JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 483 Determination of Lead by Hydride Generation Atomic Absorption Spectrometry With In Situ Concentration in a Zirconium Coated Graphite Tube Yan Xiu-ping and Ni Zhe-ming" Research Center for Eco-Environmental Sciences Academia Sinica P. 0. Box 934 Beijing China A method is described for the determination of lead by the in situ concentration of lead hydride on a Zr coated graphite tube with subsequent detection by electrothermal atomic absorption spectrometry. The method gives a six-fold enhancement of sensitivity with respect to that using a pyrolytic graphite coated graphite tube for the sorption of lead hydride. The characteristic mass (i.e. the mass of analyte which provides a defined peak absorbance of 0.0044 A) is 52.8 pg.The relative standard deviation for ten replicate measurements is 2% at the level of 3 ng of lead. An absolute detection limit (30) of 242 pg is obtained. The proposed method has been applied successfully to the determination of lead in some reference materials and a tap water sample. Keywords Electrothermal atomic absorption spectrometry; in situ concentration; zirconium coated graphite tube; lead hydride generation The generation of volatile covalent hydrides of a number of elements (ie. As Bi Sn Se Sb Te and Pb) for determina- tion by atomic absorption spectrometry (AAS) has proved extremely useful because it serves to separate the metal from other potentially interfering matrix components in the sample and can also be used as a method of concentra- tion.l12 However owing to the low yield of lead hydride and its poor thermal stability the conditions for lead hydride generation are critical and the sensitivity is low compared with that for other hydride-forming elements.Several papers3-l0 have been published on the determination of lead by hydride generation AAS (HG-AAS) in which oxidants such as dichromate hydrogen peroxide and peroxodisul- phate in acidic solution have been used in order to improve the generation efficiency of lead hydride and yield better results. Recently a niore sensitive method was developed by using nitroso-R salt medium for the generation of lead hydride. In situ trapping procedures which utilize the graphite furnace as both the concentration medium and atomization cellI2-l7 have been proved to be the most sensitive atomic spectrometric methods available for the detection of As Se Sb and Sn,18 particularly wherl a Pd coated graphite tube is used for the absorption of the hydride~.l~-*~ The combina- tion of hydride generation in the presence of oxidants with in situ concentration in the graphite tube is expected to give greater enhancement of sensitivity for the determination of lead compared with the method using HG-AAS.However few papers have been published on the determination of Pb by use of this technique. Aroza et al. 23 used HG electro- thermal atomic absorption spectrometry (HG-ETAAS) with in situ concentration in the conventional graphite tube in order to determine Pb and found it was five times more sensitive than a method based solely on HG-AAS.9 Stur- geon et aL2 proposed a method for the determination of Pb based on the generation of Pb(C2H,) using NaB(C2H,) with its subsequent trapping in a graphite furnace which gave a lower blank signal and detection limit.However the method included the synthesis of NaB(C,H,) which is not commercially available. Graphite tubes treated with compounds of refractory metals such as Zr La Mo and W have considerable advantages over conventional tubes for example enhanced sensitivity and decreased matrix interference^.^^-^^ In this work an attempt has been made to use a Zr coated graphite tube for the sorption of lead hydride generated in an H202-HN03 medium. The sorption temperature of lead hydride in such a tube was substantially lower than that reported previously and also the sensitivity was greatly enhanced.23 Experimental Apparatus A hydride generator (HG-100) made by the Research Center for Eco-Environmental Sciences Beijing China was used.Its construction and function have been described previously. Hydride generation was accomplished in a continuous mode in a mixing cell by using two channels of a peristaltic pump to deliver the sample and NaBH solu- tions. Silicone rubber tubing 4 mm i.d. x 36 cm was used to connect the outlet of the hydride generator with the quartz tube. The tip of the quartz tube was inserted into the sample introduction port at the centre cif the graphite tube and held in contact with the opposite interior wall. The hydride generated was stripped from the solution by an argon gas stream and was absorbed on the inside wall of the Zr coated graphite tube.A Perkin-Elmer Model 4000 atomic absorption spectro- meter equipped with an HGA-400 graphite furnace and a Model 056 chart recorder was used for the measurement of Table 1 Recommended experimental conditions Parameter Value Parameter Value Wavelength Bandwidth Lamp current Carrier gas flow rate Uptake rate of sample solution 283.3 nm Uptake rate of NaBH solution 3.7 ml min-' 0.7 nm HN03 concentration 0.5% v/v 6% m/v 480 ml min-' H,Oz concentration 1.5% m/m 3.7 ml min-' Sorption temperature 300 "C 3 mA NaBH concentration * To whom correspondence should be addressed.484 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1991 VOL. 6 1.0 0.8 $ 0.6 0.4 al -e a .- +- - cr" 0.2 Table 2 Furnace programme for coating zirconium - - - - - Step Temperature/"C Ramp time/s Hold time/s Internal gas flow 1 120 5 150 Normal 2 8 00 10 30 Normal 3 2000 5 10 Normal analyte absorbance in the peak height mode under gas stop and maximum power conditions.A lead hollow cathode lamp was used. The sample introduction port was enlarged to a diameter of 2.5 mm with a drill bit. Argon was used as both the purge and carrier gas. The recommended experi- mental conditions are listed in Table 1. All data obtained from sample analysis were based on the standard additions method. Reagents All the reagents were of analytical-reagent grade. De- ionized water was used throughout. A Pb stock solution 1000 pg ml-l was prepared by dissolving metallic Pb in 20 ml of 1 + 1 nitric acid and diluting to 1 1 with water.Working solutions were prepared fresh every day by diluting appropriate aliquots of the stock soh t ion. Sodium tetrahydroborate solutions of 4 and 6% m/v were prepared daily or as required in de-ionized water and were used without further filtration or stabilization. Coating the Graphite Tube With Zirconium Soak a pyrolytic graphite coated graphite tube in a 5% ZrOC12.8H20 solution for 48 h and dry it with an infrared lamp. Then repeat the following procedure five times on the dried tube inject 100 pl of 5% ZrOCl2.8H20 solution into the graphite furnace with an Eppendorf microlitre pipette fitted with a disposable polypropylene tip and heat the furnace through use of the furnace temperature programe given in Table 2.A tube treated in this manner can stand 150-200 firings. Sample Decomposition Accurately weigh 0.100 or 0.250 g of a national standard reference material (Research Centre for Eco-environmental Sciences Chinese Academy of Sciences) into a polytetra- fluoroethylene (PTFE) container add 1 ml of concentrated nitric acid and allow to stand overnight. Add 1 ml of 72% perchloric acid and 1 ml of concentrated hydrofluoric acid. Cover the container with a PTFE cover place it in a stainless-steel bomb (62 x 36 mm id. 44 mm 0.d.) and seal the bomb tightly with a screw closure in order to prevent gas leakage. Place the bomb in an oven heat to 180 "C and maintain at this temperature for about 6 h. Remove the bomb from the oven cool to room temperature remove the PTFE container take off the cover and heat the container on a hot-plate at about 200 "C and evaporate to near dryness.Finally add a suitable volume of 0.1 mol dm-3 hydrochloric acid and dissolve the residue by warming for about 10 min. Transfer the solution into a 10 ml calibrated flask and dilute to volume with 0.1 mol dm-3 hydrochloric acid. Prepare a reagent blank in parallel. Hydride Generation and GFAAS Measurement The Zr coated graphite tube was heated to 100 "C and kept at this temperature for 5 s. The tip of the quartz tube was inserted from the outlet of the hydride generator into the sample introduction port at the centre of the tube and was held in contact with the opposite interior wall. The furnace was heated to the adsorption temperature the peristaltic pump was then started and the sample and NaBH Table 3 Furnace temperature programme Step Temperature/"C Ramp time/s Hold time/s Internal gas flow 1 100 2 5 Normal 2 300 5 110 Normal 3 2200 0 5 Gas stop solutions were pumped into the cell.The hydride and hydrogen generated were swept into the furnace with argon. Collection of the hydride continued for 100 s at the adsorption temperature the quartz tube was withdrawn from the graphite furnace and the analyte was atomized at 2200 "C for 5 s. The quartz tube was inserted into the graphite tube and withdrawn from it automatically. The furnace temperature programme is summarized in Table 3. Results and Discussion Optimization of Operating Conditions In order to achieve maximum sensitivity two types of parameter were optimized by studying the effect of one variable at a time while keeping the others constant (i) parameters that may affect the efficiency of the lead hydride generation; and (ii) those that may affect lead hydride adsorption on the Zr coated graphite tube.Parameters Affecting Efficiency of Lead Hydride Generation The influence of various concentrations of HN03 on the efficiency of lead hydride generation is shown in Fig. 1. As can be seen the optimum concentration of HN03 lies within the range of 0.4-0.6% when the concentrations of H202 and NaBH were kept at 1.5 and 4% respectively. Thus a concentration of 0.5% v/v HN03 was selected for further experiments. The effect of the concentration of H202 on lead absor- bance is shown in Fig. 2. The maximum sensitivity was .L 0.4 - l t 0.2 1 I I I I 1 0 0.2 0.4 0.6 0.8 1 .o Concentration of HNO (% v/v) Fig. 1 Effect of HN03 concentration on the peak absorbance of 2.5 ng of Pb with 4% m/v NaBH and 1.5% m/m H202 I I I I I I L I 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Concentration of H,O (YO m/m) Fig.2 Effect of H202 concentration on the peak absorbance of 2.5 ng of Pb with 4% m/v NaBH and 0.5% v/v HN03JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 1.0 c" 0.8 e 2 0.6 n $ 0.4 al m .- 4- - u" 0.2 485 - - - - - 0 1 2 3 4 5 6 7 8 9 1 0 Concentration of NaBH (% m/v) 0 30 60 90 120 1 Time/s Fig. 3 Effect of NaBH concentration on the peak absorbance of 2.5 ng of Pb with 0.5% v/v HNO and 1.5% m/m H202 obtained at 1.5- 1.7% m/m H202 when the concentrations of NaBH and HN03 were kept at 4 and 0.5% respectively.Fig. 3 shows the effect of the concentration of NaBH on the efficiency of hydride generation at 0.5% v/v HN03 and 1.5% m/m H202. The optimum concentration of NaBH lies within the range 5-7% m/v (Fig. 3). Therefore a concentra- tion of 6% m/v NaBH was chosen for further study. The absorbance increases with an increase in the carrier gas flow rate up to 460 ml min-* above which the absorbance remains unchanged. A flow rate of 480 ml min-l was used. Parameters Affecting Lead Hydride Adsorption The sensitivity increases by a factor of 6.3 for the Zr coated graphite tube compared with that obtained by using the uncoated tube. The Pd coated tube gives similar sensitivity to the Zr coated tube. However the latter tube is preferred because zirconium carbide is a refractory material and after coating once with zirconium the tube can stand more than 100 firings without losing sensitivity.As palladium volatilizes at a temperature close to the atomization temperature (2200 "C) it should be added every time before collecting lead hydride. Aroza et al.23 found that the optimum sorption tempera- ture for lead hydride was higher than 600 "C when an uncoated graphite tube was used. However lead hydride can be adsorbed on the Zr coated graphite tube at much lower temperatures. Fig. 4 shows the influence of sorption temperature on lead absorbance. As can be seen the signal remains constant in the sorption temperature range of 100-500 "C. Note that even at 100 "C lead hydride can be effectively adsorbed on the Zr coated graphite tube. An adsorption temperature of 300 "C was selected for further experiments in order to avoid prolonged heating of the graphite tube at higher temperatures. After the hydride generation a period of time is required for sweeping the residual hydrides into the graphite tube.The effect of collection time was investigated under the I 4 I TemperaturePC I I 0 200 400 600 800 Fig. 4 Effect of trapping temperature on the peak absorbance of 2.5 ng of Pb with 1.5% m/m H202 0.5% v/v HNO and 6% m/v NaBH 0 Fig. 5 Effect of collection time on the peak absorbance of 3 ng of Pb with 1.5% m/m H202 0.5% v/v HN03 and 6% m/v NaBH Table 4 Interferences of foreign ions on lead absorbance (5 ng ml-l of Pb) Added as [Interfering ion] Relative [Pb"] absorbance SeO (HCl) 2 0.84 5 0.68 20 0.55 NaAsO 20 1 .oo Sb203 (HCl) 20 1 .oo K,TeO 2 I .oo Cd(N03)2 10 1 .oo 20 0.79 50 0.66 WN03 13 10 1 .oo 20 0.79 ZnSO 20 1 .oo 80 0.92 Fe203 (HNO3) 20 1 .oo 100 0.86 CoC12'6H20 20 1 .oo Ni(N03)2 20 1 .oo 80 0.88 (NH4)6M07024*4H20 10 1 .oo CU(N03)2 5 1 .oo mo3 5 00 1 .oo NaNO 500 1 .oo CaCO (HN03) 500 1.00 MgO (HNO3) 5 00 1 .oo K2S04 500 1 .oo NaCl 100 1 .oo 5 00 0.90 NaF 50 000 1.09 NH,C10 70 000 0.95 50 0.84 recommended conditions.The results shown in Fig. 5 indicate that above 60 s the absorbance of the analyte remains unchanged. A retention time of 100 s was chosen for further experiments in order to ensure that all of the lead hydride had been adsorbed completely. Interferences The interferences of different ions on the determination of Pb were studied and the results are shown in Table 4.Interferences due to hydride-forming elements such as Se and Te may be explained by competitive reactions with NaBH to form the corresponding hydrides. The transition metals Cd Cr Fe Ni and Cu inhibit hydride generation probably owing to coprecipitation of insoluble interfering corn pound^.^^^^^ Alkali alkaline earth metals and Sod2- do not interfere with the determination of Pb up to a concentration ratio of interferent to PbI1 of 500:l. The interferences of F- and C104- are not serious (see Table 4).486 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 199 1 VOL. 6 Table 5 Determination of lead in reference materials Concentration/pg g-' Sample Determined' Certified GBW 08501 Tea 1.04 k 0.06 1.06 2 0.10 (China) Tea leaves 1.02 k 0.05 1 .OO * 0.04 (China) GBW 08401 Coal fly ash 34.0 2 0.12 33.8 * 4.4 (China) GBW 08571 Mussel 1.90k 0.08 1.96 * 0.09 (China) GBW(E) 08001 * Precision expressed as standard deviation based on three determinations.Since the concentration ratios of interferent to Pb" in the sample are usually lower than those used in the interference study and the standard additions method is used no separation procedure was necessary. Sample Analysis The proposed method was applied to the determination of Pb in different types of reference materials and a tap water sample. The results in Table 5 show that the values of Pb determined in the reference materials are in good agree- ment with the certified values. A Pb concentration of 1.59kO.04 ng ml-l determined in a tap water sample by using the recommended method is in excellent agreement with that of 1.63 k 0.13 ng ml-'obtained by an independent determination using ETAAS.32 Analytical Figures of Merit The characteristic mass of the proposed method (ie.the mass of analyte which provides a defined peak absorbance of 0.0044 A) is 52.8 pg. The absolute detection limit based on the variability of the blank (ie. 30) is 242 pg. This leads to a detection limit of 0.44 ng ml-l for a solution flow rate of 3.7 ml min-* pumped for 9 s. The detection limit can be improved by introducing larger volumes of sample solution into the continuous hydride generator. The precision is 2% relative standard deviation for ten replicate measurements at the level of 3 ng of Pb.The regression equation of y=O.O833x (where y=the peak absorbance x=the analyte mass in ng) with a regression coefficient of 0.9986 is obtained from the calibration graph. The linear working range spans about two decades extending to 15 ng. The reagent blank was found to be 0.925 f 0.059 ng. Conclusion Lead hydride can be effectively adsorbed on the surface of a Zr coated graphite tube at relatively low temperatures. The sensitivity accuracy and precision are significantly im- proved compared with those obtained with the uncoated tube. The proposed method can also be used for the determination of other hydride-forming elements such as tin.31 The authors thank D. T. Wu for sample preparation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 References Robbins W.B. and Caruso J. A. Anal. Chem. 1979 51 889A. Godden R. G. and Thomerson D. R. 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Anal. At. Spectrom. 1991 6 385. Shan X.-Q. and Ni Z.-M. Can. J. Spectrosc. 1982 27 75. Paper I /O I2 7.50 Received March 18th 1991 Accepted May 9th I991