
Ref 6
10 7
7 5
4 mr
8.3 14.0
0.13% 0.076%
5 X 10 1 X 10 5
8
—
White
CO
4 X 10 7
9a
NSLS X-26
Kirkpatrick-Baez
10.0
10%
3 X 10 7
NSLS X-26
1.1 4mr
4-17
—
2 X 10 9
Wolter
10.0
NSLS X-26 CXRM
Photon Factory
stages have l-μπι step sizes. The sam ple is observed with a Nikon SMZ-10 binocular zoom microscope equipped with a high-resolution DAGE/MTI TV camera. The field of view on the TV monitor is 2-10 mm, and the optical resolution is about 10 μπι. Lawrence Berkeley Laboratory (LBL), in collaboration with BNL, per formed studies at the NSLS X-26C beam line using a Kirkpatrick-Baez optical system made at LBL and pro duced a focused XRM (FXRM). The system consisted of mutually perpen dicular 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 ΙΟ-μπι2 beam spot. The system was de signed for operation of a 10-keV energy with a 10% bandwidth. The photon fluence was measured to be 3 X 109 photons/s. A diagram of the mirror ar rangement is shown in Figure 2b.
«105
10
9" 11
a Value cited here is slightly larger than that given in Reference 9 and reflects a more detailed integration of the energy spectrum. 6 Extrapolated from values given in Reference 9.
Figure 3. Photograph of the collimated X-ray microprobe used on the NSLS X-26 beam line. 346 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989
Scanning techniques The XRM can collect data in a variety of modes. The simplest, and the one for which the calculated and experimen tally observed sensitivities are quoted above, analyzes a single small area on a target by collecting an entire fluores cence spectrum for a time period on the order of minutes. The spectra are ana lyzed as discussed below for peak areas and the concentrations determined, usually by comparison with appropri ate standards. A spectrum obtained from a sample of a National Institute of Science and Technology (NIST) Stan dard 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 pic ture 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 first 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 pro cessing 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 includ ing the fluorescence peak for the de sired element. At each pixel the com puter program determines the back-
Figure 4. T y p i c a l SR-induced X-ray f l u o r e s c e n c e s p e c t r u m produced by irradiation of NIST SRM 1 5 7 1 , O r c h a r d L e a v e s .
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ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 5, MARCH 1, 1989 · 347 A
INSTRUMENTATION ground-corrected peak areas for each ROI on line and stores only these val ues. The data are normalized using the integrated ion chamber current at each pixel. As many as 32 ROIs can be de fined for each spectrum. Because of the time required to move the stage and process the data, the minimum practi cal 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 elec tron image made with a scanning elec tron microscope and shows the major minerals pyrrhotite (Fei- x S), chalcopyrite (CuFeS2), and pentlandite (Fe4.5Ni4.5S8). 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 cor responds to a thermal temperature scale that for each element progresses from black (zero counts) through dark red, orange, and yellow to white (high est count of that element in the scan). The Pd image corresponds to the pent landite and may indicate pentlandite grains under the surface of the chalcopyrite. The use of white radiation and the higher Pd Κα 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 multi channel 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 chan nel advance signal to the MCS is de rived 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 sta tistics and not the time required for step-scanning the stage. However, ex tensive hardware is required to analyze a number of elements simultaneously.
Figure 5. Scan of a rock section of an ore deposit. Upper left image: SEM backscattered electron image of an ore deposit section (512X512 pixels of 25-μπι resolu tion); other images: 40 X 40 pixel scan of 30-μηι resolution and 2 s/pixel. The images represent the net counts per Κα fluorescence peak for Fe, Cu, Ni, Pd, and Se, respectively. The color scale is a thermal temperature scale.
Typical results from the CXRM and FXRM Illustrative spectra obtained with the X-26 CXRM and FXRM show the dif ferences produced by the use of white and monoenergetic exciting radiation. The spectrum obtained from a 20-ftm thick section of gelatin that contained several elements at a concentration lev el 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 making com parisons. The number of counts in the peaks representing the characteristic fluores-
Figure 6. X-ray fluorescence spectrum produced from prepared trace elements in a 20-μιη thick section of gelatin using the collimated X-ray microprobe (9).
348 A · ANALYTICAL CHEMISTRY, VOL. 6 1 , 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 spec tra from proton-induced X-ray emis sion (12) and a program by Kajfosz and Kwiatek (13) that uses a set of parabo las to fit the background. Petersen et al. (14) have described another pro gram, XSPEK, which is also designed for use with white beams. Calibration of the technique In principle, XRF analysis is an abso lute method in which concentrations are determined through knowledge of photoelectric and scattering cross sec tions and accurate determination of the experimental parameters. Using SR, Tolkiehn and Petersen (15) deter mined 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, Cz, of an element Ζ of atomic weight A in a target of thickness Τ cm can be expressed as KR C.«· 1
Lw° w* +
«
«
:
where Κ is a constant composed of Avogadro's number, the atomic weight of the analyte, the fluorescence produc tion cross section, and the detector effi ciency including geometry; and (μ/ρ)0>ζ are the mass absorption coefficients for the excitation and fluorescence ener gies in the matrix. The beam and detec tor each look at the surface of the target at 45° to preserve the beam-detector angle of 90°. The quantity R is the count rate of the characteristic X-ray of the analyte. I0 is the rate at which excitation photons impinge on the tar get and is proportional to the ioniza tion chamber count. The thin target approximation, in which one takes the linear term in the expansion of the exponential argu ment, yields Cz = 0.7
KR/I0(PT)
where ρ Τ is the target thickness in g/ cm2. The constant Κ is evaluated using an appropriate thin standard of the type discussed below. The thick target approximation as sumes that the exponential term is zero and results in the relationship C, = KR
//„ W°
\p)i
Figure 7. X-ray fluorescence spectrum produced from prepared trace elements in a 20-μηι thick section of gelatin using the focused X-ray microprobe. The peak at 10.5 keV is scattered from lead in the hutch into an unshielded detector (9).
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 size quantity that is much greater than the interac tion volume. Corrections for differences in matrix materials and target thickness can be made using programs adapted from those used in electron microprobe work. Lu et al. (16) adapted the NRLXRF program (17) for use with a white SR beam. Good results were ob tained in a comparison of concentra tions 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 stan dards (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-μπι2 beam spot at the X-26C line using the LBL Kirkpatrick-Baez optics. The uniformity of standards produced by MicroMatter Go. was investigated by Themner et al. (19) using a 200-μπι2 photon-induced X-ray emission beam spot. These foils were evaporated layers of elements or binary compounds on polycarbonate films at loadings of 50 μg/cm 2 . A unifor mity of ±1-2% was found for a series of
12 elements. Gordon (20) produced a multielemental standard to simulate biomedi cal thin tissues by depositing a mixed solution of five elements on a Millipore filter of 1.2-μηι pore size. The loadings were 1 cm2. In this latter run, the 10-keV mono chromatic beam was below the absorp tion edge of the other elements present. The center portion of the filter was punched out and analyzed to deter mine the concentrations of the ele ments by an appropriate independent analytical technique such as neutron activation analysis or a standard proce dure on an extracted solution of the filter. Sensitivity and minimum detection limits Two parameters should be considered in the evaluation of the XRM. The sen sitivity of the instrument in terms of the data acquisition rates (number of counts/s/ppm/sample area) is impor tant in determining how the instru ment can be applied. The related pa rameter, the MDL, is useful in the de sign of experiments. The XRM can be set up in different ways to optimize these factors for particular applica tions. The sensitivity for detection of a spe cific element is set by the photon flux of appropriate energy that can be deliv-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989 · 349 A
INSTRUMENTATION 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 ener gy-dispersive Si X-ray detectors. In an arrangement used for the CXRM and FXRM instruments, the detector had an area of 30 mm 2 and was placed 2-4 cm from the beam spot for a solid angle of 0.019-0.075 sr. Detection effi ciency using this method is about 100% for energies above 3-4 keV. Given this information, the sensitiv ity can be readily evaluated. Consider the detection of iron at the 1-ppm level in l-μπι 3 volume of material with a den sity of 1 g/cm3. The number of iron atoms in that volume is only 104. Using the white beam flux value of 3 X 107 photons/^rnVs, as discussed above, a numerical integration gives a sensitiv ity of « 0.6 counts/s/ppm. To achieve rates above 1 Hz and to make the mea surement possible in a finite time, it will be necessary to increase the num ber 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 back ground. For the synchrotron source, the backgrounds are dominated by 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 re sults using the NSLS with an 8:1 ellip soidal 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 be obtained with a spatial resolution of 30 μπι and a counting time of 60 s. A comparison of the MDL values ob tained using CXRM and FXRM was made by Jones et al. (9). A 20-μπι 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-μιη resolution, storage ring current of 67 mA, and ac quisition time of 100 s. Results obtained with the two in struments were comparable. For exam ple, 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 simi larities in performance can be antici
pated, because the actual photon flux delivered to the sample is about the same with the two methods. The differ ences result from the differences in the spectral distributions of the two beams that are used. The measured values in either sys tem can be compared with the calcula tions of MDL. If the MDL of 90 ppb for iron calculated by Gordon and Jones (2) for a sample of 2 mg/cm 2 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 resolution of 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-μπι gold wire. The width of the fluorescent spot was deter mined by calibrating the microscopy/ TV system by moving the sample stage a known distance. The spatial resolu tion of the FXRM was found to be 1019 photons/cm 2 /s. The photon beam was a fil tered white beam at the Cornell CHESS facility and had an average en ergy of 15 keV. Measurements on a re nal 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 1015 photons/cm 2 /s, and run times of 300 s were used for a fluence of 3 Χ 1017 pho tons/cm 2 . 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 measure ment 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 composition of biological and other materials bombarded by photon beams will be important for the
INSTRUMENTATION future development of the field of X-ray microscopy. Applications 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, such as micrometeorites from deep sea sedi ments, Greenland melt lakes, or possi ble cometary debris collected from the stratosphere. The trace element com positions of these particles and parti tioning between phases helps to deter mine the conditions that existed when the particle was formed. Analysis of these small particles by measuring the whole particle composition or by aver aging several points is easily done with the XRM in the point imaging mode. Analysis of the particles cannot be done with the electron probe because of its lack of sensitivity at the required ppm level. Use of other, more sensitive methods such as SIMS is ruled out by the rarity of the material, which makes a nondestructive analysis method im perative. There are many similar problems in terrestrial geochemistry. Rock 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 cases. Its use for the study of trace elements in coal has been reported by White et al. (31, 32), who measured the trace ele ment contents of iron sulfides in En glish East Midland coals. The samples were grains of sulfides with thicknesses of 20-200 μπι that were polished on both sides and mounted on trace-ele ment-free silica disks. Data were ob tained for 13 elements. One particular ly interesting result was the discovery of a strong correlation between the ar senic 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 ef fects 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 ad-
Figure 8. Relative concentrations of gallium observed in scan across a thin section of rat tibia diaphysis and growth plate. Time per point was 300 s (33).
Figure 9. Contour map of the gallium counts per 2-s time interval in rat tibia after ad ministration of gallium nitrate. Scan is 70 X 40 pixels of 50 X 50 Mm each. The color scale for the contour lines is the thermal tempera ture scale discussed in text (33).
354 A · ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 5, MARCH 1, 1989
ministered to the animal. Gallium ni trate 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 biological pro cesses are not yet fully known. In this experiment the XRM was used to pro duce both line scans and maps of the gallium distributions. Figure 8 shows a line scan across the diaphysis, and Fig ure 9 shows a map of the gallium distri bution. Maximum concentrations oc cur in regions where new bone is being formed at the highest rates. Extensive measurements of the bone concentra tions at different doses may make it possible to better understand the phar macodynamics of the drug in humans. Several groups have reported mea surements on tissue sections. Kwiatek et al. (34) and Pounds et al. (35) mea sured 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 μπι and had MDLs of 100 ppb or less. The results help to give some idea of base line concentrations of trace elements in the normal animals and of the effects of lead on the normal trace element levels. Some preliminary biological experi ments have been carried out at the Daresbury laboratory by Prins et al. (36). One experiment measured cadmi um 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 com plete. Beeuwkes et al. (37) measured trace element concentrations in cat cardiac myocytes contained in a simple wet cell continuously fed by a nutrient solu tion. It was found that measurements could be made while maintaining the cell in vivo. The ease with which speci mens can be studied in a wet environ ment at atmospheric pressure is a sin gular advantage of the XRM for appli cation to biomedical experiments. The high-intensity microbeams de scribed here can also be used for com puted microtomography experiments to produce maps of the linear attenua tion coefficients or trace element con centrations of a material with high spa tial resolution. A tomogram (38) of a freeze-dried caterpillar head is shown in Figure 10. The tomogram dimen sions are 177 X 177 pixels of 30-μΐη spatial resolution. The density scale is
Figure 10. Computed microtomogram of a lateral section of a freeze-dried caterpil lar head. Pixel size is 30 μπ\ square; the tomogram is a 177 X 177 matrix. The color scale is a thermal tempera ture scale, and white indicates highest density material.
a thermal temperature scale. These examples show the usefulness of the XRM. They also illustrate how its use complements that of the elec tron microprobe, not only because of the higher sensitivity and reasonably good spatial resolution of the XRM, but also because of the nondestructive nature of the analysis. Conclusions Synchrotron X-ray microscopy is an analytical instrument with unique ca pabilities that can be applied effective ly to many different types of experi ments. Today's first-generation instru ments by no means represent the ultimate in XRM performance. Possi ble modifications in almost all aspects of the instrument should result in or ders of magnitude improvements in sensitivity and resolution over the next five to 10 years. The basis of the instrument, the syn chrotron storage ring, will undergo marked improvements in this time pe riod. The introduction of new types of high-energy undulators at the Stanford facility and, in the future, at the Ad vanced Photon Facility at Argonne Na tional Laboratory as well as the Euro pean Synchrotron Facility in Grenoble, France, will improve the photon flux at the sample by a factor of 100 to 1000 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 inci dence mirrors. It is hard to estimate the magnitude of change to be expected, but it could be 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 10 to 100 should be possible here also. In most cases the detectors used in the XRM have been relatively unso phisticated. More widespread applica tion 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 astrono my or plasma research. When these methods diffuse into the XRM re search 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 μπι3 in the very near future. The sci entific 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 Basic Energy Sciences, Division of Chemical Sciences, supports research on the development of new ana lytical techniques using synchrotron radiation un-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 5, MARCH 1, 1989 · 355 A
INSTRUMENTATION der Contract No. DE-AC02-76CH00016. The Na tional Institutes of Health, Division of Research Resources, supports use of the X-Ray Microscopy Facility for biomedical experiments under Grant No. P41RR01838. The National Science Founda tion supports use of the X-Ray Microscopy Facili ty for measurements on geological problems under Grant No. EAR-8618346. The National Aeronau tics and Space Administration supports use of the X-Ray Microscopy Facility for measurements on extraterrestrial materials under Grant No. NAG9-106. We thank our colleagues, A. L. Hanson, J. B. Hastings, L. Grodzins, J. G. Pounds, M. L. Rivers, J. V. Smith, S. R. Sutton, A. C. Thompson, R. D. Giauque, J. H. Underwood, and P. Spanne, for many helpful discussions and the use of unpub lished data as examples for this article. Their con tributions have been essential in developing the X-ray microscopy project at the Brookhaven NSLS X-26 beam line. For information on the use of the NSLS labora tory, contact Susan White-DePace, NSLS, Build ing 725B, Brookhaven National Laboratory, Up ton, NY 11973; (516) 282-7114. For user informa tion at the CHESS laboratory, contact Penny Ellison, CHESS, Wilson Laboratory, Cornell Uni versity, Ithaca, NY 14853; (607) 255-7163. Contact Katherine Cantwell, User Research Administra tion, SSRL, P.O. Box 4349, Bin 69, Stanford, CA 94305; (415) 854-3300, ext. 2874, for information about the SRRL.
References (1) Winick, H.; Doniach, S., Eds.; Synchro tron Radiation Research; Plenum Press: New York, 1980.
(2) Gordon, B. M.; Jones, K. W. Nucl. Instrum. Methods 1985, Β10/Π, 293. (3) Thompson, A. C ; Underwood, J. H.; Wu, Y.; Giauque, R. D.; Jones, K. W.; Riv ers, M. L. Nucl. Instrum. Methods 1988, A266, 318. (4) Gohshi, Y.; Aoki, S.; Iida, Α.; Hayakawa, S.; Yamaji, H.; Sakurai, K. Jpn. J. Appl. Phys. 1987,26, L1260. (5) Van Langevelde, F.; Lenglet, W.J.M.; Overwater, R.M.W.; Vis, R. D.; Hvizing, Α.; Viegers, M.P.A.; Zegers, C.P.G.M.; van de Heide, J. A. Nucl. Instrum. Methods 1987, A257,436. (6) Cornell High Energy Synchrotron Source Newsletter, March 1988. (7) Stephenson, G. B. Nucl. Instrum. Methods 1988, A266, 447. (8) Habenschuss, Α.; Ice, G. E.; Sparks, C. J.; Neiser, R. A. Nucl. Instrum. Meth ods 1988, A266, 215. (9) Jones, K. W.; Kwiatek, W. M.; Gordon, B. M.; Hanson, A. L.; Pounds, J. G ; Riv ers, M. L.; Sutton, S. R.; Thompson, A. C.; Underwood, J. H.; Giauque, R. D.; Wu, Y. In Advances in X-ray Analysis; Barrett, C. S.; Gilfrich, J. V.; Jenkins, R.; Russ, J. C ; Richardson, Jr., J. W.; Predecki, P. K., Eds.; Plenum Press: New York, 1988; Vol. 31, pp. 59-68. (10) Giauque, R. D.; Thompson, A. C ; Wu, Y.; Jones, K. W.; Rivers, M. L. Anal. Chem. 1988,60,855. (11) Gohshi, Y.; Aoki, S.; Iida, Α.; Hayaka wa, S.; Yamaji, H.; Sakurai, K. Advances in X-Ray Analysis 1988,31,495. (12) Johansson, G. I. X-ray Spectrom. 1982 11 1984 (13) Kajfo'sz, J.; Kwiatek, W. M. Nucl. In
strum. Methods 1987, B22, 78. (14) Petersen, W.; Ketelsen, P.; Knôchel, A. Nucl. Instrum. Methods 1986, A245,535. (15) Tolkiehn, G ; Petersen, W. Nucl. Instrum. Methods 1983,215, 515. (16) Lu, F.; Smith, J. V.; Sutton, S. R ; Rivers, M. L. Chem. Geol., in press. (17) Criss, J. Technical Report No. DOD00065,1977; Naval Research Laboratory. (18) Pella, P. Α.; Newbury, D. E.; Steel, E. B.; Blackburn, D. H. Anal. Chem. 1986,58,1133. (19) Themner, K.; Lôvestam, N.E.G.; Tapper, U.A.S.; Malmqvist, K. G. Nucl. Instrum. Methods 1987, B22,126. (20) Gordon, Β. Μ., Brookhaven National Laboratory, unpublished results. (21) Grodzins, L. NeuroToxicology 1982,4, 23. (22) Sparks, Jr., C. S. In Synchrotron Radi ation Research; Winick, H.; Doniach, S., Eds.; Plenum Press: New York, 1980; Chapter 14. (23) Wall, J. S.; Hainfeld, J. F. Ann. Rev. Biophys. Chem. 1986,15, 355. (24) Legge, G.J.F.; Mazzolini, A. P. Nucl. Instrum. Methods 1980,168,563. (25) Mazzolini, A. P.; Legge, G.J.F.; Pallaghy, C. K. Nucl. Instrum. Methods 1981,191, 583. (26) Sealock, R. M.; Mazzolini, A. P.; Legge, G.J.F. Nucl. Instrum. Methods 1983,218,217. (27) Vis, R. D. Scanning Microscopy 1988, 2,977. (28) Vis, R. D.; Lenglet, W. J. M.; DeMol, J. G. N. Proc. IV. Intern. Symp. on the Medical Applications of Cyclotrons, Tur ku, Finland, in press.
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INSTRUMENTATION (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) Chevallier, P.; Jehanno, C; Maurette, M.; Sutton, S. R.; Wang, J. J. Geophys. Res. 1987,92(B4),E649. (31) White, R. N.; Spears, D. Α.; Smith, J. V. Abstracts of Papers, 1987 GSA Meeting, Phoenix, AZ; No. 122607. (32) White, R. N.; Spears, D. Α.; Smith, J. V., unpublished results. (33) Bockman, R.; Repo, M.; Warrell, R.; Pounds, J. G.; Kwiatek, W. M.; Long, G. J.; Schidlovsky, G; Jones, K. W. In XRay Microscopy II; Sayre, D.; Howells, M.; Kirz, J.; Rarback, H., Eds.; SpringerVerlag: Berlin, 1988; p. 391. (34) Kwiatek, W. M.; Long, G. J.; Reuhl, K. R.; Hanson, A. L.; Jones, K. W.; Pounds, J. G. Toxicologist 1987, 7(1), 302a. (35) Pounds, J. G.; Long, G. J.; Kwiatek, W. M.; Jones, K. W.; Gordon, Β. Μ.; Han son, A. L. In X-Ray Microscopy II; Sayre, D.; Howells, M.; Kirz, J.; Rarback, H., Eds.; Springer-Verlag: Berlin, 1988; p. 425. (36) Prins, M.; Dries, W.; Lenglet, W.; Davies, S. T.; Bowen, K. Nucl. lustrum. Methods 1985, BIO 111, 299. (37) Beeuwkes, III, R., personal communi cation. (38) Spanne, P.; Rivers, M. L. Nucl. lu strum. Methods 1987, B24/25,1063. (39) Bavdas, M.; Knochel, Α.; Ketelsen, P.; Petersen, W.; Gurker, N.; Salehi, M. H.; Dietrich, T. Nucl. lustrum. Methods 1988, A266, 308.
See us at PITTCON Booth 4546
cluded kinetic studies of electron transfer reactions in aqueous solu tions and nuclear reaction studies. His work in the field of proton-induced Xray fluorescence led to the application of synchrotron radiation to chemical analysis. CORRECTION NMR Imaging of Materials John M. Listerud, Steven W. Sinton, and Gary P. Drobny (Anal. Chem. 1989,6i,23A-41A) This article contains an error in the caption for Figure 6. In addi tion, equation 18b was inadver tently omitted. Corrected versions appear below:
Keith W. Jones (left) is a senior physi cist and head of the Division of Atomic and Applied Physics in the Depart ment of Applied Science at BNL. He received an A.B. degree from Prince ton University and an M.S. degree and Ph.D. in nuclear physics from the Uni versity of Wisconsin at Madison. His research work has been in nuclear, atomic, and applied physics using ion and photon beams.
Figure 6. Selective sequences con sisting of amplitude-modulated rf pulses and slice gradients, (a) Selective excitation scheme used to convert mz to my within a slice and (b) selective refocusing pulse that converts (mx,my) to (mx,—my) within a slice.
Barry M. Gordon (right) is a chemist in the Department of Applied Science at BNL. He received a B.S. degree from the University of California, Los Angeles, and a Ph.D. in physical chemistry from Washington Universi ty in St. Louis. His interests have in
^D _ „-(1/8)·(σ χ /ΔΧ) 2
J* = e
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(18b)
