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JOU RNAL OF RE SEARC H of the National Bureau of Standards Vo l. 85. No. 1. January-February 1980
Absolute Isotopic Abundance and the Atomic Weight of a
Reference Sample of Thallium
L. P. Dunstan, J. W. Gramlich, I. L. Barnes,
National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
and
W. C. Purdy
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,
yie lding an atom ic weight of 204.38333 + 0. 000 18.
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. Introduction
Since 1962, th e Inorgani c Analytical R esea rch Di vi sion of
the National Bureau of Standards ha s been co ndu ct ing a
lo ng term program of absolute isotopic ab und a nce ratios
a nd a to mi c weight det e rmination s usin g the ma ss spectro-
metri c me thod. Previou s atomic we ight d et er min a tion s in-
clud e silver (lll, c hlorin e [2], co pp er [3 ], bromine [4],
c hromium [5], magnesium [6], lead [7], boron [8], rubidium
[9], rh e nium [10], silico n [11], pota ss ium [12], a nd stro n-
tium [13]. Th e pr ese nt work ex te nd s th e s tudy to thallium.
Th e d ete rmination of the ab so lut e isotopic abundance
and a tomic weight of any eleme nt to a high level of accuracy
re quir es the deve lopm e nt of highly pr ecise che mi ca l assay
1 Figures in brackets ind ica te li terature refere nces at the end of this paper.
a nd mass sp ec trom etri c procedur es. Th e mass spec trom e-
ters use d for th e isotop ic a bundan ce m eas ur e men ts a re ca l-
ib rated for bias by using syn thetic mixes of kn ow n isotopic
com pos ition, pre par ed from n ea rly pur e se p ar ated iso topes.
Exten sive r esea r ch [1 4] h as demonstrated that this bia s is
du e primarily to mass d epende nt iso topi c fractionation and
to a lesser d egree to no n-lin ear iti es in th e m eas ur e me nt cir-
cuit. The m eas ur ed bia ses are u sed to ca lc ulat e a ca libr a-
tion fac tor which is then ap pli ed to th e observed iso topi c
ratio to yield the abso lut e iso topic a bundan ce ratio of a
r efere nce s ample. The atomic weight of th e sa mple ca n th en
be ca lc ul a ted by s umming th e pr o du ct of the nuc lid ic
masses reported by Wap stra and Bo s [15] a nd th e cor re-
spo ndin g atom fr ac tion s of th e individual iso top es. F or
more general app li ca tion s it is necessa ry to es tabli sh th e
limits of variation in na tur e a nd high purit y co mm ercial
s ampl es.
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The first thallium atomic weight determination was pub-
lish ed in 1863 by Claude A. Lamy. [17] Since that time, at
least fifteen independent determinations have been re-
ported from either chemical or mass spectrometric data.
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.
- 84 BaSO.
--^ TICI = 1.
- 12 AgCI
--^ Til = 1.
1864 Werther [i 9) AgI
--^ TI =0.
1872 Crookes [20) TlN0 3
--^ 2TI^ , =0.
- 15 1893 LePierre [21) Tl,
^ 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).
Most of the chemical determinations of the atomic weight of
thallium were performed prior to 1934. In all cases, the
atomic weight was determined by ratioing the weight of
thallium or one of its compounds to an e quivalent weight of
another eleme nt or compound. The most common method
involved the conversion of a known amount of thallium
chloride to silver chloride. The weight of thallium chloride
was then ratioed to the weight of silver co nsumed, and the
atomic weight of thallium was calculated using the accepted
atomic weights of silver and ch lorin e.
An example of one of the more accurate chemical atomic
weight dete rminations is that of Baxter and Thoma s [16]. In
this experiment, thallous sulfate was recrystallized several
times and converted to the chloride. The chloride was then
recrystallized several times and prepared for weighing by
distillation in nitrogen followed by refusion in nitrogen. The
purified thallous chloride was weighed and dissolved in hot
water. After dissolution was complete, a nearly eq uival ent
amount of pur e silver was added to precipitate the free
chloride. The end point was determined nephelometrically
through the addition of hundredth normal solutions of sil-
ver and chloride.
A history of the chemically determined atomic weight of
thallium is given in table 1. The only determination afte r
1934 was that of Rodriques and Magdelena [26] in 1960.
This group employed high precision density determinations
of thallous chloride as the basis of their atomic weight
determination.
Table 2 lists the atomic weight de terminations based on
mass spectrometric measur e ments of the relative isotopic
abundance of natural thallium. A search of the literature
yielded only five published isotope abundance measure-
ments. The measur e ment made by White and Cameron [31]
is still listed as the best measurement from a single natural
so ur ce by the international Commission on Atomi c Weights
[27]. The presen t work marks the first tim e that calibrated
mass spectrometry has been used for a det e rmination of the
atomic weight of thallium.
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
ph ase is s hown in figure 1.
'"M
0
'"E
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 -
s ur e of 2 x W-^6 torr before s tarting th e analysis. Liquid
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
wa s approximately 2 m V {lO" Q r es istor}. Throu g h fo c us ing
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
by 5 min. At t = 5 min ., th e inten s ity of the lOsTI was in-
c r ease d to 250 m V. The s ignal was then allowed to grow un-
til it r eac hed 10 V. If th e sig nal re quir ed less than 9 1 /2 min.
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
th e 20sTI was th en d ec reas ed to 2.5 V. At t = 30 min th e ion
b ea m wa s fo c used , and at t = 35 min. th e ratio
m eas ur eme nt s were b eg un. Ratio se ts are tak e n at t = 35 ,
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
t = 45 min. a dditi o nal ba se lin e data we re tak e n to ascertain
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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Aft er thi s tr ea tm e nt th allium rat ios of approximately 2.
were obse rv ed. Thi s effect wa s not totally surpri s in g s in ce
th e i1i ca ge l tec hniqu e for th allium had co nsistently yi e ld ed
ra ti os 0.2 perce nt lower th an the tungst en fi lament pro ce-
dur e. Th e addition of s ili ca gel to th e thallium on th e tun g-
sten fi lamen t yield ed ra ti os of app ro ximat ely 2.378.
Th e prese n ce of th e co mm o nl y found impuriti es sodium ,
potassium, and sili co n were shown to have a detrim ental
effect upon th e isotopi c ratio meas urement of thallium.
Therefor e, great ca re wa s taken to ensure that th ese im -
purities were not present in sufficient quantit y to affec t th e
ratio measuremen ts.
Large amounts (l f1g)of so dium or potassium wi ll shift th e
observe d lOST 1/^103 T I ratio to a higher va lu e. Howeve r, th e
fi lament c urr ent requ ired to io ni ze and volatilize th e sa mple
is much higher th a n normal (2.2A) or approximately 825°C.
In addition, th e sig nal grow th is abno rm a ll y s lu ggish. It is
very important to monitor th e fil ame nt curren t in thi s case
becau se large sod ium and potassi um be am s have been o b-
served to s putt er th alliu m off th e source ca usin g a dramat ic
memo ry e ff ec t , es p ec ia ll y on sma ll samp les. If 1 !1 g eac h o f
so dium and potassium is loaded onto a tu ngsten fil am ent
and dri ed without thallium , pea ks will still be see n at masses
203 and 205. Th e ratios of th ese peaks we re fo und to re fl ec t
th e co mpo sition of th e sa mpl es whi ch had b ee n analyzed
s in ce th e la st so urce cleanin g.
2.2 Purification of the Separated Isotopes
Elec tromagne ti ca ll y separa ted 203 TI and 20s TI isotop es
were ob tain ed from th e Isotop es Di vision, Oak Rid ge Na-
ti onal La boratory of th e Uni on Ca rbide Nuclear Compan y.
Th e 2O)T I iso tope wa s rece iv ed in a sealed ampo ul e in th e
form of thallium metal, and th e 20s TI isotop e wa s receive d in
th e form of thall ous oxid e. Th e 203 TI wa s designat ed as
se ri es R and D, samp le 000101 and th e 2OS T! wa s d es ign ated
as se ri es 152102.
Includ ed with e ach iso tope wa s a ce rti fi ca te of a nalysis
whi ch co ntain ed a statement of iso top ic purit y as we ll as a
se miqu anti tati ve spec trogra phi c analys is. Th e che mi cal
analys is indi cated that mo st eleme nt al impuriti es co uld be
present at levels up to 0.1 percent and that s ili co n, whi ch
co uld interfere with the mass spec tr ome tri c anal ys is of thal-
lium , was present at a leve l of 0.08 per ce nt. Th e method
used fo r th e assay of th e th allium se par ated iso tope so lu -
ti on s depend ed upon th e quantitativ e precipita ti on of
th allium chromate, and thu s a purification proce dur e wa s
developed to redu ce th e levels of lead, bar ium , s ilv er, zinc ,
co pp er, bismuth, and merc ur y whi ch form in so lu ble chro-
mates.
Th e t ec hniqu es of so lv ent ex traction , el ec trod epos ition,
and fu sion und er hydro gen gas were utilized to purify the
th allium se parated isotopes. Each iso tope (about 1 g) wa s
tran sferred to a cove red Te fl on beake r. Tw enty grams of
a qua regia wer e added to di ssol ve and ox idi ze th e th a ll i um
to th e tri valen t ox id ation state. Aft er a ll of th e th alliu m wa s
di sso lved, indi ca ting that th e oxida ti on was co mpl ete, th e
co ver and th e s id es of th e beaker were rin se d with about 5
ml of water a nd the so luti on was eva porat ed to dr yness at
ap prox im ately 80°C to avoid reduction of th e thallium to
th e more stabl e unival ent oxidat ion state. Eleve n gram s of
con ce ntrat ed (9 M) HBr we re add ed follow ed by di lution to
100 g wi th wa ter. Th e thallium wa s extracted into two 50-
mL po rti ons of methyl iso butyl keton e (MIBK), and th e
orga ni c layer was was hed twi ce with 25 mL porti ons of I N
HBr. S in ce th e thallium co uld not be qu antitatively back ex -
tracted into an aq u eo us so lution, th e MIBK laye r was trans-
ferred to a l SO mL qu artz beaker and evapo rated to dryn ess
(qu art z wa s use d beca use of th e eve ntual necess ity of evapo -
rating sulfuri c ac id). Th e res idu al organ ic mate ri a l was
d es troyed by dig es ti on wi th a mixture of 20 g of co nce n-
trated H N0 3, 5 g con ce ntr ated H2S0 4 , and 5 g co nce ntrated
HC 10 4 • Ov erni ght digest ion yielded a cl ea r so luti o n whi ch
was evaporated to dryn ess.
Th e th alliu m wa s redu ce d to th e uni va lent state with
H2S0 3. Tw e nt y grams of H2S0 3 (V IV , 2 + 98) wa s added to
di sso l ve th e r es idue and H2S0 3 was add ed until th e odo r of
S0 2 co uld be de tected. Th e so luti on wa s th en eva porated to
dryn ess.
Th e residu e wa s taken up in 100 g of 0.05N HCI0 4 , and
th e th a ll ium was plated anod ica ll y as Tb0 3 onto a la rge
pla ti num ga uze el ec trode using a s in gle 0.75 mm pl a tinum
wire as th e ca th ode a t2 .0 V.
Wh en th e anodi c depositi on wa s co mp lete, th e T I20 ) was
stripped from th e anode us in g co nce ntrat ed HN0 3; 2 g of
HCI0 4 were added, and th e so lut ion wa s evapora ted to
dryness. Tw e nt y gram s of H 2 S0 4 (V I V, 2 + 98) were add ed
to th e residue, th e th allium wa s red uced with 4 mL H 2 S0 3,
and th e so lution wa s eva porated on ly to fum es of H2S0 4 •
Th e soluti on was dilut ed to 20 g wi th wa ter a nd th alliu m
wa s plated ca th od ica ll y onto hi gh purity pla ti num wire. Th e
meta l wa s co ll ected at interval s a nd stored und er water to
preven t air oxida ti on.
As a fina l ste p, th e metal wa s kn eaded into a lump und er
wat er and transferred to a ta red quartz boa t. Th e thalliu m
metal was fu se d at 350 -4 00 °C in a tub e furnace und er a
fl ow of hydr oge n gas for 2 h ac co rd in g to Br auer [39J. Aft er
coo lin g, th e boat and th e meta l were weigh ed.
2.3 Preparation and Analysis of the Separated Isotope Solutions
The purified thallium i so topes, in th e metallic form , were
dissolved in 30 g of 8N HNO) and transferred to 200 mL
quartz flasks. The solutions were dilut ed with water to yield
a co n ce ntr at io n of app rox im ately 0.024 mmol th allium per
..
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
was added to each, and the so lution s were digested for 112 h.
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
reproducibility was within ± 0.005 mg. A buo ya ncy correc-
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
(l + 49) for a nal ys is by thermal ionizati o n ma ss sp ec trom -
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'
- Th e standard error of th e ave rage is calc ulated to be 0.0000015 mmol Tl /g so ln. and th e uncertainty of the value of co ncen tration at the 95 per- ce nt confid en ce level is 0.0000030 mmol Tl / g so ln.
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
se ts d esc rib ed above, yields a value of ± 0.0000030 mmol
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
average of fo ur determinations is ± 0.0000015 mmol Tl / g
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
Each of th e se parated isotope solutions were analyzed
fo ur lim es on each of two mass spec trom et ers (# 1 and #4 ) by
Ope rator s 1 and 2. Th e tw o aliquots of eac h isotope were
analyzed in alternate fashion and no differenc e wa s seen
betwe en th e so lutions taken befo re and after th e prepara-
tion of the ca libra~ion mixes. Th e ma ss spec trom eter
so urc es were cl ean ed between th e analyses of th e 203T l and
20sTI as a precaution against the possibility of contamina-
tion from so urc e parts, although back to back analyses of
the two se parated isotope s on the sam e source failed to yield
any e vidence of contamination. The corrected isotopic com-
posi tions of th e two isotopes are shown in tabl e 6. Although
the measured uncertainties were le ss than 0.1 perce nt, an
un ce rtainty of 0.2 percent was u sed for the statistical analy-
sis of the ratio dete rmination. This increased uncertainty
wa s used to tak e into account any possible biases and non-
lin ea rities which might be e ncount e red in th e meas urem e nt
of l arge ratio s. The es timat e of 0.2 percen t wa s ba se d on th e
mass spectrometric ratio measur eme nts of uranium calibra-
tion standards.
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
Six calibration samples were prepared by mixing weighed
portions of the "Tl 203" and "Tl 205" so lution s as de-
scribed in sect. 2.4. Th e calibration standards were pre-
pared so that thre e were higher and two were lower than the
observed 2osTl/2 0 3 Tl ratio of the standard. In addi ti on, a
sixth standard was prepared to yield a 1:1 ratio. The
weighed aliquots were delivered into 33 mL screw cap
Teflon PF A bottles.
The absolute isotopic composi tions of the ca libr ati on
s tandards are shown in table 7. The isotopic ratio of each
ca libration sample was calculated from the isotopic analysis
of the separated isotopes and thallium concentration of
each separated isotope so lution. Each calibration sample
was thoroughly mixed and eva porated to dryness on a hot-
plate. The thallium was oxidized to the triv alen t state with
aqua regia and the solutions were evapo r ated to dryness a t
a low temperature (about 80°C) followed by dilution with
(1 + 9) HN0 3 to produce a concentration of 1 mg of thalliun
per gram of solution. Prior to the mass spectrome tric ana-
ly sis of the mixes, a 1:10 dilution was effected to produce
solutions which were 100 J.lg / mL in thallium concen tration.
2.7 Isotopic Analyses of the Calibration Mixes and the Standard Sample
Two comp lete 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 consi sted of four analyses of each ca libration mix
and 24 analyses of the reference s tandard. The samp les
were run in a pattern alternating randomly selected mixes
with the standard.
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.
I
,j
t-
~
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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