Sample Containers for Shipment of Allocated Particles
Two basic types of containers are available for holding sample particles during transfer from the curatorial laboratory to various research laboratories. The first type, the SEM- or STEM-type container, is designed to act both as a storage device and as a mounting device for electron microscopic investigations but can also serve some other types of analyses. The second type, the Glass Slide Droplet (GSD-1) container, is simply a containment device used to hold a particle until the researcher can transfer the sample to a storage or mounting device that is appropriate for his research.
To expedite sample allocations, each requestor is encouraged to accept allocated particles in one of the two basic formats and to perform other necessary transfer or remounting of particles in his laboratory. However, under special circumstances, the curatorial staff may be able to modify existing devices or utilize devices provided by the sample requestor in order to complete the allocation (see Special Containers).
In all cases, sample mounts sent to researchers are accompanied by photographs and sketch maps that document the location and appearance of each sample particle on each mount.
Use of Silicone Oil and Epoxide Resins
High-viscosity silicone oil (Dow-Corning Series–200, kinematic viscosity of 500,000 centistokes) is used to coat stratospheric dust collectors and also as a transfer medium during the laboratory processing of individual sample particles. On the inertial impaction collectors, the oil acts as a chemically inert trap for particles that would otherwise bounce off the collection surfaces during sampling. As a transfer medium, the oil ensures positive adherence of the sample particle to the transfer probe (usually a fine-tipped glass needle) and mounting surfaces and, thereby, greatly reduces the chance of particle loss (a major concern during "dry" transfers).
Before it is used, the silicone oil is thinned with ultrapure Freon-113 and filtered through a Nucleopore polycarbonate membrane of 0.4-micrometer pore size in order to remove particulate impurities. Upon evaporation of the Freon, the silicone oil returns to its normal, viscous state. Although it is a convenient collection and transfer medium, the silicone oil also presents a possible source of contamination. Consequently, considerable effort should be made to rinse all silicone oil from each sample particle before it is analyzed. For particles shipped in droplets of silicone oil (see GSD-1 Containers), washing must be performed by the researcher in his laboratory.
Particles shipped “dry” (see SEM- and STEM-Style Containers ) are washed in the curatorial laboratory before shipment. Silicone oil is soluble in various liquids although hexane is the solvent preferred in the curatorial laboratory. Details of hexane washing procedures can be obtained by contacting the curatorial staff.
Epoxide resins that are particularly resistant to attack by hexane (Vac-Seal epoxy, Perkin-Elmer Ultek part number 288-6CXX) are used to bond selected particle-holding substrates to some container devices. In each case, epoxy is used sparingly and its contact with the actual particle-holding surfaces is carefully avoided.
All three varieties of mount under this category are designed specifically to fit the sample holder of a JEOL-100cx Scanning Transmission Electron Microscope. However, given suitable adapters and sample chamber geometries, the same mounts can also be used in other types of electron microscopes. Each mount is constructed of sintered, high-purity graphite. The basic block (SEM-1) is machined to produce two other varieties (SEM-2, STEM-1) of the mount (Fig. 1). For shipping, each mount is individually housed in a screw-cap-style glass vial (Fig. 2) that is doubly wrapped in heat-sealed nylon bags.
Regardless of the variety of mount used, once the mount is rinsed free of silicone oil the sample particle adheres to the substrate only through electrostatic attraction or related contact forces that are peculiar to small particles. It is important to bear in mind that those forces can be broken by strong jolts (for example, by dropping the mount) with resultant movement or loss of the sample particle. Although small (< 20 micrometer), irregular particles can be expected to survive long-distance mail transfers on "dry" mounts, large (> 20 micrometer) irregular or spherical particles (any size) are at greater risk of loss from "dry" mounts. High-risk particles are more safely transferred in oil-droplet containers.
SEM-1 is a solid block and contains no holes or cavities. Because the mount surface is black and rough at the microscopic scale (relief on the order of > 10 micrometers), sample particles placed directly on the graphite would be difficult to relocate and analyze. Consequently, SEM-1 is most usefully employed as a support for smoother, more highly reflective, or more fragile substrates (metal foils, synthetic films, etc.) that better facilitate particle analyses that do not require transmission of radiation through the sample. The desired substrates are attached to SEM-1 by epoxy bonding.
SEM-2 contains a rectangular hole (with vertical walls and rounded inner corners) that penetrates the block, thereby permitting transmission of light or other radiation through the block’s middle zone. For curatorial use (including preliminary examination of particles by scanning electron microscopy), the rectangular hole is covered on the top surface of the block by attaching (epoxy bonding) a 10-micrometer-thick polycarbonate filter membrane (Nucleopore, 0.4-micrometer pore size). The attached filter membrane then receives, by two successive steps of vacuum evaporation, a coat of carbon that is overlain by a grid pattern of aluminum. The carbon film provides electrical conductivity for electron-beam analyses whereas the aluminum grid pattern provides a reference system for uniquely locating several individual particles on the same mount. The porous membrane substrate facilitates the hexane-rinsing steps that are needed to remove silicone oil from sample particles.
STEM-1 contains two conical holes that meet near the middle thickness of the block to form a narrow platform that is designed to support a circular grid of the type used in transmission electron microscopy. The two types of grids most commonly used in the curatorial laboratory possess either a standard rectilinear mesh (TEM-A) or a single, oval-shaped hole (TEM-B) (Fig. 1). To act as effective holders of cosmic dust particles, TEM-A grids must be covered by a thin substrate with a characteristic pore size that is smaller than the sample particle size; most commonly, the substrate of choice is a holey film of carbon. The curatorial laboratory does not possess the time or resources to produce holey carbon films on TEM grids. To the extent that they are useful, bare copper grids can be provided in either the TEM-A or TEM-B style. However, researchers requiring special grids (gold, beryllium, graphite, etc., or coated with holey carbon films) are expected to furnish those grids.
Normally, a sample particle is placed either on a wide, flat area of a TEM-B grid or over a hole in a TEM-A grid that has been provided (by the sample requestor) with a holey film of carbon across the gridwork. After receipt of the sample particle, the grid can then be containerized and shipped in either of two formats. A grid can be placed loose in the polyhedral cavity of a covered plastic tray or , alternatively, permanently anchored within the circular cavity of the SEM-1 mount. In the tray container, the grid stands on edge along a diagonal (dimension slightly larger than "d" of TEM-A, Fig. 1) of the cavity so that the particle-bearing surface of the grid does not contact the walls of the cavity. Upon receipt of the tray container, the researcher can use fine-tipped tweezers to remove the grid from the cavity for transfer to a different mount surface. In the STEM-1 mount, the grid is firmly fixed (by bonding with tiny droplets of epoxy) to the circular platform and, for all practical purposes, must be used by the researcher in that format.
At the request of the researcher, sample particles allocated on TEM-style grids can either be left in silicone oil (remaining from the transfer process) or washed with hexane prior to shipment.
GSD-1 isolates the sample particle in the bottom of a pre-formed circular cavity of a 1 x 3-inch ( ~ 25 x 76-mm) rectangular glass slide (Fig. 3). The particle is contained within a droplet of silicone oil that also serves to anchor the sample to the concavity of the glass slide. A flat glass slide is attached (with Teflon tape) to the top of the concavity slide to seal out contaminants. In addition, the glass-slide "sandwich" is doubly wrapped in heat-sealed nylon bags. After shipping, the two glass slides can be readily separated and the particle can be retrieved from the concavity slide by means of a fine-tipped needle. GSD-1 is the simplest and safest container available for shipping individual particles to researchers who are independently equipped to manipulate small particles.
The curatorial laboratory is designed to process, catalog, and allocate particles for scientific research, rather than to directly perform such research. Accordingly, each sample requestor is encouraged to accept allocated samples in one of our standard formats and to perform subsequent transfer work and sample preparation in his own laboratory. In the interest of advancing science, though, the curatorial staff is receptive to requests for allocation of particles in formats different from those described above, particularly in support of the initial allocation to a newly approved sample requestor. Any sample requestor who anticipates the need for special handling of his allocated particles should communicate those needs to the curatorial staff at the earliest possible opportunity, preferably as part of each new sample request.