1682
Anal. Chem. 1982, 5 4 , 1682-1666
Determination of Bismuth in Environmental Samples by Flameless Atomic Absorption Spectrometry with Hydride Generation Dong So0 Lee SCrlppS Instltution of Oceanography, Unlverslty of Callfornla -San
Dlego, La Jolla, California 92093
A method Is described for the measurement of plcogram amounts of blsmuth In envlronmental samples. The blsmuth Is reduced In solutlon by sodium borohydride to blsmuthlne, strlpped wlth helium gas, and collected In sltu In a modlfled carbon rod atomizer. The collected bismuth Is subsequently atomized by lncreaslng the atomlzer temperature and detected by an atomlc absorptlon spectrophotometer. The absolute detectlon llmH Is 3 pg of bismuth. The preclslon of the method is 2.2% lor 150 pg and 6.7% for 25 pg of blsmuth. Analytlcal results are presented for natural waters, shells, marine algae, and sedlments.
Bismuth in seawater has been determined either by anodic stripping voltametry (1,2) or by spectrophotometry after preconcentration by ion exchange and dithizone extraction (3). At the concentrations found in these previous investigations, the methods are utilized close to their detection Iimits and are vulnerable to contamination as a result of using large amounts of reagents. About 20 ng/L of Bi are the reported concentrations. These values appear high and are questionable due to the high geochemical reactivity of Bi and its rarity in nature. Hydride generation and atomic absorption detection provide a very sensitive method for metalloid analyses. Collection of the hydride in a liquid nitrogen trap and then subsequent rapid introduction into an atomizer improved the sensitivities of such elements as arsenic and tin by an order of magnitude compared to the continuous method (4-6). However, such a collection method is only applicable to the elements whose hydrides are stable enough to be handled at ambient temperature. Bismuthine is unfortunately too unstable for this technique. An earlier attempt to collect BiH3 in a liquid nitrogen trap was unsuccessful in this laboratory since it decomposed upon warming instead of volatilizing. Only 5-15% of the trapped BiH3 was volatilized. Herein, a heated graphite tube was used to collect the generated BiH3. All experimental parameters for BiH3 generation and atomic absorption detection were optimized such that 72% of the generated BiH3 was collected in the tube reproducibly. The collected BiH3 was atomized by increasing the tube temperature. This method is so sensitive that most natural waters can be analyzed without preconcentration. However, open ocean waters contain Bi close to the detection limit, 0.06 ng/L. Such samples were preconcentrated by partial precipitation of calcium and magnesium by adding NaOH. The detection limit can be reduced to 0.01 ng/L or less. Bismuth was determined in a suite of environmental samples: waters, shells, algae, and marine sediments. Typical Bi concentrations in ocean waters are 0.1 ng/L, 2 orders of magnitude lower than the previously reported values.
EXPERIMENTAL SECTION Apparatus. The apparatus for the generation and collection of the bismuthine is illustrated in Figure 1. The sample is 0003-2700/S2/0354-1682$01.25/0
contained in a hydride generator into which helium is introduced through a fritted bubbler and which has a side port for the injection of sodium borohydride solution. This port is closed off by a septum held in a nylon Swagelok union (6.4 mm 0.d.). Different size generators (25-200 mL) can be attached to the apparatus. The carbon rod atomizer is a modification of a Varian-Techtron CRA-90 for the collection of the gaseous bismuthine. A hole (2 mm i.d. for the front half and 4 mm i.d. for the rear half) was drilled through the middle of the front carbon rod electrode. The carbon rod cell was positioned so that ita sample injection port matched with the electrode hole. The hydride generator was connected to the carbon rod atomizer by 15 cm long, 5 mm 0.d. nylon tubing. A piece of 6 mm i.d. Tygon tubing was used for the connection between the generator and nylon tubing while 3 mm 0.d. quartz tubing and a strip of Teflon tape were used between the atomizer and the tubing. The generated bismuthine is stripped from the solution by a helium gas stream, adsorbed on the inside walls of the carbon rod atomizer and then atomized and detected by an atomic absorption spectrophotometer. A Varian-Techtron Model AA-6 atomic absorption spectrophotometer equipped with a Linear Instruments Corp. Model 252 strip chart recorder and a bismuth hollow cathode lamp was used to measure absorbance. The operating parameters were: lamp current, 7 mA; wavelength, 223.1 nm; slit width, 0.2 nm; He carrier gas, 45 mL/min during stripping and 6 mL/min during atomizing. The temperature settings of the carbon rod atomizer were: drying, 300 "C for 90 s; ashing, 350 "C for 60 s; atomizing, 1850 "C for 2 s with 800 OC/s ramping. So that we could determine the optimal conditions for hydride generation using 207Bias a tracer, the apparatus was modified by replacing the carbon rod atomizer with a 27 mm diameter Millipore filtering unit (Figure 2). The generated bismuthine was collected on a 27 mm diameter Whatman filter paper which had been dampened with 5 drops of 2% KMn04 solution just before introduction to the apparatus. The paper was removed from the filter holder, transferred to a 25-mL glass scintillation vial and its 570 kV y-ray radiation was measured by a ND 100 multichannel analyzer (Nuclear Data Inc.). Standards and Reagents. All solutions were prepared with doubly distilled water. A 1 ng/mL working bismuth standard solution and a 1.1 nCi/mL 207Bitracer solution were prepared by consecutive dilutions of a 1000 mg/mL atomic absorption standard (VWR Scientific Co.) and of a 10 pCi z07Bisolution (Amersham Radiochemical Co.), respectively, with 0.1 N HC1. It is assumed that the stable Bi and ?-07Biequilibrate in all experiments where 2wBiis used. Since the chemistry of bismuth is the +3 state under the conditions, the assumption is probably valid. Nitric, perchloric, and acetic acids were redistilled by G. Frederick Smith Chemical Co. (Columbus, OH). Reagent grade hydrochloric (Baker Chemical Co.) and hydrofluoric and sulfuric (Mallinckrodt) acids were used as received. NaOH (6 N) was purified of bismuth prior to use by coprecipitation with lanthanum hydroxides. Most of the Bi blank stemmed from this reagent. Four milliliters of 15% lanthanum nitrate solution was added to 1 L of 25% NaOH. The solution was stirred for 30 min and allowed to settle overnight. On the next day, the supernatant solution was taken and stored in precleaned Teflon bottles. A single coprecipitation decreases bismuth from 0.2 ng/mL to less than 0.005 ng/mL. The lanthanum nitrate solution had been purified through La(OH)3coprecipitation by adding NaOH. Ultrapure sodium borohydride (Alfa Chemical Co.) was dissolved in water to make a 3% solution, to which 0.1 mL of 6 N NaOH per 100 mL of reagent was added as a stablizer. 0 1982 Amerlcen Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 54. NO. 11. SEPTEMBER 1982
1683
1
n Hydride Generafor
Flgure 1. Apparatus for the determinatlon of bismuth by hydride generation.
I
2
3
4
5
6
Normality of Acid Figure 3. Influence of different acids and their strength on the yield of the bismuthine generation, based upon the recovery of 207Bi: hydrochloric acid (0, O * ) , acetic acid (A).suifurlc acid (*). and nitric acid (A).Asterisk indicates reacted with 2 mL of 1.5% NaBH, while others were reacted with 2 mL of 3% NaBH,.
2 7 m m Millipore Filter Unit
50 > [r
0
Hydride Generotor
Flgure 2. Apparatus for collecting tba hydride of "'Bi
tracer.
Procedure. The hydride generator is normally filled with 50 mL of seawater or freshwater. If less than 50 mL of sample is used, the final volume is brought to 50 mL with distilled water. The solution is acidified with 2 mL of 6 N HCI and purged with the helium earrier gas for a minute to remove the air in the system. Two milliliters of 3% NaBH, is injected into the generator with a hypodermic syringe over a period of 80 s. The Program cycle of the carbon rod atomizer is initiated at the beginning of boIOhrdride Thirty seeon* before atomization'the helium flow is decressed hy closing the stopcock of the main helium inlet This step can be omitted for the samples high Bmount8 of bismuth such as the digested solutions of sediments or algae. Concentrating Bismuth from Seawater. Bismuth is eoprecipitated from 1 L of seawater by adding 1 mL of 6 N NaOH. If the sample has been preacidified for preservation, additional NaOH is required to neutralize it. The solution is stirred for 15 min with a magnetic stirrer and the resulting milky precipitate is dowed to settle overnight. The supernatant is decanted with the aid of an aspirator. The precipitate whase volume ranges from 30 to 40 mL is dissolved by adding 2 mL of 6 N HCI and transferred to the hydride generator. The container is rinsedwith l mL of 6 N HCI and distilled water and poured into the generator. The solution is brought to 50 mL, mixed hy means of the carrier gas stream, and reacted with NaBH,. Sediments, Shells and Macro Algae. Adequate amounts (generally 0.5 g of sediment and macro algae, 5 g of shell) of oven-drid (115 "c)material were mmPletelY dig.sted with "08, HCIO,, and HF-HCI on a hot plate. The residue was redissolved in 50 mL of 1 N HCI. Aliquots of the solution (0.5 mL or less for d e n t , 5 or less for shell and nmro algae) pipea into the generator, followed by 1 mL of 6 N HCI, diluted to 50 mL with distilled water, and reacted with NsBH,.
RESULTS AND DISCUSSION Optimizing Bismuthine Generation. All of the experiments in which bismuthine generation was optimized were
I
2
Amount of 3% NaBH,
3
4
imL1
F I p e 4. InRu~nceof the amwms of NaW, on the y M of biimuthine generation at 0.1 N HCi medium, based upon the recovery of 20'Bi.
done in both distilled water and seawater matrices. However, identical results were obtained in both matrices. The efficiency of the hismuthine generation was sensitive not only to the acids used but to their acidities (Figure 3). Generally, dilute acids were more effective than mncentrated ones regardless ofthe acids wed, M~ of the acids tested gave the same optimal recoveries (ca. 95%) a t similar acidities, 0.1-0.2 N. However, 0.1 N HCI was chosen since it gave the lowest Fernandez (7) reported thatBiH3 generation is independent of the acidity in the range from 1to 6 N HCI. These different results may evoke from the dissimilar reaction systems. Figure 4 shows the BiH, generation efficiency as a function of amount of NaBH, added in 0.1 N HCI solution. Although more than 0.12 of N ~ B H , required for complete recovery, 0.06 g (2 mL of 3% solution) was used herein to minimize the 'lank' With the recommended reaction conditions, BiH3 generation Was Very fast. It Wm complete within 100 S including the time for NaBH, addition (Figure 5). For routine analysis, 150 8 of reaction and stripping time were used. Collection and Atomization of Bismuthine. The generated bismuthine was stripped from the solution by the He gas stream and in the The co~~ection efficiency was not sensitive to atomizer peratures between 25 and 350 "C and to He flow rates between 40 and 100 mL/min. The efficiency decreased significantly above 400 'C, which is probably due to volatilization of the collected bismuth. Below 40 mL/min He flow, longer collection times are required for optimal recovery. For routine analysis, the atomizer was heated to 300 "C for 90 s a n d then
1664
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
Table I. Bismuth in Some USGS Standard Rocks Compared with Recommended Values amt of bismuth, pgig recommended sample this work value
0.05
AGV-1
BCR-1 DTS-1 G-2
0.058 ?: 0.006 0.049 f 0.003 0.006 t 0.001
0.042 k 0.002
0.057 0.050 0.010 0.043
0.01
50
100
150
T r a p p i n g time (sec.) Figure 5. Influence of the reaction and stripping time on bismuthine generation (0.1 ng of Bi in 50 mL of 0.1 N HCI solution was reacted with 2 mL of 3% NaBH,).
to 350 "C for 60 s by using drying and ashing steps, respectively, and He flow was 45 mL/min. Under the above conditions, 72 % of the generated BiH3 was collected in the carbon rod atomizer on the basis of 207Bitracer experiments. Eighty-five percent of the collected BiH3 was in the supporting electrode tube and the rest was in the carbon rod cell. Though BiH3 collection was nonquantitative, it was so reproducible that no measurable change in sensitivity was observed for more than 100 measurements. It is not clear in which form the BiH3 was collected, whether as BM3 itself and/or an elemental form after decomposition. The latter is more likely for the following reason. If BiH3 is collected, hydrides of other elements would be expected to be collected in the system as efficiently as BiH3. However, only a very small fraction of SnH, was collected. Further, the collection efficiency for SnH4 was very sensitive to the atomizer temperature in contrast to that of BiH3 An atomizer heated at 300 "C collected 5 times more SnH4 than the unheated one. This suggests that the hydrides must be decomposed to be collected in the atomizer. The collected BiH3 was atomized by increasing the atomizer temperture to 1850 "C for 2 s with a 800 "C/s ramp rate. Sensitivity and Detection Limit. The sensitivity of the method is 10 pg/O.O044 A. This is in excellent agreement with the manufacturer's specification of 7 pg/0.0044 A considering the incomplete collection of BiH3 (72%) in the atomizer. The absolute detection limit is 3 pg of bismuth at the 95% confidence level. This corresponds to a concentration detection limit of 0.06 ng/L for a 50-mL sample volume. When a liter of seawater is preconcentrated by base precipitation, the concentration detection limit can be lowered to 3 pg/L. Blank values (2-4 times larger than the base line noise) typically range from 5 to 10 pg of bismuth depending upon the amounts and the source of the reagents used. NaOH used in the precipitation of seawater must be purified since the blank value stems mainly from it. It is necessary to check the HC1 purity since the Bi concentration ranges over more than an order of magnitude from bottle to bottle. N&H4 generally has a low blank. Precision, Accuracy and Linearity. The precision of the method, evaluated by replicate analyses of solutions containing 25 and 150 pg of bismuth, were 6.7% (n = 9) and 2.2% (n = ll),respectively. Due to the lack of certified aqueous standards for bismuth, the accuracy of the method was evaluated by analyzing some of the U.S. Geological Survey (USGS) rock standards (8). The standards were totally digested and analyzed following the procedure for sediment analysis. As shown in Table I, the values obtained in this work are in excellent agreement with the recommended values.
The calibration curve was linear from the detection limit (3 pg) to 1 ng of bismuth, corresponding to a linear range of nearly 3 orders of magnitude. The linear range can be expanded by decreasing the detector sensitivity through increasing the He carrier gas flow during atomization. For example, a 50 mL/min He flow decreases the Bi sensitivity by a factor of 3 without losing the hydride generation and the trapping efficiencies. Alkaline Coprecipitation Recovery. Alkaline coprecipitation of seawater was chosen to concentrate bismuth from seawater for the following reasons: (1)the precipitate can be reacted directly with NaBH, after dissolution, (2) it is relatively contamination-free since it requires the least number and amounts of reagents and minimum number of manipulation steps, (3) the reactions can be carried out in a single container, the sample bottle itself, which also minimizes the possibility of contamination. The efficiency of the alkaline coprecipitation was investigated by spiking 207Bito 1L of Scripps Pier water prior to NaOH addition. The average Bi recovery was 99.9 f 1.6% for six independent measurements. Interferences. High concentrations of many metals are known to cause negative interference in bismuthine generation (9, IO). Among them are cobalt, copper, gold, molybdenum, nickel, palladium, platinum, selenium, silver, and tellurium. These metals would be expected to cause interference with this method as well. However, the concentrations of these metals in most natural waters are many orders of magnitude below those causing interference. Although these metals are abundant in sediments, their concentrations in the final solution for BiH3 generation decrease by many orders of magnitude as a result of the large dilution required due to the high sensitivity of the method. Completeness of BiH3 generation was tested by spiking all types of samples analyzed in this work with 207Bibefore NaBH, addition. No reduction of BiH3 generation was observed compared to distilled water. Background correction is usually essential for graphite furnace atomic absorption. However, this is not the case in this method since very few elements which form hydrides during BiH3 generation can be trapped in the carbon rod atomizer. Any volatile organics trapped in the atomizer will be decomposed during ashing. Seawater Storage. Two kinds of sample containers, conventional polyethylene (CPE) bottles and Pyrex flasks, were tested for bismuth loss during storge of seawater. The containers were filled with Scripps Pier water, spiked with 207Bi,adjusted to the desired pH, and stored at ambient temperature until analyses. Samples were then transferred to Pyrex beakers, coprecipitated according to the procedure for concentrating Bi from seawater, and analyzed by a ND 100 multichannel analyzer. The containers were washed with 70 mL of 3 N HC1 to measure the Bi adsorbed on the container walls. The results are shown in Figure 6. No loss of Bi was observed over a period of 8 weeks when seawater was acidified at a pH of around 2 and stored in CPE bottles. However, substantial loss was observed within a few weeks when sea-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
1685
Table 111. Bismuth in Macro Algae and Shell species kelp (unknown) macrocrystis (unknown) mussel shell mussel shell ( M . edulis) oyster shell (C. uirrrinica) 4
6
0.74
La Jolla, CA, May 2, 1977 San Diego Bay, CA, April 21, 1977 Galveston Bay, TX, SeDt 1978
( M . californianus)
2
amt of Bi, nglg (dry wt) 5.3 8.9
location and collection time San Onofre, CA, 1980 San Onofre, CA, 1980
2.3 4.2
8
Time (week)
Figure 6. Bismuth storage experlments in conventional polyethylene bottles: 0 ,acidified at a pH of 2 with HCI; W, precipitated by addlng 1 mL of 6 N NaOH per liter; A, at neutral conditlon. Asterisk indicates slight precipitate loss during decantation.
Table 11. Bismuth in Sediments from Narragansett Bay and from Pacific Ocean amt of Bi, depth in depositional wgk core, cm perioda (dry wt) Narragansett Bay (41"3!3' N , 71" 19' W ; water depth, 8 m) 0-1 '19 72-1 9 74 0.44 'I 9 6 5-1 9 6 7 0.42 3 -4 8-9 1952-1954 0.57 12-14 1939-1944 0.54 20-22 1919-1924 0.64 26-29 1902-1909 0.59 35-38 pre-1900 0.49 49-54 pre-1900 0.27 Pacific Ocean (35" 31' N, 123" 19' W; water depth, 4300 m ) surface 0.10 Pacific Ocean (55" 35' N, 169" 23' W; water depth, 1598 m ) surface 0.12 a Determined by unsupported Z1oPb (13). water was stored in CPlE bottles at neutxal or a t alkaline conditions. The missing Bi was adsorbed on the container walls. On the other hand, when seawater was stored in Pyrex flasks, no loss of Bi was observed over a period of 4 weeks at all conditions, acidic, neutral, or alkaline. However, CPE bottles were adopted in .this work for convenience, but the waters were acidified uplon collection. T o check the possibility of leaching Bi from the CPE bottles, Scripps Pier waters containing 0.13 ng of Bi per liter were stored after acidification. No indication of leaching Bi from the walls was observed over a time period of 4 weeks. Analyses of Enviroinmental Samples. Open ocean waters were collected by 30-L Go Flo samplers hung on kevlar line. The waters were passed through 0.4-pm precleaned Nuclepore filters, acidified immediately it0 pH 2 with hydrochloric acid, and stored in CPE bottles which had been cleaned for a week in 60 'C 3 N HC1. Bismuth concentrations in marine sedments from Narragansett Bay and from North Pacific Ocean are found in Table 11. Although the Narragansett Bay sediments showed higher bismuth concentrations than their North Pacific Ocean counterparts, no change in bismuth concentrations was observed for the last 70 years. This suggests that there are no significant anthropogenic inputs. The results of algae, shell, and natural water analyses are presented in Tables I11 and IV. Generally, coastal waters contained more bismuth b,y about an order of magnitude than
Table IV. Bismuth in Environmental Waters amt of Bi, ng/L collection dissample time solveda total Seawater Pacific Ocean (17" 30' N , 109" 00' W; water depth, 3550 m) Oct 31, 1981 0.053 surface 2500 m below surface Nov 3, 1981
