Abstract:
This present invention provides a sample changer for automatically changing from one sample to the next in support of high throughput X-ray powder diffraction data acquisition in a capillary geometry. The samples are mounted on the outer rim of a disk or turntable, with each sample having its capillary axis parallel to a radius emanating from the center of the disk on which the samples are mounted. Each sample is mounted on a separate motor shaft which permits the sample to be rotated about the longitudinal axis of the capillary during data acquisition. To change from one sample to the next, the disk or turntable is rotated about its axis thereby presenting in turn each new sample to a X-ray beam. When the data acquisition is completed for that sample, a new sample is rotated into the beam and its data collection is carried out.

Description:
TECHINCAL FIELD 
     The present invention relates generally to sample changers for X-ray diffractometers, and in particular to an automatic sample changer for use with capillary geometry powder-diffraction systems that permits high-throughput acquisition of X-ray powder patterns. 
     BACKGROUND OF THE INVENTION 
     Powder X-ray diffraction techniques are used to measure the structural properties of a wide variety of materials. For a general review of X-ray diffraction, see B. E. Warren, X-ray Diffraction, Dover, 1990. Typically, a beam of X-rays is passed through a sample of randomly-oriented microcrystals to produce a pattern of rings on a distant screen. The pattern correlates with the structure of the molecules comprising the microcrystals. 
     Powder diffraction may be used for the identification of the structural phases in a sample which may contain multiple structural representations of a material. Powder diffraction may also be used as a means of verifying that a powder sample contains a material whose structure has been determined by a single crystal X-ray diffraction measurement. Typically, a powder X-ray diffraction pattern is calculated for a known material, after which a new measured powder pattern is compared to the known material&#39;s pattern which is then used to verify that the sample measured is representative of the expected compound. 
     The development of synchrotron radiation sources has lead to advancements in powder X-ray diffraction techniques. See, e.g., Synchrotron Radiation Research, H. Winick and S. Doniach, Eds., Plenum, N.Y., (1979); Synchrotron X-ray Powder Diffraction, J. B. Hastings, W. Thomlinson, and D. E. Cox, Journal of Applied Crystallography, 17, 85(1984); and Powder Diffraction, D. E. Cox in G. S. Brown and D. E. Moncton, Eds., Handbook on Synchrotron Radiation, Elsevier, 3, 155 (1991). A synchrotron is a cyclic particle (electron, proton, heavy-ion) accelerator in which a particle is confined to its orbit by a magnetic field that gradually strengthens as the particle&#39;s momentum increases. Acceleration is produced by an alternating electric field that is in synchronism with orbital frequency. A consequence of the acceleration is the emission of synchrotron radiation. This radiation is a broad-spectrum (white) emission that resembles the white radiation of an X-ray tube but is immensely more intense (100 to 10 4  times the intensity of a conventional characteristic line). See, e.g., X-ray Structure Determination—A Practical Guide, pp 14-15, Stout and Jensen, 2 nd  edition, Wiley Interscience (1989). 
     There are currently several powder X-ray diffraction instruments at synchrotron radiation sources around the world where powder X-ray diffraction is practiced. The most favored geometry for carrying out powder X-ray diffraction at a synchrotron radiation source is high resolution geometry. Typically, a sample is mounted in either a flat plate or capillary geometry and X-ray beams are directed at the sample. X-rays diffracted by the sample are analyzed by an analyzer crystal before being counted by an X-ray detector. Such geometries, where the diffracted beams are analyzed by diffraction, utilize standard commercial powder X-ray diffraction instruments. 
     The analyzer crystal is typically referred to as a “diffracted beam monochromator” and is typically of a mosaic material such as graphite or lithium fluoride. Owing to the properties of synchrotron radiation, the analyzer crystal of choice for the synchrotron radiation powder diffraction method is a perfect crystal of either silicon or germanium. Such perfect crystal analyzers are typically not used for traditional tube X-ray sources, because the signal rates are too low, (typically being several orders of magnitude lower than those for a synchrotron radiation experiment). The data collection times for traditional tube X-ray sources are also prohibitively long. 
     High resolution powder X-ray diffraction measurement at a synchrotron radiation source provides numerous advantages over traditional measurement systems using X-ray tubes. In particular, the energy of the X-rays can be freely chosen with a synchrotron radiation source; the angular resolution of this method is far superior to the X-ray tube based technique; it is possible to more accurately determine the positions of the measured diffraction peaks; one can better discriminate against fluorescence from those samples that fluoresce since only a narrow band of energies around the elastically scattered X-rays from the sample are counted, thereby reducing background interference; and background interference from air scatter in the neighborhood of the sample is also reduced. The combination of the above advantages has made powder X-ray diffraction techniques at synchrotron radiation sources a favored technique for complex materials analysis problems. In some cases powder X-ray diffraction data has made it possible to solve unknown structures of materials directly from powder diffraction data. See, e.g., F. Favier, et al., Inorganic Chemistry, 37, 1776-1780 (1998). 
     As mentioned above, when collecting X-ray powder diffraction data, it is possible to mount the samples in either a flat plate geometry or a capillary geometry. Both geometries have advantages and disadvantages, and it does not hold true that one geometry can be used for all samples. However, the availability of synchrotron radiation sources and the potential for using X-ray energies higher than the traditional molybdenum K-alpha radiation available from X-ray tubes has made the capillary geometry a favored geometry at synchrotron radiation sources for many analysis problems. 
     In a capillary geometry system, the sample is mounted in a capillary of, for example, 1 millimeter in diameter, and the energy of the X-rays is adjusted to optimally penetrate the thickness of the sample with the proper amount of absorption. In either the flat plate or capillary geometry systems, it is often necessary to rotate or rock the sample during data acquisitior. This is because the samples often do not contain an appropriate statistical representation of all possible orientations, due, for example, to preferred orientation effects, and therefore oscillating them during data acquisition can help to alleviate this problem. 
     A recent development in the data collection capabilities at synchrotron radiation sources is the use of multi-element detectors to measure powder patterns. See, e.g., J. L. Hodeau, et al., SPIE Proceedings, 3448, 353-361 (1998). In this high-resolution diffraction experiment performed with analyzer crystals, nine analyzer crystals and nine detectors have been mounted in parallel to detect the diffracted X-rays. Such multi-element detectors speed up the data acquisition process considerably, making it possible to rapidly analyze a sample. To further speed up the analysis of samples, it would be desirable to have a sample changer which automatically changes multiple samples mounted in a capillary geometry, while rotating the samples during data acquisition. 
     U.S. Pat. No. 4,770,593 to Anderson (“Anderson”) discloses a changer for a flat plate geometry utilizing a conventional X-ray tube. U.S. Pat. No. 4,641,329 to Green, et al. (“Green et al.”) discloses a holder for a capillary geometry sample for a commercial powder X-ray diffraction instrument. These two patents address conventional X-ray diffraction with an X-ray tube source. Anderson-teaches a means for changing samples in an unattended manner in a flat plate geometry, while Green et al. teach the use of a conventional diffractometer with a capillary geometry. Neither patent teaches, nor suggests, an efficient high volume capillary geometry sample changer for use with a synchrotron X-ray source. 
     Creagh et al. teach a sample changer with multiple independently rotating capillary tubes for use with a synchrotron radiation powder X-ray diffractometer. See, e.g., D. C. Creagh et al., Journal of Synchrotron Radiation, 5, 823-825 (1998). Creagh et al. do not, however, teach a device for holding a vast plurality of samples for extended analysis periods. In fact, the very nature of the device taught by Creagh et al. limits the amount of samples that one can position on the sample changer before inaccuracies and alignment problems come into play. A sample changer which more effectively mounts, changes, rotates, and collects data from a plurality of samples using high resolution geometry at a synchrotron radiation source would therefore be highly desirable. 
     SUMMARY OF THE INVENTION 
     This present invention provides a sample changer for automatically changing from one sample to the next in support of high throughput X-ray powder diffraction data acquisition in a capillary geometry. The sample changer of the invention can be used with either a conventional X-ray source or a synchrotron radiation X-ray source, though the latter is preferred for high resolution measurements. During data acquisition, the samples are each rotated about the longitudinal axis of the capillary, which is aligned along the axis of rotation for the scattering angle (typically called two-theta). In a preferred embodiment, the samples are mounted on the outer rim of a disk or turntable, with each sample having its capillary axis parallel to a radius emanating from the center of the disk on which the samples are mounted. Each sample is mounted on a separate motor shaft which permits the sample to be rotated about the longitudinal axis of the capillary during data acquisition. To change from one sample to the next, the disk or turntable is rotated about its axis thereby presenting in turn each new sample to the beam. The sample changer may include switching means which begin the sample rotating about the longitudinal axis of the capillary just prior to, or concurrent with, the sample being exposed to the X-ray beam. When the data acquisition is completed for that sample, a new sample is rotated into the beam and its data collection is carried out. 
     In another embodiment of the invention, samples are mounted on the edge of a rectangular platform. The axis of each sample is perpendicular to the edge of the platform and parallel to the other samples. Samples are changed by translating the platform linearly parallel to the edge on which the samples are mounted, thereby presenting in turn each new sample to the beam. Like the previously described embodiment, the samples may be continually rotated about the longitudinal axis of the capillary in which they are housed, or they may be activated to rotate just prior to, or concurrent with, exposure to the beam. When the data acquisition is completed for that sample a new sample is translated into the beam and its data collection is carried out. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     For a better understanding of the invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic illustration of a prior art capillary geometry X-ray diffraction technique; 
     FIG. 2 is a diagrammatic illustration of a prior art sample changer for a capillary geometry X-ray diffractometer; 
     FIG. 3A is a diagrammatic illustration of one embodiment of a sample changer according to the invention; 
     FIG. 3B is a diagrammatic illustration of another embodiment of the invention; 
     FIG. 4A is an orthographic view of the embodiment shown in FIG. 3A; 
     FIG. 4B is a front view of another embodiment of a sample changer according to the invention; 
     FIG. 5 is a cross sectional view through the center of a rotation device according to the invention, excluding shading lines for clarity; 
     FIG. 6 is a cross sectional view of the embodiment shown in FIG. 4A, taken along line  6 ′ 6  of FIG. 4A, excluding shading lines for clarity; 
     FIG. 7 is an orthographic view of a sample changer system according to the invention; 
     FIG. 8 is a diagrammatic illustration of another embodiment of a sample changer according to the invention; and 
     FIG. 9 is a diagrammatic illustration of yet another embodiment of a sample changer according to the invention. 
    
    
     Like reference numerals refer to corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagrammatic illustration of a capillary geometry X-ray diffraction technique  100  known in the art. Typically an X-ray beam  102  is directed at a capillary tube  104  containing a sample to be analyzed. During data aquisition, capillary tube  104  is rotated about its longitudinal axis (rotation illustrated  108 ) so that a statistical average of the sample can be diffracted and analyzed. The X-ray beam  102  diffracts when encountering the sample, forming a diffracted beam  110 . The angle  106  formed between the diffracted beam and the directed beam  102  is typically given the designation 2θ. 
     FIG. 2 is a diagrammatic illustration of a sample changer  200  for a capillary geometry X-ray diffractometer known in the art (sample changer  200  is the same as that disclosed in Creagh et al., supra). A limited number of capillary tubes  104  are located at various positions on a disk  202 . The capillary tubes  104  are mounted on the disk  202  such that their longitudinal axes are perpendicular to the surface of the disk. Both the disk  202  and each of the capillary tubes  104  are rotatable as indicated by the arrows  208  and  108 , respectively. This device mounts samples onto large goniometer heads (not shown) which permits independent adjustment of their orientations prior to data collection. The rotational axes of the capillary tubes  104  lie parallel to the rotational axis of the disk  202 . This arrangement limits the number of samples that can be simultaneously mounted on the disk  202 , because if the samples  104  are mounted too close to one another about the periphery of the disk  202 , X-ray beam  102  will simultaneously irradiate more than a single sample  104 . 
     For ease of explanation, the following convention will be used to describe the motions of the components. A positive Z direction is along an X-ray beam from the X-ray source. A positive Y direction is upwards in the vertical direction. The positive X direction then defines the right handed coordinate system. Therefore, an observer standing with his back to the radiation source has the positive Z axis in front, the positive X axis to his left and the positive Y axis up. The rotation convention is that a rotation about the X direction (pitch) is called theta (θ), a rotation about the Y direction (yaw) is called phi (φ), and a rotation about the Z direction (roll) is called chi (χ). These axes are illustrated in FIG.  2 . 
     As illustrated in FIG. 2, if we take the origin of the coordinate system to be the sample, then the X axis runs through the longitudinal axis of the capillary of the sample and the X-ray detector&#39;s scattering angle, called two theta, rotates about the X axis. The traditional theta angle of the sample rotates about the X axis, and the samples spin about this axis. 
     FIG. 3A is a diagrammatic illustration of one embodiment of a sample changer  300  according to the invention. In this embodiment, a plurality of capillary tubes  304  are disposed about the circumference of a turntable  302 . Each capillary  304  contains a sample for analysis (not shown). The longitudinal axis of each capillary extends along a radial which extends from, and lies substantially perpendicular to, a central axis  310  that is perpendicular to the center of turntable  302 . Preferably, the plurality of capillaries  304  are disposed equally about turntable  302  (i.e., the angles between the longitudinal axes of all neighboring capillaries are the same). The turntable  302  is rotatable  308  about the central axis  310  (Z axis). The capillary tubes  304  are disposed within rotation devices (not shown, discussed in more detail below), such that they are rotatable  306  about their longitudinal axes. In use, an X-ray beam  312  remains in a single position and the turntable  302  is rotated  308  to sequentially expose each capillary tube  304  to the X-ray beam  312 . Diffracted beams  314  are then analyzed for each sample. 
     FIG. 3B is a diagrammatic illustration of another embodiment  350  of the invention. Capillary tubes  304  are positioned along the edge of a platform  352  perpendicular to central axis  358  (Y axis). The platform can be translated along the central axis  358  as indicated by the arrows  364 . As in the previous embodiment each capillary tube  304  is rotatable  306  about its longitudinal axis. To sequentially expose each capillary tube  304  to an X-ray beam  312 , the platform  352  is translated along the central axis  358 . Platform  352  is optionally translatable along the X axisor rotatable about central axis  358 . 
     FIG. 4A is an orthographic view of the embodiment  300  shown in FIG.  3 A. Both a front view  402  and a side view  404  of the turntable  302  are shown. Capillary tubes  304  are disposed about the periphery of turntable  302 , which is rotatable about central axis  310  (Z axis). 
     FIG. 4B is a front view of another embodiment  406  of a sample changer according to the invention. In this embodiment, turntable  302  forms a ring with radial spokes  408  emanating from the center of the ring to allow for the rotation of the turntable  302  about the central axis. Capillary tubes  304  extend both outwards from the outer perimeter of the ring and inwards towards the central axis from the inner perimeter of the ring. After analyzing either the outer or inner ring of capillary tubes  304 , either the turntable  302  or the X-ray source (not shown) must be translated in line with the next ring of capillary tubes  304  to be analyzed. In this way more capillary tubes  304  may be positioned on a single turntable  302 . 
     FIG. 5 is a cross sectional view through the center of a rotation device  500  according to the invention, excluding shading lines for clarity. The rotation device is used to rotate a capillary tube  304  about its longitudinal axis  514 . The capillary tube  304  contains a sample  504  which is to be analyzed. The capillary tube  304  is preferably a commercially available capillary tube, such as those supplied by the Charles Supper Company, Natick Mass., U.S.A. The capillary tubes  304  may be as small as 100 micrometers to as much as 5 millimeters in outer diameter. The wall (outer diameter less the inner diameter, of the capillary tube) thickness is preferably 10 micrometers. The capillary tube  304  is preferably made from a X-ray transparent material. The capillary tube  304  is connected to a coupler  506  at a first connector  516 . The coupler is preferably cylindrical. First connector  516  is preferably a bore located in the coupler  506  along a rotational axis  514  which coincides with the longitudinal axis of the capillary tube  304 . The bore may be a hole drilled in one end of the coupler  506  to receive the capillary tube  304 . The hole is drilled precisely along the rotational axis  514  and is sized to match the diameter of the capillary used, whether 0.5 millimeter, 1.0 millimeter, etc., with a snug fit. The capillary tube  304  is securely held in the bore by means of a suitable tacky or adhesive material such as glue, wax, or other deformable or compressible material foam, rubber, etc. The capillary tube  304  may also be held in said bore by any other means, such as by a magnetic force, etc. One skilled in the art will appreciate that any suitable means for connecting the coupler to a capillary tube may be used. 
     A second connector  518  on the coupler  506  connects the coupler  506  to a shaft  510  of a motor  508 . The second connector  518  may also comprise a bore in the coupler  506  and the coupler  506  may be secured to the shaft  510  by any suitable means, such as by means of a set screw  512 . The motor  508  is preferably a commercially available DC electric motor such as that available from Micromo, Clearwater Fla., U.S.A. The first and second connecters lie precisely along the rotational axis  514  to guarantee the rotation of the capillary tube  304  about the rotational axis of the motor. One skilled in the art will appreciate that any suitable means for connecting the coupler to the shaft  510  may be used. 
     In use, motor  508  rotates motor shaft  510  about central axis  514 . Since coupler  506  engages both motor shaft  510  and capillary tube  304 , coupler  506  and capillary tube  304  also rotate about central axis  514 . 
     The rotation direction and speed of motor  508  can be varied. The objective in rotating motor  508  is to ensure that an appropriate statistical average over all orientations of the crystallites in the sample is achieved. Preferably, an encoder is connected to the motor shaft to count the number of revolutions at each discrete setting of the detector position two theta (2θ). Preferably the shaft rotates the same number of revolutions at each setting of the detector position two theta (2θ). 
     FIG. 6 is a cross sectional view of the turntable  302  shown in FIG. 4A, taken along line  6 ′ 6  of FIG. 4A, excluding shading lines for clarity. As can be seen from this sectional view, rotation devices  500  are equally spaced around the perimeter of the turntable  302 . The number of rotation devices  500  which can be mounted around the edge of the turntable  302  is only limited by the size of the rotation devices  500  and the diameter of the turntable  302 . The rotation devices  500  are mounted with their rotational axes lying along a radial extending substantially perpendicular to a central axis passing through the center of the turntable  302 . Holes are preferably drilled around the edge of the turntable  302  to match the diameter of the rotation devices  500  which are slid into the holes. A set screw, or other suitable means, holds the rotation device  500  in place in the turntable  302 . By sliding each rotation device  500  into its respective hole to the appropriate depth the sample may be accurately positioned relative to the edge of the disk to guarantee that the capillary tubes  304  are exposed to the X-ray beam when the turntable  302  is rotated. The angle formed between adjacent rotational axes of the rotation devices  500  is preferably about 7.5 degrees. A number of apertures  602  may be provided around the center of the turntable  302  for reproducibly mounting the turntable  302  onto the remainder of the sample changer system, discussed below in relation to FIG.  7 . Alternatively, any other suitable means may be used to securely and accurately position the turntable  302  onto the remainder of the sample changer system. 
     FIG. 7 is an orthographic view of a sample changer system  700  according to the invention. Sample changer system  700  is shown from both the side  702  and the front  750 . Turntable  302  is mounted onto a rotational mechanism  710  which in turn is mounted on a vertical slide  706 . Vertical slide  706  is free to move in the vertical direction (Y axis) along a vertical beam  704  as indicated by the arrow  720 . The rotational axis of the rotational mechanism  710  coincides with the central axis of the turntable  302 . The rotational mechanism  710  is preferably an electric motor. The rotational mechanism  710  rotates the turntable  302  about its central axis, thereby exposing each capillary tube to the X-ray beam. 
     The vertical beam is connected to a lateral slide  714  by means of a gusset  708 . Lateral slide  722  is free to move along a lateral beam  712  as indicated by the arrow  722  (Z axis). The lateral beam  712  is connected to a horizontal slide  718  which is free to move along a horizontal beam  716  as indicated by arrow  7224 (X axis). The vertical  706 , lateral  714 , and horizontal  718  slides allow for the precise positioning and aligning of the turntable  302  and sample to any point in a three dimensional space. The sample changer is finally mounted on a base  726 . The sample changer may also be mounted on rotational means which are capable of rotating the sample changer about the three axes. 
     The axis of rotation of the turntable  302  is parallel to the Z direction, or parallel to the X-ray beam. The turntable&#39;s angle can be set accurately enough to bring a new sample into the beam with its axis on the axis of rotation of the an angular 2θ detector within a suitable level of accuracy. This level of accuracy depends upon the diameter of the capillary, and, for example, would be about 20 micrometers for a capillary of 500 micrometers. The preferable radial position is set at a radius of approximately 180 millimeters relative to the axis of rotation of the rotational mechanism  710 . A 20 micrometer displacement of the sample at a radius of 180 millimeters corresponds to an angle of about 0.006 degrees. Therefore it is necessary to be able to set the angle of the rotational mechanism  710  to a finer precision than this value. A commercially available rotational mechanism  710  such as the Huber Model 410 available from Blake Industries, Scotch Plains, N.J., is suitable for this purpose. A standard rotary encoder mounted on the turntable is suitable to guarantee that the turntable is set to the correct angle and therefore the sample is in the beam. 
     The slides  706 ,  714 , and  718  translate the mounting disk in the X, Y, and Z directions respectively. This is necessary to provide an alignment of the sample with respect to the beam. Commercially available linear slides such as Models 3020500XR and 3020600XR available from the Daedal Division of Parker-Hannifin, Irwin, Pa., are suitable for this purpose. 
     FIG. 8 is a diagrammatic illustration of another embodiment of the invention. In this embodiment a rotor  800  of platforms, similar to the platform discussed in relation to FIG. 3B, can be rotated  808  about a central axis  804  and translated along the central axis  804 . In use a column of capillary tubes  304  mounded on the edge of the platform  808  are analyzed one at a time by moving the platform  808  along axis  804 . Thereafter, the rotor  800  is rotated  808  about axis  804  and another column of capillary tubes on another platform is analyzed. 
     FIG. 9 is a diagrammatic illustration of yet another embodiment of the invention. In this embodiment either multiple turntables or rotors  902  are positioned about a central turntable  302 . Each individual turntable or rotor  902  is then analyzed as described in relation to either FIGS. 4-7, or  8  respectively. 
     The sample changer system is operated in the following manner. A new sample  504  is mounted in a capillary tube  304  of the appropriate diameter. The capillary tube  304  is mounted in the coupler  506  and held in place with a suitable adhesive such as wax or glue. The coupler  506  is mounted onto the shaft of the motor  508 . The rotation device  500  is inserted into the turntable  302  and adjusted so that the powder sample is located at a known distance from the edge of the turntable  302 . The turntable  302  can be either mounted on the rotational mechanism  710  or on a separate bench while the rotation devices  500  are being mounted onto it. This process is repeated for all motor locations around the edge of the turntable  302 , for as many locations as can be accommodated by the diameter of the turntable  302  and the size of the motors. It is convenient to have two different turntables so that one can be used to acquire powder X-ray diffraction data while a second one is being used to mount a series of samples. Mounting each sample so that the powder is a known standard distance from the edge of the mounting disk allows a simple rotation of the turntable to a new sample at the correct position to the beam. Suitable manufacturing of the disks and the turntable with location pins permits a one time alignment of the  706 ,  714 , and  718  slides for any disk installed. 
     Once a new turntable of samples is mounted on the rotational mechanism  710  then computer controlled data acquisition can be performed. After a data set is acquired on the first sample, a second sample is selected by rotating the turntable to the next sample. The new data set is acquired and once complete a third sample is selected, and so forth. When all of the samples on a disk have been analyzed then a new disk can be installed on the rotational mechanism and a new data collection sequence begun. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the present invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.