1530
ANALYTICAL CHEMISTRY
possible, for the condenser can be filled with dilute electrolyte and electrically heated by inserting two electrodes. The tube indicated by dotted lines can be connected through a differential manometer with the pot for measuring the pressure drop through the distilling column. The manually operated regulator for the products valve is sh0R-n as 12 in Figure 1 and in detail in Figure 2. One turn of the regulating knob moves the glass-rod valve stem I mm. The divided scale on the ring attached to the knob permits an accurate adjustment of the valve.
valve 2 is slowly opened and the air and other contaminating gases or vapors are removed. Xeedle valve 2 is then closed while stopcock 7 is opened t o the counting gas. As the needle valve is slowly opened, the sample and its well are flooded with counting gas. This operation is usually repeated. Another sample having been introduced into the exposed well, the sample change is rotated another go", which brings the first sample to the preflush position where it is continually bathed in the gas flowing from the counting chamber.
VACUUM
/I 1-
Flow Counter Sample Changer with Positive Outgassing. Frederick C. Hickey, O.P., Medical Research Laboratory, Providence College, Providence, R. I.
COUNTING GAS
II
use of a flow-type counter with porous carbon-14Iporous labeled precipitates collected in stainless steel dishes or on disks of glass or stainless steel (the latter obtained from N THE
lllicro Metallic Corp., Glen Cove, S . Y.) by the method of Van Slyke (1, 4 ) )the uncertainty regarding the complete effectiveness of preflushing must be resolved by repeated counts. Especially in the Geiger region, small amounts of residual air or water vapor have a pronounced effect on the counting rate ( 8 ) . Much time is frequently wasted before it is certain that all air and water vapor have been eliminated. I n order t o eliminate this source of worry and wasted time, as far as possible, the sample changer herein d e scribed was designed. Originality is claimed only for the outgassing feature, which can be incorporated with any of the many designs of flow counter described in the literature, such as that devised by Robinson (3). CONSTRUCTION
Four positions of the sample changer are provided.
Figure
1,B, shows them in clockwise order from the bottom: loading,
outgassing, preflushing, and counting. The counter head is machined from solid stainless steel. The counting gas, in this case 99% helium-1% isobutane, upon entering the manifold is offered two paths. The first is through needle valve 6 into the counting chamber, thence through the groove machined in the lower face of the counter head to the preflush chamber, and finally through holes drilled in the head to the glass bubbler which indicates the rate of flow. The second path, offered the gas when two-way stopcock 7 is open, leads the counting gas through needle valve 2 to the outgassing chamber. With stopcock 7 rotated 90", the counting gas is excluded from the outgassing chamber and vacuum is applied instead. Adjustment of needle valve 2 permits the control of the rate of evacuation or filling t o prevent disturbing the sample. The vacuum gage mounted directly above the outgassing chamber facilitates this control. The counting chamber is 1 inch in diameter and 1.5 inches long. The anode, consisting of a 0.25-inch loop of 0.002-inch tungsten wire, is positioned approximately 0.25 inch from the lower end of the chamber. The sample changer or lower plate is machined from "bearing bronze." The four sample wells, 1 inch in diameter and 5/16 inch deep are of somewhat unusual design as shown in C and D, Figure 1. Small semicircular niches are routed into the walls of the wells nearest the edge of the disk. An annular depression in the bottom of the well allows the counting disks to be tilted by a slight pressure from a needle, n-hile the niche permits access t o the tweezers as indicated in D. The bottom of each well is numbered for identification, the 8ame number being stamped in the lower position on the edge of the sample changer, so the well can be identified even with a sample in position. The upper numeral indicates the well actually under the counting chamber. A taper pin extending through the counter head and into the sample changer holds the wells in proper register under the counting chamber. The head and sample changer are lapped together by hand to secure a gas-tight seal. OPERATIO\
In practice the sample is introduced into the exposed well a t the front of the instrument. Taper pin 5 is raised and the sample changer rotated 90" clockviise, Tvhich brings it under the outgassing position. With stopcock 7 open to the vacuum line, needle
Figure 1. Diagram of Flow Counter with Positive Outgassing 1. Sample well under counting chamber 2. Counting gas control valve 3. Cable connector Sample well in loading position Register taper pin 6. Preflush control valve 7. Two-way stopcock 8. Passage from counting to outgassing chamber
:
Finally, with another quarter turn of the sample changer, the sample is brought under the counting chamber. Needle valve 6 controls the rate of flow so that approximately one bubble per second passes through the mineral oil bubbler. Sufficient presEwe is maintained in the line from the pressure regulator to needle valve 2 t o avoid drawing air back through the bubbler when the evacuated sample is being flushed with gas. RESULTS
Using a scaler set a t 4-mv. input sensitivity and operating a t 1250 volts, the instrument gave a plateau of 250 volts in length with a rise of 7.6% per 100 volts. The background with no auxiliary shielding was 15 counts per minute. In use, no drift toward higher counting rates, indicative of the gradual elimination of contaminating gases was observed. In one series of 17 different samples of cholesterol digitonide labeled with carbon-14 and mounted on stainless steel sintered disks, the average standard deviation !vas \Tithin the statistically predicted value. LITERATURE CITED
w.,
(1) Bernstein, and Ballentine, R., Rev. Sci. Instr.. 20, 347 (1949). (2) Korff, S. A., and Present, R. D., Phys. Rev., 65, 280 (1944). (3) Robinson, C. V., Science, 112, 198 (1950). (4) Van Slyke, D. D., Steele, R., and Plazin, J., J . B i d . Chem., 192, 770 (1951).
