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Anal. Chem. 1989, 6 1 , 1867-1869
Atomic Absorption Determination of Lead at Picogram per Gram Levels by Ethylation with in Situ Concentration in a Graphite Furnace R. E. Sturgeon,* S. N. Willie, a n d S. S. B e r m a n Division of Chemistry, National Research Council of Canada, Montreal Road, Ottawa, Ontario K I A OR9, Canada
A method Is described for the atomic absorptlon spectrometrlc determlnatlon of lead In natural waters and blologlcal tlssues based on the generation of Pb(C2H6)4using NaB(C,with its subsequent trapplng In a graphite furnace at 400 OC. Quantltatlon Is achleved by uslng a simple callbration graph prepared from aqueous standards having a sensltlvlty of 0.150 f 0.006 A ng-'. An absolute detection limit (3a)of 14 pg Is achleved. Preclslon of determlnatlon at 100 pg/mL Is 4 % relatlve standard deviation. Results are reported for the determlnatlon of Pb In a sulte of marlne reference materials.
are much greater than for the alkyllead species, its ease of ethylation and the absence of significant sample manipulation make this approach attractive for coupling with in situ concentration and atomization procedures using a graphite furnace (15-19). The latter offers substantial advantages over conventional purge and trap methodologies with furnace or heated quartz cell detection systems including simplicity of operation and use of small sample volumes, high sensitivity, and a substantial increase in detection power. The application of such in situ metal trapping to the determination of lead in natural waters and biological materials is presented here.
Impressive advances in the detection of ionic alkyllead compounds (1-2 ng/L range) have been obtained by using derivatization techniques (butylation) to form volatile tetraorganolead compounds followed by chromatography with atomic absorption detection (1,2). These procedures involve sample preconcentration (using 500-mL to 1-L volumes) and solvent phase transfers, which lead to potential losses, restrictions on maximum volumes used for analysis, and considerable time and effort. Rapsomanikis et al. (3)recently reported a novel derivatization purge and trap atomic absorption spectrometric procedure based on the solution ethylation of aqueous methylead ions by sodium tetraethylborate (NaBEtJ which eliminated the need for prior analyte concentration while permitting the entire sample to be derivatized, trapped, and determined with a minimum of handling. Detection limits obtained for Me,Pb+ and Me2Pb2+,viz. 0.18 ng/L and 0.21 ng/L, respectively (based on a 50 mL sample), are the lowest values reported to date. More reliable atomic spectrometric methods for detection of inorganic lead are required in order to access the extreme trace levels present in many samples of environmental interest. Hydride generation procedures are commonly resorted to in an effort to enhance concentration detection limits for several elements, varying success has been reported in the case of lead (4-13) using both continuous flow and purge and trap techniques. Early studies by Vijan and Wood ( 6 ) noted the instability of plumbane and the poor reaction efficiency with NaBH, reagent. Use of various oxidizing and complexing agents appears necessary for efficient generation (12)but even with these, reported conversion efficiencies of inorganic lead to plumbane range from 27 to 91 % (8-1 0,13) often accompanied by severe interelement interferences (7, 10, 11). The accuracy of such analyses in the picogram to nanogram per gram range depends primarily on the ability of the analyst to obtain a true estimate of contamination blanks introduced during the collection, transport, and handling of samples (14). In the laboratory, the latter step must be kept to an absolute minimum. Although the possibilities of Pb2+contamination
Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrometer was fitted with an HGA-500 graphite furnace and Zeeman effect background correction. A Perkin-Elmer lead electrodeless discharge lamp operated at 10 W was used as the line source. Absorption was measured at the 283.3-nm line. A nominal spectral band-pass of 0.7 nm was used. Standard Perkin-Elmer pyrolytic graphite coated tubes were modified for use by increasing the diameter of the sample introduction hole to =2 mm. A custom-made Pyrex cell was used to generate Pb(Et)4,which was transferred, via a quartz delivery tube, into the sample introduction hole of a preheated furnace tube. The design and operation of the cell have been detailed elsewhere (16-19). Reagents. A lo00 mg/L stock solution of inorganic lead was prepared by dissolving high-purity lead granules (Johnson Matthey) in concentrated HNO, and diluting to 1 M "0,. Working standards were prepared by serial dilution in deionized distilled water (DDW) (Barnstead Nanopure system) containing 0.1 M HNOP High-purity subboiling distilled HNO,, CH&OOH, and HC104 were prepared in-house. Isothermal distillation of reagent grade "*OH into a receiver vessel of cold DDW was used to produce a high-purity 12 M product. A 1M buffer solution of ammonium acetate was prepared from high-purity reagents and adjusted to a pH of 5.5 using excess acid. A 0.5% (m/v) solution of NaB(Et)4 in DDW was prepared as required. A Sure/Pac cylinder of B(Et), (Aldrich),a lecture bottle of EtCl (Matheson),sodium metal (Fisher), and anhydrous ether (Fisher) were used to synthesize NaB(Et)4. Several marine reference materials were analyzed for total Pb including National Research Council of Canada (NRCC) open ocean seawater (NASS-2),coastal seawater (CASS-2),estuarine water (SLEW-l), river water (SLRS-l), dogfish muscle (DORM-l), and proposed nondefatted lobster hepatopancreas tissue LUTS-1 (20). Additionally, a sample of water collected from the Western Scheldt estuary (salinity 12%0)as part of the International Council for Exploration of the Seas (ICES) 6th Round Intercalibration for Trace Metals in Seawater (21)was analyzed. Procedures. Sodium tetraethylboron was synthesized according to the procedure outlined by Honeycutt and Riddle (22). Approximately 14 mL of B(Et), was transferred under Nz pressure, using a septum inlet T and stainless steel needle tubing, into a N,-purged 250-mL round-bottom three-necked flask containing 30 mL of anhydrous ether. The flask was continuously purged with dry Nz through one neck. A second neck was fitted with an angled side arm containing 5 g of freshly prepared sodium sand in ether. The third neck supported a reservoir for anhydrous ether
* Author to whom all correspondence should be addressed. 0003-2700/89/0361-1867$01.50/0
EXPERIMENTAL SECTION
Published 1989 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
Table I. HGA Program time, s hold
program
step
temp, "C
ramp
generation-
1
1 1
3
400 400 400
1 2 3
400 1600 2700
collection
2O
atomization
1
1
0 1
int gas, mL/min
60
14 29 119
300 100 150
40
9 3 1
0 0 300
0
20 "
" " " 1400 1800 2200 2600 Atomlratlon Temperature,'C
O 0 . 5 % NaB(Et), added at 4 mL/min.
and bypass line for escaping NP Ethyl chloride was bubbled continuously into the solution at a rate of about 200 mL/min. The flask was suspended in an ultrasonic bath and chilled to 5-10 "C by periodic addition of ice. The sodium sand was added in four portions over approximatelya 5 h period. The purple product mixture was then allowed to warm to room temperature and transferred to 30-mL vials for centrifugation at 2500 rpm for 20 min to remove NaCl. The filtrate was evaporated down to about 20 mL under a stream of N2 and chilled at -78 "C to crystallize sodium tetraethylboron etherate which on heating at 11Cb115 "C and 1 mmHg gave ether-free sodium tetraethylboron. Estimated yield was 60%, assuming the product was pure. No further purification was attempted. The reagent was transferred, under a N2 atmosphere, to sealed vials in 1-g aliquots which were subsequently stored at 4 "C in the dark. CAUTION: B(Et), is pyrophoric. All sample and analytical manipulations were conducted in a class 100 clean room environment. Ten-milliliter aliquots of CASS-2, NASS-2, and SLEW-1 were transferred directly to the generator cell, and 200 pL of acetate buffer was added. Fivemilliliter aliquota of SLRS-1 and ICES water were used together with 5 mL of DDW and 200 pL of buffer. The biological reference materials DORM-1 and LUTS-1 were solubilized in PFA pressure vessels by using a HN03-HC104 mixture with heating in a microwave oven, as described elsewhere (23). Nominal 0.5-g subsamples (dry weight) were taken and following dissolution were diluted to 50.0 mL in 1 M "03. Blanks were run concurrently with each set of sample decompositions. Total lead was determined by using l.WmL and 1WpL aliquots, respectively, of the dissolved LUTS-1 and DORM-1 samples, These were diluted to 10.0 mL in the generation flask and 200 pL of buffer was added. The sequence of operations describing generation, collection, and atomization of Pb(Et)4 is similar to that reported for the hydride-forming elements (16-19) and will not be repeated here with the exception of details pertinent to the Pb system. The graphite furnace program is given in Table I. Lead is collected at 400 "C. Two milliliters of NaB(Et)4was metered into the cell at a flow rate of 4 mL/min with a peristaltic pump. Peak-height absorbance measurements were found adequate in all cases. Measurement of integrated absorbance was less precise (12% relative standard deviation w 4% at 1ng absolute). Standard calibration curves prepared from spikes of inorganic Pb added to 10 mL of DDW containing 200 pL of buffer were used to permit quantitation of samples.
RESULTS AND DISCUSSION The ease with which inorganic lead salts can be ethylated in good yield in aqueous solution was noted by Honeycutt and Riddle (22)and Rapsomanikis et al. (3). An autooxidationreduction reaction occurs with production of Pb(1V) and Pb, i.e. 4NaB(EtI4
+ 2Pb2+
-
Pb(Et),
+ 4B(Et)3 + 4NaC1 + Pbo (1)
The reaction proceeds to 50% efficiency with respect to quantitative conversion of inorganic lead to tetraethyllead (TEL). Triethylborane is also reported to be capable of ethylating Pb2+to TEL, thus the stoichiometry of the above equation is not exact (24).
o graphite A 4 pg Pd
60
2o
0
t 200 400 600 Deposition Temperature,"C
800
Figure 1. Effect of system variables on the generation, deposition, and TEL (see text for details).
atomization of
Figure 1 shows the relative peak height signals obtained in response to changes affecting generation, trapping, and atomization of TEL. Generation efficiency was independent of pH provided the latter was >4. Rapsomanikis et al. ( 3 ) reported an optimum pH of 4.1 in their studies. Scavenging of TEL (or its decomposition products) on the graphite tube surface was relatively insensitive to temperature over the range 200-600 "C. Both new pyrolytic graphite coated tubes and older worn tubes (over 200 heating cycles) exhibited the same collection characteristics. The range of deposition temperatures could be extended considerably (100 to >800 "C) in the presence of 4 pg of reduced Pd, which had been previously aliquoted, dried, and reduced (at 800 "C) on the tube surface. Similar observations have been noted when Pd was used to aid in the sequestering of analyte hydrides in the preheated graphite tube (25). Since Pd did not significantly enhance the performance characteristics of the system (peak absorbance and integrated signal increased by 10 and 12%, respectively), its use was discontinued in favor of a less complex procedure. Maximum power heating to an optimum atomization temperature of 1600 "C was used. The abrupt decline in response below a setting of 1400 "C favored the selection of 1600 "C as a compromise between extended tube lifetime and assurance of a robust technique. All experiments were conducted by using 2 mL of a 0.5% (m/v) solution of NaB(Et)(. With up to 10-mL sample volumes, generation of TEL was independent of NaB(Et), concentration above 0.2% (m/v). As sample volumes increased to 20-40 mL, the optimum reagent concentration was found to increase to 0.5% (m/v). Concurrently, the cell purging time (step 3 of the furnace program) had to be increased from 120 s for 10-mL sample volumes to 240 s for 40-mL sample volumes. Relative to that obtained with 10-mL volumes, signal response decreased as sample volume increased, Le., 87% at
ANALYTICAL CHEMISTRY, VOL. 61,
Table 11. Analytical Results5 sample
result
accepted value
SLRS-1 (ng/mL) SLEW-1 (ng/mL) CASS-2 (ng/mL) NASS-2 (ng/mL) ICES (ng/mL)
0.110 f 0.008 (5) 0.030 f 0.002 (5) 0.021 f 0.003 (5) 0.038 f 0.003 (8) 0.20 f 0.01 ( 5 )
0.106 f 0.011 0.028 f 0.007 0.019 0.006 0.039 f 0.006 0.19 f 0.04*
DORM-1 (/lg/g) LUTS-1 (pg/g)
0.45 f 0.02 (5) 0.074 f 0.003 (10)
0.40 f 0.12
*
0.078 f 0.016'
Mean and one standard deviation; numbers in parentheses ara numbers of replicate samples. *Mean result of 20 participating ICES laboratories (21). GFAAS direct analysis results. (I
Table 111. Figures of Merit sensitivity, A/ng 0.150 f 0.006 cell blank, ng 0.074 f 0.0042 3a detection limit, pg 14 concentration LOD (10-mL sample), ng/mL 0.001 linear range, ng 0.014-2 reproducibility, % RSD at 1 ng 4 system efficiency, % 58 f 3 20 mL, 77% a t 30 mL, and 74% a t 40 mL. Aqueous NaB(Et)4 solutions were stable for about 1week: when stored at 4 "C in darkened polypropylene screw-capped bottles. Signals equivalent to -75% of thaw obtained by using freshly prepared 0.5% (m/v) solutions of NaB(Et)( were observed for 1 week old reagent. Analytical Blanks. The primary source of the blank wati determined to be the NaB(Et),. Absolute cell (reagent and manipulation) blanks were found to be 74 f 4 and 77 f 5 pg when two different batches of NaB(Et)4were synthesized and used. It was noted that, when freshly prepared, the blank level from aqueous solutions of this reagent was about twice this value. Permitting the solution to stand overnight presumabhy allowed volatile T E L impurities to degas. Analytical Results. Table I1 summarizes the analytical results for the determination of total P b in a number of marine reference materials. In all cases, calibration was against simple working curves prepared by generating T E L from spiked 10-mL aliquots of buffered DDW. The accuracy of this approach is evident from a comparison of these data with the certified, accepted, or consensus values given for these samples in Table 11. The procedure used for the ethylation of lead is remarkably free of interferences in comparison to current hydridization techniques (11). The latter technique suffers marked signal reduction in the presence of such cations as Ca, Mg, Na, and the first-row transition elements. Signals from 1ng of Pb(I1) were found to be unaffected by the presence of 106-fold excesses of Ca2+,Na+, and Mg2+and 5000-fold excesses of Fe3", C P , Ni2+,Mn2+,As3+, and Zn2+. Copper was the only elepent tested for which a signal suppression was noted (-15% a t 1000-fold excess (1 pg absolute)). The procedure is thus sufficiently robust that direct calibration against external standards may be made for most natural waters as well as solutions of dissolved sediments and biological materials. Figures of Merit. Table I11 summarizes analytical figures of merit. Absolute peak absorbance sensitivity as determined from the slopes of calibration curves run in several tubes, using two different lots of synthesized NaB(Et), reagent, averaged 0.150 f 0.006 A/ng (Le. 28 pg/0.0044 A). This figure is 58 f 3% of that obtained by direct injection of 1ng of P b as an aqueous solution (20 pL) into the furnace with atomization under identical conditions. Comparison of both peak height and integrated absorbance measurements resulted in the same estimate of overall efficiency. Inspection of eq 1reveals that
NO. 17, SEPTEMBER
1, 1989
1869
the overall efficiency of the ethylation reaction is expected to be 50%. However, subsequent ethylation of any redissolved Pbo by NaB(Et), or B(Et)3 reagent in solution would be expected to increase this yield. The generation system is thus 58% efficient overall (generation, transfer, trapping), but this appears to be the maximum that can theoretically be achieved. With signal integration, an absolute sensitivity of 90 f 11 pg/0.0044 Ass was obtained. Atomization conditions were not optimized for signal integration. The estimated procedural detection limit for inorganic lead, , 14 pg. This based on the variability of the blank ( ~ c T ) is corresponds to a concentration detection limit of 1 pg/mL in natural water, assuming a 10-mL sample. Sub-picogramper-milliliter detection limits may be readily achieved by taking larger sample aliquots. Precision of determination is better than 5% RSD on determinations 70-fold (i.e., 1 ng) above the LOD. The linear working range spans over 2 decades, extending to 2 ng. Higher analyte concentrations are accessible by working with smaller sample volumes or by introducing an internal purge gas flow during atomization.
CONCLUSION Extreme trace concentrations of lead in environmental samples become accessible on a routine basis utilizing ethylation-in situ graphite furnace atomic absorption spectrometry (GFAAS) trapping procedures. Concentration factors of 500 are readily achieved in 3-4 min (10-mL samples vs conventional 2 0 - ~ Laliquots for GFAAS techniques) and sample volume requirements are significantly curtailed. The detection limit achieved (1 pg/mL) is, to our knowledge, the lowest reported for inorganic lead.
LITERATURE CITED Chau, Y. K.; Wong, P. T. S.; Kramar, 0. Anal. Chim. Acta 1883, 746, 211-217.
Chakraborti, D.; DeJonghe, W. R. A,; Van Mol, W. E.; Van Cleuvenbergen, R. J. A.; Adams, F. C. Anal. Chem. 1884, 5 6 , 2692-2697. Rapsomanikis, S . ; Donard, 0. F. X.; Weber, J. H. Anal. Chem. 1886, 5 8 , 35-38.
Thompson, K. C.; Thomerson, D. R. Analyst 1874, 99. 595-601. Fleming, H. D.; Ide, R. G. Anal. Chim. Acta 1878, 8 3 , 67-82. Vijan, P. N.; Wood, G. R. Analyst 1878, 707,966-973. Vijan, P. N.; Sadana, R. S. Talanta 1880, 2 7 , 321-326. Yamauchi, H.; Arai, F.; Yamamura, Y. Ind. Heanh 1881, 79, 115-124.
Ikeda, M.; Nishlbe, J.; Hamada, S.; Tujlno, R. Anal. Chim. Acta 1881, 725, 109-115.
Jin, K.; Taga, M. Anal. Chim. Acta 1882, 743, 229-236. Castillo, J. R.; Mir, J. M.; Val, J.; Colbn. M. P.; Martlnez, C. Analyst 1885, 170, 1219-1221.
Castiilo, J. R.; Mir, J. M.; Martinez, C.; Val, J.; ColBn, M. P. Mikrochim. Acta 1885, I , 253-263. D'UlIvo, A.; Fuoco, R.; Papoff, P. Talanta 1886, 33, 401-405. Patterson, C. C.; Settle, D. M. Accuracy in Trace Analysis: Sampling, Sample Handling and Analysis; Natl. Bur. Stand. ( U S . )Spec. Publ., 1976, No. 422, Vol. 1, pp 321-351. Lee, D. S. Anal. Chem. 1882, 5 4 , 1682-1686. Sturgeon, R. E.: Willie, S. N.; Berman, S. S. Anal. Chem. 1885, 5 7 , 2311-2314. Willie, S. N.; Sturgeon, R. E.; Berman, S. S. Anal. Chem. 1888, 5 8 , 1140- 1 143. Sturgeon, R . E.; Willie, S. N.; Berman, S. S. J . Anal. At. Spectrom. 1886. 7 , 115-118. Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Anal. Chem. 1887, 5 9 , 244 1-2444. Berman, S. S.; Sturgeon, R. E. Fresenius' 2. Anal. Chem. 1888, 332, 546-546. Berman, S. S.; Boyko, V. J. ICES 6th Round Intercalibration for Trace Metals in Seawater ( G IT MIS W) ; ICES Coop. Res. Report No 152; National Research Council of Canada: Ottawa, ON, Canada, 1987. Honeycutt, J. B., Jr.; Riddle, J. M. J . Am. Chem. SOC. 1861, 8 3 , 369-373. Nakashlma, S.; Sturgeon, R. E.; Wiiiie. S. N.; Berman, S. S. Analyst 1888, 773, 159-163. Honeycutt, J. B., Jr.; Riddle, J. M. J . Am. Chem. SOC. 1880, 8 2 , 3051-3052. Sturgeon, R. E.; Willie, S. N.; Sproule, G. I.; Robinson, P. T.; Berman, S. S. Spectrochim. Acta, Part 8 ,in press.
RECEIVED for review March 17,1989. Accepted June 2,1989. This is NRCC No. 30432.
