the enlarged fluorographs do not exhibit sharl)ly defined contours. Even a t its present state of development, however, it, is evident that by integrating all the fluorescence spectral parameters into a stereoenvelope, the technique yields a "stereofingerprint" of pure fluorescent compounds. hccordingly, it should permit unique characterization of even very closely related structures. The major contour pattern of the polynuclear hydrocarboni5 appears to be characteristic of the basic skeletal ring structure. By combining the method with such isolation t,echniques as gas chromatography, paper and thin layer chromatography, electrophoresis, etc., it should be possible to make positive identification and determination of highly fluorescent substances at levels far below those required for other techniques of characterization. For this purpose, when the pure compound is
available for comparison, the spectra need not be corrected for the distortions inherent in the instrumental parameters. It is recommended, however, that publication of data, corrected or uncorrected, be referenced to a convenient standard, such as quinine, so that the reader can a t least orient the data in terms of the response of his own instrument. Continuing development is directed toward increasing scanning speed, improving sensitivity and resolution, and extending the technique to recording of phosphorescence phenomena. The last involves a fourth parameter, decay time, which may well be integrated into a stereographic representation. The improved technique will be applied to some thirty polynuclear hydrocarbons now a t hand and to as many others as we can obtain. It is believed that when sufficient data,
corrected for instrumental distortions, have been acquired, not only will specific identification of these hydrocarbons be possible, but also fluorescence characteristics can be correlated with detailed structural features to aid in structure determination of new fluorescent compounds. LITERATURE CITED
(1) General Electric Co., Semiconductor
Products
Department,
Schenectady,
N.Y., Transistor Manual, 6th edition,
p. 194, 1962. (2) Ibid., p. 167. ( 3 ) Ibid.. D . 196. (4)Saw&, E., Hauser, T. R., Stanley, T. W., Intern. J . .4ir Pollution 2, 253 (1960). ( 5 ) Van Duuren, 3. L., J . S a t . Cancer Inst. 21, 1 (1958).
Pithburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 7 , 1963, Pittsburgh, Pa.
Improved Techniques for Routinely Counting l o w levels of Tritium and Krypton-85 Mary L. Curtis and H. L. Rook, Monsanto Research Corp., Mound Laboratory, Miarnisburg, Ohio
beta emitters R in the gas phase, ofcounted in the ADIOACTIVE ASSAY
Geiger or proportional region as part of the count,er filling, is a well-known analytical method (1, 3 ) . The application of this method to routine counting, using new techniques for precisely and rapidly analyzing tritium in helium-3, and krypt,on-85 in xenon, will be described. An analytical method for krypton-85 by gamma counting will also be presented. The tritium analyses are made in conjunction wit'h Mound Laboratory sales of highly purified He3. The Kr85 in xenon analysis was developed to evaluate the effectiveness of methods of purifying xenon intended for use in scintillation counters. Numerous samples varying from 1 0 - 2 to lo-'* mole yo are analyzed daily by these techniques. Small amounts of nonradioactive impurities, such as water vapor, osygen, nitrogen, argon, and carbon dioxide, are also present in these gases
added in parallel with the existing 47-pf. capacitor to eliminate double pulsing. For proportional counting, the output of the counting tube was connected to the input of the pre-amplifier of the counter. A schematic of the vacuum system (used for gas transfer) is shown in Figure 2. A 2- X 2-inch XaI(T1) well crystal and a Model 39-12 Radiation Instrument Development Laboratory 400-channel analyzer were used for the gamma analyses of Kr*5. Tritium Analysis. GEIGERCOUNTI K G OF TRITIUM I N HELIUM-3. For counting tritium, two counting tubes L A n o d e Terminal
Cathode Terminal T
b
;
y
r
lnlernol Silver Mirror
EXPERIMENTAL
Apparatus. T h e counting tubes (Figure 1 ) were made of 2-inch 0.d. borosilicat,e glass tubing. T h e y were silvered internally by the Brashear process (2) to the same length as the 2mil diameter tungsten anode. For Geiger counting, shielded probes were used to connect the output of the counting tubes to the Geiper input of a Xuclear AIeasurements Corp. Model PC-313 proportional counter. h 105-ohm resistor was added in series with the esiRting 68,000-ohm resistor in the Geiger input circuit to attenuate the pulses, and a 56-pf. capacitor was
Figure 1 . tube
U Borosilicate glass counting
were evacuat'ed and filled with a mixture of 98y0 helium and 2% isobutane to a pressure of 340 torr. After the background was determined, the tubes were placed over the standard volumes on the sample introduction system, and evacuated with stopcock J closed, which separates the cold from the hot side of the vacuum system (Figure 2). Stopcocks B and C (over the standard volumes) and K (leading to the vacuum rack) were closed, and t,he sample was allowed to expand into the standard volumes from the gas sample container through A . The pressure indicated by the micromanometer as read by a cathetometer was measured, stopcocks D and B were closed, and B and C were opened, allowing the sample to espand into the counting tubes. The amount of gas in t'he tubes a t atmospheric pressure and room temperature (20' C.) was computed from the pressure and comparative volumes. The counting tubes were transferred to tapered joints JT and A' on the cold side of the vacuum rack, and the helium isobutane mixture was added to a pressure of 340 torr by: opening stopcock H and the valve controlling the counting gas inlet; opening the stopcock on one of the counting tubes and allowing the counting gas to enter until the mercury reached a predetermined point on the manometer; and closing the stopcock on the counting tube and the valve on the counting gas inlet. Counting gas was introduced into the second tube containing a duplicate sample in a similar manner. A voltage plateau was determined for each tube filling since plateaus vary slightly from sample to sample. The tritium concentration is given by: VOL. 36, NO. 10, SEPTEMBER 1964
2047
H3 (mole %)
=
(c.p.m./cc.) (100)VI (5.32 X 10") V,
where
Vl
effective volume of the counting tube as defined by the inside diameter and anode length. V B= total volume of the tube. 5.32 X 10lz = disintegration rate i,er cubic centimeter of tritium =
,411 counting rates were corrected for coincidenre. A resolving time factor was determined for each counting tube. A sample of about 2 x 106 counts per minute was analyzed, diluted by a factor of 100 and re-analyzed. After repeating the complete operation to guard against dilution error, the resolving time was calculated from the difference between the high and low counting rate samples. The resolving times were in the range of minute for the Geiger region. AHSORPTIONOF TRITIUM.Care was taken to prevent oil vapor from the vacuum pumps from ccndensing on the vacuum system, sample containers, and counting tubes. Oil vapors formed a film to which tritium adhered, making it impossible to get a representative sample and resulting in serious background problems. During the development of the analytical techniques, as much as 25% of the tritium in the samI)le was adsorbed on the walls of the ccntainer. This was evidenced by a ral id decrease in counting rate with time uhen a counting tube was filled nith sample and counting gas. After several hours equilibrium was reached and the counting rate remained constant. W'hen the sample was removed and the tube filled with counting gas, the process was reversed. The same phenomenon was observed in sample dilution flasks. Diluting the sample with protium did not minimize the effect. Oil vapor condensation was minimized by adding stopcock J (Figure 2 ) to the vacuum system, and closing stopcock5 J and L when the liquid coolant had to be removed to de-gas the cold trap. J was also kept closed during sample introduction and evacuation of the hot side of the system, so that the left side of the system remained cold. Thus tritium adsorption on the walls of the system, which would introduce tritium with counting gas as desorption takes place, is prevented. Since sample concentrations varied by a factor of lolo, maintaining Ion backgrounds was essential. The tubes could be effectively decontaminated by allowing the equipment to stand in air, then evacuating and repeating the process, or cleaning the tubes with xylene and alcohol. Counting tubes were resilvered when the background became excessive. ACCURACY OF TRITIUM.\NALYSIS. In the range of 10-8 mole yo or greater, the relative standard deviation
2048
ANAlYTlCAL CHEMISTRY
Figure 2.
Vacuum
I!
system
of this method of analysis is +2%, between duplicate measurements of a large number of samples and between multiple analyses of single samples. From theoretical considerations of wall and end effects, and from comparison of analyses with counting tubes of different dimensions, the absolute accuracy wa5 estimated to be within 5%. This was verified by diluting a sample containing 53y0 tritium a5 determined by mass spectrometry. The r e d t s obtained by counting were within 2% of those computed from the mass spectrometer value. Low LEVELTRITIUM COUNTING.A separate vacuum system, counting tubes, and sample bottles were retained for levels of 1 0 - ~mole Yo tritium or less. Isobutane (7 torr) was introduced into 2-liter counting tubes and sample gas (tritium in helium-3) was added to a pressure of 340 torr. Samples containing mole yo tritium were counted with precision. Krypton-85 Analyses. PROPORTIONAL COCNTIXG O F KRYPTON-85 I N XENON. When counting samples of Kr55 in xenon, the best results were obtained using methane as the counting gas and counting in the proportional region a t a pressure of 25 torr. The apparatus and techniques are the same as those used for tritium counting. Serious contamination problems encountered in the early phases of the work were solved, as in the case of tritium, with procedures described. The Kr85 concentration is given by. (using the nomenclature of Equation 1) K+5 (mole %) =
(c.p.m./cc.) (100) VI 3.51 X 10" Vz
where 3.51 X 10l2 is the disintegration rate per cc. of Kr85. GAMMACOUNTINGOF KRYPTON-85. Samples containing 5 x lo-' mole % or more of Kr85 were analyzed by gamma counting. The samples in identical containers were placed in the well of the NaI crystal previously described. The gamma counting rate, in counts per minute per mm. of sample pressure
-
To Pumps
in the KrS6 photopeak, was multiplied by a predetermined factor to yield mole yo Kr85. The factor (3.67 X was determined by comparing the gamma counting rate of a n iinber of samples with results from internal gas proportional counting. .\CCURACY O F KRYPTOS-8.5 .\NALYSIS. The accuracy of the methods for Krg5 analysis was tested by gamma counting a series of dilutions of a sample of 0.267% Krg5 as determined by mass spectrometry. Results agreed within 2% of the mass spectrometry value. Since the amount of KrS5,as determined by gamma counting, was calculated using a factor dependent upon proportional counting results, the accuracy of both counting methods was established. CONCLUSION
The equipment and techniques described above have been in use for routine analysis for about a year. They have proved to be fast and trouble free. One hour is required for a complete analysis with duplicate samples by Geiger or proportional counting. Gamma counting KrS5 requires about 5 minutes per sample. ACKNOWLEDGMENT
The authors acknowledge with gratitude the encouragement and advice of L. L. Rentz, the contribution in the early phases of the work of Elton hlurphy, and the modifications in the electronic circuitry made by Fred J. Vescio. LITERATURE CITED
(1) Bernstein, W., Ballentine, R., Rev. Sci. Instr. 21, 158 (1950). ( 2 ) Hodgman, C. I),, Wemt, R . C., Sellev, S. M.,"Handbook of Chemistry"and Physics," 43rd ed., p. 3293, Chemical Rubber Publishing Co., Cleve-
land, 1961.
( 3 ) Mann,
W .R., Parkenson, G. B., Ret'.
Sci. Instr. 20, 41 (1949).
