The atomic weight of europium

International Journal of Mass Spectrometry and Zon Processes, 103 (1991) 193-202

193

Elsevier Science Publishers B.V., Amsterdam

THE ATOMIC

TSING-LIEN

WEIGHT OF EUROPIUM

CHANG and QIU-YU QIAN

Department of Chemistry, Peking University, Beijing 100871 (People’s Republic of China)

MO-TIAN ZHAO and JUN WANG National Research Centre for CertiJied Reference Materials, Beijing 100013 (Peoples Republic of China)

(Received 18 April 1990)

ABSTRACT Absolute isotopic abundances of europium in 12 samples from various origins were determined by using thermal ionization mass spectrometry. For this purpose, synthetic mixtures of independently known isotopic compositions prepared from chemically pure and nearly isotopically pure separated europium isotopes served to calibrate the mass spectrometer. The resulting true ‘s’Eu/‘s3Eu ratio is 0.90673 + 0.00235, which yields the mean isotopic abundances (47.554 f 0.065) at.% “‘Eu and (52.446 k 0.065) at.% IS3Eu at the 95% confidence limit. From these data and the relevant nuclidic masses the atomic weight of europium was calculated to be 151.9695 f 0.0013.

INTRODUCTION

Atomic weights of the elements are important natural constants and should be determined as accurately as possible. Consequently, the mass spectrometric method displaced stoichiometry during the 1940s because it could add one or more significant figures to these constants. The mass spectrometric measurement, however, involves instrumental bias due to mass discrimination. Since the 196Os, calibrated mass spectrometry has improved further the value of atomic weights by at least one more significant figure. The atomic weight of a polynuclidic element A,(E) can be found using the formula A,(E)

=

XJIMi

(1) where J;: and Mi are the fraction and the mass of nuclide. Nuclidic mass data are generally accurate to better than several tenths of a part per million. It is clear that the chemist should find all the nuclidic fractions (isotopic compositions) of the element. In the present work we made a calibrated mass spectrometric measurement of the atomic weight of the element europium, 0168-1176/91/$03.50

0 1991 Elsevier Science Publishers B.V.

194 TABLE 1 Chemical impurities (%) in samples A and B Element

Sample A

Fe Ni Pb Sn Ga Nd Gd

0.003 0.003 0.002 0.004 0.004 0.02 0.02

Sample B

0.003 0.001 0.003 0.02 0.01

which has only two stable isotopes with masses of 151 and 153, and there is no interfering mass from the neighbouring elements. Various minerals and chemicals of europium were examined for any possible isotopic variations. The IUPAC 1985 assessment of the atomic weight of europium is 151.965 f 0.009 [l]; this is based on the experimental data of six papers [2], among which the most precise measurement is that made by Hess [3] and the most recent data are those reported by Hollinger and Devillers [4]. However, none of these data were obtained with the calibration of mass spectrometry. EXPERIMENTAL

Enriched isotopes Two samples of enriched europium isotopes in the form of oxides were supplied by the U.S.S.R. Techsnabexport with quality certificate Nos. 194 and 220 issued on 16 March 1989: sample A was specified to contain 97Sat.% 151Eu, and sample B 99.2at.% ‘53E~. Our measurement showed that the former contained 97.556at.% “‘Eu and the latter 99.215at.% ‘53E~. The certified impurities are listed in Table 1. The analysis of the rare earth impurities by a VG Plasmaquad mass spectrometer indicated the presence of 0.0157% Ce, Pr, Nd, Sm, Gd, Tb and Dy in sample A, and 0.0186% of the same elements in sample B. Considering the other impurities present as oxides, the total impurities in sample A amount to 0.0364% and those in sample B to 0.0273%. The error is estimated to be 0.01%. These impurities pertain, however, to the lower limit owing to the detection limit of the analytical methods. Therefore the chemical purity of sample A is 99.964% Eu,03 and that of sample B is 99.973% Euz03. Preparation of six synthetic mixtures In the preparation of two standard solutions A and B, about 0.2 g of each isotopic reagent was dried and carefully weighed with calibrated weights and

195 TABLE 2 The weight of the isotopic reagent and the standard solution Sample

Reagent (g) 15’E~203

A B

0.172 773

‘53Euz0j

Solution (g)

0.171519

77.829 08 77.170 62

the tare technique on a microbalance, which permits estimation down to 1 pg. All the weighings were reduced to vacuum, and the precision was 0.02 mg. The oxides were then dissolved in 1.6 mol 1-l nitric acid (high purity grade). The weight of the isotopic reagents and the corresponding standard solutions are given in Table 2. The data in Table 2 together with the isotopic ratio R,s,,,a and the known nuclidic masses (15’Eu = 150.919847 and ls3Eu = 152.921225 [5]) yield the concentration C of europium in the standard solutions A and B as given in Table 3. The weighings for the preparation of six mixtures are shown in Table 4. Mass spectrometric analysis

In the automatic thermal ionization mass spectrometer (Finnigan MAT 261) the ion optical system has a mass dispersion corresponding to a 46cm radius of a conventional magnetic sector field, and it produces stigmatic focusing. With a standard entrance slit of 0.2 mm and exit slit of 0.6 mm, the resolution is 500 at 10% valley. Einzel lenses are used for axial and radial focusing of the ions at 10 kV ion energy. A rotating sample turret carries 13 filament inserts. Complete shielding prevents cross-contamination between samples. We utilized the double-filament insert: two parallel rhenium filaments (8 mm long, 0.7mm wide and 0.04mm thick) are set at a distance of 1 mm from each other. Both filaments were degassed under vacuum by heating with a current of 4 A for 20 ruin, then with 5 A for 5 min, and finally with 6 A for TABLE 3 The concentration

of europium (C) in the standard solutions A and B

Standard solution

Atomic weight of Eu

Molecular weight of Euz03

c (~01

A B

150.96807 152.90528

349.93434 353.80876

12.6830 12.5605

Eu per gram solution)

196 TABLE 4 Weight of solutions of enriched materials used to prepare calibration mixtures Mixture

1 2 3 4 5 6

Solution A (g)

Solution B (g)

1.179114 1.240398 1.100146 1.407825 1.112446 1.234575

0.969474 1.125329 1.136492 1.306873 0.931874 1.128603

1 min. After cooling, a drop of about 0.75 ~1 of the sample solution containing 1.5 pg europium was loaded onto the centre of the sample filament, which was then heated with 0.7 A for 10 min. After inserting the turret into the basic instrument, the system was evacuated down to lo-* mbar. Thereafter the ionization filament was heated with a current gradually increasing up to 5.2-5.7 A over 20 min. The sample filament was heated very carefully by increasing the current very slowly up to 1.0-l .5 A, lest the sample would splatter. Two Faraday cups (Nos. 3 and 5) collect the “‘Eu+ and ‘53Eu+ ions simultaneously. The integration time was set at 8 s. The baseline was monitored at masses 151.5 and 152.5 and the peak correction was automatic. As the ratio R,5,,,a was observed to decrease gradually with time, only the values at the start of regular heating were collected. In the mass spectrometric analysis, each of the eight samples was used in five replicate loadings and each loading was measured for six blocks, each of which consisted of ten values of R,,,,,,, . Therefore, 300 recorded ratios formed a mean R’5’/‘53with a 2 SD of less than 0.08%. All the observed data are listed in Table 5. The data of sample A (l-6) were checked using another MAT 261 TABLE 5 Observed isotopic ratios (Rls,,rs~) of the enriched isotopes and mixtures Sample

R 1511153

SD (“So)

40.50387 0.008029472 1.196778 1.089352 0.960533 1.064609 1.176143 1.080898

0.66 0.75 0.28 0.13 0.52 0.08 0.45 0.31

197 TABLE 6 Correction factors (K) for mixtures l-6 Mixture

K

1 2 3 4 5 6

0.985 827 0.984 847 0.985 101 0.985 690 0.985 206 0.985 309

Mean SD

0.985 33 0.000 37

instrument installed in the Beijing Institute of Uranium Research. The observed data agreed with those given in Table 3 to within 0.1% on average, but seemed to be inferior and therefore, were not considered further. Calculation of the correction factor K

The correction factor K = Rtrue/Rmeas for the mass discrimination calculated for each mixture by using the equation

K CA1

KRAB=

w*c* 1 -

f%R,

1 ZR,)

’

w,cB 1 zRB

+ w,cBb

-

can be

(2)

1 ZR.)

in which all the measured values of RA, RB and R,B (shown in Table 5) are converted into R,,, through multiplication by K. On rearrangement we have

R AB

W,C,R,(l

+ KR,) + w,c,&(l

+ KR,)

= wAcA(1

+

KRB)

+

wBcB(1

+

KR,)

(3)

By substituting the values of CA and C, given in Table 3 and the six sets of W, and W, values given in Table 4 we evaluated the values of K for the six mixtures (see Table 6). The dependence of C on R, or RB is insignificant. If KRA and KRB are employed in place of R, and RB in the calculation of C, CA varies by 3.9 ppm and C, by 1.3 ppm, resulting in the constancy of K at less than 1 ppm. This implies the validity of the method of calibration of mixtures prepared from highly enriched isotopes of known chemical purity. The magnitude of K arising from the evaporation of europium nitrate can be understood by a comparison with the case of lithium iodide [6], since the

198 TABLE 7 The K values for europium nitrate and lithium iodide [6] Isotopic molecules

ml Iml

K

151E~(N03)~/‘53E~(N03)3 6LiI/7LiI

0.9941 0.9925

0.98533 0.98591

ratio of the isotopic molecules evaporated is proportional of the molecular weightsm, and m2 (see Table 7).

to the square root

Minerals and chemicals

An attempt was made to examine the three chief minerals containing europium. The world’s largest rare earth (RE) deposit is the bastnaesite, (RE) C03F, which occurs in Beiyun’ebo, Inner Mongolia. This deposit has been estimated to be more than 100 million tons [7]. Its main content is ceria earths. Another extensive deposit is the RE ionic adsorption kaolinite [7], which occurs in the Jiangxi and Guangdong Provinces. The latter is a new mineral with the tentative formula Al, Si, OS(OH), x (RE) X3, and is moderately rich in yttria earths. The third mineral is monazite, (RE) P04, which occurs in Hunan Province. It contains mainly ceria earths. Another mineral examined occurs in Kirghiz, U.S.S.R. These minerals are used for the production of europium oxide by various factories as shown by the upper part of Table 8 l

TABLE 8 Samples of minerals and chemicals Sample

Mineral or chemical

Location

Factory

I

Bastnaesite

Beiyun’ebo

II

RE adsorption kaolinite

Gongnan

III

RE adsorption kaolinite

Gongnan

IV

RE adsorption kaolinite

Qingyuan

V VI

RE adsorption kaolin&e Monazite

Jixi Yueyang

VII VIII IX X XII

RE mineral Euro, Eu, 0, Eu, 0, Eu, 0,

Kirghiz

Baotou Iron Steel Works, Inner Mongolia Jiujiang Non-ferrous Metals Works, Jiangxi Institute of Rare Earth Research, Nanchang Yangjiang Rare Earth Refinery, Guangdong Zhujiang Refinery, Guangzhou Taojiang Rare Earth Refinery, Hunan in U.S.S.R. Rare Earth Products Ltd., UK Fluka, Switzerland Yaolong Chemicals Work, Shanghai Gansu Rare Earth Corp., Lanzhou

199 TABLE 9 Observed isotopic ratios (Rlsillss) of the minerals and chemicals studied Sample

R 151/153

SD x 1O-6

I II III IV V VI VII VIII IX X XI XII

0.920 167 0.920 432 0.920 206 0.920 447 0.920 331 0.920 220 0.920 140 0.919 985 0.920 335 0.920 318 0.920 166 0.920 0 17

261 184 313 175 153 366 393 250 224 262 354 219

(samples I-VI). Sample VII is, however, europium acetate hydrate. All these samples are 99.99% pure, except sample V which is 99.95% pure. The other five samples (VIII-XII) of europium oxide are prepared from unknown minerals. All are 99.99% pure, except sample VIII which is 99.999% pure. In the mass spectrometric measurement, each of the 12 samples was used in five replicate loadings, as for the eight calibration samples described above. The measured R ,5,,,s3 values are listed in Table 9 . Each value given in the table is the mean of 300 recorded ratios, the corresponding SD is also given. As an example of Iive replicate loadings of a single sample, the results for bastnaesite value of a single loading had a 2 SD are shown in Table 10, in which the R,,,,,,, of about 0.00015. The 12 measured values of R ,5,,,53 pertaining to europium from different TABLE 10 Recorded data of the measured ratio Rlsll15, for the basnaesite of Baiyun’ebo Loading No.

1 2 3 4 5 Mean SD a I,, evaporation;

R 151/153

Filament current”(A) I,

4

1.46 1.19 1.36 1.67 1.76

5.22 5.04 5.40 5.75 5.89

4 ionization.

0.9205 162 0.9199487 0 9202320 0.9198709 0.9202675 0.9201671 + 0.0002606

TABLE 11 Summary of the calculation of the atomic weight of europium” Values

Atomic weight = 151.96949

Overall limit of error,b +[2(S; + $)“*

+ B]

Uncertainty components Mass spectrometric analytical error, 2&n

Limits to error in chemical analysis, 2&

Error in composition of separated isotopes, B

(I) f 0.00132 (II) * 0.00131 (VI) f 0.00133

(I) f 0.000199 (II) k 0.000178 (VI) + 0.000235

+ 0.00073

&-0.000539

(I) f0.0647 (II) f 0.0644 (VI) f 0.0652

(I) * 0.0100 (II) + 0.0089 (VI) f 0.0117

f 0.0364

f 0.0269

(I) + 0.00235 (II) + 0.00234 (VI) + 0.0237

(I) f 0.000362 (II) k 0.000323 (VI) + 0.000427

+ 0.001325

f 0.000979

Nuclidic masses “‘Eu = 150.919 847 (24) ‘53Cu = 152.921 225 (24) Atom percent ‘j’Eu = 47.5542 lS3Eu = 52.4458 Isotopic ratio “‘Eu/ n3Eu = + 0.90673

a (I) Bastnaesite; (II) RE adsorption

kaolinite; (VI) monazite. b An additional

term, BN = 6SD [5] for the atomic weight.

201

origins are very close to one another. The final mean ratio is 0.920 230, which when multiplied by the correction factor K = 0.98533 yields the true R,,,,,Ss of 90673 for the element europium as a normal material of terrestrial occurrence. CONCLUSION

The uncertainties were treated as described previously for the determination of the atomic weight of gallium [8]. The results are shown in Table 11 . Three representative samples (I, II and VI) were taken to calculate the mass spectrometric analytical error (Table 11, column 3) so that the overall limit of error has three corresponding values (Table 11, column 2). The limit of the error in the chemical analysis was calculated as f 0.05% for the entire chemical assay, including errors of chemical composition and weighings. Therefore the tinal results of this work are the following. Isotopic ratio ‘5’E~/153E~= 0.90673 f 0.00235 Atom percent “‘Eu = (47.554 + 0.065)% ‘53Eu = (52.446 f 0.065)% Atomic weight A,(Eu) = 151.9695 f 0.0013 The atomic weights of europium determined by different workers during 1947-198 1[2] range between 15 1.963 and 151.9655. All those values are lower than ours simply because the essential correction for mass discrimination was neglected in all cases. Therefore, the new value of A,(Eu) obtained in this work might suggest a revision of the current IUPAC assessment of 151.965 + 0.009

PI. ACKNOWLEDGEMENTS

We wish to thank the National Natural Science Foundation of China for a grant to support this work, and Dr. Kevin J.R. Rosman, Curtin University of Technology, Perth, for comments on the first draft of the manuscript. REFERENCES 1 Atomic weights of the elements 1985, Pure Appl. Chem., 58 (1986) 1677. 2 P. De Bitvre, M. Gallet, N.E. Holden and I.L. Barnes, Phys. Chem. Ref. Data, 13 (1984) 809. 3 D.C. Hess Jr., Phys. Rev., 74 (1948) 773. 4 P. Hollinger and C. Devillers, Earth Planet. Sci. Lett., 52 (1981) 76.

202 5 A.H. Wapstra and G. Audi, Nucl. Phys., Ser. A, 432 (1985) 1. 6 E. Michiels and P. De Bievre, Int. J. Mass Spectrom. Ion Phys., 49 (1983) 265. 7 X. Ren, in G. Xu and J. Xiao (Eds.), New Frontiers in Rare Earth Science and Applications, Vol. 1, Science Press, Beijing, 1985, p. 39. 8 L.A. Machlan, J.W. Gramlich, L.J. Powell and G.M. Lambert, J. Res. Nat. Bur. Stand., 91 (1986) 323.