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Benzene synthesis by low temperature catalysis for radiocarbon dating
Geochimica
et
Cosmochimira Acts, 1963,
Vol. 27, pp. 797 to 804.
Pergamon PressLtd. Printedin Northern Ireland
Benzene synthesis by low temperature catalysis for radiocarbon dating JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP and DOXALD W. HOOD Department of Oceanography 8r Meteorology, Agricultural and Mechanical College of Texas, College Station, Texas (Received
13 August
1962; in revisedform 21 December 1962).
Abstract-A method is described for the ambient temperature synt’hesis of pure benzene from acetylene in 50-60 per cent yields that, is suitable for low-level liquid scintillation counting. Application of this method t,o carbon dating essentially eliminates previous problems encountered in the use of liquid scintillation counting for this purpose. The method extends the sensitivity of the carbon dating method and yet, roquircxs only standard commercially available counting systems.
liquid scintillation counting, as applied to radiocarbon dating, was first introduced by ARNOLD (1954). Samples in the liquid form allow high sensitivity and precision because of the large amounts of carbon which may be incorporated into each analysis. Since 1954, many solvents have been investigated for liquid scintillation dating, but each has had its limitations. Liquid scintillation counting has been restricted by such difficulties as complicated sample preparations, explosive hazards during synthesis, the relatively small amount of dating carbon incorporated into the solvent molecule and the undesirable quenching characteristics of the solvent itself or of certain impurities introduced during the synthesis. Benzene, long recognized as a good scintillation solvent, was first used by TABIERS (1960) who utilized the high temperature pyrolitic reaction for converting acetylene into benzene. This produced a mixture of hydrocarbons with benzene as the major component,. The yield from acetylene to benzene was 50 per cent or less. It was necessary to distill and ext’ract the product to remove impurities which caused quenching of the scintillation system. Thus, a synthesis which readily produces pure benzene in greater than 50 per cent yield would offer pronounced advantages. This paper describes a synthesis of pure benzene in 50 to 60 per cent yields. The method involves an ambient temperature catalytic conversion of acetylene into benzene, essentially identical to the method described by SHAPIRO and WEISS (1957). This method almost entirely avoids the difficulties encountered previously in liquid scintillation radiocarbon dating. LOW-LEVEL
Gmeral procedure for bemene synthesis The reaction steps and yields for the preparation of benzene are shown in Table I. Reactions (1) through (5) have been described earlier by SUESS (1954), and by TAMERS et al. (1961). The critical reactions (5) and (6) have been studied and 6
797
798
JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP, and DONALD W. HOOD
certain improvements have been incorporated in these procedures. They will be described in some detail. The SrC, obtained from the high temperature reduction of S&O, with magnesium was passed through a 10 mesh screen to obtain a satisfactory particle size for introducing the solid carbide into water at reduced pressure. Fig. 1 is a diagram of the equipment for converting carbide into benzene. The ground SrC, was placed in flask A which was connected by means of rubber stoppers and flexible Tygon tubing to flask B, containing two liters of water. A vacuum was 2 to the acetylene generating flasks A and B. first applied through stopcock Stopcock 2 was closed and carbide was allowed to drop into the water at a eontrolled rate until a pressure of approximately 0.5 atmosphere resulted. A vacuum was then applied to the generating system through stopcocks 2, 3, and 4 so that Table 1. Chemical st,eps and yields in benzene preparation
Yield
Overall yield
(%)
(%)
3 (NH&CO, + SrClz + &CO, + 2NH,Cl 4 2SrCOs + 5Mg + SrC, + SrO + 5iMgO
95-98
95-98
5 SrCz + H,O + C,H, 6 3C,H, + C,H,
90 50-60
88 44-53
Reaction
1 Carbon & CO, 2 CO, + SNH,OH -c (NH,),CO,
+ SrO
gas would pass through traps C and D. Trap C was cooled with dry ice-alcohol to remove water and trap D was cooled with liquid nitrogen to collect the acetylene in the solid form. A vacuum was continually applied through stopcock 4 in order to remove the relatively large amount of hydrogen produced along with the acetylene. In this fashion 400 g of carbide could be hydrolyzed and the resulting acetylene collected in the solid state in approximately 10 minutes. This generator was designed to handle no more than 400 g of carbide at one time. After closing stopcock 2 and removing the vacuum system from stopcock 4, the liquid nitrogen was removed from trap D and the vaporized acetylene was allowed to pass into one or more of the previously evacuated acetylene reservoirs, F. Flow indicator G and trap H were purged carefully with dry nitrogen. Next the acetylene gas was displaced from reservoirs F by means of water passing through valve 6, the gas being caused to pass at a controlled rate through flow indicator G, trap H cooled with dry ice-alcohol and into catalyst chamber I where the conversion to benzene took place. Care was taken at all times to prevent water and oxygen from entering the catalyst chamber since these materials deactivate the catalyst. As the acetylene gas contacted the activated catalyst an exothermic reaction was accompanied by a purple coloration of the catalyst. This reaction front was seen to advance along the column and no acetylene escaped from the column until all the catalyst had assumed the dark color. As soon as all the acetylene had been
Benzene synthesis by low temperature catalysis for radiocarbon dating
8-l
r=
799
800
JOHN
E. KOAICES, A. F. ISBELL, J. J.
Fig. 2. Apparatus A. B. C, D,
and DONALD W. HOOD
for preparation of diborane activated benzene from catalyst column.
Vacuum pump H,O & B,H, freeze trap and C,. H,O bubblers and D,. Stopcocks to catalyst columns
E. Valve F. Quick disconnect
STIPP
catalyst
G. Cold box ( -40
and for removal
of
to -SO’%)
H. 0, regulator I. B.&I, cylmder
J. Heat tape oven R. Enclosed well vented hood
into the column, stopcock 7 was closed, and column I was disconnected between stopcock 7 and trap H. The column was placed in the same oven J ILS that shown in Fig. 2, heated to 100°C and the benzene collected in a liquid N, cooled trap under reduced pressure. After 15 minutes all the benzene was collected in the trap. The rate at which acetylene was passed into the catalyst was found to be a critical factor in determining the yield of benzene. A rapid acetylene flow produced a high catalyst contact temperature, a dark coloration of the catalyst and a flow benzene yield. A slow acetylene flow or mixing the acetylene with an inert gas such as nitrogen resulted in a lower catalyst contact temperature, a lighter coloration of the catalyst and benzene yields of as much as 60 per cent or occasionally more. The low benzene yield resulting from a high acetylene flow rate is believed to be the result of the formation of high molecular weight hydrocarbons. The more highly polymerized materials were not removed completely from the column even at 700°C and no serious attempt has been made to identify these byproducts. In those instances where the original carbon sample is small, it is best to pass oxygen through the column at elevated temperatures, burn the polymer to CO, and recycle this CO, to increase the yield of benzene. The purity of the recovered benzene was checked on numerous occasions with the aid of a Beckman Model GC-2A gas chromatograph employing an 8 ft Fluoropak passed
Benzene synthesis by low temperature catalysis for radiocarbon dating
801
column operated
at 160°C and a Silicone column at 100°C. No material other than benzene was found to be present in deteeta.ble amounts. The catalyst employed was type S-46 cracking catalyst from Houdry Process Corporation. This was obtained in the form of l/8 in. silica-alumina pellets. The catalyst was dried and activated with diborane essentially according to the method described by SHAPIRO and WEISS (1957, 1958). Fig. 2 shows the equipment used The catalyst column was a 3 x 65 cm Pyrex glass tube. One for this purpose. end was fitted t,hrough a 2 mm vacuum stopcock to a 1817 ground glass joint, F,. The opposite end was connected through stopcocks D, and E to a vacuum system. Stopcocks D, and D, were of 2 mm bore and had a compression type Teflon plug to prevent the possibility of the benzene being contaminated with stopcock grease. Although such stopcocks are not normally recommended for high vacuum work, they were found to be satisfactory for the vaccum required in this application. The catalyst chamber held approximately 230 g of catalyst, which was sufficient to produce 3-4 g of benzene. The catalyst was dried by heating at 300°C under a vacuum of 5 mm or less for three hours. It was found that a temperat,ure of 150°C’ or greater under vacuum was adequate to remove all the associated water in the catalyst. The drying and activation of the catalyst took place in a good fume hood in such a manner that it was unnecessary to disconnect the column at any t’ime. It was possible to handle two or more coIumns simultaneously. After drying, the columns were cooled to room temperature, evacuated, and diborane was admitted at the rate of approximately 30 ml per minute. When the diborane contacted the catalyst, hydrogen was evolved. The diborane flow was continued until the hydrolysis of diborane was noted in bubbler C,, which had been connected to the column through joint F, after the drying operation. The diborane flow was stopped, stopcocks D, and D, were closed and the diborane was allowed to continue to react in the column. Periodicall~r D, was opened briefly to allow some gas to escape. After two hours the catalyst was ready for use after excess diborane had been flushed from the column with dry nitrogen and the column had been heated to 150°C under vacuum for one hour. It was necessary to remove completely any excess diborane since according to the GALLERY CHEMICAL COMPASP, diborane will react with benzene at 100°C to produce triphenylborane: 6(&H, + B&C, -
2B(C,H,c,), + 6H,
If the catalyst columns were to be stored for any period of time, they were filled with dry nitrogen or helium at one atmosphere. Such columns, after storing as long as two months, have given results comparable to freshly activated columns. Although approximately five hours were required for the actual drying and activation of each column, only about 15 minutes of actual man hours were required and several columns could be activated at one time. The diborane gas is available commercially from the CALLERY CHEMICAL COMPAKY. If it is kept at a temperature of -3OY! or lower, the disproportiormtion into higher borane hydrides is very slow. WTe stored the diborane cylinders in a commercial freezer which bad been modified to maintain a temperature of -45°C.
802
JOWNE. NOAKES,A. F. ISBELL,J. J. STIPP,and DONALI)W. HOOD
Because of the toxic and inflammable properties of diborane, involving it were carried out in a well ventilated fume hood.
all operations
COUKTISG PROCEDURES Countillg vials were chosen on the basis of their ability to contain the benzene on storage and to give reproducible ba~ekground counts. None of the vials tested yielded entirely satisfactory results. Vials having plastic screw caps lost benzene through the caps at a slow rate. This loss became serious after a storage period of approximately two weeks. When the samples were frozen during storage, they gained weight apparently as a result of water adsorption. The background count of vials was found to vary rather widely. This made it necessary to carry out a background count of each vial and to discard all vials having high counts. The liquid scintillation spectrometers employed were PACKARD TRI CARB manual series 314A and automatic series 314E. Both these instruments were found to be stable and satisfactory for low-level counting. The photo-multiplier voltage was set at 930 volts and a window setting of 9-50 volts, which provided approximately 50 per cent counting efficiency and a background of approximately 15 cpm depending upon the individual vial. Since the background of the vials was pre-determined, the automatic counter with print-out recorder was found especially suitable. Several screw cap vials and one sealed vial, containing commercial benzene and phosphors, were placed in the counter with a pre-set counting time of 100 min and the instrument was then allowed to run automatically until each vial had accumulated at least 1000 total min. The screw cap vials were then filled with the samples to be dated and were counted in the manner described. A modern sample sealed into a vial having a modern count rate of 6.67 c/m[g of carbon with a background of 15.45 cts/min. was counted along with each group of unknowns. Since each vial was counted for 100 min int’ervals, any flu~tLlation in natural background variation between samples tended to average out. The sealed vials acted as a continuous check against any long t,erm shift of counter efficiency, and provided data for correcting the calculations, if necessary. Typical results obtained The complete date list obtained by this laboratory will soon be submitted for publication in Rdiocarbon; however, some results on samples previously dated by the United States Geological Survey, by the gas counting technique, and presented in Table 2, indicate the statistical uncertainty of this method as compared to the liquid scintillation technique. The ?V-723 date was det’ermined on 6.4459 g of carbon as benzene in a counting time of 1440 minutes to give 1.811 & 0.027 c/m/g with a background of 15.455 & 0,125 c/m. The absolute specific activity of modern age corrected I850 wood has been determined as 13 d/m/g/ o f carbon on a 9.1134 g sample of carbon as benzene. SUMMARY AXD CONCLUSIONS The general availability of adequate counting facilities and the ease with which carbon in a sample can be convert,ed into benzene by the method here described
Benzene synthesis by low temperature
catalysis for radiocarbon dating
803
makes a radiocarbon dating by liquid scintillation counting a most attractive method. The procedure described in this article is now being used routinely in two laboratories and is under development in several others. Inorganic and organic carbon samples of geologic, archeologic, and marine origin have been converted to benzene and dated (STIPP et al., 1962). In addition to this dating, we have carried out inter-laboratory calibrations with geologic samples from the U.S.C.S. A date list from the laboratory is soon to be submitted for publication laboratories, in Radiocarbon,. Table 2. Comparison of dates obtained by liquid scintillation by this laboratory and by gas count,ing by the U.S. geological survey Sample No.
Location
--
Liquid scintillation ----....10,820
& 190
Gas counting 10,960 * 300
W-723
Grand Forks, N.D.
W-859
Sheep Creek Alaska
5,925 & 275
5,940
5 250
W-823
Hutchins Creek fllinois
5,535
4,840
+ 300
2 175
A number of possibilities still remain for improving this method of carbon dating. It is believed that the yields reported here can be improved and we have seen step (6) occasionally give much higher yields than the 50-60 per cent mentioned in this paper. We are at present attempting to determine what important factors govern these yields and something concerning the mechanism of the acetylene to benzene reaction in the hope that the yields may be signi~ca~ltly increased. Relatively minor modifications in the counting technique may also improve significantly the precision of this method. For example, more efficient scintillators are becoming available from various sources and the use of photomultiplier tubes sealed in quartz, rather than glass envelopes, would improve the figure of merit. The generally accepted figure of merit, W/B, in which N is the net sample counting rate and B is the background has been considerably increased through the use of liquid samples. The utilization of a large carbon sample in the form of benzene greatIy increases the total activity of the sample. The background count may also be reduced by the use of smaller counting vials or by reducing the volume of the counting solution in a standard 20 ml vial (MCDOWELL, 1962). AcklzowZedgements-Dr.
E. &I. DAVIS, Anthropology Depart,ment,, University of Texas, made available the University of Texas Radiocarbon Dating Laboratory to help in the perfection of the method. The work was initiated with Grant No. NSF-10232 with the National Science Foundation and completed with Grant Xo. bob-2119(04) with the Office of Naval Research and with the Texas A & M College Fund for Organized Research. REFERENCES ARNOLD J. R. (1954) Scintillation counting of natural Science 119, 155. CALLERY CHEMICAL COMPANY, B,H, Handling Brochure.
radiocarbon;
the counting
method.
804
JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP and DONALD W. HOOD
DIETHORN W. Doctoral Dissertation, Carnegie Tech (1956), A methane proportional counter system for natural radiocarbon measurements. LEGER C. and TAMERS M. A. (1962) The counting of naturally occurring radiocarbon in the form of benzene in a liquid scintillation counter. Int. J. Appl. Rad. Iso. NO_IIES J. E., ISBELL A. F. and HOOD D. TV. (1961) Conversion of carbon dioxide to benzene for liquid scintillation counting. Trans. Amer. Geophys. Union. 42, 266. MCDOWELL, L. L. (1962) Private communication. PACK-&D INSTRUMENT COMPANY, INC. (1961) Private communication. SHAPIRO, 1. and WEISS, H. G. (1957) Cyclization of acetylene D, to benzene D,. J. Amer. Chem. sot. 79, 3294. STIPP J. J., DAVIS E. M., NOAKES J. E. and HOOVER T. E. (1962) University of Texas radiocarbon dates-l. Radiocarbon suppl. June 1962. TAATERSM. A., STIPP J. J. and COLLIER J. (1961) High sensitivit)y detection of naturally occurring radiocarbon-I: Chemistry of the counting sample. Geochim. et Cosmochim. Scta 24, 266. TA~IERS, Iii. A. (1960) Cl4 dating with the liquid scintillation count,er: A total synthesis of t,he benzcne solvent. Science 132, 668-669. WEISS, H. G. and SHAPIRO, I. (1958) Tncreascd act,ivity of silica alnmina catalyst)s. J. Amer. Chem. Sot. 80, 3195-3198.
et
Cosmochimira Acts, 1963,
Vol. 27, pp. 797 to 804.
Pergamon PressLtd. Printedin Northern Ireland
Benzene synthesis by low temperature catalysis for radiocarbon dating JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP and DOXALD W. HOOD Department of Oceanography 8r Meteorology, Agricultural and Mechanical College of Texas, College Station, Texas (Received
13 August
1962; in revisedform 21 December 1962).
Abstract-A method is described for the ambient temperature synt’hesis of pure benzene from acetylene in 50-60 per cent yields that, is suitable for low-level liquid scintillation counting. Application of this method t,o carbon dating essentially eliminates previous problems encountered in the use of liquid scintillation counting for this purpose. The method extends the sensitivity of the carbon dating method and yet, roquircxs only standard commercially available counting systems.
liquid scintillation counting, as applied to radiocarbon dating, was first introduced by ARNOLD (1954). Samples in the liquid form allow high sensitivity and precision because of the large amounts of carbon which may be incorporated into each analysis. Since 1954, many solvents have been investigated for liquid scintillation dating, but each has had its limitations. Liquid scintillation counting has been restricted by such difficulties as complicated sample preparations, explosive hazards during synthesis, the relatively small amount of dating carbon incorporated into the solvent molecule and the undesirable quenching characteristics of the solvent itself or of certain impurities introduced during the synthesis. Benzene, long recognized as a good scintillation solvent, was first used by TABIERS (1960) who utilized the high temperature pyrolitic reaction for converting acetylene into benzene. This produced a mixture of hydrocarbons with benzene as the major component,. The yield from acetylene to benzene was 50 per cent or less. It was necessary to distill and ext’ract the product to remove impurities which caused quenching of the scintillation system. Thus, a synthesis which readily produces pure benzene in greater than 50 per cent yield would offer pronounced advantages. This paper describes a synthesis of pure benzene in 50 to 60 per cent yields. The method involves an ambient temperature catalytic conversion of acetylene into benzene, essentially identical to the method described by SHAPIRO and WEISS (1957). This method almost entirely avoids the difficulties encountered previously in liquid scintillation radiocarbon dating. LOW-LEVEL
Gmeral procedure for bemene synthesis The reaction steps and yields for the preparation of benzene are shown in Table I. Reactions (1) through (5) have been described earlier by SUESS (1954), and by TAMERS et al. (1961). The critical reactions (5) and (6) have been studied and 6
797
798
JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP, and DONALD W. HOOD
certain improvements have been incorporated in these procedures. They will be described in some detail. The SrC, obtained from the high temperature reduction of S&O, with magnesium was passed through a 10 mesh screen to obtain a satisfactory particle size for introducing the solid carbide into water at reduced pressure. Fig. 1 is a diagram of the equipment for converting carbide into benzene. The ground SrC, was placed in flask A which was connected by means of rubber stoppers and flexible Tygon tubing to flask B, containing two liters of water. A vacuum was 2 to the acetylene generating flasks A and B. first applied through stopcock Stopcock 2 was closed and carbide was allowed to drop into the water at a eontrolled rate until a pressure of approximately 0.5 atmosphere resulted. A vacuum was then applied to the generating system through stopcocks 2, 3, and 4 so that Table 1. Chemical st,eps and yields in benzene preparation
Yield
Overall yield
(%)
(%)
3 (NH&CO, + SrClz + &CO, + 2NH,Cl 4 2SrCOs + 5Mg + SrC, + SrO + 5iMgO
95-98
95-98
5 SrCz + H,O + C,H, 6 3C,H, + C,H,
90 50-60
88 44-53
Reaction
1 Carbon & CO, 2 CO, + SNH,OH -c (NH,),CO,
+ SrO
gas would pass through traps C and D. Trap C was cooled with dry ice-alcohol to remove water and trap D was cooled with liquid nitrogen to collect the acetylene in the solid form. A vacuum was continually applied through stopcock 4 in order to remove the relatively large amount of hydrogen produced along with the acetylene. In this fashion 400 g of carbide could be hydrolyzed and the resulting acetylene collected in the solid state in approximately 10 minutes. This generator was designed to handle no more than 400 g of carbide at one time. After closing stopcock 2 and removing the vacuum system from stopcock 4, the liquid nitrogen was removed from trap D and the vaporized acetylene was allowed to pass into one or more of the previously evacuated acetylene reservoirs, F. Flow indicator G and trap H were purged carefully with dry nitrogen. Next the acetylene gas was displaced from reservoirs F by means of water passing through valve 6, the gas being caused to pass at a controlled rate through flow indicator G, trap H cooled with dry ice-alcohol and into catalyst chamber I where the conversion to benzene took place. Care was taken at all times to prevent water and oxygen from entering the catalyst chamber since these materials deactivate the catalyst. As the acetylene gas contacted the activated catalyst an exothermic reaction was accompanied by a purple coloration of the catalyst. This reaction front was seen to advance along the column and no acetylene escaped from the column until all the catalyst had assumed the dark color. As soon as all the acetylene had been
Benzene synthesis by low temperature catalysis for radiocarbon dating
8-l
r=
799
800
JOHN
E. KOAICES, A. F. ISBELL, J. J.
Fig. 2. Apparatus A. B. C, D,
and DONALD W. HOOD
for preparation of diborane activated benzene from catalyst column.
Vacuum pump H,O & B,H, freeze trap and C,. H,O bubblers and D,. Stopcocks to catalyst columns
E. Valve F. Quick disconnect
STIPP
catalyst
G. Cold box ( -40
and for removal
of
to -SO’%)
H. 0, regulator I. B.&I, cylmder
J. Heat tape oven R. Enclosed well vented hood
into the column, stopcock 7 was closed, and column I was disconnected between stopcock 7 and trap H. The column was placed in the same oven J ILS that shown in Fig. 2, heated to 100°C and the benzene collected in a liquid N, cooled trap under reduced pressure. After 15 minutes all the benzene was collected in the trap. The rate at which acetylene was passed into the catalyst was found to be a critical factor in determining the yield of benzene. A rapid acetylene flow produced a high catalyst contact temperature, a dark coloration of the catalyst and a flow benzene yield. A slow acetylene flow or mixing the acetylene with an inert gas such as nitrogen resulted in a lower catalyst contact temperature, a lighter coloration of the catalyst and benzene yields of as much as 60 per cent or occasionally more. The low benzene yield resulting from a high acetylene flow rate is believed to be the result of the formation of high molecular weight hydrocarbons. The more highly polymerized materials were not removed completely from the column even at 700°C and no serious attempt has been made to identify these byproducts. In those instances where the original carbon sample is small, it is best to pass oxygen through the column at elevated temperatures, burn the polymer to CO, and recycle this CO, to increase the yield of benzene. The purity of the recovered benzene was checked on numerous occasions with the aid of a Beckman Model GC-2A gas chromatograph employing an 8 ft Fluoropak passed
Benzene synthesis by low temperature catalysis for radiocarbon dating
801
column operated
at 160°C and a Silicone column at 100°C. No material other than benzene was found to be present in deteeta.ble amounts. The catalyst employed was type S-46 cracking catalyst from Houdry Process Corporation. This was obtained in the form of l/8 in. silica-alumina pellets. The catalyst was dried and activated with diborane essentially according to the method described by SHAPIRO and WEISS (1957, 1958). Fig. 2 shows the equipment used The catalyst column was a 3 x 65 cm Pyrex glass tube. One for this purpose. end was fitted t,hrough a 2 mm vacuum stopcock to a 1817 ground glass joint, F,. The opposite end was connected through stopcocks D, and E to a vacuum system. Stopcocks D, and D, were of 2 mm bore and had a compression type Teflon plug to prevent the possibility of the benzene being contaminated with stopcock grease. Although such stopcocks are not normally recommended for high vacuum work, they were found to be satisfactory for the vaccum required in this application. The catalyst chamber held approximately 230 g of catalyst, which was sufficient to produce 3-4 g of benzene. The catalyst was dried by heating at 300°C under a vacuum of 5 mm or less for three hours. It was found that a temperat,ure of 150°C’ or greater under vacuum was adequate to remove all the associated water in the catalyst. The drying and activation of the catalyst took place in a good fume hood in such a manner that it was unnecessary to disconnect the column at any t’ime. It was possible to handle two or more coIumns simultaneously. After drying, the columns were cooled to room temperature, evacuated, and diborane was admitted at the rate of approximately 30 ml per minute. When the diborane contacted the catalyst, hydrogen was evolved. The diborane flow was continued until the hydrolysis of diborane was noted in bubbler C,, which had been connected to the column through joint F, after the drying operation. The diborane flow was stopped, stopcocks D, and D, were closed and the diborane was allowed to continue to react in the column. Periodicall~r D, was opened briefly to allow some gas to escape. After two hours the catalyst was ready for use after excess diborane had been flushed from the column with dry nitrogen and the column had been heated to 150°C under vacuum for one hour. It was necessary to remove completely any excess diborane since according to the GALLERY CHEMICAL COMPASP, diborane will react with benzene at 100°C to produce triphenylborane: 6(&H, + B&C, -
2B(C,H,c,), + 6H,
If the catalyst columns were to be stored for any period of time, they were filled with dry nitrogen or helium at one atmosphere. Such columns, after storing as long as two months, have given results comparable to freshly activated columns. Although approximately five hours were required for the actual drying and activation of each column, only about 15 minutes of actual man hours were required and several columns could be activated at one time. The diborane gas is available commercially from the CALLERY CHEMICAL COMPAKY. If it is kept at a temperature of -3OY! or lower, the disproportiormtion into higher borane hydrides is very slow. WTe stored the diborane cylinders in a commercial freezer which bad been modified to maintain a temperature of -45°C.
802
JOWNE. NOAKES,A. F. ISBELL,J. J. STIPP,and DONALI)W. HOOD
Because of the toxic and inflammable properties of diborane, involving it were carried out in a well ventilated fume hood.
all operations
COUKTISG PROCEDURES Countillg vials were chosen on the basis of their ability to contain the benzene on storage and to give reproducible ba~ekground counts. None of the vials tested yielded entirely satisfactory results. Vials having plastic screw caps lost benzene through the caps at a slow rate. This loss became serious after a storage period of approximately two weeks. When the samples were frozen during storage, they gained weight apparently as a result of water adsorption. The background count of vials was found to vary rather widely. This made it necessary to carry out a background count of each vial and to discard all vials having high counts. The liquid scintillation spectrometers employed were PACKARD TRI CARB manual series 314A and automatic series 314E. Both these instruments were found to be stable and satisfactory for low-level counting. The photo-multiplier voltage was set at 930 volts and a window setting of 9-50 volts, which provided approximately 50 per cent counting efficiency and a background of approximately 15 cpm depending upon the individual vial. Since the background of the vials was pre-determined, the automatic counter with print-out recorder was found especially suitable. Several screw cap vials and one sealed vial, containing commercial benzene and phosphors, were placed in the counter with a pre-set counting time of 100 min and the instrument was then allowed to run automatically until each vial had accumulated at least 1000 total min. The screw cap vials were then filled with the samples to be dated and were counted in the manner described. A modern sample sealed into a vial having a modern count rate of 6.67 c/m[g of carbon with a background of 15.45 cts/min. was counted along with each group of unknowns. Since each vial was counted for 100 min int’ervals, any flu~tLlation in natural background variation between samples tended to average out. The sealed vials acted as a continuous check against any long t,erm shift of counter efficiency, and provided data for correcting the calculations, if necessary. Typical results obtained The complete date list obtained by this laboratory will soon be submitted for publication in Rdiocarbon; however, some results on samples previously dated by the United States Geological Survey, by the gas counting technique, and presented in Table 2, indicate the statistical uncertainty of this method as compared to the liquid scintillation technique. The ?V-723 date was det’ermined on 6.4459 g of carbon as benzene in a counting time of 1440 minutes to give 1.811 & 0.027 c/m/g with a background of 15.455 & 0,125 c/m. The absolute specific activity of modern age corrected I850 wood has been determined as 13 d/m/g/ o f carbon on a 9.1134 g sample of carbon as benzene. SUMMARY AXD CONCLUSIONS The general availability of adequate counting facilities and the ease with which carbon in a sample can be convert,ed into benzene by the method here described
Benzene synthesis by low temperature
catalysis for radiocarbon dating
803
makes a radiocarbon dating by liquid scintillation counting a most attractive method. The procedure described in this article is now being used routinely in two laboratories and is under development in several others. Inorganic and organic carbon samples of geologic, archeologic, and marine origin have been converted to benzene and dated (STIPP et al., 1962). In addition to this dating, we have carried out inter-laboratory calibrations with geologic samples from the U.S.C.S. A date list from the laboratory is soon to be submitted for publication laboratories, in Radiocarbon,. Table 2. Comparison of dates obtained by liquid scintillation by this laboratory and by gas count,ing by the U.S. geological survey Sample No.
Location
--
Liquid scintillation ----....10,820
& 190
Gas counting 10,960 * 300
W-723
Grand Forks, N.D.
W-859
Sheep Creek Alaska
5,925 & 275
5,940
5 250
W-823
Hutchins Creek fllinois
5,535
4,840
+ 300
2 175
A number of possibilities still remain for improving this method of carbon dating. It is believed that the yields reported here can be improved and we have seen step (6) occasionally give much higher yields than the 50-60 per cent mentioned in this paper. We are at present attempting to determine what important factors govern these yields and something concerning the mechanism of the acetylene to benzene reaction in the hope that the yields may be signi~ca~ltly increased. Relatively minor modifications in the counting technique may also improve significantly the precision of this method. For example, more efficient scintillators are becoming available from various sources and the use of photomultiplier tubes sealed in quartz, rather than glass envelopes, would improve the figure of merit. The generally accepted figure of merit, W/B, in which N is the net sample counting rate and B is the background has been considerably increased through the use of liquid samples. The utilization of a large carbon sample in the form of benzene greatIy increases the total activity of the sample. The background count may also be reduced by the use of smaller counting vials or by reducing the volume of the counting solution in a standard 20 ml vial (MCDOWELL, 1962). AcklzowZedgements-Dr.
E. &I. DAVIS, Anthropology Depart,ment,, University of Texas, made available the University of Texas Radiocarbon Dating Laboratory to help in the perfection of the method. The work was initiated with Grant No. NSF-10232 with the National Science Foundation and completed with Grant Xo. bob-2119(04) with the Office of Naval Research and with the Texas A & M College Fund for Organized Research. REFERENCES ARNOLD J. R. (1954) Scintillation counting of natural Science 119, 155. CALLERY CHEMICAL COMPANY, B,H, Handling Brochure.
radiocarbon;
the counting
method.
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JOHN E. NOAKES, A. F. ISBELL, J. J. STIPP and DONALD W. HOOD
DIETHORN W. Doctoral Dissertation, Carnegie Tech (1956), A methane proportional counter system for natural radiocarbon measurements. LEGER C. and TAMERS M. A. (1962) The counting of naturally occurring radiocarbon in the form of benzene in a liquid scintillation counter. Int. J. Appl. Rad. Iso. NO_IIES J. E., ISBELL A. F. and HOOD D. TV. (1961) Conversion of carbon dioxide to benzene for liquid scintillation counting. Trans. Amer. Geophys. Union. 42, 266. MCDOWELL, L. L. (1962) Private communication. PACK-&D INSTRUMENT COMPANY, INC. (1961) Private communication. SHAPIRO, 1. and WEISS, H. G. (1957) Cyclization of acetylene D, to benzene D,. J. Amer. Chem. sot. 79, 3294. STIPP J. J., DAVIS E. M., NOAKES J. E. and HOOVER T. E. (1962) University of Texas radiocarbon dates-l. Radiocarbon suppl. June 1962. TAATERSM. A., STIPP J. J. and COLLIER J. (1961) High sensitivit)y detection of naturally occurring radiocarbon-I: Chemistry of the counting sample. Geochim. et Cosmochim. Scta 24, 266. TA~IERS, Iii. A. (1960) Cl4 dating with the liquid scintillation count,er: A total synthesis of t,he benzcne solvent. Science 132, 668-669. WEISS, H. G. and SHAPIRO, I. (1958) Tncreascd act,ivity of silica alnmina catalyst)s. J. Amer. Chem. Sot. 80, 3195-3198.