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 14724 14770 14897 14799 14670 14709 14801 14666 14711 14837 14786 Mean: 14743 =t84 Relative standard deviation:
14804 14665 14674 14794 14842 14778
10.5i%
27243 27200 27189 27014 26935 2i115 27663 27245 27331 27152 26688 27432 27501 27101 Mean: 27186 f 322 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., Co~npt. 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
the reaction was given by urea and many other amino compounds. Domnas (4) has obtained a reaction, though to a lesser extent, with such compounds by buffering them to p H 4.2 then adding bleaching powder solution followed by sodium phenate. EXPERIMENTAL
(A) Hypochlorite-phenol reaction. Reagents and technique of color development are exactly according to Scheurer and Smith (8). Standards and unknown solutions are diluted to contain
