Determination of Bromine in Silicate Rocks by Epithermal Neutron Activation Analysis C. K. Unni" and J-G. Schilling Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 0288 1
An eplthermal neutron actlvatlon analysls (ENAA) scheme lnvolvlng a single-step bromlde preclpltatlon coupled wlth Ge(LI) spectrometry has been developed for detennlnlng trace amounts of Br In slllcate rocks. The precislon (2a) of the method estimated from repllcate analyses of rock standards Is within f10% for concentratlons of Br exceedlng 0.1 ppm. The observed values for Br In W-1 and JB-1 ere very close to the reported values in literature. Comrnerclal grade aluminum foil was used as sample contalner and flux monltor for Br analysis.
T h e necessity of studying halogen degassing of the earth by volcanism prompted us t o develop a method for the determination of bromine in silicate rocks with special reference t o basalts. Bromine content of silicate rocks is not as well known as t h a t of C1. For instance, there are still no data reported on Br in submarine basalts. Yet, Cl/Br ratios can provide valuable insight into the evolution of the present day ocean through volcanism during geologic time (1,Z). The gap is partly due t o the non-availability of highly sensitive and accurate methods for the determination of trace amounts of Br in igneous rocks. Earlier determinations of Br in igneous rocks were performed by conventional analytical techniques mostly by Selivanov (3)and Behne (4). Filby ( 5 )and Sugiura (6) have questioned the validity of these earlier Br analyses by wet chemical methods which have inherently low sensitivity and high reagent contamination problems. Neutron activation analysis (NAA) has subsequently been employed by several investigators (5, 7-15) t o determine Br abundances in terrestrial rocks, meteorites, and lunar samples. All neutron activation methods published so far consist of post-irradiation chemical separations including several solvent extraction steps and precipitation of AgBr before beta or gamma counting the radioactivity of Br nuclides. T h e only nondestructive epithermal neutron activation analysis (ENAA) scheme developed by Brunfelt and Steinnes (16)lacks sensitivity for Br concentrations below 0.2 ppm. I n this paper, we describe an ENAA method based on the nuclear reaction S'Br(n,y)s%r (Table I), involving a single-step bromide precipitation coupled with Ge(Li) spectrometry. ENAA enhances t h e activity of 82Br,and reduces the total activity of irradiated rocks considerably compared to thermal NAA. T h e procedure eliminates time-consuming radiochemical separations adopted by previous workers. Also, for t h e first time, the simultaneous use of commercial grade aluminum foil as sample container and flux monitor for NAA has been made. I t is based on the 14.1 h '*Ga produced from Ga impurity in A1 foil, which can be conveniently monitored by Ge(Li) spectrometry.
EXPERIMENTAL Apparatus. Neutron irradiations were performed at the 2-Megawatt Research Reactor (neutron flux, 4 X 10" neutrons/cm2/s) belonging to the Rhode Island Nuclear Science Center. Gamma spectra of samples, standards, and A1 foil flux 1998
ANALYTICAL CHEMISTRY, VOL. 49, NO.
13, NOVEMBER 1977
monitors were acquired by a 35 cm3 Ge(Li) detector (resolution, 2.1 keV for 1332 keV y-ray of 6oCo,efficiency relative to 3 in. X 3 in. NaI(T1) detector for the 1332-keV photopeak of 'Wo source placed at a distance of 25 cm from the detector, 6.8%) coupled to a 4096 channel Analog to Digital Convertor and fast magnetic tape read-out system. Reagents. Bromine Carrier Solution (20 mglmL). Dry reagent grade KBr (Baker Analyzed) for 2 h at 110 OC and cool in desiccator. Dissolve 7.44697 g of KBr in 250 mL of deionized water. Bromine Standard Solution (20 qg/mL). Dilute 1 mL of Br carrier solution to 1 L with deionized water after addition of a few drops of "*OH. Silver Nitrate Soluticn (1 M). Dissolve 169.87 g of AgN03 (Baker Analyzed) in 1L of water and store in amber colored bottle. Preparation of Samples a n d Standards for Irradiation. Cut 3 X 3 cm of A1 foil (Anaconda aluminum), 0.02 mm thick, and soak in dilute HN03 (3 M) for 15 min to remove surface impurities. Wash the foils three to four times with deionized water, drain, and dry. Pipet out 0.1 mL of Br standard onto a tared foil, and evaporate to dryness at 80 "C. Fold the sides of the foil carefully so that the evaporated standard is in the center, and well covered. Place two pieces of A1 foil one above the other, fold three sides and weigh. Transfer about 0.3 g of rock powder into the improvised capsule, fold the remaining side, weigh, and record the weight of the sample accurately. Sample size at this point is about 12 x 1 2 mm. The foils serve as sample container and flux monitor. Place samples and standards in a cadmium container to shield thermal neutrons and irradiate for 14 h at a flux of 4 X 10" neutrons/cm2/s. Cool for 16 h. Radiochemical Separation. Pipet 1.0 mL of Br carrier into a 30-mL nickel crucible, add 2 drops of 1 M KOH to prevent volatilization of Br during heating, and evaporate to dryness a t 80 "C. Unfold the Al foils and transfer rock powder quantitatively into the crucible. Wash both pieces of foil several times with tap water, discard washings, and dry. Add 5 g of NaOH pellets to the sample. Fuse the sample over burner for 7 min with caution, cool the mixture, dissolve in hot water, and transfer to a 50-mL centrifuge tube. Centrifuge off the hydroxide precipitate containing mostly Fe and Mg, and wash it once with 10 mL of deionized water. Combine supernatant containing Br and wash solutions. Add 2 drops of sodium bisulfite to reduce any Br atoms present to bromide, 2 drops of thymol blue indicator, and lower pH to 2 by the addition of concentrated nitric acid (-8.0 mL). The solution changes color from blue t o pink. Precipitate AgBr carrying radioactive Br by adding 2 mL of AgN03 solution. Filter AgBr through pre-treated and tared glass fiber filter (Gelman). Wash several times with 1M HN03,deionized water, and acetone. Dry the precipitate for 15 min at 110 OC, mount on Petri dish and count on Ge(Li) detector for 4000 s. Store y-spectra on magnetic tape. y-spectra of AgBr separated from KBr standard and standard rock JB-1 are given in Figure 1. Unwrap the foil containing standard and leach it with hot water containing 1 mL of Br carrier. Add a few drops of dilute "03. Remove foil from beaker after washing it several times with deionized water and dry. Precipitate AgBr as before, and count for exactly the same time and in the same position as the sample. Press and pelletize the dried A1 foils which also serve as flux monitors. Count on Ge(Li) detector for lo00 s. Record y-spectra on magnetic tape. A typical y-spectruni of irradiated A1 foil is given in Figure 2. Calculations. y-spectra are subjected to analysis by Gamanl (19) and peak areas of 82Brfrom samples and standards and 72Ga
Table I. Nuclear Characteristics of Br, Al, and Gaa Target nucfide
a
Cross section for n,y reaction, barns
Isotopic abundance
Thermal
%
l:Br
50.52
8.5
i:Br !iBr
50.52 49.48
!:Br
Epithermal
Product nuclide
7-rav energy,-keV
Half-life
153
i:Br
16.8 min
2.9 2.0
41
;","lar ::Br
4.42 h 35.3 h
49.48
3.0
34
!:mBr
6.05 min.
::A1 $TGa
99.99 60.20
0.235 1.90
0.18 12.3
:!A1 :yGa
2.32 min. 21.1 min.
i:Ga
39.80
5.00
21.6
::Ga
14.1 h
way intensity %
6.6
616 666 37 554 771 46 777 1779 1039 1051 630 834 2202
1.1
38 71 84 0.26 84 99.9 0.5 0.5 24.5 95.8 26.2
From Filby et al. ( I 7) excluding epithermal cross sections which are from Steinnes (18). Table 11. Br Content of Standard Rocks Sample
Mean, ppma
Concn, ppm
W-1
0.396 0.388
0.392
0.4 0.41 0.40 0.496 0.15
20
0.031 0.059 i 0.033 0.60
13 15 21
BCR-1
JB-1 1
1
I
,
230c
1200
400
2800
C H A N N E L S
Flgure 1. Gamma spectra of AgBr separated from KBr (2 pg Br) and standard rock JB-1 (268 mg). Irradiation time, 14 h; counting time, 2000 s a
- 1
I I 1
1
1
1
400
I
,
I
I
I
1200
1
I
,
,
2000
,
/
2800
I
.
,
,
3600
C H A N N E L S
Figure 2. Gamma spectrum of 14 h; counting time, 800
time,
irradiated AI foil (180 mg). Irradiation s
from A1 foils are obtained. For the purpose of calculations, areas under 554 and 777 keV peaks are summed for '*Br after correcting for background. The 834-keV photopeak, corrected for background, from '*Ga is used for monitoring neutron flux variations among samples and standards during irradiation. Br abundances are then computed after taking into consideration decay and dead time corrections also. R E S U L T S AND D I S C U S S I O N Table I1 gives the Br contents of standard rocks, the diabase W-1 and the basalts BCR-1 and JB-1. The average value of 0.392 ppm for W-1 compares well with the magnitude of 0.4 ppm suggested by Flanagan (20), and 0.4 ppm obtained by Reed and Allen (8). It is slightly lower than Filby's value of
Literature values
Br, ppm
i
0.004
0.089 0.077 -t 0.010 0.082 0.071 0.066 0.568 0.550 f 0.019 0.528 0.579 0.528 0.568 0.539 0.555 0.540 0.541
Standard deviation reported is
Ref 8 8 5 20
117.
0.496 ( 5 ) . For BCR-1, the measured concentrations vary between 0.066 and 0.089 ppm with a mean of 0.077 f 0.010 ppm. The high standard deviation (13.5%) is caused by the decreasing sensitivity of the method at lower Br concentrations. The mean value differs significantly from 0.15 ppm given by Flanagan (20). Laul et al. (13) and Krahenbuhl e t al. (15)obtained for BCR-10.031 ppm and 0.059 f 0.033 ppm, respectively. T h e analytical uncertainties in these cases are very high. Br in JB-1 is in the range 0.5284.579 ppm for 9 determinations with a mean of 0.550 f 0.019 ppm. The reported value of 0.60 ppm by Ando et al. (21) is in close agreement with our range. Subsequently JB-1 was used in all irradiations along with unknown samples and synthetic standards for crosschecking Br values and analytical precision. The method has good precision as seen in Table 11. I t is better than *lo% (2a) for Br concentrations above 0.10 ppm. This also includes errors introduced by counting statistics. The accuracy of the method can be judged from its agreement to other reliable values reported in literature for standard rocks, and is comparable to that of precision. T h e detection limit of the method is estimated to be 5 ppb of Br in silicate rocks. The above method has been successfully applied to the study of Br in basalts from Iceland, Reykjanes Ridge, and ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
1999
Hawaii ranging in composition from tholeiites to alkali series and nephelinites. T h e method is simple and rapid involving only sample fusion, a single bromide precipitation, and counting, as opposed to the cumbersome and time-consuming procedures hitherto reported in literature. Chemical yields monitored through the recovery of AgBr by gravimetry vary between 90% and l o o % , and are much higher than the yields (60-80%) reported by other investigators. With proper precautions, chemical yield determinations can be avoided in routine analysis if high accuracies are not required. A d v a n t a g e s of ENAA. From the nuclear data given in Table I, it is seen t h a t has higher cross section in the epithermal region of the reactor neutron spectrum. An “advantage factor” of about 20 (on the basis of an interfering nuclide which follows 1 / V law also in the epithermal region) has been cited by Steinnes (18)for “Br by ENAA. Moreover, many of the major elements in geological materials have low “advantage factors” resulting in a considerable reduction of total activity of irradiated specimens. The activities due to interfering nuclides such as 24Na,%c, jlCr, j6Mn, jgFe, 14’La, and 1 5 2 Eare ~ thus reduced to a great extent. Indirectly, hazards in post-irradiation sample handling are minimized, while the activity of 82Bris enhanced. S o u r c e s of E r r o r . Some of the probable sources of error in the analysis of Br in silicate rocks are self-shielding effects due to resonance neutron absorption in the sample, interfering nuclear reactions, contribution from thermal neutron fission of 235U,Br loss by recoil, and uncertainties in chemical yield determination by gravimetry. Elements of high neutron absorption cross sections are not present in significant concentrations either in rock specimens or standards, thus causing little self-shielding effects. Self-shielding due to Br in samples and standards is also negligible since Br is present in trace concentrations. Moreover, major elements such as 0, Si, Al, Fe, Ca, Mg, Na, K, Ti, and P show no significant resonance absorption in the energy range of interest (0.4 eV to 1 meV) and do not cause any resmance interferences with resonances of Br (18). Effects due to conflicting nuclear reactions such as 82Kr(n,p)82Bror ‘’Rb (n,cu)”Br are also negligible. Kr concentrations in silicate rocks are extremely low, and most often below detection limits, and in the case of Rb the contribution from (n,cy) reaction was determined by Filby ( 4 ) to be insignificant for rocks ranging in composition from granite G-1 (Rb = 220 ppm) to diabase W-1 (Rb = 22 ppm). Fast neutron flux is also low at the position of irradiation of specimens. The fission product yields from 235Ufission by thermal neutrons are only 0.06 and 0.28% for the mass regions 80 and 82, respectively (22). Even this contribution is reduced considerably in the ENAA of Br. The close agreement of our Br values for standard rocks with reliable values reported in literature (Table 11) suggests that Br loss from samples and standards by recoil due to the nuclear reaction 27Al(n,cu)24Na in A1 foil is not a serious problem. Five grams of NaOH used for sample fusion introduces 50 pg C1, since NaOH pellets (Baker Analyzed, Reagent) contain 0.001% chloride as impurity. This results in an enhancement of chemical yield of AgBr by 0.2%. In addition, C1 present in the sample (20-30 Fg C1 in 200-300 mg rock powder) increases the chemical yield of AgBr by 0.2%. Thus, increase in chemical yield due to coprecipitation of C1 from NaOH and rock specimens with AgBr is only 0.670, and is negligible.
2000
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
Figure 1 gives the y-spectra of AgBr separated from JB-l along with that of Br standard. Both spectra show identical y-ray photopeaks due to %r, thus ensuring the radiochemical purity of AgBr separated from sample. A1 Foil-Sample C o n t a i n e r and Flux Monitor. Commercial grade A1 foil was Pound to be a convenient and inexpensive flux monitor and sample container for NAA. I t contains several impurities like Ga (-0.014%), which follows A1 closely through metallurgical operations. Table I shows that 21 min ioGa and 14.1 h 72Gacan be produced with thermal and epithermal neutrons. In the present work 72Gaactivity induced in A1 foil was used for measuring flux variations of samples and standards during irradiation. These measurements are more representative than that using usual high purity Fe or Co wires, because in the former case the entire sample or standard is covered with the flux monitor. Figure 2 gives the y-spectrum of irradiated A1 foil after 24 h of cooling. It clearly shows the photopeaks due to 72Ga(630 and 834 keV) besides those due to 24Na. The coefficient of variation of specific activity of Ga from several sections of Al foil from one roll was within 1 2 % . While this work was in progress, Parekh and Heimann (23)reported independently the use of commercial grade A1 as a versatile reactor flux monitor for thermal, epithermal, and fast neutrons. However, the simultaneous use of A1 foil as sample container and flux monitor in NAA of solid samples has not been reported so far. ACKNOWLEDGMENT We are thankful to A. F. DiMeglio and staff of the Rhode Island Nuclear Science Center for providing facilities for this work and P. R. C. McMahon for typing the manuscript. LITERATURE C I T E D A. R. McBirney, Rev. Geophys. Space Phys., 9, 523 (1971). C. K. Unni, Ph.D. Thesis, University of Rhode Island, 1976, p 272. L. S. Seiivanov, Dgkl. Akad. Nauk SSSR, 28, 809 (1940). W. Behne, Geochim. Cosmochim. Acta, 3, 186 (1953). R . H. Filby, Anal. Chim. Acta, 31. 434 (1964). T. Sugiura, Bull. Chem. SOC.Jpn., 41, 1133 (1968). K. W. Liiberman and W. D. Ehmann, J. Geophys. Res., 72, 6279 (1967). G. W. Reed, Jr.. and A. 0. Allen. Jr.. Geochim. Cosmochim. Acta, 30, 779 (1966). G. W. Reed, Jr., and S. Jovanovic, Proc. Second Lunar Sci. Conf., 2, 1261 (1971). G. W. Reed, Jr., and S . Jovanovic. Geochim. Cosmochim. Acta, 37, 1007 (1973). S. Jovanovic and G. W. Reed, Jr.. Proc. Fourth Lunar Sci. Conf., Suppl 4 , Geochim. Cosmochim. Acta, 2, 1313 (1973). A. 0.Brunfelt and E. Steinnes, Talanta, 18, 1197 (1971). J. D. Laul, R. R. Keays, R. Ganapathy, and E. Anders, Earth Pknet. Sci. Lett., 9, 211 (1970). U. Krahenbuhi, R. Ganapathy, J. W. Morgan, and E. Anders. Science, 180, 858 (1973). U. Krahenbuhl, J. W . Morgan, R . Ganapathy, and E. Anders, Geochim. Cosmochim. Acta. 37. 1353 (1973). A . 0 . Brunfelt and E . Steinnes: Ana;. Chim. Acta. 48, 13 (1969). R. H. Filby, A. I. Davis, K. R. Shah, G. G. Wainscott, W. A. Haller, and W. A. Cassatt, Gamma ray energy tables for neutron activation analysis, Washington State University, 1970. E. Steinnes, “ E p i t h m l neutron a c t b a r n analysis of geological material”, in “Activation Analysis in Geochemistry and Cosmochemistry”. A. 0. Brunfelt and E. Steinnes, Ed., Oslo, 1971, pp 113-128. J. L. Fasching, private communication, 1971. F. J. Fianagan, Geochim. Cosmochim. Acta, 37, 1189 (1973). A. Ando, H. Kurosawa, T. Ohmori, and E. Takeda, Geochem. J . , 5 , 151 (1971). C. D. Coryeil and N. Sugarman, “Radiochemical Studies: The Fission Products”, McGraw Hill, New York, N.Y., 1951. P. 0. Parekh and M. Heimann, J . Radioanal. Chem., 1 4 , 357 (1973).
RECEIVED for review May 19, 1977. Accepted August 5 , 1977. This research was supported by the National Science Foundation, under NSF Grant OCE 76-01577.
