Abstract:
A cryogenic transfer vial for storing and loading a crystal sample on a goniometer includes a cryogen retainer that inhibits spillage of the cryogen when the vial is inverted during sample loading and retrieval. The retainer may be an adsorptive material located in a region of the vial near a sample location, or may be a baffle arrangement within the vial for containing the cryogen.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of x-ray diffraction and, more specifically, to the mounting of cryogenically cooled crystallography samples on an x-ray diffraction goniometer. 
     BACKGROUND OF THE INVENTION 
     In recent years, it has become common to use x-ray diffraction for the structural determination of biological macromolecules (proteins). To protect the protein crystals from radiation damage, it is desirable to cool them to cryogenic temperatures. This is typically accomplished by storing them in a liquid nitrogen dewar and then transferring them onto a goniometer equipped with a device which cools the sample during the measurements by blowing cold nitrogen (or helium) gas onto the sample. However, of course, the samples must be maintained at cryogenic temperatures while they are transferred from the storage dewar onto the goniometer. To transfer the samples to the goniometer, cryogenic transfer vials have been used. An example of a typical prior art vial is shown in FIG.  1 . 
     The prior art sample vial  10  consists of an outer container  12 , typically made of a plastic material, and a magnetic base  14 . Attached to the base  14  is a sample holder that extends into the vial  10 , and includes a fastener  18  at the end that holds a crystal sample  20  in place. In the preferred embodiment, the fastener is a surgical nylon loop (10-20 μm), but also may take other forms. As shown in the figure, the base/sample holder combination has a shape and size that allows it to form a seal with the container  12 , with the sample holder projecting into the container interior. In order to maintain the sample at cryogenic temperatures, the container is filled with a cryogen  22 , typically liquid nitrogen. The vial is vented to allow the escape of boiled-off cryogen. This cryogen maintains the crystal at a sufficiently low temperature, but causes difficulties in practice. The standard goniometer receives the magnetic base  14  of the vial, but is located such that the sample holder  16  projects outward at a horizontal angle. Thus, as the container  12  is removed from the base, much or all of the cryogen spills out and is lost. Once the cryogen is lost, the sample crystal will rapidly warm up towards room temperature and can be damaged. It is possible also to remove the base from the upright container with a hand tool such as “cryotongs.” In this case, there is no cryogen used. Rather the cryotongs are cooled to liquid nitrogen temperatures and the thermal mass of the cryotong head keeps the sample cold during the transfer process. 
     However, to remove the base in this manner and successfully transfer it to the goniometer requires an inordinate degree of quickness and dexterity, and makes the process more difficult and subject to failure. There are also a limited number of “inverted sample” goniometers in service that can be used to position the base so that the sample holder faces downward. This allows the container  12  to be removed without spilling the cryogen. However, such goniometers have drawbacks including higher cost, less geometric access to the sample and less flexibility in orienting the sample, and thus the use of this solution is limited in practice. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a cryogenic transfer vial is provided for use with a crystal sample material and allows transfer of a sample holder to a goniometer without spilling of a cryogen in the vial, such as liquid nitrogen. The vial includes an outer container surface within which the sample material is enclosed. A container lid that seals to the outer surface also serves as a base, typically magnetic, for a sample support to which the sample material is attached. With the lid in place on the vial, the sample support extends into the vial, such that the sample material is suspended at a sample location within. Within the container outer surface is a cryogen retainer that surrounds the sample location and that retains the cryogen in that region, particularly against the force of gravity. The retainer prevents any significant spillage of the cryogen if the transfer vial is inverted and opened, such as during placement of the sample support lid in a goniometer, and removal of the outer container. 
     In one embodiment of the invention, the cryogen retainer is an adsorptive material located within the outer container surface. The adsorptive quality of the material easily holds the cryogen in place, even if the vial is inverted. Such a material may be, for example, a carbon foam that fills a portion of the vial, but not the sample location. The foam may be below the sample in the vial, and may also partially surround it. In another embodiment, the retainer may be a baffle that, in conjunction with the outer surface, forms a region surrounding the sample location within which the cryogen is contained. The surrounding region is vented, and may have an opening at an end away from the container lid. This would allow for filling of the vial with cryogen, while still ensuring that the majority of the cryogen is retained when the vial is open and inverted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view of a prior art transfer vial used for cryogenic sample transport; 
     FIG. 2 is a schematic view of a cryogenic vial according to the present invention that uses as a cryogen retainer an adsorptive material that partially surrounds the sample; 
     FIG. 3 is a schematic view of a cryogenic vial according to the present invention that uses an adsorptive material located in the vial below the sample; and 
     FIG. 4 is a schematic view of a cryogenic vial according to the present invention that uses an internal baffle as a cryogen retainer. 
    
    
     DETAILED DESCRIPTION 
     Shown in FIG. 2 is a first embodiment of a vial according to the present invention. Some of the components of the FIG. 2 vial can be identical to those of the prior art vial shown in FIG.  2 . For example, outer container portion  12  and magnetic base  14  are the same, as are the sample holder  16  and fastener  18 . Like the FIG. 1 vial, that of FIG. 2 is configured so that the base  14  forms a seal with the container portion  12 , and the sample holder projects into the interior of the vial. Thus, when sealed, the sample holder maintains a sample crystal  20  well within the vial. 
     In addition to these components, the vial also has an adsorptive material  23 , such as a foam. An appropriate foam material may be carbon-based foam. Such a foam functions as a medium for holding the liquid nitrogen, and can hold a temperature of −150° C. for about fifteen seconds. However, the foam medium  23  prevents the cryogen from spilling out of the container  12 , particularly against the force of gravity when the vial is inverted. Therefore, the base  14  and sample holder can be applied to a goniometer having a horizontal sample support positioning, or even one that directs the sample support upward. When the magnetic base  14  is applied, and the container  12  is removed, there is no spillage of the cryogen. Those skilled in the art will recognize that, although the FIG. 2 embodiment is described in terms of using a foam, any material capable of absorbing and retaining the cryogen would be appropriate for use in place of the foam. 
     Shown in FIG. 3 is another variation of the embodiment using an adsorptive foam as the cryogen retainer. In this figure, all of the reference numerals are the same as those shown in FIG.  2 . However, the foam  23  does not encircle the sample location. Rather, the foam  23  resides beneath the sample in the vial (if the opening of the vial is considered the “top” ). While this embodiment may not provide the same level of thermal transfer as that of FIG. 2, it is sufficient for keeping the sample at cryogenic temperatures, and minimizes the chance of accidentally contacting the foam with the sample and possibly damaging it. 
     Another alternative embodiment of the invention is shown in FIG.  4 . As in the embodiment of FIGS. 2 and 3, the vial shown in FIG. 4 makes use of magnetic base  14  connected to sample support  16 . Similarly, the crystal sample  20  is attached to the sample support with the same type of fastener  18 . However, in the FIG. 4 embodiment, the container  24  includes an internal baffle  26 . The baffle  26  is preferably cylindrical in shape, and rotationally equidistant from the outer surface of the cylindrical container  24 , although those skilled in the art will recognize that many different baffle shapes may work just as effectively, as would different container shapes. The baffle  26  and the outer wall of the container  24  form a reservoir in which a cryogen  22 , such as liquid nitrogen, may be trapped. This reservoir retains the cryogen when the container  24  is inverted, preventing the spillage that is common in prior art transfer vials. As such, the magnetic base may be applied to a goniometer having a horizontal or even upright sample holder direction, and the outer container removed from the base without the cryogen being spilled. 
     As in the container of FIG. 1, the container  24  may be constructed of a plastic material, as may be the baffle  26 . The baffle  28  is vented to allow evaporated cryogen to escape. In one version of this embodiment, the end  28  of the baffle is open, so that the cryogen may be added to the vial prior to use, and accumulate in the space between the baffle and the outer container wall. Although this might result in a small amount of cryogen being spilled during application of the base  14  to the goniometer, due to the presence of some cryogen in the interior of the baffle portion, the majority will be retained in the space between the baffle and the outer container wall. 
     While the invention has been shown and described with regard to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.