Combined Nickel and Phosphate Modifier for Lead Determination in Water by Electrothermal Atomic Absorption Spectrometry YUPING XU* AND YANZHONG LIANG Department of Geological Sciences, University of Illinois at Chicago, 845 West T aylor Street, Chicago, IL 60607, USA. Email: yuping@uic.edu The effectiveness of nickel nitrate plus ammonium of a suitable modifier.3,13–20 These criteria include suppression of the interference associated with sample matrices, increase of dihydrogenphosphate as a combined modifier for the determination of lead in a variety of water samples by ETAAS pyrolysis temperature without Pb loss and enhancement of Pb signals.Therefore, further work in this area is deemed necessary. was evaluated. Optimization of the temperature program, modifier mass and pyrolysis hold time for the determination of In this work, we investigated the effectiveness of a combined nickel nitrate and ammonium dihydrogenphosphate modifier for lead was carried out.The results indicate that the combined modifier allows the quantitative stabilization of Pb in water the determination of Pb in natural waters. The objective was to develop a valid and feasible analytical scheme for the direct samples up to 1200 °C during the pyrolysis step. In comparison, the maximum pyrolysis temperature without the measurement of Pb in drinking and surface waters with complex matrices, some of which interfere with Pb determination. modifier is 900 °C or lower.The modifier further reduces the background absorbance caused by sample matrices and significantly enhances the sensitivity of Pb determination. The EXPERIMENTAL observed detection limit is 0.14 mg l-1 with a sample volume of Instrumentation and Reagents 10 ml. The characteristic mass or sensitivity of the proposed method is 7 pg. The tolerable amounts of various interferents A Perkin-Elmer (Norwalk, CT, USA) Model 503 atomic absorption spectrometer equipped with an HGA-2100 graphite such as chloride, sulfate and carbonate in the presence of the modifier are high enough for the determination of lead in a furnace controller and a deuterium arc background corrector was used for all atomic absorption measurements.Collection variety of waters. The recoveries of spiked Pb in tap water and waters from variably contaminated waters from a ditch, a lake of absorbance signals and peak integration were achieved with PeakSimple II software via a data acquisition board (SRI and a river in the Chicago area were 89–101%.The proposed method has several advantages over commonly used methods Instruments, Torrance, CA, USA). The wavelength, spectral bandwidth and lamp current used for Pb determination were with H3PO4 or Mg(NO3)2+NH4H2PO4 as modifiers. set according to the recommendations of the instrument manu- Keywords: L ead; chemical modifier; atomic absorption facturer. Pyrolytic graphite-coated graphite tubes (Perkin- spectrometry ; water; interferences Elmer, Part No.B0135653) with a L’vov graphite platform Lead is toxic to humans and animals, especially to young (Perkin-Elmer, Part No. B0121091) were utilized throughchildren. As a result of worldwide accumulation, Pb presents a out. When chemical modifiers were used, they were injected serious environmental and health hazard.1 In natural waters, separately from sample solutions. Pb concentrations often range from 1 to 30 mg l-1.2 Hence Aliquots used for the samples and modifiers were 10 ml in all highly sensitive analytical techniques are necessary to determine cases except when indicated otherwise.The internal gas flow was the Pb concentrations in these waters. Because of its sensitivity, interrupted during the atomization stage. The graphite furnace versatility, speed and specificity, ETAAS has been extensively temperature program for Pb determination is given in Table 1. utilized in the direct analysis of water for numerous elements.3 Chemicals with the highest available purity but at least of In recent decades, ETAAS has been the US Environmental analytical-reagent grade were used to prepare solutions in deion- Protection Agency (USEPA) method of choice for elemental ized water. Standard solutions were made from a commercial analysis including Pb in water samples.4,5 However, problems stock standard solution of 1000 mg l-1 prepared from Pb(NO3)2 arise in analyzing samples composed of complex matrices.(VWR Scientific, Chicago, IL, USA).Working standard solutions Chemical interference encountered in a pulse-heated electrother- were obtained by dilution to volume with deionized water. mal atomizer frequently causes depression of absorbance signals Chemical modifier solutions of Ni, La and Mg were prepared due to the co-volatilization of the analyte with the matrices.6 from their nitrate salts. Palladium modifier solutions were Currently available ways of reducing such interferences include prepared by diluting a 5% Pd(NO3)2 stock solution with platform atomization,7 probe atomization,8 Zeeman effect back- deionized water. Modifier solutions of ammonium dihydrogenground correction9 and chemical treatment of the sample in the phosphate (NH4H2PO4) were prepared by dissolving the salt graphite furnace.10,11 Chemical modification is preferred as a simple approach to alleviating interferences encountered especi- Table 1 ETAAS temperature program for Pb determination in water samples using the Ni(NO3)2+NH4H2PO4 modifier ally in the direct determination of volatile elements in samples with significant amounts of matrices.The modifiers can act as Step 1 2 3 4 pyrolysis aids and delay the vaporization of the analyte before Temperature/°C 130 1000 2400 2650 the graphite tube is nearly isothermal,12 the ideal condition for Ramp time/s* 2 2 1 1 atomization. To date, a variety of modifiers have been suggested Hold time/s 40 50 5 5 for Pb, including palladium,13 palladium plus magnesium,3 Ar gas flow/ml min-1 300 300 0† 300 ammonium phosphate,14 ascorbic acid,15 lanthanum16 and others.17–20 Despite considerable efforts having been made to * Estimated.search for valid Pb modifiers in the past, only limited success † A continuous flow of 300 ml min-1 was used for the USEPA Method 7421. has been reported for modifiers that meet some or all the criteria Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 (471–474) 471in deionized water. The above chemicals were obtained from Aldrich (Madison, WI, USA) and had a purity of 99.999%. All the reagents and deionized water were tested for Pb prior to the experiments and no detectable amounts of Pb were found. Water Samples and Their Characterization Three surface water samples were taken from the Lake Calumet area, 15 miles south of downtown Chicago, IL, USA. The surface and ground waters in the area have been substantially contaminated with slag wastes from local steel companies.In addition to the slag wastes, other solid waste materials such as household trash, demolition debris and fly ash were used as fill materials to convert the extensive coastal marshes into Fig. 1 Effects of chemical modifier on the pyrolysis and atomization usable industrial and residential properties. The wastes were temperatures for the analysis of 5 mg l-1 Pb standard solution. also disposed of in several landfills in the area.Interstate Combined modifier, 100 mg ml-1 Ni(NO3 )2+10 mg ml-1NH4H2PO4 . Highway 94 lies immediately to the west of the area, where it Curves A and C show the relationship between integrated Pb peak absorbance and pyrolysis temperature with a fixed atomization tem- receives road deicing agents and automobile exhaust fumes perature of 2400 °C, (A) with and (C) without the modifier. Curves B from the highway. Lake Calumet connects Lake Michigan and D depict the relationship between Pb signal and atomization through the Calumet River. temperature with a fixed pyrolysis temperature of 600 °C, (B) with One sample was taken directly from Lake Calumet, a second and (D) without the modifier. from the Calumet River and the third from a ditch near the highway.The ditch water appeared to have come from surface runoff, although mixing with the groundwater is possible before of the combined modifier, no significant differences for the Pb the water was discharged to the ditch.These samples were signal were observed with atomization temperatures between filtered through 0.2 mm syringe filters (Gelman, Ann Arbor, MI, 1800 and 2400 °C (curve B in Fig. 1). However, the integrated USA) using a syringe filter assembly (VWR Scientific, USA). absorbance of Pb without the modifier increased with increase They were kept at 4 °C for a brief period of time before analysis. in atomization temperature (curve D).Considering the dif- The tap water used in this study was taken from the campus of fusion effect at high atomization temperatures,21 an atomizthe University of Illinois at Chicago. The source of the water is ation temperature of 2400 °C and a pyrolysis temperature of Lake Michigan. The water sample was taken after running the 1000 °C appear to be the optimum for Pb determination. tap for a few minutes and analyzed without further treatment. These temperatures were adopted in subsequent experiments, All the samples were analyzed for their pH values with an including Pb determinations in water samples and recoveries Orion (Cambridge, MA, USA) pH meter and their concen- of spiked Pb from the samples.trations of F-, Cl-, SO42-, PO43- and NO3- with a DX100 Ion Chromatograph (Dionex, Sunnyvale, CA, USA). No phos- Pyrolysis time phate was detectable in the samples. The hold time for pyrolysis in routine analysis is usually set at about 30–40 s.However, complex matrices in some water RESULTS AND DISCUSSION samples produce high background absorption that can be Selection of Chemical Modifiers beyond the correction capacity of a Zeeman effect corrector.17,22 Therefore, it often requires a longer hold time for the determi- Several preliminary experiments were conducted to select a nation of trace volatile elements (e.g., Pb) in these water samples. valid chemical modifier for the measurement of Pb in the water The approach of increasing the pyrolysis time has been found samples.The modifiers tested include commonly used Pd, to be successful in many cases and has overcome the problem Pd+Mg, phosphate, La and Ni. In view of their stabilization of background absorption.5,23 However, the validity of using and the enhancement effects on the Pb signal in standard this approach depends on the effectiveness of chemical modifiers solutions and the water samples, Ni(NO3)2 and NH4H2PO4 in stabilizing the analyte during the pyrolysis.The relationship were selected as a combined modifier for further investigation. between Pb recovery and pyrolysis hold time is depicted in An initial screening of various concentrations of Ni(NO3)2 and Fig. 2 for 2 ng Pb-spiked ditch water (the sample with the most NH4H2PO4 was done to provide an adequate combination for complex matrix) in the presence of Ni(NO3)2 plus NH4H2PO4 the optimization of the temperature program discussed below. as the modifier. The recovery was in the range 85–94% when the holdtime increasedfrom 30 to 110 s.The maximumrecovery Optimization of ETAAS Conditions of 94% occurred at a hold time of 70 s.Therefore, a pyrolysis T emperature program time of 70 s was adopted in subsequent work. The temperatures of pyrolysis and atomization were first optimized using a Pb standard solution of 5 mg l-1 in both the presence and absence of modifiers. The optimization results are shown in Fig. 1. For the optimization of the pyrolysis temperature, an atomization temperature of 2400 °C was used.When 100 mg ml-1 Ni(NO3)2 with 10 mg ml-1 NH4H2PO4 was added to the standard solution, a pyrolysis temperature up to 1200 °C could be used without Pb loss (curve A in Fig. 1). In comparison, pyrolysis temperatures over 900 °C in the absence of the modifier (curve C) resulted in significant Pb loss. In addition, the sensitivity of Pb determination with the addition of Ni(NO3)2 and NH4H2PO4 was enhanced about twofold or more compared with Pb determination without the modifier.In order to optimize the atomization temperature, a Fig. 2 Influence of pyrolysis time on the recovery of 2 ng lead-spiked ditch water. The maximum recovery of 94% occurs at 70 s. fixed pyrolysis temperature of 600 °C was used. In the presence 472 Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12Table 2 Test of interferences in the determination of Pb in 5 mg l-1 Concentration and ratio of modifiers Pb standard solutions spiked with selected interferent ions.The The modifier concentration has been known to influence recovery is the calculated percentage based on the integrated absorbance. The recovery of the interference-free standard solution significantly the sensitivity of metal determinations.22–24 In was 100%. Standard deviations were calculated from four replicates addition to increasing the possibility of contamination of the graphite tube, a high modifier concentration generally Interferent concentration/ Recovery depresses the absorbance signal owing to secondary adsorption Interferent Compound used mg ml-1 (%) at the cooler ends of the graphite tube.25,26 On the other hand, Cl- NaCl 0.91 93±7 a higher modifier concentration stabilizes the analyte to higher SO42- Na2SO4 1.01 89±5 pyrolysis temperatures.27 Therefore, careful optimization of the ClO4- Fe(ClO4)3 0.84 94±5 modifier concentration is essential in overcoming the problems CO32- Na2CO3 0.68 91±8 while achieving higher sensitivity and thermal stability.In this work, a standard Pb solution of 5 mg l-1 was used to examine the influence of increasing concentrations of Ni(NO3 )2 and have the ability to reduce the impact of sulfate anions in NH4H2PO4 on the Pb absorbance signals. The results are natural waters. The reaction mechanism between the modifier shown in Fig. 3. It should be noted that the optimization of and sulfate is unknown.In addition to sulfate, the modifier Ni(NO3)2 and NH4H2PO4 concentrations was accomplished can also effectively tolerate up to 0.91 mg ml-1 Cl-, separately. During the optimization of the Ni(NO3 )2 concen- 0.84 mg ml-1 ClO4- and 0.68 mg ml-1 CO32-. The high tration the NH4H2PO4 concentration was fixed at 10 mg ml-1 recovery of 93±7% for Cl- in Table 2 indicates that the and during the optimization of NH4H2PO4 concentration the modifier would be effective for Pb determination in chloride- Ni(NO3)2 concentration was fixed at 100 mg ml-1.For the rich water samples. purpose of comparison, the Pb absorbance without any modi- fier is also shown in Fig. 3. These results show that the Limit of Detection and Characteristic Mass maximum signal enhancement is achieved at an Ni(NO3)2 concentration of 100 mg ml-1 and an NH4H2PO4 concen- The detection limit for the proposed method is 0.14 mg l-1, tration of 10 mg ml-1. The combined modifier provides much calculated as three times the standard deviation of a 10 ml higher signal enhancement than an Ni(NO3)2 or NH4H2PO4 blank solution for 10 determinations.The characteristic mass modifier alone. The effect of Ni(NO3)2 concentrations greater or sensitivity of Pb detection, defined as the mass of Pb which than 25 mg ml-1 on the integrated Pb absorbance is not very yields an integrated absorbance signal of 0.0044, is 7 pg. significant. Similarly, NH4H2PO4 concentrations in the range 8–20 mg ml-1 have almost an equivalent effect on the signal Sample Analyses and Method Comparison enhancement. In the light of this finding, 100 mg ml-1 Using the proposed method, we determined the Pb concen- Ni(NO3)2 and 10 mg ml-1 NH4H2PO4 were chosen as the trations in and recoveries from tap water and three surface combined modifier for the interference tests below.water samples from the Lake Calumet area in Chicago. Table 3 gives the results for Pb in various water samples and the Non-spectral Interferences concentrations of some major anions in the samples.The The frequently encountered non-spectral interferent ions and samples were found to have Pb concentrations in the range compounds on the signal of the analyte element were investi- 0.78–2.48 mg l-1. The recoveries of 2 ng of Pb added to the gated in this work. Various interferent ions were added to a samples were 89–101%. 5 mg l-1 Pb standard solution, and the Pb recovery was The results obtained from this study were compared with measured for each interferent.The results are given in Table 2, those from two commonly used ETAAS methods for Pb where the RSD was calculated from four replicates. Appar- determination (Table 3). The first is the standard method ently, the combined modifier of 100 mg ml-1 Ni(NO3)2 and recommended by the USEPA.5 It involves the addition of 10 ml 10 mg ml-1 NH4H2PO4 can tolerate sulfate up to 1.01 mg ml-1 in a 1 ml sample of H3PO4 as the modifier and a 20 ml sample with only a 5–10% depression of the signal generated by injection with continuously flowing purge gas in the absence 5 mg l-1 Pb.Sulfate is one of the most troublesome interferent of a platform. The second method uses 2 mgMg(NO3)2+10 mg ions for Pb determination.17,28 The above modifier appears to NH4H2PO4 as a combined modifier.29 The maximum ashing temperature was found to be about 1000 °C under the instrumental conditions used in this study.When used to analyze 5 mg l-1 Pb standard solution at an ashing temperature of 900 °C and an atomization temperature of 2000 °C, the latter method has a detection limit of 0.39 mg l-1 and a characteristic mass of 15 pg. These values were significantly higher than those obtained in this study. The analytical performances of these two methods with the water samples used in this study are shown in Table 3. The Pb concentrations and recoveries measured for the four water samples with the USEPA method were 1.01–2.57 mg l-1 and 79–90%, respectively.For the ditch water with a complex matrix, the recovery with the USEPA method is significantly lower than that with the proposed method. The method using the Mg(NO3)2+NH4H2PO4 modi- fier also gives significantly lower recoveries of 2 ng of Pb added to the water samples. In particular, the method cannot overcome the interference from the complex matrix of the ditch Fig. 3 Signal enhancement in response to the amount of modifier water. As a result, the peaks obtained during the analyses were used for the determination of Pb in 5 mg l-1 standard solution.A fixed not sufficiently well resolved to give meaningful results. This concentration of 10 mg ml-1 NH4H2PO4 was used in Ni(NO3)2 comparison demonstrates the effectiveness and applicability of optimization (left columns) and a fixed concentration of 100 mg ml-1 the proposed modifier for the direct determination of Pb in Ni(NO3)2 was used in NH4H2PO4 optimization (middle columns).The far right column is the Pb signal without any modifier. natural waters with complex matrices. Journal of Analytical Atomic Spectrometry, April 1997, Vol. 12 473CONCLUSION It is reasonable to suggest that Ni(NO3)2 plus NH4H2PO4 can act as an effective chemical modifier for the determination of Pb in several types of waters by ETAAS. Based on the experimental results, the pyrolysis temperature can be set to as high as 1200 °C, which is compatible with or higher than those used with the most common chemical modifiers such as Pd and Pd+Mg.The combined Ni(NO3)2+NH4H2PO4 modifier also enhances the Pb signal and reduces matrix interferences. The recovery data for different water samples appear to verify the effectiveness and applicability of the proposed method for the direct measurement of Pb in water samples with complex matrices. This work was supported by the Campus Research Board of the University of Illinois at Chicago and by the Searle Environmental Health and Safety Fellowship (Illinois, USA).REFERENCES 1 Jaworski, J. F., in L ead, Mercury, Cadmium and Arsenic in the Environment, ed. Hutchinson, T. C., and Meema, K. M., Wiley, New York, 1987, pp. 3–16. 2 Concon, J. M., Food T oxicology : Contaminants and Additives, Marcel Dekker, New York, 1988. 3 Welz, B., Schlemmer, G., and Mudakavi, J. R., J. Anal. At. Spectrom., 1988, 3, 695. 4 Association of Official Analytical Chemists, Official Methods of Analysis of the Association of Official Analytical Chemists, AOAC, Arlington, VA, 15th edn., 1990. 5 USEPA T est Methods for Evaluating Solid Waste: L aboratory Manual—Physical/Chemical Methods, SW-846, USEPA, Washington, DC, 3rd edn., 1986. 6 Ni, Z. M., and Shan, X. Q., Spectrochim. Acta, Part B, 1987, 42, 937. 7 L’vov, B. V., Peliva, L. A., and Sharnopolskii, A. I., Zh. Prikl. Spektrosk., 1977, 27, 395. 8 Gir, S.K., Littlejohn, D., and Ottaway, J. M., Analyst, 1982, 107, 1095. 9 Slavin, W., Carnrick, G. R., Manning, D. C., and Pruszkowska, E., At. Spectrosc., 1983, 4, 69. 10 Ediger, R. D., At. Absorpt. Newsl., 1975, 14, 127. 11 Shan, X. Q., and Ni, Z. M., Acta Chim. Sin., 1981, 39, 575. 12 Hinds, M. W., Katyal, M., and Jackson, K. W., J. Anal. At. Spectrom., 1988, 3, 83. 13 Shan, X. Q., and Ni, Z. M., Can. J. Spectrosc., 1981, 26, 219. 14 Zong, Y. Y., Parsons, P. J., and Slavin, W., Spectrochim.Acta, Part B, 1994, 49, 1667. 15 Ebdon, L., and Lechotycki, A., Microchem. J., 1986, 34, 340. 16 Fletcher, I. J, Anal. Chim. Acta, 1983, 154, 235. 17 He, B., and Ni, Z. M., J. Anal. At. Spectrom., 1996, 11, 165. 18 Bu-Olayan, A. H., Al-Yakoob, S. N., and Alhazeem, S., Sci. T otal Environ., 1996, 181, 209. 19 Cabrera, C., Lopez, M. C., Gallego, C., Lorenzo, M. L., and Lillo, E., Sci. T otal Environ., 1995, 159, 17. 20 Donat, J. R., Lao, K. A., and Bruland, K. W., Anal.Chim. Acta, 1994, 284, 547. 21 Perez-corona, M. T., Beatriz De La Calle�-Guntin�as, M., Madrid, Y., and Camara, C., J. Anal. At. Spectrom., 1995, 10, 321. 22 Liang, Y. Z., Li, M., and Rao, Z., Fresenius’ J. Anal. Chem., 1997, 357, 112. 23 Ni, Z. M., He, B., and Han, H. B., Spectrochim. Acta, Part B, 1994, 49, 245. 24 Liang, Y. Z., Li, M., and Rao, Z., Anal. Sci., 1996, 12, 629. 25 Thomaidis, N. S., Piperaki, E. A., and Efstathiou, C. E., J. Anal. At. Spectrom., 1995, 10, 221. 26 Frech, W., Li, K., Berglund, M., and Baxter, D. C., J. Anal. At. Spectrom., 1992, 7, 141. 27 Mandjukov, P. B., Vassileva, E. T., and Simeonov, V. D., Anal. Chem., 1992, 64, 2596. 28 Welz, B., Schlemmer, G., and Mudakavi, J. R., J. Anal. At. Spectrom., 1992, 7, 1257. 29 Manning, D. C., and Slavin, W., Appl. Spectrosc., 1983, 37, 1. Paper 6/07046I Received October 16, 1996 Table 3 Lead concentrations and recoveries of 2 ng Pb added to natural waters from various sources. Standard deviations were calculated from four replicates. Anion concentrations were obtained from ion chromatographic analyses. No phosphate was detected Pb found*/mg l-1 Recovery* (%) Sample A B C A B C pH SO42-/mg l-1 Cl-/mg l-1 F-/mg l-1 NO3-/mg l-1 Lake Calumet 1.37±0.09 1.39±0.13 1.59±0.21 89±4 84±6 70±9 7.74 56.8 50.4 0.59 6.3 Calumet River 0.78±0.12 1.01±0.17 ND† 101±8 89±5 78±8 7.58 44.8 38.8 0.25 6.4 Ditch water 1.41±0.12 1.29±0.19 —‡ 93±8 79±9 —‡ 7.79 564 216 17.2 3.9 Tap water 2.48±0.04 2.57±0.22 3.82±0.37 95±2 90±6 85±7 7.78 28.1 12.5 1.27 1.5 * A, This method; B, EPA Method 7421; C, 2 mg Mg(NO3)2+10 mg NH4H2PO4.29 † ND=not detected. ‡ Pb peaks too complex to resolve. Accepted December 17, 1996 474 Journal of Analytical Atomic Spectrometry, April 1997, Vol