Anal. Chem. 1996, 68, 3008-3014
Determination of Tetraethyllead and Inorganic Lead in Water by Solid Phase Microextraction/Gas Chromatography Tadeusz Go´recki† and Janusz Pawliszyn*
Department of Chemistry and Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
A new method for the determination of tetraethyllead (TEL) and ionic lead in water by SPME has been developed. TEL is extracted from the headspace over the sample. Inorganic lead is first derivatized with sodium tetraethylborate to form TEL, which is extracted in the same way as pure TEL samples. The analytical procedure was optimized with respect to pH, amount of derivatizing reagent added, stirring conditions, and extraction time. The detection limit obtained for TEL was found to be 100 ppt when using FID and 5 ppt when using ion trap MS (ITMS). The detection limit for Pb2+, limited by the nonzero blank, was found to be 200 ppt. Linear calibration curves were obtained for both analytes when FID was used for detection. For lead they spanned over 4 orders of magnitude. ITMS offered excellent sensitivity and selectivity, but the calibration curves were nonlinear when the m/z ) 295 ion was used for quantitation. The method has been verified on spiked tap water samples. An excellent agreement was found between the results obtained for standard solutions prepared using NANOpure water and spiked tap water samples. Lead is one of the most dangerous environmental pollutants. Since 1700, global annual lead production has grown from 9.25 × 107 kg to more than 3.048 × 109 kg.1 This increase is almost entirely related to the massive use of organolead compounds as antiknocking agents and production of Pb acid storage batteries.2 Lead contamination is a public health problem, prevalent among children in urban metropolitan areas.3 In children, lead in the bloodstream was related to lowered intelligence and behavioral dysfunctions. In adults, Pb exposure causes high blood pressure with all its consequences, such as increased risk of heart attacks and strokes.4 Lead in drinking water is a priority pollutant. The EPA has set a standard of 50 µg/L and an action level of 15 µg/ L.5 † On leave from the Faculty of Chemistry, Technical University of Gdansk, Gdansk, Poland. (1) Rhue, R. D.; Mansell, R. S.; Ou, L.-T.; Cox, R.; Tang, S. R.; Ouyang, Y. Crit. Rev. Environ. Control 1992, 22 (3/4), 169. (2) £obin ˜ski, R.; Adams, F. C. Anal. Chim. Acta 1992, 262, 285. (3) Mielke, H. W.; Anderson, J. C.; Berry, K. J.; Mielke, P. W.; Chaney, R. L.; Leech, M. Am. J. Public Health 1983, 73, 1366. (4) Driscoll, W.; Mushak, P.; Garfias, J.; Rohenberg, S. J. Environ. Sci. Technol. 1992, 26, 1702. (5) U.S. EPA Office of Water, Document no. EPA/810-F-93-001; GPO: Washington, DC, 1993.
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Tetraethyllead (TEL) was first introduced by General Motors for use as an antiknocking agent in gasoline in 1923. Almost global use of this and other lead-based antiknocking additives in gasoline for nearly 50 years has made Pb perhaps the most widely distributed toxic heavy metal in the urban environment.1 Alkyllead compounds are ubiquitously present in the environment. They were found in air, atmospheric aerosols, rainwater, and surface waters as well as sediments and dusts. They are released into the environment from motor vehicles, accidental spillages, effluents from R4Pb manufacture, evaporation at gas stations, and other minor sources.6 Although many countries have moved away from the use of leaded gasoline, it is far from being eliminated entirely. The detection of alkyllead compounds in the environment is certain to remain an important feature of environmental monitoring in the foreseeable future.7 Tetraalkylleads (TAL) are not very stable in the environment. Their decomposition in aqueous solutions occurs in sequential stages as follows:
R4Pb f R3Pb+ f R2Pb2+ f Pb2+
(1)
with the decomposition being faster in the presence of light.1 Ionic alkyllead compounds are much more stable. Both nonionic and ionic alkyllead compounds are toxic to all forms of life. In general, TEL and tetramethyllead are more toxic to animals, while ionic alkyllead compounds are more toxic to plants.8 The number of analytical techniques that can be used for lead analysis in water is considerable. However, at the concentration levels found in environmental samples the choice is much more limited. Only three techniques enable sub-ppb detection limits: atomic absorption spectrometry (AAS), anodic stripping voltammetry (ASV), and inductively coupled plasma-mass spectrometry (ICPMS).7 The cost of AAS instrumentation is relatively high, and that of ICPMS is prohibitive. None of these techniques can be easily adopted to field measurements. ASV instrumentation can be portable, yet natural water samples must usually be mineralized prior to final determination due to adverse matrix effects caused by organic compounds, a task that is not easily accomplished in the field. Determination and speciation of alkyllead compounds (mainly ionic) is an even more difficult task. Differential pulse ASV (DPASV) necessitates the use of pure solvents without interfering (6) Radojevic, M.; Harrison, R. M. Sci. Total Environ. 1987, 59, 157. (7) Hill, S. J. Tech. Instrum. Anal. Chem. 1992, 12, 231. (8) Fallon, R. D. Bull. Environ. Contam. Toxicol. 1994, 53, 603. S0003-2700(96)00127-8 CCC: $12.00
© 1996 American Chemical Society
ions, and preferably in the absence of Pb. Another possibility is derivatization of ionic species by butylation with Grignard reagent and subsequent GC analysis with suitable detection (usually AAS). This method requires sample preconcentration and transfers, restricts the volume analyzed according to the requirements of the GC system, and demands considerable operator time and effort.9 Rapsomanikis et al.10 pioneered the use of sodium tetraethylborate (STEB) for in situ ethylation of inorganic lead and mercury, as well as ionic alkyllead and alkylmercury compounds, in the aqueous phase. Since then, the method has also been used for the derivatization of tin, organotin compounds, cadmium, rhodium, germanium, and thallium.9 Lead derivatization is believed to proceed according to the reaction9
4NaB(C2H5)4 + 2Pb2+ f (C2H5)4Pb + 4(C2H5)3B + Pb + 4Na+ (2) The formed (C2H5)3B also alkylates Pb salts; therefore, ethylation yield is higher than 50%.11 Similar reactions lead to ethylation of ionic alkyllead compounds. Tetraethyllead formed in reaction 2 is a volatile product that can be easily analyzed by GC. However, it must first be extracted from the aqueous phase. This can accomplished by purging and trapping,10 or by liquid/liquid extraction.11 In the latter case, the sensitivity is adversely affected by the fact that only a small aliquot of the extract can be injected to a GC column. Solid phase microextraction (SPME) seemed to be a natural candidate for the extraction of TEL formed in reaction 2. SPME utilizes a small, fused-silica fiber coated with a polymeric stationary phase for analyte extraction from the matrix. The fiber is mounted for protection in a syringelike device. As the stationary phase is a liquid, analytes are absorbed by it until an equilibrium is reached in the system. The amount extracted under these conditions is dependent on the partition coefficient between the sample and the coating. SPME has been used for many applications, including, the determination of substituted benzene compounds,12,13 caffeine in beverages,14 volatile organic compounds in water,15 polyaromatic hydrocarbons and polychlorinated biphenyls,16 chlorinated hydrocarbons,17 phenols,18,19 and pesticides.20 Sampling can be carried out directly from liquid samples, from their headspace, or from headspace over solid samples.21 Headspace sampling avoids extraction of nonvolatile compounds that would adversely affect GC analysis and enables faster equilibration than sampling directly from aqueous phase.21 (9) Rapsomanikis, S. Analyst 1994, 119, 1429. (10) Rapsomanikis, S.; Donard, O. F. X.; Weber, J. H. Anal. Chem. 1986, 58, 35. (11) Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Anal. Chem. 1989, 61, 1867. (12) Arthur, C. L.; Killam, L. M.; Motlagh, S.; Lim, M.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1992, 26, 979. (13) Potter, D. W.; Pawliszyn, J. J. Chromatogr. 1992, 625, 247. (14) Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J.; Arthur, C. L. J. Chromatogr. 1992, 603, 185. (15) Arthur, C. L.; Pratt, K.; Motlagh, S.; Pawliszyn, J.; Belardi, R. P. J. High Resolut. Chromatogr. 1992, 15, 741. (16) Potter, D.; Pawliszyn, J. Environ. Sci. Technol. 1994, 28, 298. (17) Chai, M.; Arthur, C. L.; Pawliszyn, J.; Belardi, R. P.; Pratt, K. F. Analyst 1993, 118, 1501. (18) Buchholz, K.; Pawliszyn, J. Environ. Sci. Technol. 1993, 27, 2844. (19) Buchholz, K.; Pawliszyn, J. Anal. Chem. 1994, 66, 160. (20) Boyd-Boland, A. A.; Pawliszyn, J. J. Chromatogr. 1995, 704, 163. (21) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843.
Morcillo et al.22 reported the use of SPME for the determination of butyl-, phenyl-, and cyclohexyltin compounds in aqueous samples after derivatization with STEB. Sampling was carried out directly from aqueous samples. Their results were characterized by poor precision, RSDs ranging from 24.1 to 68.8%. Cai and Bayona23 used SPME for speciation of mercury in fish and river samples after derivatization with STEB. Both direct and headspace sampling were used. Low detection limits and good precision were reported. This paper presents the first application of SPME for the determination of tetraethyllead and inorganic lead after in situ derivatization with STEB in aqueous samples. The method is fast and very simple. It has sub-ppb detection limits and is characterized by good precision. The entire procedure can be easily adapted for field measurements, as the only additional equipment necessary to perform the analysis when a field-portable GC is used is a magnetic stirrer and an SPME device. EXPERIMENTAL SECTION Instrumentation. The SPME device was obtained from Supelco, Inc. (Bellefonte, PA). Fibers coated with 100 µm thick poly(dimethylsiloxane) were used. A GC-3500 gas chromatograph (Varian Associates, Sunnyvale, CA) equipped with septum programmable injector (SPI) was used in experiments with FID. The analyses were carried out using a 30 m × 0.25 mm × 0.25 µm SPB-5 column (Supelco) and hydrogen as the carrier gas. A Star chromatographic system (Varian) was used for data acquisition and processing in FID experiments. Ion trap MS experiments were performed using a Varian Star 3400 gas chromatograph with a SPI injector coupled to a Varian Saturn 4D ion trap MS system, controlled by a computer with dedicated software. Both automatic gain control and fixed ionization time were used. A 30 m × 0.25 mm × 0.25 µm Omegawax column (Supelco) and helium as the carrier gas were used in these experiments. A VWR Dylastir magnetic stirrer (VWR Scientific of Canada, Ltd.) and PTFE-coated stir bars were used for stirring the samples. Materials and Methods. Tetraethyllead was purchased from Sigma Chemical Company (St. Louis, MO). A 1 mg/mL standard solution of TEL in methanol (BDH Inc., Toronto, Ontario) was prepared by adding 10 mg of TEL (handled with a microsyringe) to a preweighed 15 mL screw cap vial containing 10 mL of methanol. The vial was closed with a Mininert valve (Supelco). The remaining standard TEL solutions (0.1 and 0.001 mg/mL) were prepared in vials of the same type by appropriate dilutions of the most concentrated standard. Working aqueous TEL solutions were prepared by adding the appropriate volume of one of the methanolic solutions to 20 mL of water purified by a NANOpure ultrapure water system (Barnstead/Thermolyne, Dubuque, IA). The samples were prepared in 40 mL amber vials sealed with Teflon-lined silicon septa (Supelco). After each analysis, the vials were thoroughly washed with NANOpure water and baked for 15 min in an oven at 105 °C. Sampling was performed by exposing the SPME fiber to the headspace over vigorously stirred samples for a predetermined time. After sampling, the fiber was withdrawn into the needle, (22) Morcillo, Y.; Cai, Y.; Porte, C.; Bayona, J. M. Sixteenth International Symposium on Capillary Chromatography, Riva del Garda (Italy), September 27-30, 1994, p 804. (23) Cai, Y.; Bayona, J. M. Sixteenth International Symposium on Capillary Chromatography, Riva del Garda (Italy), September 27-30, 1994, poster.
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and the SPME device was transferred to the GC. The desorption temperature was 250 °C. Standard lead solutions were prepared from analytical reagent grade lead nitrate (J. T. Baker Canada). A stock 1 mg/mL solution was prepared by dissolving 79.9 mg of Pb(NO3)2 in 50 mL of NANOpure water. Standard solutions containing 0.1, 0.01, and 0.001 mg/mL Pb2+ were prepared by appropriate dilutions of the stock solution. They were stored in 10 mL Teflon vials (Alfa Aesar, Ward Hill, MA). Sodium tetraethylborate was purchased from Alfa Aesar. All operations involving manipulation of dry STEB were performed in a glovebag under dry nitrogen. A 1% solution of STEB was prepared daily by adding a certain amount of the reagent to a preweighed 10 mL Teflon vial, weighing the vial (tightly sealed), and adding the necessary amount of NANOpure water. Derivatization experiments were performed in 18 mL custommade quartz vials. The vials were made of a piece of 20 mm o.d. quartz tubing, sealed at one end to form a flat bottom. PTFE thread sealing tape was wound around the top of the vial to assure tightness. Polyethylene caps were used. The caps had holes drilled in the center, of a diameter equal to needle diameter of the SPME device. For the derivatization experiments, an appropriate amount of one of the standard Pb2+ solutions was added with a microsyringe to 10 mL aqueous solution containing 1 mL of pH 4 buffer (Baxter Diagnostic Corp., Toronto, Ontario) and 1 drop of 1:100 HNO3 (BDH Inc.). Alternately, acetic buffer prepared according to Perrin and Dempsey24 was used. In the experiments aiming at blank reduction, the aqueous solution contained 0.40 mL of ultrapure HCl, 1:100 (J. T. Baker Canada), instead of the buffer and HNO3. According to the specifications, the concentrated HCl contained 1 µg/L lead; thus the increase in Pb2+ concentration from the acid corresponded to 0.4 ppt. NANOpure water was used for all experiments. After 0.4 mL of the STEB solution was added, the vial was immediately closed and placed on the magnetic stirrer. The SPME fiber was placed in the headspace over the vigorously stirred sample. The remaining procedure (including vial cleaning) was identical to that used for TEL determination. Safety Considerations. Sodium tetraethylborate is pyrophoric; therefore it should be handled only in a glovebox or a glovebag under inert gas atmosphere. No data are available on STEB toxicity, but chemical protective clothing (gloves, lab coat, safety goggles) should be worn when one is working with this compound. It should be stored in a tightly sealed container under inert atmosphere in a desiccator. Tetraethyllead is highly toxic and, therefore, should be handled only in a fume hood, using appropriate protective clothing. Special care should be taken to avoid breathing TEL vapors. It should be stored in a tightly sealed container in a cool dry place. Lead salts are toxic; therefore, chemical protective gear should always be worn when they are handled. RESULTS AND DISCUSSION As tetraethyllead is the product of derivatization of Pb2+ ions, the suitability of SPME for the determination of TEL in water was first examined. TEL is a semivolatile, nonpolar compound with a boiling point around 200 °C and very poor solubility in water.25 (24) Perrin, D. D.; Dempsey, B. Buffers for pH and Metal Ion Control; Chapman and Hall: London, 1979. (25) The Merck Index; Merck & Co., Inc.: Rahway, NJ, 1989.
3010 Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
Figure 1. Extraction time profile for 100 ppb TEL solution sampled from headspace with a 100 µm PDMS fiber. Conditions: 30 m × 0.25 mm × 0.25 µm SPB-5 column; carrier, H2, 30 psi; injector temperature, 250 °C; oven temperature program 40 °C for 1 min, then 20 °C/min to 120 °C, and hold for 1 min; detector, FID.
Such compounds can be sampled both directly from water and from the headspace over an aqueous sample. In view of further experiments involving addition of derivatizing reagent to aqueous solutions, headspace sampling is much more desirable. The equilibration times in SPME headspace analysis are much shorter for volatile organic compounds (VOCs) than in direct sampling, and fiber lifetime is also longer. For these reasons, only headspace sampling was used in the experiments. The experiments were performed on a 100 ppb standard TEL solution at room temperature. As one of the future goals of this project is to employ the method developed for field measurements, FID was selected for analyte detection, since many field-portable GCs are equipped with this detector. Figure 1 presents an extraction time profile for TEL in water. Each point on the graph represents a mean of three determinations. A relatively high carrier gas pressure of 30 psi, yielding linear flow rate of ∼130 cm/s, was chosen for the gas chromatographic analysis. Under these conditions, the retention time of TEL was 3.4 min. Since hydrogen was used as the carrier gas, the high linear flow rate did not cause a significant loss in column efficiency. On the other hand, it allowed the analysis time to be significantly shortened and provided better conditions for analyte desorption from the fiber.26 In fact, no carryover was observed in all the experiments after a 1 min desorption. It follows from Figure 1 that equilibrium was reached in the system after ∼7.5 min. In all further experiments with TEL, an extraction time of 10 min was used to assure complete equilibration. Repeatability of results plays a crucial role in SPME analysis due to its equilibrium character. It has been established that stirring can be the major factor affecting the repeatability. For headspace analysis, stirring should be vigorous (the vortex should reach at least half-way down the water column), and its rate should (26) Go´recki, T.; Pawliszyn, J. High Resolut. Chromatog. 1995, 18, 161.
be kept constant in all experiments. The actual stirring rate required depends on the dimensions of the vial and the stir bar. For the 40 mL vials and 25 mm long stir bars used for headspace sampling of TEL, 800 rpm was sufficient. Another potential problem is related to heating of the stirring plate at high revolutions. The temperature of the sample can change quite significantly (by a few degrees C) when the vial stays in contact with the stirring plate, which affects the amount of the analyte extracted. Larger peak areas are usually observed when the sample heats up from the plate, as the water/headspace partition coefficient increases with temperature, while the gas/ coating partition coefficient is almost unaffected, since the temperature of the headspace remains virtually unchanged during the short time required for extraction. The prediction that the amount extracted should be lower when both the sample and its headspace are at higher temperature has been confirmed experimentally. A simple solution to heating of the sample during extraction is to place the vial at some distance from the stirring plate (∼5 mm). The heat flux transferred to the sample is greatly reduced in this way. In general, the temperature of the samples throughout all the experiments should be kept constant within (1 °C. If the variations of ambient temperature can be higher than that, the use of a constant-temperature bath might be necessary. The precision of results for TEL extraction was estimated for a 50 ppb solution and n ) 7 analyses under conditions controlled according to the above description. The value of RSD ) 2.43% was found for the FID detector. The calibration curve for TEL in water proved to be linear in the entire range examined, from 0.1 to 100 ppb, as evidenced by the value of the linear correlation coefficient r ) 0.998 35. Higher concentrations were not examined due to the poor solubility of TEL in water. The experimentally determined limit of detection (defined in this case as a concentration producing a 50% increase in the area counts of the peak observed for the blank) was 100 ppt. In order to achieve greater sensitivity and lower detection limits, it is necessary to use more specific detection. Experiments were therefore performed with TEL detection by ion trap MS (ITMS). Due to the requirements of the vacuum system, a helium pressure of 10 psi was used in this case. This did not significantly increase the analysis time, as on the column used (Omegawax) TEL was not retained as strongly as on the less polar SPB-5 column. Similarly, no carryover was observed after a 1 min desorption at 250 °C. Figure 2 presents the calibration curve determined for the 0.005-100 ppb range. The m/z ) 295 ion was used for quantitation. The curve is nonlinear in the entire range examined. In the low ppt range, a positive deviation from linearity is observed. This is because with increasing amount of tetraethyllead in the trap the relative abundance of the ions changes, m/z ) 295 becoming more abundant than at low amounts. At even higher concentrations ions of almost all masses are observed, and m/z ) 294 becomes the most abundant peak, most probably due to secondary reactions in the trap. The relative abundance of m/z ) 295 decreases, and this results in the negative deviation from linearity observed at higher concentrations. The phenomena are related to the instability of the tetraethyllead molecule, which on decomposition forms very reactive free radicals. It is this property that gives TEL its antiknocking properties. The reasons for nonlinearity discussed above are
Figure 2. Calibration curve for TEL sampled from headspace over an aqueous sample with a 100 µm PDMS fiber and analyzed by ITMS. GC conditions: Omegawax, 30 m × 0.25 mm × 0.25 µm; carrier, helium, 10 psi; injector temperature, 250 °C; oven temperature program, 40 °C for 1 min, then 20 °C/min to 120 °C, and hold for 1 min. ITMS conditions: EI, mass range 50-400, 5 min acquisition, manifold and transfer line temperature, 220 °C, scan time, 0.3 s, AGC target, 25 000. Each point represents a mean of two determinations.
Figure 3. Mass spectra recorded for the analysis of a 20 ppb TEL solution at the beginning, apex, and end of the TEL peak. Conditions as given in Figure 2.
illustrated in Figure 3. At the beginning and end of the peak, when the number of ions originating from TEL is small, the spectra are very close to their library counterparts. At the peak maximum, a severely distorted spectrum is observed. Third-order polynomial regression produced the following line equation: y ) 4 × 10-09x3 - 0.001x2 + 99.832x, with R2 ) 0.99840. Although less practical than linear calibration, quantitation based on nonlinear calibration can still be used for TEL analysis by ITMS, especially since this method offers excellent sensitivity. This is evidenced by the limit of detection (LOD) achieved. Figure 4 presents chromatograms for total ion current and m/z ) 295 extracted ion chromatogram obtained for a 5 ppt TEL solution. The peak observed at the retention time of 4.1 min has a signal to noise ratio of 167 (as reported by the software) at the m/z ) 295 trace. The excellent S/N is mainly due to the fact that very few background ions of this mass were observed; hence the background noise was very small. The area of the peak was 741 (arbitrary units). Assuming that a minimum peak area that can be reliably quantified is 150, the quantitation limit is 1 ppt, while the limit of detection defined as S/N ) 3 is 0.1 ppt. The quality Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
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Figure 4. Total ion current (top) and m/z ) 295 (bottom) chromatograms obtained for headspace extraction of a 5 ppt TEL sample.
Figure 5. Time profile obtained for Pb2+ derivatization/extraction. Chromatographic conditions as given in Figure 1. Each point represents a mean of three determinations.
of the mass spectrum obtained at the peak apex is very good in spite of the very low concentration, as illustrated in the same figure. Having established the conditions for TEL analysis, experiments with Pb2+ derivatization were started. Figure 5 presents the time profile obtained for headspace extraction of a 20 mL, 50 ppb sample containing 2 mL of acetic buffer, to which 1 mL of 1% STEB solution was added. The course of the curve is markedly different from that obtained for TEL extraction, illustrated in Figure 1. An almost linear increase in the amount extracted is observed up to ∼15 min, after which the curve practically levels off. The different shape of the time profile is not surprising considering that an additional step of derivatization is included. Initial experiments with Pb2+ determination were performed in glass vials. However, it was observed that the blank value obtained for these vials was relatively high. Most probably this was related to derivatization of lead leached from the glass walls. Polyethylene vials were tried, yet they did not allow visual control of the stirring process. Besides, the slightly convex shape of the vial bottom hindered stirring. Teflon vials had properly shaped bottoms but, similarly to polyethylene vials, did not allow visual control. Custom-made quartz vials were therefore used in all subsequent experiments. 3012 Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
Several factors were found to affect the amount of TEL extracted from the samples. One of them is pH. Rapsomanikis10 found pH 4.1 to be optimal; Sturgeon11 established that ethylation proceeds in the pH range from 2 to 9. The optimum pH found in our experiments was 4.5. Peak areas of TEL extracted from standard Pb(II) solutions at pH 4 and 5 were lower by ∼10%. STEB solutions are very strongly alkaline; hence, a relatively large amount of pH buffer must be added to the samples in order to keep the desired reaction. However, as the buffer itself can be the source of contamination, to keep its amount low in all further experiments the samples were initially acidified by addition of 1 drop of HNO3, 1:100, and the amount of the pH 4 buffer was kept at 1 mL/10 mL of sample. Under such conditions, the final pH of the sample with STEB added was 4.5. The amount of STEB added can also play a significant role. No significant differences in peak areas were observed for fresh 1% STEB solutions when 0.35-0.45 mL of the reagent solution was added. At lower and higher amounts, the peak areas of TEL were lower. A 0.4 mL addition was used in all further experiments. The last factor found to very significantly affect the results was the stirring rate. In order to achieve high sensitivity and repeatability, it is necessary to use very vigorous stirring. The stirrer used had a maximum speed of 2500 rpm, controlled by a dial calibrated in arbitrary units from 1 to 5. The speed could therefore be only estimated by the depth to which the vortex created by stirring reached. Peak areas obtained for the vortex reaching to the stir bar (close to maximum setting), half-way down the water column, and a quarter of the way down the water column, were 1, 0.66, and 0.41, respectively. The stirring rates in this experiment were 1800, 1200, and 900 rpm, respectively. It should be kept in mind, however, that the intensity of stirring is related also to the dimensions of the vial and the stir bar. The above figures correspond to a vial of 17 mm i.d. equipped with a 16 mm long stir bar. The decrease in peak areas observed was not related to longer equilibration time at less vigorous stirring, as similar results were obtained for both 15 and 25 min of extraction. STEB undergoes hydrolysis in acidic solutions. Apparently when the stirring is not vigorous enough, a significant amount of the reagent is lost due to decomposition and is not available for derivatization. All further experiments were performed at the optimized settings (30 m × 0.25 mm × 0.25 µm SPB-5 column; carrier, H2, 30 psi; injector temperature 250 °C; oven temperature program, 40 °C for 1 min, 20 °C/min to 120 °C, hold for 2 min; detector, FID; 10 mL samples containing 1 drop of HNO3 (1:100) and 1 mL of pH 4 buffer; 0.4 mL of 1% STEB solution added). The calibration curve obtained for lead in the 5-1000 ppb concentration range was linear, with a correlation coefficient of 0.999 13. The curve had a nonzero intercept due to a blank value corresponding to ∼5 ppb Pb2+. Both Sturgeon11 and Feldman27 found that STEB can contribute significantly to the blank. However, the blank values they obtained were significantly lower. It has been confirmed by an experiment in which the amount of STEB was changed, while all other parameters were kept constant, that the reagent is the actual source of the blank. However, as FID is nonselective, the identity of the blank peak was not certain. The determination of the blank was repeated, therefore, on the more polar Omegawax column. In this case the blank value cor(27) Feldman, B. J.; Mogadeddi, H.; Osterloh, J. D. J. Chromatogr. 1992, 594, 275.
responded to ∼1 ppb Pb2+. This indicates that an unidentified substance coelutes with TEL on the SPB-5 column. Looking for ways to further reduce the blank, the buffer and the nitric acid were eliminated from sample preparation, and the sample was adjusted to the required pH with ultrapure HCl. The blank value determined in this case corresponded to ∼300 ppt. This value is consistent with previous findings.27 The limit of detection of Pb2+ found for such a system is ∼200 ppt. The correlation coefficient of the calibration curve in the 0-2000 ppt range was found to be 0.994 39. The precision of determination of the Pb2+ concentration was estimated by analyzing n ) 7 samples containing 50 ppb Pb2+. The value of RSD found was 5.04%, which is very good considering the relatively complex character of the analysis. According to the stoichiometry of reaction 2, only 50% of the Pb2+ present in the sample should be derivatized. This means that the amount of TEL extracted from a sample containing Pb2+ of a given concentration (mass/volume) as a result of derivatization should be lower than the amount extracted from a solution containing the same concentration of TEL, even though lead constitutes only ∼2/3 of the molecular mass of tetraethyllead. Interestingly, in all cases the amount extracted from samples containing Pb2+ was significantly higher than that for TEL. This indicates that the yield of the reaction was actually higher than 50%. A similar phenomenon was observed by Sturgeon,11 who suggested that this may be due to subsequent ethylation of redissolved Pb0 by STEB or (C2H5)3B formed in reaction 2. ITMS was also used for the determination of Pb2+ by SPME. The identity of the derivatization product was confirmed by library search with very high probability (996 out of 1000). Traces of diethylmercury were also found, probably as a contamination from the reagent. Other components identified in the sample also originated from STEB. They were mainly aromatic compounds, including benzene, toluene, propylbenzene, 1-ethyl-3-methylbenzene, and pentylbenzene. Figure 6 presents calibration curves for Pb2+ determination with automatic gain control (AGC). As higher concentrations were analyzed than for TEL, it was possible to use both total ion current (top) and m/z ) 295 (bottom) signals for quantitation. The dependence determined for TIC can be treated as linear, with a correlation coefficient of 0.995 79. The dependence for m/z ) 295 is nonlinear. When a fixed ionization time of 300 µs was used, nonlinear dependences were obtained for both TIC and m/z ) 295. The reasons for nonlinearity are the same as for TEL determination by ITMS, discussed above. Blank values corresponding to ∼1 ppb and due entirely to tetraethyllead were observed. This confirms the finding that an unknown substance originating from STEB coelutes on the SPB-5 column with TEL, contributing to the increased blank values observed for this column. Finally, the method was examined on tap water samples. In all cases levels of lead lower than 1 ppb were found. The blanks did not differ statistically significantly from those for NANOpure water. To confirm the suitability of the method for the analysis of tap water, a calibration curve was determined for the same range of concentrations as for NANOpure water (5-1000 ppb). The calibration curve was linear in the entire range examined with a correlation coefficient r ) 0.998 80. The slope of the curve was within the limits of experimental error the same as for NANOpure
Figure 6. Calibration curves for Pb2+ determination by ITMS obtained for total ion current (top) and m/z ) 295 (bottom) with AGC, target set at 25 000. Conditions as given in Figure 2.
water (2420 vs 2480 for tap water and NANOpure water, respectively), which indicates that a relatively simple matrix like tap water does not affect the derivatization/extraction processes. This seems quite obvious considering that the amounts of the buffer, nitric acid, and STEB added to the samples are several orders of magnitude higher than the amount of any native matrix component; therefore, in general all samples can be treated as having normalized matrices. CONCLUSIONS The research presented demonstrates that SPME can be very successfully used for the determination of tetraethyllead and inorganic lead after derivatization with sodium tetraethylborate at sub-ppb levels. The detection limits obtained for lead with the simple experimental setup and instrumentation presented are comparable to those obtained using much more complicated (and expensive) instrumentation, and the precision of results is very good. The method can be easily adopted for field measurements, as most portable GCs are equipped with FID, and the only additional pieces of equipment necessary to perform the analysis are a magnetic stirrer and SPME holder. STEB can be prepared in the form of tablets that are added to the sample directly before the extraction. In this way, problems related to STEB handling can be easily eliminated. According to the results of Rapsomanikis,10 the same methodology should be applicable to the determination of ionic alkyllead species, which significantly broadens the scope of the procedure developed. Taking into account that similar methods have already been developed for the determination of organomercury22 and organotin23 compounds, it seems that Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
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SPME has a great potential in the analysis of selected dangerous inorganic and organometallic environmental pollutants. Sturgeon11 found that the derivatization process is not affected by a large excess of Ca2+, Na+, Mg2+, Fe3+, Cr6+, Ni2+, Mn2+, Al3+, and Zn2+ and only slightly affected by a 1000-fold excess of Cu2+. On the other hand, sampling from headspace eliminates the potential adverse effects that nonvolatile organic compounds may have on extraction and chromatographic separation. The applicability of the method for the analysis of tap water samples has been confirmed. Its usefulness for more complex samples, like blood, urine, or soft drinks, needs to be examined. When more selectivity is required, detectors other than FID can be used. Ultimate selectivity can be achieved by hyphenated techniques, like GC/MS, GC/AAS, or GC/AES.
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ACKNOWLEDGMENT The authors thank Supelco Canada, Varian Inc., and the Natural Sciences and Engineering Research Council of Canada for financial support. The assistance of Haodan Yuan in obtaining the data for tap water is acknowledged. Presented in part at the 1996 Pittsburgh Conference, March 4-7, Chicago, IL, and the 18th International Symposium on Capillary Chromatography, May 20-24, 1996, Riva del Garda, Italy. Received for review February 7, 1996. Accepted May 30, 1996.X AC9601270 X
Abstract published in Advance ACS Abstracts, August 1, 1996.
