1270
Anal. Chem. 1981, 53, 1270-1274
Separation of Plutonium in Urine without Sample Ashing for Determination by a-Spectrometry Ivan K . Kressin Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545
Present methods for the analysis of plutonlum In urine requlre ashing the organic constituents of the sample, a time-conmlng procedure and a major source of error. A method was developed to quantitatively separate 25 fCi of plutonium directly from 800 cm3 of urlne by copreclpltatlon with calcium oxalate. Wlthout ashlng, the oxalate precipitate Is directly dlssoived in 7.2 M HNO, and the plutonium Is Isolated by anion exchange, electrodeposited, and then measured by cu-spectrometry. The results are in excellent agreement with results obtained by exlstlng procedures, and for 194 samples the average recovery of the plutonlum tracer was 91 % with a standard deviation of f10. The average recovery of the plutonium Isotope in quality control samples was 101% when corrected for the 242Putracer recovery.
The potential health hazards of plutonium have been recognized, and it has been our practice a t the Los Alamos National Laboratory to monitor workers for exposure to plutonium (1). One of our methods is to analyze a worker's urine, and the method must be capable of detecting 25 fCi (0.056 dpm) in 800 cm3 of urine in order that the health protection controls can be properly evaluated. Instruments have been developed to accurately detect 25 fci of plutonium, but these instruments cannot detect 25 fCi of plutonium in a urine matrix. Therefore, the problem is the separation and isolation of 25 fCi of plutonium from a liter of urine. White, Handler, and Smith (2) reported the urine of a normal adult over a 24-h period to have the constituents listed in Table I. The composition of samples will vary widely due to illness, medication, or strenuous exercise, and although freshly voided urine may also appear clear, it may contain proteins and cells from the lining of the genitourinary tract. T o achieve the required detection limit in urine with such a large variation of cellular elements and mucus, present methods (3-11) eliminate the organic components by wet or dry ashing. In addition to being very time-consuming, ashing is one of the major causes of erratic results. Veselsky (12) presents a review of the problems of various types of bioassay samples. Lisk (13)points out that sample preparation, ashing, element isolation, and concentration are usually the most critical steps in trace element analysis of biological samples. Gorsuch (14) presents a comprehensive review of sample ashing and the recovery or losses caused by different ashing procedures, temperatures, and other operating parameters for the destruction of organic matter for trace element analysis. Wet ashing of urine samples requires digesting the sample for several days in hot "0,; whereas dry ashing can be done in several hours. However, plutonium tends to form refractory compounds a t 500 "C, which is the temperature required for dry ashing. Therefore, dry ashing must be carefully controlled followed by several days of digesting the sample in hot HN03 to ensure complete dissolution of the plutonium. A turnaround time of 8-15 days is generally required for sample analysis. 0003-2700/81/0353-1270$01.25/0
Table I. Composition of Average 24-h Urine of a Normal Adult component sodium
amt 2-4 g
phosphate
potassium
1.5-2.0 g
inorganic sulfate
magnesium
0.1-0.2 g
organic sulfate
calcium
0.1-0.3 g
urea
iron
0.2 mg
creatinine
ammonia
0.4-1.0g of N 0.1-0.2 g
peptides
uric acid
component
amino acids
amt 0.7-1.6 g
of P 0.6-1.8 g of s 0.06-0.2 g of s 6-18 g of N 0.3-0.8 g of N 0.3-0.7 g of N 0.1-0.15 g
The object of the present study was to develop a method with results as good as present methods but with a much shorter turnaround time. Separation of plutonium by extraction generally takes less time than precipitation or anion exchange methods, but we were not able to extract femtocurie amounts of plutonium directly from a liter of urine with tri-n-octylphosphine oxide (TOPO), triisoctylamine (TIOA), or di(2-ethylhexy1)phosphoric acid (HDEHP). Campbell and Moss (6) separated plutonium from the aqueous portion of the sample by using alkaline earth phosphate precipitation; and the method, with modifications, has proved to be reliable and accurate. However, the flocculent precipitate carries most of the organic constituents; therefore, before additional separation steps can be carried out to separate the plutonium, the organic components are eliminated by wet or dry washing. Oxalate precipitates are crystalline and do not carry large amounts of other material from solution, but plutonium also forms B soluble complex (15,161 with oxalate ion so there was some doubt that small amounts of plutonium would carry with a calcium oxalate precipitate. Scott and Reynolds (17)used calcium oxalate to separate plutonium from environmental samples, and Larson et al. (18, 19) precipitated calcium plutonium oxalate from urine that was 1.6 M in "OB, but these investigators ashed the resulting precipitate before continuing with the separation of plutonium. Koshland et al. (20) used calcium oxalate to coprecipitate plutonium from urine, but the results were not reproducible (11). Because ashing is the critical step in the procedure and time-consuming (21),an analytical method was developed to eliminate the ashing. The procedure that was developed uses calcium oxalate as the carrier to separate plutonium from urine, and without ashing the precipitate, the oxalate precipitate is dissolved directly in 7.2 M "03. The plutonium is isolated by anion exchange on maroporous resin, electrodeposited, and then measured by a-spectrometry.
EXPERIMENTAL SECTION Apparatus and Reagents. The bottles used to collect the urine were 16-02. Ball jars, flint glass, wide mouth, with a molded screw cap, Mold. No. 1324 from Riekes Container Co., Phoenix, AZ. The bottles were discarded after one use. The bottles were 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981 45+2mm
+
Tr-7
Flgure 1. Anion exchange column.
packaged four to a kit when submitted to the employee for a urine sample. Special cardboard boxes for making up the kits can be obtained from United Container and Display Co., Los Angeles, CA. Small individual stirrer hot plates such as the Thermolyne midget stir hot plate were used for stirring solutions during heating along with 1000-cm3tall form beakers such as Corning No. 1060 or Kimble No. 14030. A clinical centrifuge to hold 100 cm3 round-bottom centrifuge tubes was required to collect the precipitate. The glass column used for the anion exchange is shown in Figure 1. The capillary tip section, because of the suction created, increases the flow sufficiently that by using 16 columns in a rack, 16 samples can be sorbed and washed and the plutonium eluted in about 2 h. The equipment used for the electroplating has been described (22,23). Plutonium-242 was the tracer (24,25)used to determine recovery of plutonium. This isotope can be obtained from the Isotope Sales Department, Joe E. Ratledge, Supervisor, Oak Ridge National Laboratory, Oak Ridge, TN. Specify "Livermore High Purity Plutonium-242" tracer grade. The 10% oxalic acid solution was prepared by dissolving 200 g of oxalic acid in water and diluting to 2000 cm3with water. The 1%oxalic acid solution was made by diluting the 10% stock solution. The Ca(N03)zsolution (1g of Ca(N03)z.4HzO/cm3)was prepared by diluting 453 g of the salt to 450 cm3 with water. Normally, a 1-lb bottle of Ca(N03)z.4 HzO holds 450 cm3 of solution; therefore, the reagent bottle was filled with water to dissolve the salt, and this gives a Ca(N03)zsolution of the proper concentration. The solution was filtered through a fast flowing fiiter paper (Whatman No. 54) to remove the insoluble sediment. A 25% NaOH solution was prepared by diluting the 50% reagent solution with an equal volume of water. The eluting solution for plutonium was 0.36 M HC1-0.01 M fluoride and prepared by diluting 31 cm3 of concentrated HC1 and 230 mg of ammonium bifluoride (NH4F.HF) to 1OOO cm3with water. This solution was stored in a polyethylene bottle. The 5% NaHS04 solution was prepared by dissolving 50 g of NaHSO4.HZ0in HzO and diluting to 1000 cm3. The macroporous anion exchange resin used was Bio-Rad AGMP-1, 50-100 mesh resin. (Since this work was completed, it has been found that there is a large variation in the flow rate between different batches of macroporous resin. The flow rate through the column should be 2-3 cm3/min, and AGMP-1 or AG-1, X-4 resin, 100-200 mesh, may be used for the anion exchange.) To remove the fines, we poured a pound of resin into a 2WO-cm3beaker, added water to give a volume of about 1600 cm3,and stirred the slurry on a magnetic stirrer for 3-5 min. When the bulk of the resin had settled, the water was slowly decanted
1271
along with the fines. The washing was repeated twice and then the resin was poured into a column large enough to hold a pound of resin. The resin was washed with 4 L of 2 M NaOH and then 1 L of HzO. The resin was converted to the nitrate form by washing with 4 M HN03solution until a AgN03test of the wash showed the absence of chloride and then washed with an additional 4 L of HN03solution. The resin was then returned to the beaker and washed once more with HzO to remove any fines and then stored as a water slurry. Procedure. After the sample was collected (26) and poured into a 20W-cm3graduated cylinder, each bottle was rinsed with a minimum amount of 7.2 M HN03 and the rinse solution added to the sample. To avoid excessive foaming when the HN03 was added to the urine, we added 5-10 drops of 1-octanol to the urine before the HN03 rinse solution. After the sample was mixed, a measured aliquot containing 500-800 cm3of urine was transferred to a 1000-cm3tall form beaker and 242Putracer added to the sample. The magnetic stirrer was started to rapid stirring, the heater was set to give a solution temperature of 70-85 "C, and 2 cm3of Ca(N03)z.4H20solution was added. Slowly 25% NaOH solution from a polyethylene wash bottle was added until a copious white precipitate of calcium phosphate was obtained and then 25 cm3of NaOH solution added in excess. Ten cubic centimeters of 30% HzOzwas added and the stirrer set to a slow stir. After 60 min of heating, 5-10 drops of 1-octanol was added, the stirring speed was again increased to rapid stirring, and by using a polyethylene wash bottle, concentrated HN03was slowly added to the sample until the precipitate dissolves. The solution may have a slight turbity due to ferric phosphate or other salts that dissolve slowly, but the analyst could easily distinguish when the calcium phosphate was dissolved. Immediately an additional 1 cm3 of concentrated HN03 per 100 cm3 of solution was added. One hundred cubic centimeters of 10% oxalic acid solution was added, the stirrer set to slow stirring, and then 1-2 cm3 of concentrated NH40H added to initiate nucleation, using fresh NH40H from the stock bottle each day. Stirring and heating were continued for an additional 30 min and then the solution was cooled to ambient temperature. After 2 h or when the solution had cooled to ambient temperature, the supernatant solution was decanted by pouring and the precipitate transferred to a 100 cm3 round-bottom centrifuge tube. The precipitate was washed into the tube with a 1%oxalic acid solution, making sure to let all of the solution from the beaker drain into the centrifuge tube. The precipitate was centrifuged and then the supernatant liquid decanted. The precipitate was washed with 1%oxalic acid solution, and the tube swirled to thoroughly mix the precipitate and wash solution. The sample was centrifuged and then the wash solution decanted, making sure that all of the solution drained from the centrifuge tube. The inside walls of the 1000-cm3beaker were washed with 7.2 M HN03, swirled, and then the HN03wash solution from the beaker added to the centrifuge tube containing the oxalate precipitate; enough 7.2 M HN03 acid was added to the centrifuge tube to give a total volume of 70-80 cm3. The oxalate precipitate-7.2 M HNOBmixture was stirred until all of the precipitate had dissolved, and this mixing was done by adding a 5/8-in.magnetic stirring bar to the centrifuge tube and stirring. After the precipitate had dissolved, the tube was capped with a polyethylene stopper (no. 6) and heated to 75-95 OC for 90 min in a hot water bath or an aluminum heating block. After heating, the solution was ready for anion exchange isolation of the plutonium. The macroporous resin was supported in the column with a plug of glass wool, and the resin and HzO added to the column until the level of resin in the column remained constant. An additional 50 cm3of HzOwas passed through the column to ensure that the level of resin in the column had stabilized. Fifty cubic centimeters of 7.2 HN03was added to condition the resin for the anion exchange of plutonium as the hexanitraw complex. Most samples will contain a small amount of a flocculent organic precipitate that does not interfere in the anion exchange, but occasionally a sample will contain a large amount of precipitate, which requires the solution to be filtered as it is poured onto the column to prevent the precipitate from stopping the flow of sample through the resin column. These solutions were filtered by placing a 75-mm fluted funnel with a fast flowing hardened filter paper (Whatman no. 54 or 541) on top of the column reservoir. When
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
Table 11. Oxalate Method vs. Alkaline Earth Phosphate Method for a Composite Urine Sample 242PU) recovery, % 92 85 87 94 85
av RSD, %
89
oxalate method amt of 239Pu fCi
45 59 54 59 90
207 230 239 266 270
*4
242
amt of 238Pu fCi
* 26
11
61
AEP method amt of 239Pu, fCi
242Pu recovery, % 90
234 230 221 261 27 5
81
92 87 86 i
17
88
*3
244 9
28
the solution in the reservoir had dropped to the top of the resin column, the reservoir and centrifuge tube were washed with 7.2 M "OB, adding the washings to the column. The reservoir was washed three additionaltimes with 7.2 M "OB, each time letting the solution flow to the top of the resin column before adding the next wash. After the last wash solution has dropped to the top of the resin column, the column was washed with an additional 200-250 cm3of 7.2 M HN03, which was done by filling an 8-02. small-mouth polyethylene bottle with 7.2 M HN03and upending the bottle onto the top of the reservoir. When all the wash solution had passed through the column, the plutonium was eluted with 35-40 cm3of 0.36 M HCl-O.01M fluoride eluting solution. Then 2 cm3 of 5% NaHS04 and 2 drops of 70% HC104were added to the eluted plutonium. The NaHS04prevents the plutonium from baking onto the beaker during fuming (22,27). The solution was stirred and then heated gently the first 30 min to avoid bumping and then fumed to dryness on a hot plate with a surface temperature of 160-185 "C. The beakers with the Pu-NaHS04 were washed twice with 6 M HC1 and each time evaporated to dryness on a hot plate with a surface temperature of 120-150 "C. After the second evaporation with HCl, the plutonium was ready for electrodeposition (22) and a-spectrometry. The samples were counted for 50000 s (833 min) by using solid-state detectors. The a-spectrometer system used has an average background count of two counh per 5OOOO s with a detector efficiency of 32%;therefore, at the 95% confidence level, a value of 15 fCi would be a positive result for any one determination.
RESULTS The object of this study was to provide a method for the determination of plutonium in urine with a detection limit and accuracy equal to present methods but with a shorter turnaround time. There are no standard urine samples containing plutonium and even successive voidings from the same person may not contain the same amount of plutonium; therefore, several methods were considered to prove the merits of the oxalate precipitation method. One is to add known amounts of plutonium to urine and analyze for the plutonium, and this is the way quality control (QC) samples are prepared. Another method is to prepare a composite urine sample from people who excrete plutonium and then analyze aliquots of the composite urine, The method could also be tested by taking individual samples, dividing a sample in half, and analyzing the sample by the existing method and the oxalate precipitation method. The method was tested by analyzing aliquots of a composite urine and also by dividing samples in half. The alkaline earth phosphate (AEP) method has been described (6, 24, 28) and has proved over many years to give reliable results; therefore, the oxalate method was compared to the AEP method, which was the referee method. Aliquots of a composite urine sample were analyzed because in this manner the oxalate method and the AEP could be directly compared, and the deviation within each method could also be obtained. The results between the two methods using a composite urine sample appear in Table 11. The analysis of the composite urine sample by the oxalate method gave an average value for the 239Puof 242 fCi f26,
amt of 23*Pu, fCi 95 63 54 104 68
*
23
77 28
* 22
Table 111. Oxalate Method vs. Alkaline Earth Phosphate Method for Individual Urine Samples
sample A B C D
oxalate method AEP method %2Pu, =2PU, recov- amt of amt of recov- amt of amt of ery, 239Pu, 238Pu, ery, 239Pu, lj8Pu, % fCi fCi % fCi fCi 56 45 76 73
477
