fourth to one third of the KBr (5 to 7 mg) from the far end of the pile is transferred to a microdie. A micropellet is pressed and IR spectra are obtained in the usual manner. DISCUSSION
Some experimentation by the investigator is necessary to determine the proper solvent for washing a particular unknown onto the KBr. The selection of the solvent, of course, depends upon the TLC support used and upon the substance to be identified; however, most organic compounds can be effectively transferred from either a silica gel or alumina support with ethyl ether, dichloromethane, or acetone. The time required to elute the sample onto the KBr is generally less than two minutes. The hazard of water condensing on
the KBr because of solvent evaporation has presented no real problem in our laboratory. The direct transfer technique has been successfully used in the identification of a variety of TLC separated compounds. The spectra obtained have consistently shown good resolution and low background noise. A typical spectrum obtained by this approach is shown in Figure 2. This spectrum of an acetone adduct ( 4 ) of BLE-25 (an antioxidant found in cornmercial rubbers) illustrates the resolution obtainable by the direct transfer technique. RECEIVED for review August 25, 1967. Accepted October 2, 1967. (4) Sadtler Standard Spectra No. 3257, Sadtler Research Labora-
tories, 1517 Vine Street, Philadelphia, Pa., 1963.
Three-Dimensionally Rotating Sample Holder for 14-Million Electron Volt Neutron lrradiations F. F. Dyer, L. C. Bate, and J. E. Strain Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37831 BECAUSE THE NEUTRON FLUX of 14-meV neutron generators changes markedly with distance from the target and irradiation time, measurement or reproduction of the neutron dose to which samples are exposed is difficult. Several techniques have been devised to minimize errors in activation analyses resulting from this nonuniform flux. Separately irradiated standards and samples can be normalized to equal neutron doses using a neutron counter or monitor foils. Analyses of homogeneous samples by such techniques yield results with uncertainties of +2% ( I ) . When the induced radionuclide is long lived enough to permit counting of all samples, simultaneous irradiation of unknowns and standards is possible. By moving the specimens during irradiation, one can make certain that all are exposed to the same average neutron dose. A device to rotate two samples in two dimensions was described by Mott and Orange ( I ) , their data indicate that under carefully controlled conditions identical homogeneous samples yield results with uncertainties approaching counting statistics. Priest et al. (2) recently described a method of irradiation in which several samples are simultaneously rotated about a common center and moved laterally through the neutron flux. Uncertainties of about +0.5 to +l.O% were reported for measurements of l8F induced in Teflon samples. Although fairly uniform irradiation of samples was demonstrated, no comparison was mad ebetween experimental errors and those expected from counting statistics. In the present study a new type of sample holder was developed that moves several samples in three dimensions by rotating them around two axes. The rotator is simple and inexpensive to make and use. Results obtained with the rotator have shown that highly uniform irradiation can be obtained and that measurement errors closely approach expected counting statistics. EXPERIMENTAL
The sample rotator (Figure 1) consists of two concentric plastic cylinders, the inner of which holds the samples (molded or machined material, powders in vials, etc.). The internal (1) W. E. Mott and J. M. Orange, ANAL.CHEM., 37,1338 (1965). (2) G . L. Priest, F. C . Burns, and H. F. Priest, Zbid.,39, 110 (1967).
TO VACUUM
,,,,l
MOTOR
MOTOR
Figure 1. Sample rotator for 14-meV neutron irradiation
dimensions of the sample cavity are a/4 inch in length and 1 inch in diameter. Samples are loaded and unloaded manually. Both cylinders are rotated: the outer end-over-end by a motor driven hollow shaft attached perpendicularly to the outer well, the inner by an air jet that turns it on its axis. The air jet is generated by applying a vacuum through the hollow drive shaft of the outer cylinder; air enters the outer cylinder through a hole bored tangentially to the vaned surface of the inner cylinder, strikes this surface, and causes the inner cylinder to rotate. The vacuum also holds the outer cylinder cap in place against the outer cylinder lid seat (see Figure 1). Vacuum requirement is modest, and the speed of rotation is regulated to several hundred revolutions per minute by a needle valve in the vacuum line. The outer cylinder has operated at 60 r.p.m. Small variations in rotation rates of VOL. 39, NO. 14, DECEMBER 1967 o
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Table I. Results (Counts/Gram at to) Obtained for Irradiations in Configurations A and B Configuration A Configuration B Sample counts/grama counts/gramb 1 1,166,694 1,729,241 2 1,170,265 1,736,081 3 1,166,466 1,727,494 4 1,168,516 1,719,719 5 1,166,080 1,713,666 RSDc 0.15% 0.51% NO.1 ~.16% RSDCd a Results obtained with CaF2. Results obtained with samples composed of Teflon disks. Relative standard deviation of values in table. d Relative standard deviation expected from counting statistics. 0
either cylinder do not affect the uniformity of neutron exposure. I n our experiments neutrons were produced by a Texas Nuclear generator in which the target bombardment inch in diameter. area was approximately The uniformity of the neutron dose to which samples can be exposed in the rotator was tested by measuring the 18F induced in cylinders X inch) of Teflon and pressed CaF2. Some of the Teflon cylinders were prepared by stacking five. disks (lis inch thick by inch in diameter) punched from Teflon sheet. Each irradiation was made with five cylinders held in place in the rotator in polyethylene vials. Irradiations were performed with the rotator fixed in each of the three configurations shown in Figure 2. In each configuration the rotator center was about 11/,, inch from the generator target and centered on the target. Irradiations were made for about 15 minutes and radioactivity measurements were begun after a decay period of 20 minutes for Teflon and 2 hours for CaFz (to allow decay of 44K). Measurements were made on each of the total samples and the individual disks to determine the intersample precision and the intrasample variation of activity along each sample. The samples were gross gamma counted in a 2- X 2-inch NaI(T1) well detector. Results were normalized to counts per gram at the time counting was begun on the first sample. A half-life of 109.72 minutes was assumed for 18P (3). The number of counts collected for each sample ranged between 106 and 3 x 108 so that the relative standard deviation expected from counting statistics ranged between 0.3 and 0.06 %. RESULTS AND DISCUSSION
Some typical results for total sample counts obtained with the rotator in configurations A and B (Figure 2) are given in Table I. The values shown for configuration A pertain to pressed cylinders of CaF2; those for configuration B refer to samples composed of Teflon disks. All measurements on Teflon, regardless of the irradiation configuration, yielded slightly lower intersample errors for solid samples than for segmented samples. Relative standard deviations calculated from the normalized count data and from counting statistics for other measurements were all comparable to those values presented in Table I. Measurements of Teflon disks irradiated in configurations A and B showed a significant activation gradient along the samples. For the A configuration the middle disks were about 4% less radioactive than the ends. This result was ex(3) J. D. Mahony and S. S. Markowitz, J. Inorg. Nucl. Cliern., 26(6),
901-10 (1964). 1908
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’ \
I
Figure 2. Irradiation configurations of sample rotator with respect to neutron generator target A . Outer cylinder rotated in plane perpendicular to target plane 3. Outer cylinder rotated in plane parallel to target plane C. Outer cylinder rotated in plane inclined (at angle 0) to target plane
pected because the neutron flux approximately follows the inverse square law with distance from the target. Assuming the inverse square law, one can show mathematically that an average of the neutron doses received by the ends is larger than the neutron dose received by the middle. For the B configuration the middle disks were about 4% more radioactive than the ends. This result was also expected because the middle disks are slightly closer at all times to the generator target. In neither the A nor B configurations were significant differences observed between disks that occupied corresponding positions in the rotator. Results found for the A and B configurations indicated that homogeneous irradiation might be obtained with the outer cylinder of the rotator inclined about 45 O with respect to the generator target plane (configuration C). Test proved that the angle 0 (Figure 2) required was slightly less than 45’. Results obtained with an angle of about 40” are presented in Table 11. It can be seen from these data that the activation gradient along the samples was practically eliminated. The mean values of the disks for each of the five positions in the rotator indicate that the middle disks may have been about 0.3% less radioactive than the ends. The mean values for the samples also indicate a decrease of about 0.5% from sample A to sample E. Since the disks were measured in the order given in Table 11, part of the trend among samples might have been due to small errors in decay corrections resulting from an error in the assumed half-life of 18F. To determine if systematic differences among samples and disk positions are significant when compared to the random measurement error, a two-way analysis of variance (ANOVA) was made on the data of Table I1 ( 4 ) , assuming them to be homoscedastic. The results of this test showed that a sample bias and a disk-position bias were significant at the 0.05 but not at the 0.025 level of significance. Although small differences among samples and disk positions seem probable, most of these effects can be attributed to disks AS, B5, and E2 which differ from the overall mean much more than the other disks. The results presented in Tables I and I1 have demonstrated the ability of the rotator to average out the inhomogeneous neutron flux of neutron generators for multiple sample activation. Careful control of the deuteron beam on the generator target is thought to be unnecessary. The activation of (4) C. A. Bennett and N. L. Franklin, “Statistical Analysis in. Chemistry and the Chenlical Industry,” pp. 358-79, Wiley, New York (1961).
Table 11. Results (Counts/Gram at to) Obtained for Irradiations of Teflon Disks in “C” Configuration Sample A B
C D E
Mean values for disk positions
1,795,139 1,780,039 1,785,347 1,789,938 1,795,082
2 1,789,340 1,780,487 1,781,633 1,781,237 1 ,771 ,291
Disk position 3 1,787,169 1,795,936 1,787,399 1,782,322 1,775,712
4 1,795,579 1,793,509 1,784,355 1,783,152 1,799,226
5 1,801,852 1,805,095 1,787,875 1,789,427 1,788,919
Mean values for samples 1,793,815 1,791,013 1,785,599 1,785,215 1,782,046
1,789,109
1,780,797
1,785,708
1,787,164
1,794,633
1,787,482
1
both homogeneous and inhomogeneous samples can be precisely controlled. Homogeneous samples can be irradiated with the rotator in any configuration if the samples to be compared are placed in corresponding positions in the rotator. Long ( 5 / 8 inch) and/or inhomogeneous samples must be irradiated in the C configuration if they are to be uniformly activated. The sample rotator in its present form is primarily useful for activation analysis of elements that produce radionuclides with half-lives of several minutes or hours. It would not, for example, be useful for measuring trace amounts of oxygen which forms 7.4-second *6N. For such analyses sample transfer by a pneumatic system to and from the rotator would be required. Although the rotator might be designed for pneumatic sample transfer, there would be little advantage in irradiating more than two samples simultaneously when short-lived radionuclides are measured. Some increase in sensitivity would be achieved by enlarging the rotator for increased sample size. Measurements have
shown that a sample in the rotator (configuration C) is exposed to w 1 / 8 of the fast neutron flux value it receives when taped to the generator face plate. A further decrease in flux exposure would be obtained in a larger rotator. Calculations, based on the inverse square law, indicate that for samples longer than inch to be uniformly activated, the rotation angle in the C configuration would have to be altered. There are, of course, sources of error in fast neutron activation analysis -e.g., neutron self-shadowing and gamma-ray self-absorption by the sample-not caused by irradiation configuration. Although the rotator does not eliminate the effect of neutron self-shadowing, the well known technique of irradiating standards and unknowns of similar matrix (composition and density) can be fully exploited to reduce self-shadowing, errors in activation analysis.
RECEIVED for review July 27, 1967. Accepted September 5, 1967.
Device for Transfer and Dilution of Radioactive Gases Lawrence A. Elfers and Mark Herman1 National Center for Air Pollution Control, 1055 Laidlaw Avenue, Cincinnati, Ohio 45237
THE MOST
COMMON applications of radionuclides in tracer studies involve their use in liquid media. Their transfer and dilution present little or no contamination problem (1). Radioactive gases are being employed as tracers in air pollution studies in this laboratory, and others (2) have applied them in studies of analytical methods and instruments. The tracer gas must often be transferred from a glass ampoule into an appropriate cylinder so that the gas can be diluted under high pressure. To accomplish this task, an inexpensive device was designed, which, in one operation, breaks the glass ampoule, transfers the tracer gas into a steel cylinder, and dilutes it under high pressure with inert gas. The device, shown in Figure 1, consists of a series of No. 316 stainless steel pipe sections and fittings, joined in such a
Present address, Chemical Engineering Department, University of California, Berkeley, Calif. (1) R. T. Overman and H. N. Clark, “Radioisotope Techniques,”
McGraw-Hill,New York, 1960. (2) P. Urone, J. B. Evans, and C. M. Noyes, ANAL.CHEM., 37,1104 (1965).
fashion as to provide a barrel for the guidance of a projectile and a larger compartment for the glass ampoule. The projectile, a stainless steel ball, is propelled by the pressure of the diluent gas. One-way check valves must withstand pressures higher than the maximum diluent gas pressure. Prior to use, the receiving cylinder is evacuated. The apparatus is divided at the 11/4inch union (No. 9). The steel ball is then inserted into the appropriate half of the system; and the glass ampoule, containing the radioactive gas is placed in the other half. The two are then reconnected by means of the stainless steel union. After valve No. 22 is closed, valves No. 3 and No. 26 are opened. The shutoff valve on the high pressure diluent gas cylinder (No. 1) is then opened with a brisk twist, and the pressurized gas causes the steel ball to shoot forward and break the glass ampoule. A sintered stainless steel filter (No. 15) (pore diameter 60 p ) prevents broken glass from entering the receiving cylinder. After the pressure in the two cylinders has reached equilibrium (about 1000 psi), the cylinder valves are closed and the pipe system is slowly vented through the vent valve (No. 22) into an appropriate scrubber system before the gas in the pipe is released to a fume hood. VOL. 39, NO. 14, DECEMBER 1967
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