Determination of selenium in canadian food items by cyclic instrumental neutron activation analysis

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Determination of Selenium in Canadian Food Items

by Cyclic Instrumental Neutron Activation Analysis

Y. SHk* E. E. Suww, N, J. HOLZBECHER, AND A. CHATT Trace Ana lys i s Research Centre, Depar tment of Chemistry,

Dalhousie University, Halifax, NS, B3H 4J3, Canada

ABSTRACT

Instrumental neutron activation analysis (INAA) and pseudo- cyclic INAA (PCINAA) have been used to determine the selenium content of a variety of Canadian food items. Use of the 162-keV gamma ray of short-lived 77roSe in INAA allowed relatively simple and rapid determinations and was suitable for many of the foods. PCINAA was found to give lower detection limits and was used for the low-selenium food samples. Both internal and external quality assessments were used to evaluate and assure the accuracy and precision of the methods developed.

Index Entries: Selenium; food; neutron activation analysis; quality assessment.

INTRODUCTION

Selenium is well known as an essential nutr ient for humans , but it is potentially toxic as well. The safe range of intake of this e lement is rather narrow. Low levels of se lenium intake have been repor ted to be associated with Keshan disease and increased risk of cancer and ischemic heart disease, whereas high intakes can result in toxicity (1). Therefore, a R e c o m m e n d e d Dietary Allowance (RDA) of 55 ~tg/d for adul t females and 70 ~tg/d for males has been established in the United States (2). In Canada, the Recommended Nutr ient Intake (RNI) level is 50 ~tg/d for adults (3). Variations in the a m o u n t of se lenium in soil in different geo- graphical areas lead to widely varying se lenium contents of food. Low

*Author to whom all correspondence and reprint requests should be addressed.

Biological Trace Element Research 3 7 7 Vols. 71-72, 1999

378 Shi et al.

levels of soil selenium have been reported to be responsible for the low- selenium content of agricultural products, and their consumption leading to low dietary intake of selenium (4,5).

In order to understand better the role of selenium in human nutrition and metabolism, neutron activation analysis (NAA) and other analytical techniques are being developed for the determination of selenium in foods as well as other biological samples (6-12). When high neutron flux is available, selenium is often assayed by NAA using the long-lived 75Se isotope (119.8 d). Although the target isotope 74Se has a high thermal neutron absorption cross-section of 51.8 b, it has an abundance of only 0.87%. Therefore, a lengthy irradiation at a high neutron flux and long decay and counting periods are required if the 75Se nuclide is to be used. There are also potential interferences for most of the intense gamma rays emitted by 75Se caused by overlapping gamma rays from several nuclides (13). An alternate approach that can considerably reduce the total experimental time and cost is the use of the 77roSe nuclide in rou- tine NAA. Its target isotope, 76Se, has a 10 times higher natural isotopic abundance, but the cross-section is 2.5 times lower than that of 74Se. The half-life of 77mSe is only 17.4 S, allowing saturation activity to be reached in a short time, thus leading to enhanced sensitivity. There is a potential spectral interference for the 161.9-keV gamma ray of 77mSe from the 162.3-keV gamma ray of 116rn2In, which has a half-life of only 2.18 s. How- ever, a short decay time of 20 s or so can eliminate this potential interference. Furthermore, indium is rarely detected in biological materials such as food and diets. Cyclic Instrumental NAA (CINAA) is particularly suitable for elements having short-lived nuclides to achieve higher precision and lower detection limits (14). In CINAA, a sample is irradiated for a short time and rapidly transferred to a detector for a short period of counting, and then the entire process is immediately repeated for an optimum number of cycles. Pseudo-cyclic INAA (PCINAA) is a variation in which several minutes to days are allowed to elapse between the rep- etition of the irradiation-decay-counting cycles.

A large number of food and diet samples are being analyzed for selenium in our laboratory, requiring the development of rapid and sen- sitive INAA methods. It is important that the selenium levels be measured under an extensive quality assurance program. Both internal and external quality assessments of selenium measurements in foods and diets by conventional INAA and PCINAA are also described here.

MATERIALS AND METHODS

Freeze-Drying of the Food Samples Between 10 and 70 g of food samples, frozen before storage, were

thawed at room temperature for about 24 h before weighing; then, they

Biological Trace Element Research Vols. 71-72, 1999

Determination of Selenium in Canadian Food Items 379

were weighed into 125-mL Nalgene jars for freeze drying. The Nalgene jars and the Teflon-coated spatulas used to transfer the food samples were carefully washed with HC1 followed by HNO3 before use. A piece of Ultraclean Teflon sheet (Clean Room Products Inc., 10 cm x 10 cm) with two small holes was used to cover each jar, and a Teflon sheet was used to line the freeze-drier trays to prevent contamination during the freeze-drying, which was performed in a tray-type freeze-drier (Virtis model 50SRC6). The samples were lyophilized for about 1 wk; then they were reweighed, and their moisture contents were calculated. Homoge- nization was carried out using a porcelain mortar and a pestle. The homogenized samples were stored in the Nalgene jars and capped tightly to avoid absorption of water.

Each homogenized food item was weighed into a small polyethylene vial (1.2 mL) with a full-vial geometry. This Vial was then capped, heat-sealed, and placed in a medium-sized vial (7 mL). An empty small vial was placed as a spacer on the top of the vial containing the sample. The medium-sized vial was then capped and heat-sealed for irradiation.

Elemental Comparator Standards

The water used was first distilled in a quartz apparatus and then further deionized using an ultrapure deionization column purchased from the Fisher Scientific Company. This distilled deionized water (DDW) was analyzed for inorganic contaminants by INAA; the levels were below the detection limits of INAA. The DDW was used for mak- ing solutions, diluting solutions, and for washing all apparatus.

All elemental comparator standards used in this work were prepared from the plasma emission spectroscopy standard solutions purchased from SCP Science. A stock solution of a desired concentration was prepared from the 1000-ppm standard by dilution. The diluted standard solutions of 1 mL were pipeted into 1.2-mL (small) polyethylene vials, which were then capped and heat-sealed. Eppendorf pipets were carefully calibrated prior to use for dilutions and transfers. The comparator standards were of identical geometry and contained 500, 1000, and 2000 ng of selenium.

Reference Materials A number of standard reference materials (SRMs) were obtained

from the U.S. National Institute of Standards and Technology (NIST); these include NIST SRM 1567a Wheat Flour, SRM 1577b Bovine Liver, SRM 1568 Rice Flour, and SRM 1548 Total Diet. The mass of the materials used varied between 200 and 800 rag, depending on the minimum mass recommended and the amount needed to maintain a full-vial geometry. The materials were weighed directly into small polyethylene vials, capped, and then heat-sealed. These small sample vials were then


380 S h i et al.

placed inside medium-size (7 mL) polyethylene vials and heat-sealed as describe above.

Irradiations and Counting

All samples and standards were irradiated in the inner pneumatic irradiation sites of the Dalhousie University SLOWPOKE-2 reactor (DUSR) facility. At these sites, the thermal flux is 5 x 1011 n/cm2/s and the epithermal flux is 2 x 1011 n/cm2/s when the DUSR is operated at half-power of 8 kW. The stability, homogeneity, and reproducibility of DUSR neutron flux have previously been described (15). In conventional gamma-ray spectrometry, the gamma spectra of the irradiated materials were recorded using a 60-cm 3 Canberra Ge(Li) semiconductor detector with a resolution of 1.88 keV at the 1332-keV photopeak of 6~ a peak- to-Compton ratio of 35:1, and an efficiency of 9.5%. This detector was used in conjunction with an APTEC multichannel analyzer. The 161.9-keV gamma ray of 77roSe was used for assaying selenium and it was free from interference under the experimental conditions employed. The selection of timing parameters, namely the irradiation time (ti), decay time (td), counting time (tc) and the number of cycles (n), depended mainly on the concentration of selenium as well as the major elements present and on the sample matrix. For most of the food samples, these parameters were ti = 30 s, ta = 10 s, tc = 30 s, and n < 3. A decay time of 1 wk between cycles was used. Parameters such as ti = 6 to 12 s, t~ = 10 s, tc = 30 s, and n _ 3 were also used for the samples with high radioactivity after irradiations.

RESULTS AND DISCUSSION

INAA and PCINAA Methods for Selenium

A number of methods were evaluated for the determination of Se in food samples. Table 1 shows the analytical performance of four different INAA. PCINAA with three cycles was found to give the lowest detection limit and high sensitivity. The qualitative detection limits were calculated using the procedure described by Currie (16). In order to measure the widely varying selenium content in a number of Canadian food items reliably, two INAA methods were selected. A conventional INAA method involving only one shot irradiation-decay-counting was satis- factory for samples with relatively high selenium content. However, selenium contents in many of the vegetables, fruits, and other food items were so low that the conventional INAA method could hardly give reliable results. A PCINAA method was used for those low-selenium food samples.

Furthermore, the detection limit for the measurement of selenium varied from sample to sample, mainly because of the Na and C1



Table 1 Evaluation of Various Methods for Selenium

Using a Composite Diet Sample

381

Method t~-td-tc, S Nuclide Counting

used technique

Sensitivity,

counts gg~

INAA 17h-15.6d-12h 7 5 S e Conventional 9066

INAA 30-10-30 s 7rinSe Conventional 2780

INAPt 30-15-30 S 77roSe Anti-coincidence 1470

PCINAA 30-10-30 s, 77roSe Conventional 6900

3 cycles

D.L,,

ng/g

190

110

120

60

D.L. = detect ion limit.

Table 2 Detection Limits and Selenium Concentrations

in Some Food Items Analyzed bv PCINAA Food item 1 cycle, ng/g 2 cycles, ng/g 3 cycles, ng/g

Se conc. D.L. Se conc. D.L. Se conc. D.L.

Evaporated Milk, Canned N.D. 34 41.5 32 40.5 26

Ice cream 62 60 47.6 43 51.5 33

Cheese, Cottage 61.7 58 62.0 48 59.3 39

Broccoli N.D. 25 27.2 12 23.4 9.8

Pineapple, Canned N.D. 14 15.4 11 16.7 9.2

Puddings 48.2 47 42.2 35 45 30

Dinners ( meat or poultry + N.D. 17 16.8 11 18.1 9.4

vegetables)

Pancakes 92 90 111 70 79 53

Pizza 111 98 155 94 150 75

N.D. = not detected.

contents in the food samples. Activities of 24Na and 38Cl can be high, leading to elevated backgrounds, which increase the detection limit. The selenium concentrations and detection limits for some food samples (Table 2) show that lower detection limits were obtained after each successive cycle. After three cycles, most of the measured selenium concentrations were two times greater than the detection limits; however, the selenium contents were below or close to the detection limits for the same food samples when only conventional INAA was used. The improvement of measurement can also be seen from the 162- keV photopeak of 77rnSe in the gamma-ray spectrum after each cycle (Figs. 1 and 2).


382

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Shi et al.

250

*E 200 0 0

0 150

E ",1 ~" 100

50

. / ~ ,~ G' ~ ~ 1 A ,', ! I �9

3 cycles

2 cycles

1 cycle

I ' I I I I

110

Fig. 1.

900 - -

130 150 170 190

Energy, keV

The PCINAA gamma-ray spectra of ice cream.

210

800

700

"E 600 O ,y_o 500 O

400 t~ E = 300 e , -

200

100

0 10

Fig. 2.

k..,~. , / L /

3 cycles

2 cycles

1 cycle

I I I I I

130 150 170 190

Energy, keV

The PCINAA gamma-ray spectra of pancakes.

210

Determination of Selenium Using Different Masses of the Sample

A n u m b e r of food items were analyzed using different masses. The purpose of this exper iment was to evaluate the homogene i ty of the food



Table 3 Selenium Concentrations Using Different Masses of the Sample

383

Sample -1.6 g -4).9 g ,-~0.4 g ---0.2 g

ng/g ng/g ng/g ng/g

Cheese 305 _+ 14 346 + 23 432 + 23 284 + 24

Organ Meats, Liver and Kidney 1344 + 24 1178 + 24 1441 + 36 1249 + 43

Eggs, boiled 370 + 10 327 + 10 434 + 15 465 + 20

Fish, Fresh Water, Fresh or Frozen 417 + 11 402 + 11 415 + 15 475 + 20

Potatoes, Boiled Without Skin 33.8 + 3 20.9 + 2 34.5 + 4 33.2 + 5.0

Cooking Fats and Salad Oils < 8 < 5 < 16 < 17

Cereals (mixed) (baby food) 96.9 + 5 60.0 + 4 105 + 8 69.7 + 8.6

Egg Breakfast on a Bun or Bagel or 230 + 10 188 + 10 260 +16 242 + 19

Muffin or Croissant

items after freeze-drying and to determine if there was a possible blank contamination in the procedure. The selenium concentrations obtained by using four different masses of each sample was in fairly good agreement, considering the low levels of this element (Table 3). The results also sug- gest that the homogeneity of the food samples after freeze-drying was good and that there is no significant blank contamination in the procedure.

Selenium Content of Some Canadian Food Items Analyzed by INAA and PCINAA

About 135 Canadian food items were analyzed by the methods developed in this work. In general, the selenium levels in vegetables, fruits, oils, and beverages are low, whereas the levels are relatively high in meats, fish, cereal and its products, eggs, and poultry. The selenium concentrations, expressed as the mean and standard deviation of three replicates, of typical samples from each food group obtained by using INAA and PCINAA methods are shown in Table 4.

Quality Assessments for Selenium Determination

A large number of food and diet samples are being analyzed in our laboratory and the selenium levels of these samples vary greatly. It is important that the selenium concentration be measured under an extensive quality assurance program. Both internal and external quality assessments of selenium measurements in foods by INAA and PCINAA have been performed. Internal quality assessment was done by analyzing comparator standards along with the food samples. Control charts were


384 Shi et al.

Table 4 Selenium Content of Some Canadian Food

Items by INAA and PCINAA

Food sample Se content,

ng/g + SD

Milk, 1% 36 + 4.7

Beef, ground 241 + 11

Eggs, boiled 327 + 29

Fish, Marine, Fresh 367 _+ 8

Soups <25

Flour, Wheat 391 _+ 23

Potatoes, Raw 29 + 3.7

S~awberries 6.3

Cooking fats < 13

Water < 13

Candy, Suckers <20

Frozen entrees 68 + 11

Fish burger 190 + 12

1.5

E 1.4 ' - ' l

r

1.3

,,- 1.2 o

'- 1.1

o. 1 e . -

= 0.9 o O

0.8

0.7

------If �9 [ I

I �9 �9 �9 �9

I I I IiiI II �9 I II I �9

�9 I

I

I I I I I I

+ 3 s

+ 2 s

Avg.

- 2 s

-3s

0 5 10 15 20 25 30 35

Selenium internal standard number

Fig. 3. Internal quality assessment chart for selenium.

constructed. One such chart involving 29 analyses is s h o w n in Fig. 3. All the m e a s u r e m e n t s were wi th in + 2 s. External qual i ty assessment was also pe r fo rmed . Four SRMs of biological origin wi th a w i d e range of se len ium content were used. The means of each of those materials are



Table 5 External Quality Assessment for Selenium

385

Reference material Measured value, ng/g Certified value, ng/g

NIST SRM 1567a 1080 + 60 1100 + 200

Wheat Flour

NIST SRM 1577b 705 + 47 730 + 60

Bovine Liver

NIST SRM 1568 296 + 25 400 + 100

Rice Flour

NIST SRM 1548 206 + 93 245 + 5

Total Diet

given in Table 5, along with their certified values. It is evident that both the precision and accuracy of the method are good.

CONCLUSIONS

The INAA method employed using the short-lived nuclide 77mSe for the determination of selenium in individual food items was found to be simple and rapid. A PCINAA method with three cycles gave superior detection limits and was used for those food items with relatively low selenium content. The methods are capable of producing results with good precision and accuracy, as shown by internal and external quality assessments, respectively.

REFERENCES

1. O. A. Levander, A global view of human selenium nutrition, Annu. Rev. Nutr. 7, 227-249 (1987).

2. National Research Council, Recommended Dietary Allowance, 10th ed., National Acad- emy Press, Washington, pp. 230-234 (1989).

3. Health and Welfare Canada, Nutrition Recommendations, The Report of the Scientific Review Committee 1990, Canadian Government Publishing Centre, Ottawa, pp. 165-167 (1990).

4. G. Yang, K. Ge, J. Chen, and X. Chen, Selenium-related endemic diseases and the daily selenium requirement of humans, World Rev. Nutr. Diet. 55, 98-152 (1988).

5. H. J. Robberecht and H. A. Deelatra, Selenium in human urine: determination, spe- ciation and concentration levels, Talanta 31, 497-508 (1984).

6. D. Authur, Selenium content of Canadian foods, Can. Inst. Food. Sci. Technol. J. 5, 165-169 (1972).

7. J. Kumpulainen, A.-M. Reittila, J. Lehto, and P. Koivistoinen, Electrothermal atomic absorption spectrometric determination of selenium in foods and diets, J. Assoc. Off. Anal. Chem. 66, 1129-1135 (1983).

8. J. Kumpulainen and K-E. Saarela, Determination of selenium in staple foods and total diets by electrothermal atomic absorption spectrometry without solvent extrac- tion, ]. Anal. Atomic Spectrom. 7, 165-170 (1992).


386 Shi et al.

9. L. S. McDowell, P. R. Giffen, and A. Chatt, Determination of selenium in individual food items using the short-lived nuclide 77m Se, J. Radioanal. Nucl. Chem. 110, 519-529 (1987).

10. W, Zhang and A Chatt, A quality assurance programme for the determination of selenium in foods by instrumental neutron activation analysis, in Proceedings of the Inter- national Symposium on Harmonization of Health Related Environmental Measurements Using Nuclear and Isotopic Techniques, International Atomic Energy Agency, Vienna, pp. 421-433 (1996).

11. J. W. Finley, L. Matthys, and T. Shuler, Selenium content of food purchased in North Dakota, Nutr. Res. 16, 723-728 (1996).

12. M. S. Alaejos and C. D. Romero, Analysis of selenium in body fluids: a review, Chem. Rev. 95, 227-257 (1995).

13. M. Makarewicz and R. Zeisler, Enhanced sensitivity for the determination of selenium by INAA, Biol. Trace Element Res. 43, 95-102 (1994).

14. N. M. Spyrou and S. A. Kerr, Cyclic activation: the measurement of short-lived iso- topes in the samples of biological and environmental samples, J. Radioanal. Chem. 48, 169-183 (1979).

15. D. E. Ryan, A. Chatt, and J. Holzbecher, Analysis for trace elements with a SLOW- POKE Reactor, Anal. Chim. Acta 200, 89-100 (1987).

16. L. A. Currie, Limits of qualitative detection and quantitative determination, Anal. Chem. 40, 586-593 (1968).


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Determination of selenium in canadian food items by cyclic instrumental neutron activation analysis