Anal. Chem. 1081,
/-\,
.............. k 1 ' *.. ........ ...... B
.............
I
0
l
l
of NaCl containing 5 pg of thiourea. The present atomizer is suitable for lower concentration level with small sample size. Therefore, this is too sensitive to extend on a concentration basis without any modification.
LITERATURE CITED
.............
l
l
l
l
l
l
'
l
1
0.5 Time ,
Czobk. E. J.; Matwsek, J. P. Anal. Chem. 1978, 50, 2-10. Churella, D. J.; Copeland, T. I?. Anal. Chem. 1978, 50, 309-314. Segar. D. A.; Gonralez. J. G. Anal. Chlm. Acta 1972, 58, 7-14. Sturgeon, R. E.; Berman, S. S.; Desaulnlers, A.; Russell, D. S. Anal. Chem. 1979, 57, 2364-2369. McArthw, J. M. Anal. Chlm. Acta 1977, 93, 77-83. Mennlng, D. C.;Slavln, W. Anal. Chem. 1978, 50, 1234-1238. L'vov, 8. V. Spectrochlm. Acta, Part B 1978, 338, 153-193. Regan, J. G.; Wanen, J. Analyst(London) 1978, 707, 220-221. Hydes, D. J. Anal. Chem. 1980, 52, 959-963. Ohta, K.; Suzuki. M. Talanta 1975, 22, 465-469. Amos, M. D.: Bennett, P. A.; Brodie, K. G.: Lung, P. W.; Matowek, J. P. Anal. Chem. 1971, 43, 211-215. Ohta. K.; Suzuki, M. Anal. Chlm. Acta 1978, 96, 77-82. Ohta, K.; Suzukl, M. Talanta 1978, 25, 160-162. Ohta, K.; Suzukl, M. Talanfa 1979, 26. 207-210. Ohta, K.; Suzukl, M. Anal. Chlm. Acta 1979, 710, 49-54. Ohta, K.; Suzuki, M. Fresenlus Z. Anal. Chem. 1979, 298, 140-143. Krasowskl, J. A.; Copeland, T. R. Anal. Chem. 1979, 57, 1843-1848.
A
\
sec.
Figure 7. Atomization profiles of varying amount of Cu in 0.5 p g of NaCi: (A) 1 pg of Cu, (B) 5 pg of Cu, (C) 10 pg of Cu. All samples were atomized in the presence of 5 p g of thiourea.
employed. A 1-pL sample containing 0.5 pg of NaCl was handled by use of 5 pg of thiourea. In the present atomizer the concentration detection limit is restricted by the small sample volume (1pL). Therefore, detection limit is given in absolute terms (picograms). As little as 0.6 pg of Cu could be detected. For Mn the detection limit was 0.6 pg in 1pg
13
53, 13-17
RECEIVED for review July 10,1980. Accepted October 6,1980. Supported by the Ministry of Eduction, Science and Culture, Japan, through Grant-in-Aid for Special Project Research.
Determination of Boron and Lithium in Nuclear Materials by Secondary Ion Mass Spectrometry W. H. Christie,' R. E. Eby, R. J. Warmack, and Larry Landau Analytical Chemistry Division, Oak RMge Natlonal Laboratory, Oak Rklge, Tennessee 37830
Secondary Ion mass spectrometry (SIMS) has been used for the rapld, accurate analysis of B and LI In varlous nuclear materials. The problem of sample charging observed In the analysis of Insulator materials has been overcome by dlstributing the sample as a thln film on a conducting substrate. No sample chemistry Is required for Isotopic measurements and only sample dissolution Is necessary for the application of isotope dilution methods for quantitative analysis. The high sensitivity of SIMS for B and Ll makes it possible to analyze sufficiently small radioactive samples so that radiation is reduced to acceptable levels for safe handling. The preclslon of SIMS isotopic analysls for natural B samples Is about 0.5% and is about 1.0% for natural LI samples.
In many instances, secondary ion mass spectrometry (SIMS) allows one to perform mass and isotopic analysis on samples that are not amenable to other mass spectrometric techniques (e.g., surface ionization, electron impact, etc.). Collins and McHugh (I) demonstrated the principle of sputtered ion analysis as a technique for isotopic measurements of elements that perform poorly in thermal ion sources. In our laboratory we frequently encounter samples that are either radioactive, refractory, or in unknown chemical form or that occur with some combination of these features. In this paper we discuss the applicability of SIMS to the rapid, accurate determination of Li and B isotopic ratios in difficult sample types. The significant advantage that accrues from the use of SIMS for 0003-2700/8 110353-0013$01.OO/O
isotopic analysis of these materials is the complete elimination of any chemical sample preparation steps. The difficulty in preparing these types of samples for conventional mass spectrometric analysis (e.g., electron impact, thermal emission) is treated in some detail by Wichers (2) et al. and in the ASTM Standards Handbook (3). We further show that for some samples quantitative results can be obtained by using isotope dilution techniques with minimum sample chemistry. Three specific applications to nuclear materials will be discussed first, the quantitative determination of B and its isotopic composition in borosilicate glasses; second, the determination of the isotopic composition of B and Li in irradiated nuclear-grade aluminum oxide/boron carbide composite pellets; third, the quantitative and isotopic determination of B and Li in highly radioactive solutions of unknown composition. The first example arises because, in certain instances, solutions of fissionable uranium are stored in vessels that are of unsafe configuration from the standpoint of nuclear criticality. To ensure the nuclear safety of these containers, they are loaded with circular Raschig rings made of borosilicate glass. The *OB in this glass has a large cross section for neutron adsorption and thereby reduces the nuclear reactivity of the stored solutions. These Raschig rings are sampled at regular intervals, and the loB/l1B ratio is determined to verify that no significant neutron producing event has occurred. The second example stems from the analysis of aluminum oxide/boron carbide composite pellets. These materials are used in nuclear reactors as neutron shims (control rods) and 0 1980 American Chemical Society
14
ANALYTICAL CMMISTRY. VOL. 53. NO. 1. JANUARY 1981
usually contain from 1.5-4.0 w t % B. Significant difficulty is encountered in preparing samples of this materid suitable for conventional mass analysis because of the high aluminum content. It is not unusual to do the required separation chemistry and find that the final sample loaded into the maes spectrometer will not generate sufficient ion intensity to allow analysis. We have shown that the SIMS technique can be applied to these materials to provide rapid isotopic analysis of B and any 7Li that might he present aa a result of %(n, a) reaction. The last example concerns the analysis of liquid solutions of unknown composition. Samples of this type are Been infrequently and are usually the result of an accident of some sort. Because of their unknown nature and the desire to establish their nuclear history, rapid analytical results are desired. The SIMS technique can be applied to samples of this type to provide both isotopic and quantitative information. In the SIMS technique i t is recognized that some method of surface charge compensation must be employed in the analysis of insulator materiala. Thia is most frequently achieved by using 0-primary ions because of the steady-atate, self-hiasing effect created by the emission of secondary electrons from the sample surface ( 4 ) during primary ion bombardment. This effect allows a continuous, constant emission of sputtered secondary ions. When a true surface analysis of an unaltered insulator surface is required, 0primary beams or some other form of surface charge compensation must be used. In this work we are concerned with overcoming the charging problems encountered with the insulator samples studied but are not constrained to performing a surface analysis. McHugh (5) has shown that microsize insulator particles adhering to a conducting carbon or tantalum substrate can be analyzed with an ‘9 primary beam. He observed that use of an primary beam resulted in significant particle lm due to particle charging and subsequent electrostatic repulsion. Morgan and Werner (6) have solved the conductivity problem by mixing powdered insulator samples with graphite prior to SIMS analysis. SIMS requires a sample with a reasonably smooth, flat surface (rough sample surfaces cause distortion in the electrostatic ion extraction field that can result in unpredictable anomalies in ion extraction efficiencies). The sample p r e p aration technique that we describe meeta the flat sample surface requriement and provides for uniform ion extraction. To demonatrate that the SIMS method provided the required accuracy in these isotopic determinations, it was necessary to show that no uncorrectable isotopic fractionation took place during sputtering and that the unique, simple sample p r e p aration method introduced no uncontrollable variations. EXPERIMENTAL SECTION Appparatw. The secondary ion maas spectrometer used in thin m r k (Manufactured by Applied Research Laboratories, Sunland. CA) was based on the design of Liebl(7); the capabilities of this instrument have been described by Andersen and Hinthorne (8) and McHugh (9). The reader interested in the details of this instrument should consult these references. In simplifiedterms, this instrument is basically two maas spectrometers. One spectrometer focuses a mass-analyzed primary ion beam onto the sample surface. Where the primary beam strikes the sample surface, ions are formed by various sputtering processes. The sputtered ions are collected by an extraction electrode and direaed by an electrostaticlens into a doublefocuaing m a s spectrometer where maas analysis is accomplished. Preliminary Experiments on Sample Preparation and Analysis of Borosilicate Glasses. The first technique we examined in the analysis of borosilicate glaases was tailored to meet the SIMS requirement of a flat sample surface. Small pieces of the sample, 0.&1.0 cm, were prepared by cr~ahingRaachig ring
electron micrograph showing dlstrlblrtlon 01 bcrosllicate glass particles on a pyrolytic graphite svrlace. Figure 1. Scanning
that had been used in uranium storage tanka and virgin ringsthat contained normal boron. Several pieces of each sample were encapsulated into an epoxy resin and polished flat. After ultrasonic solvent cleaning, a 50-nm carbon film was evaporated onto the mount surface to render the sample conducting. A relatively intenae (25 nA. Os+)primary beam was used to p r e sputter a region 100 X 80 pm to a depth of several thousand angstroms. Thia wan done to ensure removal of surfaceimpuritiesand allow measurement of the “bulk boron. We then used a 1-nA, ISOprimary beam to eliminate sample charging effects, and analytical data were acquired from a 50 x 40 pm region. Counting rates approaching l@counts s-l were obtained for IlB in the natural standard. This initial experiment was very encouraging beeawe we were able to demonstrate that the isotopic composition of B could be determined directly from the glass at a 1.0% precision level and required no cumbersome chemical preparation steps. The mechanid polishing of the samples is straight forward but is time consuming and requires specialized equipment. Sample Preparation. Our final sample handling technique waa simply to grind the sample to micron particle size and distribute it in a very thin layer on a conducting substrate. In this mode the sample behaves like a conductor and no surface charge compensation is required, dowing the analpie to be anomplished with whatever primary bombarding species that is convenient. Grinding was achieved by crushing 0.2 g or leas of sample in a chromium mortar. The sample was ground for only a few minutee as it is not necessary to achieve a uniform particle size. The powdered material was placed in a small plastic vial and 0.S1.0 mL of distilled water added. This mixture was shaken to ensure particle suspension and was then allowed to stand for 30 a. A 1-pL micropipet was used to draw up 0.250.50 pL of liquid from the top surface of the suspension. This withdrawn liquid was usually a pale gray or white color and contained only sample fmes from the grinding proceas. This suspension waa then deposited on a conducting substrate. Pyrolytic carbon wss the mast common substrate material, but in some experimenta Au and Pt surfaces were used. After evaporation of the water, roughly circular spots of submicron sized particles were typically distributed aa shown in Figure 1. The large surface area generated in the grinding
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table I. SIMS Isotopic Analysis of Powdered Borosilicate Glass With 0,' and 0-as Bombarding Species "B/l0B atom ratios sample identification O,+primary 0 - primary 1 2 3 4 5 6 7 8
av i SD
0.246 0.247 0.244 0.248 0.251 0.247 0.249 0.245 0.247 * 0.002
0.248 0.248 0.248 0.249 0.245 0.246 0.248 0.247 0.247 t 0.001
process allowed enough surface leaching of the sample to occur so that when the water evaporated the remaining solid was self-cemented to the conducting substrate. Observation of the sputtering region during analysis showed that no visible particles were ejected from the sample surface because of repulsive electrostatic charge buildup. A thin film that appeared discontinuous when viewed under low magnification was best for analysis when primary beam. using an 02+ SIMS Analysis Procedure. The Applied Research Laboratories ion microprobe accepts a circular sample mount up to 2.54 cm in diameter. We usually used pyrolytic carbon planchets scribed with eight identifying marks equally spaced around the disk circumference. These marks, visible through the instrument microscope, provided positive sample identification during analysis. Eight samples, as approximately 0.15-0.20 cm diameter spots were loaded adjacent to the identifying marks, and a standard (one or more) was loaded in the center of the disk. Analysis was accomplished by rastering a region 100 X 80 Mm with a 5-nA, 02+primary beam. Hydride formation during the sputtering process would cause a mass interference for both Li and B isotopic analysis. To minimize this possibility, since exposure to water was part of the loading procedure, we allowed samples to remain at instrument residual pressure overnight before analysis. Prior to and during analysis, a copper plate in close proximity to the sample surface was cooled to liquid nitrogen temperature to minimiie the partial pressure of hydrogen-containingspecies in the sputtering region. Work by Magee and Wu (IO)suggest that hydride formation can be eliminated by sputtering in ultrahigh vacuum where the hydrogen partial pressure is less than 1.3 X Pa. Using the Applied Research Laboratories ion microprobe for isotopic analysis requires that a bias correction be made to correct for mass fractionation, detector response, and other unidentified mass-dependent discriminations that occur. During analysis, a constant distance (0.51 mm) is maintained between the secondary ion extraction electrode and the sample surface in an attempt to maintain constant sample geometry. Standards for each element are then analyzed, and a bias correction factor is determined that can be used to correct sample data. Isotopic ratio data for B and Li are acquired by electrostatically switching the two isotopes onto the detector 50 times a second. During each cycle, a separate scaler is gated on to accept and integrate ion counting pulses from each isotope. Scaler live time is preset to provide 10-s counting times for each isotope. Upon completion of the 20-s total counting period, the acquired data are transferred to a PDP-11/34 computer where corrections are made for counting system pulse-pair resolving time. This is followed by printing of a count-loss corrected isotopic ratio on the system printer as the next data collection cycle is occurring.
RESULTS AND DISCUSSION SIMS Analysis of Borosilicate glasses. To demonstrate that 02+ primary beams can be used without sample charging problems affecting the analytical data, a comparison of 02+ and 0- results was made. 02+ primary beams are the most common mode of analysis because better focused, higher current density beams can be obtained. Table I shows that 02+ can be used for insulator analysis (with proper sample
15
Table 11. SIMS Isotopic Analysis of Natural Boron Glass Powder Standards bias e run loB/llBa correction no. raw av SDb %RSDC n d factor 1 2 3 4 5 6 7 8
av SD
0.2697 0.2687 0.2702 0.2689 0.2634 0.2709 0.2706 0.2695 0.2690 0.0024
0.0003 0.0009 0.0007 0.0010 0.0009 0.0016 0.0006 0.0007
10 10 10 10 10 10 10 10
0.11 0.33 0.26 0.37 0.34 0.59 0.22 0.26 0.31
0.9155 0.9189 0.9138 0.9182 0.9374 0.9114 0.9124 0.9161 0.9180 0.0083
SD = standard a Natural loB/l1Btaken as 0.2469. deviation. RSD = relative standard deviation. n= number of replicates. e Bias factor = 0.2469/(10B/11B raw av). Table 111. SIMS Isotopic Analysis of Borosilicate Glass Powders sample atom ratio % R S D ~ nc loB/l1Bav SD a no. 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9
0.2469 0.2468 0.2495 0.247 2 0.2469 0.2478 0.2467 0.2497 0.2456 0.2474 0.2457 0.2467 0.2455 0.2465 0.2470 0.2463 0.2469 0.2463 0.2469 0.0009
0.0005 0.0005 0.0019 0.0007 0.0013 0.0013 0.0008 0.0006 0.0008 0.0006 0.0009 0.0009 0.0016 0.0009 0.0006 0.0005 0.0010 0.0006 0.0009
0.20 0.20 0.76 0.28 0.53 0.52 0.32 0.24 0.33 0.24 0.37 0.36 0.65 0.25 0.24 0.20 0.41 0.24 0.35
10 10 10 10
10 10 10 10 10 10 8 10 10
10 10 10 10 10
% abs error
100(Y - X)/Y 0.00 -0.04 1.05 0.12 0.00 0.36 -0.08 0.41 -0.53 0.20 -0.49 -0.08 -0.57 -0.16 0.04 -0.24 0.00 -0.24 -0.01 0.34
av SD % RSD = percent relative SD = standard deviation. SD. n = number of replicates. x = I0B/l1Baverage, natural l0B/I1Btaken as 0.2469 = y . loading) with no appreciable compromise in data quality. We define an isotopic bias factor as
BIAS FACTOR = (normal a/b)/(measured Za/Zb) (1) where a and b represent the two isotopes being measured and I is the observed ion intensity. Normal represents the accepted value of the isotope ratio in the standard being used for the calibration. The bias correction factor for isotopic boron analysis was determined from a powdered borosilicate glass containing natural boron. The results of this study are reported in Table 11. A run shown in Table I1 was the result of averaging 10 (n) ratios. The standard deviation (SD)therein reported represents the internal precision for the 10 ratios making up the raw average. The average of the eight determinations reported in Table 11has an SD of about f l % .We found that this could be improved by paying careful attention to instrument set-up conditions. Specifically, it was necessary to focus the secondary ion spectrometer so that rectangular peak shapes were obtained. It was then necessary to ensure that the two isotopes of interest were centered about the detector slit by manually
18
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table IV. Comparative Quantitative Analysis of Boron isotope atomic sample dilution SIMS absorption no. wt%B wt % B 1 2 3 4 5
3.94 3.88 3.98 3.87 3.94
3.91 3.90 3.92 3.90 3.88
Table V. SIMS Isotopic Analysis of Li and B in Al,O,/B,C Composite Pellets ppm atomic
Li
llB
1
45 35 38 49 46 43
83.4 83.4 82.9 82.9 83.8 84.3
fine tuning the secondary magnet to allow centering of the electrostatic sweep control for each mass. The data reported in Table 111 reflect the improved precision (now about 0.5%) obtained by correctly tuning the secondary spectrometer and probably represents the best precision obtainable with this instrument for the type of samples studied. To examine the applicability of the SIMS method for quantitative analysis, we dissolved five samples of borosilicate glass powders containing natural B in aqueous HF using the procedure of Burdo and Snyder (11). Ten microliters of each sample was then loaded on to a pyrolytic C planchet. A second loading of 10 pL of each sample was loaded at another location on the planchet. Before the samples were dried, 10 pL of a O ' B spike solution was injected into each sample drop while viewing under a low-power microscope. The two solutions were mixed by drawing the solution up into the spike pipet several times. The spiked and unspiked samples were then dried as usual in a laminar flow hood and were subjected to SIMS analysis. The quantitative results thus obtained from standard isotope dilution calculations are compared in Table IV to results for the same samples by use of conventional atomic absorption methods. SIMS Analysis of A1203/B4CComposite Pellets. Aluminum oxide/boron carbide composite pellets are used as neutron shims (control rods) and contain from 1.5 to 4.0 wt % B which may or may not be enriched in log. Lithium is of interest in these materials because 'Li is formed by n, a reaction on loB during irradiation. The fact that these materials have been irradiated necessitates hot cell handling. The pellets are powdered in a ball mill and for conventional analysis are subjected to a sealed tube dissolution procedure to get B into solution. This step is followed by a chemical separation to remove Al, which interferes with conventional mass spectrometry of B. Using SIMS, we have treated the powder produced in the ball mill treatment in a manner analogous to that described earlier for the analysis of borosilicate glasses. Only a few milliRoentgen of radiation is involved for each sample loaded. Li and B isotopic data obtained directly from powdered samples by using SIMS analysis are reported in Table V for six irradiated A1203/B4C composite pellets. The limiting factor fo the Li analysis in these samples appears to be a small variable contribution of the species C2+ to the m / z 6 position. This arises from C sputtered from the B4C and C planchet and is estimated to be less than 20 ppm from the analysis of unirradiated &03/B& samples. Mon-
total B wt, ppm
method isotope dilution spark source a
3314 3565 3211 3235 3978 3050
spectrometry microtitration mannitol procedure isotope dilution
SIMS
total Li
atomic %
sample 2 3 4 5 6
Table VI. Analysis of a Three-Mile Island Water Sample and a Comparison of SIMS with Other Techniques"
method flame emission
spectrometry isotope dilution SIMS
wt, ppm
isotopic B I1B/I0B (atom) -4
4.07
isotopic Li atom fraction
