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IS I , ." J. B e Evans, 3. E, Quinlan and J, E, Willard
~ e ~ a r t ~ e i t of Chemistry, University of Wisconsin, Madison, Wieconsia
C m I C A L EFFECTS OF NUCLEAR TRANSFORMAT I O N S AND THEIR USE I N MAKING LABELED COMPOUNDS
The chemical e f fec t s of nuclear transformations
can be u t i l i zed t o introducte the radioactive atoms
produced i n t o chemical species useful f o r t racer
studies. Gas chromatography of fers a means of sep-
arat ing the tagged compounds w i t h high specif ic .
ac t iv i ty . A chromatographic apparatus f o r separating
and identifying labeled compounds i s desoribed,
Comparisons are made of this with other labeling
techniques.
4 J
Since 1934 when gs i l a rd and Chalmers (31) reported t h a t 1/ 3d
radioiodine produced by the i r radiat iol i of e thy l iodide with ,
neutrons appears i n a form which can be p a r t i a l l y extracted
w i t h water, it has been recognized tha t nuclear ppocesses can$ 3
cause chemical reactions, @ Typical such processes which w i l l be considered i n thi'i
a 1 a e discussion include the following: C 1 (n,\b)Cl (37 .min. );
88 ~ r " [ n , % ) ~ r
1s 'l (36 hr.1; I (n, #)1"~(25 min);
14 1 4 I
N (n,p)C (5600 y); ~i'(n,Cf)~'(12 y); ~ e @ ( n , p ) ~ ' ~ l 2 y).
The notation s ign i f i e s t h a t the s tab le isotope a t tlie l e f t
captures a neutron and emits a gannna ray, proton o r W ~ P d par t ic le , forming the radioisotope
- -
which indicated
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
r i g h t . The h a l f - l i f e i s given i n the parentheses a t the r i g h t .
I n each case the r ad ioac t ive product atom i s born with high
energy which allows it t o hrenk i t s parent chemical bond and
subsequently t o r e a c t with surrounding molecules, A s i l l u s -
t r a t e d by Figure 1, a wide v a r i e t y of r a d i o a c t i v e l y l abe led
compounds may r e s u l t . These a re a source of t r a c e r s f o r research,
development and product ion use, i f they can be separa ted from
each o the r with s u f f i c i e n t l y high s p e c i f i c a c t i v i t y , I n the
fol lowing paragraphs we s h a l l review b r i e f l y t h e evo lu t ion of
knowledge a s t o t h e c h a r a c t e r i s t i c s and mechanisms of such
reac t ions , s h a l l descr ibe the use of gas chromatography f o r
sepa ra t ing very h igh spec if i c a c t i v i t y l abe led compounds, s h a l l
compare t h i s method of l a b e l i n g with c e r t a i n o the r methods,
and s h a l l c i t e r e fe rences which may be u s e f u l t o those d e s i r -
ing f u r t h e r acquaintance with the f i e l d , A more comprehensive
d iscuss ion of baaio invoot iga t ions of ehernfcal e f f e c t s of
nuclear t ransformations has been given i n e a r l i e r reviews ( 2 , 3 ) *
References here t o work p r i o r t o these reviews w i l l be l i m i t e d
t o a .few examples,
Mechanism
Although it was a t f i r s t perhaps thought t h a t t h e rup tu re
of t h e carbon-iodine bond i n e t h y l iodide,. observed by S z i l a r d
and Chalmers (31) , r e s u l t e d from t h e momentum c a r r i e d by the
neutron a s i t e n t e r e d the iodine atom it was soon r e a l i z e d t h a t
the process could occur even w i t h neutrons of only thermal
energies . The rupture was then asc r ibed t o the r e c o i l energy
given t o the i odine atom a s a r e s u l t of emission of one o r more
e z s @02
12 7 l a e gamma rays i n the I (n;g)I process. For an 8 Mev. gamma
ray t h i s r e c o i l energy i s 6170 kca l . per mole, which i s more
than one hundred times g r e a t e r than t h e energy of the carbon-
iodine bond, Consis tent with these Ideas was the observat ion
(4.) t h a t i n some gaseous a l k y l h a l i d e s nea r ly 100% of the
halogen atoms which capture neutrons s p l i t t h e parent bond and
appear i n inorganic form. These atoms can be d i s t ingu i shed
from those which have not captured neutrons, because t h e ( n , ~ ) .
process changes them t o r ad ioac t ive i so topes .
I n l i q u i d o r s o l i d organic media from 20% t o nea r ly 100%
of the r ad ioac t ive atoms which have s p l i t t h e i r parent bonds
r e e n t e r organic combination, Early t h e o r i e s a sc r ibed t h i s t o
a " b i l l i a r d b a l l collision^^ mechanism (22) i n which t h e r e c o i l
atom was pos tu la ted t o l o s e i t s energy by a head.*on c o l l i s i o n . with an atom of equal mass, t h e s t ruck atom being knocked on,
leaving behind a r a d i c a l which combined w i t h the r ad ioac t ive
r e c o i l atom, It was necessary t o d i s c a r d t h i s hypothesis when
i t was observed i n s e v e r a l l a b o r a t o r i e s t h a t : a ) Halogen atoms
. ac t iva ted by neutron capture can e n t e r organic combination not
only by replacement of halogen atoms bu t by rep lac ing various
o the r atoms and. r a d i c a l s (4, 11, 12, 23) including hydrogen
atoms; b ) Low.concentrations of atom scavengers a l t e r the
chemical f a t e of r ad ioac t ive halogen atoms produced by the ( n , ~ )
process i n organic compounds i n d i c a t i n g t h a t many of the r e c o i l
atoms do not e n t e r s t a b l e combination u n t i l a f t e r they have
l o s t t h e i r excess energy and d i f fused i n thermal equi l ibr ium
wi th the medium ( 1 3 ) .
These f a c t s suggested t h a t the r e c o i l atom must form a
t r a c k o r pocket of r a d i c a l s of var ious s o r t s , wi th one of which
i t may combine a f t e r becoming thermalized, I f i t does not f i r s t
C A I e n t e r s ta ,h le combination by a "hotl1 r e a c t i on, Conc.urrently,
i n v e s t i g a t o r s became aware o f t h e f a c t t h a t the atoms produced
by the (n,X) process of ten , and perhaps always, c a r r y a posi-
t i v e charge . ( 3 3 ) a s a r e s u l t of 'some of t h e energy of t h e
nuclear process being made a v a i l a b l e f o r e j e c t i n g e l e c t r o n s
from the o r b i t s . This charge might lead t o r e a c t i o n as a r e -
s u l t of the energy l i b e r a t e d on n e u t r a l i z a t i o n o r a s a r e s u l t
of ion-molecule r eac t ions ( 2 8 ) . To explore the' r e l a t i v e e f f e c -
t iveness of r e c o i l energy and of charge i n causing r e a c t i o n , 80
the products r e s u l t i n g from a c t i v a t i o n by the B r (11.4. hr .1
I T 8 0 2+ B r (18 min.) isomeric t r a n s i t i o n i n s e v e r a l media were
'7 9 80 compared wi th those formed by the B r ( n , ~ ) ~ r process ( 1 0 ) ~
80 The isomeric t r a n s i t i o n i s known t o produce B r atoms with
an average charge of 910 (34). I n a number of t e s t s the pro- .
ducts 'of a c t i v a t i o n by the two processes have been i n d i s t i n -
guishable . Unfortunately t h i s r e s u l t does not d i s t i n g u i s h . .
d e c i s i v e l y between t h e charge and recoil ,mechanisms s ince atoms
from the isomeric t r a n s i t i o n may acqui re some k i n e t i c e n e r g y
from coulomb repuls ion , a l though t h i s i s much lower than t h e
eneragy acquired. from r e c o i l i n t h e ( n , 6 ) process. Recoil atoms 7 9 7 8
of y e t much h igher energy produced by the B r (n,2n)Br
r e a c t i on produce s imi l a r r e s u l t s (30) . Consequently, the
quest ion of t h e separa te con t r ibu t ions of charge and r e c o i l
energy i s n o t y e t solved al though t h e r e i s evidence from some
gas phase r e a c t i o n s t h a t both rnay be important ( 2 1 ) ,
S tudies of iodine, bromine, and ch lo r ine a c t i v a t e d by the
(n ,$ ) process i n the gas phase show t h a t such atoms a r e able
t o undergo unique processes unl ike those observed f o r halogen :3
atoms a c t i v a t e d i n any o t h e r way ( 1 6 ) , I n gaseous hydrocarbons
o r a l k y l h a l i d e s they can d i sp lace hydrogen atoms and organic
groups i n bimolecular, non-radical r e a c t i ons t o produce a
mult i tude of l abe led compounds from a s i n g l e type of organic
gas , It seems probable t h a t these r eac t ions a re ion-molecule
processes (28), al though the p o s s i b i l i t y cannot be excluded
t h a t they a r e due t o atoms i n an exc i t ed s t a t e not encountered
i n photochemical s tud ies . I t i s poss ib le t h a t a s i m i l a r
unique mechanism may c o n t r i b u t e i n p a r t t o the formation of
the m u l t i p l i c i t y of products observed i n condensed phases,
Carbon and Hydrogen Labeling
The above o u t l i n e of the development of knowledge of t h e
chemical e f f e c t s of nuclear t ransformations i n organic systems .
has d e a l t wholly wi th halogen atoms, These have received
predominant a t t e n t i o n i n such i n v e s t i g a t i o n s u n t i l r e c e n t l y
because the r ad ioac t ive spec ies have s u i t a b l e h a l f - l i v e s and
a re convenient ly made, wi th s a t i s f a c t o r y cap tu re c r o s s sec t ions ,
by neutron i r r a d i a t i on 'of the n a t u r a l l y occuring i so topes ,
Also, when s t a b i l i z e d i n organic combination they do not e a s i l y
exchange wi th o the r molecular forms, o r 'hydrolyze,
From the s tandpoint of preparing r a d i o a c t i v e l y l abe led
compounds f o r use i n research , development and production con-
t r o l i t would o f t e n be d e s i r a b l e t o be ab le t o employ carbon,
hydrogen, oxygen o r nf trogen. Unfortunately n i t rogen and
oxygen a r e excluded f r o m such use by the very s h o r t h a l f - l i v e s
- 005 4 -A
of t h e i r r a d i o i s o t o p e s . Carbon and hydrogen each has a n a t u r -
a l l $ occu r ing i s o t o p e which c a p t u r e s neu t rons t o form a r a d f o - 13 14 2 3
a c t i v e i so tope , f e e . C (n,)oC (5600 y ) and H (n ,a )H ( 1 2 Y ) ,
b u t i n each c a s e t h e n a t u r a l abundance of t h e c a p t u r i n g iso , tope
i s s o low, t h e c r o s s s e c t i o n f o r neu t ron c a p t u r e is s o low,
and t h e h a l f . l i f e i s s o long, t h a t i t i s i m p r a ~ t i c a l ~ t o use i
t h e p roces s f o r producing u s e f u l amounts of t h e r ad i+c t i , ve
s p e c i e s . There a r e however s e r v i c e a b l e t r a n s m u t a t i o q p roces se s 1 4 3
t by which C and H may be produced from o t h e r elemerjts. These
14 14 6 3 3 3 f a r e N (n ,p )C , L i (n,c.q)H . a n d He (n ,p )H . The n a t u r a l
14 abundance of N i n n i t r o g e n i s 99.6% and i t s c r o s s s e c t i o n
f o r t h e ( n , p ) p roces s w i th thermal neu t rons is . 1.76 b a r n s ; 6
t h e n a t u r a l abundance of L i i s 7.5% and the c r o s s s e c t i o n . 945 3
ba rns ; He , a decay produc t of tri tium, may be purchased a t a
reasonable c .os t , i t s c r o s s s e c t i o n i s 5400 b a r n s , A s i n t h e
case of t h e r a d i o a c t i v e halogen atomo produced by t h e (n,Y) 14 3
proces s , t h e C and H a r e born w i t h h igh , r e c o i l energy and
a r e presumably p o s i t i v e l y charged, t h e r e c o i l energy be ing
much g r e a t e r because t h e e n e r g e t i c p a r t i c l e e j e c t e d i s a massive %
pro ton o r a l p h a p a r t i c l e r a t h e r than a gamma ray . I
It h a s been shown t h a t by exposing n i t r o g e n compounds i n 14
hydrocarbon s o l v e n t s t o neu t rons i t i s p o s s i b l e t o form C - l a b e l e d molecules i nvo lv ing replacement of normal C o r N atoms
1 4 by C , and a l s o t o form molecules w i t h l a r g e r and s m a l l e r
carbon s k e l e t o n s t h a n t h e s u b s t r a t e (29, 40 ) . T r i t o n s formed 6 3
by t h e L i (n ,q )H process i n f i n e l y ground mix tu re s of l i t h i u m
sa l t s w i t h o rgan ic compounds form l a b e l e d molecules of t h e
s u b s t r a t e a s w e l l a s o t h e r , p r o d u c t s ( 2 6 ) . The d i s t r i b u t i o n of
t he , l a b e l on t h e d i f f e r e n t ca rbon atoms of t h e molecules formed 3
5 i s s e l e c t i v e r a t h e r t han s t a t i s t i c a l . When mix tu re s of He .
with methane, e thane 03 propane a r e i r r a d i a t e d w i t h neu t rons m .
t h e tritium formed i s 'found i n compounds r e q u i r i n g b o t h
. . l eng then ing and s h o r t e n i n g of t h e carbon c h a i n s (15, 27) ,
S e p a r a t i o n by Gas Chromatography
P r i o r t o t h e a v a i l a b i l i t y of g a s chromatography it was
impossible t o s e p a r a t e w i thou t c a r r i e r o r s o l v e n t unweighable
amounts of l a b e l e d compounds such a s a r e i n d i c a t e d by t h e peaks
of F igu re s 1 and 2. With gas chromatography, however, t h e
only s o l v e n t r e q u i r e d i s a c a r r i e r gas , such a s helium, from
'which t h e unweighable components can be removed, w i th appro-
p r i a t e c o l d t r a p s , a s t hey come o f f t h e column. It h a s been
demonstra ted tha t e s s e n t i a l l y c a r r i e r - f r e e components a r e
s e p a r a t e d by t h e chromatographic column a s e f f e c t i v e l y a s com-
ponents p r e s e n t i n macro anlounts (9 , 1 0 ) . This i s i l l u s t r a t e d
by F igu re 1 where a l l o f t h e r a d i o a c t i v i t y peaks a r e f o r .
l l c a r r i e r - f r e e l t p roduc ts , excep t t hose under t h e d o t t e d l i n e s
which i n d i c a t e added c a r r i e r s d e t e c t e d by thermal c o n d u c t i v i t y .
Fo r t hose compounds which a r e s u f f i c i e n t l y v o l a t i l e and s t a b l e
t o be s e p a r a t e d by gas chromatography the use of chemical
. . e f f e c t s of n u c l e a r t r ans fo rma t ions f o r producing h i g h s p e c i f i c
a c t i v i t y t r a c e r s i s , then,, no l onge r s u b j e c t t o t h e s t r i n g e n t
l i m i t a t i o n s fo rmer ly impos'ed by t h e s e p a r a t i o n problem,
I d e n t i f i c a t i o n of P,roduc t s . Before gas chromatography was
a v a i l a b l e i t had been demonstrated t h a t n u c l e a r ' t r a n s f o r m a t i o n s
i nvo lv ing ha logen atoms can l e a d t o a v a r i e t y of l a b e l e d com-
pounds i n a l k y l h a l i d e media, This was d-one by t h e r e l a t i v e l y 5 (J(P'7
- 8 -
l a b o r i o u s pro.cess of add.ing c a r r i e r s f o r su spec t ed produc ts
!: t o t h e i r r a d i a t e d sample, d i s t i l l i n g and observ ing t h e r a d i o -
-. . a c t i v i t y i n succes s ive f r a c t i o n s of t h e d i s t i l l a t e (6 , 11). E
With gas chromatography one can f i r s t determine. t he number of
c a r r i e r - f r e e compounds p r e s e n t by making a chromatogram of the '
r a d i o a c t i v i t y peaks from s a y a 50 m i c r o l i t e r sample of t h e
i r r a d i a t e d m a t e r i a l . To ano the r sample he c a n then add non-
r a d i o a c t i v e macro p o r t i o n s of su spec t ed produc ts whose o r d e r
o f . emergence from the column has been p r e v i o u s l y determined.
I By s imu l t aneous ly moni to r ing t he r a d i o a c t i v i t y and h e a t conduc-
t i v i t y changes ' i n t h e e f f l u e n t gas from t h e column, u s i n g a
. . two pen c h a r t r e c o r d e r t o g ive a r e c o r d such a s . t h a t of F igure
1, t h e r a d i o a c t i v i t y peaks a r e e a s i l y . i d e n t i f i ed. ' Each of t h e 1 .i
l a b e l e d . . peaks i n F igu re 1 has been charaacterized by th i s method,
Often s fgn i f i c a n t guesses a s ' t o , t he i d e n t f t y of peaks may be
made from a knowledge t h a t t h e compounds come off ' a p a r t i c u l a r
column i n t he o r d e r of some pr 'operty. Those .of F igu re 1 emerge
i n t h e o r d e r of t h e i r b o i l i n g p o i n t s . There i s the p o s s i b i l i t y
t h a t two compounds mag n o t , be, r e s o l v e d . by t h e column, i n which
c a s e t h e s u p e r p o s i t i o n of t h e thermal c o n d u c t i v i t y and r a d i o -
a c t i v i t y peaks may . l e a d t o a f a l s e conc lus ion . Where t h e r e i s
doubt, a's f o r example i n the nC,II1lBrfl peak .of F igu re 1 t h e I . I ..._ . components of t h e peak should be t r apped and run th rough a
column of a d i f f e r e n t type.
Equipment, The equipment used i n o b t a i n i n g t h e chromato-
grams of ~ ig t .&e 1 i s diagrammed i n F igure 3 .' . The column c o n s i s -
t e d of 12 f e e t of 4 mn. I . D . g l a s s t u b i n g c o i l e d ' i n t o a double
s p i r a l e i g h t inches l ong and f o u r inches i n d i ame te r and packed
with 40-60 mesh Johns -Manville C-22 f i r e b r i c k coa t ed w i t h 40%
'of i t s weight of General E l e c t r i c SF-96 ( 4 0 ) s i l i c o n e o i l ,
During t h e 30 minute s e p a r a t f o n , r u n t h e t empera ture oP the
h e a t i n g b a t h around t h e column was r a i s e d g r a d u a l l y from 40' t o
ZOO0 t o a l low components of s u c c e s s i v e l y h i g h e r b o i l i n g p o i n t
t o move through the column a t a d e s i r a b l e r a t e . The e f f l u e n t
gas passed t o t h e bo t tom.cf a t h i n wa l l ed 1 m l , g l a s s th imble
which f i t i n t h e w e l l 0 f . a NaI ( T l ) s c i n t i l l a t i o n c r y s t a l ; The
l a t t e r f e d th rough a p h o t o m u l t i p l i e r tube, ' p r e a m p l i f i e r , l i n e a r
a m p l i f i e r and r a t e . m e t e r t o t h e c h a r t r e c o r d e r , From t h e
s c i n t i l l a t i o n c o u n t e r the gas passed i n o r d e r through t h e thermal
. c o n d u c t i v i t y s ens ing c e l 1 , a l i q u i d a i r t r a p ( i n which t h e
c o m p o n e n t o under a n a l y s i s were f r o z e n from t h e hel ium s t r e a m ) ,
t h e thermal c o n d u c t i v i t y r e f e r e n c e c e l l , and t h e f lowmeter.
The f low r a t e was main ta ined a t 60 rnl,/min, by i n c r e a s i n g t h e
p r e s s u r e from 1 2 t o 1 9 lb. /sq, i n , a s t h e temperature i nc rea sed .
P l a c i n g , t h e r e f e r e n c e c e l l o n . t h e e f f l u e n t s i d e r a t h e r t h a n t h e
i n p u t s i d e of t h e system avoid.ed v a r i a t i o n s i n t h e base l i n e of
t h e c h a r t r e c o r d e r a s t h e r educ ing va lve .was a d j u s t e d 'dur ing a
run. The n e g a t i v e peak inmed ia t e ly fo l l owing t h e a i r peak of
.F igu re 1 i s due t o t h e f a c t t h a t a i r i s n o t removed' by t h e
l i q u i d a i r t r a p , The tubes from t h e coiwnn t o t h e s c i n t i l l a t i o n
c o u n t e r and from t h e ' c o u n t e r t o t he c e l l were wound with nichrome
wi re and a s b e s t o s and e l e c t r i c a l l y h e a t e d t o p reven t condensa t ion
of t h e l e s s v o l a t i l e compone'nts.
Equipment Modi,f i c a t i ons , Many modif i c a t i ons of t h e above
equipment a r e of course p o s s i b l e , and i n some c a s e s a r e neces -
s a r y t o a l low t h e s e p a r a t i o n of d i f f e r e n t chemical s p e c i e s and
t h e moni tor1ing of d i f f e r e n t t y p e s and e n e r g i e s of r a d i a t i o n ,
The aluminum c l a d s c i n t i l l a t i on c o u n t e r w i t h g l a s s th imble
i n tho w e l l g i v e s h i g h e f f i c i e ' n c y f o r t h e coun t ing of gamma
r a y s and h igh ' energy b e t a r a y s b u t i s u s e l e s s . f o r count ing
t h e weak b e t a r a y s f'rom tri t i m o r carbon-14. For t r i t i u m it
i s neces sa ry t o have t h e compound t o be counted i n s i d e t h e
coun te r . This was accomplished i n ' the exper iment of F igu re 2
(15) by us ing methane, a good coun te r gas , t o c a r r y t he sample
through t h e chrorria.tographic column and t h e n a l l owing i t t o
f low through a ' p r o p o r t i o n a l c o u n t e r which c o n s i s t e d of a n
i n t e r n a l l y s i l v e r e d g l a s s sphere of 1 0 m l , vo l . (14) . It cad^ b w c Y i ~ ' r t)/
i s i n t e r e s t i n g t o n o t e t h a t the,peak f o r t r i t i a t e d methane
shows up w e l l even w i t h methane a s t he c a r r i e r gas.
The use of methane has t h e d i sadvantage t h a t i t s h e a t con-
d u c t i v i t y i s no t a s much d i f f e r e n t from t h a t of most compounds
of i n t e r e s t a s i s t h a t of helium, and t h a t t h e p o t e n t i a l r e -
q u i r e d f o r coun t ing i s h i g h e r t h a n t h a t of mix tures of t he
r a r e gases w i t h hydrocarbons. Wolfgang and Rowland (27, 4 2 )
have avoided t h e s e d i sadvantages by us ing he l ium a s t he gas
t o c a r r y t h e sample t h rough t h e chromatographic column and
h e a t c o n d u c t i v i t y c e l l , f o l l o w i n g which methane i s b l e d i n t o
t h e s t ream t o make a good coun t ing mixture . The mixed gas i s
then passed t h rougha c y l i n d r i c a l b r a s s p r o p o r t i o n a l coun te r ,
These a u t h o r s have a l s o g iven a u s e f u l a n a l y s i s of t h e e f f e c t
of chamber volume and f low r a t e on t h e s e n s i t i v i t y of d e t e c t i o n
of t h e r a d i o a c t i v i t y peaks. Wilzbach, Kaplan and Riesz (38)
have avoided t h e n e c e s s i t y , fo r a s p e c i a l c o u n t e r gas by employ-
i n g a s t h e i r d e t e c t o r a f low type i o n i z a t i o n chamber i n con-
j unc t ion w i t h a v i b r a t i n g r e e d e l e c t r o m e t e r . I n a d d i t i o n t o
01s 03-0
b - -. - 11 - opera t ing a t low vol tage with any c a r r i e r gas t h i s u n i t should
, , t o l e r a t e gaseous s p e c i e s which form negat ive ions , such a s
- - - .. a l k y l halides., -whf ch s t o p the- opera t ion of -proportional -cow-ters, \b-
although such spec ies may poss ib ly reduce t h e d e t e c t i o n e f f i c -
,.\ . u iency r a t e s somewhat. When an i o n i z a t i o n chamber i s used which
is small enough so t h a t a s i g n i f i c a n t f r a c t i o n of the b e t a
p a r t i c l e s reach the wa l l s the s e n s i t i v i t y of the chamber w i l l
vary a s the composit ion of t h e flow gas v a r i e s ; the l a r g e r
the ,number of e l e c t r o n s per molecule i n the gas the l a r g e r
w i l l be the number o f ' ion p a i r s produced per b e t a emit ted . i n
the chamber.
Kokes, Tobin and Emmett ( 1 9 ) were probably the f i r s t i n -
v e s t i g a t o r s t o sepa ra te r ad ioac t ive ly l abe led compounds by gas
chromatography and analyze the e f f l u e n t by both counting and
hea t conduct iv i ty . They analyzed macro amounts of hydrocarbons 1 4
labolod w i t h C . Thcir radf o a c t i v i t g d e t e c t o ~ l was a mica
window Geiger tube arranged s o t h a t the gas sample flowed
through a compartment enclosing the window.
For many systems an important r e q u i s i t e of the radf oactfv-
i t y d e t e c t o r 1 s t h a t it be ab le to operate a t e l eva ted tempera-
t u r e ' s o t h a t it can be hea ted t o prevent ' compounds of low
v o l a t i l i t y from condensing. In taking the .data of Figure 1
the thimble' i n the wel l of the s c i n t i l l a t i o n counter was main-
t a i n e d a t 200' ( 9 ) . This i s n o t an id .eal arrangement because a
it r i s k s the p o s s i b i l i t y of cracking an expensive c r y s t a l and
because t h e s e n s i t i v i t y of t h e c r y s t a l drops a f t e r a prolonged
period of hea t ing the we l l , I t has, however, been used success-
f u l l y f o r hundreds of experiments, The s e n s i t i v i t y remains
s u f f i c i e n t l y cons tant f o r the time requi red f o r a run. When it
drops, due t o long hea t ing , i t r e t u r n s t o i t s normal value on
cooling. Another method ,of counting ene rge t i c b e t a p a r t i c l e s
o r gamma rays ini)a h o t f lowing gas i s t o pass the gases through
an e l e c t r i c a l l y heated t h i n wal led g l a s s tube ad jacen t t o a t h i n
walled Geiger ' counter (9, , lo ) . The counter is mounted h o r i -
z o n t a l l y below the flow tube and the two a re separa ted by a
t h i n shee t of aluminum f o i l thus al lowing the counter t o operate
a t . nea r room temperature, In genera l Geiger counters and pro-
p o r t i o n a l counters do not operate s a t i s f a c t o r i l y above about
75' C. Therefore f o r experiments such a s those of Figure 2
the gases from the chromatographic column were passed through
a l eng th of unheated tubing before e n t e r i n g the counter tube,
Wilzbach and coworkers (38) have operated t h e i r flow i o n i z a t i o n
chamber a t 110' but f i n d t h a t the leakage current becomes
excessive a t higher temperature;. I t I s t o be expected t h a t
t h i s may be remedied by using d i f f e r e n t i n s u l a t o r s o r , by o the r
changes i n the m a t e r i a l s of cons t ruc t ion . ,
A very promising prospect f o r the manufacture of flow
counters operable a t temperatures up t o 200' C , o r h igher i s
the type of tube developed by L. B. Clarke, Sr: of the Naval
Research Labora tor ies ( 7 ) . These tubes have cathodes cons is -
t i n g of f i l m s deposi ted by passing a mixture of s t ann ic ch lo r ide
and methyl a l coho l vapors through the heated g lass s h e l l .
Co l l ec t ion of ~ r o d u c t s . The requirements f o r sample
c o l l e c t i o n a r e simply a s e r i e s of small t r a p s which can be
cooled w i t h l i q u i d a i r o r o t h e r r e f r i g e r a n t and through which
the e f f l u e n t stream from t h e chromatographic column can be
d i r e c t e d i n t u r n a s successive peaks appear, The s imples t ' u n i t
i s a small U tube with a ground j o i n t which can a t t a c h t o the
e f f l u e n t end of the column. Manifolds of var ious s o r t s with . ,
-*
severa l t r a p s i n p o s i t i o n f o r convenient r ap id switching from
f-, one t o another have been devised.
Spec i f i c ~ c t i v i t y of Products. Those l a b e l e d molecules
which a r e formed by s u b s t i t u t i o n of the r e c o i l atom f o r a
s i n g l e atom of the same element i n the t a r g e t molecule ( a s i n
the formation of l a b e l e d n-propyl bromide from n-propyl bromide
o r of t r i t i a t e d propane from propane) a r e l i m i t e d i n s p e c i f i c
a c t i v i t y by d i l u t i o n by the i n a c t i v e molecules o r i g i n a l l y present .
Attempts t o increase the s p e c i f i c a c t i v i t y by longer i r r a d i a t i o n s
a t h igher f l u x e s may r e s u l t i n major decomposition by r a d i o l y s i s ,
The l abe led compounds which a r e d i f f e r e n t from the t a r g e t mole-
*. - cule ( a s i n the formation of methyl bromide from n-p.ropyl
bromide o r of t r i t i a t e d methane from propane) would be present
i n t h e " c a r r i e r - f r e e t 1 s t a t e (I .e, undi lu ted by non-radioact i ve
molecules) i f the s t a r t i n g m a t e r i a l were abso lu te ly pure and
i f none of the i n a c t i v e spec ies i n quest ion were formed by the
a c t i o n on the s t a r t i n g compound of gamma rays, f a s t neutrons
and r e c o i l atoms, I n p r a c t i c e such r a d i o l y t i c a c t i o n cannot be
avoided and non-radioact ive molecules a r a produced which s r e
i d e n t i c a l t o some o r a l l of t h e s p e c i e s produced by the r a c o i l
l a b e l i n g reac t ion , thereby lowering the s p e c i f i c a c t i v i t i e s of
- . these spec ies ,
The r a t i o of i n a c t i ve molecules produced by r a d i o l y s f s t o
r ad ioac t ive molecules produced by t h e l abe led r e c o i l atoms
depends on: 1) The G value (molecules formed pe r 100 ev
- 14 - absorbed) f o r t h e p a r t i c u l a r r a d f o l y s i s s t e p ; 2 ) The r a t i o of
neu t rons t o gamma r a y s from t h e neu t ron sou rce ; 3 ) The r e c o i l
e n e r g i e s of t h e p a r t i c l e s from t h e n u c l e a r p roces s which forms
t h e l a b e l i n g atom; 4) The neu t ron c a p t u r e c r o s s s e c t i o n f o r
t h e p rocess which produces t he l a b e l i n g atom; 5 ) The ccncen t r a -
t i o n of t h e neu t ron c a p t u r i n g atom r e l a t i v e t o o t h e r atoms.
In carbon and t r i t i u m l a b e l i n g a l l of t h e energy of t h e n u c l e a r
p rocess ( n , p o r n;ty) i s u s u a l l y absorbed i n t h e system s i n c e 1 4
t h e e m i t t e d pro tons oi7 a l p h a p a r t i c l e s , a s w e l l a s t h e C and 3
H atoms have s h o r t ranges i n m a t t e r , whereas i n t h e (n , x ) proces se s t h e e m i t t e d gamma r a y i s only wsakly absorbed and
t h e r e c o i l energy of t h e tagged atom i s r e l a t i v e l y ' l o w . I n a 6 3
t y p i c a l c a se of l a b e l i n g by t h e L i (n,?)H p roces s t h e r a t i o
of r a d i o a c t i v e t r i t i u m . atoms formed t o .molecules of t h e sub- '6
s t r a t e decomposed by t h e r e c o i l i n g p a r t i c l e s may be 1 0
The l o n g e r t h e h a l f l i f e of a r a d i o a c t i v e s p e c i e s t h e more
atoms must be produced t o g ive a u s e f u l d i s i n t e g r a t i o n r a t e f .or
t r a c e r purposes, i . e . the l o n g e r i s t h e r e q u i r e d i r r a d i a t i o n
time. Consequently t h e l o n g e r t h e h a l f l i f e t h e lower w i l l be
t h e s p e c i f i c a c t i v i t y expressed a s t h e r a t i o of d i s i n t e g r a t i o n
r a t e t o i n a c t i v e molecules. - 1'4 E
For t h e r ea sons enumerated above C and H cannot be p r e -
pared by r e c o i l l a b e l i n g w i t h n e a r l y a s h i g h s p e c i f i c a c t i v i t y
a s can t h e halogens . T y p i c a l s p e c i f i c a c t i v i t i e s r e p o r t e d i n 6 3
t h e l i t e r a t u r e a r e 2 x 1 0 H d i s i n t e g r a t i o n s p e r minute pe r
mi l l i g r am of g a l k c t o s e prom i r r a d i a t i o n of a mix ture of Li,CO, 1 4
and g a l a c t o s e (25) and 420 C dep,m./mg, of a c r i d i n e from .
. i r r a d i a t i o n . o f t h a t compound ( 3 9 ) .
Other Labeling Methods
By Ioniz ing Radiation. By use of t h s chemical e f f e c t s of --- -. .
.C. nuclear t ransformat ions f t i s poss ib le t o produce l abe led mole-
cu les much more complex than can be read1 l y produced by chemical P\
syntheses . The m e t h ~ d has the disadvantage t h a t i t r e q u i r e s a
h igh i n t e n s i t y neutron source such a s a nuclear r e a c t o r ,
hrilzbach (37 ) has pioneered a new method of la:).eling with tritium
which promises t o have a l l of t h e advantages of t h e nuclear
a c t i v a t i o n method f o ~ t h i s element while being simpler t o ca r ry
, out and l e s s sub jec t t o reduct ion i n s p e c i f i c a c t i v f t y due t o
r a d i a t i o n damage, He has shown t h a t it i s possfb le t o l a b e l
complex compounds wi th t r i t i u m by simply allowing the des i red
compound t o s tand i n c o n t a c t with tritium gas , Rates of forma-
t f o n of the t r i t i a t e d form of the o r i g i n a l compound as h igh a s
2.2 m f l l i c u r i e s per curie-day exposure have been o b a ~ r v e d and
y i e l d s of t r i t i a t e d r a d i o l y s i s products aye s t i 11 g r e a t e r ,
When these spec ies can be separa ted f n high puri-ty each i s a
p o t e n t i a l l y use fu l t r a c e r compound. Among the labe l3d coxpounds
prepared have been c h o i e s t e r o l ' w i t h a s p e c i f i c 2-c t iv i ty of 64
mc./g. and d i g i t o x i n with a s p e c i f i c a c t i v i t y of 90mc ./g.
In the Wilzbach method the tritium e n t e r s combination a s
a r e s u l t of t h e i o n i z a t i o n and e x c i t a t i o n of molecules of the
medium by t h e b e t a r a d i a t i o n of the tritium (1, 24) . I d e n t i c a l
r eac t ions can be induced i n s h o r t e r times by i r r a d i a t i n g a
mixture of t r i t i u m and the organic compound with an in tense ex-
t e r n a l source of gamma r a d i a t i o n ( I ) , Mixtures of tritium with
methane y i e l d not only t r i t i a t e d methane but a igo t r i t i a t e d
I
hydrocarbons of each c h a i n l e n g t h up t o a t l e a s t C6 , and when . .
tritium i s a l lowed t o s t a n d w i t h h e x a n e birth c h a i n , l eng then ing
a n d . chaan de.gradatio;? occG.~, ' .' . . .
L A t approximately t h e . sam.e ti me t h a t tile unique r e a c t i o n s
A noted above have come. t o l i g h t t h e f o l l o w i n g ~ S s e r v a t i o n s have
been r epo r t ed : 1) Hydrocarbon ions formed by t h e e l e c t r o n beam
i n t h e i o n i z a t i o n chamber of a mass, spec t rome te r~show very h i g h + =I-
c r o s s s e c t i o n s f o r r e a c t i o n s such a s CH, + CH4-C,H, + H2
(28); 2 ) 0 rgan ica l l .g bound t r i t i u m i s formed.. when' gaseous T, a t
about 5 0 microns p r e s s u r e f s s u b j e c t e d t o a . f i e l d of on iy 200
volts/cm. between. e l e c t r o d e s c o a t e d w i t h va r ious o rgan ic comp~unds
' ( f o r t imes t o o s h o r t t o al low d e t e c t a b l e r a d i a t i o n induced 41' '1 4+
l a b e l i n g ) ( % ) ; 3 ) C i ons formed i n a mass spec t rometer and
a l lowed t o Impinge on s o l i d benzene y i e l d bo th benzene and 1 4 .
I s t o luene l a b e l e d w i t h C ( 2 0 ) .
Each of t h o t lunconvent ional l l r e a c t i o n s c i t e d i n t h i s s e c -
t i o n a s w e l l a s those i n i t i a t e d by n u c l e a r t r ans fo rma t ion ,
i nvo lves a mechanism of ac t t i va t i on which produces i o n s , The
sugges t i on i s obvious t h a t it may be pos . s ib le t o d . iscover many
u s e f u l l a b e l i n g r e a c t i o n s which. can be i n i t i a t e d by al10,wing
a lpha , b e t a o r gamma r a y s o r a n e l e c t r i c a l d i s c h a r g e t ,o i o n i z e
mix tu re s of s imple t agged molecules with o t h e r molecules . Con- 1 4 1 4 14 1 4
ven ien t source molecules may inc lude C 0, C 0,; C H4, C, H,, , 36 3 6 3 6 3 6 82 82
H2S , S 02, H C 1 , C l , , HBr , B r , , v a r i o u s t r i t i a t e d
(.
compounds, and many o t h e r s .
By Chemical Syn thes i s and By Exchange; The p r e p a r a t i o n of
l a b e l e d compounds by t h e use of n u c l e a r t r ans fo rma t ions and by
i o n i z i n g r a d i a t i o n both invo lve equipment and t e c h n i ques which
a r e u n f a m i l i a r t o many chemists . These methods g e n e r a l l y pro-
duce complex ~ n i x t u r e s of l a b e l e d compounds f rom which t h e d e s i r e d
components must be s c r u p ~ ~ l o u s l y purf f l e d be fo re use. Furthermore, . . if t h e molecule d e s i r s d h a s s e v e r a l atoms of she elemen? t o be
tagged tlze l a b e l i s usua1l.y rando~nly o r s em1 -r;andomly d i s tri bu-
t e d , For such r ea scns 'the g r e a t m a j o r i t y of a l l l a b e l e d com-
pounds produced f o r t r a c e r s t u d i e s t o d a t e ha1:e been those which
a r e sirnple enough f o r cliemical s y n t h e s i s , I n o r d e r t o o b t a i n
h igh y i e l d s from t h e r a d i o a c t i v e s t a r t i n g compcund, t o p reven t
unnecessa.ry d i l u t i o n of t h e tagged element w i t . h non-radioactive
i s o t o p e s , and t o avoid con tamina t ion of th.e l a b o r a t o r y , i t i s
o f t e n d e s i r a b l e . t o dks ign s y n t h e t i c procedures m i t a b l e f o r use
w i t h i-nilligram q u a n t i t i e s of r e a g e n t s , u s i n g vacuun l i n e tech.-
n iques ( 5 ) . I n a n e x t e n s i v e review of chemical and b i o s y n t h e t i c
l a b e l i n g methods Thomas and Turner ( 31 ) have c i t e d over f o u r
hundred i l l u s t r a t i v e r e f e r e n c e s , In such syn theses of compousc:l.:;
l a b e l e d w i t h ca rbon o r hydrogen i t i s customary t o s t a r t w i t h 1 4 3
BaC O3 o r H, gas , which a r e a v a i l a b l e from t h e a tomic energy
commission l a b o r a t o r i e s i n - s e v e r a l c o u n t r i e s , o r w i t h s imple 1 4 3
compounds of C o r H, a v a i l a b l e from commercial s u p p l i e r s .
I l l u s t r a t i v e of s imple chemica l l a b e l i n g p roces se s which
have 'been found u s e f u l i n our l a b o r a t o r y a r e t h e fo l l owing . 3 6
1) Many organ ic c h l o r i d e s may be e a s i l y l a b e l e d w i t h C 1 by 3 6 3 6
exchange w i t h A 1 C 1 3 ( 2 ) . AgCl i s p r e c i p i t a t e d f rom a s o l u t i o n 3 6
of H C 1 ob t a ined from tl?e Oak Ridge Na t iona l Laboratory , d r i e d
by f u s i o n under vacuum, and h e a t e d wi th A 1 me ta l t o produce 3 6
A 1 C 1 3 t o which, t h e o rgan ic c h l o r i d e i s exposed on t h e vacuum 8 2
system. 2 ) Labe l ing of o rgan ic bromides w i th B r may be
Or5
8 2 accomplished i n a n analogous f a s h i o n , t h e A l B r , be lng conven-
e 2 i e n . t l y prodil.ced by r e a c t i o n . of A 1 ~:i t ,h By, 1r!?!.cn has been formed
by i r i - a d i a t f on of lf.q,-did e lementa l F3pZ ?.XZ a rj72-cle a:? r e a c t o r , 8 2
3 ) Photocharnical reeztlox!.~ haire been 933!5 t.0 ayod~j-ce IBr and
9 % 82 and Bi-, , a t e lsvateci t,c.rr,p";>a.t~ires w i t h a ?4/iacciz laxp, !j.) HBr
may be p ~ e p a r s d by th.3 ~ ~ p i d c x s h z i ~ g s r ~ h i c h ozcurs c n a f x i n g 8 2
t ank HSr and Br, fo l iowed by removal of t h e E r a by con.l;act wf.i;h
mercury. 5 ) O ~ g a n i 2 f odides appear - to be qua.n.ti t a t i v e l g con-
v e r t e d t o khe c o r r e s p o n l i n g r a d i o b r c a l d e s by sin2le replacemer1-3 e.2 d .2
r e a c t i o n s w i th Bp, (17)e 6 ) B I ar:2 E C1 ( 8 1 have jeer1 ? r e - 5 0
pared by h a a t i n g mix tures of H, and 2, and of 11, + Z C 1 ever a
h o t p la t inum w i r e , 7 ) Aroaatkc compounds may be l a b s l e d w i t h 3 I
t r i t i u m by h e a t i n g wl.th HI C l i n t h e p r e s m c e of a c a t a l y s t such 14
a s SnC1, (8). 8 ) E t h y l bsnzenes l a b e l e d w i t h S 8 . ~ 3 produced 1 4
by p a s s i n g e. gaseous mix.tv.re of benzsne and C e t h y l ch?-~;.ido
ove r anhydrous aluminum c h l o r i d e (3 ) ,
B i o s y n t h e s i s , Labeled compounds t o o complex t o be p repared
by chemical s y n t h e s i s have i n a number of i n s t a n c e s been made 1 4
by f e e d i n g C 0, t o growing p l a n t s , Rather h i g h s p e c i f i c
a c t i v i t i e s of randomly l a b e l e d compounds can be ob t a ined by
s t a r t i n g with young p l a n t s , The u se of o t h e r growing organisms 1 4 3 2
f o r f n c o r p o r a t i n g C and o t h e r rad ioe lements , p a r t i c u l a r l y P 36
and. S . , has been r e p o r t e d by many i n v e s t i g a t o r s . Some of t h e i r
r e s u l t s a r e c i t e d by Thomas and Turner ( 3 2 ) and by Kamen (18)
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(1953 )
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N e w York, 1949,
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r ' l .I 1
RADIOACTIVITY-
. .
-. Figu re 1. Gas chromatogram of r a d i o a c t i v e p roduc ts of neu t ron
i r r a d i a t i o n of n-propyl bromide p l u s 5 mole $ bromine; d o t t e d
l i n e shows compounds added f o r , i d e n t i f i c a t f on of t r a c e amounts
of r a d i o a c t i v e s p e c i e s ,
Ftgure 2 , ,Gas chromatogram of . t r f ' t i a t e d -products from 3 3
H & ' ( ~ , ~ ) H r e a c t i o n i n C,H,. (56crn. C3H8, 6 mm. Ha , 5 mme 1 a
H,; 1 hr . i r r a d i a t i o n i n Argonne CPS r e a c t o r a s 2 x 1 0 -2 -1
neutron crn, see , ; s e p a ~ ~ a l ; e d on s I l i c a g e l es1umn)a
TEMP BATH
Figure 3, Gas chromatographic apparatus for identification
of radioactive compounds produced as a consequence of nuclear
reactions,
