Oxygen-18 Determination by Counting Delayed Neutrons of Nitrogen-17 SAADIA AMIEL and MAX PEISACH
P.0.B. 527,
Soreq Research Esfablishmenf, Israel Atomic Energy Commission,
b Neutron counting of 4.14-second nitrogen-17 i s used for the analysis of oxygen-1 8 in solutions containing known of amounts lithium-6. Nitrogen-1 7 i s produced b y the successive nuclear reactions LiG(n,a)t; O'*(t, a)N1' on irradiation with thermal neutrons. The total delayed neutron emission of a sample irradiated to saturation a t a thermal neutron flux of 1 O % / C ~ . ~ neutrons per second i s about 4 X mg. of lithium-6 per atom per cent oxygen-1 8. The nitrogen-17 formation i s proportional to the atom fraction of oxygen-1 8 and i s independent of the sample size.
lo5
by measuring the delayed neutron emission from nitrogen-17. Seutrons can be counted selectively and efficiently without interference from 6- or yray background. Furthermore, neutron emission in radioactive decay is a sufficiently rare phenomenon t o be specific for the detection and measurement of a particular neutron precursor. The formation of nitrogen-17 in neutron irradiated samples was recently used for the analysis of lithium-6 in aqueous systems (4). Nitrogen-17 decays as follows: x 1 7
8-
4.14 sec.
017* +
prompt
0'6
HF, INPORTAXE of oxygen-18 as a T t r ? cer for olvgen in various fields of research created the need for suitable analytical techniques. The common method of determining oxygen-18 content is by mass spectrometry (IO), b u t radioactivation techniques have been tried as well ( 1 1 ) . Although mass spectrometry is a very sensitive high precision method, its use is sometimes restricted ,1 the relatively high cost of the equipment; additional drawbacks are the need t o use m c u u m apparatus, the relatively long procedure, and the destruction of the sample analyzed. It frequently happens t h a t the more rapid methods of analvsis by radio activation are sufficient and are preferred even though they are less accurate and sensitive than mass spectrometry. I n the main, analysis of ouygen-18 by radioactivation is carried out by nieasuring the activity of oxygen-19 or fluorine-18 (6). These methods are based on y-ray spectrometry, coincidence counting, or, t o a more limited ehtent, autoradiography where beta eniiqsion is measured by the blackening of photographic emulsions (?). The main limitation of all these methods is the interference from other components in the sample. nhich frequently may be major constituents. Recently attempts to determine ourgen-18 were reported in ~ l i i c hthe number of particles emitted during a nuclear reaction were measured-e.g., neutrons (S) from the reaction 015(a,n)Se21and alpha particles ( 6 ) from the reaction 0 1 8 ( d , a ) Y s . I n this n ork an attempt was made t o away ouygen-l8 in water samples
Rehovoth, lsrael
+ n,
Qm
= 4 57 n1.e.v.
It can be formed directly from oxygen isotopes with fast neutrons in a reactor, by the reactions (1): O ~ ' ( Rp, ) Y and 018
7 ,
Qm = - 7 . 9 3 m.e.v. (1)
(n,d ) N17, Qm =
- 13.77 m.e.v.
(11)
With thermal neutrons nitrogen-17 cannot be produced directly, but in the presence of lithium-6, secondary reactions with the 2.7-m.e.v. tritons from Lie(n,ol)t can lead t o nitrogen-17 formation by the reactions 018
( t , a) W
7 ,
Qm
= 3 . 8 2 m.e.v.
(111)
p ) N17, Qm = 0.15 m.e.v.
A , = [Li6]ianaQIn A,
=
an (IV)
The cross section of reaction I for unmoderated fission neutrons is not well established and has on separate occasions been reported t o be 5 and 9 pb. (9, I S ) . The cross section for reaction I1 is not known, but can be taken t o be about two orders of magnitude smaller than for reaction I. because of the corresponding drop in the abundance of neutrons in fission with the appropriate energy. I n natural water, the contribution of reaction I1 might account for a few per cent of the total nitrogen-17 produced by fast neutrons. Nevertheless, in oxygen-enriched water where the ratio of oxygen-18 to oxygen-17 may be higher than in natural water, the relative contribution of reaction I1 to the total yield of nitrogen-17 may become important or even comparable to t h a t of reaction I ( 2 ) . The number of delayed neutrons
(1)
where [Lis]
and in the presence of nitrogen by N 1 6 (1,
obtained by reaction I from the nitrogen-17 produced from a 1-gram sample of natural water irradiated to saturation in a fission spectrum neutron flux of 102n/cm.2-second should be about 325, when the cross section is taken as 5 pb. When lithium-6 is introduced into the water, the formation of nitrogen-17 proceeds predominantly by reaction I11 because of its high effective cross section with thermal neutrons (4). A 1-gram natural water sample containing 1 mg. of lithium-6 yields about 9000 delayed neutrons by this reaction when irradiated to saturation in a thermal neutron flux of 10L2n/cm.2second. It follows t h a t in water samples containing lithium-6 irradiated in a mixed flux, containing thermal and fast neutrons caused by partial moderation, the production of nitrogen-17 by reactions I and I1 is relatively small and reaction I11 could then be used for the measurement of the ouygen-18 content of the water. The rate of formation of tritons by thermal neutron bombardment of lithium-6 is given by
= =
un,@ =
the number of tritons produced per second, the number of atoms of Li6 the thermal neutron flux per per second, the thermal reaction cross section of Lie(n, a ) t .
Since the range of tritons is very small ( 1 4 ) , the probability of a triton producing a n atom of nitrogen-17 is proportional t o the atom fraction of oxygen-18 provided that the solution is sufficiently dilute with respect to lithium t o ignore collisions with the solute. It is assumed t h a t the energy loss of the triton per collision with a n oxygen atom is independent of the isotopic composition of oxygen and that the hydrogen composition is kept constar,t. Thus A,
0:
Atf(OIE)
(2)
where A,, f(0'8)
=
the number of nitrogen-17 atoms produced per second = the atom fraction of oxygen-18
VOL. 35, NO. 3, MARCH 1963
323
combining Equations 1 and 2 one gets A,
gn,a
On [Lie]j ( 0 l 8 )
which, for a known constant thermal neutron flux reduces to A,, 0: [Lie]f ( 0 l 8 )
(3)
Thus, the rate of production of nitrogen-17, in a sample of Li8-Hz0l8 irradiated with thermal neutrons, should be independent of the water content and should be proportional to the total lithium-6 content and the atom fraction of oxygen- 18. The above derivation implies that the analysis of the oxygen-18 concentration in water can be carried out with thermal neutrons with any sample size. The only proviso is that the lithium concentration should be low for two reasons; first, to avoid extra energy loss of the tritons by collision with the relatively light lithium, which will result in a lower effective cross section, and second, to obviate attenuation of the irradiation neutron flux within the sample by selfshielding. In a well-thermalized flux, interference by reactions I and I1 is made negligibly small. When a mixed neutron flux is used for the analysis care should be taken to subtract the nitrogen-17 yield produced with fast neutrons by the direct reactions. Such a correction can be made by reirradiating the sample in a cadmium box which will absorb Table I.
I
EXPERIMENTAL
Preparation of Samples. Samples of water containing up to 97.04 atom per cent oxygen-18 were obtained from the Weizmann Institute of Science, Rehovoth, Israel, and were analyzed by mass spectrometry. From the supplied water, samples of lower oxygen-18 concentration were prepared by mixing known weights of enriched and natural water. Lithium-6 was obtained as metal containing 96.1 atom per cent Lie from Oak Ridge National Laboratory, Oak Ridge, Tenn., U.S.A., and from it all lithium salts were prepared. Synthesized lithium salts were checked for lithium content by chemical analysis, due account being taken of the decrease in atomic weight with increasing proportion of lithium-6. The water samples used in this investigation contained lithium-6 as io-
Calibration of Neutron Count from Oxygen-1 8 Enriched Samples I1 I11 IV V VI VI1 VI11
0 18 0 17 atom % pg.atoms 97.04 66.53 30.13 21.06 17.39 13.53 9.317 7.085 4.257 1.965 0.204
thermal neutrons and thus effectively eliminate the contribution from reaction I11 without changing those from reactions I and 11. Another way of correcting for the above mentioned interference relies on the fact that the yield of nitrogen-17 from reactions I and I1 is proportional to the heavy oxygen content or sample size, while that from reaction 111 is independent of this parameter and depends on the lithium-6 content. Accordingly by reirradiating the sample with a changed lithium-6 concentration, the net effect of reaction I11 could be obtained.
222.3 166.5 114.9 93.7 76.9 65.3 424.7 44.9 39.4 27.6 29.8
Li6 mg.
Xeutron counts UnScreened Difference Ratio screened Cd (IV) - (V) (IV)/(VI) Lie as nitrate or carbonate
0.3915 0.5102 0.01368 0.01357 0.01722 0.03431 0.1119 0.1553 0.1142 0.3064 0.3214
228832 206485 2844 2046 2082 2986 7666 6523 2956 3563 430
4074 4084 447 348 300 282 14G9 273 80 141 50
224758 202401 2397 1698 1782 2704 6197 6250 2876 3422 380
1,018 1.020 1.186 1,205 1.168 1.104 1.237 1.043 1.028 1,041 1.132 5858
Counts per mg. of Li6 per atom c7- nu
(?fj/
(I)(III)
Mean: Standard deviation: 1105 Relative standard deviation 88.29 77.32 65.16 56.42 35.73 9.33 2.52 0.204
324
207.1 187.1 27.8 147.3 104.6 43.2 127.8 20.8
Li6 as iodide 5850 0.6366 332961 0.5187 237846 4576 0.01264 5027 203 n.. 41.11 145407 3010 __._ _. _. . . 62749 1623 0,2994 367 0.2059 12289 322 0.0990 1685 0.2353 318 37
ANALYTICAL CHEMISTRY
327111 1.018 233270 1.019 4824 1.042 1.021 ~ 142397 ~ i.027 61126 1.031 11922 1.236 1363 281 1.132 Mean: 5855 Standard deviation: =kt229 Relative standard deviation 1 3 . 9 % ~~
~~
5916 5963 5815 5945 5952 5824 5944 5680 5916 5683 5796
=k 1 , 8y0
5820 5816 5857 6110 5714 6209 5463 5554
dide, nitrate, or carbonate. Because of the hygroscopic nature of lithium iodide the preparation of its solutions was carried out in a glove box by weighing the water sample before and after the addition of the salt. With the other lithium salts, stock solutions of known lithium-6 content were prepared with water highly enriched with oxygen-18. Small weights of the stock solutions were used to prepare samples with decreasing atom per cent oxygen-18 water. Errors of the order of 0.2 mg., which were inherent in the direct addition of about 10 mg. of LiI to 1-gram water samples, were excluded in this second procedure. In all cases the lithium-6 concentration was kept well below the level where self-shielding might introduce unnecessary errors. Sample Containers. The irradiation sample containers were polyethylene vials 51 mm. in length and of 8.5-mm. i.d. The vials were closed with caps which themselves could serve as miniature vials 7.3 mm. deep and with 8.3-mm. i.d. Samples were placed either in the main body of the vial or, when small samples had to be irradiated, into the caps, and were heat-sealed with polyethylene disks before irradiation. By placing small volumes of solution into the caps, the decreased free volume of the container restricted the movement of the sample thus ensuring better reproducibility of irradiation position. Sealed samples were mounted in polyethylene carriers (rabbits) which were used in the pneumatic tube facility of IRR-1 (the Israel 5 h i w . swimming pool reactor a t Soreq). Irradiation. The rabbits were irradiated a t the face of the reactor core where the thermal flux was 4.7 X 10l2 n/cm.*-second and the fast (fission spectrum) neutron flux was 1.1 X 10la when the reactor operated a t 1 Mm., the power level a t which all the experiments were carried out. Samples were irradiated for 30 seconds to reach saturation activity of nitrogen-17 and counted for 25 seconds after a delay of 5 seconds. Cadmium boxes of 0.03-inch wall thickness mere used when discrimination against thermal neutrons was necessary. Counting. The counting assembly consisted of a ring of six BlOF, neutron counters connected in parallel and embedded in a block of paraffin 40 X 40 X 40 cm. with a cavity in the center of the ring along the axis of the counters. During counting the sample was positioned half-may along the cavity in the center of the block. Pulses from the counters were fed through an amplifier to a fast scaler which was started autoniatieally by an electronic timer 5 seconds aftei the end of irradiation. The timer was triggered by the rabbit leaving the irradiation position. Discrimination against y-rays was checked by placing a 6-mc. Cos0 source in the counting position, Photoneutron emission due to (y,n) reactions induced by high energy y-rays from activation of sample and container impurities on the deuterium of the paraffin block, were negli-
gible. Stability was checked daily with a standard RaD-Be neutron source. A counting rficiency of about 7y0 was obtained for a known spontaneous fission neutron source. The counting duration of 25 seconds was sufficient to record most (>98%) of the neutron counts obtainable from nitrogen-17. RESULTS
Calibration. h'eutron activities obtained, with and without cadmium screening, from 1-gram water samples of varying isotopic composition of oxygen and containing known amounts of l i t h i u m 4 are given in Table 1 where the more consistent results with nitrate and carbonate are separated from those with iodide. The difference between the two measured counts as given in Column VI of Table I was taken as the net delayed neutron count induced by thermal neutrons in the irradiated samples. The value, normalized to unit atom per cent OI8 (Column VIII), gave a mcan value of 5858 counts per mg. of Lib per atom per cent 01*for samples irradiated a t a power level of 1 Mw. (rel. std. dev. 1.13%). The fact that this value is constant irrespective of what lithium compound was used proves that the neutron count obtained per mg. of Lie is directly proportional to the atom fraction of oxygen-18 over the entire range as given in Equation 3. The relative standard deviation of the results for samples prepared by adding solid lithium iodide t o the water was about 4%, largely due to the hygroscopic nature of the salt; but results from samples prepared by mixing weighed solutions of lithium and water had a relative standard del iation of about 2%, the over-all relative standard deviation of the nietliod. Some results of analyses on water samples of known concentration of oxygen-18 are given in Table 11. The individually calculated error of the determinations, reaffirm that the method has a relative standard deviation of about 2%, but the errors in replicate analyses tend to fall together rather than to give a random spread, thus showing that errors in sample preparation make up a large part of the overall error. Effect of Sample Size. T o determine the extent of escape of tritons with diminishing sample dimensions, which would show up as a decreased neutron count, weighed portions from 1 to 1000 pl. of a single solution were irradiated and counted. The solution contained 25.1 mg. of lithium-6 per ml. in water enriched to about 15 atom per cent oxygen-18. The variation of the neutron count per unit volume with the sample size is shown in Figure 1. I t was found that the neutron count per unit volume is independent of
400
-I I I I I I I I I m 7 7 " 7 \ r r
I
I
I
I
I -l-T-i-i7
I
i
11 I I I
-
P
a
-
a
a
d
.-g300 - .-EE
-
L
0)
a c
200
-
-
a
"
? I
100
-
; , I
I
I l l l l l l l l l l l l l l l
'v
1
I
1
sample size down to less than half a drop, 10 pl., below which the activity falls rapidly with decreasing sample size. The concentration of lithium-6 in the solution used for this test was much greater than for the main investigation. Such a high concentration was necessary to obtain sufficient counts from small solution volumes without resorting to repetitive irradiations and cumulative counting. The expected loss of neutron counts due to self-shielding of lithium-6 was observed for sample volumes exceeding 400 pl. (not shown in Figure 1) and increased to about 17% for a volume of 1000 p1. The observed self-shielding in this series of experiments consists in practice of three effects; self-shielding by the solute during irradiation, attenuation of the tritons produced in the sample, and the absorption of the moderated neutrons of nitrogen-17 during counting. All three effects depend on the concentration of lithium-6 and the shape of the sample. When cadmium screens are used in the experiment, a n apparent self-shielding effect of up to ~ 1 0 % is observed as the result of absorption of thermalized neutrons in the counting assembly.
1
I
1
1
I
I
I
1
1
1
1
1
Table 11. Comparative Analysis of Oxygen-1 8 in Water
Atom 700 1 8 Known Found 61.11 40.42
neutron count of nitrogen-17 in aqueous solutions of lithium-6 irradiated with thermal neutrons is proportional to the atom fraction of oxygen-18 in the water, provided the lithium-6 concentration is kept constant. This relationship, which has been derived in Equation 3, appears to be valid over the entire range of concentration of oxygen-18; furthermore, the count per milligram of lithium-6 does not seem to be affected
I
by variations in the lithium-6 conccntration (Column VIII, Table I). Type of Neutron Flux. As the irradiations were carried out with a mixed neutron flux, the activities obtained in unscreened samples (Column IV, Table I) were higher than expected by reactioii III alone. This difference is due to reactions I and I1 which proceed with fast neutrons, the contribution of which is eliminated by taking the difference between the neutron counts obtained from the bare and cadmium screened sample (Column VI, Table I). The values thus obtained give a measure of the nitrogen-17 formation by a pure thermal neutron flux whereas the values obtained from unscreened samples also include the contribution of the O"(n,p) and the 0l8(n,np)reactions as well as an additional contribution of reaction I11 induced by the epicadmiurn neu-
DISCUSSION
It has been verified that the delayed
I
15.17 13.77 4.843 0.204
(natural)
Error, yo
62.94 62.27 61.97 39.57 39.53 39.77 15,26 14.87 14 83 14.05 14.03 13.67 4.836 4,854 4.855 0.198 0.209 0.199 0.206 0.200
VOL. 35, NO, 3, MARCH 1963
+3.0 +1.9 +1.4 -2.1 -2.2 -1.6 $0.6 -2.0 -2.3 +2.0 +1.9
-0.7 -1.4 +2.3 +2.5 -2.9 +2.5 -2.5 $1.0 -2.0
a
325
100 200 I
10
100 A ~ O ~C"S
1000
per atom Li6
Figure 2. Variation of activity parameter, P (see text), with atom ratio of 0lsto Lie
trons. The ratio of these two values, (Column VII, Table I) gives the extent of interference by fast neutrons. For about 1 gram of oxygen-18 enriched samples containing about 0.2 to 0.5 mg. of lithium-6 the interference amounts to 2 to 475, but with lower lithium-6 content the effect intensifies and may reach about 20%. The formation of nitrogen-17 by reactions I and I1 depends on the content of heavy oxygen isotopes and hence on the sample size, while that by reaction I11 is independent of volume. For analysis, the use of smaller size water samples with the same lithium-6 content should reduce the extent of interference by fast neutrons corresponding to the decrease in the total amounts of oxygen-17 and -18 in the sample. The relative contribution of fast neutrons to the formation of nitrogen-17 as indicated by the values in Column VI1 of Table I is representative of the ratio of slow to fast neutrons prevailing during the irradiation of the sample. A neutron flus with a different energy distribution should yield different results, but the contribution of the thermal neutrons as obtained by difference betxeen counts of bare and screened samples (Column VI, Table I) is proportional to the atom fraction of osygen-18 and is independent of the energy distribution of the reactor neutrons. Clearly, the use of a well thermalized neutron flus eliminates the need for cadmium screening and a double irradiation, since the count obtained from the unscreened sample is due only to reaction 111. Ratio of Oxygen-18 to Lithium-6. To obtain neutron counts t h a t are neither too high a n d may be subject t o coincidence losses nor too low and
326 *
ANALYTICAL CHEMISTRY
300 400 500 600 700 800 900 1000 Volume solution
10000
(microliters)
Figure 3. Variation of concentration of lithium-6 with volume, for spheres in which self-shielding effect amounts to
2%
subject to relatively large statistical errors, the lithium-6 content of samples may be suitably adjusted. It then becomes necessary t o establish t h a t t h e results are not affected by differences in t h e oxygen-18 t o lithium-6 ratio. This can be derived from Equation 3, which can be transformed into:
where f(Ol8) = [018]/[H20]and where square brackets are used to denote the number of atoms 0l8and molecules of water. -4plot of the ratio [018]/[Li6]against the parameter P = A,[H20]/[Li6]2 should be linear. Such a test was carried out and the results, plotted in Figure 2, show that the relationship holds over a range of ratios from about 1 to l o 4 and hence that the nitrogen-17 yield per mg. of Li6 per atom per cent 0l8 is independent of the relative composition of the nuclides that take part in the reaction. This extends the validity of Equation 3, beyond the range covered by Table I. Effect of Sample Size and Selfshielding. The nitrogen-17 activity per unit volume solution induced by thermal neutrons is independent of sample size as is shown in Figure 1. This suggests t h a t samples as small as a drop or even less can be used for analysis. The increase in sample activity obtained by using higher concentration of lithium-6 will be partially counteracted by self shielding which will introduce an error in the analysis. This effect as calculated for spherical samples, and approximated t o other geometrical shapes (8, I @ , is a function of three factors, which are the reaction cross
section, the concentration, and a geometry factor of the form 4V/S where S is the surface area of the sample of Volume V . It follows that as the volume of a sample decreases the effect of self shielding diminishes and larger concentrations of lithium-6 become tolerable. Figure 3 shows the calculated variation of lithium-6 concentration with sample volume for spheres in which the self shielding effect is 2%Le., where the error introduced by ignoring the effect of self-shielding reaches the value of the standard deviation of the calibration samples. The attenuation of the tritons by lithium-6, which may be appreciable, is not taken into account. Accuracy and Sensitivity. The accuracy and sensitivity of t h e determination of oxygen-18 can be derived from t h e statistical considerations of t h e delayed neutron count of t h e sample, provided the statistical error is t h e major error in the determination. The neutron emission of t h e sample, integrated from 5 seconds after the end of irradiation, irradiated t o saturation at a thermal neutron flux of 10*3n/cm.2-secondis178,200 neutrons, per mg. of lithium-6 per atom per cent oxygen-18. This suggests that for example, a drop size sample of water (say, 33 pl.) of 1 atom per cent oxygen-18 containing 40 pg. of lithium-6 can be analyzed with a standard error of 4% in a single irradiation a t lo%/ cm.2-second ivhen counted with an efficiency of 10%. For larger samples where the lithium-6 content can be increased the count increases correspondingly and the errors become smaller. Repeating the measurement and accumulating the counts decreases the statistical fluctuations and thus in-
creases the accuracy of the analysis; ten such repetitions could be carried out in about 10 minutes. B y shortening the period of delay after irradiation, a larger number of counts can be accumulated with the same effect. A higher count and hence a better sensitivity is also obtainable by increasing the lithium-6 content of the sample, but as errors due t o self shielding increase as well, accuracy may be sacrificed for the sake of sensitivity. The use of higher thermal neutron fluxes and better counting efficiencies remain the important factors for improving sensitivity. I n practice the sensitivity and accuracy of analysis are determined by additional sources. The most obvious ones are instability of reactor flux, fluctuations in the duration of irradiation, delay of counting due to the instability of the various timers, and instability of the electronic circuitry and counters. Errors from all these sources are reflected in the reproducibility of the counts obtained from inany repetitions of a single sample during the course of a day (Table 111). By using replicate samples, errors due t o sample location and to the variations in the containers are included (Table 111). The relative standard deviation amounting t o about 0.6% in the former and about 1.2% in the latter are not sufficient to account for the value of 2.0% obtained for the calibration samples. The experimental errors in sample preparation appear to be the more iniportant errors of this method but the errors in the assay by mass spectrom-
Table 111.
Reproducibility of Experimental Results Counts obt,ained from repeated irradiations Single sample Replicate samples 14579 14614 14804 27243 27200 14724 14770 14665 27189 27014 14897 14799 14674 26935 2i115 14670 14709 14794 27663 27245 14801 14666 14842 27331 27152 14711 14837 14778 26688 27432 14786 27501 27101 Mean: 14743 =t84 Mean: 27186 f 322 Relative standard deviation: 10.5i% Relative standard deviation:
27659 27020 27209 26850 27179 27123 27058 i l ,197,
LITERATURE CITED
(2) iimiel, S., Gilat, J., Israel Atomic Energy Comm. Rept. IA-755 (1962). (3) Amiel, S., Nir, A., Israel Patent Bppl. 15,594 (1961). (4) Amiel, S., Welwart, Y., ANAL.CHEM. 35, in press. (5) Amsel, G., Smulkomki, O., Covzpt. Rend. 251, 950 (1960). (6) Fleckenstein, A., Gerlach, E., Janke, J., Marmier, P., Naturwissenschuften 46, 365 (1959). (7) Fogelstrom, I., Holm-Hansen, O., Tolbert, B., Calvin, M.,Intern. J . A p p l . Radiation Isotopes 2, 280 (1957): (8) Gilat, J., Gurfinkel. Y., Israel Atomic Energy Comm. Rept. IA-756 (1962). (9) Henderson, W. J., Tunnicliffe, P. R., Nucl. Sci. Eng. 3, 145 (1958). (10) Lapidot, A., Pinchas, S., Samuel, D., Proc. Chenz. SOC.(London) 1962, 109. (11) hleinke, W. W., ANAL. C H E x 32, 104R (1960). (12) Xisle, R. G., Nucleonics 14, No. 3, 86 (1960). (13) Roys, P. -4.,Shure, IC., Nucl. Sci. Eng. 4, 536 (1958). (14) Sher, R., Floyd, J. J., Phys. Rev. 102, 242 (1956).
(1) Ajzenberg-Selove, F.,‘ Lauritsen, T., Nucl. Phys. 11, 221 (1969).
RECEIVEDfor review July 5, 1962. ilccepted SovembPr 8, 1962.
etry of the samples used for calibration may be comparable and should be taken into account. Effects due t o foreign materials have not been taken into account, as the presence of foreign materials in samples t o be analyzed for the isotopic composition of oxygen is unlikely. However, if corrections have to be made, the corrections reported for lithium-6 analysis (4) can be applied. ACKNOWLEDGMENT
We thank the operating crew of IRR-1 for the reactor irradiations, and David H. Samuel and Fritz S. Klein of the Weizmann Institute of Science for the supply and analysis of enriched water samples.
Nonspecifkity of Hypochlorite-Phenol Estimation of Ammonium in Biological Material J. T. WEARNE Department o f Biochemisfry, Royal Perth Hospital, Perth, Western Australia
b Hypochloriie plus phenol yield a colored product with ammonium, giving a sensitive method for estimation of the latter in pure solution. However, if other amino compounds are present, reaction may take place with these if the hypochlorite is added first, and a t low pH.
B
in 1859 (1) described the production of a blue color by addition of sodium hypochlorite and phenol to ammonium solutions. Many studies and modifications have been made since (S, 11, 14). The reaction is sensitive and has been considered specific for ammonium, ERTHELOT
though two authors (7, 14) report that color is produced by amino acids in high concentration. The reaction has been used with satisfaction for the estimation of ammonium produced from Kjeldahl digestions, and less satisfactorily for the estimation of urea, after treatment with urease to produce ammonia. The latter application has been criticized for poor reproducibility. Scheurer and Smith (a),working x i t h Kjeldahl digests, modified the method by using chlorine water as the source of hypochlorite ion, improving the sensitivity and obviating the need for heating. Bolleter, Bushman, and Tidwell ( 2 ) have used a similar mudificatior,
and suggeqt the rcactions by nhich an indophenol blue is produced. Khile investigating blood urea niethods, the chlorine n-ater and sodium phenate reagents of Scheurer and Smith were applied to protein-free filtrates of plasma treated nith urease. Good colors were obtained, but i t was then found that identical colors were obtained if the urease treatment n a s omittedi.e., the reaction appeared to take place with urea itself. The results obtained with the filtrates approximated closely to the “nonprotein nitrogen” or urea-nitrogen, depending on the type of filtrate used. Tests with pure solutions showed that VOL. 35, NO. 3, MARCH 1963
327
