occur gradually over a long period of use was found to cause an upward shift in the ratio data of as much as 0.07 percent.

The high temperature drying was accomplished using a pyrometer to adjust the filament temperature to 860 °C. Since this temperature is at the low end of the pyrometer range, this stage of the drying had to be performed in a room where near darkness could be obtained. The use of the pyrometer for temperature control was believed to be a key factor in obtaining a highly precise isotopic ratio measurement. The dependency of the 205T1/203T1 ratio on the filament temperature during the high temperature drying phase is shown in figure 1.

The drying procedure was as follows: The thallium solution was dried on the filament using currents of 1A for 10 min, and 3A for 5 min. and a heat lamp. The filament was then transferred to a darkened room. A Class 100 clean air hood (airflow 15 linear m/s) was set up over a drying box, which was designed for manual current adjustment, and the filament was mounted at a 45° angle to allow a better view of the filament surface. Using an optical pyrometer for temperature adjustment, the filament was heated at 860 °C for 1 min., producing a darkened filament containing a thin line of tungsten oxide along each edge.

After loading the sample into the source of the mass sper trometer the system was allowed to pump down to a presure of 2 × 10 torr before starting the analysis. Liqui nitrogen was then added to a source cold finger which further reduced the pressure to less than 1× 10 torr. Becaus the thallium ionizes at a very low temperature (about 700 °C) a pyrometer cannot be used to precisely set the filamer: temperature during the analysis, so instead, the filament current was increased until the intensity of the 205 Tl peas was approximately 2 mV (1011 Q resistor). Through focusing of the ion beam and gradual increase of the filament cur rent, the intensity of the 205Tl peak was increased to 100 m by 5 min. At t = 5 min., the intensity of the 205 Tl was in creased to 250 mV. The signal was then allowed to grow until it reached 10 V. If the signal required less than 912 min or greater than 13 min. to reach 10 V, the run was aborte. before any data was taken since, under these conditions data which was high by 0.05 percent had been observed. Upon reaching 10 V, the signal was reduced to 3 V and al lowed to grow to 7 V which generally required 2 to 3 min. The signal intensity was then reduced to 2.5 V. After 2 min.. the signal intensity was increased to 10 V and allowed to grow to 30 V over a period of 1 min. The signal intensity of the 205 Tl was then decreased to 2.5 V. At t = 30 min the ior beam was focused, and at t = 35 min. the rati measurements were begun. Ratio sets are taken at t = 35 40, 48, and 53 min. Each of the ratio sets described above consisted of five ratio pairs of data taken over a period of 5 min. The computer was programmed to delay 8 s after switching peaks and then to take 15 intensity measuremen's (one/s) on top of the peak before switching peaks again. At t = 45 min. additional baseline data were taken to ascertair that no baseline shifts had occurred during the measurement of the first two ratio sets. The average of the four ratio sets was recorded as the 205T1/203T isotopic abundance ratio of the sample.

The rate of isotopic fractionation during the 25 min. over which ratio data was measured is very small, generally or the order of two to three parts in ten thousand. Even when the precision within an analysis is very high, the difference between successive analyses may be very large (approx mately 0.3%). The traditional method of minimizing be tween run differences is strict parameter control. The procedure described herein was designed and tested to yield a high degree of internal and external precision; however, inconsistencies will result unless all parameters are rigidly controlled.

The data obtained during the analysis of some thallium minerals and high purity materials indicated that silica contamination could affect the observed isotopic ratio. The examination of a solution of Th2CO, which had been stored in a borosilicate glass flask for over a year, yielded 205T]/203T ratios of 2.380 consistently until it was treated with HF.

After this treatment thallium ratios of approximately 2.382 were observed. This effect was not totally surprising since the silica gel technique for thallium had consistently yielded ratios 0.2 percent lower than the tungsten filament procedure. The addition of silica gel to the thallium on the tungsten filament yielded ratios of approximately 2.378.

The presence of the commonly found impurities sodium, potassium, and silicon were shown to have a detrimental effect upon the isotopic ratio measurement of thallium. Therefore, great care was taken to ensure that these impurities were not present in sufficient quantity to affect the ratio measurements.

Large amounts (1 μg) of sodium or potassium will shift the observed 205T1/203T1 ratio to a higher value. However, the filament current required to ionize and volatilize the sample is much higher than normal (2.2A) or approximately 825 °C. In addition, the signal growth is abnormally sluggish. It is very important to monitor the filament current in this case because large sodium and potassium beams have been observed to sputter thallium off the source causing a dramatic memory effect, especially on small samples. If 1 μg each of sodium and potassium is loaded onto a tungsten filament and dried without thallium, peaks will still be seen at masses 203 and 205. The ratios of these peaks were found to reflect the composition of the samples which had been analyzed since the last source cleaning.

2.2 Purification of the Separated Isotopes

Electromagnetically separated 203T1 and 205T isotopes were obtained from the Isotopes Division, Oak Ridge National Laboratory of the Union Carbide Nuclear Company. The 203T1 isotope was received in a sealed ampoule in the form of thallium metal, and the 205 Tl isotope was received in the form of thallous oxide. The 203T was designated as series R and D, sample 000101 and the 205 Tl was designated as series 152102.

Included with each isotope was a certificate of analysis which contained a statement of isotopic purity as well as a semiquantitative spectrographic analysis. The chemical analysis indicated that most elemental impurities could be present at levels up to 0.1 percent and that silicon, which could interfere with the mass spectrometric analysis of thallium, was present at a level of 0.08 percent. The method used for the assay of the thallium separated isotope solutions depended upon the quantitative precipitation of thallium chromate, and thus a purification procedure was developed to reduce the levels of lead, barium, silver, zinc, copper, bismuth, and mercury which form insoluble chro

mates.

The techniques of solvent extraction, electrodeposition, and fusion under hydrogen gas were utilized to purify the thallium separated isotopes. Each isotope (about 1 g) was

transferred to a covered Teflon beaker. Twenty grams of aqua regia were added to dissolve and oxidize the thallium to the trivalent oxidation state. After all of the thallium was dissolved, indicating that the oxidation was complete, the cover and the sides of the beaker were rinsed with about 5 ml of water and the solution was evaporated to dryness at approximately 80 °C to avoid reduction of the thallium to the more stable univalent oxidation state. Eleven grams of concentrated (9 M) HBr were added followed by dilution to 100 g with water. The thallium was extracted into two 50mL portions of methyl isobutyl ketone (MIBK), and the organic layer was washed twice with 25 mL portions of IN HBr. Since the thallium could not be quantitatively back extracted into an aqueous solution, the MIBK layer was transferred to a 150 mL quartz beaker and evaporated to dryness (quartz was used because of the eventual necessity of evaporating sulfuric acid). The residual organic material was destroyed by digestion with a mixture of 20 g of concentrated HNO3, 5 g concentrated H2SO4, and 5 g concentrated HCIO4. Overnight digestion yielded a clear solution which was evaporated to dryness.

The thallium was reduced to the univalent state with H2SO,. Twenty grams of H2SO, (V/V, 2 +98) was added to dissolve the residue and H2SO, was added until the odor of SO2 could be detected. The solution was then evaporated to dryness.

The residue was taken up in 100 g of 0.05N HClO4, and the thallium was plated anodically as Tl2O3 onto a large platinum gauze electrode using a single 0.75 mm platinum wire as the cathode at 2.0 V.

When the anodic deposition was complete, the T20, was stripped from the anode using concentrated HNO3; 2 g of HCIO4 were added, and the solution was evaporated to dryness. Twenty grams of H2SO4 (V/V, 2 + 98) were added to the residue, the thallium was reduced with 4 mL H2SO,, and the solution was evaporated only to fumes of H2SO4. The solution was diluted to 20 g with water and thallium was plated cathodically onto high purity platinum wire. The metal was collected at intervals and stored under water to prevent air oxidation.

As a final step, the metal was kneaded into a lump under water and transferred to a tared quartz boat. The thallium metal was fused at 350-400 °C in a tube furnace under a flow of hydrogen gas for 2 h according to Brauer [39]. After cooling, the boat and the metal were weighed.

2.3 Preparation and Analysis of the Separated Isotope Solutions

The purified thallium isotopes, in the metallic form, were dissolved in 30 g of 8N HNO3 and transferred to 200 mL quartz flasks. The solutions were diluted with water to yield a concentration of approximately 0.024 mmol thallium per

gram of solution for the 203Tl isotope and 0.020 mmol per gram for the 205Tl isotope. The solution of 203Tl was desig. nated "Tl 203" and the solution of 205 Tl as "TI 205". Samples of the "Tl 203" and "Tl 205" were withdrawn for the analysis of impurities by isotope dilution spark source mass spectrometry (IDSSMS). Each sample was spiked with 3 × 10 g of each of the following separated isotopes: 206 Pb, 144Nd, 137 Ba, 125Te, 123Sb, 117Sn, 113In, Cd, 108 Pd, 107 Ag, 97 Mo, 91Zr, 86Sr, 69Ga, 64Cu, 58Ni, 54Fe, 52 Cr, 47Ti, 40Ca, 39K, 24Mg. The solution was blended to ensure equilibration of the spikes with the elements present in the sample, and the solution was evaporated to dryness. The thallium was oxidized with aqua regia and the solution was evaporated to dryness. Two grams of concentrated HBr were added to the sample, and following dissolution, water was added to increase the volume to 10 mL. The thallium was extracted into 10 mL of MIBK. The aqueous layer was drawn off and reserved, and the organic layer was washed with 10 g of IN HBr. The aqueous fractions containing the impurities and separated isotopes were combined and evaporated to dry ness for analysis by IDSSMS. Additional aliquots containing 2 mg of thallium were taken for the analysis of mercury and bismuth by atomic absorption spectrometry.

Table 3 shows the results of these analyses as well as that of a sample of doped thallium which had been purified by the same method used for the separated isotopes. This sample of pure thallium metal had been doped with 0.1 percent of 26 elements including Ag, Bi, Ba, Cu, Hg, and Pb to determine the efficiencies of the purification procedure.

The only element that was detected at a level high enough to affect the thallium chromate assay procedure was lead at 80 ppm in the "Tl 203" solution. Two additional aliquots of the "TI 203" were withdrawn and analyzed for lead by isotope dilution mass spectrometry [40]. Because lead chromate is much more insoluble than thallium chromate, a correction was applied to the "TI 203" solution or the basis of quantitative precipitation of the lead.

2.4 Assay of the Separated Isotope Solution

The quartz flasks containing the purified separated isotopes were vigorously shaken to ensure thorough mixing of the solutions. Four weighed aliquots of about 30 g for the "TI 203" solution and 37 g for the "Tl 205" solution, each containing approximately 0.7 mmol of thallium, were with drawn by the following method. The polyethylene stopper which had been used to seal each flask was removed and replaced with a prepunctured stopper. An 18 gauge, 16 cm long platinum needle with a Kel-F hub was inserted into the solution. A 20-mL polyethylene hypodermic syringe was at tached to the Kel-F hub of the needle. The plunger of the syringe had been covered with a 130 μm thick skived Teflon tape to prevent contamination. After withdrawing the desired amount of solution the syringe was disconnected from the hub and the tip was capped with a Kel-F hub. Any static charge that might be present on the plastic syringe was dissipated by wiping it with a damp lint-free cloth, and the syringe and contents were weighed on a semimicrobalance to± 0.02 mg. The solution was then delivered to a 400-mL Teflon beaker and the syringe was again capped, wiped. and weighed. The weight of the sample was determined from the weights of the syringe before and after delivery of the sample. Since 30-mL or more of each solution were required to produce a 0.7 mmol sample, two loadings of the 20-mL syringe were weighed for each sample.

The aliquoting procedure involved the withdrawal of samples for the determination of isotopic composition, for assay of the solution concentration, and for the preparation of calibration standards. The entire process was carried out within five hours and the aliquoting pattern shown in table 4 was followed to ensure that the concentration and isotopic composition of the separated isotope solutions did not change during aliquoting. This process was carried out first for the Tl 203" solution and then for the "Tl 205" solu tion. Nearly twice as much "Tl 205" was to be added to the mixes, and therefore the target ratio would be more easily achieved with the addition of a larger quantity of solution.

Each sample was assayed as follows: Ten grams of H2SO. (V/V, 2 +98) were added to each beaker and the solutions were evaporated until fumes of H2SO4 were observed. The solutions were cooled, diluted to 10 g with water, 4 mL of H2SO3 were added to each, and the solutions were again

evaporated until fumes of H2SO4 were visible. The solutions were diluted to 60 g with water, 1 mL of 10 percent K2CO3 was added to each, and the solutions were digested for 2 h. Glass stirring rods were placed in each beaker and the thallium was precipitated by adding 2 g of concentrated NH4OH, followed by the dropwise addition of 1 g of 10 percent K2CrO4 to each with constant stirring.

The solutions were allowed to stand at room temperature for approximately 18 h. Each solution was then filtered through a tared 15-mL fine fritted glass crucible. The filtrate containing the soluble thallium was collected in a 100mL Teflon beaker. After all the solution had been filtered, the T2CrO4 precipitate was washed three times with approximately 30-mL of 50 percent (v/v) ethanol-water mixture. The precipitate was dried for 2 h. at 125 °C and reweighed. Further drying at 125 °C yielded no change in weight of the Tl2CrO4 precipitate.

The crucibles were weighed to ± 0.002 mg on a microbalance. To eliminate any errors due to static charge, the crucibles and tares were reweighed cyclically until the reproducibility was within ± 0.005 mg. A buoyancy correction for the glass crucibles was made by averaging the change in weight of two empty tare crucibles. The air weight of the Th2CrO4 was converted to vacuum weight using a measured value of 6.983 as the density of the precipitate at 22 °C. The millimoles of thallium present in the precipitate were determined using the calculated atomic weight for thallium and the 1975 atomic weight values for chromium and oxygen. The formula weights used were for 203T12CrO4 and for 205Tl2CrO4.

After filtration of Th2CrO4 was complete, the soluble portion and washings were returned to the original 400 mL beaker and evaporated to a volume of approximately 10 mL. The solutions were made acidic with concentrated HNO, (color change from yellow to orange) and a small amount of ethanol was added to reduce Cr* to Cr+3. The solutions were transferred to weighed polyethylene bottles, diluted to 80 to 100 g and aliquoted. The aliquots were spiked by weight with 203T1, and the resulting solutions were evaporated to dryness. One gram of aqua regia was added to oxidize the thallium and, after evaporation, the residues. were taken up in IN HBr. The thallium, as HT1C14, was extracted into methyl isobutyl ketone (MIBK) and evaporated

This method of determining the concentration of thallium solutions was previously tested on solutions containing a known amount of "natural" thallium. A thallium master solution was prepared from high purity (99.99%) thallium metal (SRM 997) and seven sets of four samples were withdrawn from this master solution, each on a different day over a period of one month. In addition, one more set of four was determined just before the assay work was begun on the separated isotope solutions. This extra set allowed the analyst to be certain that the experimental conditions were still under control. The final set which was completed 11 months after the first set was assayed, showed no evidence of any bias. The uncertainty (ts) of 31 individual determinations is 0.029 percent and the ts of the set averages is 0.014 percent. Comparison of the calculated and measured concentrations indicated a positive bias of 0.028 percent which would have a negligible effect on the ratios.

Pooling the results of the analysis of the separated isotope solutions shown in table 5 with the results of the eight sets described above, yields a value of ± 0.0000030 mmol Tl/g solution for the standard deviation of an individual determination (7 deg of freedom). The standard error of the average of four determinations is ± 0.0000015 mmol Tl/g solution.

' Student T test at a 95 percent confidence limit.

2.5 Isotopic Analyses of the Separated Isotope Solutions

Each of the separated isotope solutions were analyzed four times on each of two mass spectrometers (#1 and #4) by Operators 1 and 2. The two aliquots of each isotope were analyzed in alternate fashion and no difference was seen between the solutions taken before and after the preparation of the calibration mixes. The mass spectrometer sources were cleaned between the analyses of the 203Tl and 205Tl as a precaution against the possibility of contamination from source parts, although back to back analyses of the two separated isotopes on the same source failed to yield any evidence of contamination. The corrected isotopic compositions of the two isotopes are shown in table 6. Although the measured uncertainties were less than 0.1 percent, an uncertainty of 0.2 percent was used for the statistical analysis of the ratio determination. This increased uncertainty was used to take into account any possible biases and nonlinearities which might be encountered in the measurement of large ratios. The estimate of 0.2 percent was based on the mass spectrometric ratio measurements of uranium calibration standards.

TABLE 6. Isotopic composition of the thallium separated isotopes.

2.6 Preparation of Calibration Samples

Six calibration samples were prepared by mixing weighed portions of the "Tl 203" and "TI 205" solutions as described in sect. 2.4. The calibration standards were prepared so that three were higher and two were lower than the observed 205TI/203T1 ratio of the standard. In addition, a sixth standard was prepared to yield a 1:1 ratio. The weighed aliquots were delivered into 33 mL screw cap Teflon PFA bottles.

The absolute isotopic compositions of the calibration standards are shown in table 7. The isotopic ratio of each calibration sample was calculated from the isotopic analysis of the separated isotopes and thallium concentration of each separated isotope solution. Each calibration sample was thoroughly mixed and evaporated to dryness on a hotplate. The thallium was oxidized to the trivalent state with aqua regia and the solutions were evaporated to dryness at a low temperature (about 80 °C) followed by dilution with (1 + 9) HNO3 to produce a concentration of 1 mg of thalliun per gram of solution. Prior to the mass spectrometric ana lysis of the mixes, a 1:10 dilution was effected to produce solutions which were 100 μg/mL in thallium concentration.

2.7 Isotopic Analyses of the Calibration Mixes and the Standard Sample

Two complete sets of analyses of the calibration mixes and the standard samples were made (one by Operator 1 on Instrument #1 and one by Operator 2 on Instrument #4. Both analysts used the procedure outlined in section 2.1. Each set consisted of four analyses of each calibration mix and 24 analyses of the reference standard. The samples were run in a pattern alternating randomly selected mixes with the standard.