9
Anal. Chem. 1981, 53,9-13
P b and Mg, the ultrasonic nebulizer-miniburner system gave 5-fold improvements in detection limits; here, the noise levels were related to the flame background emission and electronic noise rather than the nebulizer. For all laser measurements, the laser beam was focused in the center of the observation region; cylindrical focusing was tried but gave no gain in signal. In the case of Mg and Pb, further improvements in detection limits would result by use of a dye laser with larger spectral irradiances a t the excitation lines; however, the decrease in detection limits with increase in laser spectral irradiances will only occur as long as source-related noise, e.g., scatter noise, is unimportant. For Ca and Sr, an increase in laser spectral irradiance would not improve the detection limits and in fact would be expected to cause a degradation in them because of the increase in scatter related noise. In Table IV, concentrational detection limits obtained with laser excited AFS (LEAFS) and the ultrasonic nebulizerminiflame system are compared to detection limits obtained by several other methods. The ultrasonic nebulizer-minitlame burner system using LEAFS is shown to be comparable for several techniques and much better than others. In Table V, absolute detection limits (picograms) are shown for LEAFS by use of the ultrasonic nebulizer and miniflame burner vs. other spectroscopic methods. As can be seen, the ultrasonic system gives at least 1 order of magnitude in the worst cases and up to 3 orders of magnitude improvement in the best cases. This shows that due to the low sample uptake rate and high efficiency of fog transfer, that the ultrasonic nebulizerminiflame burner system is capable of performing trace analysis on small samples without preconcentration steps. In Table VI analyses of NBS standard reference materials using LEAFS with the ultrasonic nebulizer-miniflame burner system are presented, along with the certified values. In this study, both the Molectron UV-14 and the Chromatix CMX-4
lasers were used. As can be seen, the accuracy and precision of this method were extremely good with no indication of interferences for the real sample analyses performed here. Thus this system should be useful for measurement of very low concentrations of elements in real samples with a precision and accuracy not available by most other methods.
ACKNOWLEDGMENT The authors thank Mr. A Grant and his staff for construction of all optical support equipment and h4r. Rudy Strohschein for construction of the nebulizer chambers. LITERATURE CITED (1) Wineforher, J. D. ACS Symp.Ser. 1978. No. 85. (2) Weeks, S. J.; Haraguchi, H.; Whefonhr. J. D. AMI. Chem. 1978, 50, 360-368. (3) Fraser, L. M.; Wfotdner, J. D. Anel. Chem. 1972, 44, 1444-1451. (4) Havath, J. J. MS Thesis, Unlverdty of Florida, Qainesvlle, FL. (5) Owerrs. L. E. Technical Repat AFWlR-67-400; WrlghtPattm At Force Bass. OH, 1966. Face Base, OH, 1971. . S M . 0. W.; Parsons, M. L. J . Chem. E&. 1973, 50. 679-681. Bower, N. W.; Ingb,J. D. Anel. cham.1976, 48,666-692. IUPAC, Nomendatwe, Symbols. Unlts and W Usage h Spectro1Analy84, Part 11. 1975. Epstein, M. S.; Ralns, T. C.; Menls, 0. Can. J . Specbpsc. 1975, 20, 22-26. Epsteh, M. S.; Nkdel, S.; Omenetto, N. D.; Reeves, R.; Bradshew, J. D.; W f W d n e C , J. D. Anel. chem.1970, 51, 2071-2077. Metthson. J. M. Anel. cham.1972, 44, 1715-1716. . . PSrkMlmer Atomlc Absw~tionMefatwe; PerkirrELnec Cap.: Norwak, CT. (14) Wfordner. J. D.; Fkztrgerald, J. J.; Omenetto, N. Appl. Specfrosc. 1975, 29,369-363. (15) Bamana, P. W. J. M.; [kBoes, F. J. specbodrtn.Acta, Pari B1975. 3019,309-334.
RECEIVEDfor review July 7,1980. Accepted October 1,1980. Research was supported by Grant No. AF-AFOSR-F49620-
8oc-ooo5.
Elimination of Alkali Chloride Interference with Thiourea in Electrothermal Atomic Absorption Spectrometry of Copper and Manganese Masaml Sutukl,
Klyohlsa Ohta, and Tatsuya Yamaklta
Department of Chemkfry, Faculty of Engineering, Mie University, Kam%rsmcho, Tsu, Mleken 514, Japan
Interferences of NaCI, KCI, and NH&l on the atomizatlon of Cu and Mn have been studied in a molybdenum mkrotube atomlzer. Interferences of CaCI, and MgCI, were also examined. Chloride Interference on Cu is removed by adding thkuea as maw modifier, whkttmdfectolchkrkleon Mn is compensated by background correction. The atomhation pra4iles show the complex atomlzatkm process for Cu h akall chlorides dlfferlng from Mn. As to the effect of thlourea as modifier for Cu, ll can be assuned that the fonnatkm d Cucl is prevented by hydrogen sulflde generated through the decomposHlon of thiourea In the atomizer. The detectlon llmlt for Cu In 0.5 pg of NaCl was 0.6 pg In the presence of thiourea and that for Mn was also 0.6 pg.
The interferences caused by a chloride matrix are serious in electrothermal atomic absorption spectrometry. Great 0003-2700/81/03539$01.00/0
efforts have been devoted to remove or minimize such interferences. Czobik and Matousek (1) reported the resulte of a detailed study on the interference effect of metal chlorides in furnace atomic absorption with both a slow conventional and a fast response detection system and showed that treatment with phosphoric acid was effective for removal of chemical interference in the Cu-NaCl system. Churella and Copeland (2) examined the concentration-dependent interferences of several alkali and alkaline earth halides in electrothermal atomic absorption spectrometry of Cu by use of a carbon cup and showed that the addition of Na202eliminated or substantially reduced the interferences caused by halides. A selective volatilization technique was applied for removing chloride matrix interferences (3). However, loss of sensitivity caused by covolatilization of Cu and Mn with NaCl was observed. Sturgeon et al. ( 4 ) described the combination of selective volatilization and matrix modification techniques for direct determination of Mn and Zn in seawater by graphite 0 1980 American C2wmlcal Sodety
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JANUARY 1981
furnace atomic absorption. Charring with NH4Cl and a slow temperature rise technique are effective for elimination of chloride matrix interference on Mn (5). NHICl was also used to determine P b in a chloride matrix with the molybdenumcoated graphite furnace as matrix modifier (6). Apart from methods involving the removal of the bulk of the interfering chloride, L'vov (7) described a method involving binding free chlorine into molecules which dissociate at low temperatures by adding an appropriate component to the sample and illustrated by the releasing action of LiN03 on the depression of T1 absorption in NaCl. Reduction of matrix effects by the addition of a soluble organic reagent was also proposed (8,g). The purpose of the present work is to study the interference of chloride matrices on electrothermal atomization of Cu and Mn in a molybdenum microtube atomizer and its elimination by the addition of thiourea. EXPERIMENTAL SECTION Instrumentation. Atomic absorption measurements were made with a Nippon Jarrell-Ash 0.5m Ebert-type monochromator coupled to an R106 photomultiplier tube (Hamamatsu T V Co.) and a fast-response amplifier which was assembled in this laboratory. The output signal from the amplifier was fed to a microcomputer (SORD M223) through an AD converter (DATEL ADC-HX 12BCC) and multiplexer (DATEL MX-808). The memory capacity of this computer is Moo0 words and the basic cycle time is 500 ps. A JASCO 0.25-111 Czerny-Turner-type monochromator (CT 25N) coupled to a fast-response amplifier and a storage oscilloscope (Kikusui Electronics Corp. 5516ST) with a time constant of 0.5 ps was also used for interference study. A molybdenum microtube (20 rnm long and 1.5 mm i.d.) (10) was used as atomizer. This microtube was machined from a molybdenum sheet (0.05 mm thick). The microtube atomizer was e n c l d in a Pyrex chamber (300 mL volume) which had two silica end-windows to allow transmission of the light beam. The chamber was purged with argon a t a flow rate of 480 mL/min and hydrogen at a flow rate of 20 mL/min. The microtube was mounted on two supports so that there was no localized variation in tube temperature. The power for heating the atomizer was applied by a stepdown transformer. Hollow cathode lamps (Hamamatsu TV Co.) were used as the source. The radiation from the source was unmodulated for fast tracing of the output from the photomultiplier. In order to limit radiation from the microtube reaching the photomultiplier, we placed a slit in front of the microtube. The spectral lines used were 324.8 and 279.5 nm for Cu and Mn, respectively. The atomizer temperatures were measured with a photodiode (Hamamatsu TV Co. S641). The signal from the photodiode was calibrated with an optical pyrometer (Chino Works) and is recorded with the absorption signal simultaneously. Samples were injected into the microtube atomizer by use of a 1-pL glass micropipet. Reagents. All chemicals used were analytical reagent grade. Water was distilled and deionized. Stuck solutions, 1 mg/mL, of the elements of interest were prepared by dissolving 1.ooO g of pure metal in a minimum amount of "OB and diluting to 1 L. Solutions of the matrix compound, 50 mg/mL, were prepared as needed. Working solutions were freshly prepared just before use by diluting appropriate volumes of stock solutions. General Procedure. The pure metal solution was run first and then metal solutions containing salt were run in order of increasing salt concentration. Each sample was atomized by heating to give a final temperature of 2200 "C after drying at 100 "C for 10 s. All the atomization signals were stored in a microcomputer. Signals were subject to background (noise) subtraction, compensation of base line, and smoothing. Then the resulting signals were displayed on a cathode ray tube (CRT). The programming was also made to indicate the well-defined appearance and peak temperatures of atomization profiles. The atomization signals were traced on a storage oscilloscope separately to observe the atomization profiles. R E S U L T S A N D DISCUSSION Effect of Hydrogen. Amos et al. (1Z) showed that, by replacing the nitrogen or argon inert gas shield around a
%A 100
50
0
100
0
1.0
2.0
Time , sec Flguo 1. OsciUoscope traces for effect of hydrogen Row rate on akdzatkn of 50 pg of Cu: (1) rx) Wovea added; (2) 5 pg of W N e a added (A) 500 mUmin of Ar, (B) 5 mL/min of H2 and 495 mUmh of Ar, (C)20 W m h r of H2 and 480 mL/min of Ar, (D) 200 ml/min of H2 and 300 Wmin of Ar, (E) 500 mL/min of H2, (F) background for Ar (no Cu present), (G) temperatwe increase.
carbon filament atomizer with an argon-hydrogen mixture, improved sensitivity could be obtained for many elements, together with a reduction in interference effects and suppression of background absorption. For a metal atomizer, it is necessary to mix hydrogen with the argon purge gas in order to protect the atomizer from oxidation by traces of oxygen in the argon. This served to prolong the life of the atomizer. Therefore, the dependence of atomization profiles for Cu and Mn on purge gas flow rates was examined. Figure l(1) shows the argon and hydrogen flow-rate dependence. T h e atomization temperature for Cu shifted to higher regions with increasing hydrogen flow. The lowest atomization temperature of Cu was shown in pure argon atmosphere. However, instability of the base-line signal and poor reproducibility of signals resulted. Hydrogen contributed to stabilization of the signal. The dependence of atomization profile for Cu in the presence of thiourea on hydrogen flow rate is shown in Figure l(2). In this instance, the effect of varying hydrogen flow on Cu atomization was reduced, although its effect remained. Thiourea also contributed to a highly reproducible atomization profile. The appearance and peak temperatures of profiles were 1260 and 1720 K, respectively, for pure Cu and 1190 and 1740 K, respectively, for Cu in the presence of thiourea. Thiourea appears to have some effect on the atomization temperature of Cu. Cu is coordinated by the sulfur atom in thiourea, and sulfide may be formed from its complex by heating in the atomizer. The gaseous Cu atoms may arise through the thermal dissociation of sulfide. A similar effect of hydrogen was observed for Cu in the preaence of NaCL The shift of atomization temperature to higher regions with increasing hydrogen flow is characteristic of volatile elements such as Pb, As, Sb, Se, and T e (Z2-Z6). The addition of hydrogen in purge gas lowered the atomization temperature for Mn. Figure 2 shows the argon and
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
11
%A
100
50
0
100
50
1.0 2.0 Time , sec Fbwr 2. OscYLoscope traces for effect of hydrogen Row rate on atonhtion of 50 pg of Mn: (1) no Warm added; (2) 5 ecg of Warea added (A) 500 W m i n of Ar, (B) 20 ml/min of H2 and 480 W m l n of Ar, (C) 50 m l l m h of H, and 450 mLlmin of Ar, 0) 500 Wmh of H, (E) temperatwe hcrease.
0
hydrogen flow-rate dependence. Thiourea appears to have little effect on atomization of Mn. Matrix Modification. Many workers have shown that the addition of various substances to a solution containing chloride reduced the matrix interference in atomization of Cu, Mn, and Pb. Substances which have been proposed involve H$04 ( I ) , Naz02(2),NH4Cl ( 5 , 6 ) ,and soluble organic reagents (8,9). We have investigated the application of thiourea for effective atomization of elements in group 5A and 6A and the s u p pression of interference from diverse elements (12-16). An attempt was made for the use of thiourea to modify the chloride interference on Cu. Figure 3 shows the interference of NaCl (0.5pg) on the atomization profile of Cu (50 pg) and the effect of thiourea (5pg) for modification of matrix interference. In Figure 3(2) the ratios of Cu absorption signal (integrated) of pure Cu to t h w of Cu in the presence of NaCl were 1.5 and 0.98 in the absence and presence of thiourea, respectively. The signal of Cu in the presence of NaCl was restored to the level obtained with a pure solution of Cu provided thiourea was added. These results show that thiourea is an effective modifier for chloride interference in the atomization of Cu. The reproducibility of the atomization profile of Cu in the presence of NaCl was invariably poor unless thiourea was present. A proper amount of thiourea was 5 pg for the present atomizer. A large amount of thiourea led to lower Cu absorption. Incomplete modification of chloride interference with thiourea was shown for 1 pg of NaCl (Figure 4). In this case, the signal of Cu was restored to only 88%. The measurement with peak height led to serious error. Thiourea was more effective for less than 0.5 pg of NaCl irrespective of amount of Cu. Thioacetamide provided a result similar to thiourea, but urea did not serve a~ matrix modifier. Similar interference effects of NaCl and ita modification with thiourea were observed for P b (16). The effect of NaCl on Cu and P b was not compensated with background correction.
3. CRT display for atomlzatlon of Cu in NaCI: (1) no ttJouea added; (2) 5 pg of thiourea added (A) Cu (50 pg), (B) Cu (50 pg) in the presence of 0.5 fig of NaCI, (C) temperatwe lnmease.
Flgvo 4. CRT display for atomization of Cu in NaCI: (1) no thkuea added; (2) 5 pg of Wouea added (A) Cu (50 pg), (B) Cu (50 pg) in the presence of 1 pg of NaCI, (C) temperatwe increase.
This shows the complex nature of the interference process taking place in the atomizer. Figure 5 shows the interference effect of KC1 on Cu atomization and ita modification with thiourea. The interference pattern for KC1 differed from that for NaC1. In this case,the profile of Cu shifted to lower temperature region and signal enhancement was observed. However, it is apparent that thiourea acta as matrix modifier for Cu. The permissible amount of KC1 was lower than that of NaCl. The limiting amount of KC1 was 0.05pg irrespective of the amount of Cu tested. The effects of NH4Cl, CaC12,and MgC12 on Cu were
12
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
I
r
I
Time sec atomization of Cu in KCI: (1) no thiourea added; (2) 5 pg of thiourea added (A) Cu (50 pg), (B) Cu (50 pg) in the presence of 0.05 pg of KCI, (C) temperature increase.
sec Mn in NaCI: (1) no thiourea added; (2) 5 pg of thiourea added (A) Mn (50 pg), (B) Mn (50 pg) in the presence of 1 pg of NaCi, (C) temperature increase.
also modified by the addition of thiourea. The permissible amounts were 10, 0.05, and 0.5 pg for NH4Cl, CaC12, and MgCI2, respectively. Although the effect of thiourea was observed for the amounts which exceeded the permissible amounts of these salts, Cu absorption was not restored to that of pure solution. The effect of chloride matrix on Mn differed from Cu. Figure 6 demonstrates the effect of NaCl on atomization of Mn. The small absorption in the lower temperature region is molecular absorption due to NaC1, and it disappears by background correction over the concentrations tested in the present work. Thiourea showed no appreciable effect on Mn atomization in the presence of NaC1, although reduction of absorption due to NaCl in the lower temperature region was observed. The permissible amount of NaCl was 1 pg. The effects of other chlorides on Mn atomization were tested. Thiourea served as matrix modifier for KC1, NHICl, CaC12, and MgC12. The permissible amounts were 1, 30,0.1, and 1 pg, respectively. Many workers postulated that the main cause for matrix interference is either a physical loss of the analyte before the analyte-containing compound can be decomposed to produce the free metal or a chemical loss of the analyte through vapor-phase reactions with the compounds of the sample matrix. However, Krasowski and Copeland (17)have not observed the chemical loss of the analyte through vapor-phase reactions, and they proposed the occlusion of the and* in the inorganic matrix crystals for the principal matrix interference mechanism. Formation of molecular monochlorides has been suggested as a probable method of interference by L’vov (7). Hydes (9) found that the process of reduction of chloride interference for Cu with organic acids is one which promotes atomization from the oxide rather than the chloride salt of the analyte. Occlusion process is not likely in a microtube atomizer where the temperature is sufficiently higher than the melting point of the salt. The reduced absorption for Cu observed in the presence of NaCl may be due to the fact that Cu forms relatively volatile chloride which evaporates along with NaCl a t a comparatively
low temperature and then partially dissociate in the gaseous phase. Hydrogen forms a relatively stable molecule HCl to remove chlorine from the system. However, the concentration of HCl in the microtube atomizer may not be higher, because the formation of HC1 from NaCl is not thermodynamically easy. The probable process of NaCl interference on Cu does not explain the enhancing effect for KC1. Czobik and Matousek (I) found that the atomization temperature of Pb is lowered in the presence of NaCl, while the Cu absorption is depressed. They interpreted the depression of Cu absorption by the vapor-phase chloride formation, which resulted from high concentration of chlorine in the furnace up to temperatures in excess of Cu atomization temperature of 1430 OC, and the shift of P b absorption peak by insufficient formation of chlorine from NaCl molecules a t 750 OC where the atomic absorption peak of P b appears. KCl is analogous to NaCl in the physical properties such as boiling and melting points. Therefore, the different interferences of NaCl and KC1 on Cu are difficult to be elucidated completely by the above process. The mechanism of chemical interference due to alkali chlorides appears to be complex. We must await the further experimental results concerning the interference of alkali chlorides on various elements for exact interpretation of interference process. The modification mechanism of thiourea for chloride interference differs from those of organic acid and other chemicals. Hydrogen sulfide is generated by heating of thiourea in the microtube atomizer. It can therefore be presumed that the preferential formation of sulfide serves as an effective formation of atoms without dissipation in molecular form. Hydrogen sulfide also appears to be available for Mn atomization in the presence of some halides. Detection Limits of Cu and Mn in NaCl. The detection limits for Cu and Mn in NaCl matrix were established. Figure 7 demonstrates the CRT display for 1,5,and 10 pg of Cu in 0.5 pg of NaCl and 5 pg of thiourea. Detection limit was defined as that concentration giving an absorption signal: background ratio of 2 under the experimental conditions
Figure 5. CRT display for
Time
Flgure 6. CRT display for atomization of
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 a t 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
