T o overcome the difficulties of transferring this easily hydrolyzed material from the vessel to the source, a #22 thin-walled Teflon (DuPont) tube was attached to the atomizer. The other end was immersed in the fluid while the source was running. The positive pressure in the source allowed dry H e t o purge the feed tube of moisture bearing air. To further minimize the hydrolysis of the samples, it was required that they be handled in 50-ml vessels having a 5-ml neck. To readily aspirate this dense liquid, it was necessary t o maintain a head of less than 1 cm. The work here was performed with the fluid at that level.
Analytical curves for Cu, Al, Fe, Si, and Ti in BBr3 were prepared. In Table I1 the concentration range covered and the analytical line employed for the analyses are shown. The lowest concentrations at which quantitative determinations were made were those which, under our experimental conditions, gave a line having a maximum transmittance of 90%. The precision of the analysis is excellent. Based on fourteen determinations of Si and Ti in BBr3, the relative standard deviation at the 95% confidence level was shown t o be 4.6%.
RECEIVED for review May 27, 1968. Accepted July 15, 1968
Microdetermination of Mercury by the Oxygen Bomb Combustion Method Masahiko Fujita, Yasushi Takeda, Tadao Terao, Otomatsu Hoshino, and Tyunosin Ukita Faculty of Pharmaceutical Sciences, Unicersity of Tokyo, Hongo, Tokyo, Japan
ORGANICmercury compounds are widely used as fungicides
EXPERIMENTAL
against rice blast disease. I t is, therefore, very important from the standpoint of public health to determine the mercury residue in rice grains. In order to determine the mercury in rice, it is necessary to digest the organic material. The digestion method generally applied has been the wet digestion procedure ( I ) , but this procedure is tedious and takes a long time for complete digestion of the materials; also, it is sometimes accompanied by a loss of mercury during the digestion (2). The determination of mercury by a combustion method has been reported by Schoeniger (3) and several other investigators ( 4 ) . Though these methods are useful in their rapidity and accuracy, the amounts of samples available are too small, because the combustion is performed in a glass flask. In the present work, combustion has been carried out in an oxygen-filled bomb made of stainless steel to facilitate the treatment of larger amounts of organic material. By this method, satisfactory recovery of mercury from samples COP taining known quantities could be achieved. No interference from the metal bomb was observed in the mercury determination. The combustion was carried out in the presence of 1.ON nitric acid previously added to the bomb to absorb the oxidation products. Without the nitric acid, reproducibility of the results was unsatisfactory. The oxidation products were subsequently reduced by addition of hydroxylamine hydrochloride and urea solution. The mercury was extracted with dithizone solution, and the mercury dithizonate was submitted to column chromatography for separation from excess dithizone (5), and was determined colorimetrically.
Apparatus. Combustion was carried out in a bomb designed by Fujiwara and Narasaki (6). The spectrophotometric determination was carried out in a Beckman DU spectrophotometer using 1.00-X 0.50-cm quartz cells. Reagents and Chemicals. Solvents used were distilled before use. All chemicals used were of the highest purity obtainable. Dithizone solution: Dithizone (5.1 mg) was dissolved in 100 ml of carbon tetrachloride. Urea solution: Ten grams of urea was dissolved in 100 ml of deionized water and the solution was washed with dithizone solution and carbon tetrachloride successively until the extracts became colorless. Hydroxylamine hydrochloride solution: Forty grams of hydroxylamine hydrochloride was dissolved in 100 ml of deionized water. This solution was washed with dithizone solution and carbon tetrachloride as in the case of urea solution. Aluminum Oxide Column Chromatography. This was carried out by the method of Ishikura and Yokota ( 5 ) . The column (0.3- x 10-cm) was packed with 0.3 gram of aluminum oxide of activity grade 3 (7). Combustion of Samples and Preparation of Sample Solutions. The amounts of sample appropriate for one combustion are shown in Table 11. The samples were wrapped in a sheet of rice paper of 7- X 7-cm size and tied with cotton thread and put in the sample cup. In the case of oils, the sample was poured directly into the platinum cup and the end of the cotton thread was immersed in the sample. The remaining part of the thread was passed through the coil of platinum wire as a fuse. Forty milliliters of 1.ON nitric acid was then added to the bomb, wetting down the inside surface. The bomb was assembled and oxygen was passed into it until the pressure had built up to 25 kg/cm2. The sample was ignited by passing a small ac current through the platinum coil under a potential difference of 12 volts. After ignition, the bomb was shaken for 15 seconds and cooled in an ice
(1) Committee on Editing Methods of Analysis, “Official Methods of Analysis of the Association of Official Agricultural Chemists,” 10th Ed., Association of Official Agricultural Chemists, Washington, D.C., 1965, p 375. (2) Analytical Methods Committee, Analyst, 90, 515 (1965). (3) W. Schoeniger, Mikrochim. Acta, 1955, 123. (4) B. C. Southworth, J. H. Hodecker, and K. D. Fleischer, ANAL. CHEM.,30, 1152 (1958). (5) S. Ishikura and K. Yokota, Chem. P/iunn. Bull., 11,939 (1958). .
I
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ANALYTICAL CHEMISTRY
(6) S. Fujiwara and H. Narasaki, Japan Analyst, 10,1268 (1961). (7) H. Brockmann and H. Schrodder, Ber., 74,73 (1941).
bath below room temperature. After cooling, the gases were released through a small orifice t o atmospheric pressure. The contents of the bomb were transferred to a 300-ml beaker. The inside surface and the parts of the bomb were rinsed out into the beaker with 160 ml of 1.ON nitric acid. The combined solution was transferred to a 500ml separatory funnel. Two milliliters of the urea solution and 5 ml of the hydroxylamine hydrochloride solution were added. The mixture (sample solution) was used for the following analysis. Determination of Mercury. Six milliliters of the dithizone solution was pipetted into the sample solution and the funnel was shaken vigorously for 1 minute. Carbon tetrachloride layer was filtered through a cotton plug. A 5.0-ml aliquot of the filtrate was put on the aluminum oxide column and the column was washed with 0.5 ml of carbon tetrachloride. The mercury dithizonate was eluted with the mixed solvent of carbon tetrachloride : chloroform (1 : 1). The first 1 ml of eluate was taken in a 1-ml volumetric flask containing 1 drop of glacial acetic acid. By these procedures, the mercury dithizonate was separated from the excess dithizone. The absorbance of the eluate at 490 m p was measured using the solvent as reference. RESULTS AND DISCUSSION
Recovery of Mercury from Rice Bran. The recovery test was performed using the rice bran samples which contained known quantities of mercury. The rice bran samples were prepared by adding 0.05 t o 0.15 ml of 0.001 mercuric chloride in 1.ON nitric acid to 1 gram of rice bran t o make mercury contents ranging from 0.5 to 1.5 pg. Recoveries of added mercury, as shown in Table I, averaged 93%. The background amount of mercury, 0.16 pg, which was originally contained in the rice bran used, was subtracted from the total amount of mercury to obtain the net amount. Preparation of Calibration Curve. A calibration curve was obtained from the colorimetric determinations using a standard mercury solution (1 pg of mercury per ml prepared from mercuric chloride in 1.ON nitric acid). Aliquots containing 0.2, 0.8, 1.4, and 2.0 pg of mercury were diluted to 200 ml with 1.ON nitric acid. To each was added 5 ml of hydroxylamine hydrochloride solution and 2 ml of urea solution. Each was extracted with dithizone solution, and the mercury dithizonate was chromatographically separated from the extract o n the aluminum oxide column, and the absorbance was determined as described above. The absorbance was found to be linear against 0 to 2.0 pg of mercury. Analysis of Samples. Unpolished rice grain, rice bran, vegetable oils, normal human hair, and liver, kidney, and brain of mercury-poisoned rats were analyzed by both the present, combustion method and usual wet digestion method. The results are shown in Table 11. The results indicate that there is no significant difference between values obtained by the two methods. It was found that the maximum amounts that could be burned completely by this combustion method were 4 grams for rice grain and 2 grams for rice bran. Vegetable oils which were hardly digested by the usual wet digestion method were completely burned by our method. Copper and the noble metals are known to form complexes with dithizone in 0.1N nitric acid. As copper is widely found in plants and animals as a natural component, and also found as pesticide residue, it is possible that certain samples for the
Added, 0
0.5 0.5 0.5 1.0 1.0 1 .o 1.5 1.5 1.5
pg
Table I. Recovery of Mercury Added as HgClt to 1 Gram of Rice Bran Mercury, pg Mercury, % Total Net Added Average found found recovered, recovered, (A) (A - 0.16) 2 0.16 .. . . , 0.58 0.63 0.67 1.01 1.23 1.13 1.59 1.51 1.47
0.42 0.47 0.51 0.85 1.07 0.97 1.43 1.35 1.31
84 94 02 85 07 97 95 90 88
93
96
91
Table 11. Analysis for Mercury Content Bomb combustion Wet digestion Wt of sample Hg found, Hg found, analyzed, Samples PPm PPm grams Hair 4.0 4.60 0.50 Hair 6.5 6.35 0.50 Rice bran 1.58 1.72 2.00 Rice bran 1.58 1.6@ 2.00 Rice bran 1.45 1.51 2.00 Rice bran 1.45 1.55a 2.00 Unpolished rice 0.61 0.80 4.00 Unpolished rice 0.097 0.18 4.00 Unpolished rice 0.11 0.17 4.00 Liver 24.6 25.99 0.34c Kidney 19.2 13.07 0.2@ Brain 2.17 2.27 0 . 405c Brain ... 2.95 0.62~ Salad oilb ... trace" 1 .OO Sesame oilb ... trace" 1.00 = The data obtained by combustion in the bomb from which inner plutinum vessel was detached. b The samples were poured directly in the platinum cup and burned. The samples were dried in cucuo over phosphorus pentoxide.
present mercury analysis may contain an appreciable amount
of copper. But the copper is known to be separated from mercury by the chromatography involved in the abovementioned procedure (5). Therefore, in this method, contamination with copper had n o effect on the determination of mercury. I n conclusion this method has been found convenient in its rapid digestion of the samples, accuracy, and availability for relatively large amounts of sample. ACKNOWLEDGMENT
The authors express their gratitude to Shizuo Fujiwara of the University of Tokyo and Hisatake Narasaki of Saitama University for the rental of the combustion bomb. RECEIVED for review April 4, 1968. Accepted July 1, 1968. This research was supported by a grant from the Ministry of Health and Welfare, Japan.
VOL. 40, NO. 13, NOVEMBER 1968
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