Because ammonium perchlorate is anisotropic (4,it is necessary to take one picture, Figure IC, 45" from Figure 1B. Grains a t cxtinction positions in Figure 1B will show up in Figure 1C. The white areas in the photomicrographs, Figure 1, Band C, represent the ammonium perchlorate crystals in the thin section of the propellant. Careful examination of the photomicrographs in Figure 1 will reveal corresponding grains of ammonium uerchlorate in the three pictures. The round and irrecularlv shaoed dark areas in Figure 2 represent t h e undissolved nitrocellulose ball powders in the thin section of the propellant.
Nitrocellulose ball powders absorbed less x-r ays than the surrounding materials Alumirmm particles absorb almost llLL1. . . ~~icrerure, U L ~ K radiation all soft x-rays; LL..~-c~-. can pass through them to expose the film. The areas under the aluminum particles will be white on the developed microradiogram. The smaller white areas in Figure 3 represent the aluminum particles, while the larger areas probably represent aggregations of aluminum particles. The method described in this paper can be used to take photomicrographs and microradiographs of 8 thin section of a propellant to show the size, shape,
and position of such ingredients as ammonium perchlorate crystals, aluminum particles, and nitrocellulose ball powders in the thin section. LITERATURE CITED
(1) Hess, W. M., Nmelco Reporter, 9, No. 1, 3 (1962).
(2) Rigby, G. R., "The Thin-Section Mineralogy of Ceramic Materials," p. 39, The British Refractories Research Aasociation, 1948.
This paper has been clewed for open publication by the Directorate for Security Review, Department of Defense. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965
Removable Mylar Window for Electron-Probe Microanalyzer
,
.L
T. E. Reichard and W. 5. Coakley, Central kesearcn veporrrnenr,
of most microanalyzers are housed in separate vacuum chambers which are connected to the electronbeam column via a relatively small channel through which x-rays pass from specimen to spectrometer. It is advantageous to have this channel, at different times, either completely open, solidly closed, or separated by a thin Mylar vacuum-tight window. An unobstructed opening is essential for passage of the longer x-ray wavelengths used for analysis of light elements; a Mylar window is needed when the spectrometer is opened for alignment of diffracting crystals, counter tubes, etc.; and the closed position is convenient for overnight shutdown. A three-position valve which provides for these three conditions, "instantly" interchangeable under vacuum, has been installed in our electron probe. The modification was made at relatively low cost, nith very little alteration of the existing instrument, and with only a one-day interruption of normal operation. Although this valve was designed specifically for the Cambridge unit ( I , 2 ) , variations of it may be applicable to other electron probes. H E X-RAY
I
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monsonro
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AIR VENT
SPECTROMETERS
Telectron-probe
i&79 .
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/PLUG
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POSITIONING
SPECIMEN 6-MICRON MYLAR I
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Figure 1.
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I Cross sedtinn thrnunh :entr !r of valve, shown in open position
The principal components are shown in Figure 2-i.e., the valve body with a rectangular O-ring groove cut into its inside cylindrical wall, and the plug with its end caps. The longitudinal edges of the two rectangular plug ports are rounded slightly and polished, bo ride smoothly over the gremed O-ring. A sheet of 6micron-thick Mylar is cemented over one of the plug ports with epoxy resin. The end caps are attached with epoxy also, so they can be removed via epoxy solvent if the Mylar needs to he replaced. The valve
is operated by a small lever which keys into a square hole in one end cap. I n the Cambridge electron probe, the valve is mounted inside the brass tuhe which couples the spectrometer to the base of the lens column just above the specimen chamber. This coupling tube, which previously held the permanent Mylar window, is modified to serve as the valve housing, The opera& ing lever is brought out through a small O-ring seal between the objective-lens housing and the x-ray aperture assembly. The coupling tube and the
,I
in the configuration of a stopcock with an O-ring s r d aronnd its bore opening. Figure 1 shows a cross section through the center of the valve, in open position, h mechanical stop aligns t,he hlylar window and "open" posit,ions a t the two limits of a 90-degree rotation of the ryl'ndrical plug. I n the center (454egrre) position, the port is solidly hlnnkrrl off. h small vrnt is provided to allow air to rsrape from the sliace undm the Mylar r h r n bhe spectrometer is pre-cracuated with t,he valve in closed posit,ion.
Valve Figure 2. body and Plug
VOL 37. NO. 6, MAY 1965
773
knurled ring wbich clamps the spectrometer onto i t are the only parts of the existing instrument which are altered, Mthough the limited space available dictates a very compact design, the valve opening is more than sufficient to pass the full solid-angle of x-rays, as defined by the standard rotary aperture in fully-open position.
Complete detailed drawings of the valve assembly, as designed for the Cambridge electron probe, will he provided by the authors on request.
LITERATURE CITED
(1) Cambridge Instrument Co., Ltd., 13 Grosvenor Place, I>ondon, S.W. 1, "The hlicroscan X-Ray Anslyaer (Mark
II)," 1SR1.
(2) Duncnmb, P., nklford, I ) . A,, "x-nay
ACKNOWLEDGMENT
We thank J. n. Reynolds for fahrication of the valve.
Microscopy and X-Ray Microanalysis," Engstrom, A,, Cosslett, V. H., Pntke, H. H., &., mevier, pD. 353364. Amsterdam, 1960.
An All-Glass Coupled Column for Larsje-Scale Chromatographic Separations
0.William
Berg, Department of Chemical Engiineering and Applied Chemistry, University of Toronto, Toronto 5, Canada
a column conT slstmg . of a series of individual columns of successively decreasing HE COUPLED COLUMN, '
volume, was described first by Claesson ( 1 ) and Hagdahl (8). A recent review is also available (3). A high-capacity column with superior separating ability can he assembled using slightly modified standard components of industrial flanged-glass pipe (Figure 1). For the use with aqueous solutions no modification is necessary, apart from the addition o f a stopcock. Figure 2a depicts an all-metal threestage coupled column according to Hagdahl. The shaded area indicates the absorbent. The individual columns are connected to each other t y threaded capillary couplen. Figure 2b shows how such a column can be simulated by using standard fittings of flanged industrial glass pipe. The corresponding parts are joined with horizontal lines. These parts are in descending order: hose connector, not shown in Figure 26 hut in Figure 4, straight length of pipe, reducer, length of pipe, reducer, length of pipe. Finally, the lower end is closed with a stopcock, not shown. Clearly, the reducer has to be modified to incorporate the capillary. The easiest way to accomplish this is to insert a plug, a cylindrical piece of an inert material--e.g., polyethylenewith a suitable hole drilled through it. Such a design is shown in Figure 3a. The plug is indicated by the shaded area. Unfortunately, the upper rim of the reducer has a somewhat smaller diameter than the inner part of the tubing. Consequently, the plug has to he elastic. With aqueous solutions this type of construction works well. The addition of a stopcock produces then an exeellent coupled column of any desired capacity. With organic solvents, this design is less desirable as the plug swells. Consequently, an all-glass construction was adopted (Figure 3b). Efforts should be made to make the upper part of the capillary section flat, as this supports the filter paper disk, used to prevent adsorbent flow to the column below. Table I summarizes the parts used to 774
ANALYTICAL CHEMISTRY
hnild a five-section coupled column. The fittings up to 0.75-inch nominal diameter were supplied by Sentinel Glass Co., Hathoro, Pa., while the larger sizes were supplied by Q.V.F. Glass (Canada) Ltd., Searhorough, Ontario. The latter brand of pipe fittings is to he preferred for the larger sizes; actually the smallest diameter available is 5/s inch, as the pipe sections are available in 1-inch increments from 6 inches up to 12 inches. For borosilicate glass (Corning) the smallest availatle diameter is 1 inch. The stock lengths increase in &inch increments from 6inch sections. The gaskets are of the Teflon (Du Pont) envelope type. They should he of the machined type rather than the split variety. The latter ones have a slightly smaller diameter than the corresponding fittings. The stopcock used to close the smallest column section has a plug of Teflon. Actually, a variety which also incorporates a needle valve is to be preferred as occasionally the flow rate adjustment with the ordinary stopcock is somewhat time consuming. The top of the column is closed with a standard hose connector (Figure 4). The construction is apparent from the figure. The shaded area is a short length of heavy rubber tubing. An all glass ring seal could he envisaged between the glass tubing leading to the pump and the connector. This type of seal is preferable &s it acts as a safety valve in case the pump is turned on and the stopcock is not opened. The complete five-section column can he filled with about 600 grams of alumina. This is sufficient for the separation of at least IOgramsofsample. It is seldom necessary to use all five sections. With additional fittings, stopcocks, and connectors, any suitable combination of the columns can be used. Figure 1 shows a three-section b OvercVIIview of 3-section (0.5, 0.75, and 1 inch) column fed b y Figure 1 .
bellows pump
