ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
2015
Determination of Bromine in Biological, Soil, and Geological Standard Reference Materials by Instrumental Epithermal Neutron Activation Ernest S. Gladney” and Daniel R. Perrin University of California, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, New Mexico 87545
Bromine is measured in National Bureau of Standards, Canadian Geological Survey, and U.S. Geological Survey reference materials. Detection limits of 50 ppb can be achieved in some matrices by this purely instrumental technique. Interferences from W, As, Sb, Mo, and Fe are discussed.
The measurement of bromine at concentrations encountered in typical environmental matrices requires sophisticated and sensitive analytical procedures. Classical methods usually involve destructive solution of the sample and titration of bromate by liberation of iodine ( 1 ) . More recently, X-ray fluorescence ( 2 ) , optical emission spectroscopy ( 3 ) , and ion-selective electrodes ( 4 ) have been used. None has sub-ppm detection limits, and the latter two still require sample destruction. Nuclear activation methods present the best nondestructive, high-sensitivity techniques for environmental Br measurements. Proton induced X-ray fluorescence (5, 6), photon ( 7 ) , thermal neutron (8-1 71, and epithermal neutron (18-21) activation methods have all received attention. Although ppb detection limits are reported for the first procedure, little capability has been demonstrated with real environmental samples, except for air particulates. Photon activation with bremsstrahlung radiation from linear accelerators has adequate sensitivity but is not nearly as widely available to investigators as reactor-based activation methods. Part-per-billion detection limits have been reported for very long thermal neutron irradiations (10). Bromine loss during such long irradiations has been observed (22),making these analyses subject to large potential errors. Thermal neutron irradiations also produce copious quantities of 24Naactivity in environmental samples, especially biologicals, which severely interferes with instrumental measurements of Br and poses unnecessary radiation exposure for analysts undertaking chemical separations. Similar detection limits (5 ppb) have recently been reported for an epithermal activation/chemical separation method for Br in silicate rocks (21). Such sensitivity is not, however, required for many environmental applications. Chemical separations necessarily result in sample destruction and significantly increase the time required per analysis. The purpose of this paper is to explore the analytical utility of epithermal neutron irradiation in a rapid, nondestructive analysis for Br in a variety of environmental matrices. In contrast to thermal neutron activation analysis, epithermal irradiation reduces the 24Naactivity dramatically. Epithermal neutrons are those with kinetic energies of 0.1 eV to 1 MeV. Epithermal neutrons are present in reactor fluxes and require only some “filter” to absorb the thermal portion of the neutron energy spectrum. Cadmium and boron are especially good thermal neutron filters (23). Epithermal activation is usually done by wrapping the sample with Cd 0003-2700/79/0351-2015$01.00/0
Table I. Advantage Factors for Epithermal Neutron Irradiation in Cadmium and Boron Filtered Systems element
Cd filter ( 2 5 )
B filter
%Na Br Br
1.0 21 21 16 34 46 44 22
1.0 63 126 92 62 68 4 20 16
76As Sb lz4Sb 9 9 1 8 7 W
~
~
foil and inserting the package into the reactor neutron flux. The work by Brune and Jirlow (24) and Steinnes (25) examining the relative activation of elements in thermal and epithermal neutron fluxes is based upon this epi-cadmium method. These authors have developed an “advantage factor” which shows that relative to 24Na, and szBr are produced some 20 times more often in epithermal than thermal fluxes. This results largely from the fact t h a t Br has strong crosssectional resonances in the epithermal neutron energy region, whereas Na does not. By cutting off the thermal neutrons, induced 24Naactivity is diminished much more than that of the Br isotopes. By changing the filter material from Cd to natural B, a larger portion of the low energy neutron spectrum is absorbed. The low energy cutoff for Cd is 0.4 eV, while that of B is about 300 eV. The substitution of B for Cd further enhances the relative activation of Br as compared to Na. Our measurements yield the advantage factors shown in Table I for 80Br and 82Br for our B-filtered system. While 82Bris more often used, Br concentrations may also be determined using @%r(t1I2 = 17.4 m). This approach, while rapid, has drawbacks since the strongest transition in BOBr, the 618-keV line, is fed by only 6% of the decays, while the 778-keV line in is fed by 83% of the decays. Furthermore, the 618-keV y-ray sits atop the Compton edge from 2.6 h 56Mn and is also interfered with by the 620-keV doubleescape peak from the 1642-keV line of 38Cl ( t l l z= 37 min). This is particularly a problem with biological matrices in which C1 is relatively abundant. In general we feel that while using 80Br saves times, it is less sensitive than 82Br,and that the latter is the preferred isotope for quantitative measurements.
EXPERIMENTAL One-gram samples of various environmental standards were encapsulated in polyethylene rabbits and irradiated for 5 min to 2 h in the epithermal neutron facility of the Los Alamos Omega West Reactor. The neutron filter is permanently installed in the reactor flux and samples are pneumatically transferred in and out. The filter is constructed from a 50:50 volume mixture of powdered elemental B and Al, hot pressed into a cylindrical configuration. The shield wall is 2.5 cm thick, providing about 2.3 g/cm2 of B. This gives an estimated lower energy filter cutoff of 280 eV and provides essentially total absorption of thermal 1979 American Chemical Society
2010
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
Table 111. Bromine Concentrations in Canadian Certified Reference Soils
c
N
soil ref. no.
soil type
Regosolic clay SO-2 Podzolic B Horizon SO-3 Calcareous C Horizon SO-4 Chernozemic A Horizon SO-1
t I
9 rl tl tl t' t i y
y
Y
@q +I c.1
2 my rim
tl
t-mwwc.1mGww d
bottle no.
Br, ppm, recommended X i a value
133 711 97 903 495 1023 103 441
1.3 i 0.1
1.4 i 0.2
1.4i 0.2
14.8 i 1.0 15.2 3 1.5 5 . 5 i 0.7 4.8 T 0.2 5.5 i- 0.3 5.8 f 0.5
15 t 2 5 . 2 - 0.8
5.6
t
0.5
Table IV. Bromine Concentrations in USGS Geochemical Exploration Reference Materials sample no.
B 5 PPm, X+a
description
GXR- 1
jasperoid, Drum Mountains, Utah soil, Park City, Utah hot spring deposit, Humboldt County, Nev. porphry copper, Utah soil ( B zone), Somerset County, Maine soil ( B zone), Davidson County, N.C.
GXR- 2 GXR-3 GXR-4 GXR-5 GXR-6
0.39
i
0.07
3.0 i 0.1
