The atomic weight of gallium

403 I~er~knal JournaI of Mass Spccrrometry and Ion PhJsia. 2I (1976)4Ow Q Ekcvier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE

J_ R.

ATOMIC

DE

LMXER

WEIGHT

ANI

Ii_ 3. R,

Deparznwx~ of Physicsz W-tern AustmIia] (First received 24

OF GALLIUM

ROSh¶AN

AustraIian Insrirufe of Technolos_v, South Bent&


June1975;in finalform 24 February1976)

ABSIRACT

The isotopic composition of gallium in two synthetic mixtures prepared from highly enriched 6gGa and 71Ga tracer have been measured by solid source mass spectrometry_ This has enabled the isotopic composition of gahium to be determined, independent of instrumental mass discrimination_ An atomic weight for gallium of 69_724+_0_003 has been calculated from the absoiute isotopic composition, and we would recommend the adoption of this value in place of the currently accepted value of 69_72+0_01_

The atomic weights of elements which possess more than one stabIe isotope can be dctermincd from a knowkdge of the isotopic composition of the eIement and the atomic masses of the nuclides The atomic masses are now known with high accuracy [l]_ Unfortunately the absolute isotopic composition of many eIements have never been accurately determined, and this therefore limits the accuracy with which the atomic weight can be determined_ A more fundamental limitation is imposed by the fact that some elements display natural variations in

isotopic composition, PI-

thus preventing a unique estimation of their atomic weights

An inherent problem in mass spectrometric analysis is to isolate machineinduced mass discrimination from real isotopic fractionation produced by mechanisms which may be of importance to our understanding of the early history of the solar systemSince atomic weight determinations require an accurate assessment of the extent of instrumental discrimination, it is necessary to measure the mass discrimination factor for the particular element under study using the method of

kmization best suited for the mass =pxtrometric anaIysis_ Three methods have been used to measure the instrumental mass dbxriminationr calibrated isotopic standards; the doubIe spike technique; and by the preparation of synthetic standards_ The present study is concerned with the absolute isotopic composition of @Iium in order that the atomic weight of gallium may be accurately determined_ De Iaeter [3 J measwred the isotopic composition of gaIIium in six iron meteorites, the standard syenite SY3 and a terrestriaI standard, using thermal ionization techniques_ The isotopic abundances of meteoritic and terrestrial gallium were identica1 to within O-II :A- An estimate of the extent of mass spectrometer-induced discrimination was made by De Laeter [3] on the basis of repeated measurements on the National Bureau of Standards, Rubidium Standard NBS 9&I, and ratio of 0_6641~0_0005 was thereby estimated_ au “absolute*’ ‘rGaj6’Ga Unfortunatety no suitable NBS isotopic standards arc presently avaiIabIe in the same mass region as gaIIium, and the double spike technique cannot be appIied to elements which, like gaIIium, oniypossess two isotopes. It was therefore decided to make up a number of synthetic mi.xturesof gaIIium from isotopicaIIyenriched samples of 6gGa and “Ga which were obtained from the Oak Ridge NationaI Laboratory (ORNL), in order to measure the absolute isotopic composition of gaIIium_

EXPERlSfEtWAL

PiWGDURE

The Laboratory Standard was preRared from a spectroscopicaliy pure sample of gallium metaI which was first cleaned with aqua regia and then taken CarefuIIy into soIution in 6 M HCI such that the concentration of the final solution was aaxnateiy known. Ahquots of the Standard solution, containing approximateIy 1 pgof gaIIi~wereauaIysedin a305cm radius of curvatures 90” magnetic sector soiid source mass spectrometer using the silica gel technique [4]_ Previously outgassed zone refined rhenium filaments were used throughout the analyses_ Using the silica gel technique, microgram quantities of gailium produced ion beams of approximate strength 5 x IO- ” A for at least 24 h_ The mass spectrometer was fitted with both electron multiplier and Faraday cup facilities, and measurements were made on both coIIection modes for each sample analysedThe output current was amplified using a vibrating reed electrometer, and the resultant signal then digit&d and fed on-line to a minicomputer_ For the electron muftipller and direct cohection (criticahy damped) modes- IO9 and IO” Q input resistors, respectiveIy, were employed in the electrometer_ The amplifying system was checked for linearity and speed of response. Themass spectn~ was scanned by cycIicalIy varying the magnetic fieidData was collected in sets of ten spectral scans, and the mean and standard deviation of each set of data was determined before the next set of data was collected-

Ten sets of data were colkzcted for both the eIectron multiplier and direct colIection modes_ The mass spectrometer parameters were unaltered during the series of measurements described in this paper. The ORNL tracers wexe obtained in the form of Ga203 and the isotopic analyses reported for the oxides by ORNL are given in TabIe 1, The isotopic composition of each tracer was also measured in the mass spectrometer used for TABLE 1 lSOTOPIC

co.wosmos isorope

Scpmrd

OF SEPARATED

GALLWU

lsoropic

ana[wik

ISOTOPES

-GCt

69Ga 99-75 fO.05

“Ga

69Ga 020 yO.02 0/m “Ga 99.80&0_02 %

7’Ga

Ikotopic anatysis (measured)

(ORIVL)~~ Of_

0.25fO.05 %

69Gik 99.754*0.004

oA

“Ga

oA

0.246f0.004

6pGa 0313 &OX@%O/_ “Ga 99.787~0.004 oA

* The Iknits are an csprcssion of the precisionof the measurementonly. The error is estimated at less than 1 O/_from known sources of systematic errors-

the project, and corrected by the mass discrimination factor determined from the experiment itself_ The corrected isotopic abundances were then used to determine the final

mass discrimination

correction

factor as discussed

beIow_ The corrected

isotopic abundance of the gallium tracers measured in the project are also given in Table I_ Before making up the synthetic mixtures, the tracers were heated for 24 h in an oven at 120 “C and then allowed to cool in adesiccator overphosphorous pento.xide_ TABLE 2 DATA

PERTNEXT

-I’D THE

PREPARATlON

Aromic Wright of 69Ga in 69Ga=0, Atomic weightof ‘;Ga in ‘*Ga=Ot Atomic weight of 0 in GarOS Molecular weightof 69Gaz0s Mokcular weight of 7’Ga101i hfklure A Weight of 69Ga10, Weight of llGazOs Calculated y1Gaj6gGa ratio

OF SYXTHETIC

AlLyTURES

-65.9305 I -_1o_ooOo6 +70.9204 1_0_OooO5 i 59994 ~0.0001 185.8592 ~0.0004 189.8391 &0.0004 27.603 ~0.010 mg 15.135 ~50.010 mg OS388 ~0.001 I

~uixrm-eB Weight of 69GazOlr Weighr of y’GaZO, Calculated =*Gf19Ga ratio

10.010mg 12.665 ~0.010 mg 0.6434 to.0013

19.325

* CklcuIatcdfrom the measured isotopic abundances listed in Table I and the atomic masses listed in Ref, [I].

Two synthetic mixtures were prepared by weighing and mixing the appropriate amounts of 69GaQ3 and “Gaz03 to obtain 7’G#9Ga ratios approximately the same as the Standard value_ The dry mix of the tracers was dissolved in IO M NaOH in a teflon beaker, taken to dryness, redissolved in 10 M NaOH and then transferred to a container in 6 M HCL All the weighings were made with a microbalance (Met&x Model MS), checked with caEbrated weights. The measured weights and their estimated errors are listed in Table 2 together with other relevant information to enable the 71Gaf9Ga ratios of the two synthetic mixtures to be calculated-

RESULTS AND DlSCUSStOS

The values for the “Gaf”‘Ga ratios for the two synthetic mixtures were calculated according to the reiationship rrt, J? + t’epGa

=

hyl

:’

bYi59 -

(1 -Fc59)

Af69

ff~(i--F&i fl

z9F.,

where W is-the weight of GarOs, lli is the motecular \w5ght of Ga103r F is the moie fraction of the gallium isotope, and the subscripts 69 and 71 refer to the isotopes and oxides of 69Ga and “Ga, respectively_ The uncertainty in the calculated 7’Gaj69Ga ratios for mixtures A and B are listed in Table 2, The atomic \wzightof oxygen was assumed to be that of the natural element, and to have the value tnd uncertainty recognized by the International Commission on Atomic Weights of’ IUPAC_ The uncertainties in the caIcuiated ratios arose mainly from weighing errors and uncertainties in instrumental discrimination_ Table 3 lists the measured isotopic ratios for the Iwo s_yntheticmixtures and the Laboratory Standard_ Since the concentration of the synthetic mixtures and the Laboratory Standard solutions were known, approximately equal quantities of gallium were loaded in the mass spectrometer for each sample analysedTime-dependerit fractionation was minimal during the individual analyses, but a strict time sequenoe was adopted for each sxnpIe_ Measurements were not commenced until the 69Ga ion beam had attaine3 stability at a predetermined electrometer reading,and the electronmultiplierand Faradaycupcollection modes were used for equival!ent periods of timeThe isotopic ratios listed in Table 3 are the means of approximately 100 individual sweeps_ The errors quoted for each ratio represent two standard errors of the mean: The‘ standards have been measured on a number of occasions in order to give an indication of the precision of the measurements, The average

Laboratory

Mixture

standard

A

MixtureB

0.6?967~0.00062 0.6.5037f0.00012 0~65021f0.00024 o-65008i0_ooo20 O-~987frOXtOO23 0.650rX~0.00021 ‘Mean0.65014~0.00070

0.65831i0.00027 0.6588l~O.OOOO8 0.65909~0.ooO20 0.65874~0.00079

052696;0.00012 0.52738~0_00016 0_!52924t0.ooo22 M~~10.52786~0.0024

o.53372io.oooo4 0.53415~0.oOow OS3546iO.OQOll o.53444*o.oo1s

0_62727+0_m13 0.62954~OXOO20 0.63145~0.00006 Mic;m0_6~34~~0_0042

0.63539~0.00003 0s63643i0.00016 0.63882~0.00007 O-636SSt0.0035

71Ga/6gGa ratio is the mean of the replicate analyses, whilst the associated error is at the 95 72 confidence level. The error shown here is the major source of error in this work and is due to differing mass fractionation in the spectrometer from one sample to the next_ It will be noted that the Faraday cup collecrion mode gives a systematically higher 7’Ga16gGa ratio than the electron muitipiier collection mode. The permil deviation between the two sets of measured values for all the data listed in Table 3 is 6.2 y& per mass unit; and this difference is apparentIy almost entirely due to the different ion-eiectron conversion efficiencies in the electron multiplier_ The permil deviations between the measured 7iGa!6qGa ratios for the fwo synthetic mixtures and the calculated ratios are IO-2 and 4-I ?& per mass unit, for the electron multiplier and Faraday cup collection modes for mixture A, and IO-9 and 5-l y&, per mass unit for the corresponding modes for mixture B. The correlation between the two sets of data is excellent, so that mean values of 10X& 3.5 yW and 4_623_0y&, can be used to correct the Laboratory Standard ratios listed in Table 3_ Using the “Gai6’Ga ratio for the Laboratory Standard determined by the electron multiplier and the IO-5 ym per mass unit correction factor, we obtain an absolute isotope ratio of 0.6641 t_O_OO46_Sin&My using the Faraday cup 71Gaj6gGa ratio, this gives an absolute isotope ratio of 0_664S+O.0040. The agreement between the absoIute isotope ratios for the electron multiplier and Faraday cup collection modes is excelIent, and the results have therefore been pooled to give a final absolute “Gaj6’Ga ratio of 0_6645~0.0030. This corresponds to an isotope abundance of 6gGa = 60.078 0/0and "Ga = 39,922 %_ Using

the masses of Wapstra and Gove [i ] this gives an atomic weight for gallium of 69_724+0_002 at the 95% confidence Ievel, The final 7’cja/63Ga ratio of 0_6645+0_0030 is therefore independent of mass spectrometer-inzhtced discrimination effects_ It is in good agreement with the value of 0664I~0.0005 estimated by De Laeter [3] from a knowledge of NBS isotopic standank measured in the mass spectrometer used in this project, There is one a%htional potential source of error in the work described above_ This arises from possible variations in the purity and stoichiometry of the Ga203 tmcen used to prepare the synthetic mixtures. If the purity and stoichiometry of the 69Ga20, and ‘tGa,03 tracers are approximately equal, then there is no significant chant to the atomic weight determination listed above: which has been CakzuIatedon the asumption that both oxides are stoichiometricand of high purity_ This assumption is a reasonable one, since both tracers were prepared by ORNL in the same manner_ Gallium forms three oxides: Ga20, GaO and Ga203. The normal oxide Ga203, is the most stabk at all tempentu,bes and can only be converted to the other oxide forms under extreme conditions [SJ_ ORNL convert the purified gaIEum tracer to Ga203 by precipitating Cia(OH), with aqueous ammonia, dissolving the precipitate in nitric acid, evaporating the nitric acid solution to drynw adding an ex-s of o.xaIic acid. heating carcfuliy to decompose the nitrate, and finally heatirzgto a constant weight at 1000 “C_ No fiIter paper was used which might cause e tiuction of the Ga,03 to a lower o.xide_ An independent check of the purity and stoichiometry of the tracers was made by calibrating known quantities of the 69Ga302 and “Ga203 tmccrs against a known quantity of the Laboratory Standard using the isotope dilution technique. The results of this experiment showed that there was no variation in purity or stoichiometry of the two tracers within experimental errors. Diffcrcntial scanning ca1orimett-ywas also used for the 69Ga10, and “GazO, tracers to see if any evidence of water of crystahixation or non-stoichiometry could be observedThe results were negative_ However, we have assessed the effect of possible variations in purity and stoichiometry of the tracers within the limits of our experiu&ntal errors, on the determination of the atomic weight of gallium- This has the efkct of increasing the 95 7; confidence limits associated with the atomic weight to 69.724+0.003Thus, our best estimte of the atomic weight of gallium is 69.724t0.003 which we recommend should now replace the existing value of 69-72*0_01 (IUPAC [6])-

ACKNOWLEDGEMENTS

The authors would like to thank Dr V_ M_ Johnson from ORAL

for informa-

tion about the Ga203 tracers, and Mr B. Sturman from the Chemistry Department

409 of the W_ A_ Institute of Technology for assistance with the differential scanning calorimeter- This research was supported by the Australian Research Grants Committee_

REFEREXCES

1 Z 3 4 5

k H_ Wapstra and N_ B_ Gove- NutI_ Data Tables. 9 (1971) 267_ P_ de Bi&rc, Z. AML Chem., 264 (1973) 36% J_ R_ De Laxer, Geochim, Co.mzoch~nz_Acru, 36 (1972) 735. A. E. Cameron, D_ H_ Smith and R L. Walker, Anal- Chenz., 41 (1969) 52% K_ Wade and A_ J_ Banister, in J_ C_ Bail*. H_ J_ Emelelis, R Nyholm and A_ F- Trotm~VoL 1. Pergamon., O?rford, 1973, p_ 993 Dickenson, (E&_) Comprehentire Inorganic Chemistry_ 6 IUPAC Commission on Atomic Weights, Pure Appi. Chems 30 (1972) 639.