Determination of selenium in standard rocks by neutron activation analysis

Geochemical notes

283

AJcnowledgements--Dr. PETER ROBINSON of the University of Massachusetts provided field control, and Drs. P. M. HURLEY and H. W. F ~ R B ~ N of the Massachusetts Institute of Technology provided use of the mass spectrometer and X-ray spectrograph respectively. The Bureau of General Research, Kansas State University, provided financial assistance.

Department of Geology Kansas State University Manhattan, Kansas, U.S.A.

DOUGLAS G. BROOKINS

REFERENCES BILT.INGS M. P. (1956) The Geology of 1Vew Hampshire, Part I I . Bedrock Geology. Concord, New Hampshire Planning and Development Committee, 203 pp. BROOKINS D. G. (1963) R b - S r geochronological investigations in the Middle H a d d a m and Glastonbury quadrangles, Conn. Ph.D. Thesis, Mass. Inst. Technology, Dept. Geology and Geophysics, 213 pp. BROOKINS D. G. (1965) R b - S r study of the Ammonoosuc volcanics Monson gneiss problem, New England (abstract). I n Program for the Geological Society of America Annual Meeting, Kansas City, pp. 18-19. BROO~rNS D. G. and HURLEY P. M. (1965) R b - S r geochronological investigations in the Middle H a d d a m and Glastonbury quadrangles, Eastern Connecticut. Am. J. Sci. 268, 1-16. FAUL i . , STERN T . W . , THOMAS i . H. and ELMORE P. L. D. (1963) Ages of intrusion and metamorphism in the l~orthern Appalachians. Am. J. Sci. 261, 1-19. RODGERS J. (1952) Absolute ages of radioactive minerals from the Appalachian region. Am. J. Sci. 250, 411-427. RODGERS J., GATES R. M. and ROSENFELD J. L. (1959) Explanatory text for preliminary geologic map of Connecticut, 1956. Conn. Geol. Nat. History Survey Bull. 84, 64 pp. WASSERBU'RG G. J. and HAYDEN R. J. (1955) A~°-K4° dating. Oeochim. Cosmochim. Acta 7, 51-60. Z A ~ T ~ N R., S~-~DER G., STERN T. W., ~'L~RvI~ R. F. and BUCKNA~ R. C. (1965) Implications of new radiometric ages in Eastern Connecticut and Massachusetts. U.S. Geol. Survey Prof. Paper 525-D, 1-10.

Geochimicaet CosmochimicaActa 1967, Yol. 31, pp. 283 to 285. PergamonPress Ltd. Printed in Northern Ireland

Determination oi selenium in standard rocks by neutron activation analysis INTRODUCTION THE abundance of selenium in terrestrial rocks is generally very low. Highly sensitive analytical methods are therefore essential for studies of the geochemistry of selenium. During the past few years several investigators have found it useful to apply neutron activation analysis in studies of the selenium distribution in various types of materials (F~EMA-N et el., 1959; LEDDICOTTE, 1959; BOWENand CAWSE, 1963; DA~rr.and STEr~rNES, 1965). SCHr~DEWOLF (1960) and A~ArwA (1966) have determined the selenium content of stony meteorites using neutron activation combined with radiochemical procedures based on ion exchange and a number of precipitation steps. I n the present work another specific separation method for selenium, namely distillation with a mixture of hydrochloric and hydrobromic acids, has been adapted to activation analysis of geochemical samples. The method is based on 120 d Se 75 formed by (n, ~)-reaction, and has been applied to a series of igneous rock standards distributed by the U.S. Geological Survey.

284

Geochemical notes EXPERIMENTAL PROCEDURE

F i n e l y crushed rock samples of 500 m g were w r a p p e d in a l u m i n i u m foil a n d irradiated for 14 d in a thel~nal n e u t r o n flux of 1 × 10 TM n/em2/sec in the reactor J E E P - 1 (Kjeller, Norway). A selenium standard, consisting of a b o u t 0.5 ml of a solution prepared b y dissolving SeO~ in dilute nitric acid (5 ~g Se/mi) sealed in a silica ampoule, was irradiated together w i t h the samples. A f t e r seven days delay, to reduce the a c t i v i t y of short-lived nuclides, the samples were t r e a t e d according to the following procedure: The sample is transferred to a nickel crucible containing 1 m l of selenium carrier (20 m g Se), carefully e v a p o r a t e d to dryness and fused w i t h 5 g sodium hydroxide. T h e fusion cake is rem o v e d from the crucible using a few ml of w a t e r and transferred to a distillation apparatus.

2000

401 S u m

peak

1500

Z 3 0 U

1000 121+136

500

I 100

I 200

I 300 Energy

~,~I 400

. 500

(KeV)

Fig. 1. G a m m a - r a y s p e c t r u m of 120 d Se ~6 isolated f r o m a n e u t r o n irradiated rock sample. After 20 ml concentrated sulphuric acid has been added dropwise (caution), 5 m l of 1 : 1 hydrochloric acid/hydrobromic acid m i x t u r e is added and t h e solution h e a t e d to incipient fumes of SO 3. The distillation flask is cooled, and t h e addition of hydrochloric acid/hydrobromic acid and subsequent distillation is repeated twice. The combined distillate is transferred to a 250 ml beaker, and selenium precipitated f r o m h o t solution ~4th N a t I S O 3. I f the precipitate is not formed immediately, more hydrochloric acid is added. The selenium is collected on a tared m e m b r a n e filter p r e t r e a t e d w i t h 6 N HCI, a n d washed w i t h 1 N HCI and water. After drying for 10 rain at l l 0 ° C , t h e filter is weighed and transferred to a counting vial. The chemical yield is usually 7 0 - 9 0 % . The silica ampoule containing t h e s t a n d a r d solution was broken, and 3-4 aliquots of 100 each were w i t h d r a w n b y means of a micropipette and transferred to separate counting vials. The g a m m a - a c t i v i t y measurements were performed b y m e a n s of a 3 × 3 in. well-type l~aI(T1) scintillation detector connected to an " I n t e r t e c h n i q u e " 400 channel gamma-spectrometer. The counting period for each sample was usually 30 rain. T h e g a m m a - s p e c t r u m of Se ~5 isolated from a rock sample is shown in Fig. 1. The radiochemical p u r i t y p r o v e d to be v e r y good, as no extraneous a c t i v i t y could be d e t e c t e d in a n y sample. Tellurium is expected to a c c o m p a n y selenium in t h e chemical procedure, b u t t h e tellurium content of t h e actual samples was evidently too low to cause any significant interference in t h e a c t i v i t y measurements. The calculations were based on t h e 0.40 MeV coincidence sum peak, and the p e a k areas were e v a l u a t e d according to the m e t h o d of COVET.n (1959).

Geochemical notes

285

RESULTS AND DISCUSSION I n Table 1 are shou~n t h e results obtained for the concentration of Se in the U.S. Geological S u r v e y series of s t a n d a r d rocks. The precision of a single d e t e r m i n a t i o n is a b o u t ~-10~o for concentrations exceeding 0.02 p p m . I t is difficult to state the accuracy of the results, as no values of the selenitun c o n t e n t h a v e so far been published, either for the extensively analysed ~V-1, or for the newer standards. I t is Lmlikely, however, t h a t significant s y s t e m a t i c errors are introduced dtn'ing the irradiation. No appreciable shielding effects are e x p e c t e d to il~luence the results, as none of the m a j o r elements of the rocks are strong n e u t r o n absorbers, and as a dilute solution of selenium is used Table 1. Content of seleniLun in some s t a n d a r d rocks (ppm) Diabase W- 1

Basalt BCR- 1

Nepheline Syenite STM- 1

0.098 0.104 0.114 0.123 0.110

0.093 0.112

0-012 0.008

0.103

0.010

Granodiorite GSP- I

Andesite AGV- 1

Peridotite PCC- 1

0-005

0-055 0-063

0.007 0-009

0.023 0.021

0.003 0-004

0-005

0-059

0.008

0.022

0.004

Granite G-2

Dtmite DTS- 1

as a standard. F u r t h e r m o r e , the nuclide Se 75 is not being p r o d u c e d by any interfering nuclear reaction. The m o s t probable errors, duo to flux in_homogeneity within the irradiation container and to a v e r y low cotmt rate, are r a n d o m errors, and will be included in the spread of duplicate determinations. The m e a n values presented here are therefore p r o b a b l y accurate to ± 10% for concentrations above 0-02 p p m , and :t:20% for the rest of the samples. The sensitivity for the detection of selenium b y the conditions described here is a p p r o x i m a t e l y 0.0005 pg. I t is the e x p e c t a t i o n of the authors t h a t the present m e t h o d will be useful in further studies of the distribution of selenium in terrestrial rocks, and t h e r e b y contribute to a b e t t e r unders t a n d i n g of the geochemistry of selenium.

l]li~eralogical-Geological i~luseum University of Oslo Oslo 5, Norway Institutt for Atomenergi, Isotope Laboratory Kjeller, Norway

A.O.

BRU~rFELT

E. STEI~ES

Acknowledgement--The w o r k was carried out while one of the authors (A. O. B.) held a research fellowship g r a n t e d b y the Norwegian Research Council for Science and the Humanities. REFERENCES AKXIWA If. (1966) A b u n d a n c e s of selenium, tellerinm, and indium in meteorites. J . Geophys. lees. 71, 1919-1923. Be,YEN H . J . 1~. and CAws~, P. A. (1963) The d e t e r m i n a t i o n of selenium in biological material b y radioactivation. Ana/yst 88, 721-726. COW~Ln D. F. (1959) D e t e r m i n a t i o n of g a m m a - r a y abundance directly from the total absorption peak. Ana/yt. Ghem. 31, 1785-1790. D~ J. B. and ST~.rs~.s E . (1965) D e t e r m i n a t i o n of selenium in animal tissue b y n e u t r o n a c t i v a t i o n analysis. R e p o r t K R - 9 5 . FINEI~k-NN I., LJUNGREN K., FO~SS~,~G H . G. and E ~ w ~ L. G. (1959) A c t i v a t i o n analysis for selenium in ore concentrates, slags and waste gases obtained in metallurgical industry. Int. J. Appl. Radiat. Isotopes 5, 280-288. LEDDICOTT~, G. W. (1959) T h e radiochemistry of selenium. R e p o r t N A S - N S 3030, pp. 21-27. SCm~DEWOLF U. (1960) Selenium and teUrium content of s t o n y meteorites by n e u t r o n activation. Geochim. Gosmochim. Acta 19, 134-138.