removed to eliminate noise from that source from interfering with the alpha detector. In Figure l , A is the preamplifier taken from the standard Packard counter. 3 is the multiplier phototube; in this cme a Dumont No. 6292. C is a Mumetal shield with felt lining used to enclose the multiplier phototube both for protection and for the exclusion of light. D is a disk of vitreous quartz with a 90-micron thick CsI-TI activated scintillation crystal attached. E is the housing proper for the entire alpha detector unit. This piece was machined from a single block of high-purity aluminum 6l/* inches long by 3 3 / 4 inches in diameter. The housing is attached to a 1-inch thick Lucite collar for attachment to the tripod and for proper positioning of the preamplifier and multiplier phototube unit. F of Figure 1 is the bottom of the unit, which is also the sample holder. This piece is likewise made of aluminum, gold plated, to provide easier cleaning with less contamination from one sample to the next. The gold plating was used because gold can be expected to be more free of lead impurities than stock aluminum. There is a small depression in the center of F which is just large enough to accommodate the sample planchet, G. The sample planchet holds approximately 200 mg. of material and presents an area of exactly 1 sq.
cm. for counting. The sample planchet
fits into the depression of F and the brass ring, H, clamps the rubber ring of F against the base of E such that a
vacuum-tight seal is obtained and, a t the same time, the sample is held a t a highly reproducible separation from the scintillator crystal. This separation is approximately 2 mm. in this unit. The scintillator crystal is mounted with the quartz side toward the multiplier phototube, using a silicone oil of high viscosity. The CsI side of the scintillator crystal is sealed to the far side of the hole in E by rubber cement to form a vacuum-tight seal from above. Evacuation is made through a small hole in E connected to the vacuum nipple shown in the housing. Although the cesium iodide crystal has a lower output pulse than that of zinc sulfide, it was chosen because of its greater efficiency, due to its higher atomic number and density. The geometry of the counting chamber is such that essentially all alphas (2 pi) are detected by the scintillator crystal. This is true within the error imposed by counting statistics. A constant R = a / E of 0.496, given by Geological Survey Bulletin No. 1097 A, is used to convert counts per square centimeter per hour, E, from the thick
source emitter to alpha disintegrations per milligram per hour, a, for thin source emitters. This resulting a value is used with the lead concentrations in parts per million to obtain an age for the sample. For our work, calibration of the counter is carried out using U. S. Geological Survey analyzed samples with alpha activity of 100 to 1500 counts per hour per square centimeter. Our counts on these samples have agreed within counting error with those counts obtained by the Geological Survey. Samples of zircons separated from various mining exploration samples have ranged from 80 to 1000 counts per square centimeter per hour. A background of 40 to 60 counts per hour is customary with good stability from day to day. This background has recently been reduced to 20 to 30 counts per hour. Counts are ordinarily taken on samples and standards for 4 hours each, with 1hour background readings between samples to give a total of 4 hours on the background for each series of four to six samples. Samples encountered to date have had a minimum of 4 intensity to background ratio which provides counting accuracies of *IO% or less in 4hour counting periods.
Remote Operation of Single-Pan Balance for Weighing in Inert Atmospheres J.
E. Barney II, Spencer Chemical Co. Research Center, Merriam,
Kan.
years, increased attention Istruction, has been given to the design, conand operation of dry boxes N RECENT
for handling moisture- and oxygen-sensitive materials. Dry boxes constructed of plastic or stainless steel, in which the atmosphere can be maintained a t a dew point of -70" C. or lower and the oxygen content can be kept a t less than 5 p.p.m., may be purchased from a number of companies. Several very satisfactory dry boses have been described (1, ,%?)that can be constructed in a well-equipped shop. Comparatively little effort has been devoted, however, to the problem of weighing samples in the inert atmosphere available in dry boxes. Most balances, when left in dry boxes containing such moisture- and/or oxygensensitive compounds as TiC18, TiClr, and AlC13, will corrode rapidly, with subsequent loss in accuracy of weighing. In general, most workers (3) have used differential weighing techniques, wherein the actual weighings are made outside the dry box, or they have used inexpensive trip or torsion balances. For analytical weighings, however, the former technique is 'time-consuming, 1294
a
ANALYTICAL CHEMISTRY
Figure 1, Schematic diagram showing side view mounting of balance E. a/,-inch plywood base EA. Mettler H5 balance D. 1.
Manostat dry box 1 -inch angle iron supports L. 3/8-inch Plexiglas supports for motors M. Reversible synchronous motors S. '/,-inch lag bolts SS. Snap action switch W. W a l l
while the latter is not accurate enough A novel apparatus that permits rapid, accurate analytical weighings in a dry box has been devised. A Mettler H5 analytical balance (Fisher Scientific Co., St. Louis, Mo.) is mounted above a dry box on a platform suspended from a wall. Holes cut in the platform and in the top of the dry box permit a weighing pan to be suspended inside the dry box. Small, reversible synchronous motors drive the pan-release knob and the vernier control knob of the balance through flexible couplings. Direction and angular distance of travel of these knobs are controlled by foot-operated momentary switches, and by snap-action switches activated by pins on the knobs. A side view of the apparatus is shown in Figure 1. Only one snap-action switch is shown; actually, four are required, two for each knob, in order to limit travel in both clockwise and counterclockwise directions. Also, the mirror used by the operator to view the optical scale of the balance is not shown. Any large truck mirror is satisfactory. It is conveniently mounted on one of the angle-iron braces.
The circuit diagram for operation of the reversible synchronous motors is shown in Figure 2. To operate the control knobs, the operator depresses either foot switch 3 or 3'. The synchronous motor then drives the knob either clockwise or counterclockwise until the desired position of equilibrium is obtained. If the operator accidentally moves the knobs too far in either direction, the pins on the knobs contact snap-action switch 2 or 2' and immediately stop the motors. In manipulating the vernier knob, the operator should reverse the motor momentarily to obtain precise control, as these motors tend t o overrun slightly. The I-r.p.m. motor drives the vernier control, while the 4-r.p.m. motor drives the pan-release knob. To weigh a sample on the balance, the operator first enters the dry box through the glove ports and removes the plug from the hole in the top of the dry box. It is convenient to tap the hole and use a machine screw as a plug. Argon is then caused to flow out the hole a t a rate sufficient to prevent air from entering. The auxiliary balance pan in the dry box is then coupled to the balance pan by means of the stirrup provided by Mettler plus a 12-inch section of bare No. 12 copper wire. The sample is then placed on the balance pan, and is weighed by techniques customarily used on single-pan balances. If the sample weighs more than 1200 mg., one or more of the weight control knobs must be operated by another person. At the moment a reading is taken, the flow of argon must be stopped, as it disturbs the equilibrium of the balance. For this reason, a toggle valve should be provided on the
llo v.
Figure 2. Circuit diagram for remote operation of balance controls 1. Hurst reversible synchronous motor, Type R.S.M.-1 (1 r.p.m.) or R.S.M.-4 ( 4 r.p.m.) 2,2'. Unimax snap-action switcher, Type USMW, normally closed 3,3'. Linemaster foot switches, Model T-5 1 -S, normally open
argon inlet line inside the dry box so that the operator can control the flow of argon. The Mettler H5 single-pan balance is especially well suited for remote operation, because it can be equipped for weighing beneath the balance a t moderate cost. The 1200-mg. optical scale provides a wide range for weighing with manipulation of only one knob. The construction of the balance also permits easy adjustment for any tare. The wires connecting the two pans must not touch either the balance platform or the dry box. Experience has shown that once the proper adjustments have been made, few additional changes are necessary. With the arrangement
shown, the balance can also be used for weighing operations outside the dry box. Because argon must not flow out of the hole in the dry box during the actual weighing, trace amounts of air may enter the box. Thus, efficient removal of oxygen and water from the atmosphere inside the dry box is a necessity. In the dry box used in conjunction with the Mettler balance, sodium-potassium eutectic (available from MSA Research Corp., Callery, Pa.) is used to remove water and oxygen. It is stored inside the box in a stainless steel pan, and the surface is kept free of oxides by a stainless steel skimmer driven by a 4-r.p.ni. synchronous motor. A midget blower that delivers 15 cu. feet per minute moves the dry box atmosphere; it must be turned off when weighings are being made. Argon (Spencer Chemical Co., available from Southern Oxygen Co.) used for the dry box atmosphere contains so little water and oxygen that no purification train is necessar,y. This balance has been used successfully for weighing Tic13 and Tic14 with no detectable oxidation or hydrolysis of the sample. Weighings routinely require 2 to 4 minutes, once the sample is in the dry box and the operator has entered the box through the glove ports. LiTERATURE CITED
(1) Gibb, T. R. P., Jr., ANAL.CIIEM.29,
584 (1957). (2) Sherfey, J. M., Ind. Eng. Chen. 40, 435 (1954). (3) Williams, A. F., Park, T. O., Analyst 85,126 (1960).
A KBr Powder Trap for Gas Chromatographs for Obtaining Infrared Spectra Herman W. Leggon, Parma Research Center, Union Carbide Corp., Parma 30, Ohio of gas chromatoIliquidgraphic techniques to the study of and solid organic compounds at N THE APPLICATION
temperatures up to 400" C., the finding of unknown components is extremely common until the system has been explored thoroughly. Errors in sample reporting are minimized by establishing or verifying the identity of these unknowns by obtaining their infrared spectra. A number of traps (1-6) have been reported for capturing desired components issuing from the chromatograph either by a freeze-out technique or by simple condensation. Samples are then transferred to an infrared spectrometer and scanned, A trapping technique was needed in this laboratory which would permit the component to be run in a KBr
pellet in the infrared spectrometer. Such a technique was developed. Any piece of straight glass tubing capable of quick attachment and/or detachment to the outlet of the chromatograph is appropriate. A straight piece of open glass tubing with a 7 / 1 6 ground glass female outer joint is satisfactory for the Kromo-Tog Model K-2 used in this laboratory.
KBr POWDER7
"TEFLON" PLUNGER
7 / 1 5 GROUND GLASS JOINT'
Figure 1 .
A KBr trap
7
The trap is prepared by inserting a Teflon plunger to the desired depth into the joint end of the glass tube. Next a weighed quantity of infrared quality KBr is added carefully through a small funnel or with the aid of glazed paper into the other end of the tube. The tube is lightly tapped, while in a vertical position, to pack the KBr powder against the plunger and make a plug in the tube (Figure 1). The Teflon plunger is removed and the prepared trap is stored in a desiccator to protect it from moisture. When the desired component begins to emerge from the detector cell, the trap is quickly attached to the heated outlet of the gas chromatograph. As the carrier gas passes through the loosely packed KBr, the eluted fraction is deposited on the powder. After the component has been trapped, the Kl3r VOL 33, NO. 9, AUGUST 1 9 6 1
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