Preparation and Testing of Standard Soils Containing Known Quantities of Radionuclides C l a u d e W . Sill a n d Forest D. H i n d m a n Health Services Laboratory U S Atomic Energy Commission ldaho Falls, ldaho 83401
A general procedure for preparation of standard soils containing a known quantity of any given radionuclide is described. Four separate standards have been prepared from three different soils using plutonium-239 to demonstrate the reproducibility and reliability of the procedure. Extensive analyses using plutonium-236 tracer show that the standards contain the exact concentration calculated to have been added, that they are not detectably inhomogeneous on samples as small as 1 gram, and that homogeneous standards of lower concentrations can be prepared exactly by weight dilutions with the unspiked soils. Of a total of 56 determinations made on 1- and 10-gram aliquots of the four individually spiked standards and two others made by dilution, only four determinations showed distinct signs of inhomogeneity with the particular method of preparation and sample size employed. Of the remaining 5 2 measurements, all agreed with the calculated value within three standard deviations of the determination, and 42 were within two standard deviations. The dramatic effect of heat treatment on the leachability of the plutonium is demonstrated. Also, an alternative method for preparation of solid standards for members of the natural uranium and thorium series is suggested.
Because of the public attention being focused on current radioactive waste burial practices and the impact of both accidental and routine releases of plutonium on the public health, the accurate and reliable determination of plutonium in soil is becoming increasingly important. Analytical procedures used around the world vary greatly in their characteristics and applicability, and adequate standards are not available t o permit satisfactory intercomparison of their performance. Liquid standards freeze, evaporate. deteriorate, or suffer breakage or other misfortunes on prolonged use and/or storage. More importantly. liquid standards do not permit evaluation of the ability of‘ the procedure to convert insoluble and refractory forms of the nuclide sought to the ionic form before chemical separations are attempted. Tracers are almost a!ways added in water-soluble form and will not correct for losses in insoluble material with which the tracer did not get exchanged. Similarly, solid standards would he helpful to prove whether or not, and under what conditions. plutoniu m can be leached selectively from the insoluble matrix with common acids including hydrofluoric acid. Solid standards are generally prepared by homogenizing a large quantity of some naturally-occurring material and then analyzing the final product as many times by as many different procedures as are required to convince the sponsor t h a t he has the “best value” for each element or radionuclide being determined. When the analyses are relatively simple and consistent results are obtained from several independent methods. such standards are undoubtedly reliable. However, when different procedures give different results, it is generally very difficult to prove to everyone’s satisfaction which values are correct, be-
cause each laboratory can generally produce a series of highly precise numbers to support its own contention. Furthermore, adequate interlaboratory comparisons are organized relatively infrequently. Consequently, most of the time, proof of accuracy on actual samples rests solely on simple replication among aliquots of a few samples by a single procedure within a single laboratory and frequently by a single analyst. Because many samples by their very nature cannot be made homogeneous, which is particularly true of soil containing plutonium arising from nearby reactors or processing facilities, replication of analyses on actual samples will not necessarily identify a n accurate procedure even when one is being used. If a solid standard of any desired type of material could be prepared that contained a n exactly known quantity of any desired radionuclide, that could be preconditioned in any desired way, and that is homogeneous a t the smallest sample size to be used, the accuracy of any procedure could he determined unequivocally in the smallest laboratory with a few replicate determinations. Furthermore, solid standards of virtually infinite stability can he made available immediately off-the-shelf over long periods of time. It is the objective of the present paper to show that such standards can indeed be prepared for soil with remarkable ease, and with probably greater accuracy than can be confirmed by subsequent analysis. Specifically, the objective was to prepare a soil containing any nonvolatile radionuclide a t a concentration .that is known or can he determined by means other than direct chemical analysis for the radionuclide sought in order to prevent uncertainties in the analytical procedure from being passed on to the standard. The only analysis made in the preparation of the present standard soils is the standardization of the pure tracer used to spike the soil. The standard sample concept for soils has been questioned on the grounds that no two soils are exactly alike. t h a t the chemical and physical forms of the nuclide sought are unknown and cannot he duplicated, and that solids cannot be made sufficiently homogeneous to permit exactly known and reproducible concentrations to he obtained by weight dilutions. It matters very little if no two soils are exactly alike if the analytical procedure can he made to handle the worst possible case. If necessary. several standards can he made to simulate several different “worst possible cases.’‘ Similarly, if the sample is completely decomposed chemically by a procedure that is known to dissolve the most refractory and intractable forms of the nuclide being sought, the original chemical and physical forms will have been destroyed and are no longer relevant. That solids can be made to act almost like liquids with respect to homogeneity and dilution is demonstrated below.
EXPERIMENTAL Instrumentation. A windowless 2-a proportional flow counter using methane as the counting gas was used in the standardization of the radioactive tracers. An alpha spectrometer having a 450-mm2 surface barrier detector and a resolution of about 25-keV
full-width-at-half-maximum peak height ( I ) was used to determine t h e isotopic composition of t h e tracers. Standardization. The radioactive tracers were obtained in a dilute nitric acid solution free of radioactive impurities: nonvolatile substances, ammonium salts, organic matter, and anions other t h a n nitrates., Selected procedures for purification of radionuclides of thorium through californium to meet the exacting requirements of high sensitivity alpha spectrometry will be published elsewhere (2). Each solution was standardized by counting in a 2-n counter aliquots prepared both by direct evaporation and by electrodeposition. T h e 2-7r counter was standardized using a U.S. National Bureau of Standards source containirig arnericium241 electrodeposited on platinum having a total uncertainty in the certified value of 1.0%. Two 1-ml aliquots of the selected radionuclide containing about 5 X lo4 d p m each were placed in 60-ml beakers for subsequent electrodeposition, and two other aliquots were placed directly on 2-inch mirror-finished stainless steel plates for direct evaporation a t the same time a s the aliquot t h a t was added to spike the soil. The pipet used had been treated with a silicone water-repellent to eliminate drainage and calibrated “to contain” by blowing the residual liquid from the tip. The pipet had been demonstrated repeatedly to be capable of delivering its calibrated volume to within 0.1%. The solutions on the steel plates were evaporated very slowly to dryness under a 250-watt infrared l a m p to minimize losses due to spraying. When the solutions had evaporated nearly to dryness, 2 or 3 drops of concentrated nitric acid were added to keep the activity spread as uniformly a s possible over the plate during the final evaporation to dryness. The dry plates were heated over a blast burner just to the first red glow, and the temperature was then lowered quickly by placing the plate on a cold steel surface to minimize oxidation of the plate. T h e plate was placed in the 2-n counter immediately after cooling to avoid any possibility of absorption of water vapor from t h e air, and counted for 30 minutes to give about 8 x lo5 total counts so t h a t the statistical precision wculd be adequate for the most precise work to be accomplished. T h e total disintegration rate of the solution being standardized was determined by comparison of its counting rate to that of the americium-241 standard counted under identical conditions and to about the same number of counts. The plated activity was then counted in the alpha spectrometer, again for approximetely the same number of total counts, t o determine the fraction of the total activity present coming from the particular nuclide of interest and to determine what other nuclides might be present in case mixed standards were to be prepared. The final concentration in the soil was then calculated from the totai activity added, the fraction of the total resulting from the nuclide of interest. and the total weight of soil used. This value is referred to subsequently as the “calculated value.” All counting errors from that on the a m ericium-241 standard on were propagated t o the final soil concentration according to the general relationship t h a t the fractional error in the function involved is equal to the square root of the sum of the squares of the fractional errors in each of the independent variables ( 3 ) . A correction was also made for the differences in backscatter between platinum and stainless steel ( 4 ) . T h e other two aliquots placed in 50-ml beakers were electrodeposited ( 5 ) and the resultant plates analyzed by alpha spectrometry t o determine the isotopic composition with better resolution than is possible with plates prepared by direct evaporation. The improved resolution is necessary in some cases such as in the determination of plutonium-238 in the presence of plutonium-236. However, because virtually all electrodeposition procedures give losses ranging from as little as 0.5% to 15% or more depending on the exact conditions and the nuclide present, the electrolyte from each electrodeposition was treated t o restore the original conditions and reelectrodeposited to recover as much as possible of the activity not electrodeposited originally. If the losses from the original electrodeposition were over 1 or 2%. the second electrolyte was again retreated until little more activity was recovered. indicating complete recovery. The sum of t h e activities on all of the (1) C. W. Sill and D. G . Olson. A n a / . Chem.. 42, 1596 (1970) ( 2 ) C. W . Sill. “Purification of Radioactive Tracers for Use In High Sen-
sitivity Alpha Spectrometry,’’ U.S. Atomic Energy Commission, Idaho Falls, Idaho, in preparation. (3) R. J. Overman and H . M . Clark, “Radioisotope Techniques,” McGraw-Hill, New York. N . Y . . 1960, p 109. (4) J. M . R. Hutchinson. C. R. Naas, D. H. Walker, and W . B Mann. Int. J . Appi. Radiat. isotopes. 1 9 , 51 7 (1968) ( 5 ) K. W . Puphal and D. R . Olsen, Ana/. Chem. 44, 284 (1972) 114
electrodeposited plates from a given run invariably agreed within the statistics of the measurements with the activity obtained by direct evaporation. The two methods of standardization thus complement each other, electrodeposition giving the best resolution but direct evaporation‘ of properly prepared solutions giving the most accurate and reliable results in the shortest and most convenient way. Preparation of Standard Soils. Select two or three times as much soil from the desired geographical area as the total quantity of standards t o be prepared and dry overnight a t about 110 “C. Grind up the soil clods, dry further if necessary, and screen using a No. 200 U.S. standard sieve (74-micron opening). Discard the rocky residue not passing through the screen. Blend the entire batch of prepared soil thoroughly so that all subsequent standards will be identical except for t h e particular nuclide added. Rescreen about 600 grams of the -200 mesh stock soil through a combination of No. 200 and No. 326 sieves until about ninetenths of the material has passed a second time through the 200mesh screen to ensure absence of larger or irregular particles t o facilitate rescreening of the final spiked material. Remove the 200-mesh screen and continue shaking the 325-mesh screen (43micron opening) until little more of the fine material passes through. Place 100.0 grams of t h e fraction between 200 a n d 325 mesh into a weighed 250-ml platinum dish. Combine the remaining fractions in the two screens a n d pan and reserve to add to the spiked fraction to reconstitute the original soil exactly. Add small portions of water t o the soil in the platinum dish and stir thoroughly until a smooth paste is obtained. Add 1 ml of a solution containing about 5 X lo* d p m of t h e desired purified, carrier-free radionuclide in 10% nitric acid dropwise with continuous stirring from a pipet t h a t has been treated with a silicone water-repellent and calibrated “to contain” to a t least 0.1%.Mix each drop of the radioactive solution thoroughly throughout the wet soil with a heavy glass stirring rod before adding the next. Draw and reserve two identical aliquots of the solution for subsequent standardization. Heat the mud under a n infrared lamp while stirring thoroughly and continuously until the mass becomes immobile to prevent separation of any liquid phase t h a t could evaporate to form areas of higher concentrations on “hot spots” of activity. Finally. dry the sample thoroughly, either under the infrared lamp a t about 80 “C or in a drying oven or muffle furnace at any higher temperatures when special heattreating effects are desired. Cool and transfer as much of the spiked soil as possible from the dish to a weighed piece of glazed rolling paper. Weigh the paper and contents to determine the quantity of spiked soil present a t the beginning of the grinding and screening operation. The quantity of soil remaining in the dish can be determined by reweighing the dish. Grind the dried material until it again passes entirely through the 200-mesh screen. It will be necessary to t a p the screen and pan sharply against a solid bench top to keep the screen from glazing over with the fine powder obtained on grinding. Collect the screened material on another weighed piece of glazed paper and weigh it to determine the fraction recovered. Mechanical losses from dusting. from the mortar and pestle, and the screen can easily be held to about 1%.Because this material should be essentially homogeneous, the very small loss incurred can be corrected for from the before and after weights of product obtained. Mix the spiked soil with enough more of the same unspiked -200 mesh soil, including t h a t remaining from the secondary hand screening. to give a total of 1800 grams. Blend the mixture overnight in an efficient blender such as t h e Patterson-Kelley twinshell blender to ensure homogeneity. The platinum dish retains a slightly higher fraction of the total activity used than of the total soil used because of direct evaporation of solution on the sides of t h e dish. T o determine this correction, add a few milliliters of 48% hydrofluoric acid t o the dirty dish and evaporate t o dryness. .4dd 3 grams of anhydrous potassium fluoride and fuse over a blast burner. Transpose the fluoride cake to a pyrosulfate fusion, and precipitate, mount, and count the barium sulfate as described previously (6). Correct the activity added to the soil for the quantity remaining in the dish. All the thorium and radium nuclides in the soil will be precipitated with the barium sulfate b u t the error is negligible with the small quantity of soil and large quantity of activity used. When prepared as directed. the sample contains about 28 dpm/gram of the radionuclide selected. Smaller concentrations can be prepared either by using a smaller quantity of the radionuclide initially or by blending weighed quantities of the higher (6) C. W. Sill and R. L. Williams, Ana/. Chem.. 41, 1624 (1969)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 , JANUARY 1974
standard with additional weighed quantities of the same -200 mesh soil used to prepare the original standard. Similarly, standard soils containing exactly known quantities of several nuclides in any proportions can be prepared by thorough blending of weight aliquots of the respective standards. When samples are prepared with concentrations below a few dpm/gram, the soil should be obtained from a t least 2 feet below the surface of undisturbed ground t o minimize corrections that must be made for the radionuclides present from global fallout.
Table I . lnterlaboratory Comparison of Plutonium Determinations in Standard Plutonium Soil No. 1 Laboratory Sample, g 239Pu.d p m / g n 238Pu,dpm/gn 35.1 f 0.4 This Lab 10 0.60 f 0.04 35.3 f 0.4 0.56 f 0.04 A
1
0
5
C
0.5
D
3
E
1
F
1
G
10
RESULTS AND DISCUSSION Each standard plutonium soil was analyzed repeatedly over a period of several weeks using a procedure that will be described in detail elsewhere (7). A 10-gram sample of soil is dissolved completely in the presence of plutonium236 tracer by fusing in a platinum dish with anhydrous potassium fluoride. The fluoride cake is then transposed with concentrated sulfuric acid to a pyrosulfate fusion with elimination of hydrogen fluoride and silicon tetrafluoride. The pyrosulfate cake is dissolved in dilute hydrochloric acid and all alpha-emitting nuclides are coprecipitated with barium sulfate. The barium sulfate is dissolved in an aluminum nitrate fusion and plutonium is extracted into Aliquat-336. After scrubbing the organic extract with 8M nitric and 10M hydrochloric acids, plutonium is stripped with a perchloric-oxalic acid solution, electrodeposited, and determined isotopically by alpha spectrometry. Because both potassium fluoride and pyrosulfate fusions are known to dissolve the most refractory and intractable plutonium compounds, thus assuring complete exchange between the tracer and the plutonium in the sample, the chemical yield, counting efficiency, counting time, etc. are the same for both isotopes and cancel out. In addition to the activity of plutonium-236 added, only the total number of counts of plutonium-239 and plutonium-236 recovered are necessary to calculate the concentration of plutonium-239 in the sample. However, the yields are invariably larger than 95%. The plutonium-236 tracer used in the analyses was standardized in exactly the same way as the plutonium239 solution used to spike the soil. Approximately 300 dpm of plutonium-236 was used both in the standardization and in tracing 10-gram samples of the nominal 30 dpm/gram standards, and about 12 dpm was used with the 1- and 10-gram aliquots of the 0.5 dpm/gram standard so that the statistical uncertainty in the plutonium-236 recovered would always be equal to or less than that from the plutonium-239 being determined. Counting times up to 1200 minutes were generally used for the high standards and up to 5000 minutes for the lower standards to permit differences between the analytical and calculated values to be identified as sensitively as possible. The absolute uncertainty of the standard soils is probably about 270, the linear sum of about 1% statistical uncertainty a t the 99% confidence level in the standardization and the 1% assigned by the National Bureau of Standards to the standard americium source on which the standardization is based. However, the uncertainties in the americium standard and in the backscatter corrections are the same for both the standardization of the plutonium-239 used to prepare the soil standards and the plutonium-236 on which the subsequent analytical determinations depend. Consequently, only the statistical counting errors in both values are considered when the analytical and calculated values are compared. Unless otherwise specified, the uncertainty given is the standard Puphal, and F. D. Hindman, "Simultaneous Determination of Alpha-Emitting Nuclides of Radium through Californium in Soil," U.S. Atomic Energy Commission, Idaho Falls, Idaho, in preparation
(7) C. W. Sill, K. W.
35.3 32.6 34.0 34.6 32.9 34.4 33.1 35.1 35.1 34.6 36.2 35.1 31.3 33.4 34.7 34.1 9.6 9.3 8.9 33.1 32.7 33.8
f 0.4 f 1.0 f 1.0 f 1.0 f 0.7 f 1.0 f 1.7 f 3.4 f 3.4 f 3.3 f 4.0 f 4.2 f 4.1 f 1.3 f 1.4 f 1.4 f 0.9 f 0.8 f 0.8 i 0.7 f 0.7 f 0.7
0.49 f 0.04 0.56 f 0.14 0.84 f 0.21 0.91 f 0.23 1.29 f 0.15 1.80 f 0.27 1.75 f 0.56 0.91 f 0.14 0.71 f 0.12 0.69 f 0.1 1
...
... 1.31 f 0.26 1.42 f 0.28 1.35 f 0.27
Calculated values, 34.8 f. 0 1 and 0 58 i 0 01 dprnlgram, respectiveiv
deviation of each individual determination resulting from propagation of all counting statistics only. The first standard soil was ignited for 4 hours a t 1000 "C after spiking, but before diluting to the final weight, to simulate material involved in a fire or that remaining after laboratory ignition to burn off organic matter. The material was used in a laboratory intercomparison program under the sponsorship of the U.S. Environmental Protection Agency. The results of the seven laboratories responding and those obtained in the present laboratory are given in Table I. As mentioned above, relatively large samples, long counting times, and total sample decomposition were used in the present laboratory to keep the statistical uncertainty small to see just how closely the best analytical values obtainable would agree with the calculated one. The analytical results obtained for both plutonium-239 and plutonium-238 agree remarkably well with the calculated values. Unfortunately, the statistical uncertainties of the results from the other laboratories are too large to provide a very exacting comparison of the results. However, except for Laboratory F, every one of the experimental results for plutonium-239 was within three standard deviations of the calculated value and twelve of the eighteen results were within one standard deviation. The results for plutonium-238 were generally too high, probably due to failure to correct adequately for the plutonium-238 added with the plutonium-236 tracer or i n the instrument background. Because the only source of plutonium-238 in this sample was from the tracer used to spike the soil, in which the ratio of plutonium-239 to plutonium-238 was 65, the plutonium-238 found cannot be greater than one sixty-fifth of the plutonium-239 found. This determination provides an excellent check on how well blank corrections are made. Again, the standard deviations are too large to tell how good the comparison might have been. It is extremely significant that Laboratory F was the only one of the group that did not use hydrofluoric acid or potassium fluoride fusions in the decomposition of the
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
115
Table 1 1 . Determination of 239Puand 238Puin Highest Plutonium Standard from Soil No. 2 239Pu.dpm/gramQ
32.8 f 0.3 32.3 f 0.2 32.4 f 0.2
2-3aPudpm qram'
32.8 f 0.3 32.4 f 0.2 32.8 f 0.2
0.49 f 0.02 0.507 f 0.011 0.53 f 0.02
Table V. Determination of 239Puin Lowest Plutonium Standard from Soil No. 3 Prepared by Dilution of Highest Standard Sample size,
0.51 f 0.02 0.49 i 0.02 0.52 f 0.02
10
'Calculated values. 32.7 f 0.1 and 0.51 1 f 0 005 dpmlgram, respectively. Sample size, 10 grams
Table I l l . Determination of 239Puin Lowest Plutonium Standard from Soil No. 2 Prepared by Dilution of Highest Standard 239Pu,dpm/grama
0.65 f 0.02 0.62 f 0.02 0.77 f 0.01*
0.61 f 0.02 0.67 f 0.01* 0.57 f 0.01
0.62 f 0.01 0.63 f 0.02 0.60 f 0.01
a Calculated value, 0.602 f 0.003 dpm/gram. Sample size. 10 grams. *Outside acceptable statistical limits.
Table I V . Determination of 239Puand 238Puin Highest Plutonium Standard from Soil No. 3 Sample size, grams
10
1
10''
239Pu,dpm;grama
29.5 f 0.3 29.7 f 0.3 29.5 f 0.2 29.9 f 0.4 28.9 f 0.4 29.4 f 0.4 29.0 f 0.3 29.5 f 0.3 28.9 f 0.3
29.4 f 0.3 29.5 f 0.2 29.0 28.9 28.4 29.5 29.4 29.4
f 0.4 f 0.5 f 0.5 f 0.3 f 0.3 f 0.3
238Pu.dpm/grama
0.45 f 0.02 0.43 f 0.02 0.40 f 0.02 0.51 f 0.08 0.52 f 0.06 0.39 f 0.07 0.46 f 0.02 0.44 f 0.02 0.46 f 0.02
0.44 f 0.03 0.47 f 0.02 0.47 f 0.07 0.35 f 0.07 0.50 f 0.08 0.44 f 0.02 0.48 f 0.02 0.45 f 0.02
aCalculated values, 29.8 f 0.1 and 0.46 f 0.01 dpm/gram, respectively. Different solution of 236Pu,separately purified and standardized. Yields, 95.0, 96.5, 97.8, 97.7, 95.8. and96.0 f 0.8%, respectively.
-
sample. The extremely low value obtained is predictable in view of the repeated observations (7) that ignited plutonium oxide is leached very incompletely by common acids unless hydrofluoric acid is used extensively. Undoubtedly, some of the other results are slightly low for the same reason, even in the presence of hydrofluoric acid, when larger samples or shorter digestion times are employed. A second standard was prepared from a different soil to see if the excellent results obtained in this laboratory on the first trial could be reproduced. However, the heat treatment was reduced to 1 hour a t 700 "C to approximate more closely the conditions normally used in laboratory ignitions. Also, an aliquot of this standard soil was diluted approximately 54-fold with the same unspiked soil to see if standards of lower concentration could be prepared by direct dilution without detectable loss of homogeneity or accuracy of the diluted standard. As shown in Table 11, the higher standard gives analytical values for both plutonium-239 and plutonium-238 that are within the expected experimental and statistical uncertainties of the analysis in every case without the slightest indication of inhomogeneity or significant deviation from the calculated value on 10-gram samples. However, the lower standard, which was diluted 934 times overall from the original 100 grams of spiked soil, shows distinct signs of inhomogeneity. The 116
239Pu,dpmfqram'
yrdllls
1
0.502 0.491 0.516 0.62 0.59
f 0.01 1 f 0.013 f 0.011
f 0.03* f 0.02*
0.503 f 0.01 1 0.499 f 0.01 1 0.516 f 0.011 0.51 f 0.03 0.54 f 0.03
0.494 f 0.009 0.509 f 0.008 0.505 f 0.013 0.54 f 0.02 0.52 f 0.03
'Calculated value, 0.503 f 0.003 dpm/gram. *Outside acceptable statistical limits. data are shown in Table 111. Although seven of the nine analyses agree very well with the calculated value, the two values marked by asterisks are clearly outside the statistical limits of measurement with any reasonable probability. Blanks were run before each analysis, using the identical reagents, glassware, electrodeposition equipment, and detector to be used in the subsequent determination to avoid any question of contamination. Blanks were invariably less than 2 or 3 counts in the plutonium-239 energy region in 103 minutes. Apparently, a similar inhomogeneity was not observed on the higher standard because of the much smaller effect of a few particles of higher plutonium concentration on the much higher activity present. The first two standard soils were prepared from the material that had been passed through a 200-mesh sieve only. Because about 75% of the soil would also pass through a 325-mesh sieve, it seemed likely that a particle problem could arise during drying of the original spiked mud due to formation of small clods containing much of the finer material that would not necessarily be reduced during grinding any finer than that required to let it pass through the 200-mesh screen. Consequently, a third standard plutonium soil was prepared using only the fraction passing through the 200-mesh screen but not passing through the 325-mesh screen for spiking as described in the recommended procedure. Also, the 100 grams of spiked soil was dried a t only 110 "C for 2 hours before dilution to 1800 grams total for two reasons. First, it was desired to see if a standard soil could be made from which the plutonium could be leached quantitatively with nitric and/or hydrochloric acids to permit its use in testing those analytical procedures employing leaching techniques. Second, if significant leaching c,ould be obtained by any given procedure, repeating the analysis on another aliquot after ignition to burn off organic matter would demonstrate dramatically to the user of the procedure the severe effect of strong heating on the leachability of plutonium and the consequent uncertainties resulting .from the use of such procedures. A 60-fold less concentrated standard was also prepared by thorough blending of 30.0 grams of the third standard with an additional 1770 grams of the same unspiked soil for 2 days in a P-K twin-shell blender to test the effect of removing the -325 mesh fraction before spiking. Because homogeneity necessarily depends on the size of sample taken for analysis, analyses were made on both 1- and 10-gram samples of both standards. As with the first two standard soils, analyses of the high standard, prepared by only 18-fold dilution of the initial spiked material, are remarkably reproducible and agree with the calculated value as well as reasonable experimental uncertainties permit. The data are shown in Table IV. There is no indication of inhomogeneity on either size of sample. However, unlike the second standard soil, the same conclusions now apply equally well t o the 10-gram aliquots of the lower standard prepared by an additional
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
Table VI. Leachability of Plutonium from Standard Soil No. 3a Acid Soluble Heat treatment
Plutonium standard
Residue
Total
%
dpmlgram
dpmlgrarn
%
2 hours at 110°C
Highh Lowb
29.2 0.452
f 0.5 f 0.018
98.0 f 1.6 89.9 f 3.6
0.89 f 0.04 0.024 f 0.004
3.0 f 0.1 4.8 f 0.8
30.1 f 0.5 0.476 f 0.018
101.0 f 1 . 6 94.7 f 3.4
1 hour at 700 "C
Highh Low0
19.0 f 0.3 0.256 f 0.013
63.8 f l.O 50.9 f 2.5
11.4 f 0.3 0.246 f 0.012
38.3 f 1 .O 48.9 f 2.3
30.4 f 0.4 0.502 f 0.018
102.1 f 1.4 99.8 f 3.5
4 hours at 1000°C
Highb Lowh
5.8 f 0.1 0.071 f 0.005
19.5 f 0.3 14.1 f 0.9
23.4 f 0.2 0.422 f 0.013
78.5 f 0.7 83.9 f 2.5
29.2 f 0.3 0.493 f 0.014
98.0 f 0.8 98.0 f 2.7
4 hours at 1ooo"c
HighL Low
4 hours at l0OODC 4 hours at 1000 "C
dpmlgram
17.6 f 0.2
59.1 f 0.7
12.0 f 0.4
%
f 1.3 ...
40.3
...
...
...
Highd Lowd
15.4 f 0.1 0.281 f 0.015
51.7 f 0.4 55.9 f 2.9
14.2 f 0.3 0.224 f 0.011
47.7 f 1.0 44.5 f 2.1
HighL' Highf'
19.2 f 0.2 18.5 f 0.2
64.4 f 0.7 62.1 f 0.7
9.9 f 0.2 10.9 f 0.2
33.2 f 0.6 36.6 f 0.6
29.6
f 0.5
99.4 f 1.6
...
...
29.6 f 0.3 0.505 f 0.019
99.4 f 1.1 100.4 f 3.6
29.1 f 0.3 29.4 f 0.3
97.6 f 1.0 98.7 f 1.0
Ten grams of soil was boiled (I Calculated values are 29.8 & 0.1 and 0.503 & 0.003 dpm/gram of 239Pufor the high and low standards, respectively. for 2.5 hours with 100 ml of aqua regia. Ten grams of soil was simmered in a platinum dish for 2 hours with 95 ml of concentrated nitric acid and 5 ml of 48% hydrofiuoric acid. Ten grams of soil was moistened with concentrated nitric acid and evaporated to dryness with 40 ml 48% hydrofluoric acid in about 1 hour. e Ten grams of soil heated to near boiling for 16 hours with 100 ml of either 95-to-5 or 50-to-50 of concentrated hydrofluoric acid and 8 M nitric acid.
60-fold dilution, or a total dilution from the original spiked material of 1080. These data are shown in Table V. Apparently, the use of two sieves to limit the size distribution of the particles on which the activity is dispersed is both effective and necessary. The results in Table V show that a detectable inhomogeneity does exist a t the 1-gram level, although not necessarily due to individual particles of higher activity. Two of the six analyses are clearly outside the statistical uncertainties shown. However, counting times of 3000 to 5000 minutes were used to obtain the statistical uncertainties shown. With more reasonable counting times, the inhomogeneity would probably be undetectable because of the larger standard deviation. However, if such low-level standard soils are desired for use with only 1-gram aliquots, with the long counting times and/or large statistical uncertainties that result, they can be made better by reducing the activity added initially to lower the large dilution factors required. For example, the data of Table IV show that the standard soil is not detectably inhomogeneous a t the 1-gram level after only 18-fold dilution of the original spiked material, and with much better statistical certainty than any soil containing less activity could possibly have even with long counting times. There is no reason to suspect that a smaller quantity of activity used to spike the soil initially would be any less homogenizable under the conditions used. Because several of the analytical results on the second and third standards in Tables I1 and IV seemed to run 1 to 2% lower than the calculated values, a fourth standard soil was prepared exactly like that of Table IV, including the activity added, except that it was dried with an infrared lamp only. Both the plutonium-236 tracer and the plutonium-239 spike were restandardized very carefully in an effort to see if the bias was real. Analytical values of 29.2, 28.9, 29.1, 29.4, 29.1, and 29.0 f 0.3 dpm/gram were obtained in excellent agreement with the values obtained on the previous standard in Table IV. It seems unlikely that as much as 1% of the 100 grams of spiked soil was lost during grinding and screening. On the other hand, it is difficult to see how the analytical values could be low because all losses should be accurately corrected for with the large quantity of tracer used. Note A d d e d in Proof. Subsequent experience has demonstrated that weighing the spiked soil before and after the
grinding and screening operation gives too low a correction for the actual losses incurred. Frequently, the weight obtained after grinding and screening is slightly higher than it was before, even though there is visible evidence of loss of several tenths of a gram on the mortar and pestle. Apparently, there is sufficient mass contributed from the mortar during grinding, particularly if a large porcelain one is used, to more than offset the real losses of spiked soil that have occurred. A much more accurate and reliable correction can be obtained by determining the actual activity lost as described for the residual material in the platinum dish. Rinse the screen and pan with a little water, and clean the mortar and pestle with nitric and hydrofluoric acids, using a rubber policeman if necessary. Add both rinses to the same platinum dish' containing the residual spiked soil and precipitate and count the combined activity on barium sulfate as described. After correction, the calculated value will then agree with the analytical value exactly. The results of leaching tests performed on both the high and low standards from soil No. 3 shown in Tables IV and V are given in Table VI. Both the acid-soluble fraction and the residue were analyzed to ensure complete accountability of the plutonium known to be present. When the heat treatment was limited to the 2 hours a t 110 "C used to dry the sample originally, over 95% of the plutonium was leached out of the soil by boiling with aqua regia for 2.5 hours. Standard plutonium soils for use with leaching procedures should be dried at temperatures below 100 "C for no longer than is required to dry the sample. As either the temperature or duration of heating is increased, the plutonium leached by aqua regia clearly decreased to 15 to 20% a t the highest temperature used. Such a low leaching rate is particularly significant because the present standards were prepared by drying water-soluble carrierfree plutonium nitrate uniformly on particles only 43 to 74 microns in diameter and the "particle size" of the plutonium particles must be extremely small, even if the plutonium had hydrolyzed before drying. The leaching rate of plutonium in soils taken in the vicinity of plutonium-processing facilities should be much lower because of the much larger size of the real plutonium particles present. As shown elsewhere plutonium present in soil from global fallout is easily leachable, apparently because of the small particle size and because the extremely high
(a,
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temperatures resulting from detonation of nuclear devices decompose the plutonium oxide to lower oxides that are more easily soluble. Also, as shown in Table VI, use of hydrofluoric acid is clearly beneficial in increasing the quantity of plutonium dissolved after severe heat treatment. However, about one-third of the plutonium still remained in the residue under the best conditions tried. In all the tests using hydrofluoric acid, a large siliceous residue remained after the treatment, as well as some precipitated calcium fluoride, both of which undoubtedly retarded dissolution. It seems likely that repeated filtration and treatment of the residue until virtually all of the visible siliceous matrix has been dissolved will be necessary before dissolution of refractory plutonium compounds can be expected to be complete. The data of Table VI also show clearly that there is no detectable difference in rate of leaching of plutonium from either the high or low standards with or without hydrofluoric acid, a difference of 60-fold in plutonium concentration. This is understandable because the heat treatment and size of the plutonium particles is the same regardless of their concentration. Thus, hydrofluoric acid cannot be considered to be an effective or reliable reagent for quantitative dissolution of plutonium compounds that have been heated strongly, short of complete sample decomposition. Plutonium-239 was chosen to use in preparing the first soil standards not only because of the greater current interest in this nuclide but also because availability of the excellent tracer, plutonium-236, permitted the most accurate proof-of-principle. Adequate tracers are not available for many other radionuclides. However, because of its general nature, the procedure described above should be equally applicable without proof to the preparation of standard soils containing exactly known concentrations of any other radionuclide, such as other transuranium elements, strontium-90, or other fission products, etc. An alternate method is available for preparation of solid standards for members of the natural thorium and uranium series which are themselves present in significant quantities in most soils. Such standards would be particularly helpful to laboratories concerned with milling operations for the production of uranium and thorium during which large quantities of solid and liquid wastes containing the radioactive daughters are generated and must be controlled. During an investigation on the determination of lead210 (8), a sample of pitchblende was obtained from a deposit known to contain primary, unaltered mineral. The sample was pulverized, passed through a 200-mesh screen, (8) C W SillandC P Willis.Anal Chern, 37, 1661 (1965)
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blended thoroughly for 24 hours, and analyzed for uranium by an accepted volumetric procedure demonstrated to give accurate results on this type of material (9). Ten analyses on five separate bottles gave a value of 0.973 f 0.002% U308. Using accepted values for the abundances and half-lives of uranium-238 and -235, the specific activity of the sample can be calculated to be 6057 dpm/ gram for the uranium-238 chain and 283 dpm/gram for the uranium-235 chain. Analysis of the material for lead210 gave an average value of 5.98 x lo3 dpm/gram which is 98.7% of the calculated uranium equivalent and is not statistically different from 100%. Separate measurements of the polonium-210 content by two different investigators using different procedures about six years apart gave average values of 6.00 f 0.18 X lo3 dpm/gram for seven determinations on samples ranging in size from 0.1 to 1.5 grams (8) and 5.95 X 103 dpm/gram on three 1-gram samples ( I O ) . Neither average is statistically different from the value calculated from the uranium concentration. More recently, Percival and Martin analyzed this same sample of pitchblende extensively for radium-226, thorium-230, actinium-227, protactinium-231, and thorium227 during development of a combined procedure for these nuclides. The procedure and results will be reported in detail elsewhere (11). However, all analytical results agreed with the calculated values to within two standard deviations of the determination. From these extensive analyses obtained by several different investigators using different procedures over several years' time, it seems conclusive that this material is in secular equilibrium, is homogeneous, and constitutes an accurate and reliable solid standard for the uranium milling laboratories.
ACKNOWLEDGMENT The authors thank G. J. McNabb and K. W. Puphal for the many electrodepositions required and J. S. Morton and R. J. Kelson for assistance with the alpha spectrometry. We also thank R. F. Smiecinski and the Environmental Protection Agency for making the results of the interlaboratory comparison on the standard plutonium soil No. 1available to us. Received for review January 29, 1973. Accepted July 23, 1973. (9) C. W. Sill and H. E. Peterson, Anal. Chem.. 24, 1175 (1952). (10) R . P. Bernabee and C. W. Sill, "Determination of Polonium-210 in Environmental Samples," U.S. Atomic Energy Commission, Idaho Falls, Idaho, in preparation. (11) D. R. Percival and D. B. Martin, "Sequential Determination of Radium-226, Radium-228, Actinium-227, and Thorium Isotopes in Environmental and Process Waste Samples," U.S. Atomic Energy Commission, Idaho Falls, Idaho, in preparation.
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