adsorbent. However, this loss is more than offset by the increased counting efficiency which is possible. That is, the polystyrene film can be sectioned and counted in a simple gas flow counter, whereas the glass plate must be assayed by an end-window type counter, unless specialized equipment is available (6). The fact that the polystyrene solution does not disturb the radioactive solute spot was verified by radioautography. After development, the plate with its glass support was covered with a piece of sheet plastic which was secured to the back of the glass with Scotch Tape. A piece of Kodak Type K K x-ray film was sandwiched between the plasticcovered adsorbent and another piece of support glass. Strong rubber bands were used to hold the sandwich firmly together. The x-ray film was exposed to the thin layer chromatogram for 12 hours and developed by the reconimended methods. The sheet plastic was then removed and the polystyrene mixture was poured over the chromatogram. The smooth side of the resulting polystyrene film was placed in contact with x-ray film in a press (4). The
exposure time was increased to compensate for the polystyrene absorption. The two autoradiograms were compared visually. X o distortion or migration of the spot was observed for any of the radioactive solutes investigated (glucose, mannose, mannitol, and sorbitol). The advantage of using the polystyrene film for radioautography is the ease in handling of the chromatogram. The disadvantages are the loss in sensitivity due t o absorption by the polystyrene and the fact that the solute material cannot be conveniently recovered. If recovery of the material is necessary, then exposure of the x-ray film to the chromatogram on the glass plate is the best method of radioautography. Some workers (5) place the film in direct contact with the adsorbent. However, the plate can be handled without disturbing the adsorbent layer if it is covered with sheet plastic. Sheet plastics tested for this use and the percentage of C14 beta-radiation absorbed by each of them are: 0.00025 Mylar (22%), 0.0005 Mylar (39Y0), Saran Wrap (&Yo) and Handi-Wrap (24%). Handi-Wrap was chosen for
the work above because of its low percentage of absorption and because i t produced a cover free of wrinkles. These techniques provide simple, lowcost methods of radioautography and radioassay which have general application in thin layer chromatography but are particularly useful for water soluble substances. LITERATURE CITED
(1) Barrollier, J., Naturwissenschaften 48,
404 (1961).
( 2 ) Csallany, A. S., Draper, H. H., Anal. Biochem. 4. 418 11962). I , (3) Lichtenbkrger, W., Z. Anal. Chem. 185(2), 111 (1961).
(4) Lotz, W. E., Gallimore, J. C., Boyd, G. A., iVucleonics 10, 28 (March 1952).
(, 5,) Mangold. H. K.. Kammereck. R.. "Proc&dings-l961' International 'Syrn:
posium on Microchemical Techniques," Vol. 2, p. 697, John Wiley, New York, 1962. (6) Randerath, K., "Thin-Layer Chromatography," p. 65, Academic Press, h'ew York, 1963. (7) Squibb, R. L., N a t w e 198, 317 (1963). WORKsupported in part by a grant from the National Science Foundation.
Preparation of Nonmetallurgical Specimens for Electron Probe Microanalysis
T. E.
Reichard and W . S. Coakley, Monsanto Company, St. Louis, Mo.
1"
ELECTRON probe microanalysis of nonmetallic materials the specimens often require specialized handling because of small size, irregular shape, fragility, porosity, low physical stability and/or low electrical conductivity. The following techniques were developed and found useful for electron probe studies of diverse materials such as catalyst pellets, contaminant particles, pigment dispersions, "hard" biological specimens, detergent granules, and semiconductor materials. Analysis of Existing Surfaces. A cylindrical aluminum pill with a suitable depression is used for mounting small beads, granules, "grit," bits of metal, etc., which are t o be retrieved after analysis. The depression is almost filled with a soluble gluee.g., Duco cement or a solution of Formvar or Parlodion-premixed with graphite to a paste consistency. The particles are placed on the paste and pressed flush with the outer rim of the pill. The graphite-glue hardens to a stable mounting with high electrical conductivity, and the specimen(s) can be dissolved free after analysis. As a quicker alternative for analysis of microscopic amounts of grit or specks which need not be retrieved, the particles are pressed directly into the flat surface
316
ANALYTICAL CHEMISTRY
Plllrtlc Box
Spec1 men G r a n u 1es
" R e t a i n i n g Wall" (lapel
/'
..,
DOUble-Adh's/Ve Tape
Figure 1. Vacuum-imbedding nique for void-free mountings
tech-
of a solid aluminum pill, using a briquetting press. When ends or edges of elongated or lamellar particles are to be analyzed, a piece of double-coated transparent tape (Scotch Brand Yo. 665, adhesive on both sides) is placed on a microscope slide, and the specimen surface of interest pressed into the adhesive. A short cylindrical ring cut from */r-inch diameter aluminum, copper, or brass tubing is centered around the sample, and also pressed into the adhesive. The open space is filled with indium metal chips which, because of their softness, can be hand pressed and molded around the specimen without displacing or damaging it. The tape is then peeled off and
the mounting is solvent cleaned for analysis. Analysis of Cross Sections. Electrically conductive epoxy resins (loaded with finely divided silver, graphite, or aluminum) are very useThey ful as imbedding media. harden without heat or pressure and do not damage fragile specimens. Heavily silver-loaded epoxies are generally preferred for their high thermal and electrical conductivity ; however, they produce a relatively high level of continuous x-ray emission. When minimal background emission from the mounting is an important consideration (as for improved contrast in x-ray scanning images) graphite or aluminumloaded epoxies are used. Conductiveepoxy imbedding is especially helpful for analyses of particles with coarse, porous structures wherein a conventional vapordeposited aluminum or carbon coating may fail to form a continuous electrically conductive surface layer. When several specimens are to be studied in cross section, several 'i4-inch rings are placed on a small metal plate and half-filled with epoxy mixture. Specimens are then placed in desired orientations and epoxy added to fill the rings, stirring somewhat to wet the
specimen and work out bubbles. The epoxy binds the ring mountings to the plate in a very convenient form for grinding, polishing, cleaning, etc., after which the individual mountings are pried off the plate for analysis. For larger specimens, or for a wider selection of particles and structures than is provided by 1/4-inch mountings, a sheet of mylar (which serves as a parting layer) is taped flat onto a microscope slide and covered with double adhesive tape. Particles are sprinkled onto the tape and pressed slightly into the adhesive. Individual particles of interest can be placed in desired positions and orientations. Conductive epoxy is “puddled” over the particles. The adhesive prevents particles from being swallowed up and randomly distributed in the epoxy. Mylar and tape are stripped from the hardened mounting which is then ground to the desired
depth for analysis of particle cross sections. Some materials are difficult to wet with epoxy, leaving undesirable voids and bubbles. Vacuum-desiccation of the sample particles prior to mounting improves their epoxy wettability and greatly reduces the formation of voids. Vacuum Imbedding. I n mounting detergent materials for cross-section analyses, some voids occur, even with previously desiccated particles, because of air entrapment by the irregular shaped granules. The following technique produces completely voidfree mountings with improved physical stability. Previously desiccated granules are placed on double adhesive tape as for the “puddling” technique, but in the bottom of a square, transparent plastic box. The hinged box lid is set a t an angle as shown in Eigure 1, and a “retaining wall” built up with tape.
The lid is filled with epoxy-hardener mixture, and the entire assembly placed in a vacuum chamber. After a short evacuation, the volatility of the hardener causes the epoxy to foam up and flow over the sample particles, filling the entire volume of box and retaining wall. Readmission of atmospheric pressure then collapses the foam over the sample, closing voids and forcing the resin into all accessible pores and crevices in the granules. ACKNOWLEDGMENT
The authors thank A. H. Heraog, D. E. Hill, and R. A. Rodgers for helpful ideas and suggestions. Material taken from a paper presented at the 15th Mid-America Symposium on Spectroscopy, Chicago, June 1964.
Apparatus for Precise Automatic Recording of Electrocapillary Curves Allen J. Bard and Harvey B. Herman, Department of Chemistry, The University of Texas, Austin, Texas
of electrocapillary llLl curves using a dropping mercury electrode (D.M.E.) is a familiar operaEASUREMENT
tion in polarographic and other electrochemical studies. These curves are useful in studying adsorption and the electrical double layer, as well as being applicable to the analysis of dilute solutions of surface active substances ( I ) . Previous studies [ ( I , 2) and references contained therein] have indicated that under proper conditions surface tension values based on drop time measurements correspond very closely to those obtained from drop weight and Lippmann electrometer measurements. The determination of the drop time a t many different potentials is a tedious task however, and several suggestions for automatic determination of this quantity have been made.
Ri’ha proposed a method based on the photoelectric measurement of the drop time and photographic recording of the electrocapillary curve employing a mirror galvanometer ( 5 ) . Instrumentation for photoelectric measurement and digital presentation of drop times was described by Corbusier and Gierst ( I ) , but construction of the apparatus, as well as manual plotting of the curve, was still required. This communication describes an apparatus based on principles similar to those of Corbusier
9.8
-
u’ 9.6
-
*
9
and Gierst, but using commercially available apparatus and allowing automatic digital and analog recording of the electrocapillary curve. EXPERIMENTAL
A block diagram of the apparatus is shown in Figure 1. The light beam of a small bulb is interrupted by the fall of the mercury drop. This causes a momentary drop in the output current of the silicon photovoltaic light sensor, a Texas Instruments LS-222 sensor.
: Ii-9.4
o
g 9.2 P
DIG- TOANALOG
*
-
POLAROQRA . O d - = - - t J
1
Figure 1 . Apparatus for automatic eledrocapillary curve measurements
I
0
I
I
I
E,
volt
-0.2
Figure 2.
-0.4
I
I
-0.6
I
I
I
-0.6
vs. S . C . E .
Typical results obtained with apparatus
Solution contained 1 M Dotarsium nitrate and 1. Tetra butylammonium iodide concentration, 0 2. 0.042 m M 3. 0.083 m M VOL. 37, NO. 2, FEBRUARY 1965
317
