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Neutron Activation ~ n a l ~ s i s of Gold In Teak

(Tectona grandis) . . .

C h e r n i s t ~ l v l yslon

Office of The Atomic Energy for Peace Bangkok, Thailand

NEUTRON ACTIVATION ANA1,k'SIS O F GOLD I N TEAK (TECTONA GRANDIS)

Chemistry Division, Office o f the Atomic Energy for Peace.

Abstract

The atnount of gold in teak had been determined by using neutron

-~ activation technique. The neutron f l u x utilized was of the order of

1 0 ~ ~ n / c m ~ / s e c . As usual the spectrum of the energy peak of gold was

identif ied using a gamma-nlultichannel pulse height analyzer. The sensiti

yity fo r gold was approximately loa7 g. Iti t eak especially Na, As, Cu, Mn

and La were in terfer ing elements in the energy region on 0.4-0.8 illev.

With the result , the non-destructive analysis o r ~ l y was not possible and

radiochemical separation was found essential. The gold content of teak was

0.58+ 0.0037 ppln.

~ntroduction Plan t chemists and biochemists have occupied themselves with studies

mainly on organic compounds present in plants such as tannins, pigment,

oils, alkaloids, enzymes, gums, waxes and resin. T h e hardwood distillation

industry was developed as ear ly as 19 th century with the a im of producing

acetic acid and methyl alcohol. Softwoods a re somewhat d i f fe ren t f rom

hardwoods in the distribution of chemical compounds of distillation products

containing marked-distinction of methoxyl groups.

Less attention has been paid to the significance of the inorganic

c o n ~ p o u l ~ d s which a re normally not major components of plant constituents.

With t h e sensitivity and rapidity of the modern technique of neutron

activation analysis, it appears also possible t o study the inorganic relat ionship

between soil and plant.

The determination of gold in t eak by the use of lieutron irradiation

of t h e the material followed by a -scintillation spectrometry was f i r s t

at tempted a t the Chemistry Division of the Office of Atomic Energy f o r

I'eace in 1966, since the presence of gold was unexpectedly recorded while

working on wood analysis.

Although the gold content in t eak is less than a microgram per gram

of sample involved, a h i c h is hardly a n amount worth considering f o r

commercialization, t h e application of t h e new technique is sowething to be

tr ied out. T h e analysis of specific t eak samples was possible through the

cooperation of t h e Royal Forest Department and it is hoped t h a t the resul ts of inorganic analysis in plant would be of some value t o t h e studies of t h e

propert ies of wood as carried out a t t h e Department.

The nuclear reaction used in this experiment is Au lg7 (n, u ) Au 198.

The half l i f e of the l a t t e r is 64.8 hours and i t decays with the emission of

gamma-ray of energy 0.412 Rlev.

~ x ~ e r i m e n t a l Procedure , T h e t eak powder "Sak Thong" (Golden teak)was f i r s t careful ly charred

over the f r e e f l ame of a Bunsen burner and then continued i n a fu rnace

unt i l a l l powder turned into ash and a l l organic matters completely removed.

Samples of the ash weighing about 0.05 g f o r each determination were

wrapped separately i n an alunlinum f o i l .

A s tandard solution containing 1 pg of gold per m l f o r comparison

was prepared f r o m a Specpure gold dissolved irr aqua regia. A few m l of

t h e s t andard solution was put into a quar tz anlpoule and t h e la t ter was

later sealed.

Both sample and s tandard were then i r radia ted i n the same aluminum container o f 60 hours i n the reactor a t a the rmal neutron f l u x of -

1 0 ~ ~ n / c m ~ / s e c . T h e irradiated sample and standard were subsequently cooled

f o r 24 hours t o allow short- l ived radioactive isotopes t o decay.

T h e sample was t r ans fe r red to a glass beaker and 20 mg of Auclg

was added a s a carr ier . T h e method of chemical separation was c a r r i e d o u t

following t h e procedure as described by D. Gibbons 1958. Alternately

the gold chloride could be reduced to metal by magnesium or zinc o r s u l f u r

dioxide, Elnery and ~ e d d i c o t t e ' ~ ) (1961). The s tandard was t r e a t e d ~ i m i l a r l ~

and radioactivity measurement was made using a a -mult ichannel pulse

height analyzer. T h e calculat ion f o r the Au198 was made a t the 0.412 Mev

of t h e gamma-ray and correction was made fo r the yield f r o m recovered

AU"' . As a control, the decay of the 0.412 Mev photopeak was taken

in to account .

P; & P; -

Iiesults

- . The rcsults of gold con te r t in t eak a r e prcsented in Table I. Tlie

accuracy of th is method is ahout+S% which is shown in Table 2. ~..

From the gamma-spectrum as sho\sn in Fig. I i t might appear tha t

Au could be cstirnated a t i t s peak no]]-destructively. However the e r ro r

would be g rea t on accout of the presence of in terfer ing elements. I n case

of in terference f rom Na, Cu and As the difficult ies could be overcome by

adjusting the decay time. - The distinct gamma-ray spectrum of irradiated sample of a s l ~ e d teak

a f t e r radiochemical separration is presented in Fig. I1 . The manganese

interference is not a problem a t a l l since the irradiated sample was cooled

fo r six days before activity measurement . ,'

The Fig. I11 shows that peaks of lanthanum could in terfere with gold.

The hal f l i f e of La-140 is 40.2:hrs. and gnmlna energies are in the regions

of 0.88,0.49, 0.81 and 1.6 Mev.

Table I

Gold content in teak

The average gold content in teak is 0.5804 * 0.0087* ppm.

* Standard deviation

Wt. of teak in gram

4.8108

4.8198

2.7885

2.6228

8.2532

2.5133

W't. of ashed teak i n gram

0.0627

0.0627

0.08618

0.03403

0.04221

0.03261

Gold in microgram

2. $225

2.8101

1.6198

1.6470

1.9226

1 . 4401

Gold i n ppm.

0 . 5648

0 .5880

0.5809

0.5898

0.5910

0 .5780

dl& b

Table ;2

Accuracy estimated froin slandard gold in the range 0.1-1.0 microgram - 10 irradiated in the thermal neutroti f lux of 10 n/cm2/sec'

The accuracy of th is method ia within 8% which is qui te satisfnctory.

Conclusion The gold content of t e ak was found a s 0.58+0.0037 ppm. The va r i a t i o i~ is

probably due to t he envirominent in which the teaks grow. Again a s the

Counts per /Jg of Au

2090.4 1 1899.9

1891.2

Au standard

0.4864

0,9728

result in terferr ing inoganic e l e ~ n e r ~ t s could dif fer .

Counts ohserx ed (Photopeak area)

208.4

321.2

1840.6 -

References 1. Gibbons, D. (I968), "The JJeiermination of Gold i n Biological hiaterials

by Neutron Activation Analysis." International Journal of Applied

Radiation and Isotopes, 4, 46-49.

9. Emery, J . F . and Leddicotte, G . K . (1961), NAS-NS 8086 Nuclear Science

Series.

FIGURE 11

GAMMA SPECTRUM OF IRRADIATED ASHED TEAI<,

AFTER RADIOCHEMICAL SEPARATION

Decay Time 20 hrs.

I 10 Thermal Neutron Flux 10 n/cm'/sec I

Channel Number

FIGURE Ill

1 anrhdnum Interference in Teat

I I I I

1 'la --- - Standxd LG

- Teak

Irradiation Time 4 hrs.

Decay Time 15 days

2 FIUX n ~ c m /set

20 40 60 80

Channel Number