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This document details the process of determining the atomic weight of thallium using mass spectrometry. the history of chemical determinations of thallium's atomic weight and the challenges faced in previous measurements. It also explains the methodology used in the new mass spectrometric determination, including the importance of filament temperature and current control, and the impact of impurities on the measurement. The document concludes with the results of the analysis and acknowledgments.
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JOU RNAL OF RE SEARC H of the National Bureau of Standards Vo l. 85. No. 1. January-February 1980
National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
and
McGill University , Montreal, Quebec, Canada
August 8. 1979
T he accep t ed atom ic we ig ht of thalli um has re main e d at a va lu e of 204.37 ± 0.03 s in ce 19 62. At thi s le ve l of unc e rtaint y, h oweve r , t he at omic weig ht becomes a limiting fa ct or to hi g h accu racy analys is. Th e ne w ma ss sp ec tr ome tri c det ermina ti on of th e atomi c we ight of th allium ha s b ee n co m plete d. A hi g h precis i on assay t ec hn ique was devel oped so that accu ra te ly known quan titi es of the 'OJTI and '·' TI se parat ed iso top es co uld be mix ed to produ ce st andards f or ca libr at io n of th e mas s spec tr ometer. Thi s assay t ec hniqu e involved th e gravim e tri c deter m in atio n o f 99.3 per ce nt of th e th allium as TI ,C rO •. The so lubl e thallium was th en aliquoted and dete rmin ed by iso t ope dilution ma ss spec tr ome tr y. Befor e making up the fin al so luti ons from which th e assay and c alibr atio n sa mpl es would be w ithdr aw n , the separa t ed iso t opes wer e purifi ed by so lvent ex tr ac tio n a nd el ec tr odeposi tio n. A t un gs te n filament s urfac e i onizatio n tec hniqu e was deve lo ped for the dete rminati o n of pr ec ise iso t op ic abundance mea s ure me nts fo r thal li um. Thi s tec hn ique a ll owed isotopi c analysis of the se pal ated iso top es, ca li brat i on s tandard s, and a natur al th alli um re fer ence sta ndard with pr ec isio ns of be tt er than 0. 1 p erce nt. T he '·'T I/ 'OJ TI abso lute iso to pi c abu nd ance ra t io of the re feren ce sa mpl e was found to be 2.387 14 ± 0.00 I 0 1,
K ey words: Absolu te rat ios; at om ic weig ht ; iso t opic a bundan ce; re feren ce s tandard; thallium; tha lliu m ch ro m ate.
1 Figures in brackets ind ica te li terature refere nces at the end of this paper.
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TABLE I. Chemically determined atomic weights of thallium (C (^) O2 = 12).
Year Inv estigator Method AtomiWeight^ c
1863 Lamy [i 7) (^) --Tl,SO. =2. BaSO.
TlCI -- = 1.669 (^) 203. AgCI 1865 Hebb erling [i8) (^) --Tl,SO. =2.
--^ TICI = 1.
--^ Til = 1.
1864 Werther [i 9) AgI
--^ TI =0.
1872 Crookes [20) TlN0 3
--^ 2TI^ , =0.
^ TlO ' 3 = 0.
2TIN0 3
--^ 2TI =0. Tl,SO.
'^ TlO 3 = 0.
Tl,SO.
--^ Tl,03 =8. H,O 204.^29
--^ TICI = 1.
1894 We ll s and Penfield [22) AgCl TlCI -- =2.2232 204. Ag TICI
1922 Honigschmid, Bir ckenback, and Koth e [23) -- = 1.6733 (^) 204. AgCI TlBr -- =2.63539 204. 37 Ag
1930 Honigschmid and Striebel [24)
--^ TICI =2.
Ag
1931 Briscoe, Kikuchi, and P ee l [25) TICI --=2. 22336 204. 38 1933 Baxter and Thomas [16) Ag 1960 Rodriquez and Precision pycno· 204. Magdalena [26) metryofTICI Average (overall) 204. Average (since 1922) 204. Calculated using atomic weights recommended by the International Commission on Atomi c Weights (1975) [27).
TABLE 2. Calculation of the atomic weight of thallium from relative isotopic abundance measurements.
Year Investigator ,o5Tl^ /^ ,o3TI^ WeightAtomic a
1931 Schiiler and Keyston [28) 2.3 204. 1931 Aston [29) 2.4 (^) 204. 39 1938 Nier [30) 2. 44 204. 1948 White and Cameron [31) 2.394 204. 1949 Hibbs [32) 2.394 204. Average 204. a Calculated using nuclidic masses from Wapstra and Bos [25).
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occur gradually over a long pe riod of use was found to cause a n upward s hift in th e ratio data of as much as 0.07 per ce nt. Th e high temp e rature drying was accomplished using a p yro met er to adjust the fil a me nt te mp er a tur e to 860°C. Si nee thi s te mp e rature is at th e low e nd of the pyromet e r ra ng e, thi s s tag e of the drying had to be p e rform ed in a room whe re n ea r darknes s co uld be obtained. Th e use of th e pyrometer for te mperature co ntrol was believed to be a key fa c tor in obtaining a highl y pr ecise isotopi c ratio me asur e - m ent. Th e de pe nd en cy of the 2osTl j2 03TI ra tio o n th e fila - m e nt te mp e rature during the high te mp e ratur e drying
0
0 ~
2 389
2 388
2 387
2386
385
2 383
2 382 800
-"-.
820 840 860 880 900 Drying Temperature 1 0 () FIG URE l. Thallium 2051203 Ratio ve rsus Drying Temperature
Th e dr y ing procedur e was as follows: Th e thallium so lu- tion was dri ed on the filament us ing c urr e nt s of IA for 10 min. and 3A for 5 min. and a h ea t la mp. Th e filament was th en tran sfe rr ed to a dark en ed room. A Class 100 cle an air hood {airflow = 15 linear m / s} was se t up over a dr y ing box, which was design ed for manual c urrent adjustment, and the filam e nt was mount ed at a 45° angle to allow a be tt er view of th e filament s urfa ce. Using an optical pyrom e ter for te m- p era tur e adjustment, the filam e nt was h ea ted at 860°C for 1 min. , produ ci ng a dark e ned filament co nt a ining a thin Ii ne of tungsten oxide along eac h e dge.
After lo a ding th e sa mpl e into the sourc e of the mas s sp ec- tromet er th e system was allowed to pump down to a pr es -
nitr oge n was th en added to a so ur ce cold fing er which fur- th e r redu ced th e pressure to l ess th a n 1 x 10- 7 torr. B eca use th e thallium ionizes at a very low te mp e ratur e {about 7 00 °C} a pyr o me te r can not b e use d to preci se ly se t th e filament te mp e ratur e during th e analysis, so in st ea d, the filam e nt c urr e nt was in crease d until the int en s it y of th e 20sTI p ea k
of th e io n b ea m and gradual in crease of th e filam e nt cur- re nt, th e int e nsity of th e 20sTI p ea k was incr ease d to 100 m V
c r ease d to 250 m V. The s ignal was then allowed to grow un-
o r greater than 13 min. to r eac h 10 V , th e run was aborted befor e a ny dat a was tak en s inc e, und er th ese co ndition s data whi ch wa s high by 0.05 p erce nt had b ee n observed. Upon r eac hing 10 V, th e sign al was re du ce d to 3 V and al- lowed to grow to 7 V which generally re quir ed 2 to 3 min. Th e s ignal int en si ty wa s th en re duced to 2.5 V. Afte r 2 min ., th e s ignal int e nsit y was in c rea se d to 10 V and allowed to grow to 30 V over a period of 1 min. Th e sig nal int e nsity of
40, 48, and 53 min. Each of th e ratio se ts described above co nsis t ed of five ratio pair s of data tak en over a pe riod of 5 min. Th e co mput er was programm ed to d ela y 8 s after s witc hing p ea ks a nd th en to take 15 inten s ity mea s ur em e nt s (on e / s) on top of th e peak before s witc hing p ea ks again. At
that no ba selin e s hift s had occurred during th e mea s ur e- me nt of th e fir st two ratio sets. Th e average of th e four ratio se ts was reco rd ed as th e 2os Tl j2 03T\ i sotop ic a bundan ce ratio of th e sa m pie. Th e rate of iso topi c fr ac tionation during the 25 min. over whi ch ratio data was m eas ur ed is very small , generally on th e order of two to thre e parts in t en th o u sa nd. Even wh en th e pre cisio n within an analysis is very high, th e difference between success iv e analyses may be very larg e (approx i- mat ely 0.3 % ). Th e tr a diti onal method of minimizing be· tw ee n run diff e re nces is stri c t parameter co ntr o l. Th e pro- ce dur e d esc rib ed he re in was designed and tested to yield a high d eg ree of int e rnal and ex te rnal pr ecisio n; h owever, in - co nsis ten cies will result unl ess all parameters are rigidl y co ntroll ed. Th e data obtained during th e analysis of so me thallium min e ral s a nd high purity material s indicat ed th at sili ca co n- tamination co uld affect th e observed i sotop ic r a ti o. Th e ex - amination of a so lution of Tl2C0 3 which had been sto red in a borosilicate glass fla sk for over a yea r, yie ld ed 2Os T I/203 TI r at ios of 2.380 co nsistently until it was tr ea ted with HF.
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2.2 Purification of the Separated Isotopes
2.3 Preparation and Analysis of the Separated Isotope Solutions
..
TABLE 4. Aliq uoting procedure used for preparation of calibration standards.
Aliquot No.
2, 4 through 9 10, 11 12
Sam pl e Use Iso topic Composition Assay Mixe s 1- Assay Isotopic Compositio n
evaporated until fumes of H 2S0 4 were visib le. The so luti ons were diluted to 60 g with water, I mL of 10 percent K2C0 3
Glass s t irr ing rods were pla ced in eac h beaker and th e thall.ium was precipitated by adding 2 g of co n cent ra te d NH 4 0H, followed by the dropwi se addition of 1 g of 10 per· ce nt K2Cr04 to eac h with constant stirring. The so luti o ns were a ll owed to stand at r oom te mp e ratur e for app r oximately 18 h. Each so lu tion was then filt e red through a tared 15-mL fin e fritt ed g l ass cr ucibl e. Th e fil- trate co ntainin g th e solu ble thallium was co ll ec t ed in a 100- mL Teflon beaker. After a ll th e so luti on h ad been filtered, the TI,Cr04 precipitate was washed thre e tim es with ap- proximately 30-mL of 50 percent (v / v) ethano l-water mix- ture. The pr ecipitate was dried for 2 h. a t 125°C and re- weighed. Furth er drying at 125 °C yie ld ed no ch ange in weight of th e TbCr04 precipitat e. The crucibles were weig h ed to ± 0.002 mg on a microbal- ance. To eliminate any errors due to s tati c charge, th e c rucibles an d tares were reweighed cycl ically until the
tion for th e glass cr ucibl es was made by averaging th e c hang e in weight of two empty tar e c ru c ibl es. Th e air weight of the TJ,Cr04 was converted to vacuum weight us- ing a measured value of 6.983 as th e density of the precipi- tate at 22°C. The mil lim oles of thallium present in the pre- c ipit ate were det erm ined us ing th e calculated atomic weig ht for thallium and the 1975 atomic weight va lu es for c1uo- mium and oxygen. The formula weights used were for 203TI,Cr 04 and for 205TI,Cr04. After filtration of TI,Cr04 was complete, the so luble por- ti on and washings were returned to the origi nal 400 mL beaker and evaporated to a volume of approx im ate ly 10 mL. The solu tion s were made acid ic with concentrated HN0 3 (color c hang e from ye ll ow to o rang e) and a sma ll amo unt o f ethanol was a dd ed to reduce Cr+ 6 to Cr+3. The so luti ons were transferred to weighed polyethylene bo ttl es, diluted to 80 to 100 g and aliquoted. The a liqu ots were sp ik ed by weight with 20 3 TI, and th e r esu ltin g so lution s were evaporated to dryness. One gram of aqua regia was added to oxidize th e th alli um and, after evapora ti on, the r es idu es were tak en up in IN HBr. The thallium, as HT lC14, was ex - tracted into methyl isobutyl ketone (MIBK) and evapo rat ed
to dryn ess. The organic matt er was d es troy ed by dig es tion with a mixtur e of per chl oric and nitri c ac ids (1:5); and th e so luti on was eva porat e d to dryness. Th e purifi ed th a llium was dilut e d to a co nce ntration of 100 Jig / mL with HN0 3
e try. The thallium found in th e so lubl e portion (in mmol) was added to th e thallium d e te rmin ed g rav im etr i ca ll y to yie ld th e total thallium in th e sa mpl e. Th e r es ult s of thi s ana lysis are s hown in tabl e 5.
TABLE 5. Co nce ntrati on of thallium isotope solutions.
Total Conc.Soln. So ln. Sam pl e Wt. Soln. Thallium Tl / g Soln. No. (g) (mmol) (mm o l) TI203 1 30 .4 2998 0.731881 0. 2 30.579 11 .735270. 3 30.74880 .73955 1. 4 30.26530 .728017. Average 0.02405051 a TI205 37. 19243 0.755083 0. 2 37.09542 .753 11 5. 3 37.79003 .767268. 4 36.35 10 0 .738048. Average 0.02030275'
Thi s method of det e rminin g the conce ntrati on of thal- lium solut i ons was previously tes t ed on so lution s con ta inin g a kn ow n amount of "natura l" thallium. A thallium ma s ter so luti on was prepared from high purity (99.99 % ) thallium me tal (SRM 997) and se ve n se ts of four samp l es were with - drawn from this master solu tio n, e ach on a diff e r ent day over a pe riod of one month. In add iti on, on e mo re se t of four was det e rmined just before the assay work was begun on th e se parated isotope so luti ons. This extra se t a ll owed th e a na lys t to be ce rt a in th a t th e e xperimental con diti ons were still under contro l. The final se t which was comp le te d 11 month s afte r th e first set was assayed, sh owed no ev i- dence of any bi as. The un ce rt ai nt y (tsf o f 3 1 individual d e- te rmin a ti ons is 0.029 percent and th e ts of th e se t averages is 0.014 p er ce nt. Comparison o f the ca lc ulat e d and m eas- ur ed co n centrations indicated a po sitive bias of 0.028 per- cen t which would hav e a n eg ligibl e effec t on th e ratio s. Pooling th e r es ults of th e analysis o f th e sepa r a te d iso - t ope so luti ons s hown in tabl e 5 with th e results o f th e e ight
TI /g so lution for th e s tandard d ev iati o n of an individual d eter mination (7 d eg of freedom). Th e s tandard e rror of th e
so lution.
J Stude nt T test at a 9S percent co nfid ence limi t.
2.5 Isotopic Analyses of the Separated Isotope Solutions
TABLE 6. Isotopic composition of the thallium separated isotopes.
Separated I so topes
" TI203"
" TI205"
I so topic Composition (atom percent) 99.26333 ± 0.00140 a 0.73667 ± 0.
0.55758 ± O.OOOOla 99.44242 ± 0. a Bas ed on experience and results of MS work on uranium , the uncertain· ty of the ratio determinations is taken to be 0.2 percent, which is much larger than the calculated 95 per ce nt co nfid ence limit , to take into account bias and non ·lin ear behavior for ratios as large as th ese.
2.6 Preparation of Calibration Samples
2.7 Isotopic Analyses of the Calibration Mixes and the Standard Sample
TABLE 7. Thallium co mposition of calibration samples.
Sample I so tope Solution WI. Soln. (g) 203T^ I^ (mmol)^ 205TI^ (mmol)^ 205TI^ /^203 TI^ Ratio I TI203 1.21897 0.02910083 0. TI205 3.54447 0.00040125 0.07156125 2.
2 TI203 1.05205 0.02511590 0. TI205 2.97292 0.00033655 0.06002185 2.
3 TI203 1.09751 0.02620125 0.00019445 (^) 2. TI205 (^) 3.15388 0.00035703 0.
4 TI203 2.16573 0.05170319 0.00038371 (^) 1. TI205 2.56360^ 0.00029021^ 0.
5 TI203^ 1.08879^ 0.02599299^ 0.00019290^ 2. TI205 3.16857^ 0.00035870^ 0.
6 TI203^ 2.16484^ 0.05 168195^ 0.00038355^ 2. TI205 6.15241^ 0.00069648^ 0.
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TABLE 10. Summary calculations of the atomic weight of thallium.
Uncertainty Components
Mass Poss^ ibl^ e^ Poss ible Values Ov^ era^ ll^ limit^ spec trometric^ sys tematic^ error^ sys tematic of e rr aTa analytical in of^ co separated^ mp^ osition che e^ rr^ or mi^ cal in error (^) iso topes analysis
Atomic Weight = 204.38333 ±0 .000176 ±0 .000074 ±0 .000028 ±0. 000074
Nuclid ic Masses ('2C= l2) 203T! = 202.972336 (^) ±0. 205Tl = 204.974410 (^) ±0.
Atom Perce nt 203T! = 29.524 (^) ±0.0088 ±0 .0037 ±0.0014 ±0. 205T! = 70.476 (^) ±0.0088 ±0.0037 ±0.0014 ±0.
Isotopic Ratio 205T! / 203Tl = 2.38714 (^) ±0.00101 ±O .00042 ±0 .00017 ±O. a The overa ll limit of er ror is th e sum of the 95 percent co nfidence limits and th e terms co ve ring effects of known sources of poss ibl e syst ematic e rror.
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