ANALYTICAL CHEMISTRY, VOL. 51, NO. 6 , MAY 1979
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Determination of Lead in Seawater by Furnace Atomic Absorption Spectrometry after Concentration with Yield Tracer Yoshimltsu Hirao, Kazuko Fukumoto, Hajime Sugisaki, and Kan Kimura" College of Science and Engineering, Aoyama Gakuin University, 6- 16- 1, Chitosedai, Setagaya-ku, Tokyo, Japan
Lead in seawater at the nanogram level was determined by graphite furnace atomic absorption spectrometry (GFAAS) after simple concentration together with radioisotope "*Pb added for yield correction. All chemical procedures as well as purification of reagents were carried out in a laboratory through which filtered air was being passed. Lead In 1 kg of acldlfled seawater was equlllbrated with "'Pb of a known radioactivity, extracted with dithizone In chloroform, backextracted with 0.1 M hydrochloric acid and subjected to GFAAS by a two-channel spectrometer. Recovery yield of lead waz found to be 60-90% from the radioactivity of '"Pb in the back-extract. Lead concentratlons thus determined were 39-1300 ng/kg with about 10% precision.
Determination of lead concentration and distribution in seawater is of great importance in geochemistry and environmental science. Extractable lead was estimated to be about 100 ng/kg in coastal seawater, about 30 ng/kg in surface water of open ocean, and about 4 ng/kg in deep seawater as a summary of the interlaboratory check (1-3). There are several methods for determination of nanograms of lead. Because of the very low concentration of lead in a high concentration of salts, preconcentration of lead is usually recommended. Recovery yield of nanograms of lead in such preconcentration steps has been assumed to be either quantitative or constant, but no direct evaluation has been made for each sample except in isotope dilution mass spectrometry. Furthermore, according to Patterson, owing to serious contamination during analysis, there may be few values which give the lead concentration in seawater properly ( 4 ) . Therefore, in this paper, extractable lead in seawater was determined through a clean laboratory technique by furnace atomic absorption spectrometry after preconcentration with t h e yield tracer. By t h e use of radioisotope '12Pb, recovery yield of lead in the preconcentration step could be measured for each sample and, hence, the preconcentration step was much simplified. EXPERIMENTAL Reagents. Water. Water was purified by a system of a stainless steel still and an ion-exchange resin column, and then by a quartz two-successive stage distillation system in a clean laboratory. Acetone, Chloroform, Nitric and Hydrochloric Acids. The organic reagents (special reagent grade) and the acids (the super special grade (SSG)) from Wako Chemical Industry Ltd. (Wako) were purified twice by an ordinary quartz distillation apparatus. Distillation was performed a t subboiling temperature by using infrared lamps and a heater. Ammonia Water. After passing through a millipore filter of 0.45-pm pore size, pure ammonia gas from a small bomb was absorbed in the purified water. Dithizone. Merck "Pro Analisi" grade dithizone was purified by the ordinary method (5) with the purified reagents. Ammonium Citrate. Citric acid (special reagent grade) from Wako was dissolved in the water. The solution was adjusted to pH 8.5 with the ammonia water. Lead in the solution was ex-
tracted with the purified dithizone in chloroform 5 times. The reagents thus purified were used throughout the experiment, unless otherwise stated. Measurement of Lead B l a n k in t h e Reagents. About 30 mL of the purified reagent was weighed in a quartz beaker and evaporated to dryness in a Teflon oven, through which nitrogen gas was being passed. The beaker was washed with 1 mL of 0.1 M hydrochloric acid. Lead concentration and recovery yield were measured by GFAAS and 'lzPb tracer technique, respectively, as described below. Lead Standard Solution. Five-nine grade of lead metal from Wako was dissolved in nitric acid. The solution was dried up and the residue was dissolved in hydrochloric acid. A 500-ppm solution was stocked and diluted to proper concentration at each measurement. Preparation of '12Pb (6). Anhydrous thorium chloride was prepared from thorium nitrate from Wako via the hydroxide by Chauvenet's method (7), as the nitrate gave a low collection efficiency. Fifty grams of the powdered chloride was put in a wide-mouth polyethylene bottle. About 1 cm above the chloride, a platinum plate (2.5 cm in diameter) was settled and 1200 V of electric potential was applied, as shown in Figure 1. After 1day, about 5 X lo3 dps of "'Pb was collected on the plate. This was sufficient for the tracer and, hence, no further condition was studied. The '12Pb collected on the plate was dissolved in 1 mL of hot concentrated nitric acid in a few seconds and used as the yield tracer. Measurement of "'Pb Radioactivity. The activity was measured by total y-ray counting with a conventional scintillation counter. Usually 'l'Pb and its daughter nuclide, '12Bi, are in radioactive equilibrium, as half lives are 10.64 h and 1.01 h, respectively. But, during the chemical separation of lead, bismuth retained 8G90% of its equilibrium. Therefore, the measurement was done. after 4-h standing. Cleaning of Teflon a n d Q u a r t z Wares. The wares were immersed successively in hot concentrated and 0.1 M nitric acid for one day each. Cleaning of Polyethylene Bottle. A new bottle was washed with soap and acetone, and stood in SSG concentrated and 0.1 M hydrochloric acid at 50 "C for 3-5 days each. The bottle was then stored in another portion of the same 0.1 M acid until use. Adsorption of Lead on the Polyethylene Bottle. It is often heard that lead in seawater is adsorbed on the washed polyethylene wall at neutral conditions, but not a t pH 1. Thus adsorption and desorption of lead on and from the wall were confirmed by using zlzPbtracer. Hydrochloric acid solution of 'l'Pb was added to the seawater sample, and the sample solution was kept a t pH 1 for a while. Then the pH value was adjusted to 8.5 with sodium hydroxide, and after a while the solution was acidified to pH 1 and warmed at 50 " C . The change in the radioactivity of the solution was measured at each stage with elapsed time. Sampling. Surface seawater samples were collected from the head of a small boat. During the sampling, the boat was pointed toward the wind direction and moved forward slowly. A cleaned bottle was hung up with a fishing rod, and nylon net and rope, and sunk into the water with a weight 3-5 m ahead of the boat. Just after the final bubble came out, the bottle was pulled up and taken from the net by polyethylene gloved hands. The bottle was double-wrapped with polyethylene bags, frozen with dry ice as quickly as possible, and stored in a freezer until analysis. Chemical Procedure. Sample seawater was thawed and acidified with hydrochloric acid to pH 1 and warmed at 50 "C for 1 day. A known amount (radioactivity) of 'lzPb was added
0003-2700/79/0351-0651$01.00/0@ 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Table I. Lead Concentration in Purified Water sample wt, g found, ng A
925 946 884 8 68 889 885
B C
D
5.7 5.7 3.3 5.6 13., 26.
chem. yield, %
blank, ng
added ng/kg
lead concn., nglkg
87 63 87 84 86 90
4.2 4.4 0.3 0.3 0.3 0.3
0 0 0 0 11.5 26.5
2.5 4.9 4.0 7.3 17., (6.2)' 32., (6.2)'
E F ' Values in parentheses were obtained by subtracting added lead from total lead. .. 2. 24
Ra x o R n d 6 P o- d 2 P b 366
;"'
55 5
02s
?io)
30
I
i
Figure 1. System for collection of *'*Pb
and equilibrated with exchangeable lead in the seawater for 2 h. After cooling, 25% ammonium citrate solution was added with the ratio of 2 mL to 1L sample, and the pH value was adjusted to 8.0-8.5 with ammonia water. The upper limit of this pH range can be 11.5. Lead in the seawater was extracted into 40 mL of dithizone solution (1mg/100 mL chloroform) from a 1-L sample in a Teflon separatory funnel and back-extracted with 10 mL of 0.1 M hydrochloric acid. The pH value of the back-extract was adjusted to 8.5, and the lead was again extracted with 5 mL of dithizone solution. The organic phase was washed with 20 mL of water of pH 8 and the lead was back-extracted with 1-5 mL of 0.1 M hydrochloric acid. The '12Pb radioactivity in the back-extract was measured for calculation of the lead recovery yield, by comparing it with the initial activity. Graphite furnace Atomic Absorption Measurement. After y-ray counting, the lead solution was supplied for GFAAS. A flameless atomizer system (NJA-FLA-100) and a two-channel atomic absorption spectrophotometer (NJA-AA-8500)of Nippon Jarrell-Ash Co. Ltd. were used. A hollow cathode lamp from Hamamatsu TV Co. Ltd. was used at the recommended current. The spectral line at 283.3 nm was selected for the measurement. Twenty microliters of sample was put into the graphite furnace. The power supply for the furnace was programed as follows: drying at 200 "C; ashing at 700 O C ; atomization at 2200 O C . Background was checked by a D2 lamp in the second channel.
RESULTS AND DISCUSSION Adsorption and desorption of lead on and from the wall of t h e polyethylene bottle were investigated by the method described in the Experimental section. At p H 1,no adsorption was seen. B u t a t p H 8.5, adsorption occurred obviously. Figure 2 shows a typical adsorption curve with a 30-mL polyethylene bottle (the w d area contacted with seawater was 50 cm2). Generally percentage adsorption of lead varies depending on the ratio of the wall area contacted with the water to the volume of the water, and more than 10% lead seems to be adsorbed before analysis. However, the adsorbed lead was leached out completely under the warm acid condition. Therefore, even if lead in the samples was adsorbed on the wall before the sample froze and after it thawed, it was clear that the adsorbed lead was desorbed completely from the wall under t h e stated condition. T h e lead blank in the reagents used was measured by the procedure described in the Experimental section. By this
v I
0
12
24
36
(hour)
Time
Figure 2. Adsorption of lead on polyethylene wall
process, more than 95% lead was recovered. Nevertheless no lead was detected (detection limit: 0.5 ppb) for all reagents, so that the lead blank for each reagent was estimated to be less than 30 ng/kg. No lead peak was observed for the solution of 212Pbtracer after 180-h collection. Therefore the tracer was of sufficiently high specific activity. In the second channel, no lead background was observed for all samples. As to the water used, the lead concentration was estimated by the "chemical procedure" in the Expermental section without citrate addition. T h e results are shown in Table I. Samples A, B, and C were analyzed immediately after Collection. Samples C, D, E, and F were collected a t the same time and the latter three (D, E, and F) were acidified and analyzed after 4 days standing a t 50 O C . Also 11.5 and 26.5 ng per kg sample were added to samples E and F, respectively. From samples A, B, and C, lead concentration in the water was estimated to be 4 ng/kg. From samples D, E, and F, the values 7.3, 6.2, and 6.2 ng/kg were obtained and seemed to be in good agreement with each other. T h e difference of 3 ng/kg between samples A, B, C and D, E, F was assumed to be dissolution of lead from the polyethylene wall a t p H 1 in 4 days, and this value was used for the evaluation of the blank from the bottle during analysis. The lead blank was also checked by the chemical procedure for each set of determinations, and the results were used for the estimation of the blank in Tables I and I1 together with the above mentioned results. For the measurement of the recovery yield with 'l2Pb radioactivity, the counting error was about 2 % . For the GFAAS, uncertainty was estimated to be 6% for the range of 5-15 ppb, from deviations from a calibration curve. Standard deviation for 10 measurements of the same lead solution was about 8% for 10 ppb. Furthermore, by considering the uncertainty in the estimation of blanks described in the preceding paragraphs, it was concluded that uncertainty of 510% was included in each determination. Seawater samples were collected around Tokyo Bay, Japan. Lead concentrations in the samples are shown in Table 11. All
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6 , MAY 1979
Table 11. Lead Concentration in Seawater around Tokyo Bay sample wt, found, chem. sampling station g ng yield, % ( A ) Arakawa Riv. 87 90 78 780 70 (B) Inner Tokyo Bay 1815 (C) Outer Tokyo Bay
( D ) Oshima Is. a
Blank A: Blank from reagents.
894 99 2090 1984 1509
330 44 61 66 58
63 86 64
71 86
blank A, np"
blank B, ngb
concn., ngikg
1.6 5.2 5.4 0.2 5.4 6.0 3.6
0.2 3 1.5 0.2 2 2 1
1300 610 580 510 42 39 42
Blank B: Blank from bottle (cf. Table I a n d text).
samples were collected in individual polyethylene bottles. The values for two 2-L samples from outer Tokyo Bay (C) were in good agreement with each other within 5% deviation. The values for three samples from inner Tokyo Bay (B) were fairly close to each other, even though sample size varied from 2 L to 100 mL. T h e difference between 610 and 510 ng/kg probably resulted from the heterogeneity of lead in the samples, since the samples looked turbid. Namely, difference in contents of the colloidal particles and in dissolution of lead from the particles during warming process may cause the difference. Chlorinity a t station B is about half of that a t C, and the middle of those a t A and C, indicating mixing of seawater (C) and river water (A). A similar trend is also seen among the lead concentrations a t A, B, and C. It is noted that lead concentrations a t C and D are in the same range. The concentration probably indicates a typical extractable lead concentration in surface seawater at about 1km off the ocean coast. Use of 212Pbas yield tracer for lead analysis has several advantages. Preparation of '"Pb tracer is very simple. Once thorium chloride (or thorium compound of high emanating power for 220Rn)and the electric supply system are prepared, the radioactivity will be supplied within 10 h. The tracer 21?Pb obtained is in high radiochemical purity and contains no detectable lead blank. The half life of 10.64 h is long enough for t h e tracer and short enough for the environment. Measurement of 212Pbis also simple. A conventional y-ray counting system is enough for the measurement. Once the activity is added to a sample, as written in the chemical procedure, the recovery yield of lead can be checked at any step by activity measurement. The fourth column in Table I1 shows the total yield thus obtained. For the ultra microanalysis of lead, handling and chemical procedures must be as simple as possible to avoid serious contaminations. Also, only the minimum excess of reagents is permissible to reduce the blanks from reagents. For these requirements, the 212Pbyield tracer technique is extremely useful. The recovery yield can be measured directly for each sample by the use of the tracer and, hence, it is unnecessary to keep the recovery either quantitative or constant. Thus
653
c4 oleo" 1.3 9.7 18.0 18.5
C l : Chlorinity.
the chemical procedure is quite simplified and the amount of reagent to be used is also much reduced. Furthermore, i t is noteworthy that the simplified procedure can eliminate the serious contamination. As for the isotope dilution technique, mass spectrometry (MS) following the chemical concentration and purification with the spike is well known. Once the spike is equilibrated with the sample, chemical yield during the separation step is not necessarily to be found, and sensitivity and precision of the method are both very high. However, in general, MS is not so common and takes much time for one analysis. In these points, the precision of GFAAS is not as good but the sensitivity is in the same range as MS, and it is commoner in laboratories and gives a faster method than MS. Therefore, the combined method of GFAAS with the preconcentration procedure, described here, is one of the best methods for analysis of lead in the exchangeable and extractable state in seawater.
ACKNOWLEDGMENT
R. Tsujino, M. Matsubara and the co-workers in Tokyo Division of Nippon Jarrell-Ash Co. Ltd. permitted us to use the GFAAS system in its best condition, for which the authors' thanks are due.
LITERATURE CITED (1) C. Patterson, Science, 183, 553(1974). ( 2 ) Participants of the Lead in Seawater Workshop, Mar. Chem.. 2, 69-87 (1974). (3) Participants of the Lead in Seawater Workshop, Mar. Chem.. 4, 389-392 (1976). (4) C. Patterson and D. Settle, Natl. Bur. Stand. (U.S.) Spec. Pub/.,422, 321 (1976). (5) E. B. Sandell, "Colorimetric Determination of Traces of Metals", Interscience, New York, 1950, p 107. (6) N. Matsuura. "Jikken Kagaku Koza", Voi. 12, N. Saito, Ed., Maruzen, Tokyo, 1956, p 419. (7) E. Chauvenet, Compt. Rend.. 148, 1267-70 (1909).
RECEIVED for review September 13,1978. Accepted December 15, 1978. Part of this paper was presented a t the 22nd Symposium on Analytical Chemistry, Nara, Japan, June 1977; also a t the 26th International Congress of Pure and Applied Chemistry, Tokyo, September 1977.
