Analyst, October, 1967, Vol. 92, pp. 611-613 61 1 Improved Determination of Microgram Amounts of Lead in Food with a Radioactive Tracer BY DONALD C. BOGEN AND MICHAEL T. KLEINMAN (Health and Safety Labovatovy, U.S. Atomic Enevgy Commission, New York, New York 10014) Microgram amounts of lead in food are determined by use of a radio- active tracer. The radioactive tracer, lead-2 12, is radiochemically purified from thorium nitrate and is used for chemical yield determination. Food sample dissolution involves freeze drying and wet ashing to minimise con- tamination and eliminate volatilisation. The stable lead is determined spectrophotometrically as the dithizone complex at 510 mp. The chemical recovery for analysis is 94 per cent., and the precision of analysis is better than 10 per cent.THE analysis of low-level lead in environmental samples by existing techniques is sub ect to uncertainty because of contamination and incomplete recovery. Although reagent ( on- tamination can be reduced in some matrices by dry ashing, Gorsuch observed losses of Lead in food dry ashed at ,500" C, but found that by wet ashing in a nitric acid system, excli sive of sulphuric acid, average recoveries of 100 per cent. were obtained.1 A technique has x e n developed that decreases contamination from reagents and corrects for losses in reco tery. This involves speeding up the diqestion procedure by freeze-drying the samples befor: wet ashing and adding a radioactive tracer. This tracer is prepared by a modification of Petrow's procedure,2 with natural thorium nitrate as the source of the radioactivity.The lead is concentrated by solvent extraction of the dihydrogen tetra-iodolead(I1) complex from 5 per cent. hydrochloric acid into 3-methyl-2-butanone.3 The organic phase is wet ashed and lead is extracted as the dithizone complex into chloroform.4 The amount of lead is determined by spectrophotometric measurement of the absorption of this complex at 510mp and the chemical recovery by gamma-scintillation counting of the lead-212 tracer. EXPERIMENTAL APPARATUS-- Spectrophotometric measurements were made with a Beckman Model DU spectrophotometer and radioactivity measurements with a Baird Atomic Model 810 scintillation counter, fitted with a Raird Atomic Model 132 scaler - timer unit. REAGEKTS- solzdion-Prepare a 30 per cent. v/v solution in toluene.A 1-kg sample is freeze-dried, wet ashed and lead-212 tracer is added. Freeze-drying was carried out with a Lab-Line Freeze Dryer. Methyltri-octanoylammonizlm chloride (Aliqzlat-336, General Mills Inc., Kankakee, Illinois) 3-Alefh.d-2- bzdanon e. Diphen-ylthiocarba~one, 0.005 per ce& 7 ~ / v in chloro f o m . Basic h f e r solzction-Mix 30 ml of 10 per cent. w-/v sodium cyanide, 75 ml of 2 per cent. w/v sodium sulphite, 340 ml of concentrated ammonium hydroxide and 605 ml of de-ionised water. Lead- 212 tracer- Prepare by dissolving 10 g of analytical-reagent grade thorium nitrate in 100 ml of de-ionised water and add a few drops of concentrated nitric acid. Evaporate the solution nearly to dryness, dilute to 55 ml and add 45 ml of 6-6 M hydrobromic acid.Lead- 212 was separated from thorium by extraction into 50 ml of 30 per cent. methyltri-octanoyl- ammonium chloride that had been equilibrated with two 50-ml volumes of 1.5 M hydrobromic acid. The organic phase was washed four times with 25-ml portions of 0.1 M hydro- bromic acid and the lead stripped from the organic phase with 25 ml of concentrated hydro- chloric acid. The aqueous phase was washed with 50ml of toluene to remove traces of methyltri-octanoylammonium chloride. The lead-containing solution was evaporated to exactly 25 ml and the radiochemical purity checked by gamma-scintillation counting. As lead-212 has a 10.6-hour half-life, it was necessary to complete the wet ashing of the sample before adding the tracer.612 BOGEN AND KLEINMAN : IMPROVED DETERMINATION OF MICROGRAM [Ana@St, VOl.92 PROCEDURE- One kilogram of the sample was dehydrated by freeze-drying for 24 to 72 hours. Subse- quently, the 30 to 50 per cent. weight-reduced sample was wet ashed with 1 to 2 litres of concentrated nitric acid in new borosilicate glassware. A cover glass placed on top of the ashing beaker provided good reflux action in destroying the organic material. After evaporat- ing the solution until nearly dry, the deposited salts were treated with 100 ml of concentrated nitric acid and 25 ml of 30 per cent. hydrogen peroxide to ensure complete destruction of the organic material. The residue was then dissolved in 100 ml of 6 M nitric acid, and the remaining salts were removed by gravity filtration on ashless filter-paper. The filter-paper containing the salts was placed in a Teflon beaker and the silica expelled by distillation with hydrofluoric and nitric acids, during which treatment the filter-paper was destroyed.A true solution was thus obtained, which was combined with the filtered solution. An amount of lead-212; tracer, sufficient to yield 20,000 counts per minute, was added to the wet-ashed sample and the solution evaporated almost to dryness. The residue was then dissolved in 500ml of 5 per cent. hydrochloric acid and 50ml of 10 per cent. potassium thiocvanate solution added. Iron, tin and other elements that might interfere in subsequent steps were removed by extraction with two 100-ml portions of 3-methyl-2-butanone. About 10 ml of 10 M sodium iodide solution were added and the lead iodide complex extracted with 200 ml of 3-methyl-2-butanone.The organic phase was evaporated to dryness and the residue wet ashed with small amounts of nitric and perchloric acids. After drying, the residue was dissolved in 4 drops of hydrochloric acid and 75 ml of basic buffer solution were added. Lead was extracted with 10-ml portions of dithizone solution until the organic phase remained blue - green for two successive extractions and was then extracted into potassium acid phthalate buffer (pH 3-4), which separated it from any bismuth.j About 75 ml of basic buffer solution were added and the lead again extracted as its dithizone complex. The solution was then transferred into a 50-ml calibrated flask, diluted to volume with chloroform and the lead determined spectrophotometrically at 510 mp.The solution was transferred into a suitable counting vial and counted in a well-type gamma-scintillation counter, after allowing 8 hours to elapse for the ingrowth of lead-212 daughters. RESULTS AND DISCUSSION Table I shows an average recovery of 94 per cent. of the lead that was added to “spiked” Table I1 indicates that the method is repro- samples after correcting for contamination. ducible to within 10 per cent. TABLE I ANALYSIS OF “SPIKED” SAMPLES Milk sample No.* 1 2 3 4 5 6 Lead added, Pg 0 100 200 300 400 500 Lead recovered, PQ 0 88 184 292 372 504 Average lead recovery 94 per cent. * Each of the six milk samples weighed 1 kg. TABLE I1 MEASUREMENT OF SPLIT SAMPLES First analysis, Second analysis, Average percentage Sample Pg Per kQ PQ Per kg deviation from mean Milk A .. .. . . 216 184 200 f 8 Fresh vegetables . . . . 298 244 271 f 10 Canned vegetables . . . . 674 708 691 2 Milk B .. . . . . 186 200 193 & 4October, 19671 613 It is necessary to determine the lead in the nitric acid used in wet ashing as this reagent is a major source of contamination. vITe carried out four determinations on the acid and found a lead concentration of 2 to 5 pg per litre. The procedure was, therefore, designed so that not more than 2 litres of nitric acid were used. The reagent contamination level of the complete technique, including sample preparation, was evaluated by analysis of several blanks and found to total 20 & 5 pg. Kehoe6 has stated a range of lead intake from the diet of 0.10 to 1.0 mg per day.Assuming a total intake of 2.0 kg of food7 the lead concentration in the diet would be in the range of 50 to 500pg per kg. It is apparent, therefore, that contamination presents a serious problem unless large sample aliquots of about 1 kg are used. AMOUNTS OF LEAD I N FOOD WITH A RADIOACTIVE TRACER I I l I l l l I i I I I 1 1 10 20 30 40 50 60 70 80 90 100 110 I20 130 140 I Time, hours 0 Fig. 1. Observed decay curve of lead-212 tracer (10-5-hours half-life) The procedure described for the preparation of the tracer provides radiochemically pure Jead-212. This was verified by gamma counting of prepared solutions of lead-212 over a period of several days. The half-life of lead-212 was determined to be 10.5 hours, as shown in Fig. 1, and this is in good agreement with the established value of 10.6 hours.* As the half-life of lead-212 is 10-6 hours and the sample preparation time is 3 to 6 days, the tracer must be introduced into the solution prepared after wet ashing. 1. 2. 3. 4. 5. 6. 7. 8. REFERENCES Gorsuch, T. T., Analyst, 1959, 84, 137. Petrow, H. G., and Cover, A., Analyt. Chew., 1965, 37, 1659. West, P. W., and Carlton, J . K., Analytica Chim. Acta, 1952, 6, 406. Morrison, G. H., and Freiser, H., “Solvent Extraction in Analytical Chemistry,” John Wiley Talvitie, N. A., and Garcia, W. I., Analyt. Chew., 1965, 37, 851. Kehoe, R. A., Avchs of Envir. Hlth, 1961, 2, 418. Rivera, J., and Harley, J. H., U.S. Atomic Energy Commission Refiovt HASL-147, Health and Strominger, D., Hollander, J. M., and Seaborg, G. T., Rev. Mod. Phys., 1958, 30 (2), Part 2, 788. Received Februavy 27th, 1967 and Sons Inc., New York; Chapman & Hall Ltd., London, 1957, p. 214. Safety Laboratory, New York, 1964.