STANDARD CHLORINE SOLUTION.Five ml of Zonite ( l x available chlorine) is diluted to one liter to make a 50-ppm chlorine solution, prepared daily as needed. WORKING CHLORINE SOLUTION.Eight ml of the standard chlorine solution is diluted to one liter to make a 0.4-ppm chlorine solution. To the first of two 50.0-ml aliquots of sample at 25 ==I 2 "C, add 1.0 ml of buffer solution, mix, and add 2.0 ml of the starch-iodide reagent. To the second aliquot, add 1.0 ml of buffer solution followed by 1.0 ml of the nitrite solution, and mix quickly. After 40 seconds, add 2.0 ml of the starchiodide reagent. Measure the absorbances of the solutions 12 minutes after addition of the starch-iodide reagent. The total available chlorine and the combined available chlorine are read directly from the straight line calibration curve, using the absorbances obtained from the two aliquots, and the differences in absorbances provide a measure of the free available chlorine. The calibration curve is a straight line in the ranges 0-0.4 ppm chlorine concentration and 0-1.4
absorbance, with an intercept on the concentration axis (abscissa) of about 0.025 ppm. The small intercept is characteristic of the linear starch-triiodide complex (5). No interference was observed from the 18 ions of those studied above in the determination of amino hydrogen that are compatible with hypochlorite and chloramines. This method permits direct and unambiguous determination of the total available chlorine and combined available chlorine present as chloramines. Differentiation of monochloramines from di- and trichloramines would be possible through the selective destruction of di- and trichloramines by bromide ion but further differentiation is not possible by this method. RECEIVED for review November 29,1968. Accepted February 20, 1969. Work supported by Federal Water Pollution Control Administration research grant WP-00235. ~
-
(5) R. E. A. Drey, ANAL.CHEM., 36,2200(1964).
A Mass Spectrometric Method for Determination of Subnanogram Quantities of Neptunium-237 Jerry H. Landrum, Manfred Lindner, and Nancy Jones Lawrence Radiation Laboratory, Unioersity of California, Livermore, Calg. 94550 UNTILthe development of the method presented here, thermalneutron activation had been in use at this Laboratory to determine subnanogram levels of 237Np. Samples containing unknown but subnanogram quantities of 2 3 7 Npwere irradiated, together with *J7Npsamples of known but comparable weights, in either the S-tube of the Livermore Pool Type Reactor (LPTR) or the trail cable facility of the General Electric Company's Vallecitos Test Reactor (GETR). In either case, the irradiations were carried out in slowly rotating cylindrical sample holders so that all samples were exposed to the same average time-integrated neutron flux. After irradiation, the induced ZasNp activity was determined in each sample by conventional radiochemical techniques involving the addition of several thousand alpha disintegrations per minute of Z3?Npthat served to determine the chemical recovery in the final purified sample. Although this method seemed to work satisfactorily, there were several serious drawbacks. The activation obtainable on the LPTR was generally only marginal for subnanogram quantities of Z37Np, although that obtainable on the GETR was adequate. However, the aluminum sample containers used in the GETR irradiations were so radioactive from the induced 24Na activity that the recovery of the neptunium samples from such containers had to be carried out behind heavy shielding and involved a master-slave unit. The subsequent chemical recovery and radiation counting of the za*Np required considerable effort and several weeks of counting and data handling before results were forthcoming. EXPERIMENTAL
The Mass-Spectrometric Method. Isotope dilution techniques have been used for many years for the determination of isotopic concentrations of a large number of elements. 840
ANALYTICAL CHEMISTRY
Briefly, a uniquely uncommon isotope of the element whose concentration is to be determined is added in known amount to the unknown specimen. Mass-spectrometric determination of the ratio of the diluent isotope to those present in the unknown then leads directly to the desired result. The method is commonly employed with stable isotopes, but in recent years it has been used with increasing success for the determination of unstable nuclei. We have successfully used the radionuclides 235Npand the long-lived isomer of 236Npas mass diluents for the determination of low levels of 237Np. The mass spectrometer used for the neptunium analyses was a double-focusing, 60-degree sector type, with a 12-inch radius. It has been described previously (1-4) with special attention given to the data-acquisition system. The source chamber of this spectrometer contains a tantalum filament (from which the sample was volatilized) and a rhenium filament on which surface ionization of the. gaseous species occurs. The voltage signal produced by collection of the ion beam on an electron multiplier was converted to frequency, digitized, and recorded in a 256channel multisumming scaler, which was swept in synchronization with the magnetic field. The neptunium sample, usually between 0.1 and 0.5 nanogram in 3N HCI solution, was simply evaporated onto the sample filament and mounted in the source chamber of the spectrometer. When the source pressure was reduced to at least 10-7 torr, ion collection was begun at a low resolu(1) J. R. Dews and R. S . Newbury, J. Geophys. Res., 7, 3069 (1966). (2) G. W. Barton, Jr., L. F. Tolrnan, and R. E. Roulette, Rev. Sci.Instrum., 31, 995 (1960). (3) G. W. Barton, Jr., L. E. Gibson, and L. F. Tolman, ANAL. CHEM., 32, 1599 (1960). (4) R. S . Newbury, G. W. Barton, Jr., and A. W. Searcy, J. Chem. Phys., 48, 793 (1968).
t
Table I. Concentrations, in Atoms/MI, of 285,6.7Np Solution Prepared from Deuteron-Irradiated 2a5U Mass Concentration (ml-1) 2.936 X 1014 (as of 11-28-67) 235 236 0.672 X 10" 237
MASS NUMBER
Figure 1. Mass spectrum of diluent 2a5Np and 2aENp solution tion of about 300. The species collected were those of the ionized neptunium atoms (rather than those of ionized molecules). The magnetic field was swept in the mass range from 235 to 238. Ordinarily, three or more mass-ratio determinations were obtainable on a single sample, each consisting of an accumulation of 80 to 100 individual sweeps. Whenever possible, a minimum of about 100,OOO ions was collected in the most intense mass peak. Because of the small size of the samples, some interference was occasionally encountered from spurious mass peaks due to molecular fragments (either organic or inorganic) which, though invisible, could possibly have been present in the final sample. Such interference effects usually disappeared after the first few mass ratio determinations at low temperature. It was found that the use of reduced volumes of pure reagents in the final stages of the chemical separation of the neptunium samples also contributed to specimens less subject to contaminant mass peaks. One obstacle PREPARATION OF 235Np AND 2 3 6 NISOTOPES. ~ to the use of the isotope dilution technique for determination of la7Np is that other long-lived isotopes of neptunium [*35Np 410d (5, 6) and 236Np5 0 0 0 ~(6)] are difficult to prepare. The method employed at this laboratory was essentially that of Gindler, Huizenga, and Engelkemeir (6). It consisted of a high-intensity deuteron irradiation of highly purified, isotopically enriched 2a5U. The produced n35Np and 23eNp were isolated through exhaustive radiochemical purification steps (7). Prior to irradiation of the 235Utarget it was necessary to remove possible traces of 237Np. Some of the details are ( 5 ) R. A. James, A. Ghiorso, and D. Orth, Phys. Rev., 85, 369
(1952). (6) J. E. Gindler, J. R. Huizenga, and D. W. Engelkemeir, ibid., 109, 1263 (1958). (7) M. Lindner, Report UCRL-14258, Lawrence Radiation Laboratory, Livermore, July 1965.
0.169 x 1014
given below. Three grams of 99.2% 235U(as U308) were dissolved, and a measured level of 239Npradioactive tracer added. This solution was then subjected to a series of radiochemical neptunium-uranium separations (7) designed to remove possible traces of 237Np(because the presence of as little as 1014 atoms of this latter nuclide would roughly equal the anticipated levels of 235Npand 236Npwhich would be produced in the deuteron irradiation). The neptunium chemical fraction from this purification procedure was isolated and, by determination of the 2a9Npand 237Nplevels, the original 235Uwas inferred to have contained 2 x 1013 atoms of 237Np. Although the 2.3-day 239Npis not an ideal radioactive tracer for determination of chemical yields, the chemical separations were completed within such a short period that the gamma radiation from this nuclide could be conveniently determined in solution with a well-type NaI scintillation counter. The uranium was next recovered and ignited to U308. was irradiated with several mamp The purified 235u308 hours of 22-MeV deuterons on the Argonne National Laboratory low-energy high-current cyclotron. The nuclear reactions pertinent to this experiment are ioooy Z35U (d,n) Z3eNp
-
za5U (d,2n) 285Np
410d
Approximately two months after the end of the irradiation the 235Ua08was dissolved and a neptunium fraction isolated in high purity from all fission products as well as from mass traces of 235U (7). The relative masses of 235, 236, and 237 were next determined on the mass spectrometer. Finally, the solution was subjected to a further radiochemical separation from mass uranium, and the mass ratios of 235, 236, and 237 again determined. No significant difference could be discerned in the relative masses before and after this final separation. The neptunium fraction was therefore considered to be free of any significant levels of mass contamination by 235u.
The resulting mass spectrum shown in Figure 1 indicates that small amounts of 237Npand 2asUwere present in the mass spectrum of the final solution. It is not clear whether the 2a7Npwas produced by some unanticipated reaction such as 2a8U(d,3n) 237Np,or whether it represented residual na7Np from the original purification procedure. At any rate, this level set a lower limit on the level of 2a7Npthat could be determined with this particular solution. Even with this limitation, it was found that the determinations of 237Np by the massspectrometric (M.S.) method were far simpler and more rapid than the neutron activation (N.A.) method. In addition, the sensitivity of the M.S. method was at least an order of magnitude greater than the N.A. method. The final stock solution contained the separated neptunium isotopes in 1.0 ml of 5N HCl. The atomic concentrations of 235Np,2a6Np(and 2a7Np)in the final solution were determined in reverse through addition of a known amount of 287Npto a known volume of the stock solution. The spectrum obtained for this resultant mixture is shown in Figure 2, and the calculated atomic concentrations of the final solution are given in Table 1. There was no detectable difference in the 235/236 mass ratios for the stock solution and that portion of the stock solution to which 2a7Nphad been added. VOL. 41, NO. 6,MAY 1969
841
Table 11. Comparison of Neutron Activation and Mass Spectrometric Methods for Determination of 2"Np in Solution
z
Results in ng2n Np/ml soh
z
N.A.o Solution g, M.S., 2asNpb 0, M.S., 2seNpc g 3 72 1 0.0206 0.85 0.0208 1.2 0.0207 1.3 2 0.0069 0.94 0.0069 1.8 0.0070 1.6 3 0.0093 0.93 0.0093 1.1 0.0094 1.2 4 0.0274 0.59 0.0265 1.1 0.0262 1.3 5 0.0466 2.8 0.0457 1.1 0.0459 1.3 6 0.0528 2.1 0.0534 1.2 0.0522 1.3 7 0.0122 0.80 0.0122 1.8 0.0114 2.1 8 0.0261 1.9 0.0279 4.3 0.0240 7.9 9 0.0235 1.5 0.0234 2.3 0.0235 2.6 Irradiations performed in the trail cable facility of the GETR in a flux of approximately 1.5 X 10" n/cm2 seconds for approximately 6 hours. b Calculations based on zasNp contained in the diluent. c Calculations based on 236Np contained in the diluent. Q
Approximately 0.1% of the quantity shown in Table I was taken for each analysis. In the mass-spectrometric analyses, it should be pointed out that there was always a contaminant peak of varying intensity at mass 238 due to the low but measurable levels of la8Uin ordinary solutions, reagents, glassware, etc. (see, for example, Figures 1 and 2). The presence of this uranium did not interfere with the determinations and it served as an estimate of possible conisotope. tamination by the less abundant 235U RESULTS AND DISCUSSION
Comparison of the Mass-Spectrometric and Neutron Activation Methods. Nine different solutions ranging in concentration from 0.01 to 0.05 ng zs7Np per ml were analyzed by both methods. To obtain good statistics at these levels from the neutron activation method, it was necessary to concentrate the neptunium from large volumes of solution. Volumes only one tenth as large were sufficient for the mass spectrometric method. The results of both methods are summarized for the nine different solutions in Table 11, and are expressed in nanograms 237Npfound per milliliter of solution. There are two independent sets of results from the mass spectrometric method, one based upon the 2a5Npand the other upon the zs6Npconcentrations listed in Table I. The standard deviations of the determinations are given in columns 3, 5, and 7. In the neutron activation method these deviations were derived from the analysis of the counting statistics of duplicate samples from the same solution. In the massspectrometric method the standard deviation was derived from repeated determinations of the mass intensities from a single sample deposited on a source filament. Thus, the uncertainties do not necessarily reflect the same inherent imprecisions. Nevertheless, it is clear that the estimated overall uncertainties of the two methods are comparable. The time required to obtain results from the N.A. method varied from about two weeks to a month, depending upon starting conditions of the initial solution. By the M.S. methods, results were easily obtained within 4 or 5 days. Of particular interest are the results derived from the use of the 236 mass as the diluent. For two reasons 236Nphas greater potential than does Za5Np. First, the 410-day halflife of za5Npmeans that a correction for decay must be applied to each determination. Second, although it did not prove to be a problem in these determinations, the presence of 2a5Uas a contaminant in the 235 mass peak is always a possibility. By contrast, even though there was initially only 2 0 z as much 2aENp as 2a5Np,contamination by za6U 842
ANALYTICAL CHEMISTRY
t
"i MASS NUMBER
Figure 2. Neptunium diluent with added Za7Np
at the mass 236 peak is very unlikely. Consequently, as a result of the 286Np,the life of such a diluent solution is essentially infinite, and the absence of a uranium mass contaminant considerably relaxes the degree of chemical purification required for reliable results. ACKNOWLEDGMENT
The authors are indebted to Milan Oselka and the operating crew of the Argonne Cyclotron and to Peter C. Stevenson whose advice was invaluable. RECEIVED for review November 15,1968. Accepted February 20, 1969. Work performed under the auspices of the U. S. Atomic Energy Commission.
