Neutron activation analysis for trace elements in unpolished rice

Analytica Chimica Acta. 87 (1976) 119-124 @Elsevier Scientific Publishing Company, Amsterdam -

NEUTRON ACTIVATION UNPOLISHED RICE

ANALYSIS

Printed in The Netherlands

FOR TRACE ELEMENTS

IN

S. J. YEH, P. Y. CHEN, C. N. KE, S. T. HSU and S. TANAKA* Institute

of Nuclear

Science,

National

Tsing Hua University,

Hsinchu

(Taiwan)

(Received 30th March 1976)

SUM-MARY Twenty-two elements in 19 unpolished rice samples harvested in Taiwan during 1973 have been determined by a neutron activation analysis technique, consisting of both non-destructive and destructive methods. After the removal of “Na and “P, chemical separation into three groups is achieved by anion exchange and distillation. The concentrations of heavy metals in different rice samples vary widely.

Since there has been increased concern over the presence of trace elements in foodstuffs, an effective method of monitoring these in biological materials by simple and reliable techniques must be developed. Rice is the most important foodstuff in daily use in the East. In this study 22 elements in unpolished rice have been determined by neutron activation analysis, consisting of both non-destructive and destructive methods. Biological materials usually contain a large amount of potassium, sodium and phosphorus; consequently the prevailing radionuclides 42K, 24Na and 32P produced may completely obscure the minor activities of interest in y-spectrometry; radiochemical separations are necessary after irradiation to remove these interfering radionuclides. The separation techniques employed are the selective removal of 24Na by hydrated antimony pentoxide [ 11, the removal of 32P by acidic a.luminum oxide [2], and separation into three groups by anion exchange and distillation. EXPERIMENTAL

Samples and standards

Nineteen unpolished rice samples harvested in 1973 in ten different prefectures in Taiwan were collected by the Bureau of Food Administration. Two different kinds of rice (Fen-Lai and Tsai-Lai) are represented by suffix numbers 1 and 2, respectively, in the sample notation in Table 4. Each *Permanent address: Institute for Nuclear Study, University of Tokyo, Tansaki, Tokyo 188, Japan.

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sample (20 g) was washed three times with-redistilled water and once with acetone, dried at 40 aC, pulverized in an agate mortar and stored in a desiccator. Portions (0.5 and 1 g) of each rice sample were sealed in a polyethylene bag and a quartz axnpoule, respectively, for non-destructive and destructive analysis. Standard solutions were prepared by dissolving the pure metals or compounds of the elements of interest in redistilled water or nitric acid to give appropriate concentrations. The standard reference for non-destructive analysis was prepared by adding 100 ~1 of each standard solution to a filter paper, and allowing it to air-dry before sealing in a polyethylene bag. The standard reference for destructive analysis was prepared by adding known amounts of each standard solution to a quartz ampoule and adjusting the volume to 2 ml with redistilled water before sealing.

Neutron

irradiation

Two samples and one standard were packed in a polyethylene bag and irradiated simultaneously in the Open-pool Reactor (THOR), National Tsing Hua University, Taiwan. Irradiation conditions for non-destructive and destructive analysis are shown in Table 1, together with the cooling and counting times.

Chemical separation

after irradiation

After irradiation, and cooling for 1 d, the sample was transferred from the quartz ampoule to a Sjostrand type wetcashing reflux apparatus 133. After appropriate carriers had been added, the sample was digested with a mixture of 14 M HNO, and 30 % H,O,. The digestion was repeated three tunes before the sample solution was distilled twice, after adding 5 ml of water, to expel free nitrogen and bromide. After the residue was dissolved in 8 M HCI, the chemical separation scheme shown in Fig. 1 gave a separation into three groups. The hydrated antimony pentachloride (HAP) was air-dried at 270 o C for 5 h, pulverized and sieved (So-100 mesh). Then 1 g of the powder was added to a column (1 cm diameter, 3 cm in depth) and pretreated with 8 M HCI. The anion-exchange resin was Dowex 1-XS (100-200 mesh) (column dimensions 0.7 cm diameter, 15 cm deep). The acidic aluminum oxide was chromatographic-grade acid alumina (E. Merck, W. Germany) (5 g in a column of 1 cm diameter, 7 cm in depth).

-y-Ray measurement Prominent -y-peaks from nuclides of interest were measured with a 3&m’ Ge(Li) detector coupled with a 4096~channel pulse-height analyser. The detector had a high resolution (2.2 keV (FWHM) for the 1332-keV 6oCo r-ray) and a peak-to-Compton ratio of 27. Peak analysis was done with the Hewlett-Packard 2116 C computer program. The nuclear data are summarized in Table 2. Special care was given to the determination of the following nuclides. Since the 844-keV peak from “Mg and the 847-keV peak horn 56Mn were superimposed on each other, the attenuated 1014-keV and

121

TABLE

1

Experimental

conditions

with thermal neutrons

Flux (n cm-’

Sample (g)

Non-destructive 0.5=

-1.

09

-2.10”

s-’ )

2m 2d

lm 2h

lo’*

Counting time

Cooling time

Irradn. time

Elements determined

5m 5m

Mg, Al, Cl, Ca, Mn Na, K, Br

1.5 d 2.5 d

50 m 50 m

6.5

50 m

AS Fe, Co, Cu, Zn MO, Cd, Sb, Hg SC, Cr, Rb, Cs, Sm

Destructive -2

1.0=

=Fast shuttle

rabbit.

Digest three

bPneumatic

times

30h

- 1012

transfer

tube.

d

=Verticai

tube.

with HNOs and l-t&

Dissolve in 8M HCI Poss through HAP column Elute with 40ml of l2M HCI Efiluent

I

I Resin Fe, MO,

Evoporote to near dryness (QJ 2 ml) Add 8M HCI Poss through Dowex I-X8 column Elute with 40 ml of 8 M HCI

I Effluent

Co, Cd,

Cu, Sb,

Zn, Au,

Evoporate to near dryness Add I ml l8M H&SO Distil twice with I ml of 48 % ond I ml of 30% H202 Distil twice with 5ml of H20

Go, Hg

Group I

I Residue Dissolve in ( I +I ) HN03 Poss through AA0 column Wash with 5Dml of HN03

I

As, Se Group 2 (ICI)

AA0 (PI

SC,

Cr.

Rb,

80,

La ,

Ce

Group

Fig. 1. Radiochemical

group separation

scheme.

3

Cs ,

,

Sm,

Eu

HEr

122 TABLE

2

Nuclear data Target nuclide

Abundance (%)

“Na =6Mg =AI “CI 4’K ‘OCa 4=SC ‘OCr 5sMn S6Fe =qCo 63cu @Zn ‘zAs ‘I Br “Rb “MO ltJCd “‘Sb

100

’ -2s ’ 52Sm ‘=Hg

11.3 100 24.5 6.9 0.185 100 4.31 100 0.33 100 69.1 48.9 100 49.5 72.2 23.8 28.9 57.3 100 26.7 0.146

Neutron capture cross-section (barn) 0.53= 0.034 0.24 0.43 1.2 1.1 23a 16 13.3 1.2 37 4.5 0.47 4.5

2.7a O.Sa 0.13 0.30 6.6 30 204 3092

Radioactive product

Half-life

r4Na =‘Mg “Al “Cl .=I( ‘9Ca 46sc s’Cr s6Mn 59Fe 6oco Yzu bsZn =As “‘Br =Rb 9gMo ‘ISCd ‘*‘Sb “*es ls3Srn 19’Hg

15.0 9.5 2.31 37 12.5 8.8 83.9 27.8 2.57 45 5.26 12.8 245 26.5 35.3 18.7 67 53 67.2 2.07 47 65

y-peak used (kev) h m m ; m d d h d y h d h h d h h h y h h

1369 1014 1779 1642, 1525 3083 889, 320 1811 1095, 1173, 511 1115 559, 555, 1076 141b 337: 564, 605, 103 68;

2168

1120

1292 1332

657 777

528 693 796 77

Yium of the cross-sections for the formation of metastable and ground state (for ‘lmBr, 83 % IT). bFrom the daughter 99mTc. =Frorn the daughter “smIn. dK-x ray from Au.

TABLE 3 Chemical yield and reproducibihty Element

Chemical yield %

R-s-d-a (%)

Element

Chemical yield %

R.s.d.= (%)

SC Cr Fe co cu Zn As

97 98 98 99 100 99 92

1.6 2.0 2.7 1.5 0.4 2.6 0.6

Rb MO Cd Sb cs Sm Hg

69 92 99 99 94 98 82

4.5 3.2 3.6 4.0 1.1 2.2 0.4

aRelative standard deviation from the mean of three measurements.

TABLE 4 Analytical data for unpolished rice harvested in Taiwan, 1973 Concentrations(%)

SampleConcentrations(p.p,m.) Na Al SC A-l A-2 B-l B-2 C-l C-2 D-l D-2 E-l E2 F-l F-2 G-l G-2 H-l H-2 I-l I-2 J-2 DBL.*

7s 16 62 79 8G 13 44 16 81 30 22 13 26 10 12 6 14 10 47 1

Cr

Mn

Fe Co

12 0.010 0.27 70 5G 21 0.012 0.20 44 21 17 0.004 0.22 65 47 26 0.004 0.26 33 12 10 0.004 0.46 37 11 11 0.003 0.24 25 13 16 0.004 0.24 62 17 13 0.004 0.33 34 15 30 0.013 0.17 32 16 19 0.003 0.20 44 16 19 0.001 0.26 47 9 22 0.002 0.23 34 13 14 0.006 0.28 42 49 21 0.006 0.48 39 19 9 0.004 0.24 30 31 10 0.007 0.31 32 13 16 0.001 0.16 30 16 14 0.002 0.17 34 1G 17 0.006 0.40 24 94 1 0.001 0.03 0.6 4

“D.L. = detection limit.

Cu

Zn As

Br Rb MO

Cd

Sb

Cs

Sm

Hg

Mg

Cl

K

Ca

0.10 303 24 0.10 2.4 4 0.18 0.50 0.0760.03 0.0013 0.008 0.19 0,0580.40 0.012 0.07 3.6 26 0.100.7 8 0.27 0.10 0.003 0.04 0.0011 0.038 0.31 0.0470.42 0.010 0.07 4.7 1S 0827 2.7 33 0.41 0.22 0.0060.16 0.0016 0.012 0.20 0,0320.36 0.019 0.07 3.0 17 0.17 2.0 18 0.42 0.20 0.005 08080.0016 0.019 0.21 0.0460.32 0.016 0.11 3.5 29 0.123.7 7 0.31 0.11 0.0840.03 0.0019 0.016 0.21 0.0780.39 0.012 0.04 3.2 24 0.11 1.3 6 0.39 0.88 0.030 0,02 0.0013 0.028 0.23 0.0270.39 0.010 0.043.2 30 0026289 22 0.40 0.15 0.0400.03 0.0030 0.031 0.21 0.0550.48 0.023 0.064.4 24 0.12 1.5 7 0.34 0.23 0.026 0.03 0.0033 0.031 0.24 0,0500.52 0.013 0.06 4,O 11 0016 1.8 8 0.48 0.11 0.012 0.02 0.0072 0,014 0.14 O.OG90.41 0.013 0.05 3.6 13 0046 1.8 9 0.90 0.36 0.023 0.02 0.0036 0.035 0.20 0,0630.33 0.017 0.02 3.0 12 0009 1.7 9 0.55 0,12 0.0020.02 0.0022 0.032 0.16 0,0490.43 0.009 0.03 2.6 16 0014 1.9 28 0.52 0.12 0.006 0.02 0.0037 0.040 0.19 0.0270.42 0.009 0.07 4.2 13 0.11 1.3 4 0.34 0.09 0.018 0.05 0.0021 0.022 0.26 0.0360.38 0.014 0.14 3.6 15 0.16 1.2 2 0.25 0.27 0.0150.26 0.0026 0.039 0.30 0.0320.40 0.022 0.10 6.0 18 0,ll0.9 3 0.52 0.24 0.009 0.050.0007 0.047 0.21 0.0240.33 0.009 0.08 4.0 21 04160.8 4 0.29 0.37 0.004 0.02 0.0010 0.017 0.21 0,0230.35 0.011 0.02 4.0 9 0817 189 7 0.37 0.11 0.0040.02 0.0030 0.021 0.19 0.0790.47 0.014 0.01 3.7 8 0027 1.4 8 0.17 0.08 0.002 0.02 0.0026 0.024 0.21 0.0340.52 0.009 0.08 3.4 19 00040.6 8 0.23 0.13 0.034 0,02 0.0012 0.023 0.31 0.0420.32 0.010 0.0050.060.50.0020.50,30.010.01 0.002 0.02 0.0001 0.004 0.0040.0030.0020.004

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1811~keV peaks were used respectively. For 64Cu determination, the 511-keV annihilation peak was used after subtracting the contribution from 65Zn by reference to the 1115keV photopeak [4] _ For 82Br, the 777-keV peak was used, because the 555-keV peak was coincident with the 559keV 76As peak. For “MO, the 141-keV peak from the daughter nuclide ggmTc was used after subtracting the contribution from “Fe by reference to the 1095-keV photopeak [4]. For r1SCd,the 337-keV peak from the daughter ‘IsrnIn was used instead of the 528-keV peak from ““Cd to raise the sensitivity by an order of magnitude [ 51. The sensitivity was also increased by measuring the 56PkeV ‘**Sb peak instead of the 621-keV lz4Sb peak, and the 68-keV x-ray and 77-keV -y-peak from lg7Hg were used instead of the 279-keV *03Hg peak. RESULTS AND DISCUSSION

Chemical recovery and reproducibility of experiment The percentage recoveries of 14 elements from the chemical separation procedure were measured by means of radioactive tracers; the results are presented in Table 3. Only Rb and Hg are not recovered almost quantitatively; a considerable part of the rubidium may be adsorbed on the HAP column and some mercury may be lost during the distillation procedure. The reproducibility of the analytical results was checked by irradiating three solution standards in one pack and applying the procedure used for samples (Table 3). Analysis of rice The data for 22 elements in 19 unpolished rice samples are summarized in Table 4, together with the detection limits of this method. The concentrations of most of the elements vary widely in the different rice samples. The differences exceed a factor of 10 for Na, SC, Co, As, Rb, Cd, Cs and Sm, and there are no clear correlations between different elements. The rice harvested in mountainous districts, I and J, contains low heavy metal concentrations in general, and relatively high concentrations of As and Br may reflect the use of agricultural chemicals in some districts. More detailed investigations are required. Comparison of the present results with those for unpolished rice harvested [6] in Fuchu city (Tokyo) reveals that Taiwan rice contains more Mn and As and less Cr and Sb than Fuchu rice. S. J. Yeh and P. Y. Chen thank the National Science Council for financial support. REFERENCES 1 2 3 4 5 6

F. Girardi and E. Sabbioni, J. Radioanal. Chem., 1 (1968) 169. E. Sabbioni, R. Pietra and F. Girardi, J. Radioanal. Chem., 4 (1970) 289. B. S&strand, Anal. Chem., 815 (1964) 36. H. Al-Shahristani and M. J. Al-Atyia, J. Radioanal. Chem., 14 (1973) 401. G. H. Morrison and W. M. Potter, Anal. Chem., 839 (1972) 44. S. Nagatsuka and Y. Tanizaki, Radioisotopes, 22 (1973) 234.