Table I. Performance Characteristics of the Prototype ( W ) Value Detector
Background current 3 . 5 X lo-@ in air (amperes) Koise level at a band 10-l' pass of 0. to 1 cycle per see. (amperes) Temperature and pressure range for less than 0.1% change of ion current in air mm. pressure 60 to 760" C. temperature 20 to 200" Response to a 10% Test gas concn. of test gas added to air. Increase in ion current, 70 0 2
coz
0.7 0.27 0.04 3.0 3.5
6.0
8.0 3.3
9.1 Toluene a These were the limits of the test; the detector will probably function without disturbance over a wider range of temperatures and pressures than those here shown.
proportions of the common gases of the air is, however, confirmed. The response to hydrocarbon gases and vapors increases at first with molecular weight
but tends to an asymptote beyond molecular weight 100. ( W ) values are approximately proportional to ionization potentials and an ebtimate of the detector response for an unknown vapor can he drawn from its ioriization potential. A detector with a minimum detectable concentration limit in the region of 0.03y0 by volume is hardly the choice for use in gas chromatography. However, the great stability of the ( W )value detector in circumstances where the ambient variables are changing and its potentially rapid response make it valuable wherever the measurement or monitoring of high vapor concentrations in air is contemplated. Applications such as anaesthetic vapor measurement during surgery or fuel vapor measurement under the dynamic conditions of a functioning internal combustion engine are examples of these. Table I shows that the detector is slightly sensitive to oxygen and for this gas gives a positive response approximately one fifth of that for a hydrocarbon vapor. This is not a disadvantage in the measurement of the aerial concentration of vapors since the proportion of oxygen is usually constant. I t is potentially useful for the measurement of oxygen concentration in closed environments where the pressure may be different from normal. The prototype detector just described was made to demonstrate the feasibility of a (U7) value detector; in the form
shown it is cumbersome and unsuited to conditions requiring a rapid response. Other and much more convenient forms are possible, including open ion chambers immersed in the atmosphere under test. The (W) value detector wili function with carrier gases other thari air, but because of the Penning effect ( 1 ), the detector behaves anomalously with pure noble gases. The Penning effect can he exploited for detection. Under some conditions using the rare gases as carriers there is an increased sensitivity to gas and vapor concentration without a loss of the useful indifference of the detector to changes in the ambient conditions. h detailed investigation of this development will be the subject of a later paper. ACKNOWLEDGMENT
I am indebted to my colleagues, Albert Zlatkis and G. R. Shoemake of the University of Houston, Texas, for their generous help and advice. LITERATURE CITED
(1) Druyvestegn, M. J., Penning, F. ll., Rev. M o d . Phys. 12, 98 (1940).
J. E. LOVELOCK Bowerchalke Kr. Salisbury, Wilts England RECEIVED for review October 28, 1964. accepted January 4, 1965. Work supported by a contract from the Royal Aircraft Establishment, Farnborough, England.
Rapid Determination of Strontium-90 in Bone Ash via Solvent Extraction of Yttrium-90 SIR: Di(2-ethylhexyl) phosphoric acid (EHPA) is a versatile and powerful tool in radiochemical analysis, especially with regard to the lanthanides, actinides, and yttrium. Peppard et al. describe the use of this reagent for separating the lanthanide elements (3, 4). Petrow et al. used it for the analysis of thorium-228, radium-228, and actinium-227 (6-7). Kauffman and Matuszek have recently described the use of EHPA in the determination of strontium-90 via extraction of yttrium-90 (2). Their procedure, however, has several limitations. First it requires the use of a liquid scintillation counter equipped with an energy discriminator. Second, relative to the use of a low beta counter, the method is somewhat insensitive. 584
ANALYTICAL CHEMISTRY
Finally, the authors indicate that zirconium-niobium-95 and uranium interfere. One would expect that thorium would also interfere in this method. I n the procedure described below, yttrium-90 is extracted from a hydrochloric acid solution of dissolved bone ash, into dilute EHPA. Dilute acid washes are used to remove calcium and phosphate, and the radionuclides of strontium, cerium, radium, promethium, cesium, cobalt, manganese, zinc, actinium, lead, barium, ruthenium, and lanthanum from the solvent. Zirconium, uranium, and thorium remain in the solvent, along with yttrium. The yttrium is then stripped from the solvent, along with some zirconium and uranium, with strong hydrochloric acid.
However, zirconium and uranium, along with the inevitable traces of iron, are extracted from, the strip solution with a quaternary amine, Aliquat 336. The yttrium-bearing solution is evaporated to dryness, mounted, and counted. Yttrium recovery is 96%. The procedure is rapid, and when samples are analyzed in groups of four, the total working time is only 25 minutes per sample. Bone ash samples weighing 10 grams are treated, but larger samples can be treated by adjusting the volume of reagents accordingly. EXPERIMENTAL
Apparatus. A PCC-10.4 proportional converter and scaler, Nuclear Measurements Corp., and a n Omni-
guard low background beta detector, Tracerlab, were used. Reagents. Di(2-rthylhexyl) phosphoric acid (EIIPA) was obtained from Vnion Carbide Chemical Corp. Prepare 1.5.1l E H P A in n-heptane as described by Petrow et al. ( 7 ) . Dilute this solution to 0.45.11 E H P A with the appropriat,e volume of n-heptane. hliquat 336, methyltricaprylammonium chloride, General Mills Chemical Co., was used. Prepare a 30 volume yo solution of Aliquat 336 in toluene and wash it once with an equal volume of 9.11 hydrochloric acid. Procedure. I t is assumed t h a t the sample is sufficiently old, so t h a t radioactive equilibrium is assured. -ish the bone in a silica boat a t 600" C. until a white ash is obtained. Pulverize the sample well to hasten its subsequent dissolution. Place 10 grams of the ash in a beaker; add 17 i d . of hydrochloric acid, and 85 nil. of water. Heat until the ash dissolves, cool the solution, and adjust the acidity to p H 1.0 wit,h dilute ammonia. Before proceeding be sure that all precipit,ated calcium phosphate is redissolved. Add the solution to a separatory funnel containing 50 ml. of 0.4551 EHI'A and extract for 1 minute. S o t e the time. Draw off the aqueous phase and either discard it or save it for fresh yt,triuni-90 ingrowth. Wash the solvent' t,hree times, for 1 minute, u-it'h 25-ml. portions of 0.50d1 hydrochloric acid. Ilisrard the wash solutions. Extract' the solvent three times, for 1 minute each, with 15-1111. portions of 9.11 hydrochloric acid. 1)iscard the solvent and combine the three extracts in a separatory funnel containing 50 ml. of 30YG hliquat, 336, and extract for 1 minute. Draw off the aqueous phase into a beaker and discard the solvent. Evaporate the solution to dryness and bake the beaker for 10 minutes a t 600' C. in a muffle furnace to destroy the organic residue. Cool the beaker, add 5 nil. of nitric acid, and evaporate the solution to a volume of 1 nil. L-sing an eye dropper>mount the solution onto a 2-inch st,ainless st'eel planchet by evaporation under a heat lamp. Rinse the beaker three t'imes with 1-ml. portions of nitric acid and mount the rinse solutions onto the planchet. Flame the dried planchet over a burner, and count t,he sample. Kote the time a t which the count commences. T o compute the strontium-90 content of the sample, use the following relationship : Sr-90, d.p.m.,'gram ash
=
c.p.m. - B - C G X 0.96 X F X TI' X -.00108L
B = detector background; C = reagent blank in c.12.m.; G = detector geometry; 0.96 = yttrium recovery factor F = beta backscatter factor, 1.58 from a stainless steel planchet; It- = sample weight, grams; t = time elapsed, in hours, between extraction and counting of yttrium.
RESULTS
T o dekrmine a cheniical yield for the procedure, known amounts of yttrium-90 were added to a bone ash solution. Prior to this, any yttrium-90 originally present in the bone ash solution was removed by extraction with EHPA. The chemical yield of yttrium, determined by analyzing 10 suc,h samples, was 967,. T o compare our 2~ internal flow counter with our end-window low background counter, the samples were counted on both instruments. The atter product for the 2~ counter is 0.79. For the end-window counter, it was 0.55. The factor for the end-window counter was constant, provided care was taken to maintain the sample in the center of the planchet. This is done by first inscribing a small circle on the planchet with a black wax pencil. The wax serves t,o cont'ain the solution, and it is easily destroyed when t,he planchet is flamed. Next, samples of purified bone ash solution were spiked with known amounts of ytt,rium-gO and analyzed. These results are given in Table I. Replicate samples of steer bone ash and human bone of unknown strontium90 content were analyzed, and these data are given in Table I1 and Table 111. The steer bone samples were sufficiently active so that they could be count,ed on the 2~ count'er, with a background of 70 c,p.m,,as well as on the low background counter, with a background of 1.2 c.1i.m. The human bone samples were only counted on the latter instrument. T o determine the purity of the yttrium-90 separated from steer bone, the five samples were combined and the decay rate was st,udied. After six half lives, the sample was still decaying with half life of 64 hours, and the indicated long-lived im1iurit.y is less than 0.25%. There was no evidence for the presence of any short lived inipurit,y. Finally, two samples of bone ash, portions of which had been analyzed a t the Health and Safety Laborat,ory of the iitomic Energy Commission using a completely different technique, were analyzed by the method presented above. The results from both groups are presented in Table IV. There is an average difference between the result,s for the two laboratories of about 157,. While the source of t'he difference is not known, it is suspect'ed that it arises from differences in counter calibration and, specifically, differences in backscatter fact,ors. DISCUSSION
Decontamination factors were determined for variouq radionuclides, including cesium-137, strontium-90, ru-
Analysis of Spiked Bone Ash
Table 1.
d.p.m. Y90 Added
Found 9,900
10,000 10.000 1 000 1,000 100 100 20 20
10.180 980
;
Table
II.
~~~
985 97 101 22 21
Replicate Analyses of Steer Bone Ash for SrS0
d.p.m. SrgO/gm.of ash Low
background counter
2~ Counter 55 5
55 57 58 56
55 1 57.2 58.5
9 2 5 6
58.9 5g.2 57.1
Av. 56 7
Table 111. Replicate Analyses of Human Bone Ash for SrgO
d.p.m. Srgo/gm. of ash 0 93 0 09
**
1 00 0 09 1 02 f 0 . 0 9 1.06 f 0 09
Table IV. lntercomparison of Analytical Results
d.p.m. Srgo/gm. of ash X.Y.U. A.E.C. 13.8 14.0 14.0 14.0 1.1 1 1 1.0 1.1
17.5
1.2
thenium-106, radium-224, thorium-228, lead-212, cesium-144, promethium-147, actinium-228, zirconium-niobium-95, uranium, cobalt-60, manganese-54, iron55, and zinc-65. The results are given in Table V. While no data were obtained for barium and lanthanum, it can be inferred from the stront'ium, radium, actinium, and cerium values that both barium and lanthanum will have decontamination factors in excess of IO3. The various elements listed were chosen for study, not because they are all necessarily bone seekers, but rather because some or all of them are found in biological mat,erials of various kinds. Of all the elements studied, only promethium has a decontamination fact,or sufficiently low to be considered as an interfering element. Sornially, VOL. 37, NO. 4, APRIL 1965
585
Table V.
Decontamination Factors
Cobalt Thorium Uranium Radium Cerium Promethium Radium Cesium Ruthenium Strontium Actinium Zinc Zirconium-niobium Iron Manganese Lead
>103 200 > 103 > 103 103 20 >io3
> 103 >io3 > 103 > 103 >io3 >io3 >io3
> 103 >io3
however, promethium occurs at levels much lower than strontium. The author has reported, for instance, that in steer bone, the promethium content is only 1% of the strontium-90 content (1). Should a sample be analyzed where a high promethium content is suspected, its interference can be easily overcome. By counting the sample through 30 mg. per sq. cm. of aluminum absorber, the promethium count can be reduced by a factor of 100. However, especially if a 2~ internal flow counter is used, this will result in a significant loss of yttrium counts because nearly all of the backscattered yttrium-90 radiation will also be absorbed. A better tech-
nique would be t,o increase the acid strengt'h in t'he EHPA washing steps from 0.5X to 0.8L11 hydrochloric acid. The yttrium recovery will only be reduced to 92%, but the promethium decontamination factor will be increased to 400. Yttrium-91, if present, can be different'iat'ed from yttrium-90 by either absorption or decay. .A far better solution is to extract the solution twice with EHPh to remove all ytt,riuiii nuclides, and then set the solution aside for a measured period of time for fresh yttrium-90 ingrowth. Two extractions will remove more than 99.9% of any yttrium initially present without loss of a measurable amount of strontium. The sensitivity of the procedure is, of course, a function of the background and geometry of the detection equipment being used, and is dependent upon the reagent' blank as well. With a geometry of 35 to 40%, a background of 1 c.p.m., and a reagent blank of 0.1 c.p.m., as little as IO-'* Curie of strontium-90 can be determined. ;is described, the procedure considers only the analysis of bone ash. It, has been successfully applied to other biological ashes, including food ash and plant ash, with no procedural alteration other than the methods used in dissolving the sample.
ACKNOWLEDGMENT
The author expresses his appreciation to Joseph Rivera of the U. S. Atomic Energy Commission Health and Safety Laboratory for furnishing some of the samples. LITERATURE CITED
(1) Eisenbud, Ll.> Petrow, H. G., Dosimetric hspects of Radium Poisoning, Ann. Rept., AEC Contract AT(30-112896,September 1964. ( 2 ) Kauffman, P. E., Matuszek, J. M.,
c.s.
Jr., Ninth Annual Meeting of the Health Physics Society, Cincinnati, Ohio, June 14-18, 1964. (3) Peppard, 1). F., >lason, G. W., Maier, J. L., Driscoll, W. J., J . Inorg. Sucl. Chem. 4, 1334 (1957). (4) Peppard, L). F., .\lason, G. W., Aloline, S. W., Ibid., 5 , 141 (1957). (5) Petrow, H. G., Allen, R. J., . ~ N A L . CHEM.33, 1303 (1961). ( 6 ) Ibid.,35, 747 (1963). (7) Petrow, H. G., Cover, .4.,Schiessle, U . J., Parsons, E., Ibid., 3 6 , 1600 (1964). HEKRYG. PETROW A. J. Lanza Research Laboratories Institute of Environmental AIedicine New York University Tuxedo, N. Y. RESE.4RCH supported in part by the Division of Biology and Medicine of U. S. A4tomic
Energy Commission Contract AT(30-1) 3086 and Sational Cancer Institute Contract C h 06989 and ES00014.
Fluorescence Analysis for Traces of Naphthace ne in Anthracene SIR: Assay of trace impurities in anthracene purified by zone refining for growth of single crystals requires methods sensitive to levels below 1 p.p.m. impurity. The fluorescence of anthracene and of anthracene-naphthacene mixtures has been studied (1, 2 , 5-8, 11-13, 1 7 ) . Saphthacene, a major impurity in anthracene, may be determined by fluorescence. Saphthacene fluorescence arises not from direct excitation of the low concentration of naphthacene present, nor from absorption of energy from anthracene emission (8, 1 1 ) . I n solid solutions the anthracene host absorbs the exciting energy and transfers it to naphthacene by sensitized fluorescence (11, 16), or by excitation migration (4-6, 8). When molecules of the two species are separated beyond crystal lattice distances either upon fubion or by a solvent, the energy transfer cannot occur so readily as in the solid solution. EXPERIMENTAL
Apparatus. A Farrand spectrofluorimeter equipped with a 150-watt Xenon arc lamp, a n R C h IP28 photomultiplier and two monochromators was employed for all measure-
586
ANALYTICAL CHEMISTRY
ments. T h e phototube was connected by a microammeter to a Varian G-10 strip recorder. Reagents. Anthracene was prepared from anthraquinone by t h e method of Fieser (3) and was purified by recrystallization and zone refining. Naphthacene (Matheson Coleman & Bell No. 9082) was used without further purification. All solvents used were spectroquality grade. Procedure. Standard solutions of anthracene and naphthacene in isopropyl alcohol were mixed in proportions to prepare 39 qolutions, each containing 0.50 mg. per ml. of anthracene, and with naphthacene content ranging from 0.6 to 4360 p.p.m. of the anthracene , content. These standard mixtures, in 4-ml. portions, were diluted with 125 ml. of isopropanol to give clear solutions. Rapid addition of 125 ml. of distilled water to 4-id. portions of the standard mixtures produced colloidal dispersions. Fluorescence measurements were made with the exciting monochromator, adjusted for peak fluorescence, at 365 mp. Tyndall and Rayleigh types of emission (10) do not present a problem in the system. L-denfriend (15) and Parker and Reese (9, 10) have discussed instrumental errors in fluorescence analysis. In the present work,
the instrument was used at constant slit width and excitation energy was kept constant. No correction for phototube response was made because quantitative measurements were made at selected, fixed wavelengths. RESULTS A N D DISCUSSION
Emission peaks for anthracene occur a t 405 mp (maximum), 430, and 450 mp; for naphthacene in anthracene at 510 mp (maximum), 540, and 575 mp. The results are qualitatively similar to those described for dilute solutions (1, 1 1 , 1 3 ) . Typical spectra for colloidal suspensions are presented in Figures 1 and 2. Colloidal Spectra. From 0.6 to 300 p.p.m. naphthacene, anthracene fluorescence a t 405 mp decreases and that of naphthacene a t 510 mp increases, as measured by the height of the emission peak (Table I). Above 300 p.p.m. napthacene, self-quenching of naphthacene emission is observed. This is caused by interaction betwsen naphthacene molecules a t the higher concentrations. Color of the colloidal suspensions changes much more noticeably than does that of the solid in either single crystal or polycrystalline form.
