Fluorometric Determination of Submicrogram Quantities of Thorium T. D. Filer Health Services Laboratory,
U.S . Atomic Energy Commission, Idaho Falls, Idaho
THECOMPOUND, morin, (Z’,4’, 3, 5, 7-pentahydroxyflavone), has been found to be a useful reagent for the fluorometric determination of submicrogram quantities of various metals, particularly beryllium and thorium (1-4). Because of morin’s sensitivity to both metals, beryllium is a particularly serious interference in the determination of thorium. For example, 7 x 10-4 pg of beryllium causes a detectable interference in the direct fluorometric determination of thorium using morin ( 4 ) . Because the Earth’s crust contains about 6 x pgimg of beryllium, the direct determination of thorium in most natural samples will be in error even on a sample size as small as 1 mg if morin is used. In contrast, 3,4’, 7-trihydroxyflavone is comparable to morin in sensitivity to thorium while being only 1/1400 as sensitive to beryllium. Therefore, the direct determination of thorium on sample sizes as large as 1 gram would not be affected by natural levels of beryllium in the present procedure. EXPERIMENTAL
Apparatus. The instrumentation and filters used have been described ( I $ ) . Reagents. The preparation of most of the reagents used has been previously described ( 4 ) . Reagents which are unique to this procedure are described below. STANDARD THORIUM SOLUTIONS.Prepare the 250 pg of thorium per milliliter standard as described ( 4 ) . Dilute 40.00 ml of the stock solution and 10 ml of concentrated sulfuric acid to 1 liter. This solution contains 10 pg of thorium per milliliter. SODIUMSULFATE-DTPA-TEA. Dissolve 110 grams of anhydrous sodium sulfate in 375 ml of water. Dissolve 6.0 grams of recrystallized DTPA in 50 ml of water and 25 ml of colorless 50 % triethanolamine (TEA). Combine the solutions and dilute to 500 ml of water. Add 10 drops of 0.01 % quinine sulfate and concentrated sulfuric acid while stirring the solution near an ultraviolet lamp until the bright blue fluorescence is restored. Filter the solution through a double glass filter paper in a small Buchner funnel. Store the solution in a borosilicate glass bottle with a polyethylenelined screw cap. ETHANOLAMINE-PIPERIDINE BUFFER. Transfer 44.0 grams of recrystallized DTPA, 50 grams of sodium citrate, and 20 grams of anhydrous sodium NaaC6H5O7 51/2 H20, sulfite to a 500-ml volumetric flask with about 300 ml of water. Add 24.0 grams of ethanolamine and 65 ml of redistilled piperidine, stopper the flask, and swirl under a stream of cold water until cool. Dilute to 500 ml and store in a tightly-stopped borosilicate glass bottle with a polyethylene-lined screw cap. 3,4‘,7-TRIHYDROXYFLAvONE SOLUTION. The preparation of 3,4’,7-trihydroxyflavone was been described (5-7). Transfer
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(1) C. W. Sill and C. P. Willis, ANAL.CHEM., 31, 598 (1959). (2) M. H. Fletcher and R. G. Milkey, ibid., 28, 1402 (1956). (3) R. G. Milkey and M. H. Fletcher, J . Amer. Chem. SOC.,79, 5425 (1957). (4) C. W. Sill and C. P. Willis, ANAL.CHEM., 34,954 (1962). (5) D. G. Roux and G. C. de Bruyn, Biochem. J., 87,439-44 (1963). (6) Katsuzo Yamaguchi, Nippon Kagaka Zasshi, 1963, 148-52. (7) Z.I. Jerzmanowska and M. Michalska, Rocrniki Chem., 35, 353-7 (1961).
6.75 mg of the flavone to a 100-ml volumetric flask and dilute to 100 ml with 95% ethanol. POTASSIUM CYANIDE.Dissolve 10 grams of potassium cyanide in 140 ml of water. Centrifuge, decant, and dilute to 1 liter with water and store in a polyethylene bottle, Procedure. Evaporate an aliquot containing 0-10 pg of thorium to dryness in the presence of concentrated sulfuric acid and sodium sulfate as described (4). Cool the sodium acid sulfate residue, add 2 ml of water, and 1 drop of 25% sodium metabisulfite. Cover the beaker with a watch glass and boil the solution until the volume has been reduced to about 0.5 ml. Remove the cover glass and rinse with a few drops of water. Add 5.00 ml of sodium sulfate-DTPATEA solution. Transfer the solution quantitatively to a 25-ml volumetric flask. Add 1 additional drop of 25% sodium metabisulfite and 1.00 ml of cyanide solution. Add 2 drops of 0.01 quinine sulfate and, while swirling the flask near a long-wave ultraviolet lamp, add 1 M sodium hydroxide dropwise until the bright blue fluorescence disappears. Add 5.00 ml of the ethanolamine-piperidine buffer, swirl, and rinse the sides of the flask with a few drops of water. Add 1 drop of a 25% hydroxylamine hydrochloride solution and 1.OO ml of 3,4’,7-trihydroxyflavonesolution, mix, and dilute to volume. Mix thoroughly and place in a constant-temperature bath at 25 “C for 20 minutes. Measure the fluorescence at 520 mp using the technique described previously ( I , 8). Permanent glass standards ( I ) can be used to reproduce the same instrumental sensitivity from day to day. The time of measurement after addition of the 3,4‘,7trihydroxyflavone is very important and should be kept within 1 or 2 minutes of the recommended value of 20 minutes for blanks, standards, and samples for highest precision. Place 1 ml of water for a blank and 1 ml of the 10-pg per ml standard thorium solution in 100-ml beakers, add 1 drop of 72% perchloric acid and 1 ml of the sulfuric acid-sodium sulfate solution. Evaporate carefully on a covered hot plate until evolution of sulfuric acid fumes has ceased and treat as described above. Subtract the blank from the standard and express the sensitivity as microgram thorium per net scale division. Correct the samples for an appropriate blank carried through the entire procedure and calculate their thorium content from the sensitivity value obtained from the standard. Sample Preparation. Because of the refractory nature of the compounds of thorium and associated elements and the inability to dissolve them even in boiling concentrated acids, pyrosulfate fusion is always employed to ehsure their complete dissolution and conversion to a soluble ionic form. AIR DUSTS. A method for the treatment of air dusts has been described (4). The dissolution of the air dust described in this procedure is applicable through the point where the sample is placed in the 25-ml volumetric flask, then the procedure is modified as follows: Add 5 ml of sodium sulfateDTPA-TEA, 1 ml of cyanide, and 5 ml of buffer solutions. Add 1 drop of 25 % hydroxylamine hydrochloride, 1.OO ml of 3,4’,7-trihydroxyflavone, and mix thoroughly. Place the flask in a constant-temperature bath for 20 minutes and measure fluorescence. (8) C. W. Sill, C. P. Willis, and J. K. Flygare, Jr., ANAL.CHEM., 33, 1671 (1961).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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Element Be Be Zr Zr Hf Ti sc A1 Ga La Y U U
Cr Cr Ba Ba Ba Sr Ce Sb Nb Nb Ta Ta Si Sn Hg
Quantity, mg 0.10 0.001 0.1 0.01 0.01 0.1 0.1 1.0 1.0 1 .o 1.0 1.0 0.1 1.0 0.1
Table I. Effects of Other Substances Error, scale divisione Blanks 10 pg Th
1.0 0.1 0.01 1 .o 1 .o 1 .o 1 .o 0.1 1 .o 0.1 1 .o 1.0 1 .o
+14.3 +0.2
+18.1 $2.0 +10.1 +1.5 +12.0 +19.0 +2.6 +6.1 +3.7 -13.6 -2.4 +29.3 $0.9 +1.1 +0.9 +0.2
+0.3 +7.9 +0.4 +8.0 +1.1 $4.2 $0.4 $0.3 $4.2 +O. 9
+11.7 $0.3 +16.0 +2.5 1-10.6 $3.5 $8.0 +16.7 $1.9 +7.0 $4.6 -42.7 -5.9 -26.0 -5.0
-58.0 -21.9 -2.1 -6.7 +6.0 -1.1 $6.8 -0.1 +1.8 $1.5 -3.7 -11.0 +1.4
Remarks Fluorescent; 7.0pg/sc. div. No interference Fluorescent; 5.4 pg/sc. div. Fluorescent; 5.0 pg/sc. div. Fluorescent; 1 .O Mg/sc. div. Fluorescent; 67 pg/sc. div. Fluorescent; 8.3 ,ug/sc. div. Fluorescent; 53 pg/sc. div. Fluorescent; 385 pg/sc. div. Fluorescent; 164 pg/sc. div. Fluorescent; 270 pg/sc. div. Pale yellow solution No detectable color of uranium Very turbid due to anhydrous Cr2(S0& Slight turbidity Very turbid Slight turbidity No turbidity detectable Slight turbidity Fluorescent; 127 pg/sc. div. Turbid. Antimonic acid precipitates Very turbid. Niobic acid precipitates Slight turbidity Turbid. Tantalic acid precipitates Slight turbidity by Tyndall effect Few flocs of dehydrated SiOzpresent Turbid Turbid, but clears when solution is made alkaline
+1.4 1 .o +O. 8 Faint blue color 1 .o +0.4 0.0 -12.2 Bright yellow in concentrated H2SO4 1 .o -3.6 1 .o -0.7 -9.7 Elemental gold precipitates 1 .o +0.4 -14.9 Added as NaHIP04 before fuming F 1 .o -0.5 +1 .o Added as NaF before fuming As 1 .o $0.3 +4.1 Blank, 25.0 sc. div.; 10 pg Th standard, 100.8 sc. div.; sensitivity,0.133 pg/sc. div. Differences larger than f0.2 sc. div. on blanks or 11.0sc. div. on standards probably indicate significant effect of added substance.
Mn cu V(+5) Au P
Q
WATER. Place 50 ml of water and 3 ml of concentrated sulfuric acid in a 125-ml Erlenmeyer flask and evaporate to fumes. Add 2 grams of anhydrous sodium sulfate, heat t o a pyrosulfate fusion, and finish as described for Air Dusts. Use a 50-pl aliquot of the prepared solution for development of the fluorescence. THORIUM ORES AND RAREEARTHS. A method for the dissolution of thorium ores and rare earths has been described (4). The procedure for the treatment of thorium ores is modified after the sample has been placed in the 25-ml volumetric flask: Add 1 drop of 25% sodium metabisulfite, 5 ml of sodium sulfate-DTPA-TEA, and 1 ml of cyanide solutions. Neutralize to the quinine end point with 1N sodium hydroxide. Add 5 ml of buffer, 1 drop of 25% hydroxylamine hydrochloride, and 1.OO ml of 3,4‘,7-trihydroxyflavone and place in a constant-temperature bath for 20 minutes before reading the fluorescence. With the exception that sodium sulfate-DTPA-TEA is used instead of sodium perchlorate-DTPA-TEA, the purified rare earths are carried through the procedure as previously described (4) until the sample is placed in the 25-ml volumetric flask. Add 1 drop of 25% sodium metabisulfite, 1 ml of cyanide, and 1 N sodium hydroxide dropwise until the quinine fluorescence disappears. Add 5.00 ml of buffer, 1 drop of 25% hydroxylamine hydrochloride, 1.00 ml of 3,4‘,7-trihydroxyflavone,dilute to the mark, mix, and place in the water bath for 20 minutes before measuring the fluorescence. RESULTS AND DISCUSSION
The effect of 3,4‘,7-trihydroxyflavone concentration and changes in alkalinity on the fluorescence of the thorium-3,4’,71266
trihydroxyflavone complex is similar t o the thorium-morin system (4). Detection Limit and Precision. The detection limit is defined at the 95% confidence level as that quantity of thorium which is equal to twice the standard deviation of its determination. To determine its value and the precision obtained with larger quantities of thorium, 10 blanks and ten 10-pg thorium standards were analyzed under the recommended conditions, including the evaporation of thorium solutions to dryness in the presence of sulfuric acid and the transfer from beaker to the volumetric flask. The results indicate a detection limit of 0.03 pg and a precision t o about 1 with lOpg, both at the 95 confidence level. Linearity. The effect of thorium concentration on the fluorescence was investigated a t a 3,4‘,74rihydroxyflavone concentration of 2.50 X 10- mole per 25 ml to determine the linearity under analytical conditions. Nonlinearity is not detectable since it is not greater than the precision of the procedure--i.e., about 1 % up to 10 pg of thorium. At 50pg, nonlinearity has increased t o about 9 %. “Nonlinearity” is defined as the per cent deviation of the observed value from the value that would be obtained by extrapolation of the linear portion of the experimental curve. As the thorium concentration is increased further, deviation from linearity becomes very pronounced. Effect of Other Substances. In anticipation of applying the present procedure to a wide variety of sample types without separations, a detailed investigation was made of the effect
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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of many other substances on both blanks and 10-pg thorium standards. The element or compound investigated was added before fuming with sulfuric and perchloric acids to determine its effect under the recommended conditions. No error could be detected on blanks and the error was less than 1 on 10-pg thorium standards in the presence of 1 mg of potassium, rubidium, cesium, magnesium, zinc, cadmium, bismuth, indium, thallium, boron, germanium, selenium, tungsten, molybdenum, calcium, silver, iron, cobalt, nickel, copper, lead, or any of the rare earths. Errors produced by other substances are shown in Table I. BERYLLIUM.In the thorium-morin procedure ( 4 ) , quantities of beryllium larger than 0.7 mpg are detectable. Even in the presence of acetylacetone, the tolerance level to beryllium is still only 0.01 pg. An extraction with acetylacetone must be used so that 0.5 pg of beryllium can be tolerated. In the present method, beryllium must exceed 1 pg before it is even detectable. This advantage of the present procedure over the morin procedure was clearly shown when a series of niobium-tantalum ores containing 0.02-0.04 thorium were analyzed for thorium. The ores were dissolved and aliquots containing 1 mg of sample were analyzed by different methods: the morin procedure with no attempt to separate the thorium from the other constituents in the ore; the present method with no separations; an entirely independent method using alpha spectrometry; and the morin procedure after the thorium had been separated from the other constituents of the ore. The last three methods had excellent agreement among the results, whereas the morin procedure with no separations was a factor of 10 higher than the rest. The most plausible explanation for the failure of the morin procedure is the presence of trace amounts of beryllium. This clearly illustrates that the morin procedure needs additional separations whenever it is used to analyze routine ore samples. In contrast, 3,4’,7-trihydroxyflavone could be used with confidence on most ore samples without any separations. IRONAND COBALT.In the absence of cyanide and hydroxylamine hydrochloride, iron or cobalt produces serious negative interference, 1 mg of iron being sufficient to eliminate the fluorescence completely. This is due to the strong absorption of both the emitted thorium-3,4’,7-trihydroxyflavone fluorescence and the exciting radiation by the highly colored ferric-DTPA or cobalt-DPTA complexes. However, in the presence of cyanide and hydroxylamine hydrochloride,
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the more stable, slightly colored ferrocyanide and cobaltous cyanide complexes virtually eliminate this interference. The effects of titanium, uranium, aluminum, rare earths, cerium, zirconium, hafnium, yttrium, lanthanum, gallium, scandium, and chromium in this procedure are the same as described in the thorium-morin procedure (4). Most of the interferences listed in Table I are not likely to be encountered in concentrations high enough to interfere when less than 1 mg of total sample is used. Because of the high sensitivity and precision of the procedure, the thorium content of many types of samples can be determined with small samples. Because of the tolerance to many other elements, no separations of any kind are required in most cases and no loss of reliability results if the sample size is kept so small that potentially interfering elements cannot acquire the concentration necessary to interfere. Zirconium, titanium, and uranium are potentially the most serious interferences because they are likely to accompany thorium in significant concentrations, they give no sensitive warning of their presence, and their separation is difficult and time-consuming. However, they can still be accommodated without loss of reliability by proper choice of sample size. For example, if the total sample taken for analysis is limited to 1 mg, thorium can be determined in concentrations greater than 3 x without detectable interference from as much as 0.1 zirconium, 1 uranium, or 1 titanium. If the sample size is reduced to 50 pg, as much as 20 uranium, 2 0 x titanium, or 2 % zirconium can be present and still permit detection of as little. as 0 . 0 6 z thorium. These concentrations would seem to include most samples to be encountered so that many valuable practical applications can be made without separation and with a high degree of confidence in the results.
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ACKNOWLEDGMENT
The author acknowledges the assistance of his associates during many helpful discussions. Special thanks are extended to R. W. Henry who conducted the screening program with the flavone derivatives and did some of the preliminary work with the thorium-3,4’,7-trihydroxyflavone complex. Also, thanks are due to E. G . Paul who prepared the original sample of the flavone. RECEIVED for review March 17, 1970. Accepted June 15, 1970.
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