Determination of Gases in Neutron Irradiated Beryllium Oxide by Gas Chromatography Hydrogen, Tritium, Helium, Nitrogen, Oxygen, and Carbon Monoxide Anthony J. Morris and Malcolm Thackray Australian Atomic Energy Commission, Research Establishment, Lucas Heights, N.S.W., Australia
A method is described which allows the determination of helium, hydrogen, tritium, oxygen, nitrogen, and carbon monoxide in irradiated BeO. The Be0 i s dis. solved in a mixture of sulfuric and phosphoric acids by refluxing in an atmosphere of carbon dioxide. The carbon dioxide is then frozen out and the residual gas is analyzed by gas chromatography and/or mass spectrometry. By the use of a delay line and a plastic scintillation detector, the tritium content of the hydrogen can be portrayed on the same chromatogram as the helium and hydrogen. Tritium remaining in the acid i s determined by liquid scintillation counting. The ratio of free to combined tritium i s approximately
1:lO. BERYLLIUM OXIDE has been studied for several years by this laboratory as a possible moderator for high-temperature gascooled reactors. The material tested to date has proved unsuitable owing to structural instability at high neutron doses. Part of this structural instability is caused by gases produced by fast neutron reactions of beryllium. Not only are helium and tritium produced by this process, but the removal of beryllium ions from the lattice results in an excess of oxygen ions. Part of this oxygen appears as gas on dissolution and part reacts with impurities to form mainly carbon monoxide. Depending on operating temperature, diffusion of some of these gases through the beryllium oxide lattice is possible, permitting formation of bubbles at grain boundaries. A knowledge of the amount and composition of gas contained in irradiated beryllium oxide is thus of value in correlation with measurement of physical properties. Most investigators of gas in irradiated beryllium oxide have been concerned mainly with the rate of diffusion of helium (1) or tritium ( 2 ) during post-irradiation annealing. Although both of these gases are completely released at sufficiently high temperatures, it has been found more convenient to employ dissolution procedures in determining their total amounts. Myer, White, and Rubin (3) determined helium in this material by dissolving it in potassium hydrogen fluoride in a sealed vessel plated with silver. Gases not condensible in liquid nitrogen were oxidized in hot copper oxide to convert hydrogen isotopes to water, which was frozen out before the remaining helium was determined by gas chromatography. Felber (4) and Hibbits (5) dissolved the beryllium oxide by refluxing with a mixture of sulfuric and phosphoric acids under an argon atmosphere. When dissolution was complete, (1) Y. Cartaret, J. Bareau, and J. Elston, Proc. Brit. Cer. SOC.,7 , 363 (1967). (2) K. T. Scott and L. L. Wassell, U . K . Report A.E.R.E., R5102,
(1966).
(3) A. S. Myer, Jr., J. C . White, and I. B. Rubin, 4th Gatlinburg Conference on Nuclear Reactor Technology, TID 7606, p 158,
all gases except helium were removed with an activated charcoal trap. Felber determined the helium by mass spectrometry and Hibbits by gas chromatography. The activated charcoal trap prevents the determination of all gases except helium because even hydrogen is partially absorbed at the temperature of liquid nitrogen. For the determination of tritium, this severe fractionation is not required because it is easily converted to water and determined by scintillation counting. This type of procedure using acid dissolution with an oxygen sweep was described by Palmer, Roman, and Whitfield (6). The method described in this report, which was developed independently, is similar to that of Hibbits, but the dissolution is carried out under carbon dioxide. (Appreciable gas pressure above the acid mixture is necessary to permit the dissolution to proceed in a reasonably short time). The advantage of using carbon dioxide is that it can be easily removed by differential freezing. Thus all the more permanent gases extracted from the sample are present in the gaseous state and can be pumped into a small volume, ready for examination either by gas chromatography or by mass spectrometry. This modification has permitted us to determine six gases, not just helium, on a routine basis. In our work, it proved difficult to determine in a single operation all gases present-namely helium, hydrogen, tritium, oxygen, nitrogen, and carbon monoxide-either by gas chomatography or by mass spectrometry. With a 5A Molecular Sieve chromatographic column, argon carrier was satisfactory for the determination of hydrogen, helium, and nitrogen but the thermal conductivity detector proved too insensitive for the determination of the quantities of carbon monoxide and oxygen encountered. Helium carrier was satisfactory for oxygen, nitrogen, and carbon monoxide but was too insensitive for hydrogen. Mass spectrometry with our MS3 instrument resolved all gases present except nitrogen and carbon monoxide (both of which have a mass number of 28). Tritium was present in insufficient concentration to be determined by mass spectrometry. For a complete and rapid determination of all gases extracted from irradiated beryllium oxide, it was therefore necessary to use both gas chromatography and mass spectrometry. After eluting from the chromatographic column, the separated gases were carried through a delay line before passing through a spiral of scintillating plastic tubing viewed by a photomultiplier tube. The tritium content of the hydrogen was determined by scintillation counting of its beta activity. The tritium was measured directly on a calibrated ratemeter or as an additional peak on the recorder. Combined tritium, remaining as water in the acid mixture, was measured by dilution of the acid, distillation of a small aliquot, and liquid scintillation counting.
Oct. 1960.
(4) F. F. Felber, Jr., 2nd Conference Nuclear Reactor Chemistry, TID 7622, p 254 (1961). ( 5 ) J. 0. Hibbits, Tuluntu, 13, 151 (1966).
(6) A. R. Palmer, D. Roman, and H. J. Whitfield, J . Nucl. Materials, 14, 141 (1964). VOL. 41, NO. 3, MARCH 1969
467
Figure 1. Apparatus for gas extraction and chromatography EXPERIMENTAL
The apparatus for gas extraction and chromatography is shown in Figure 1. Apparatus. The gas extraction system consisted of a 50-ml round-bottom borosilicate glass dissolution flask, a watercooled Liebig reflux condenser, a micro bunsen burner, and a vacuum system with manometers of mercury and di-butyl phthalate, Toepler pump, and Perkin-Elmer gas sampling valve. Gas Chromatography was performed using a 7-foot 6-inch x 1/4-inch0.d. stainless-steel column, Linde 5A Molecular Sieve (34-44 mesh British Standard), a Perkin-Elmer thermistor detector, operating at 7 V dc and 25 O C temperature, and a Negretti Zambra precision pressure regulator (7). Equipment used for tritium counting was a Nuclear Enterprises 801 plastic scintillator detector, a 2-inch diameter E.M.I. 60975/A photomultiplier operating at 1150 V, an ECKO pre-amplifier, and an ECKO N 600A ratemeter. For liquid scintillation counting, a Nuclear-Enterprises 201 liquid scintillation counter was used. Calibration of the Gas Chromatography Detectors. The gas chromatography apparatus was calibrated on a weekly basis by introducing known pressures of pure gas into the vacuum system and transferring to the gas chromatography column in a manner identical to that used for gases resulting from the dissolution. Hydrogen standards of known tritium content were used to calibrate the scintillation counter. In the early stages of the work, wet calibration runs were also performed in which known gas pressures were present in the system prior to freezing the acid mixture in the dissolution flask. These tests showed that errors due to the solubility of the various gases in the acid were very small and could generally be included in a small total pressure correction to allow for the cooling of the flask and trap. Extraction. Before starting a run, 10 ml of an qui-volume mixture of concentrated air-free sulfuric and phosphoric acids is introduced into the dissolution apparatus. The Be0 sample, usually consisting of small pellets weighing between 100 and 500 mg, is placed in an arm of the vacuum system directly above the flask. The acid mixture is frozen with solid carbon dioxide to prevent evaporation of water while the system is pumped down to about 1 mm Hg. Taps 1, 3, 4, 5, 7, 8, and 11 are closed. Carbon dioxide is admitted into the distillation flask through Tap 12 until the ( 7 ) M. Thackray and L. W. Hillen, J . Chromatog., 10,309 (1963). 468
ANALYTICAL CHEMISTRY
t secs
Figure 2. Separation of hydrogen, tritium, and helium mercury manometer reads approximately 50 cm. The evacuation and filling procedure is repeated to remove final traces of air and the sample is pushed into the flask by means of an iron rod operated by an external magnet. The acid mixture is melted and then refluxed until the sample has completely dissolved. During refluxing, Taps 6 and 2 remain open to ensure that the pressure does not exceed 76 cm of mercury. While the dissolution is proceeding the rest of the vacuum system is continuously pumped out by the mercury diffusion pump and should achieve a pressure of approximately mm Hg and should maintain this for several minutes when isolated from the pump. When all of the beryllium oxide was dissolved (high-fired material requires about 1 hour of refluxing), the acid is cooled to room temperature and again frozen with carbon dioxide. Tap 7 is opened and the COz gas is condensed by liquid nitrogen in the cold trap. The pressure of the residual gas is measured with a McLeod gauge, the volume occupied by the gas having been determined previously by expanding the known volume, V , at a known pressure into this larger volume and measuring the resulting pressure. The necessary small corrections are made to the pressure measurement to allow for cooling the contents of the flask with dry ice and the trap with liquid nitrogen. For gas chromatography, the gas is transferred with the aid of the Teopler pump to the small volume enclosed by Taps 10 and 11. When all the gas is transferred, as indicated by a negligible pressure reading on the McLeod gauge, Taps 2, 3, 5 , 6, 7 , and 9 are closed and Taps 4 and 11 are opened. The pressure of gas is readily measured on the oil manometer. The volume of gas, V , is then transferred by the Teopler pump to the gas sample valve. This represents about one-tenth of the total gas extracted. As it appears impossible to make the gas sample valve of the gas chromatograph completely vacuum-tight, the sample volume is continuously evacuated until the gas is ready for chromatography. After completion of the chromatographic analysis, the remaining gas can be transferred to a sample flask for analysis by mass spectrometry when necessary.
Table I. Determination of Gases from Irradiated Beryllium Oxide Sample I1 Sample I Be0 irradiated 8 X lozon/cm2 at 535 "C Be0 irradiated 5 X lozon/cm2at 75 "C Wt. of sample 0.1843 g Wt. of sample 0.1292 g Total gas evolved 0.308 cm3 Total gas evolved 0.091 cm3 Gas chromatography Mass spectrometry Gas chromatography Mass spectrometry 0 2 = 14.4% 0 2 = 14.7z 0 2 = 13.7% 0 2 = 13~8% Nz = 29.7% N2 = 2 8 . 5 z Nz = 13.0% (CO Nz) = 43.0% He = 32.8% CO = 30.0% He = 5.9% CO not detected Tritium = 1000 pCi/ml H2 = 23.2% Tritium = 590 pCi/ml H? = 37.0% gas gas
+
Sample no. 1
2 3
4 5
Table 11. Determination of Helium Content of Irradiated Beryllium Oxide Volume Volume of Irradiation dose of gas gas per g Sample wt, g and temperature extracted, pl BeO, pl % He by GC 0.251 0.285
0.265 0.175 0.114
2 . 0 x 1020n/cm2 at 100 "C 3 . 0 X 10'0 n/cm2 at 100 "C 7.5 x 1020 n/cmZat 100 "C
6.0 X 1020n/cmz at 600 "C 8.7 X lozan/cm2at 750 'C
Measurement of Helium, Hydrogen, and Tritium. These gases are separated and measured by chromatography at a temperature of 0 "C. Argon is the carrier gas and the flow rate is 60 ml/minute. The uncorrected retention volumes are: helium, 72 ml; and hydrogen, 102 ml. The separation of the three gases is shown in Figure 2. Tritium Detector. The system of counting tritium in an efficient gas proportional counter (5) is tedious and subject to errors from nonequilibration, because gas dilutions of the order of l o 5 are necessary owing to the high specific activity of the gas released at each extraction. If a very inefficient beta scintillation detector is used, no dilution of the gas is required. The effluentfrom the thermal conductivity detector passes through a delay line of 50 feet of l/x-inch polyethylene tubing before entering the scintillating plastic tubing in the counter. The spiral tubing is surrounded by silicone fluid in a light-tight housing and is viewed by the photomultiplier. (Both the counter and photomultiplier are enclosed in a 2-inch-thick lead castle to keep the background interference to a minimum). Pulses from the preamplifier are fed to a ratemeter and thence to the recorder. The use of the delay line enables the hydrogen peak to be recorded on the single point recorder before the recorder is switched over and placed in circuit with the ratemeter. The tritium, counted in pulses per second on the ratemeter, also appears as an additional peak on the recorder, adjacent to that due to the total hydrogen. The column effluent passes to an efficient fume hood. The background count of the plastic scintillator remained constant over long periods showing that no appreciable exchange of its hydrogen with the tritium gas occurred. This can probably be ascribed to the fact that exposure is only to elemental tritium and the contact time of a few seconds per analysis is too short to permit Wilzbach labelling to occur. Liquid Scintillation Counting of Combined Tritium. A 5-ml aliquot of the remaining acid solution is diluted to 500 ml with demineralized water and the solution is distilled until 50 ml of distillate has been collected. A 0.1-ml aliquot is diluted to 10.0 ml with dioxane-based scintillant or "scintillator." Standard tritium solutions are used for comparison. Determination of Oxygen, Nitrogen, and Carbon Monoxide. These gases are determined with the same molecular sieve column at 25 "C. The helium flow rate was 60 ml/minute.
175 314
608 116 70
700 1100 2300 660 610
8.5 6.3 8.7 29.6 26.6
He by MS 9.3 6.0 8.2 30.2 26.8
The retention volumes are: Oxygen Nitrogen Carbon monoxide
= = =
3 min 12 min 30 min
= = =
180 ml 720 ml 1800 ml
Both oxygen and nitrogen are determined by measuring their peak heights. Carbon monoxide, because of its long retention time, is eluted as a broad peak which is not suitable for the preparation of a standard graph based on peak heights. A standard graph was therefore prepared by chromatographing various aliquots of carbon monoxide and measuring the area under the peak with a planimeter. When the three gases are determined chromatographically, a suitable aliquot of the extracted gas sample is retained for the determination of helium and hydrogen by mass spectrometry. The changeover from helium carrier gas to argon carrier gas and vice versa is readily accomplished, because both gases are connected to the same common line into the gas sampling valve. The flow to each gas is controlled by opening or closing the respective Hoke valve. The plumbing arrangement is shown in Figure 1 . When the carrier gas is changed, the column is conditioned by purging it with gas overnight. RESULTS AND DISCUSSION
A complete determination of gases extracted from two different samples of irradiated beryllium oxide is shown in Table I. Oxygen, nitrogen, carbon monoxide, and tritium were determined by gas chromatography using helium as the carrier gas. Hydrogen and helium were determined by mass spectrometry. Oxygen was also determined by mass spectrometry and the results are in good agreement with results shown for gas chromaliography. The percentages of all gases (except tritium) are given on a volume per volume basis. The method of dissolving the ceramic by refluxing in the acid mixture under an atmosphere of carbon dioxide which can be easily removed by freezing in liquid nitrogen, permits all the more permanent gases to be determined. Argon was found in trace quantities and was assumed to VOL. 41, NO. 3, MARCH 1969
469
come from the air absorbed on the surface of the beryllium oxide pellet. In the irradiated samples, the ratio of oxygen t o nitrogen was significantly higher than the same ratio for air, which suggests that some of the oxygen in the irradiated beryllium oxide was released in molecular form on dissolution. In an unirradiated sample, the oxygen t o nitrogen ratio was similar to that of air. Many samples were analyzed for helium content only, because the Materials Division of the Research Establishment was mainly interested in helium content produced in the irradiated beryllium oxide. Table I1 shows the comparison of the helium content, as determined by gas chromatography and mass spectrometry, for a wide variety of irradiated samples. The helium content is proportional t o dose at 100 “C but considerable losses are apparent above 600 “C. The lowest detection limit for tritium was 6 pCi, but this was quite ade-
quate for the levels of activity encountered. The highest tritium content was about 0.18 Ci per ml of gas extracted. Most of the tritium formed (-9Oz) remained in the acid as tritiated water and this is at variance with the results of diffusion experiments ( 2 ) in which most of the tritium can be released in elemental form when B e 0 is strongly heated. The results suggest that tritium atoms are trapped in the lattice in a form which is readily oxidized by the hot acid but not by the free oxygen produced during the irradiation of BeO. The introduction of a scintillation detector in the gas chromatograph has made the method of determining the ratio of free-to-combined tritium simple and fairly rapid and suggests that the method could be used in the study of many other problems concerned with the reactivity of tritons with solids.
RECEIVED for review May 31, 1968. Accepted November 12, 1968.
Determination of Carbon Skeletons of Microgram Amounts of Steroids and Sterols by Gas Chromatography after Their High Temperature Catalytic Reduction P. M. Adhikary and R. A. Harkness Department of Paediatric Biochemistry, Royal Hospital for Sick Children and Department of Clinical Chemistry, University of Edinburgh, Scotland, U. K.
A method is described for the complete reduction of microgram quantities of steroids and sterols to their parent hydrocarbons. The apparatus consists of a siliconized glass tube containing platinum catalyst coated on siliconized glass beads. A slow stream of hydrogen is passed through the heated catalyst containing tube and the samples are injected into the catalyst bed. The reduction products are trapped for analysis by gas-liquid chromatography. The effects of a number of factors involved in the method have been investigated and its reliability has proved satisfactory. Complete reduction of many steroids and sterols could be reproducibly achieved at catalyst temperatures of 170-90 OC, but the yield was related to the structure of the starting material. The method has been used to identify the parent hydrocarbons of many steroids and sterols. CHROMATOGRAPHIC TECHNIQUES, especially gas-liquid chromatography, GLC, are very useful in the separation and quantitation of minute quantities of compounds of biological origin ( I ) . The number of possible structures which natural products may possess is much larger than the number of hydrocarbon skeletons to which their functional groups are attached. Thus complete reduction of these natural products to the “common denominator” of the hydrocarbon skeleton represents a considerable simplification (2), and the hydrocarbons produced can be characterized in microgram quantities by GLC. This type of technique has been used t o study
insect attractants (3), the queen bee substance (4, alkaloids
(3, fatty acids from wool wax (6),and to convert components of petroleum t o the saturated hydrocarbons (7). Steroids and sterols are important classes of the naturally occurring compounds, but their individual identification is complicated because they occur only in small quantities in the presence of many closely related substances. “Carbon skeleton” chromatography has been extended by the present work to the identification of steroids (8). This article describes a method for the catalytic reduction of steroids and sterols, studies factors affecting the method, and examines its reliability. It has been used for the complete reduction of about 5-10 pg of steroids and sterols, and the yields of different hydrocarbon products from several types of compounds have been studied at various temperatures. Different types of catalysts and supports for the catalysts have been investigated. The products obtained were consistent at a constant temperature and provided information about the structure of the carbon skeleton of the starting material. EXPERIMENTAL
Apparatus and Materials. HYDROGENATOR. The basic principle of the apparatus is similar to that of the “Carbon skeleton determinator” described by Beroza and Acree (3) ~
~~
(4) R. K. Callow and N. C . Johnston, Bee World, 41, 152 (1960).
(1) E. C. Homing, W. J. A. VandenHeuvel, and B. G. Creech, in “Methods of Biochemical Analysis,” Vol. XI, D. Glick, Ed., Interscience New York, 1963, pp 69-147. (2) I. E. Bush, “The Chromatography of Steroids,” Pergamon Press, London, 1961, p 300. (3) M. Beroza and F. Acree, Jr., J. Assoc, OBc. Agr. Chemists, 47, l(1964). 470
ANALYTICAL CHEMISTRY
( 5 ) M. Beroza, J. Org. Chem., 28, 3562 (1963). (6) D. T. Downing, Z. H. Kranz, and K. E. Murray, Ausr. J . Chem.,
13, 80 (1960). (7) C. J. Thompson, H. J. Coleman, R. L. Hopkins, and H. T. Rall, J. Gas Chromatogr., 5, 1 (1967). (8) P. M. Adhikary and R. A. Harkness, Biockem. J., 105, 40P
(1967).
