Anal. Chem. 1981, 53, 765-770
785
Simultaneous Determination of Boron and Lithium by Charged Particle Activation Analysis Chaturvedula S. Sastri, Rostlslav Caletka, and Warn Krivan" Sektion Anaiytik und Hochstreinigung, Universitat Ulm, 7900 Ulm, West Germany
A systematlc study of proton and deuteron activations for the determlnatlon of boron and llthlum uslng 'Be as an Indicator radionuclide Is described. The Interferences arlslng from beryllium and nltrogen In the determination of these elements are discussed. It is found that with deuteron activatlon the contributions from boron and lithlum to the total measured activity can be dfstinguished. Radiochemical procedures based on solvent extraction and Ion exchange for the speclfic separation of 'Be from irradlated nioblurn matrlx are presented. The detectlon limlts achlevable In this matrlx are at 15 ppb and 6 ppb for boron and Iithlum, respectively. Results obtained for niobium and the NBS standard (1571), orchard leaves, are given.
Many physical properties of solid materials of scientific and/or technological relevance strongly depend on the trace concentration of light elements (1). Boron and lithium are trace constituents of this kind. There exist only a few sensitive nonnuclear techniques for the determination of low concentrations of boron and lithium in solids. Among them, atomic emission spectrometry is probably the most valuable for both elements. Inductively coupled plasma emission spectrometry allows detection of boron in different metals down to 0.05 ppm (2). Limits of detection up to 0.01 pg/mL have been reported for lithium by flame emission spectrometry (3). However, the problem of reagent blank cannot be avoided in these methods. A number of nuclear techniques are known for the determination of boron and lithium. A technique of boron autoradiography based on the (n,a) reaction was described by Mapper and Bolus ( 4 ) . The neutron capture prompt y-ray technique was used for the determination of boron by Gladney et al. (5),but it requires very specialized equipment. McGinley and Schweikert (6)developed a quasi-prompt technique for boron and lithium with detection limits around 0.5 ppm each. Among other prompt methods, Oliver and Peisach (7) measured protons from (d,p) reactions on boron and Borderie et al. (8) measured y-rays from the (oc,oc'y) reaction on lithium, but these methods are also not sensitive enough at the subppm level. Charged particle activation proved to be a sensitive technique for the determination of light and medium Z elements (9-17). Among different projectiles, bacsed on sensitivity considerations,protons and deuterons are commonly used for the determination of boron and lithium and the end product measured is 7Be, llC, or 13N,depending on the nuclear reaction and the element. If we assume that proton activation is used and 'Be is the radionuclide measured, even at relatively low energies, it can be formed via the reactions 7Li(p,n)7Be,log( p d 7 B e ,and "B(p,c~n)~Be, making it difficult to say whether the measured activity is from boron or lithium when both are expected to be present. With higher proton energy this problem becomes more complicated because of additional interference coming from nitrogen and beryllium. Similar problems arise in deuteron activation and also when llC or
13N is
used as the product nuclide to identify the required element. This is often the situation in charged particle activation, where the element sought is subject to interference by another element. In such cases the practice followed is to make corrections based either on the experimental activation yields for the desired and the interfering reactions or on calculations using cross-section data (18). However, to be able to apply such corrections, the concentration of the interfering element must be known. An alternate approach is to irradiate with the same particle but at two different energies (19) in which case one may be able to distinguish the contribution from the two elements to the production of the same nuclide. The purpose of the present paper is to demonstrate the feasibility of differentiating boron and lithium and finding their individual concentrations from the knowledge of the measured total activity. As an example, their determination in a standard reference material, orchard leaves, and niobium has been considered. In addition, two procedures were developed for the specific separation of 7Be from irradiated niobium.
EXPERIMENTAL SECTION Principle. Assume that the matrix under investigation contains both boron and lithium. There are two approaches possible to solve the problem. From the Measurement of Thick Target Yields. The thick target yield for a given element is a function of irradiation energy. If we irradiate two samples of a given matrix, one at energy El and the other at energy E, (E, > El) and assuming that at E l , A B represents the activity contribution from boron and ALi represents the activity contribution from lithium, we can write the following equations:
for sample 1 at energy El AB + AL, = A1 for sample 2 at energy E2 RBAB
+ R L ~ A L ~A2
(1)
(2)
where A1 and A2 are the experimentally measured total activities of the sample at El and E,, and RB
=
thick target yield at E, (for B) thick target yield at El
RLi
=
thick target yield at E, (for Li) thick target yield at El
In separate experiments one can plot the thick target yields for boron and lithium, contained in suitable standards, at different energies. From these plots, the ratios R B and RLi, which are dependent on El and E2, can be found. The energies El v d E2 are so chosen that RB and RLi differ by a large factor, which is essential for a good precision of the analysis. The choice of El and Ez should be so made that the measured activity Ai or Az is from boron and lithium only. If there is another interfering element whose contribution to the measured total activity is significant,then it becomes necessary to irradiate three samples at three different energies El, E,, and E, to be able to formulate three equations involving the third interfering element also. By solving eq 1 and 2, the individual contributions AB and
0003-2700/81/0353-0765$01.25/00 1961 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
786
Table I. Nuclear Reactions for the Production of 'Be abundance, Q value, element nuclide % reaction MeV Li Be B C N Li Be B C N
Proton-Induced Reactions 92.5 'Li(p,n)'Be 'Be 100 gBe(p,t)7Be 1°B 20 'OB(p,a)'Be "B 80 "B(p,an)'Be 'lC 98.89 'T(p,apn)'Be I4N 99.64 I4N(p,2a)'Be
'Li
-1.6 -13.4 +1.1
-10.3 -26.3 -10.5
Deuteron-Induced Reactions Li 7.5 6Li(d,n)7Be + 3.4 'Li 92.5 7Li(d,2n)7Be -3.9 gBe 100 9Be(d,tn)7Be -17.6 ]OB 20 ,OB(d,an)'Be -1.1 80 "B "B( d,a 2n)'Be -12.5 98.89 "C(d,~~p2n)~Be-28.5 99.64 I4N ''N(d,20rn)7Be -12.7
Table 11. Nitrogen Interference on Boron and Lithium Determination (Assuming Equal Concentration of Sought and Interfering Elements) irradiation particle
energy, MeV
protons
14 16
20
deuterons
% interference
on B
on Li
4.1 12.5 51
0.4
2.1 13
18
0.4
0.1
20
1.8
0.5
Table 111. Interference-Free Detection Limits for Boron and Lithium in Niobium sensitivity detection irradi(dpm/ppm limit, a 5wA 2 h) ppb activation energy, B Li B Li type MeV proton deuteron
14 15
1200 600
12000 1350
8
15
0.8 6
a Irradiation and measuring conditions: current = 5 MA, irradiation time = 2 h, 'Be chemically separated and measured with a 3 X 3 (in.) well-type NaI detector for 5 h.
can be known and using the formula of Ricci and Hahn (20), one can determine the concentration of boron and lithium in the matrix. From the Measurement of Defined Thickness Target Yields. The second method which can be followed is based on "defined thickness" target yields. In a general case we have samples 1 and 2. For sample 1 with energy interval (E3-&) we get ALi
n
E3
U
Sample 1
u Sample 2
'
(E3 E4E41
'
Adef.B + Adef.Li = Adef.1 and for sample 2 with energy interval (E,-E,)
(3)
K B A d e f . B + KLiAdef.Li = Adef.2 (4) Here, Adef.1 and are experimentally measured defined thickness target activities of samples 1 and 2, respectively, and K B and K L i are constants similar to RB and RLi. The solution to eq 3 and 4 is similar to that of eq 1and 2. The quantitation in this case can be done according to the method of Sastri et al. (21-23). This approach is useful when the material for analysis is available only as a thin foil or sheet and not as a thick target. Sample Preparation. With the exception of beryllium, all standards were irradiated as powders pressed into thick pellets of rectangular shape. Boron was irradiated as pure H3B03 and lithium as LiF mixed with spectral pure graphite in the ratio 1:lO. Nitrogen was irradiated as NHICl and KNOBand beryllium in the elemental form as a thick foil. Niobium samples were prepared as thick targets cut with a diamond saw from a sheet or a rod; they were etched in HF + HN03 medium prior to irradiation. Orchard leave samples were prepared by mixing spectral pure graphite in the ratio 1:4 and pressing the mixture into thick pellets. The standards and samples were wrapped in 20 pm thick aluminum foils. Irradiations. They were aimed at the following: (a) study of activation curves for boron, lithium, nitrogen, and beryllium for irradiations made with protons and deuterons which enables the selection of suitable particle and ita energy and (b) analysis of niobium samples and of the standard reference material, orchard leaves (SRM 1571 from NBS). The irradiations were performed in the internal beam of the isochronous cyclotron at Kernforschungszentrum,Karlsruhe. To obtain activation curves, we performed proton and deuteron irradiations on standards in the energy range 5-20 MeV and at currents ranging from 50 to 200 nA for 5-20 min. Niobium samples were irradiated with deuterons at 5 pA current for 1-3 h. Orchard leave samples were irradiated with deuterons at 0.5-1 FA current for 1-4 h. The reason for irradiating niobium and orchard leave samples only with deuterons is given at a later stage in the text. The current was measured with a Faraday cup. The target holder was water cooled. Counting and Quantitation. After activation,the standards and samples were counted on an Ortec Ge(Li) detector which has a resolution of 1.9 keV fwhm for the 1332-keVy-line of %o and an efficiency of 20.9% relative to a 3 X 3 in. NaI detector. The detector was coupled to a Canberra "Series 80" multichannel analyzer. The standards were measured, depending on the activity level, from 600 to 10000 s. The orchard leave samples were counted for periods ranging from 60 000 to 80 000 s, following a minimum waiting period of about 2 weeks in order to minimize the matrix activity. The niobium samples were etched for a second time, after irradiation, to remove possible surface contamination. It was possible to measure 7Be instrumentally in niobium samples irradiated with 7-MeV deuterons. A chemical separation became inevitablefor all samples irradiated at 15/17 MeV where a higher Compton continuum contributed by the products of direct and secondary nuclear reactions, namely, (d,n), (d,2n), (n,y), (n,2n) etc., on major impurities zirconium and tantalum made the instrumental analysis impossible. However, to have a unified procedure, all the niobium samples, irrespective of irradiation energy, were chemically separated for beryllium. After the separation, the fractions containing IBe were counted in a well-type, 3 x 3 in. NaI detector having a shielding of low-activity lead. Separation Procedure for Niobium. After the surface contamination was removed by etching,the niobium sample was dissolved in a mixture of HF + "OB whereby HNOawas added dropwise until the dissolutionwas complete. Next, about 100 fig of beryllium carrier was added and the solution was diluted with methanol in the ratio 1:l and applied to a 0.7 X 19 (cm) column filled with Dowex-1(100-200mesh, pretreated with 10 mL of HF + CH30H (1:l)).After being washed with about 7 mL of HF + CHBOH(1:l)mixture, beryllium was eluted with 15 mL of 20 M HF. The aqueous phase containing 'Be can be directly counted by using a y-ray spectrometer with a Ge(Li) detector. As additional purification, in order to enable measurements
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY I981
6x10'
1
Lithium
J
soran
i
f
5x104
J
4x10
,
5
10
15
Y 10
2b
I
/
1
1x104
767
Nitrogen ( ~ 1 0 )
15 Energy (MeV1
20
Energy (MeV)
Figure 1.
Thick target yields for proton reactions on Li, Be, B, and N.
with a NaI detector, the aqueous phase wa13evaporated to near dryness with the addition of a few drops of concentrated H2S04 until white fumes of H2S04appeared. The residue was dissolved in 10 mL of 1%solution of Titriplex-I11and the pH was adjusted to a value between 6 and 7 by addition of NaOH. Finally 1 mL of 5% aqueous acetylacetone was added. The solution was transferred to a separating funnel, allowed to stand for 5 min, and then extracted with 3 mL of chloroform. The extraction was repeated three times, and the combined organic phase was measured for beryllium. RESULTS AND DISCUSSION Thick Target Yields and Nuclear Interferences. The nuclear reactions together with the Q values for the production of 7Be are given in Table I. The Q values are taken from Keller et al. (24). The thick target yields (Figures 1and 2) at each energy are average values of three to four irradiations. As a special case to test the reproducibility of irradiations performed on different days, with 14-MeVprotons nine samples were irradiated for boron and six samples for lithium standards, and the standard deviation from the mean activities was between 8% and 10% in both cases. The nuclear interference levels (Table 11) were obtained assuming equal concentration of the sought and the interfering elements in the sample to be analyzed. In Figure 1,the thick target yields for proton-induced reactions on boron, lithium, beryllium, and nitrogen are shown. In the energy range 5-14 MeV, the yield for lithium is about 10 times higher than that for boron but the curves show a parallel course (RB = RLi) indicating that the suggested methods for distinguishing boron from llithium cannot be applied. Beyond 14 MeV, the yield for boron increases rapidly relative to lithium, but in this energy region, 'Be can also be produced from nitrogen. The interference from nitrogen increases strongly with proton energy. At 14 MeV, it amounts to 4.4% on boron and 0.4% on lithium; it goes up to 51% and 13%, respectively, for 20-MeV protons. Also, the interference
Figure 2.
Thlck target yields for deuteron reactions on Li, Be, B, and
N.
from beryllium, based on the gBe(p,t)7Bereaction having a Q value of -13.4 MeV, has to be considered with increasing proton energy (>15 MeV). The thick target yields for the deuteron-induced reactions on the above elements are shown in Figure 2. Here, the activation trend for boron and lithium is different from that previously shown. In the energy range 5-7 MeV, the yield for lithium is somewhat lower than that for boron, but both are essentially of the same order. Beyond 7 MeV, the yield for lithium increases much more rapidly than for boron and the influence of beryllium is noticeable at an energy >20 MeV. If we irradiate boron and lithium standards as thick targets choosing 7 MeV for Eland 15 MeV for E2,we obtain R L ~ / R B = 2.5. Between 15 and 20 MeV, the yield for boron stabilizes whereas for lithium it continues to rise reaching RL~/RB = 4.3 for El = 7 MeV and E2 = 20 MeV. From the data given above it is evident that the irradiation with deuterons is superior to that with protons with regard to elimination of the interference from nitrogen. This interference is the deciding factor in setting the upper limit for the irradiation energy E2. This energy depends on the ratio of the concentration of boron and nitrogen, and/or of lithium and nitrogen. For example, in metals like zirconium, niobium, and tantalum, nitrogen is usually present at the 10-100 ppm level whereas boron is present at the 1-10 ppm level and lithium even much lower than this. At these concentration ratios, with reference to boron determination the interference from nitrogen can be neglected up to a deuteron energy of 18-20 MeV. On the other hand, in biological matrices like orchard leaves and bovine liver where nitrogen is present at percentage level, and boron and lithium are at parts-permillion level, the highest acceptable deuteron energy lies between 13 and 15 MeV. Sensitivity and Limits of Detection. From the thick target yields shown in Figures 1 and 2, sensitivities can be
768
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
Table IV. Concentration of Boron and Lithium in Niobium and Orchard Leaves total no*
no. of samples irradiated samples at different energies, MeV analyzed 7 13 15 17 Of
matrix niobium (ES-80) niobium (WCT-74) orchard leaves (SRM 1571) a Suggested value only.
7 4 8
3 2
4
B
0.04
4
0.2
2
4
derived for arbitrary experimental conditions. As an example, in Table I11 sensitivities are given for 14-MeV protons and 15-MeV deuterons assuming a beam intensity of 5 pA and an irradiation time of 2 h. The limit of detection of one element depends on the count rate, which is a function of concentration, of the other. Consequently,the best limits of detection can be obtained for an element when the activity contribution from the other element is negligible. These interference-free limits of detection are also given in Table 111. They are estimated assuming an irradiation with 5 p A flux for 2 h, a specific separation of 7Be from the irradiated matrix with yield nearly loo%, and its counting with a well-type 3 x 3-in. NaI detector. The limits of detection are expressed as a minimum detectable photopeak equal to 3a to the background in the spectral region of the above detector for the niobium matrix but they hold also for any other matrix which can withstand these irradiation conditions. For these matrices there can be small variations in the obtainablelimits because of the difference in the particle range between niobium and the chosen matrix. The limits can be improved to some extent by increasing the irradiation time and beam current. The limits are given for 14-MeV protons and 15-MeV deuterons, as at these energies boron and lithium determinations are not interfered with to any noticeable level by nitrogen and beryllium. It is known that one can obtain higher sensitivity for boron when reactions like l1B(p,n)l1C,1°B(d,n)llC,and 1°B(a,n)13N are used (25).However, these methods are subject to nuclear interference from elements like carbon and nitrogen which are usually present at higher concentration levels than boron. On the other hand, one finds that: (1) Because of relatively long half-life of 'Be compared to llC (10.3 min) and 13N (9.96 min), the separation of 'Be can be more conveniently done. (2) Unlike the measurement of llC and 13Nwhere a decay curve analysis is essential, the measurement of 7Beis specific because it emits a y-ray of 477 keV. (3) It makes the simultaneous determination of boron and lithium possible. If we compare the detection limits achievable for boron and lithium under the given irradiation conditions, lithium determination is always more sensitive than boron. Also, for lithium itself, proton activation is nearly 10 times more sensitive than deuteron activation but is not recommended for routine use because of the nuclear interference from boron which cannot be estimated and corrected for. But, in a particular matrix, if boron concentration is expected to be some orders of magnitude lower than lithium, it will be appropriate to find lithium by proton activation where concentration levels as low as 1ppb can be determined. Determination of Boron in SRM Orchard Leaves. The accuracy of the developed technique was checked by analyzing the NBS Standard Reference Material 1571, orchard leaves. Separate irradiations were made with deuterons at 7 and 13 MeV and 7Bewas measured instrumentally after the decay of matrix activity by a considerable level. The energies referred to here were those incident on the 20 pm thick alu-
concentration, ppm found certified value Li B Li
0.01 0.05
f
f
36 f 3
