69.0 mb, and 8 s / h = 0.59, where subscripts N and 0 correspond to the reactions N --c I5Oand 0 + 1 5 0 , respectively. For n N = no we would havef = 0.37-i.e., the interference error could not be neglected in this particular analysis. Our method has thus helped to appraise quickly the analytical situation and, furthermore, it can be used to solve this unfavorable case if the concentration of interference is known. Suppose then that the nitrogen concentration is independently determined in the germanium sample, for example by the reaction 14N(3He,a)13N,to be 20 ppb; assume also that a total 1 5 0 activity, D = 800 dpm, is measured at the end of an irradiation, during one half-life of l50,with a 10-pA beam of 6MeV 3He particles. We assign a 5 % error to this measurement; thus D = 800 + 40 dpm. To obtain the oxygen concentration we should first subtract from D the activity, Dx, caused by the 20 ppb of nitrogen interference. Our method provides a simple way to calculate D,; we simply compute the sensitivity, Ss,for the reaction 14N(:$He,d)150, caused by 6MeV 3He ions in a germanium sample. By substituting h, R = 14.03 mg/cm2(IO), and other known values in Equation 3 we obtain Ss = 23.0 dpm/ppb. Thus 20 ppb of nitrogen produce Dh.= 460 dpm of IjO. Since Table I11 indicates an error of 13.5 (the largest in the table) for the method when applied to this reaction, we have D N = 460 f 62 dpm. Finally, the activity of l60due to oxygen is D O = D - D N =
800 - 460 = 340 dpm; this value can now be directly used for comparison with the 150activity measured in the standard and the activation analysis would thus be completed. However, it is very important to know what is the error of D O ; it may be obtained by propagation: 1/402+622= 74 dpmLe., 22% of D o . This error is quite reasonable considering the unfavorable conditions chosen for our example : the present method can quickly show that the sought oxygen concentration is 10 ppb-Le., half that of the interference. Tables IV and V illustrate an interesting and unique characteristic of charged particle activation analysis: the influence of the range on the sensitivity, quantitatively expressed by Equation 3. The tables show that the sensitivities (column 5 ) grow faster than the average cross sections (column 4), as energy increases, instead of being directly proportional to the cross section as in neutron and photon activation analysis. Indeed, the influence of the matrix may be quite drastic; for example, the increase in yield or sensitivity due to the large range of particles in a heavy matrix like uranium (Table V) is about twice that observed in aluminum (Table IV).
RECEIVED for review September 5,1967. Accepted November 8, 1967. Research sponsored by the U S . Atomic Energy Commission under contract with the Union Carbide Corp.
__~______
Zone Refinilng for the Separation of Radioactive Trace Contaminants from Organic Compounds Bodo Diehn,' F. S. Rowland,2 and A. P. Wolf Departments of Chemistry, Unicersity of Kansas, Lawrence, Kans., and Brookhacen National Laboratory, Upton, N . Y The purification by zone refining of propionamide containing traces of radioactive acetamide as impurity was studied because of the particular difficulties involved in the separation of homologous solid compounds if conventional methods are used. The separation efficiency has been determined quantitatively as a function of travel speed of the liquid zone, number of zone passes, and impurity concentration. With the automatic zone refining apparatus described in this paper, the concentration of acetamide in propionamide was readily reduced by lo4 from its original concentration of 0.02% with a minimum of handling of the sample. The technique offers particular advantages in studies of nuclear recoil reactions in solid substances, and in other situations in which traces of very high specific activity contaminants are encountered.
THE PURIFICATION of organic compounds from radioactive contaminants to the level at which the melting point, infrared spectrum, etc., correspond to those of an authentic pure sample usually presents no special problems caused by the radioactivity. However, purification to radiochemical purity,Le., negligible contamination by radioactive impurities-can present major difficulties if the contaminant is present in trace, high-specific-activity quantities. Special cases of this kind are not uncommon, and occur frequently in the study of chemical Present address Department of Chemistry, The University of Toledo, Toledo, Ohio 43606 Present address Department of Chemistry, University of California Irvine, Calif. 92664 60
ANALYTICAL CHEMISTRY
reactions, especially noteworthy among which are those accompanying nuclear transformations. Two purification techniques currently known which effect consistently good separation from trace contaminants are derivatization and gas chromatography. The former, often involving complete chemical cycles, can frequently provide good purification (1). However, the preparation of derivatives can be very time consuming, and can also be accompanied by low chemical yields, such that recycling must be very limited in scope. Furthermore, the separation of members of homologous series may be quite difficult, Gas-liquid chromatography (2), which usually gives excellent separations, can be conveniently carried out on gases and liquids, but the handling of solid samples is in most cases unsatisfactory. A study was therefore undertaken to determine the applicability of zone refining as a means for achieving radiochemical purity. If applicable to a particular compound, the process can be completely automated, and has the additional convenience that it can be used for 1-100 gram samples per batch. Since its introduction by Pfann (3), this technique has been widely used for the purification of metallic elements, particularly in the semiconductor industry. Applications to organic (1) A. P. Wolf and R. C. Anderson, J . Am. Cliem. Soc., 77, 1608 (1955).
(2) R. J. Kokes, H. Tobin, Jr., and P. H. Emmett, Ibid.,77, 5860 ( 1955).
(3) W. G. Pfann, J. Metals, (New York,) 4, 747 (1952).
t
10-4k
10-4
-LIMIT OF DETECTION
FRACTION OF TOTAL LENGTH
Figure 1. Effect of travel speed on separation efficiency Distribution of 0.02% acetamide in propionamide after 60 zone passes. Travel speed : 0 50 mm/hr ; 0 25 mm/hr ; A 19 mm/hr compounds have also been reported (4, 5). However, while the technique has been applied to radioactive substances (&IO), no attempt has been made to utilize it for the purification of the radioactive products formed after nuclear reaction. EXPERIMENTAL Apparatus. The principal requirement of a zone refining apparatus is the provision for the passage of a molten zone through an otherwise solid ingot, accomplished in our apparatus by motion of the ingot through stationary heating coils. The organic material to be purified is poured while molten into 15 cm long, 5-mm i.d. borosilicate glass tubes which have been precut with a glass saw at 12.5-mm intervals. The sample tubes in our apparatus are arranged vertically, and travel upward for a distance of 15 cm through an alternating arrangement of four cooling and three heating coils. Thus, three separate molten zones pass through the verticallyrising sample in each upward cycle. When the upward movement is completed, the motion is reversed and the sample tubes are returned to the starting position rapidly enough (less than 30 seconds) to prevent melting during the downward movement. The cycles are repeated automatically, and recorded until the desired number of zone passes has been completed. All of the subsequent data are recorded in terms of the number of zone passes. (4) E. F. G. Herrington, “Zone Melting of Organic Compounds,” Wiley, New York, 1963. (5) W. R. Wilcox, Friedenberg, and N. Back, Chem. Rev., 64, 187 (1964). (6) M. Joncich and D. R. Bailey, ANAL.CHEM., 32, 1578 (1960).
(7) T. Nozaki, M. Tamura, Y.Harada and K. Saito, Bull. Chem. Sot. Japan, 33, 1329 (1960).
(8) H. Schildknecht and A. Mannl, Angew. Chem., 69, 634 (1957). (9) H. Schildknecht, Kerntechnik, 6 , 249 (1964). (10) P. Sue, J. Pauly, and A. Nouaille, Bull. SOC.Chim. France, 1958, 593.
-LIMIT OF DETECTION
FRACTION OF TOTAL LENGTH
Figure 2. Determination of optimum travel speed Distribution of 0.02% acetamide in propionamide after 120 hours. Number of zone passes, travel speed 0 120 passes, 50 mm/hr; A 60 passes, 25 mm/hr; a 45 passes, 19 mm/hr; 0 24 passes, 10 mm/hr; A 18 passes, 7.5 mm/hr
After completion of the separation, the sample is divided for assay into 12 fractions at the precut locations. Because the zones move from top to bottom, the impurities are concentrated at the bottom of the tube. In this apparatus, simultaneous purification of nine 2-ml samples is possible, and the temperature is variable individually for each of three groups of three samples, with an upper limit of 350” C. Travel speeds can be varied between 7.5 and 150 mm/hour. EVALUATION OF THE APPARATUS FOR SEPARATION OF RADIOACTIVE TRACER IMPURITIES Use of Radioactive Tracer Impurities. These experiments have been oriented toward the solution of separation problems in neutron-irradiated organic solids, especially those involving the presence of small quantities of impurities from the same homologous series, as with acetamide in propionamide. The use of tracer level radioactive acetamide (106 dpm 14C per mg of C) permitted quite accurate measurements of much lower impurity levels than had been possible by differential thermal analysis in previous experiments with organic compounds (6). For most of the runs concerned with the evaluation of the apparatus, solid solutions of 0.02% radioactive acetamide in pure propionamide were used. This propionamide was only of conventional organic chemical purity, and other, unidentified, nonradioactive impurities were presumably present in macroscopic quantities greater than those of the acetamide impurity. The assay method used for determining the specific radioactivity of 14Cwas that of Christman et al. (11). A low level counting apparatus (Sharp-Beckman “Lowbeta” electronics) ____ (11) D. R. Christman, N. E. Day, P. R. Hansel], and R. C. Anderson, ANAL.CHEM., 27, 1935 (1955). VOL 40, NO. 1, JANUARY 1968
61
was used to count low activity samples. The limit of the apparatus was 0.1 dpm f10 %. Optimization of Travel Speed for Separation. The initial objective for the first experiments was the empirical determination of a travel speed which would require a minimum of time for a maximum in purification. If the passage of the molten zone through the ingot is slow enough, the impurity can equilibrate throughout the zone, and the maximum separation per zone pass can be obtained. On the other hand, the longer the time allowed for equilibration, the fewer the completed zone passes per unit operation time for the apparatus. Optimization of travel speed in this manner may involve speeds somewhat faster than those permitting equilibration within the accuracy of the experiment. One sample each was run at speeds of 50, 25, and 19 mm/ hour until 60 zone passes were completed. The distribution of radioactivity in the ingots after completion of the runs is shown in Figure 1. Five samples were zone-refined at different travel speeds of the molten zones for a total running time of 120 hours each. These results are shown in Figure 2 for experiments involving 120 zone passes at 50 mm/hour, 60 at 25 mm/hour, 4.5 at 19 mm/hour, 24 at 10 mm/hour, and 18 at 7.5 mm/hour. For our apparatus and the propionamide-acetamide system, the maximum separation at constant time is achieved at about 25 mm/hour. At higher travel speeds, the crystallization boundary moves too fast and impurities are occluded, while at much slower speeds, the total number of zone passes completed is too low. Separation Efficiency. The ultimate distribution, representing the maximum obtainable separation possible for a given system (the steady-state distribution for an infinite number of zone passes), can be calculated from the exponential formula given by Pfann (IZ), in which the constants A and B are related C(x) = A exp (Bx)
B exp ( B ) - i
(3) In this theoretical formulation, the separation efficiency is independent of the ingot length until backward reflection of the impurity pile-up at the end of the ingot is propagated into the region under consideration-in crude approximation, until the number of zone passes is equivalent to y , the ratio of ingot length to length of the molten zone. For the difficult separations for which these procedures may be especially useful, the distribution coefficient may be near unity, [B ZZ 2(1 - k ) (1 - k ) 2 ]and the ultimate distribution can be approximated as
+
*)
co
= B y exp [(x
- y)BI
(4)
(12) William G. Pfann, “Zone Melting,” Wiley, New York, 1958.
62
o
ANALYTICAL CHEMISTRY
1
Figure 3. Separation efficiency for different numbers of zone passes at 25 mm/hr Distribution of 0.02 acetamide in propionamide after zone refining. Number of zone passes at 25 mm/hr: A, 15 passes; 0 , 25 passes; A, 60 passes; 0, 240 passes
and can be described by the maximum purification attainable at x = 0, as given in Equation 5.
‘(O) co
(1)
to the length of the molten zone, I, the length of the ingot, L, the distribution coefficient, k , between the crystallizing solid and the molten liquid, and Co,the initial, uniform concentration of the impurity throughout the ingot. If the lengths are given in multiples of the length of the molten zone, then A and B are given by Equations 2 and 3, in which the total ingot length is given by L = yl, and x can vary between 0 and y .
k =
FRACTION OF TOTAL LENOTH
- BY exp ( - B Y )
Examination of Equation 5 makes it apparent that good separations depend quite strongly upon having a large value of y , the ratio of ingot length to molten zone length. The value of (By) corresponding to C(0)/Co = is 11.7, indicating that the values of y must be 17,27, and 56 fork = 0.7,0.8, and 0.9, respectively,with a limiting value such that y(l - k ) E 5.8. Empirical observation of the exponential fall-off of impurity concentration in some of the samples of Figure 1 and 2 indicates that zone refining can be a very efficient purification procedure indeed for acetamide in propionamide. The impurity concentration, already very low at the start (0.02%), is decreased by a factor in excess of lo4 in the two runs at lower travel speed in Figure 1. The straight line on the semilogarithmic plot at low impurity concentrations is consistent with the expectations from Equation 1, while the experimental deviations above the exponential function at the highest purifications are not necessarily of any theoretical significance. Not only are the errors large near the limit of assay sensitivity, but such experimental deviations could arise from inclusion in the original radioactive acetamide of 0.01 radioactive propionamide. Such an amount of radioactive impurity could possibly have been formed by self irradiation of the high specific activity acetamide during storage. In our apparatus, a zone length of 5 mm was sought (y 30), and could be maintained satisfactorily in the purer portions of the ingot, although it tended to increase in length as the zone entered the less pure end of the ingot. Com-
-
4
t I
IT-=--
n
-4
!-
5 V W
a u)
W
5
l-
a
J W E
IO-^^ E I
I 0-5
0
.I
1 I I I I I I .2 .3 .4 .5 .6 .7 .8 FRACTION OF TOTAL LENGTH
\T
3
I .9
LO
2
0.5
,
1.0
L
FRACTION OF TOTAL LENGTH
Figure 4. Separation efflciency for macroscopic impurity concentrations Distribution of 3 acetamide in propionamide after zone refining. Number of zone passes at 25 mm/hr: 0, 120 passes; A, 480 passes
-
parison of the extrapolated exponential fits (C(O)/C, with the ultimate distribution expected for y = 30 indicates that the effective distribution coefficient for acetamide in propionamide is k 5 0.8. The degree of separation obtained after different numbers of zone passesat 25 mm/hour is shown in Figure 3. It is evident from these experiments that 60 zone passes are sufficient to purify the top 10% of the ingot to the limit of our assay method if only a trace amount of acetamide is present as an impurity, Calculation of the number of zone passes required to approach the ultimate distribution is complicated, and is not worthwhile for the present experiments. Clearly, the exponential behavior of the impurity concentration does not hold throughout the entire ingot, nor would it be expected to hold, as the experiments are well away from the ideal situation for such comparison with theory. The chief experimental improvements necessary for facilitating such a comparison would involve manipulation of the travel speed to ensure equilibration, and prepurification of the propionamide to remove nonradioactive impurities. Thus, while the propionamide was of good chemical quality, visual observation showed some accumulation of nonradioactive impurities, and the basic assumptions of Equation 1 concerning invariance of k , constant length of molten zone, etc., obviously did not hold. Purification of Carrier Substances by Zone Refining. While radioactive products formed in nuclear reactions are usually present in irradiated materials in even smaller concentrations than 0.02 % (13), the procedure of zone refining can be useful not only in purification of the parent substance, but also in determination of the radioactivity present in the form of other compounds through the addition and subsequent zone refining of nonradioactive carrier substances. In these cases, (13) A. P. Wolf, Ann. Rev. Niicl. Sci., 10, 259 (1960).
I
-2
lo 0
Figure 5. Purification of radioactive products from reaction with acetamide
14C
0 Distribution of I4C radioactivity in neutron-irradiated acetamide after 240 zone passes at 25 mm/hr 0 Radioactivity per gram of C from thallous acetate obtained from
acetamide A Distribution of I4C radioactivity in 10 :I mixture of propionamide and neutron-irradiated acetamide after 240 zone passes at 25 mm/hr A Radioactivity per gram of C from thallous propionate obtained
from propionamide a small amount of the irradiated parent material is added to a large macroscopic nonradioactive quantity of the compound whose presence as a trace radioactive impur ity is suspected. The original parent material is now an impurity in the larger bulk of carrier material, and the latter is freed from such impurities by the zone refining technique. Experiments were carried out with a 3% solution of acetamide of lower specific radioactivity in inactive propionamide to test the separation efficiency with macroscopic impurity levels. The results of two runs with this mixture are shown in Figure 4. It is evident that the number of zone passes must be increased significantly for higher macroscopic impurity concentrations in order to achieve satisfactory purification. Some time can be saved if the carrier can be prepurified by conventional means, such as vacuum sublimation, to impurity levels of 1 % or lower, prior to zone refining. Even without prepurification, the principal problem is only the additional time required in the automatic apparatus for purification at these macroscopic impurity levels, Application to Radioactive Impurities Formed by Nuclear Reaction. Thermal neutrons react with the 14N atoms of nitrogenous materials to form 14C atoms with excess kinetic energy. After loss of most of this excess energy in collisions with the surrounding medium, the 14C atoms react with the parent material in hot (as well as thermal) reactions (13). With acetamide parent material, such reactions can lead to radioactive acetamide, propionamide, and succinamide as well as a number of other compounds. This mixture of radioVOL 40, NO. I , JANUARY 1968
63
active compounds in the acetamide lattice furnishes a typical example for the application of the fractional fusion technique to the purification of the parent acetamide and to the determination of the radioactivity of two trace impurities in the acetamide. A 10-gram sample of acetamide was irradiated in the Brookhaven Research reactor for 140 hours. After degassing, an initial total radioactivity of 28.0 mp c/mg C (1 mpc = 2220 dpm) was calculated froni radio gas cnromatographic analysis of the gaseous fraction, plus combustion of the degassed solid. Five grams of the degassed acetamide were purified by two successive vacuum sublimations, and the purified material (4.47 mpc/mg C) filled into a sample tube and subjected to 240 passes at 25 mm/hour. The sample was then cut into 12 fractions, and the specific activities were determined for each fraction. These specific activities are shown in Figure 5 . Evidently, the top five cuts consisted of material of constant specific activity, as would be expected if the radioactivity represented only the radioactive acetamide left after remove1 of trace radioactive contaminants. The presumed radiochemical purity of the acetamide was checked by additional experimental work. The top four fractions were hydrolyzed to sodium acetate by reflux with 3 5 x NaOH for 12 hours. Acetic acid was isolated via continuous ether extraction of the acidified hydrolysis mixture, separated from the ether by preparative GLC, further purified by recycling in the GLC apparatus, and then converted to thallous acetate (14). The activity per gram of carbon of the thallous acetate is presented on the right margin of Figure 5 (solid circle) and is in excellent agreement with the value originally obtained from the material purified by zone refining. The radioactivity present as propionamide-14C was determined from a homogenized (by melting) mixture of 0.44 gram of the same degassed neutron-irradiated acetamide and 4.40 grams of pure propionamide. The mixture was sublimed twice, and then purified by 240 zone passes at 25 mm/hour. The three top segments of the purified propionamide indicated constant specific radioactivity, as shown in Figure 5. Hydrolysis and purification of these combined fractions gave thallium propionate with the specific activity indicated in the right margin (solid triangle), again in very good agreement (14) A. P. Wolf, C. S. Redvanly, and R. C. Anderson, J. Am. Chem. SOC.,79, 5717 (1957).
64
ANALYTICAL CHEMISTRY
with that obtained after zone refining alone. This experiment illustrates the combination of zone refining with carrier techniques for the isolation and measurement of the tracer impurities themselves. The determination of the activity present as succinamide*4C was carried out in analogous fashion for a different sample of neutron-irradiated acetamide by mixing 10 by weight of pile material with the carrier succinamide prepurification by vacuum sublimation, zone melting, and assay of the various cuts. Succinamide was converted to succinimide by distillation prior to fractional fusion. Again, the constant specific radioactivity in the top fractions was confirmed by derivatization, and is illustrative of radiochemical purity achievable through the zone refining technique. SUMMARY
Our experiments have been conducted largely from the empirical point of view of seeking a satisfactory purification method for radioactive components in neutron-irradiated solids. Zone refining is shown clearly by the data presented in this paper to be highly suitable for the purification of irradiated acetamide, and can presumably be applied with equal success to many other irradiated organic solids. Once the appropriate experimental parameters have been determined, purification to the limits of sophisticated assay methods is possible, even with homologous compounds as impurities. In addition, the application of the high sensitivity toward detection of radioactive tracers indicates that more detailed experimental applications of zone refining, together with comparisons to the appropriate theories, can be readily attempted in many organic systems. ACKNOWLEDGMENTS
We thank D. R. Christman and Mrs. C. Paul for radioassays, and R. Withnell for the detailed design of the zone refining apparatus. RECEIVED for review October 9, 1967. Accepted October 27, 1967. Research carried out under the auspices of the U. S. Atomic Energy Commission and submitted in partial fulfillment of the requirements for the Ph.D. degree, University of Kansas, 1964, by B. Diehn.
