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A system for the enrichment of low-level tritium in large volume hydrogen samples by gas chromatography
International
Journal of Applied Radiation
and Isotopes, 1969, Vol. 20, pp. 813-819.
Pergamon Press. Printed in Northern Ireland
A System for the Enrichment of Low-Level Tritium in Large Volume Hydrogen Samples by Gas Chromatography H. PERSCHKE, International
Atomic
Energy
M. B. A. CRESPI* Agency,
Scibersdorf
and G. B. COOK
Laboratory,
A-2444
Seibersdorf,
Austria
(Receiued 20 Mny 1969) A chromatographic system is described in which amounts of about one mole of hydrogen containing tritium at very low levels can be enriched 40 times with 100 per cent tritium The system is based on previous work by the authors with smaller samples and has recovery. It appears useful lo been tried successfully at lo-i3 mole per cent tritium concentrations. concentrate samples for measuring purposes when these are available only in up to one mole quantities. UN SYSTEME D’ENRICHISSEMENT DE FAIBLE CONCENTRATION DE TRITIUM DANS DES ECHANTILLONS DE GRAND V’OLUME D’HYDROGENF.. A L’AIDE DE LA CHROMATOGRAPHIE EN PHASE GAZEUSE On decrit un chromatographe en phase gazeuse a l’aide duquel on peut enrichir d’utt facteur 40, environ, une mole d’hydrogene avec un rendement de 100 pourcent. Cette methode est basee sur un travail precedent des auteurs ci-dessus nommes, qui concernait des petites quantites d’hydrogene et a CtC employee avec succts pour de concentration de tritium de IO-l3 pourcent de mole. Cette methode est valable pour drs mesures d’echantillons qui mettent en jeu des quantites ne depassant pas, environ, une mole. CIICTEMA &JDl OEOTAIIlEIIIIfI E:ECbMA B EO
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EIN SYSTEM ZUR ANREICHERUNG VON TRITIUM TRATION IN WASSERSTOFFPROBEN VON GROSSEM GASCHROMATOGRAPHIE
NIEDRIGER VOLUMEN
KONZENMITTELS
Es wird ein Gaschromatograph beschrieben, mit dessen Hilfe sehr kleine Mengen Tritium in etwa ein Mol Wasserstoffgas 40 ma1 mit lOO:/,iger Ausbeute angereichert werden konnen. Die Methode basiert auf einer frtiheren Arbeit obiger Autoren mit kleineren M’asserstoffmengen und wurde erfolgreich fur Tritiumkonzentrationen von lo-r3 Molprozent erprobt. Diese Mcthode ist dann von Vorteil, wenn Proben fur Messzwecke angereichert werden sollen und nur in Mengen bis etwa ein Mol verfiigbar sind. * Permanent address: Aires, Argentina. 1
Comision
National
de Energia 813
Atomica,
Avenida
de1 Libertador
8250,
Buenos
811
H. Perschke, M. B. A. Cresfii and G. B. Cook
INTRODUCTION ‘1‘HIESEPARATION of the isotopes of hydrogen by differential adsorption at low temperatures has been under study for some years and gas chromatographic peaks for all the isotopic and isomeric molecular species of hydrogen have been identified from several adsorption media.(r) rllso, a theory explaining the underlying effect has been developed by WHITE et al.@) ‘The studies have originated direct techniques for the analysis of isotope mixtures and have opened new possibilities for their preparative separation. ‘This is of special interest for the determination of tritium, in which very low concentrations are currently measured radiometrically on enriched material, and attempts have been made to process hydrogen samples of large volume with this purpose in mind. ‘l’hrce adsorbents have been tried for this purpose, namely, palladium, ferric oxide coated alumina and molecular sieves. Hoyt”) enriched tritium in hydrogen gas by zonal heating of columns filled with approx. 1400 g of palladium, having a capacity for processing 70 1. hydrogen corresponding to a 50 ml water sample. The tritium recovery reported was 60 per cent in the first 500 ml of gas evolved. HOROWITZ and GAT(~) studied the processing capacity of alumina columns and suggested a procedure for large samples in which tritium is continuously stripped from a closed loop of hydrogen carried by helium which is reloaded by exchange with the water sample. They did not try it in the laboratory, however, and the largest sample actually handled in their batch experiments is reported to be 65 crn’r NTP. Pollowing a similar approach, AKIITAR and SMITH(~) built and operated an apparatus in which small batches taken from the gas sample are directed to an alumina column and processed one after another. The enriched fractions are collected together in a separate charcoal adsorption column. The operations are automatically controlled by a time-programmed switching device and at the end of the experiment the charcoal column is desorbed and the enriched material measured. The authors succeeded in detecting at least 10-s mole per cent of the tritium enriched from a IO-” mole per cent sample processed in 10 batches, but
failed when 100 batches of a IO-r0 mole per cent sample were tried. Apparently, the partial pressure of the desired isotope was too small in the last mixture for the collector column to work efficiently. Molecular sieves are not as selective as alumina for the different isotopes but their capacity is higher, presenting potential interest for on-step processing of large samples without excessively bulky apparatus. This line of work has been followed mainly at the present laboratory, where CRESPI and PERSCHKE’Q processed samples of 200 cm3 NTP with a main separation column 165 cm long and less than .5 mm in dia. recovering 100 per cent of the tritium enriched nearly 20 times. The technique consisted in moving the adsorbed hydrogen by means of simultaneous heat displacement and helium elution through the column and then through a second, much smaller one, to improve the separation. ‘The final band, enriched in tritium, was collected in the last U-shaped retention column of small capacity from which it was later desorbed for measuring. The procedure was further developed by PERSCHKE(~) who with a main column 2 m long and 10 mm diameter recovered tritium enriched 60 times from 1.5 1. samples at 100 per cent yield. ‘l’he present paper deals with the application of the technique to samples of 20 1. volume, which appears enough for most envisaged applications. A system is described and experimental conditions are given for the quantitative recovery of the tritium contained in that volume of hydrogen. The system has been built and operated in the laboratory and has handled with negligible memory cIfect samples of levels as low as lo3 T.U.” giving enrichment factors of the order of 40. WORKING
CONDITIONS
Preliminary experiments(‘) had shown that 20 1. of hydrogen could be handled by the 3sections column described under “Apparatus” below and seen in Fig. 2, and that 96 per cent of the tritium contained in the sample could be recovered enriched ten times. To establish the * T.U. : Tritium tration corresponding mole per cent.
unit = Unit to (T)/(H)
of tritium concen= 10P1’ or lo-l6
Enrichment
I
of low-leael
tritium in large volumehydrogen samples by gas chromatography
123456 LENGTH
OF RETENTION
COLUMN(M)
FIG. 1. Tritium recovery as function of the length of the retention column for a 20.0-l. sample. (Each point average of two expcriments.) conditions for complete recovery, retention columns of 5 mm dia. and different lengths were attached to the end of the main column and several samples of the same volume processed. The results are shown in Fig. 1, where it is seen that a length of about 450 cm is necessary to retain the whole tritium activity. The capacity of a column of this length can be calculated to be 2.3 1. and the corresponding enrichment is, therefore, less than 9. Studies were also done regarding the influence of the helium flow rate and the speed of the heat displacement on the tritium yield. The figures used in the preliminary experiments (300-350 cm3 helium/min and about 2 hr total
815
warm-up period) were arrived at partly from experience with smaller apparatus and partly as a compromise between estimated theoretical values for the different diameters of the column sections. As the values used might not be optimal, it appeared advisable to investigate the effect of their variation on the separation ability of the column. Systematic experiments were then performed by attaching a retention column of fixed length to the end of the main column and measuring the percentage of tritium retained from identical samples. The results showed that the warm-up period must be of the order of 100 min or longer and the helium flow rate no less than 140-l 50 ml/min. Shorter warm-up periods give much poorer separations, and backmixing starts to appear when the helium flow rate falls below 130 ml/min leading to The flow rate was tested first erratic results. with samples of small volume (20 ml) and it was found that the separations at 150 ml/min vvere considerably better than at higher values. but Tvhen the same conditions were used on 20-l. samples, with the column heavily loaded and large amounts of adsorption heat evolved, the difference was practically nil. I’herefore, values of 1CO xnin warm-up period and 150 ml/nun helium flow rate were adopted for normal operation. It appeared obvious from the results obtained
#=
E
F
G
FIG. 2. Enrichment system (explanation in text)
He
816
H. Perschke, hf. B. A. Crespi and G. B. Cook
in these experiments that a limit of less than 10 times enrichment is the maximum attainable when the main column only is used for the separation. ,4 second processing column was then placed after the main one, in agreement with previous experience, to try to enlarge the enrichment factor. The length of this second column was chosen to be ii.6 m, which appeared enough to hold the whole tritium peak, as shown above, providing at the same time some extra length in case of small unverifiable variations in the separation efficiency of the main column. Further experimentation proved that a retention column of total capacity about 500 ml placed after the second one would retain the whole of the tritium activity present in the enriched band. This would result in an enrichment factor of 40, and the final system was established following these conditions. APPARATUS The general assembly is given in Fig. 2. It follows the general lines of the previous one described’“), to which the reader is referred to clarify details. The sample is collected in two molecular sieve traps A connected in series and capable of holding over 20 1. of hydrogen when kept in liquid nitrogen. The traps are of 600 ml volume and contain 400 g of Linde 13X, & in. pellets, each. molecular sieve Occasionally, and when more permanent sample storage is necessary, traps of pyrophoric uranium similar to G in Fig. 2 are used. ‘Ihe desorption from the traps has to be carried out keeping under control the heat evolved when the sample is readsorbed in the main column to assure a sharp band in it. This is achieved by moving down the cooling baths of the traps by means of a synchronous motor (B) switched on and off by a light barrier circuit coupled to the rotameter C and adjusted to stop the motor when the desorption flow exceeds 600 ml/min. ‘I’he main separation column D is made of glass and consists of three sections connected by short lengths of 8 mm id. tubing. The first two sections are of 28 cm length and 50 mm i.d. and the third of 30 cm length and 20 mm i.d. All three are filled with molecular sieve 5 A of 0.5-0.8 mm grain size. A similar column made from brass or copper gave unsatisfactory
results, because only when the Dewar was completely withdrawn from the column the hydrogen desorbed and moved totally without being fractionated at all. The second and third columns (E and F) made of copper tubing wound into a spiral did not show this effect. In this case however, the entire spiral remained within the covered Dewar vessel when withdrawn from the liquid nitrogen. They were filled with a finer grain (0.3-0.5 mm) of the sieve. Column E is 5% m long and 5 mm i.d. and column 17 (the retention column) 2.3 m long and 3 mm id. The main column is kept cold by a special Dewar container 115 cm long and 10 cm i.d. (dotted lines in Fig. 2) which is lowered at an adjustable constant speed by means of a synchronous motor acting on a pulley. Similar systems actuated by hand are used to remove the smaller columns from the liquid nitrogen. In these cases, a speed of 1.5 cm/min was found appropriate. The flow of the helium carrier gas is controlled by means of a differential pressure regulator (Brooks series 8900), not shown in Fig. 2, connected to rotameter C. ‘l’he pyrophoric uranium trap G is used to collect the enriched sample at room temperature as UH, after the experiment. It is made of a U-shaped stainless steel tube 30 cm long and 24 mm id. filled with 70 g of pyrophoric uranium powder dispersed in about the same amount of silica gel. ‘l’he by absorbing hydrogen trap is “prepared” from a cylinder at 330°C in uranium lumps and decomposing the UH, at 55O’C; by heating with a controlled temperature furnace. SAMPLE
PREPARATION
The procedure used to reduce the water sample to hydrogen is essentially the same already reportedt6) but a new furnace, capable of handling the large samples in a reasonable short time and built ofstainless steel, is employed. The The furnace is illustrated in Pig. 3. supporting grid, placed at 20 cm from the bottom, ensures that the magnesium filling is entirely at 550°C when the reduction starts. The position of the heating mantle allows the direct connection of the sample bulb to the furnace, excluding the cooling helix used in the previous design and providing smaller
Enrichment
of low-level tritium in large volume hydrogen samples by gas chromatography
TAINLESS
43 mm
817
STEEL
i.d.
SAMPLE
FIG. 3. Furnace for reduction of water sample.
reflux and quicker operation. For the heating mantle a Ni-chrome heating coil on a ceramic tube (Pyrostea, type 72-6023, Schweizer Isola Werke, Breitenbach bei Basel, Switzerland) was used which dissipated approximately 1000 w. With the furnace described, the quantitative reduction of 16 ml of liquid water to 20 1. NTP of hydrogen gas takes less than 40 min. MEASUREMENTS Mixtures resulting from tracer experiments were controlled by measuring directly in hydrogen form as described in.(s) Enriched
samples obtained in low level experiments were reacted with ethylene by the procedure given by PERSCHKE’~) and counted as ethane low background installation in a special described by CAMERON and PAYNE(~). RESULTS The tracer experiments made to adjust the working conditions, as reported above, were done with hydrogen of approximately O-015 pCi/l specific activity which corresponds to an absolute disintegration rate of 33.000 dpm/l in the sample before enrichment. This is about 6 . IO6 T.U. or 6 . lo-lo mole percent
H. Perschke, M, B. A. Crespi and G. B. Cook
818
tritium, far above the levels for which enrichment previous to measurement may be of interest. To check the behaviour of the system at low levels, a series of experiments were performed with two “1000 T.U.” tritiatcd water standards available at the laboratory. ‘The absolute activity corresponding to 1000 T.U. ( 1o-13 molt per cent tritium) is 5.7 dpm/l hydrogen XTX’. The results obtained are given in Table 1. ‘1’AnLE
1.
hw
Cxpcrimcnt 1 2 3 4
level
experiments with tritiated water standards Standard used (‘T.U.)
Measured value (T.U.)
930 !I30 1040
924 & 20 919 -‘; 20 1025 Ir 20 1070 -I 20
1040
The reproducibility in thcsc and other experiments was 2 per cent or better. Xlemory cffccts were conlirrned to be negligible by processing a lo5 ‘1.U. sample followed by the 930 T.U. standard. ‘l‘he result of this expcrimcnt was also within the 2 per cent of the known value. Further experiments with higher activities showed that less than 0.1 per cent of the tritium of a sarnplc containinatcs the next one processed. DISCUSSION ‘I’he results reported (‘1 show that the COIK++ sions obtained regarding the possibility to crutch tritiuin quantitatively by gas solid chromatography on niolccular sieves can bc cxtendcd to volumes as big as 20 l., which correspond to 1G g of water, with laborator) equipment of reasonable size. The enrichment takes place primarily in the main column, which dimensions determine the processing capacity of the system as a whole. The procedurc has been applied to samples of lo-l3 mole per cent tritium concentration (1000 T.U.) with good results. While no experiments have been made at lower levels, the reproducibility of the results and the low cross-contamination observed indicates that the method can probably be applied successfully to the lower ranges of natural concentrations. When considered in the frame of the most
important use of tritium cnrichmcnt, namely, preconcentration for low level nieasurerncnts the present technique oKcrs interesting possibilities in the case of samples of limited size. ‘fhcsc arise in many fields of research, such as water or hydrogen sampled from artificial satcllitcs, high atmosphere water vapour, small biological samples, residues of fusion experiments, samples produced by nuclear reactions in accelerators, dating of old wines, glaciology, ctc-. In thcyc cases, the unique characteristic of chrontatographic cnrichmcnt of recovering the total amount of tritiurn contained in the sattlplc i,, a definite advantage against other enrichment procedures. With electrolysis, f.i.. rccoveric.s are of the order of 80 per cciit for cnrichinctlt factors of 20 or smaller and dctcrioratc at higher values, bci:ig as low as 30 per cettt ill some cascs.(‘ol AI bcttcr situation is prcscntcci by thermal dilrusion cnricllmcnl,(~l”g) \vltic~It appears capable of giving high enriclnncltt factors with on!y a small loss of tritium. \L’lictt sample size is not a ploblCiI1, Ilo~vcver, it is cli&ult for tcClllliqllCS of enrichnicnt in the gaseous state to coinpcte with clectrol~~sis~ since this is able to handle liquid water and can deal with much birgger initial san~pic~ without too bulky apparattts. ‘I’hr tii!lC necessary lo pcrforni 11x2 c.hro:natcb grapllic cnrichitient of a 20-l. gii\ saiiiplc is al)out 4 ltr, ai follo~vs: ia) waler wcighirtg and reduction: -10--X1 tnitt; (1,) sarnplc adsorption: 30-40 inin; (c) scparatioii: 120--130 rnin; iti) displaccntcnt front retention column : ! 0 rllill. i2n apparatus can thus handle two snriil~lcs per 8-hr shift and it is estimated that, at the present state of autornatization, four apparatus can easily Ire served by two trained operators. ‘1%~ output of such an installation would be 011 the average of one enriched sarnplc per hour. comparing favourably in time and pcrsonncl with other currently applied tcchniqucs, including simultaneous electrolysis of several samplcs.(“” It would be advisable in this case, for economic reasons, to strip the used helium from hydrogen by means of uranium and to send it under prcssurc to a storage tank for rcusc. The combination of electroly&i \vitli gils chromatography, already suggested in 0111 previous publication’“), is also an interesting possibility of application of tlic prcscnt 20-I.
Enrichment of low-level tritium in large volume hydrogen samples by gas chromatography technique. The combination of both procedures can push the overall enrichment to values in the neighbourhood of 1000 with 70-80 per cent tritium recovery, a performance of interest for the measurement of very low activity samples for dating purposes. Acknowledgement-The
collaboration
Measuring Laboratory measurement of the low acknowledged.
of the Tritium of the I.A.E.A. for the level samples is gratefully
REFERENCES 1. AKHTAR S. and SMITH H. A. Chem. Rev. 64, 261 (1964) ; WEST D. L. and MARSTON A. L. J. Am.
them. Sot. 86, 4731 (1964). 2. WHITE D. et al. J. them. Phys. 32, 72 (1960); ibid. 34, 2189 (1961); J. Chim. Phys. 60, 29 (1963); ibid. 60, 97 (1963).
819
3. HOY J. E. Science, N.Y. 161, 464 (1968). 4. BOROWITZ J. L. and GAT F. R. ht. J. a/@. Radiat. Isotopes 15, 401 (1964). 5. AKHTAR S. and SMITH H. A. 6th ConJ: on Radiocarbon and Tritium Dating, Pullman, Wash., 1965. CONF-650652, 1965, p. 491. CRESPI M. B. A. and PERSCHKE H. Int. J. a+@. Radiat. Isotopes 15, 569 (1964). PERSCHKE H. Nature, Lond. 209, 1021 (1966). PERSCHKE H. Int. J. appl. Radiat. Isotopes 17, 358 (1966). CAMERON J. F. and PAYNE B. 6th Conf. on
Radiocarbon and Tritium
Dating, Pullman, Wash.,
1965. CONF-650652, 1965, p. 454. 10. CAMERON J. F. Radioactive Dating and Methods of Low Level Counting p. 543. IAEA, Vienna
(1967). 11. RAVOIRE J. et al. ibid., p. 639. 12. VERHAGEN B.Th. ibid., p. 657.
Journal of Applied Radiation
and Isotopes, 1969, Vol. 20, pp. 813-819.
Pergamon Press. Printed in Northern Ireland
A System for the Enrichment of Low-Level Tritium in Large Volume Hydrogen Samples by Gas Chromatography H. PERSCHKE, International
Atomic
Energy
M. B. A. CRESPI* Agency,
Scibersdorf
and G. B. COOK
Laboratory,
A-2444
Seibersdorf,
Austria
(Receiued 20 Mny 1969) A chromatographic system is described in which amounts of about one mole of hydrogen containing tritium at very low levels can be enriched 40 times with 100 per cent tritium The system is based on previous work by the authors with smaller samples and has recovery. It appears useful lo been tried successfully at lo-i3 mole per cent tritium concentrations. concentrate samples for measuring purposes when these are available only in up to one mole quantities. UN SYSTEME D’ENRICHISSEMENT DE FAIBLE CONCENTRATION DE TRITIUM DANS DES ECHANTILLONS DE GRAND V’OLUME D’HYDROGENF.. A L’AIDE DE LA CHROMATOGRAPHIE EN PHASE GAZEUSE On decrit un chromatographe en phase gazeuse a l’aide duquel on peut enrichir d’utt facteur 40, environ, une mole d’hydrogene avec un rendement de 100 pourcent. Cette methode est basee sur un travail precedent des auteurs ci-dessus nommes, qui concernait des petites quantites d’hydrogene et a CtC employee avec succts pour de concentration de tritium de IO-l3 pourcent de mole. Cette methode est valable pour drs mesures d’echantillons qui mettent en jeu des quantites ne depassant pas, environ, une mole. CIICTEMA &JDl OEOTAIIlEIIIIfI E:ECbMA B EO
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EIN SYSTEM ZUR ANREICHERUNG VON TRITIUM TRATION IN WASSERSTOFFPROBEN VON GROSSEM GASCHROMATOGRAPHIE
NIEDRIGER VOLUMEN
KONZENMITTELS
Es wird ein Gaschromatograph beschrieben, mit dessen Hilfe sehr kleine Mengen Tritium in etwa ein Mol Wasserstoffgas 40 ma1 mit lOO:/,iger Ausbeute angereichert werden konnen. Die Methode basiert auf einer frtiheren Arbeit obiger Autoren mit kleineren M’asserstoffmengen und wurde erfolgreich fur Tritiumkonzentrationen von lo-r3 Molprozent erprobt. Diese Mcthode ist dann von Vorteil, wenn Proben fur Messzwecke angereichert werden sollen und nur in Mengen bis etwa ein Mol verfiigbar sind. * Permanent address: Aires, Argentina. 1
Comision
National
de Energia 813
Atomica,
Avenida
de1 Libertador
8250,
Buenos
811
H. Perschke, M. B. A. Cresfii and G. B. Cook
INTRODUCTION ‘1‘HIESEPARATION of the isotopes of hydrogen by differential adsorption at low temperatures has been under study for some years and gas chromatographic peaks for all the isotopic and isomeric molecular species of hydrogen have been identified from several adsorption media.(r) rllso, a theory explaining the underlying effect has been developed by WHITE et al.@) ‘The studies have originated direct techniques for the analysis of isotope mixtures and have opened new possibilities for their preparative separation. ‘This is of special interest for the determination of tritium, in which very low concentrations are currently measured radiometrically on enriched material, and attempts have been made to process hydrogen samples of large volume with this purpose in mind. ‘l’hrce adsorbents have been tried for this purpose, namely, palladium, ferric oxide coated alumina and molecular sieves. Hoyt”) enriched tritium in hydrogen gas by zonal heating of columns filled with approx. 1400 g of palladium, having a capacity for processing 70 1. hydrogen corresponding to a 50 ml water sample. The tritium recovery reported was 60 per cent in the first 500 ml of gas evolved. HOROWITZ and GAT(~) studied the processing capacity of alumina columns and suggested a procedure for large samples in which tritium is continuously stripped from a closed loop of hydrogen carried by helium which is reloaded by exchange with the water sample. They did not try it in the laboratory, however, and the largest sample actually handled in their batch experiments is reported to be 65 crn’r NTP. Pollowing a similar approach, AKIITAR and SMITH(~) built and operated an apparatus in which small batches taken from the gas sample are directed to an alumina column and processed one after another. The enriched fractions are collected together in a separate charcoal adsorption column. The operations are automatically controlled by a time-programmed switching device and at the end of the experiment the charcoal column is desorbed and the enriched material measured. The authors succeeded in detecting at least 10-s mole per cent of the tritium enriched from a IO-” mole per cent sample processed in 10 batches, but
failed when 100 batches of a IO-r0 mole per cent sample were tried. Apparently, the partial pressure of the desired isotope was too small in the last mixture for the collector column to work efficiently. Molecular sieves are not as selective as alumina for the different isotopes but their capacity is higher, presenting potential interest for on-step processing of large samples without excessively bulky apparatus. This line of work has been followed mainly at the present laboratory, where CRESPI and PERSCHKE’Q processed samples of 200 cm3 NTP with a main separation column 165 cm long and less than .5 mm in dia. recovering 100 per cent of the tritium enriched nearly 20 times. The technique consisted in moving the adsorbed hydrogen by means of simultaneous heat displacement and helium elution through the column and then through a second, much smaller one, to improve the separation. ‘The final band, enriched in tritium, was collected in the last U-shaped retention column of small capacity from which it was later desorbed for measuring. The procedure was further developed by PERSCHKE(~) who with a main column 2 m long and 10 mm diameter recovered tritium enriched 60 times from 1.5 1. samples at 100 per cent yield. ‘l’he present paper deals with the application of the technique to samples of 20 1. volume, which appears enough for most envisaged applications. A system is described and experimental conditions are given for the quantitative recovery of the tritium contained in that volume of hydrogen. The system has been built and operated in the laboratory and has handled with negligible memory cIfect samples of levels as low as lo3 T.U.” giving enrichment factors of the order of 40. WORKING
CONDITIONS
Preliminary experiments(‘) had shown that 20 1. of hydrogen could be handled by the 3sections column described under “Apparatus” below and seen in Fig. 2, and that 96 per cent of the tritium contained in the sample could be recovered enriched ten times. To establish the * T.U. : Tritium tration corresponding mole per cent.
unit = Unit to (T)/(H)
of tritium concen= 10P1’ or lo-l6
Enrichment
I
of low-leael
tritium in large volumehydrogen samples by gas chromatography
123456 LENGTH
OF RETENTION
COLUMN(M)
FIG. 1. Tritium recovery as function of the length of the retention column for a 20.0-l. sample. (Each point average of two expcriments.) conditions for complete recovery, retention columns of 5 mm dia. and different lengths were attached to the end of the main column and several samples of the same volume processed. The results are shown in Fig. 1, where it is seen that a length of about 450 cm is necessary to retain the whole tritium activity. The capacity of a column of this length can be calculated to be 2.3 1. and the corresponding enrichment is, therefore, less than 9. Studies were also done regarding the influence of the helium flow rate and the speed of the heat displacement on the tritium yield. The figures used in the preliminary experiments (300-350 cm3 helium/min and about 2 hr total
815
warm-up period) were arrived at partly from experience with smaller apparatus and partly as a compromise between estimated theoretical values for the different diameters of the column sections. As the values used might not be optimal, it appeared advisable to investigate the effect of their variation on the separation ability of the column. Systematic experiments were then performed by attaching a retention column of fixed length to the end of the main column and measuring the percentage of tritium retained from identical samples. The results showed that the warm-up period must be of the order of 100 min or longer and the helium flow rate no less than 140-l 50 ml/min. Shorter warm-up periods give much poorer separations, and backmixing starts to appear when the helium flow rate falls below 130 ml/min leading to The flow rate was tested first erratic results. with samples of small volume (20 ml) and it was found that the separations at 150 ml/min vvere considerably better than at higher values. but Tvhen the same conditions were used on 20-l. samples, with the column heavily loaded and large amounts of adsorption heat evolved, the difference was practically nil. I’herefore, values of 1CO xnin warm-up period and 150 ml/nun helium flow rate were adopted for normal operation. It appeared obvious from the results obtained
#=
E
F
G
FIG. 2. Enrichment system (explanation in text)
He
816
H. Perschke, hf. B. A. Crespi and G. B. Cook
in these experiments that a limit of less than 10 times enrichment is the maximum attainable when the main column only is used for the separation. ,4 second processing column was then placed after the main one, in agreement with previous experience, to try to enlarge the enrichment factor. The length of this second column was chosen to be ii.6 m, which appeared enough to hold the whole tritium peak, as shown above, providing at the same time some extra length in case of small unverifiable variations in the separation efficiency of the main column. Further experimentation proved that a retention column of total capacity about 500 ml placed after the second one would retain the whole of the tritium activity present in the enriched band. This would result in an enrichment factor of 40, and the final system was established following these conditions. APPARATUS The general assembly is given in Fig. 2. It follows the general lines of the previous one described’“), to which the reader is referred to clarify details. The sample is collected in two molecular sieve traps A connected in series and capable of holding over 20 1. of hydrogen when kept in liquid nitrogen. The traps are of 600 ml volume and contain 400 g of Linde 13X, & in. pellets, each. molecular sieve Occasionally, and when more permanent sample storage is necessary, traps of pyrophoric uranium similar to G in Fig. 2 are used. ‘Ihe desorption from the traps has to be carried out keeping under control the heat evolved when the sample is readsorbed in the main column to assure a sharp band in it. This is achieved by moving down the cooling baths of the traps by means of a synchronous motor (B) switched on and off by a light barrier circuit coupled to the rotameter C and adjusted to stop the motor when the desorption flow exceeds 600 ml/min. ‘I’he main separation column D is made of glass and consists of three sections connected by short lengths of 8 mm id. tubing. The first two sections are of 28 cm length and 50 mm i.d. and the third of 30 cm length and 20 mm i.d. All three are filled with molecular sieve 5 A of 0.5-0.8 mm grain size. A similar column made from brass or copper gave unsatisfactory
results, because only when the Dewar was completely withdrawn from the column the hydrogen desorbed and moved totally without being fractionated at all. The second and third columns (E and F) made of copper tubing wound into a spiral did not show this effect. In this case however, the entire spiral remained within the covered Dewar vessel when withdrawn from the liquid nitrogen. They were filled with a finer grain (0.3-0.5 mm) of the sieve. Column E is 5% m long and 5 mm i.d. and column 17 (the retention column) 2.3 m long and 3 mm id. The main column is kept cold by a special Dewar container 115 cm long and 10 cm i.d. (dotted lines in Fig. 2) which is lowered at an adjustable constant speed by means of a synchronous motor acting on a pulley. Similar systems actuated by hand are used to remove the smaller columns from the liquid nitrogen. In these cases, a speed of 1.5 cm/min was found appropriate. The flow of the helium carrier gas is controlled by means of a differential pressure regulator (Brooks series 8900), not shown in Fig. 2, connected to rotameter C. ‘l’he pyrophoric uranium trap G is used to collect the enriched sample at room temperature as UH, after the experiment. It is made of a U-shaped stainless steel tube 30 cm long and 24 mm id. filled with 70 g of pyrophoric uranium powder dispersed in about the same amount of silica gel. ‘l’he by absorbing hydrogen trap is “prepared” from a cylinder at 330°C in uranium lumps and decomposing the UH, at 55O’C; by heating with a controlled temperature furnace. SAMPLE
PREPARATION
The procedure used to reduce the water sample to hydrogen is essentially the same already reportedt6) but a new furnace, capable of handling the large samples in a reasonable short time and built ofstainless steel, is employed. The The furnace is illustrated in Pig. 3. supporting grid, placed at 20 cm from the bottom, ensures that the magnesium filling is entirely at 550°C when the reduction starts. The position of the heating mantle allows the direct connection of the sample bulb to the furnace, excluding the cooling helix used in the previous design and providing smaller
Enrichment
of low-level tritium in large volume hydrogen samples by gas chromatography
TAINLESS
43 mm
817
STEEL
i.d.
SAMPLE
FIG. 3. Furnace for reduction of water sample.
reflux and quicker operation. For the heating mantle a Ni-chrome heating coil on a ceramic tube (Pyrostea, type 72-6023, Schweizer Isola Werke, Breitenbach bei Basel, Switzerland) was used which dissipated approximately 1000 w. With the furnace described, the quantitative reduction of 16 ml of liquid water to 20 1. NTP of hydrogen gas takes less than 40 min. MEASUREMENTS Mixtures resulting from tracer experiments were controlled by measuring directly in hydrogen form as described in.(s) Enriched
samples obtained in low level experiments were reacted with ethylene by the procedure given by PERSCHKE’~) and counted as ethane low background installation in a special described by CAMERON and PAYNE(~). RESULTS The tracer experiments made to adjust the working conditions, as reported above, were done with hydrogen of approximately O-015 pCi/l specific activity which corresponds to an absolute disintegration rate of 33.000 dpm/l in the sample before enrichment. This is about 6 . IO6 T.U. or 6 . lo-lo mole percent
H. Perschke, M, B. A. Crespi and G. B. Cook
818
tritium, far above the levels for which enrichment previous to measurement may be of interest. To check the behaviour of the system at low levels, a series of experiments were performed with two “1000 T.U.” tritiatcd water standards available at the laboratory. ‘The absolute activity corresponding to 1000 T.U. ( 1o-13 molt per cent tritium) is 5.7 dpm/l hydrogen XTX’. The results obtained are given in Table 1. ‘1’AnLE
1.
hw
Cxpcrimcnt 1 2 3 4
level
experiments with tritiated water standards Standard used (‘T.U.)
Measured value (T.U.)
930 !I30 1040
924 & 20 919 -‘; 20 1025 Ir 20 1070 -I 20
1040
The reproducibility in thcsc and other experiments was 2 per cent or better. Xlemory cffccts were conlirrned to be negligible by processing a lo5 ‘1.U. sample followed by the 930 T.U. standard. ‘l‘he result of this expcrimcnt was also within the 2 per cent of the known value. Further experiments with higher activities showed that less than 0.1 per cent of the tritium of a sarnplc containinatcs the next one processed. DISCUSSION ‘I’he results reported (‘1 show that the COIK++ sions obtained regarding the possibility to crutch tritiuin quantitatively by gas solid chromatography on niolccular sieves can bc cxtendcd to volumes as big as 20 l., which correspond to 1G g of water, with laborator) equipment of reasonable size. The enrichment takes place primarily in the main column, which dimensions determine the processing capacity of the system as a whole. The procedurc has been applied to samples of lo-l3 mole per cent tritium concentration (1000 T.U.) with good results. While no experiments have been made at lower levels, the reproducibility of the results and the low cross-contamination observed indicates that the method can probably be applied successfully to the lower ranges of natural concentrations. When considered in the frame of the most
important use of tritium cnrichmcnt, namely, preconcentration for low level nieasurerncnts the present technique oKcrs interesting possibilities in the case of samples of limited size. ‘fhcsc arise in many fields of research, such as water or hydrogen sampled from artificial satcllitcs, high atmosphere water vapour, small biological samples, residues of fusion experiments, samples produced by nuclear reactions in accelerators, dating of old wines, glaciology, ctc-. In thcyc cases, the unique characteristic of chrontatographic cnrichmcnt of recovering the total amount of tritiurn contained in the sattlplc i,, a definite advantage against other enrichment procedures. With electrolysis, f.i.. rccoveric.s are of the order of 80 per cciit for cnrichinctlt factors of 20 or smaller and dctcrioratc at higher values, bci:ig as low as 30 per cettt ill some cascs.(‘ol AI bcttcr situation is prcscntcci by thermal dilrusion cnricllmcnl,(~l”g) \vltic~It appears capable of giving high enriclnncltt factors with on!y a small loss of tritium. \L’lictt sample size is not a ploblCiI1, Ilo~vcver, it is cli&ult for tcClllliqllCS of enrichnicnt in the gaseous state to coinpcte with clectrol~~sis~ since this is able to handle liquid water and can deal with much birgger initial san~pic~ without too bulky apparattts. ‘I’hr tii!lC necessary lo pcrforni 11x2 c.hro:natcb grapllic cnrichitient of a 20-l. gii\ saiiiplc is al)out 4 ltr, ai follo~vs: ia) waler wcighirtg and reduction: -10--X1 tnitt; (1,) sarnplc adsorption: 30-40 inin; (c) scparatioii: 120--130 rnin; iti) displaccntcnt front retention column : ! 0 rllill. i2n apparatus can thus handle two snriil~lcs per 8-hr shift and it is estimated that, at the present state of autornatization, four apparatus can easily Ire served by two trained operators. ‘1%~ output of such an installation would be 011 the average of one enriched sarnplc per hour. comparing favourably in time and pcrsonncl with other currently applied tcchniqucs, including simultaneous electrolysis of several samplcs.(“” It would be advisable in this case, for economic reasons, to strip the used helium from hydrogen by means of uranium and to send it under prcssurc to a storage tank for rcusc. The combination of electroly&i \vitli gils chromatography, already suggested in 0111 previous publication’“), is also an interesting possibility of application of tlic prcscnt 20-I.
Enrichment of low-level tritium in large volume hydrogen samples by gas chromatography technique. The combination of both procedures can push the overall enrichment to values in the neighbourhood of 1000 with 70-80 per cent tritium recovery, a performance of interest for the measurement of very low activity samples for dating purposes. Acknowledgement-The
collaboration
Measuring Laboratory measurement of the low acknowledged.
of the Tritium of the I.A.E.A. for the level samples is gratefully
REFERENCES 1. AKHTAR S. and SMITH H. A. Chem. Rev. 64, 261 (1964) ; WEST D. L. and MARSTON A. L. J. Am.
them. Sot. 86, 4731 (1964). 2. WHITE D. et al. J. them. Phys. 32, 72 (1960); ibid. 34, 2189 (1961); J. Chim. Phys. 60, 29 (1963); ibid. 60, 97 (1963).
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3. HOY J. E. Science, N.Y. 161, 464 (1968). 4. BOROWITZ J. L. and GAT F. R. ht. J. a/@. Radiat. Isotopes 15, 401 (1964). 5. AKHTAR S. and SMITH H. A. 6th ConJ: on Radiocarbon and Tritium Dating, Pullman, Wash., 1965. CONF-650652, 1965, p. 491. CRESPI M. B. A. and PERSCHKE H. Int. J. a+@. Radiat. Isotopes 15, 569 (1964). PERSCHKE H. Nature, Lond. 209, 1021 (1966). PERSCHKE H. Int. J. appl. Radiat. Isotopes 17, 358 (1966). CAMERON J. F. and PAYNE B. 6th Conf. on
Radiocarbon and Tritium
Dating, Pullman, Wash.,
1965. CONF-650652, 1965, p. 454. 10. CAMERON J. F. Radioactive Dating and Methods of Low Level Counting p. 543. IAEA, Vienna
(1967). 11. RAVOIRE J. et al. ibid., p. 639. 12. VERHAGEN B.Th. ibid., p. 657.