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LA, CE, SM, Eu, TB, YB AND Lu CONTENT OF FIFTEEN FRENCH GEOCHEMICAL REFERENCE SAMPLES DETER= MINED BY NEUTRON ACTIVATION ANALYSIS

G.R. REDDY and D.R. PANT

Analytical Chemistry Division, Bhabha Atomic Research Centre, ?fombay, Bombay 400 085, India

La, Ce, Sm,Eu,Tb,Yb and Lu content of fifteen silicate reference standards were determined using neutron activation analysis. For samples poor in rare-earth el- ements, radiochemical neutron activation analysis was applied. In most cases, the comparison of these results with those published is satisfactory.

A number of new geochemical reference standards have been introduced in recent years. Determination of uranium content in several of these standards using neutron activa- tion and delayed fission neutron counting was reported earlier from this laboratory (1). This communication pre- sents the results of seven rare-earth elements (REE) in fifteen French reference standards determined by neutron activation analysis employing a combination of INAA and RNAA procedures.

PROCEDURE

100-150 mg sample powders were sealed in clean polythene bags. Mixed REE standard (3.66 pg La, 6.5 pg Ce,0.508pgSm,0.53pgEu, 1.03pgTb, 1.996pgYband 0.54 vg Lu) was prepared by evaporating 100 pl of a REE solution under infra-red lamp on clean polythene sheet and heat sealing. Samples and standard were packed together in a polythene container and irradiated in the D8 position of 'APSARA' Reactor of Bhabha Atomic Research Centre, Trombay at flux 10%. cm-2 sec-1 for seven hours. They were allowed to decay for two days prior to REE activities counting either directly (INAA) according to decay and counting schedule ofTable 1 or after a radiochemical group separation previously described (2) (RNAA). The activities were measured using a 45 cc Ge(Li) detector coupled to a

Table 1. Schedule of decay time and counting used in the INAA of REE in Geo-Reference samples

Rare Earth Gamma. energy Nuclide(s) used in the

their half lives keV Decay time measured and determination

1.4 days lmLa(40 h) 1596 153Sm(47 h) 103

2. 1 week 175Yb(4.2 d) 396 177Lu(6.7 d) 208

3.3 weeks 141Ce(33 d) 145 l6@Tb(72 d) 299 152~u(12 y) 1408

Canberra series 30,4000 channel analyser having a system resolution of 2.3 keV for the 1332 keV Co-60 gamma ray.

Photopeak activities were calculated manually by the method of Cove11 (3). The activities corrected for chemical yield, determined by ignition of REE oxalates to oxides and weighing, were utilised in the case of RNAA for computing the element concentrations by comparison with the standards.

RESULTS AND DISCUSSION

The rare-earth elements reported here were determined by neutron activation employing INAA and RNAA pro- cedures. The samples for INAA and RNAA procedures were selected using the intensity of 1 596 keV gamma-ray of

Geostandards Newsletter, Vol. 17, No I , Avril1993, p . I13 h 116

114

-

Table 2. REE content (ppm) of french reference standards determinated by INAA

This work(4)*

~iterawe5

This work(4)*

~iterature5

This work(4)*

~iteraturd

This work(6)*

~iterature5

This work(4)*

~iteratures

This work(4)*

~i [matures

This work(4)*

~iteratures

This work(4)*

~iteratures

This work(4)*

it era turd

This work(2)* ~iteratures

La

AV 36.3 sd 2.2 a

AV 23.55 sd 2.80

- 25

AV 66.53 sd 3.26

15

AV 53.88 sd 6.87

29

AV 319.5 sd 22

310

AV 180.2 sd 14.8 - 200

AV 77.2 sd 4.20

- 82

AV 75.2 sd 4.78

a

AV 19.88 sd 1.69

aL

AV 98.65 90

Ce Sm

1. Granite. GA 64.64 4.83 5.9 0.27 76 u 2. Granite GH 50.02 8.91 2.16 1.76

@ 9.0

3. Granite GS-N 114.83 6.24

6.50 0.764 135 - 7.7

4. Granite AC-E 134.6 23.12

1.25 3.01 - 154 24.2

5. Bauxite BX-N 519.3 18.3 46.3 1.1

520 22

6. Biotite Mica-Fe 362.5 32.2

7.5 2.28 - 430 - 33

7. Basalt BE-N 130.6 11.6 12.14 0.86 - 152 - 12

8. Basalt BR 133.5 10.63 11.78 0.31 - 151 - 12

9. Diorite DR-N 43.85 4.75 9.2 0.48 46 - 5.3

10. Diathene DT-N 153.9 9.53 134 8.4

Ell Tb Yb Lu

1.01 0.64 1.95 0.28 0.14 0.17 0.19 0.04 1.08 0.6 u

0.183 2.93 9.437 1.25 0.034 0.268 1.314 0.103 0.12 1.9 u - 1.1

1.452 1.077 1.833 0.323 0.23 0.51 0.415 0.069 1.7 u 1 5 rn

1.685 3.85 16.7 2.10 0.334 0.330 1.328 0.328 2 . o a 1 7 . 4 2$5

4.13 2.67 10.8 1.60 0.083 0.54 0.83 0.06 4.0 2.7 11 1.7

3.03 4.17 4.20 1.22 0.573 1.73 0.66 0.296 1 3 325

3.18 1.17 1.72 0.215 0.28 0.22 0.40 0.0218 3.6 1.3 u p;14

.3.29 1.26 1.91 0.233 0.31 0.19 0.33 0.012 22 1.25 1.9 w

1.46 0.807 2.34 0.35 0.18 0.29 033 0.007 - 1.45 0.8 - 2.7 - 0.4

1.75 0.56 1.78 0.24 1.4 0.57 0.8 0.14

Recommended values are underlined; others are proposed values. * Numbers in the brackets are the number of measurements.

115

Table 3, REE content (ppm) of french reference standards determinated by RNAA

This work(4)*

Lit. data’

This work(4)*

Lit. data’

This work(5)*

Lit. data’

This work(3)* Lit.

This work(3)*

Lit. data’

AV sd

AV sd

AV sd

AV

AV sd

La Ce Sm Eu Tb Yb

1. Anorthosite AN-G 2.58 5.46 0.765 0.383 0.24 1.097 0.268 0.942 0.108 0.069 0.086 0.291 - 2 Q 0.7 0.2 a85

2. Granite MA-N 0.84 4.05 0.128 0.035 0.158 0.148 0.29 1.19 0.036 0.011 0.182 0.121 0.4 0.8 0.1 0.017 0.01 0.05

3. K-Feldspar FK-N 0.95 1.42 0.094 0.43 0.017 0.10

- 1 19 0.06 Q& 0.01 0.04 0.19 0.42 0.039 0.064 0.012 -

4. Phlogopite Mica-Mg 0.32 .0.92 0.12 0.051 0.06 0.04 0.3 1 1 0.06 0.06 0.02

5. Serpentine UB-N 0.33 0.96 0.21 0.07 0.137 0.30 0.071 0.048 0.012 0.009 0.089 0.15 0.5 1 0.2 0.08 0.06 0.25

Lu

1.123 0.057 QLu

0.073 0.046 0.004

0.035 0.015 0.01

0.017 0.01

0.06 0.029 0.04

Recommended values are underlined; others are proposed values. * Numbers in the brackets are the number of measurements.

1NLa as the parameter for the relative abundance of REE. A preliminary examination of the irradiated samples indicated REE content to be high in the case of samples GA, GH, GS- N, AC-E, BE-N, DR-N, DT-N, BX-N, BR and Mica-Fe. Hence these ten samples were analysed by INAA. The indicated concentration of REE in the case of samples AN- G, MA-N, FK-N, UB-N and Mica-Mg were low for de- termination of REE by INAA and hence these five samples were separately analysed by RNAA procedure for their REE. In the INAA procedure the interference of 143 keV 59Fe and 300 keV 233Pa in the measurement of 145 keV 141Ce and 299 keV 16oTb activities respectively was cor- rected using pure 59Fe and 233Pa nuclides. However, no corrections were made for the interference of uranium fission.The levels of fission interference (27 pg Ce and 0.22 pg La/lOO pg U) are significant in samples with Ce/U and La/U ratios less than 1 .O (4). In the case of samples such as those used in this study where the uranium content is

generally less than 20 ppm (with the exception of Mica-Fe where it is close to 90 ppm), the Ce/U and La/U ratios are more favourable. In the case of MA-N, the ratio is < 1 and hence not favourable. A minimum of four independent measurements were made on each of the samples studied in this work except for samples UB-N, Mica-Mg and DT-N, of these samples UB-N and Mica-Mg were analysed in trip- licate and DT-N in duplicate.

The results of INAA and RNAA analyses (mean values with the corresponding standarddeviation) for fifteenFrench geostandards are summarised in Tables 2 and 3. The avail- able literature data (5 ) on these samples is also included in the tables to facilitate direct comparison. Fairly consistent values with good precision were obtained for samples analysed by INAA procedure (Table 2). For example, the values of precision for La and Lu the two end members of the REE group are typically 5- 12% RSD in the concentration

116

range of 20-32OppmLaand 5-25QRSD in the concentration range of 0.2-2.1 ppm Lu. Further as seen from Table 2 the data is in good agreement with the recommended values reported in tlle literature‘(5) for several of the REE deter- mined in this study.

The agreement is particularly satisfactory as seen from the data on samples GS-N, AC-E and DR-N, where rec- ommended values for all the seven REE have been reported andwiththeexceptionofEu andTbinAC-E andEu,Tband Lu in GS-N our results are within 10-15% of the recom- mended values indicating good accuracy for the methpd- ology. However, our results are consistently lower than the recommended values indicating the possibility of a sys- tematic error being present in the measurement of standard solutions which have been made with locally made push button type micro pipettes not calibrated previously. The agreement is equally good in the other cases also excepting for the Eu and Lu in Mica-Fe which are high and could be viewed as due to interference fromuranium fission since the uranium content of Mica-Fe is high (90 ppm). However, since there is no corresponding increase in the more fission sensitive La and Ce values this is not considered as true and the observed deviations are also believed to be due to calibration errors cited above.

As compared to the precision of INAA analyses, the precision of RNAA analyses is poor and typically 2040% RSD for La and 3040% RSD for Lu. A perusal of RNAA data in Table 3 shows that our results are in good agreement with the literature data (5-6) in the case of samples AN-G, FK-N, UB-N and Mica-Mg. Our results are systematically higher than the literature data (7) in the case of MA-N. Presumably due to interference from uranium fission since WandCeWratiosarelessthan 1 andhencenot favourable.

ACKNOWLEDGEMENT

The authors are grateful to Dr. R.K. Iyer, Head, Envi- ronmental Chemistry Section, Analytical Chemistry Divi- sion for kindly reviewing the original manuscript and offering valuable suggestions for its improvement.

RESUME

Les teneurs en La, Ce, Sm, Eu, Tb, Yb et en Lu ont dtc dosees par analyse par activation neutronique dans

quinze khantillons gkhimiques de rbference. Pour les khantillons pauvres en term ram, I’analyse a etd effectuee aprks separation radiochimique. Dans la plupart des cas, la cornparaison de as r6sultats avec ceux publies est satisfaisante.

REFERENCES

(1) G.R. Reddy, D.R. Pant and M. Sankar Das (1982) Uranium content of thirty three rock reference samples determined by delayed fission neutron counting method, Geostandards Newsletter, 6: 217-219.

(2) G.R. Reddy, B.L. Rao, D.R. Pant and M. Sankar Das (1976) Neutron activation analysis of 13 minor and trace elements in geological samples, Journal of Radioanalytical Chemis- try, 33: 39-51.

(3) D.F. Cove11 (1959) Determination of gamma-ray abundance directly from the total absorption peak, Analytical Chemistry, 31: 1785- 1790.

(4) A.V. Murali, R. Parthasarathy, T.M. Mahadevan and M. Sankar Das (1983) Trace element characteristics, REE patterns and partition co-efficients of zircons from different geological environ- ments - a case study on Indian zircons. Geochimica et Cosmochimica Acta, 47: 2047-2052.

(5) K. Govindaraju (1989) 1989 Compilation of working values and sample descrip- tion for 272 geostandards, Geostandards Newsletter, Spe- cial Issue, 13: 1-113.

(6) K. Govindaraju and I. Roelandts (1988) Compilationreport (1966-1987)on traceelementsinCRPG geochemical reference samples: Basalt B R Granites GA and GH; Micas; Biotite Mica-Fe and Phlogofite Mica-Mg, Geostandards Newsletter, 12: 119-201.

(7) K. Govindaraju (1992) Personal Communication.