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Anal. Chem. lQ83,55, 1050-1054
(5) Barnes, R. M.; Schlelcher, R. G. Spectrochlm. Acta, Part 8 1975, 306, 109. (6) Barnes, R. M., CRC Crlf. Rev. Anal. Chem. 1978, 7, 203. (7) Murdick, V. A., Jr.; Plepmeier, E. H. A"/. Chern. 1974, 4 6 , 678. (8) Willis, C. H.; Chandler, H. M., Jr., Introduction to Electrical Engineering"; D. Van Nostrand: Princeton, NJ, 1957. (9) Babington, R. S. Pop. Scl. 1973, May, 102. US. Patents 3421 692; 3 421 699;3 429 058;3 425 059;and 3 504 849. (IO) Fry, R. C.; Denton, M. 8. Appl. Spectrosc. 1979, 33, 393. (11) Plasma Line 1980, 1 , 2. (12) Klueppei, R. J.; Coleman, D. M.; Eaton, W. S.;Goldstein, S.A,; Sacks, R. D.; Walters, J. P. Spectrochlm. Acta, Part B 1978, 338, 1. (13) Skogerboe, R. K.; Urasa, I. T.; Coleman, G. N. Anal. Chem. 1976, 30, 500. (14) Ingle, J. D., Jr. "Notes on Basic Spectrometric Measurements", Oregon State University: Cowallis, OR, 1978;p 67.
(15) Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 206. (16) Dixon, W. J.; Massey, F. J. "Introduction to Statistical Analysis"; McGraw-Hill: San Francisco, CA, 1969;p 195. (17) Skogerboe, R. K.; Urasa, I.T. Appl. Spectrosc. 1978, 32, 527. (18) Kirkbright, G. F.; Ward, A. F. Talanta 1974, 2 1 , 1145. (19) Vickers, T. J.; Winefordner, J. D. I n "Analytical Emission Spectroscopy"; Grove, E. L., Ed.; Dekker: New York, 1972;Part 11, p 333. (20) Garbarino, J. R.; Taylor, H. E. Appl. Spectrosc. 1981, 35, 153. (21) Fassei, V. A.; Kniseley, R. N. Anal. Chern. 1974, 4 6 , I l l O A .
RECEIVED for review September 7 , 1982. Accepted February 4,1983.
Determination of Trialkyllead Compounds in Water by Extraction and Graphite Furnace Atomic Absorption Spectrometry Walter R. A. De Jonghe, Wllly E. Van Mol, and Fred C. Adams" Department of Chemistry, University of Antwerp (U.I.A.), Universiteitsplein 1, 6-2610 Wilrijk, Belgium
A slmple extraction procedure Is descrlbed for the sensltlve determlnatlon of traces of trlalkyllead compounds in water. After enrlchment of the sample by a fast vacuum dlstlllatlon technique and saturatlon of the resldual volume with sodium chlorlde, the analytes are extracted In chloroform. By Incorporation of specific purlflcatlon steps, Interference from other forms of organic and lnorganlc lead Is completely ellmhated. The flnal chloroform extract is treated wlth a sulfurlc acld solution in order lo transfer the trlalkyllead compounds present back into an aqueous solution. The analysis Is completed by graphlte furnace atomic absorption spectrometry. Under normal laboratory conditions, a detection limlt of 0.02 pg can be achieved wlth I - L samples. The method was developed to investlgate the occurrence of trlalkyllead compounds In envlronmental water samples. Experiments wlth splked lake water have establlshed that there Is a conslderable loss of lnltlally added trlethyllead chlorlde upon exposure to sunllght, while trimethyllead remains falrly stable.
I t is well established now that environmental samples may contain substantial quantities of tetraalkyllead compounds (TAL) originating from their use as antiknock agents in gasoline and possibly from natural alkylation of inorganic lead (1-5). Numerous methods have been developed to detect the occurrence of these contaminants in air, water, sediment, and biological matrices. On the other hand, only a few methods are available for the environmental determination of trialkyllead ions (TriAL), the highly toxic photochemical and metabolic dealkylation products of TAL. Those described include the extraction and spectrophotometric measurement of colored alkyllead complexes (6-10) and direct polarographic techniques (11). Individual TriAL species allegedly have been determined by thin-layer chromatography (12) or by gas chromatography with an element-specific detector (13-15). Other procedures are reviewed in detail elsewhere (16). Although proven valuable in laboratory studies dealing with only one or two species, a number of the techniques adopted
so far are difficult to apply with real samples, in which theoretically four different TriAL compounds can be encountered: trimethyllead (TriML), dimethylethyllead (DMEL), methyldiethyllead (MDEL), and triethyllead (TriEL). Moreover, for a complete analysis, they require pure standards of the intermediate species, DMEL and MDEL, the preparation of which is troublesome (17). The present study describes an improved extraction procedure based on the well-developed technique of salting out the TriAL ions as neutral species into an organic solvent (12, 14,18,19),combined with graphite furnace atomic absorption spectrometry (GFAAS). In contrast to earlier approaches where other forms of organic and inorganic lead may give rise to serious interferences, the proposed determination is highly specific for TriAL even in the presence of up to 100 bg L-l of inorganic lead salts. Stability tests of TriML and TriEL in water are discussed to demonstrate the practical applicability of the method. EXPERIMENTAL S E C T I O N Apparatus. A Perkin-Elmer 503 atomic absorption spectrometer is used, in combination with a PE HGA-74 graphite furnace atomizer. Absorbances are read as peak height from a Hitachi Perkin-Elmer 56 strip-chart recorder and from the built-in peak-read device of the spectrometer. The furnace is flushed with argon at a flow rate of 300 mL m i d internally and 900 mL min-' externally. The gas flow can be changed during the atomization stage to increase sensitivity, by means of the stopped (gas-stop), reduced (miniflow),and continuous flow settings. Solutions are injected with Eppendorf micropipets with disposable polypropylene tips. The light source is a lead hollow-cathode lamp, operated at 10 mA. All analyses are done at the 283.3-nm line with a spectral bandwidth of 0.7 nm. The miniflow mode is used in the atomization step, as this provides a suitable compromise between the sensitivity and linearity pursued. For a 2 0 - ~ Lsample injection the optimal temperature/time program is as follows: drying at 100 OC for 30 s, charring at 500 OC for 20 s, atomizing at 2300 "C for 10 s and glowing-out at 2700 OC for 10 s. For the optimization of the extraction procedure, a number of measurements are also performed by means of a Varian 3700 gas chromatograph with flame ionization detection (GC-FID). It
0003-2700/83/0355-1050$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
1 1
80
--
soturotion point
20
0
I
100
I
*OC
1051
300 LOO conc NaCl l g / l )
Flgure 1. Effect of NaCl on the chloroform batch extraction of 1 mg of TriML and TriEL from water (VorQ/Vaq= I), analysis with GC-FID:
( 0 )TriML; (0)TriEiL.
is used with a 50 cm long glass column (6.2 mm o.d., 2 mm ids), packed with 10% OV-101 on 100/120 mesh Gaschrom Q, at the following instrumental conditions: carrier gas flow rate, 30 mL min-'; air flow rate, 300 mL min-'; hydrogen gas flow rate, 30 mL min-'; injector temperature, 130 "C, detector temperature, 250 "C; and column temperature, 50 "C t o 250 "C at a rate of 20 "C min-l. Standards and Reagents. Trimethyl- and triethyllead chloride and dimethyl- and diethyllead dichloride were obtained from the Associated Octel Co. (London). TriAL standard stock solutions (100 pg mL-') are prepared by dissolving appropriate amounts in deionized, doubly distilled water. When stored in a refrigerator, these solutioins are stable for at least 4 weeks without noticeable deterioration. Working standardgi, obtained by further dilution, are prepared daily. Solutions of the dialkyllead compounds are prepared immediately before use. The standards for inorganic lead are made from a lo00 pg mL-l lead nitrate solution (Merck Titrisol 9969). The reagents used in t,he extraction procedure are all of analytical reagent quality. The buffer solution (pH 11)is prepared from ammonium citrate and ammonia (20, 21). After the extraction the chloroform can be recuperated by rotary evaporation; the material proved to be of a sufficient purity for subsequent determination. Procedure. The sample size used for analysis depends on the T r i m concentration expected to be present,. For environmental applications a 1-1, volume is advised. After shaking for 1 min with hexane (1 mL per 100 mL of water), the sample is filtrated on a Type RA Milliprefilter of 1.2-pm pore size. The water is brought into a rotary evaporator and evaporated under vacuum at 60 "C until a residual volume of ca. 15 mL remains. For a 1-L sample this takes about 3 h. Thereupon the sample is quantitatively transferred to a 100-mL separating funnel and the volume is adjusted to 25 mL with distilled water. About 8 g of NaCl is added and if necessary the pH is brought t o below 10. After this, the sample is extracted twice with 25-mL portions of chloroform (shaking for 1 min). The extracts are comlbined and 1 mL of a M dithizone in chloroform solution if4 added. Next, this organic phase is shaken for about 2 min with an aqueous solution consisting of 15 mL of an ammonium citrate/ammonia buffer and 35 mL of 0.1 M EDTA. The chloroform layer is separated off, shaken first with (distilledwater t o remove traces of EDTA and buffer and then for 1 min with 5 mL of a 0.1 N sulfuric acid solution. Analysis of this sulfuric acid extract by means of GFAAS allows the quantitation of the TriAL initially present.
RESULTB AND DISCUSSION Extraction of Trialkyllead Compounds from Water. In aqueous media the TriAL compounds are present in the dissociated form (22, 23), and as such, extraction into a water-immiscible solvent is not possible. However, through the addition of salts, e.g., NaX, the dissociation TriALX s TriAL + X may be suppressed, thus increasing the solubility in nonpolar solvents. Varying amounts of NaCl were added to 25 mL of distilled water spiked with 1 mg of TriML and TriEL to test this "salting out" effect. The samples were batch-extracted with 25 mL of chloroform, an aliquot of which was analyzed with GC-FID. In this way, a single experiment
Figure 2. Effect of chloroform/aqueous phase ratio on the batch extraction of 1 mg of TriML and TriEL from NaCI-saturated water (symbols as in Figure 1).
Table I. Effect of Different Chloride Salts at Saturation Concentration on the Batch Extraction of TriML from Water
salt
g equiv of % recovery C1- at saturation in concn chloroform
LiCI. NaCl KCl RbCl CaCl,
15.0 5.1 3.2 6.4 13.4
83 75 60 64 82
-
Table 11. Effect of Different Solvents on the Batch Extraction of TriML and TriEL from Water % recovery
solvent
TriML
TriEL
hexane toluene carbon tetrachloride diisopropyl ether butanol ethyl acetate tetrahydrofuran 1,2-dichloroethane chloroform
15 18 14
81 91 73 100 54 81
22
35 61 63 72
75
82
87 93
a t the same time yielded results for both TriAL species. Since T r i m is relatively more polar than TriEL, its extraction from aqueous media was expected to be lower. The results (Figure 1)indicate that, whereas TriEL is recovered already a t lower salt concentration, extraction of TriML requires a nearly saturated NaCl cjolution. Even then, the recovery of the latter species is not higher than 75%; in the absence of NaCl no extraction of TriML takes place. The recovery, although not quantitative, is constant to within a relative standard deviation of 3%. Moreover, by carrying out a second extraction, TriML is recovered to the same extent as TriEL. As shown in Figure 2, this cannot be achieved by merely increasing the organic/aqueous phase ratio. All the above considerations may indicate a stoichiometric side reaction, e.g., the formation of chloro complexes, under the influence of the high chloride matrix (24-28). Because the physicochemical behavior of DMEL and MDEL is likely to be intermediate between that of TriML or TriEL, the use of 8-9 g of NaCl per 25 mL of water can be expected to allow a satisfactory recovery for all TriAL compounds. Experiments with other salts did not give significantly better results than NaCl (Table I). Several solvents were also tested, but no advantages over chloroform were observed (Table 11). In order to examine the pH dependence, extractions were carried out with aqueous solutions which contained, in addition to a fixed Concentration of sodium chloride, various
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
50
I LO-
i
20
c
i
;‘I 20
‘30
8C
60
40
23
0
residual volumelm.1
Figure 3. Effect of pH on the batch extraction of 1 mg of TriML and TriEL from NaCI-saturated water with chloroform (V,,,/ V,, = 1) (symbols as in Figure 1).
Flgure 4. Effect of rotary evaporation at 60’ on the recovery of 0.5 p g of TriML and TriEL from originally 1 L of water (two consecutive chloroform extractions) (symbols as in Figure 1).
quantities of either NaOH or HC1. The results are shown in Figure 3. Over the whole pH range investigated, the TriEL species is recovered for ca. 90%. The recovery of TriML, however, appears to remain constant only up to pH 10. According to data obtained with nuclear magnetic resonance spectrometry, a t high pH TriML hydroxide may be formed (24);this probably explains the decrease in extraction recovery a t pH >lo, as the water solubility of the latter compound is about 1order of magnitude higher than that of TriML chloride (29). It is recommended therefore, when analyzing water samples, to verify the pH. The difference in extraction behavior between TriML and TriEL in alkaline medium might form the basis of a simple speciation procedure between both compounds. This possibility was not further investigated, however. GF-AAS Determination of the Extracted Trialkyllead Compounds. Preliminary studies revealed that direct GFAAS analysis of the chloroform layer is not appropriate for determining its TriAL content. Besides the analytes, traces of NaCl and-if present in the sample-of inorganic lead may also be transferred to the organic phase. Thus, the measurement is subjected to various interferences: losses of P b as PbClz during the ashing step and strong nonspecific molecular absorption during the atomization, both caused by NaC1, and results that are too high due to the presence of inorganic lead. A purification method was sought that would allow washing out the impurities and retaining the analytes. Owing to the solubility of the TriAL chlorides in water, the organic phase cannot be purified simply by rinsing with an aqueous solution. So as to prevent a possible back-extraction during the purification step, they are first converted into the corresponding TriAL dithizonates. Simultaneously, inorganic lead also becomes complexed, but this dithizonate is subsequently decomposed by EDTA upon treatment with the cleansing solution. The more stable lead-EDTA chelate is then formed, which dissolves in the water phase. The TriAL dithizonates, on the contrary, remain behind unaltered in the chloroform. Consequently, together with the elimination of the traces of NaC1, a selective removal of inorganic lead is achieved. It might be noted at this stage that an alternative and more simple procedure for TriAL measurement would consist in the direct chloroform extraction of the original water sample in the presence of dithizone and EDTA. In these conditions the overall specificity is decreased, however: both TriAL and dialkyllead compounds, further degradation products of TAL, are then extracted. Prior to the eventual GFAAS determination, it was preferred to extract the TriAL from the chloroform layer back into an aqueous solution; the measurement is less cumbersome, whereas in view of the possible enrichment factor, in addition a higher sensitivity is obtained. Because of the instability of the TriAL dithizonate at low pH (30), shaking with a dilute
Table 111. Recovery of TriML and TriEL from 1 L of Distilled Water (Two Consecutive Chloroform Extractions) amt of TriAL amt of TriAL added, ng of Pb found, ng of Pb
recovery, %
43 87 174 261 350 435 4349 43488
TriML 40 78 140 232 284 361 4066 39444
84 93 91
82 163 244 325 3253 32530
DiEL 76 143 21 3 2 84 3083 20458
93 88 87 87 95 94
93 90 80 89 81
acid solution is sufficient to decompose the complex. The TriAL portion dissolves in the aqueous solution and the dithizone liberated remains in the chloroform. In practice, this is accomplished by means of a 0.1 N sulfuric acid solution. Preliminary experiments revealed that nitric acid is less suitable for this purpose, as the GFAAS measurements in this case become irreproducible. The blank of pure HzS04“pro analysi” and “suprapur” (Merck 731 and 714) was found to be 0.4 and 0.7 pg of P b L-l, respectively. Calibration can be done with inorganic lead standards; no differences in absorbance were noticed for equal concentrations of either TriML, TriEL, or inorganic lead. It is worth mentioning that in other matrices these species may give rise to different sensitivities, e.g., in dilute phosphoric acid medium the GFAAS determination of TriAL is only one-third as sensitive as that of inorganic lead (31). Detection Limit, Precision, and Accuracy. For practical reasons, the extraction described should not be carried out with more than 25 mL of water. Since the detection limit directly depends on the volume of sample, however, the analysis of larger volumes is desirable. The procedure includes therefore a concentration step. It was found that in a rotary evaporator 1-L samples can be evaporated to less than 10 mL without loss of the analytes (Figure 4). After the enrichment, the volume is adjusted to 25 mL for quantitation of the analysis. Under these conditions, the detection limit of the method, defined as three times the standard deviation of the blank, is as low as 0.02 pg/L. In the course of the present study, different levels of TriAL were spiked to 1 L of distilled water and carried through the entire procedure. Typical results are shown in Table 111. For a total of ca. 30 synthetic samples analyzed, the overall re-
ANALYTICAL CHEMISTRY, VOL. 55,
Table IV. Recovery of Mixtures of TriML and TriEL from Distilled Water (Two Consecutive Chloroform Extractions) amt of TriAL added, ng of Pb TriML TkiEL total 0 70 139 209 278
336 252 168 84 0
amt of n i A L found, ng of Pb
recovery,
293 312 297 275 263
87 97 97 94 95
336 322 307 293 278
%
Table V. Effect of Inorganic Lead on the Blank Value of the Extraction Procedure for TriAL Compounds (Two Consecutive Chlloroform Extractions) amt of inorg Pb added, yg nil 1 10 100 1000 10000 a
blank found, ng 10 t 10 14 f 19 t 34 f 64 t
*
7 (40)a 6 (3) 11 (4) 1 5 (3) 9 (6) 10 (3)
Number of determinations.
covery amounts to 87 f 4 and 92 f 5% for, respectively, TriML and TriEL; taking into account an average recovery factor of 90%) results will hence be accurate within 5%. Moreover, the results summarized in Table IV suggest that the method can also be applied successfully to samples in which mixture!$of the TriAL species are present. The precision of the method was evaluated by analyzing six replicate samples of 1 L of distilled water containing 0.5 pg L-l of TriAL,. The relative standard deviations for TriML and TriEL were 2.6 and 3.8%, respectively. Interferences. Because of the element specificity of the GFAAS determination, it might be anticipated that only lead compounds other than TriAL constitute a major risk of interferences. Basically, three forms of lead should be considered: inorganic lead, TAL, and dialkyllead compounds. In order to demonstrate the selectivity of the method toward inorganic lead, a series of blank determinations were carried out in the presence of various amounts of lead nitrate. The results are shown in Table V. Although there is a gradual increase of the blank !value, up to 1000 pg of inorganic lead can be tolerated in the sample without exceeding the limit of detection. This is satisfactory, as in most natural waters
NO. 7,
JUNE 1983
5053
the lead concentration is several orders of magnitude below this level (32). Nevertheless, in view of the potential variability of the lead content, we recommend that simultaneous determinations are made for each type of solution to be analyzed. This remark may be especially important for highly contaminated water, e.g., highway runoff, sewage effluents, industrial drainages, etc. The effect of dialkyllead compounds (DiAL) on the blank was assessed by applying the method to 1-L samples containing 10 pg of either dimethyllead dichloride or diethyllead dichloride. For an evaluation of the results (Table VI) it is important to note that these salts, in general, contain TriAL and inorganic lead impurities. The elevated value obtained with diethyllead dichloride when using the nontreated DiAL standards does not indicate an interference. Rather, it reveals that the concentration of TriEL impurities has appreciably increased since the time of analysis by the supplier (ca. 10 months earlier). Apparently, the TriML concentration remains constant. When the standards were treated first with iodine monochloride, so as to convert the TriAL present into DiAL (20,21),the normal values are found. The interference of DiAL compounds can therefore be considered as negligible, especially since their instability probably prevents the occurrence of elevated levels in the environment. Effects of TAL on the determination of TriAL are circumvented effectively by a preliminary extraction with hexane. In earlier work it was demonstrated that an organic/aqueous phase ratio of 1/100 is sufficient for the quantitative removal of TAL from water samples (33,34).In these conditions losses of TriAL should be negligible: even drastic salting-out conditions do not result in a significant extraction. We can reasonably expect lower recoveries in the actual conditions used for TAL extraction. We have added TriAL standards to a number of lake and river water samples and have not observed any detectable difference in recovery as compared to distilled water. Similarly, the recovery remained the same when the sample was filtered before the analysis or when 0.5 mL of hydrochloric acid was added to avoid adsorption losses on the container walls. Some problems were experienced with the phase separation in the NaCl/chloroform extraction for natural water samples, but in general adding a few milliliters of methanol destroyed the emulsion which sometimes developed; addition of methanol did not change the TriAL recovery. Application. A number of natural lake and river water samples (1L) were collected in plastic bottles containing 0.5 mL of hydrochloric acid for analysis with the method de-
Table VI. Effect of DiAL on the Extraction Procedure for TriAL Compounds (Two Consecutive Chloroform Extractions) jDiAL std added
composition of DiAL std addeda species % w/w
amt of TriAL amt of TriAL expected, ng of Pb found, ng of Pb
dim ethyllead (108 pg)
untreated IC1 treated
(CH,),PbCl PbC1,
95.6 3.2 1.3
(CH3)2PbC12
98.7
(CH3)2PbC1Z
(CH, ),PbCl PbCl, diethyllead (113 pg) untreated IC1 treated
(CZH, )ZPbCl, (CZH, )3PbC1 PbC1, (CZH, 12PbCL ( C A ),PbCI PbCl,
0.0
2488
0.0
2060
