ANALYST, SEPTEMBER 1988, VOL. 113 1431

Differential Anodic Stripping Voltammetric Determination of Selenium in Hair and Flour at a Gold-film Electrode Ting-guo Wu and Wei-zhi Xiang Shanghai Medical Instrumentation College, Shanghai, People's Republic of China Fu-zheng Zhang Shanghai Light Industry College, Shanghai, People's Republic of China Jia-qi Deng" Department of Chemistry, Fudan University, Shanghai, People's Republic of China

A method for the determination of trace amounts of selenium in hair and flour by differential anodic stripping voltammetry with a gold film electrode was developed. The procedure for the digestion of hair and flour is described. Of the media tested, 0.5 M sulphuric acid was found to be the most suitable. The optimum measuring conditions were found to give the best reproducibility for successive scans and the highest selenium stripping peak. The detection limit was 0.1 yg 1-1 of selenium in solution. Keywords: Selenium determination; hair; flour; anodic stripping voltammetry; gold-film electrode

Selenium is an essential trace element. Hou and Zhu1 discovered that selenium prevents Keshan disease, a juvenile cardiomyopathy , and Schrauzer et aZ.2 reported that selenium acts as a cancer-protecting agent in the human body. Both internationally3 and in the USA4 there are generally higher cancer rates in geographic areas with low selenium levels in the soil than in areas with higher selenium levels, and interest in the relationship between selenium and human health has increased greatly in recent years. Hair and flour are normally regarded as indicators of selenium levels in the human body and in soil, respectively.~~6

Neutron activation analysis , atomic fluorescence spec- trometry, gas - liquid chromatography and atomic absorption spectrometry are generally used to determine selenium in biological samples. Andrews and Johnson7 described the voltammetric deposition and stripping of selenium at a rotating gold-disc electrode. Zhang et al.8 applied a gold electrode to the determination of selenium in natural water by anodic stripping voltammetry (ASV). Shen et aZ.9 and Huang et al. 10 were able to determine selenium in serum by cathodic stripping voltammetry (CSV) and Baltensperger and Hertz11 systematically investigated parameters for the determination of selenium by CSV. Deng and co-workers have published several papers on ASV with a gold-film electrode (GFE) for the determination of trace amounts of arsenic,l2 tellurium ,a germanium14 and copper. 15

The aim of this work was to develop a routine method based on differential anodic stripping voltammetry (DASV) with a GFE for the determination of selenium in hair and flour. No work of this kind appears to. have been published previously, although one paper16 described the determination of selenium in electrolytic copper by ASV with a GFE.

In this work, various kinds of media and conditions of determination were investigated and the optimum conditions were found. It takes only about 40 min to analyse a sample of hair or flour (including sample digestion and instrumental determination) owing to the high scan speed and differential signal, with a high sensitivity and short electrolysis time. This method is inexpensive, rapid, simple, efficient and very accurate.

Experimental Apparatus

A Model SVA-2 stripping voltammetry analyser (Shanghai

* To whom correspondence should be addressed.

No. 11 TV Factory, Shanghai, China) was equipped With a Model LZ3-100 X - Y recorder (Dahua Instrumentation Factory, Shanghai, China). The three-electrode system con- sisted of a glassy carbon working electrode (area 0.124 crnz), a glassy carbon rod auxiliary electrode (0.4 cm diameter and 2 cm length) and a silver - silver chloride (0.5 Y KCl) reference electrode, to which all potentials are referred.

Reagents Analytical-reagent grade chemicals were used.

SuZphuric acid. Eliminate selenium as follows in order to minimise the blank value.17 Add 30 ml of hydroboric acid to 200 ml of concentrated sulphuric acid in a beaker and place the beaker on a hot-plate at 280°C until fumes emerge from the sulphuric acid. After cooling, store the sulphuric acid in a quartz bottle.

Digestion solution. Prepare by mixing the sulphuric acid with perchloric acid (3 + 4).

Nitric acid. De-ionised water. Re-distilled twice from quartz. Purge gas. High-purity nitrogen. Stock standard selenium(N) solution, 1 X 10-3 M. Dissolve

0.0790 g of high-purity selenium powder in water, add 3.5 ml of nitric acid and dilute to 1000 ml with water.

Preparation of Samples Hair sample Soak a hair sample in a solution of 1% d m detergent powder for 30 min and keep it at 45°C. Wash the sample with tap water and then with purified water six times separately. Place the wet hair sample in a beaker and heat the beaker in an oven at 70 "C for 24 h. Cut the hair into small pieces and store it in a glass bottle.

Flour sample Place some flour in a beaker and then keep the beaker in a desiccator for 24h before use. Store the beaker in the desiccator between analyses.

Digestion of Samples Hair sample Place an exact amount of 0.3000 g of hair sample in a 50-ml quartz cell (also used as an electrolytic cell). Add 3.3 ml of digestion solution and 0.10 ml of 5% d m sodium molybdate

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1432 ANALYST, SEPTEMBER 1988, VOL. 113

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0.6 0.7 0.8 0.9 EN VS. A0 - AgCl

Fig. 1. Differential anodic stripping voltammogram for 40 p 1-1 of selenium(1V). Deposition potential, Ed = -0.1 V in 0.5 M H,S&; td = 15 s; and sweep rate, Y = 100 mV s-1

solution, used as a catalyst in the digestion process. Place the cell on a hot-plate at about 230 "C and heat until the colour of the digestion solution becomes bright yellow - green. Con- tinue the heating for a further 2 min, then cool the solution. Add a small volume of water and heat for a further 2 min. Transfer the solution into a 25-ml calibrated flask and dilute to volume with water.

Flour sample Place 0.5000 g of flour in the cell as above and add 3.6 ml of digestion solution and 0.30 ml of 5% mlm sodium molybdate solution. Heat the cell on a hot-plate at about 80°C for 8 rnin and then at about 230°C until the d o u r of the digestion solution becomes bright deep yellow - green. Continue to heat for a further 4 min, then proceed as for a hair sample.

Plating of the Glassy Carbon Electrode (GCE) with Gold Prior to plating, polish the electrode surface successively with fine ceria powder on a piece of chamois leather and on two pieces of filter-paper lubricated with ethyl acetate, then rinse it with water. Insert the electrode into a gold(II1) solution (2.5 X 10-4 M) in 1 M sulphuric acid. The subsequent plating procedure and the characteristics of the GCE were described elsewhere.16 A single gold film can be used for at least 200 determinations before the decrease in sensitivity requires re-polishing and the deposition of a new gold film.

Measurement Procedure Prepare a test solution containing 40 pg 1-1 of selenium. Transfer the solution into a cell and immerse the GFE in the solution. Purge the solution with nitrogen for 5 min. Condition the GFE at +1.2 V for 30 s, then deposit for 15 s at -0.1 V. Cease purging and, after 30 s, scan the GFE from -0.1 to + 1.2 V at 0.1 V s-1. Record the selenium stripping peak with the X-Y recorder (Fig. 1). Repeat from the conditioning step until a selenium stripping peak of constant height is obtained. In this way, tests of various conditions for the determination of selenium were conducted.

Results and Discussion Optimum Conditions for the Determination of Selenium Comparative tests of various media, such as nitric, perchloric and sulphuric acids and their mixtures in different propor- tions, were conducted. Only when sulphuric acid was used as the medium were a small number of cycles needed to obtain a selenium stripping peak of constant height.

Variation of the concentration of sulphuric acid from 0.05 to 1.25 M, of the deposition potential from 0 to -0.4 V, of the

Table 1. Recovery and precision

Sew Amount of Amount of found*/ C.V., Se added Se recovered Recovery,

Sample gkg-1 YO gkg-1 gkg-' YO

Flour . . 64 5.8 70 130 94 Hair . . 620 3.2 670 1260 96

* Each value represents the mean of six analyses.

conditioning potential from +1.0 to +1.5 V and of the conditioning time from 20 to 90 s was investigated. The results indicated that the optimum conditions for the determination of selenium are 0.5 M sulphuric acid as the medium, deposition at -0.1 V and conditioning of the GFE at +1.2 V for 30 s. These conditions gave the highest selenium stripping peak and the best reproducibility of successive scans.

Relationship Between Peak Height (Stripping Current) and Concentration

The peak height of the blank solution is zero with a deposition time of 30 s. The graph of peak height against concentration of selenium is linear in the range 4-100 pg 1-1 with a deposition time of 15-30 s. The detection limit is 0.1 pg 1-1 of selenium in solution.

Hamilton et al. 16 found the detection limit to be 0.1 pg 1-1 of selenium in solution using a GFE. In their work, the absolute responses (peak height vs. concentration) for selenium in 0.1 M nitric acid were within the range 1040 pg 1-1 with a deposition time of 50 s. Our deposition time of 15 s for selenium in the range 20-100 pg 1-1 is about one third of their value of 50 s. This difference in deposition time may be explained to a certain extent by the choice of 0.5 M sulphuric acid as the medium and using a high scan speed and differential signal.

Analysis of Samples Pour 25 ml of sample solution into a cell and start the DASV measurement as described above. The standard additions method is used to determine selenium in the sample. The times required for deposition of a hair sample and a flour sample are 1 and 5 min, respectively. The results are in good agreement with recently published values. 18-20

The coefficients of variation for the determination of selenium in hair and flour samples are 3.2 and 5.8%, respectively, for six replicate determinations, as shown in Table 1. The recoveries of known amounts of selenium added to hair and flour samples are also given in Table 1. The results indicate that this method is suitable for the determination of trace amounts of selenium in a variety of samples such as hair, blood, urine, water and food.

It was found that purging the sample solution with nitrogen before the determination was unnecessary, because the same results were obtained whether or not the sample solution was purged. In our view, selenium deposited on a GFE at -0.1 V may exist in the form of the intermetallic compound AuSe, in which the combined selenium would not be affected by dissolved oxygen.

Conclusions The method has high sensitivity and high precision and is simple and inexpensive. The instrumentation for DASV is inexpensive and therefore the method seems to be an attractive alternative to atomic absorption spectrometry for the determination of selenium.

A multiple stripping peak appeared when an in situ GFE was used. Therefore, the gold film must be plated on a glassy carbon electrode before the determination.

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ANALYST, SEPTEMBER 1988, VOL. 113 1433

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References Hou, S., and Zhu, Z., Huanjing Kexue, 1982,3,18. Schrauzer, G. N., White, D. A., and Schneider, C. J., Bioinorg. Chem., 1977, 7, 35. Schrauzer, G. N., and White, D. A. , Bioinorg. Chem., 1977,7, 23. Shamberger, R. J., Tytko, S. A., and Willis, C. E., Arch. Environ. Health, 1976,31, 231. Laker, M., Lancet, 1982,2,260. Laker, M., Lancet, 1982,2,449. Andrews, R. W., and Johnson, D. C., Anal. Chem. , 1975,47, 294. Zhang, F., Ji, L., and Deng, J., Huaxue Skjie, 1984, 25(4), 132. Shen, Z., Lu, R., and Shi, Q., Ying Yong Huaxue, 1984,1(5), 78. Huang, B., Zhang, H., Pu, G., Yin, F., Zhang, S., and Yang, H., Anal. Lett., 1985, 18(3B), 279.

11.

12. 13. 14.

15. 16.

17. 18.

19.

20.

Baltensperger, U., and Hertz, J., Anal. Chim. Acta, 1985,172, 49. Deng, J., Chen, X., and He, P. , Huaxue Xuebao, 1980,38,441. Deng, J., He, P., and Chen, X . , HucrxueXuebao, 1980,38,529. Deng, J., Zhang, Q. , and Wang, Z., H u u e Xuebao, 1982,40, 151. Zhang, F., Ji, L., and Deng, J., Fenxi Huuxue, 1984,12, 162. Hamilton, T. W., Ellis, J., and Florence, T. M., Anal. Chim. Acta, 1979,110, 87. Lu, Z., Xie, G., and Zeng, X . , Fenxi Huaxue, 1980,3,223. Schroeder, H. A., Frost, D. V., and Balassa, J. J., J. Chronic Dis., 1970,23,227. Schroeder, H. A., Frost, D. V., and Balassa, J. J., J. Chronic Dis., 1970,23,449. Zhang, F., Wu, T., Ji, L., and Deng, J., Henliang Fenxi, 1985, 2,21.

Paper 8l00136G Received January 19th, I988

Accepted May Idth, 1988

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