ANALYTICAL CHEMISTRY
a94
Table 111. ~
~of Sodium ~ ~onofluorophosphate l ~ ~
Fluoride theor. % Analysis'by maAufacturer (98:6% NaZPOzF, 1.4% NaF) Fluoride, found, Sample 1 Sample 2
13.20 13.64 13.50 13.64
ANALYSIS OF OTHER COMPOUNDS
A sample of monofluorophosphate was analyzed by the lanthanum fluoride method. The results are shown in Table 111. The amounts of fluorine found check well with the analysis obtained by other methods. Various laboratory samples of anhydrous rare earth fluorides and oxyfluorides were satisfactorily analyzed by the lanthanum fluoride method. The rare earth ions were quantitatively recovered from the distillation residue by precipitation with cupferron. As the method appeared to work well for inorganic compounds, it was decided to try to extend the method to organic compounds.
A 0.3-gram sample of pure p-fluoroacetanilide was thoroughly mixed with 15 grams of sodium peroxide, 1.4 grams of potassium nitrate, and 0.3 gram of sugar, and fused in a Parr bomb. The fusion residue was dissolved in 125 ml. of water and partially neutralized with perchloric acid. The basic solution was filtered to remove ferric hydroxide formed by reaction with the steel
bomb and then further acidified with perchloric acid. The fluosilicic acid i ~ distillation was begun with a portion of this solution and the remainder was slowly added to keep the distillation temperature below 155" C. The remainder of the distillation and analysis was carried out as previously described in the procedure for inorganic compounds. Results of this analysis are shown in Table IV.
Table IV.
Analysis of p-Fluoroacetanilide
.4v. 12.48
LITERATURE CITED
(1) Fischer, J., 2. anal. Chem., 104, 344 (1936). (2) hleyer, R. J., and Schulr, W., 2. angew. Chem., 38, 203 (1925). (3) Popov, A,, and Wendlandt, W., ANAL.CHEM.,26, 883 (1954). (4) Saylor, J. H., Deal, C. H., Jr., Larkin, M. E., Tavenner, 11. E., and Vosburah. W. C.. Anal. Chim. Acta. 5 . 157 (1951). (5) Willard, H. H,: and Winter, 0. B., IND. ENG.CHEW ANAL.ED.. 5 , 7 (1933). (6) Zachariasen, W. H., Acta Cryst., 4, 231 (1951). RECEIVEDfor review October 26, 1953. Accepted February 23, 1954. Presented before t h e Division of Analytical Chemistry a t the 124th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, 111.
Mass Spectrometric Determination of Thorium G. R. TILTON and L. T. ALDRICH Department o f TerrestrialMagnetism, Carnegie institution
of Washington, Washington, D. C., M. G. INGHRAM, University o f Chicago and Argonne National Laboratories, Chicago, 111.
T
HE determination of thorium in the minerals of igneous rocks is of considerable importance in the measurement of mineral age. The technique of isotope dilution adapted to these determinations is of such sensitivity and precision a.q to be of general interest and is presented here. Isotope dilution procedures have been used for a number of elements over the past few years. Some examples are europium, samarium, and gadolinium ( b ) ,nickel, zinc, selenium, krypton, and xenon ( f f ) , calcium and argon ( 6 ) , rubidium and strontium (3),lead (91, and uranium ( 1 5 ) . The present work extends the method to thorium. The material requirements and procedures necessary for the determination of any element by isotope dilution are: 1. A standardized solution of an isotopically altered carrier of the element to be determined. 2. A procedure for equilibrating a known weight of that carrier with the sample to be analyzed and extracting enough of the element to be determined for isotope analysis. 3. A mass spectrometer to perform the isotopic analyses required, These analyses include those of the carrier, the carrier plus sample, and for elements of variable isotopic composition, the sample alone. In addition a blank is usually necessary in order t o evaluate contamination. Each of the above steps is considered in detail for thorium under the appropriate sections which follow. PREPARATION AND STANDARDIZATION OF THORIUM-230 CARRIER
I n secular equilibrium the uranium-238 decay chain contains about one atom of thorium-230 to 56,400 atoms of uranium-238. Thus if a uranium ore is in secular equilibrium with its thorium daughters it will contain 17.2 y of thorium-230 per gram of uranium-238. The purity of the thorium-230 will depend upon the amount of normal thorium (thorium-232) which was incorporated into the ore a t the time of its formation. ;1number
and
of instances have been knon-n in nature where uranium ores possess thorium contents below the limits of spectroscopic detection. An ore of this type, a black uranium and vanadium bearing ore with an age of 70,000,000 years (13) from the La Sal S o . 2 ;\Kine of the Colorado Plateau, was selected as the source material for the thorium-230 carrier used in this study. The mineralogy of the ore is still under study a t the U. S. Geological Survey The ore apparentlv represents a mineral which is undescribed in the literature ( 1 2 ) . The processing of 900 grams of the ore containing 220 grams of uranium gave 750 y of thorium nith a thorium-230/thorium-232 mole ratio of 3.9. Since no elaborate precautions were taken to avoid contamination during the isolation procedure, the ratio represents only a loiver limit of the ratio actually existing in the ore. The chemical procedures follow : Separation of the Thorium-230 Carrier. Nine hundred grams of ore representing 220 grams of uranium were ground to pass 40 mesh and were digested in 1800 ml. of 8M nitric acid for 90 minutes. The remaining siliceous residues were centrifuged down and discarded. The solution was adjusted to a pH of 3 with ammonium hydroxide and the volume was brought to 4000 ml. with water. The combined oxalates were precipitated by the addition of oxalic acid (400 grams). The oxalate precipitate was filtered and washed with a 1% solution of oxalic acid. The oxalate precipitate was ignited for 2 hours a t 500" C. in a muffle furnace to convert it to oxides. The residues were then digested in 75 ml. of 8 M nitric acid. A radioactive precipitate remained which was discarded when further study showed the activity to be due to lead. A second oxalate precipitation &as performed on the supernate from the nitric acid digestion and the precipitate was ignited and digested in nitric acid as before. The combined nitric acid solutions were concentrated to 35 ml. 4 solution containing 450 grams of aluminum nitrate enneahydrate in 450 ml. of water was added. The nitrate solution was then shaken 2 minutes with 300 ml. of mesityl oxide to extract thorium and uranium into the organic phase. These elements were then extracted back into 200 ml. of water.
V O L U M E 26, NO. 5, M A Y 1 9 5 4 The aqueous extract was diluted to 400 ml., 5 mg. of F e + + +as ferric nitrate were added, and the pH was brought t o 7.5 with a saturated ammonium carbonate solution. The precipitate was dissolved in nitric acid and the resulting solution was brought to a pH of 2 and a volume of 50 ml. The thorium (with most of the uranium still present) was then extracted into 50 ml. of a 0.1M solution of thenoyltrifluoroacetone (TT-4) in benzene. layer was stripped of The benzene-thenoyltrifluoroacetone the thorium with two 25-ml. portions of 2M nitric acid. Since milligram amounts of uranium were still present, the sample was diluted to 100 ml., 5 mg. of lanthanum(II1) as lanthanum nitrate were added, and 20 ml. of a 10% solution of oxalic acid were added to the hot solution. The solution was allowed to stand 16 hours. The lanthanum oxalate precipitate was then centrifuged down and reserved. A second 5 mg. of lanthanum( 111) were added and the precipitation was repeated. The combined oxalate precipitates were treated with 10 ml. of concentrated nitric acid and 1 ml. of iOyo perchloric acid and evaporated t o a moist paste to destroy the oxalate. The residue was dissolved in a few drops of 6 M nitric acid, the volume was brought t o 25 ml. and the p H to 2. A second extraction with a 0.1M solution of thenoyltrifluoroacetone in benzene was performed which was identical to the one previously described. The thorium mas finally extracted into 35 ml. of 2 N nitric acid from the thenoyltrifluoroacetone-benzene layer. The only significant impurity remaining in the thorium a t this point was microgram amounts of lanthanum from the final thenoyltrifluoroacetone extraction. This could have been reduced by backwashing the benzene-thenoyltrifluoroacetone layer with 0.01J4 nitric acid if desired. The lanthanum did not interfere in subsequent mass spectrometric analysis of the thorium in the untreated solution. The process yielded 750 y of thorium with a thorium-230/thorium-232 ratio of 3.9 representing a chemical yield of about 20%. The bulk of the loss in yield occurred in the early stages of the procedure. The yield of thorium through the various steps was conveniently followed by measuring the 24-day thorium-234-protactinium-234 beta activity of aliquots of the solutions. T o standardize the thorium-230 carrier solution, a 100-microliter aliquot of a solution of normal thorium, thorium-232 only, containing 34.96 y of thorium per milliliter, was equilibrated with 100 microliters of the carrier solution. The thorium-230/ thorium-232 mole ratio of the pure carrier mas 3.858 with a standard error of 10.011 and the same ratio for the aliquot of carrier plus 3.496 y of shelf thorium was 0.9189 i 0.0030. From this the thorium-232 content of the carrier required for future use was calculated t o be 10.93 XI= 0.05 y per milliliter. The total thorium concentration was 52.7 y per ml. For subsequent analytical work 100-microliter aliquots of carrier solution containing about 5 y of thorium were used. CHEMICAL PROCEDURES
Chemical processing needs to be quantitative only during the dissolution of the sample and its equilibration with the isotopically altered carrier. Beyond this point the result will depend only upon the new isotopic ratio(s) produced in the carrier by the sample rather than upon the degree of recovery of the element being determined. This is strictly true onlr if contamination introduced in processing is less than a few per cent of the amount of the analyzed element contributed by the sample. When contamination is high the result depends on the assumptions that the chemical vields of both sample and blank are the same and that the contamination introduced into the sample and blank is the same. The last assumption will be true if contamination is coming from reagents, but may not be true when i t is coming from dust or glassware. It is usually possible to select a sample which contains sufficient thorium to eliminate the influence of contamination on the result. I n a determination of microgram amounts of an element i t is a tremendous advantage to the chemist to be freed from the necessity for quantitative recoveries throughout the procedure. A further advantage of isotope dilution is the fact that the chemical separation of the element to be determined need not be complete since the mass spectrometer will discriminate bet\+een impurities of a different mass. Even when an impurity peak falls on the same mass unit as the element under investigation, differences in volatility will often permit the impurity to be distilled off before or after the determination is made. A snecific
895 example of this occurs in strontium determinations. Varying amounts of rubidium are almost always present which will contribute an impurity to the strontium-87 peak. I n actual practice it is possible to distill off the rubidium in the early stages of the run and to obtain finally pure strontium peaks for analysis. I n the case of the thorium determinations discussed here it is not necessary to achieve complete separation from zirconium, cerium, or uranium as is usually required in analvtical procedures for the element. The exact amount of impurities that can be tolerated in an analysis varies with the emitting characteristics of the impurities. I n general the total amount of impurities must be reduced to an amount of the same order of magnitude as the element which is being analyzed, otherwise i t is usually found that peak intensities will vary somewhat erratically over the course of the run and reduce the accuracy of the ratio measurement. Procedures for Thorium Analyses. Once the sample was completely dissolved, an aliquot of a solution of the thorium-230 carrier was equilibrated with the solution containing the sample. From this point three methods were found useful for the separation and purification of thorium: ( a ) extraction a t a pH of 2 into a 0.1M solution of thenoyltrifluoroacetone in benzene, ( b ) extraction from a nearly saturated solution of aluminum nitrate into methylisobutylketone (hexane), and (c) precipitation of thorium on lanthanum oxalate carrier. The thenoyltrifluoroacetone extraction was adapted from the studies of Hageman (4). For the extraction from aluminum nitrate hexane s a s substituted for the mesityl oxide used by Levine and Grimaldi ( 7 ) because hexone vias much less susceptible to oxidation with the attendant introduction of carbonate during subsequent evaporation of the aqueous extracts. Radioactive tracer studies showed that about 90% of the thorium a-as extracted in each pass by extracting a solution containing 800 grams of aluminum nitrate enneahydrate in 75 ml. of concentrated nitric acid and 425 ml. of water with an equal volume of hexane. The procedures described below have been designed to give high, though not necessarily quantitative recoveries of thorium with a minimum expenditure of time in processing. Consequently precipitates were centrifuged rather than filtered, and the extraction procedures were not quantitative although this would have been possible if desired. Only the procedure for the determination of thorium in zircon gives any reasonable separation of thorium from uranium. Trial runs using the 24-day thorium-23~-protactinium-234 beta activity as tracer indicated chemical yields of 65 to 80% for the procedures. Blanks were run with all analyses using a few milligrams of iron(II1) added as ferric nitrate to carrv the thorium-230 carrier through the initial precipitations with ammonium hydroxide. The borosilicate glassware was cleaned by immersion in hot 10% sodium hydroxide for 5 to 10 minutes, followed by a rinse with distilled water and a final immersion for a similar period in hot 8M nitric acid followed by thorough rinsing with redistilled water. All cleaned glassware was wrapped or sealed with the aemiadhesive Parafilm (Marathon Paper Corp., Menasha, Wis.) when not in actual use. Determinations have been made on a granite from the Essonville, Ontario, region of the Canadian Shield and on the minerals zircon, sphene, and perthite separated from the same rock by E. S. Larsen of the U. S. Geological Survey. The procedure for the perthite determination is given in detail and the remaining procedures are outlined. Analysis of Perthite Feldspar. REAGENTS.Laboratory distilled water was redistilled in borosilicate glass. Sitric and perchloric acids Rere made by redistilling the reagent grade acids a t reduced pressure in a 20-inch quartz Vigreux column. (This precaution would probably have been unnecessary in the present work.) Reagent grades of hydrofluoric acid and borax were used without further purification. Thenoyltrifluoroacetone crvstal9 xere dissolved in reagent grade benzene to form a 0 . M solution and extracted once with 2M nitric acid before use. Hexone was extracted once with 1% nitric acid and twice with water before use. Ammonia gas was used in place of bottled ammonium hydroxide to avoid the introduction of silica or carbonate ion. The aluminum nitrate reagent was prepared by
696
ANALYTICAL CHEMISTRY
dissolving 800 grams of alunlinum nitrate enneahydrate in 75 nil. of concentrated nitric acid and 425 nil. of water. PROCEDURE.Weigh approximately five grams of 60 to 100mesh sample to the nearest milligram in a 250-ml. platinum dish. Add 50 grams of hydrofluoric acid and heat on a hot plate at, lorn temperature to a volume of 25 ml. Bdd 25 ml. of perchloric acid and fume 15 minutes with the hot plate a t medium temperature. Add 1 gram of borax in 10 ml. of water and 15 ml. of perchloric acid and fume to a moist paste a t medium heat of the hot plate. Experience has shown t'he addition of borax to aid greatly in dissolving the calcium usually present in a feldspar. Bdd 1.50 ml. of water and digest for 15 minutes. S t this point add the aliquot of the thorium-230 carrier solution. Dilute the solution to 1000 ml. and make three ammonium hydroxide precipitations from this volume by bubbling in ammonia gas from a lecture bottle with a fribted glass filter in the line. Dissolve the first and second hydroxide precipitates in 5 ml. of concentrated nitric acid. This is necessary to prevent later insolubility of potassium perchlorate. Dissolve the final hydroxide precipitate in 50 nil. of concentrated nitric acid and evaporate the solution to a volume of 15 ml. Add ammonia gas t,o bring the pH to 2 (adding wat,er to dissolve any nitrates that may separate) and then add 10 ml. of the aluminum nitrate-nitric acid reagent. Extract the thorium (together with uranium) into two 30-ml. portions of hexone. shaking 1 to 2 minutes per pass. Strip the t'horium and uranium from the hesoiie with two 10-nil. portions of water.
Allow the precipitate to stand overnight. Reserve the lanthanum oxalat,e precipitate and repeat the precipitation once more. Destroy the oxalate by adding a few milliliters of nitric acid and a few drops of perchloric acid and heating just t.o dryness. Recover the thorium by a thenoyltrifluoroacetone extraction followed by a backwash as described above for perthite. Analysis of Sphene. Fuse about, 50 mg. of the mineral in borax and digest the bead in 3M hydrochloric acid. Reserve the resulting solution and treat the small precipitate with a few milliliters of perchloric acid and 1 ml. of hydrofluoric acid. After fuming for 5 minutes, combine the two solutions. S d d the thorium-230 carrier and make an hydroxide precipitation with ammonia gas. Remove t'he silica by perchloric acid dehydration. Make a second hydroxide precipitation, dissolve the precipitate in a small amount of nitric acid, and add the aluminum nitrate reagent. Extract the thorium into hexone and strip it from the hexone with water. Finally make a thenoylt'rifluoroacetone extraction as described previously for perthite. Analysis of Granite Composite. Treat 6 grams of sample ground to pass 40 mesh for destruction of silica by the hydrofluoric acid-perchloric acid-borax met,hod described for perthite. A small residue consisting of the zircon and possibly other heavr minerals remains. Collect the residue and fuse it in borax ih the manner described for zircon. Add the thorium-230 carrier to the desired aliquot of the combined solutions. Make two hydroside precipitations with ammonia gas and purify the thorium by an extraction into hexone from aluminum nitrate sohtion followed by a thenoyltrifluoroacetone extraction. I n earlier uranium studies of the same granite, repeat determinations on I-gram samples showed that the sampling of the accessory minerals, which are present in abundance8 of 0.01 to 0.1 yo and contain most of the uranium and thorium of the rock, was not representative. Consequentl5- a larger sample was used as thc starting material. 1 3 STRUMEVT4TIOY
The zircon analyses nere perfoinied on the 12-inch, 60-(leglee high resolution mass spectrometer a t Argonne National Laboratory designed by Inghram, Hayden, and Hess. The instrument was equipped with an electron multiplier giving an increaqe in intensity of the order of lo4. Using the multiplier and surface ionization techniques I y of thorium \I as more than suficient for a satisfactory analysis B hile 25 to 50 7 would have been required nithout the multiplier. The remaining analyses were made at the Department of Terrestrial Magnetism on a 6-inch, 60-degree Sier-type instrument with an electron multiplier, which again permitted isotopic analysis of 1 y of thorium. The thorium samples were evaporated on tantalum ribbons as thorium nitrate from a 0.1111 nitric acid solution. The ribbons uqed in the present n-ork were approvimately 1 em. long and 0.001 X 0.025 inch in cross section. ..\t about 2000" C. emission of T h o + occurred which n a s used for the analyses. 230
Th230
232
CARRIER
230
232
+
1.269 9. G R A N I T E
Figure 1.
RESULTS
57' Th230 C A R R I E R
Thorium Determination
Concentrate to a volume of 15 ml. and adjust the pH to 2 using a narrow range p H paper. Extract the thorium (together with most of the uranium) into 25 ml. of a 0.1M solution of thenoyltrifluoroacetone in benzene. Backwash the thenoyltrifluoroacetone-benzene layer once with 25 ml. of 0.01M nitric acid. Finally extract the thorium into two 10-ml. portions of 2.11 nitric acid and concentrate just to dryness in a 5-ml. beaker. At the time the mass spectrometric analysis is to be made, diwolve the minuscule residue in 50 microliters of 0 . l N nitric acid. Evaporate half of this solution on a tantalum ribbon and load into the instrument. Analysis of Zircon. Fuse about 10 mg. of the mineral in borax (8) and digest the resulting bead in 3151 hydrochloric acid. Add the thorium-230 carrier and make an hydroxide precipitation with ammonia gas. Remove the silica by dehydration with grchloric acid, following the method of Willard and Cake (16). adioactive tracer studies on trial runs showed that less than 1?To of the thorium is carried on the silica precipitate obtained by perchloric acid dehydration in both the zircon and sphene procedures. Precipitate the hydroxides again by addition of ammonia gas. Dissolve the precipitate in nitric acid, bring the p H t o 1, add 1 mg. of lanthanum(II1) and precipitate lanthanum oxalate, by adding a saturated fiolution of oxalic acid to the hot solution.
.I thorium determination required measurements of the thoriuni-230/thorium-232 ratios of the pure carrier, the carrier plus sample, and a blank. -4lgebraic considerations lead to the folloniiig equation for calculation of the thorium content of a sample if contamination can be ignored:
\!-hew
P = Parts per million of thorium in t h e sample by weight R, = Thorium-23O/thorium-2:32 of the carrier R, = Thorium-230/thorium-232 of the carrier plus sample Th, = Thorium-232 in niicrogianis contained in the aliquot of carrier added io = Keight of sample taken for analysis in grams From this equation i t is seen that if both R, and R, are measured on the same instrument and under the same operating conditiong, P will be independent of any systematic bias on the part of the instrument. For this reason the zircon analyses done a t Argonne and the remaining analyses done a t the Department of Terrestrial Magnetism may be compared directly. (Th, is derived from a similar equation which is independent of instrumental bias.) I t
V O L U M E 26, NO. 5, M A Y 1 9 5 4
897
-______
were the same as those already described for the mass spectrometric determinaWeight of Thorium Content, tions except t h a t in the case of zircon Sample, Grams Re RZ Thc P. P. 11. the lanthanum oxalate precipitates were 1 269 3 858 + 0 011 0 07770 zt 0 00055 1 . 0 9 3 5 0.006 41 88 zt 0 60 dissolved and precipitated a second time . _ ~ _ _ ~ _ _ to ensure the elimination of zirconium interference in the absorption reading. Table 11. Results of Thorium Determinations (Three micrograms of zirconium produce an absorbance equal to Thorium Content, P.P.M. about 1 y of thorium.) Preliminary experiments showed that the Percentage Chem. Mass Mineral of Total Rock Cornpoaition spectrometric Colorimetric zirconium carried on a single lanthanum oxalate precipitate was Zircon run I 2170 30 2180 equivalent to less than 3 y of thorium and t h a t two precipitaZircon r u n I1 0.04 ZrSiOc 2169 i 30 ... Zircon run I11 2195 zt 30 , . . tions vould thus reduce zirconium to a level below the limit of SDhene 0.4 CaTiSiOi 5375 +c 75 5550 detection. The precipitating conditions n-ere as follows: Perthite 52 ‘~ $ ~ ~ ! ! $ ~ 0 8 0,410 & 0.008 ... Coniposit~ 100 41.88 f 0 . 6 0 44 0 Twenty-five milliliters of solution containing approximately 50 mg. of zirconium and 2 mg. of lanthanum(II1) as lanthanum nitrate was adjusted t o a p H of 1. The solution was heated t o just below the boiling point and 10 nil. of a 10% solution of oxalic acid was added with stirring. Lanthanum oxalate separated id also apparent that either weight or mole ratios may be used so on cooling and the precipit,ate vas allowed t o stand for a t least 6 hours before being centrifuged. long as one is consistent. The thorium residues were reacted with the disodium salt of The data required for the calculation of the thorium content of 2-(2-hydroxy-3,6-disulfo-l-naphthylazo)benzenearsonicacid reathe composite sample are listed in Table I where mole ratios are gent in a manner identical t o that suggested by Thomason, Perry, given. Figure 1 shows actual traces for the ratios cited i n Table and Byerly (14). The solution from the thenoyltrifluoroacetone extraction was concentrated just t o dryness in a 5-ml. beaker I. The ratios quoted are based upon ten to twelve sets of ratios and moistened with 5 drops of concentrated hydrochloric acid. similar to those illustrated. The deviations are the calculated This solution was transferred irith the aid of additional water standard errors of the sets. The blank for this determination t o a 10-nil. volumetric flask containing 1.00 ml. of a 0.1% aqueous x a s 0.009 i 0.005 1. solution of the Thorin reagent. The contents of the flask were diluted t o the mark and the t r a n m k i o n was read on a Reckm:iri Blanks carried through the various analyses ranged from 0.01 to Model DC spectrophotometei,, using a spectral width of 2 nip about 0.03 y . Accordingly the blank assigned to all determinaand a wave length of 545 mp. tions was taken as 0.02 =k 0.01 y of thorium. Since in all cases except the perthite determination more than 40 y of thorium were DISCUSSION present in the sample analyzed, the blank had no effect on the The method of isotope dilution permits accurate determinations final result. For the perthite only 1.8 y of thorium were conof microgram and submicrogram quantities of thorium. Aptributed to the analysis bj- the sample and the effect of possible proximately 3 t,o 5 days of irorking time are required for each wmtamination is reflected in the slightly larger uncertainty determination. The technique will be of greatest value where reported for this analysis in Table 11. limited amounts of sample are available or where the thorium The uncertainty quoted for perthite is given only to cite the concentration of a sample is so low that the processing of the power of the method. Since the thorium in the mineral is problarge amounts of sample that noultl be required for conventional ably derived from small inclusions of other minerals [Picciotto methods of determination a.ould give rise to excessive contamina( I O ) ] ,a pecond analysis rvould almost certainly fall outside of the tion during the analytical procedure. limits giyen in Table 11. -4reasonable estimate of the actual A comparison with radiometric techniques for the determinathorium content of the mineral n.ith inclusions would be 0.4 & tion of thorium shows that the sensitivity of the mass spectro0.1 p.p.ni. metric method is considerably greater in the present work even When the blank could he ignored, the calculated error of an though the full sensitivity of the isotope dilution technique was individual determination based on the standard errors of the not exploited. Dalton e t al. ( 2 ) have recently reported refined ratio measurements i r a ~usually xkO.8 to 1.0%. K e have instudies on the radiometric determination of thorium and found creased this error to + 1.5% in order to be conservative and have scintillation counting of lead-212 plus bismuth-212 (ThB and t:tlieri this figure as the accuracy of a single determination, The ThC) to have a much greater sensitivity than thoron gas count,ing. replicntcs tic~tc~rminatione on zircon shown in Table I1 demonstrate Calculations from their data indicate that 1 -/ of thorium could be that the resulk fall n-ell within the assigned probable error of a measured with a standard error of about ~!=15’%by scintillation single clctermination. Unpublished data of G. I,. Davis and counting. This is about ten times the standard error of the I.. T. .il
