摘要:

Direct determination of rare earth impurities in lanthanum oxide by Øuorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry with slurry sampling Chen Shizhong, Peng Tianyou,* Jiang Zucheng, Liao Zhenhuan and Hu Bin Department of Chemistry, Wuhan University, Wuhan, 430072, China Received 3rd June 1999, Accepted 25th August 1999 Slurry sample introduction with electrothermal vaporization (ETV) has been applied to inductively coupled plasma atomic emission spectrometry (ICP-AES) for the direct determination of rare earth impurities in lanthanum oxide.A polytetraØuoroethylene (PTFE) emulsion was used as Øuorinating reagent to form volatile Øuorides rather than refractory carbides of rare earth elements (REEs). The Øow path of carrier gas between the graphite furnace device and the ICP torch was improved, and the main factors affecting the analytical signals, such as the Øow rate of carrier gas and auxiliary carrier gas, matrix concentration, exposure time, vaporization temperature and vaporization time, were studied systematically. Under the optimum operating conditions, the detection limits (DL) for 14 REEs were obtained in the range of 2 ng ml21 (Yb) to 130 ng ml21 (Ce), and the relative standard deviation (RSD) is less than 5%.The recommended approach has been applied to directly analyse lanthanum oxide without any chemical pretreatment. Introduction The applications of high-purity rare earth oxides in hightechnology Æelds depend not only on the characteristics of the basic substances, but also on the purity of the compounds.1 Therefore, the development of a rapid, sensitive and reliable method for the determination of trace rare earth elements (REEs) is very essential.Of the many analysis techniques for rare earth oxides, it is now accepted that inductively coupled plasma atomic emission spectrometry (ICP-AES) offers numerous advantages as a technique for the quantitative analysis of REEs.2±6 However, the conventional ICP-AES method suffers from the following problems: (1) limited sensitivity; (2) low nebulization efÆciency; (3) spectral interference and matrix effects.Electrothermal vaporization (ETV) as sample introduction device combined with ICP-AES provides very attractive features, including high sampling efÆciency, requirement of a small sample, low absolute detection limit and direct analysis of solid samples.Unfortunately, the REEs are difÆcult to vaporize from the graphite furnace at high temperature because of the formation of refractory carbides. In previous work with ETV-ICP-AES, halogenating reagents, such as CHF3,7 CCl4 or NH4Cl,8 and Freon-12,9 were utilized to promote the vaporization and transportation of the refractory elements, and improved the detection limits of refractory elements. Fluorination assisted ETV-ICP-AES using a polytetraØuoroethylene (PTFE) emulsion as a Øuorinating reagent has been successfully applied to directly analysing solid biological, environmental and high-purity materials.10±13 In the present study, successful efforts have been made in the analysis of highpurity lanthanum oxide.The conventional Øow path of the carrier gas between ETV and ICP was replaced by a new joint design, and the main factors affecting the analytical signals, such as the Øow rate of carrier gas and auxiliary carrier gas, matrix concentration, exposure time, vaporization temperature and vaporization time, were investigated in detail.The results showed that the proposed method has some advantages in sensitivity, rapidity, reliability, no requirement for chemical pretreatment and matrix matching. Experimental Apparatus and operating conditions A 2 kW, 27°3 MHz ICP generator (Beijing Second Broadcast Equipment Factory, Beijing, China) and an ICP torch (Chang Sha Quartz and Glass Factory, Changsha, China) according to Fassel were used.A modiÆed graphite furnace vaporizer was used as the vaporization device.10±13 The evolved compounds were swept by carrier gas into the plasma excitation source through a concentric glass tube with three openings (Fig. 1) connected with plastic tube (4 mm id). The analytical signals were recorded using a WPG-100 plane grating spectrograph with 1200 grooves mm21 blazed for 300 nm (dispersion 0.8 nm mm21 in Ærst-order spectrum, Beijing Second Broadcast Equipment Factory, Beijing, China).The spectrum was photographically recorded on a photographic plate (ultraviolet I, Tianjing, China) and the blackening values of the analysis lines were measured using a microphotometer (Zeis II model, Jena, Germany). Then the blackening values were converted to signal intensity values by the characteristic curve of emulsion. The operating conditions for ETV-ICP-AES are listed in Table 1. Standard solutions and reagents The stock standard solutions of REEs with a concentration of 1 mgml21 were prepared by dissolving their specpure oxides in Fig. 1 Block diagram of the joint of the gas Øow path.J. Anal. At. Spectrom., 1999, 14, 1723±1726 1723 This Journal is # The Royal Society of Chemistry 1999dilute HCl, followed by dilution to a certain volume with water. A 60% (m/v) PTFE emulsion (dv1 mm) was provided by the Institute of Shanghai Organic Chemistry (Shanghai, China). All other chemicals used in this work were of specpure grade.Twice-distilled water was used throughout. Slurry sample preparation The La2O3 powder (50 mg) was accurately weighed into a 5.0 ml test tube; 0.75 ml of 60% (m/v) PTFE emulsion and 3.0 ml of 0.1% (m/v) agar solution were added, and then adjusted to 5.0 ml with water. The resulting mixtures were dispersed with an ultrasonic vibrator for 20 min, and the test tube was shaken prior to any sampling. Recommended procedure After the ICP had stabilized, a 20 ml sample was pipetted into the graphite furnace.After being dried and ashed, the analyte was vaporized and carried into the plasma by the argon gas, and the emission signals of the analytes were recorded. Results and discussion The interface between ETV and ICP Aschematic diagram of the modiÆed Øow path of the carrier gas is shown in Fig. 1.13 A concentric glass tube with three openings (5 cm long, 1 mm internal tube id and 1 mm interval between internal and outer tube) was connected between the ETV device and the ICP torch.In this system, the conventional Øow path of the carrier gas was divided into two parts. One was a sample carrier gas from the ETV device to transport the vaporized sample into the ICP, and the other was an auxiliary carrier gas from the outer tube of the concentric glass tube to maintain the ICP channel and to obtain the optimum analytical signal. Optimization of the gas Øow rate The effect of the Øow rate of the carrier gas and the auxiliary carrier gas on the analytical signal of Yb is shown in Fig. 2. As can be seen from Fig. 2, a lower Øow rate of carrier gas and a higher Øow rate of auxiliary carrier gas gave the optimum signal-to-background (S/B) ratio because of the effective ICP channel and high transportation efÆciency. Similar effects were observed with other elements, in the absence and presence of PTFE. Therefore, a carrier gas Øow rate of 0.3 l min21 and an auxiliary carrier gas Øow rate of 0.7 l min21 were chosen in this work.Effect of matrix concentration Fig. 3 shows the effects of matrix (La) concentration on the determination of REE impurities. It can be seen that, when the concentration of matrix (La) varied from 0 to 10 mg ml21, no obvious emission signal intensity changes of the analytes were observed. However, once the matrix (La) concentration surpasses the tolerance, the analytical signal intensities begin to increase or decrease. The decrease in the signal intensity could be attributed to two competitive reactions.One took place between the REEs determined and the Øuorinating reagent PTFE, and the other between the matrix (La) and the Øuorinating reagent PTFE, leading to incomplete vaporization of the REE impurities. The increase in the signal intensity was mainly due to spectral interference from the emission lines of La in the vicinity of the REE analytical lines, causing an increase in the signal intensity of the analytes. In real sample analysis, a sample concentration of 10 mg ml21 was used.Choice of vaporization time and exposure time At the vaporization temperature of 2400 �C, the effects of the vaporization time on the analytical signals are shown in Fig. 4. The vaporization time has an obvious inØuence on the analytical signal intensities from 0.5 to 3 s with PTFE, and then the emission intensity reaches a plateau. This indicates that the REEs could be completely vaporized within a short time.In this study, 4 s was chosen as the vaporization time. The relationship between the signal intensity and the exposure time in the presence of PTFE is given in Fig. 5. In this technique, an integrated signal was detected, and the optimum exposure time is the time necessary for complete transportation of the vaporized analytes; therefore, an exposure time of 10 s should be chosen. Table 1 The operating conditions for ETV-ICP-AES Incident power/kW 1.2 Carrier gas/l min21 0.3 Auxiliary carrier gas/l min21 0.7 Coolant gas/l min21 16 Entrance slit width/mm 20 Drying temperature/�C 100; ramp 10 s, hold 20 s Ashing temperature/�C 1000; ramp 10 s, hold 20 s Vaporization temperature/�C 2400, 4 s Exposure time/s 10 Sample volume/ml 20 Fig. 2 Effect of the Øow rate of the carrier gas and the auxiliary carrier gas on the signal-to-background (S/B) ratio. Yb, 1.0 mg ml21. Flow rate of auxiliary carrier gas/l min21: curve 1, 0.3; curve 2, 0.5; curve 3, 0.7; curve 4, 0.8; curve 5, 0.9.Fig. 3 Effect of matrix concentration on the analytical signals with PTFE: Yb, 1.5 mg ml21; Eu, Gd and Ho, 3.0 mg ml21; Pr and Sm, 5.0 mg ml21. 1724 J. Anal. At. Spectrom., 1999, 14, 1723±1726Vaporization temperature The vaporization temperature has a critical effect on the analytical signal. From Fig. 6, it can be seen that, in the absence of PTFE, poor analytical signals are obtained and do not reach a maximum until 2800 �C. However, in the presence of PTFE, stronger analytical signals are observed, and reach a plateau above a temperature of 2400 �C due to the formation of more volatile Øuorides with similar vaporization characteristics.In this study, a vaporization temperature of 2400 �C was used for simultaneous multielement determination. Detection limits and precision The detection limit (DL) is deÆned as the analyte concentration yielding a signal equal to three times the standard deviation of the background noise.The detection limits and relative standard deviations (RSD) for the proposed method (C~2.0 mg ml21, n~9) are summarized in Table 2. Table 2 Fig. 4 The analytical signal as a function of the vaporization time with PTFE: Yb, 1.5 mg ml21; Eu, Gd and Ho, 3.0 mg ml21; Pr and Sm, 5.0 mg ml21. Fig. 5 Dependence of the analytical signal on the exposure time with PTFE: Yb, 1.5 mg ml21; Eu, Gd and Ho, 3.0 mg ml21; Pr and Sm, 5.0 mg ml21. Table 2 The detection limits (DL) and relative standard deviations (RSD) for this method DL This method/ PN14/ TC-ETV15/ Element Wavelength/nm ng ml21 ng ng pg RSD (%) Ce 413.380 130 2.4 110 – 4.3 Pr 390.844 30 0.6 98 – 3.4 Nd 406.190 90 1.8 98 – 3.8 Sm 359.260 40 0.8 60 – 3.1 Eu 397.199 6 0.1 4 12 2.8 Gd 342.447 14 0.3 66 – 2.5 Tb 350.917 20 0.4 16 – 2.7 Dy 353.170 8 0.2 7 – 2.3 Ho 345.600 5 0.1 5 – 1.8 Er 337.271 7 0.2 1 34 1.8 Tm 313.126 25 0.5 0.3 – 3.0 Yb 328.937 2 0.04 1 18 1.4 Lu 307.765 30 0.6 22 54 2.7 Y 321.669 6 0.1 4 25 2.0 Fig. 6 The analytical signal versus the vaporization temperature. (A) With PTFE: Yb, 1.5 mg ml21; Eu, Gd and Ho, 3.0 mg ml21; Pr and Sm, 5.0 mg ml21. (B) Without PTFE. Table 3 The analytical results of REEs in La2O3 powder (n~5) Element Calibration curve methoda/ mg g21 PN-ICP-AESa/ mg g21 Standard addition methodb/ mg g21 Calibration curve methodb/ mg g21 Ce 112.3°6.0 115.0°7.0 116.9°8.0 110.5°8.0 Pr 73.6°3.0 75.8°2.0 69.8°5.0 72.4°4.0 Nd – – – – Sm 55.4°3.0 56.7°5.0 52.8°3.0 53.6°4.0 Eu – – – – Gd 15.5°2.0 16.7°2.0 13.4°2.0 14.2°4.0 Tb – – – – Dy 4.85°0.6 5.11°0.6 4.58°0.5 4.73°0.3 Ho 2.54°0.3 2.92°0.4 2.48°0.4 2.77°0.2 Er 5.92°0.4 6.21°0.4 5.75°0.6 5.60°0.6 Tm 12.1°2.0 14.5°2.0 12.4°2.0 13.7°2.0 Yb 1.87°0.2 2.05°0.2 1.91°0.2 1.85°0.1 Lu – – – – Y 4.21°0.4 4.53°0.3 3.95°0.5 4.03°0.4 aAnalysis after dissolving sample with HCl.bDirect analysis with slurry sampling. J. Anal. At. Spectrom., 1999, 14, 1723±1726 1725also shows the comparison of the DL of the REEs obtained by this method, conventional pneumatic nebulization (PN)-ICPAES and tungsten coil (TC)-ETV-ICP-AES.As can be seen from Table 2, the DL of the REEs obtained are comparable or better than those for the TC-ETV-ICP-AES and PN-ICP-AES methods. Sample analysis The contents of the trace REE impurities in high-purity La2O3 were directly determined according to the described method. The sample was also analysed by PN-ICP-AES, and the analytical results are listed in Table 3.The recoveries of some REEs are summarized in Table 4, and were obtained by the calibration curve method with slurry sampling. Conclusion In brief, the application of slurry sampling ETV-ICP-AES for the determination of trace REE impurities in high-purity lanthanum oxide has numerous advantages, such as: (1) elimination of sample pretreatment; (2) small sample requirement; (3) simple and rapid operation; (4) reduction of sample contamination; (5) calibration with standard solutions without matrix matching.Therefore, it is likely to become an effective method for the direct determination of trace impurities in powder samples. Acknowledgements This work was supported by the National Science Foundation and the Education Ministry Foundation of China. References 1 W. Fey and K. H. Lieser, Fresenius' J. Anal. Chem., 1993, 346, 896. 2 Z. C. Jiang and B. Hu, Fenxi Kexue Xuebao, 1995, 11(2), 62. 3 J. Y. Li, J. Yang and J. R. Dong, Guangpuxue Yu Guangpu Fenxi, 1995, 15(4), 71. 4 Z. C. Jiang, H. Chen and S. X. Zhang, Gaodeng Xuexiao Huaxue Xuebao, 1990, 11(11), 1283. 5 L. Darbha and S. Gangadharan, Fresenius' J. Anal. Chem., 1994, 348(4), 284. 6 W. S. Li, C. L. Peng, P. Yuan, W. D. Qi, Z. X. Kuang and C. H. Xu, Fenxi Ceshi Xuebao, 1998, 17(1), 18. 7 G. F. Kirkbright and R. D. Snook, Anal. Chem., 1979, 51, 1938. 8 K. C. Ng and J. A. Caruso, Analyst, 1983, 108, 476. 9 J. M. Ren and E. D. Salin, Spectrochim. Acta, Part B, 1994, 49, 567. 10 T. Peng and Z. Jiang, J. Anal. At. Spectrom., 1998, 13, 75. 11 Z. Jiang, B. Hu, Y. Qin and Y. Zeng, Microchem. J., 1996, 53, 326. 12 T. Peng and Z. Jiang, Fresenius' J. Anal. Chem., 1998, 360, 43. 13 S. Cheng, F. Li, Z. Liao, T. Peng and Z. Jiang, Fresenius J. Anal. Chem., 1999, 364, 556. 14 X. Pu, A. Pei and B. Huang, Chin. J. Anal. Chem., 1989, 17, 61. 15 K. Dittrich, H. Berndt, J. A. C. Broekaert, G. Schaldach and G. Tolg, J. Anal. At. Spectrom., 1988, 3, 1105. Paper 9/04436A Table 4 Recoveries of some rare earth elements Element Added/mg ml21 Found/mg ml21 Recovery (%) Nd 0.0 0.0 – 3.0 3.24 108 5.0 5.37 107 Eu 0.0 0.0 – 0.1 0.096 96 0.2 0.207 104 Tb 0.0 0.0 – 0.5 0.47 94 1.0 0.96 96 Lu 0.0 0.0 – 0.5 0.46 93 1.0 0.95 94 1726 J. Anal. At. Spectrom., 1999, 14, 17

ISSN:0267-9477    

DOI:10.1039/a904436a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Non-Boltzmann distribution among energy levels of singly-ionized vanadium in dc glow discharge and rf inductively coupled discharge plasmas Kazuaki Wagatsuma and YuÃetsu Danzaki The Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Received 2nd June 1999, Accepted 13th August 1999 The Boltzmann plots of ionic vanadium lines are investigated using an Ar glow discharge plasma as well as an inductively coupled plasma. Departures from a linear plot are observed with both of the plasmas, meaning that LTE conditions are not realized.It results from increased population of the 3d3(a 4F)4d 5H levels to which the V II 264.433 and 265.565 nm lines are assigned, which could be explained from resonance charge-transfer collisions between vanadium atom and argon ion. Introduction Spectroscopic diagnostics of a plasma determine the physical parameters, such as the electron temperature and concentration, by analyzing the intensity and the width of spectral lines.1 The excitation temperature based on the Boltzmann distribution is a useful criterion for selecting the discharge conditions as well as analytical lines when the plasma is employed as an excitation source in optical emission spectrometry.2 The Boltzmann plot method of measuring the excitation temperature is now being extensively applied in plasma diagnostics, because it is not necessary to estimate the absolute emission intensity and to know the concentrations of emitting species.The Boltzmann distribution among different energy states exists in a plasma being in local thermodynamic equilibrium (LTE); however, actual analytical plasmas are not always in LTE.3±8 This implies that the excitation temperature of such plasmas is not unique but sometimes varies, depending on the kind of emission lines employed. Therefore, it is important to determine whether a particular plasma is in LTE, and further to study the non-LTE characteristics if LTE is not established in the plasma.This approach would provide some idea regarding the excitation processes occurring in the plasma. A possible reason for non-LTE behaviors is that the excitation processes might include non-thermal collisions such as charge-transfer collision.9 The selective excitation/ ionization10 taking place through charge-transfer collisions does not follow LTE conditions, and thus causes a non- Boltzmann distribution among different energy states, yielding non-linear Boltzmann plots.Several researchers have indicated that particular ionic emission lines are much enhanced through resonance charge-transfer collisions in glow discharge plasmas, 11±14 derived from the selective excitation to the corresponding energy levels.10 This effect should appear in the Boltzmann plots. In this paper, we investigate Boltzmann plots of vanadium ionic lines assigned to various transitions in an Ar glow discharge plasma (Ar±GDP), and report on the departures from the normal Boltzmann distribution and the excitation mechanism.For comparison, the results obtained with an rf inductively coupled plasma (ICP) are also presented. Experimental A Hitachi P-5200 ICP system with a Czerny±Turner mounting monochromator was employed for measuring ICP-excited emission intensities. For the Ar±GDP measurements, the ICP torch was replaced with a Grimm-style glow discharge lamp. The focal length of the spectrometer is 0.75 m.The grating of 3600 grooves mm21 has a reciprocal dispersion of 0.29 nm mm21 and a blaze wavelength of 200 nm. The ICP source comprises a three-turns load coil of 26 mm inner diameter and a Fassel-type fused-silica torch having an 18 mm diameter outer tube. The glow discharge lamp was made inhouse according to the original model by Grimm.15 Its structure has been described elsewhere.16 The inner diameter of the hollow anode was 8.0 mm and the gap between the electrodes was adjusted to be about 0.3 mm. A vanadium disk (99.8%, purity) was prepared for the Ar± GDP sample.It was polished with waterproof emery papers and then rinsed with ethanol. Before making measurements, predischarges were carried out for about 10 min to remove the surface contaminants. A stock solution for the ICP measurements was prepared by dissolving 99.9% purity vanadium metal, 0.800 g, with 200 mL nitric acid (7 M) at room temperature. The sample solution containing 0.8 mg mL21 of vanadium was made up by diluting the stock solution with deionized water.The solution contained about 1.4 M niric acid. Table 1 gives the operating conditions for the Ar±GDP and the ICP in detail. Results and discussion Analytical lines In selecting analytical emission lines for the Boltzmann plots, the following conditions should be fulÆlled: large range in Table 1 Operating conditions for the Ar±GDP (a) and the ICP (b) (a) Ar±GDP– Plasma gas: Argon (99.9995%) Gas pressure: 530 Pa (4 Torr) Discharge voltage: dc 250±475 V (constant voltage mode) Discharge current: 35±70 mA (b) ICP– Rf frequency: 27.12 MHz Forward power: 0.5±1.2 kW ReØected power: Less than 20W Plasma gas: Argon (laboratory grade) Gas Øow: Outer 12 l min21 Intermediate 0.50 l min21 Carrier 0.45 l min21 Observation height: 15 mm above load coil J.Anal. At. Spectrom., 1999, 14, 1727±1730 1727 This Journal is # The Royal Society of Chemistry 1999their excitation energy, accurate gA values and line positions at closely spaced wavelengths to avoid a calibration of the wavelength dependence of the detection sensitivity. When considering these conditions we noticed a set of ionic vanadium lines (V II) at wavelengths between 252.8 and 269.1 nm, as listed in Table 2.These lines are classiÆed into three groups: 3d3(a 4F)4p 5D±3d4 5D, 3d3(a 2H)4p 3H, 1H±3d4 3H and 3d3(a 4F)4d 5H±3d3(a 4F)4p 5G transition.17 Their excitation energies range from 4.61 to 9.03 eV, and all the corresponding gA values were published by Corliss and Bozman,18 as shown in the second and fourth columns of Table 2.The Æfth and sixth columns give the intensities emitted by the ICP and the Ar±GDP, respectively. It is not possible to compare the data of the Ar±GDP with those of the ICP directly because of different amounts of vanadium in the plasmas as well as different discharge powers supplied to the plasmas. However, the relative emission intensities are very different between these two plasmas.Boltzmann plots Fig. 1 shows the Boltzmann plots of the V II lines emitted by the Ar±GDP at discharge voltages of 475 and 275 V. The intensities for levels at about 9 eV lie well above a straight line joining the other two groups of data, and are about 26104 larger than those predicted by the extrapolation of this straight line. This effect results from a great enhancement of the emission intensities of the V II 264.433 and 265.565 nm lines.We have calculated an excitation temperature based on the groups of V II lines of which excitation energies range from 4.61 to 6.88 eV, as shown in Fig. 2(a). The excitation temperatures are barely dependent on the discharge voltage supplied. Fig. 3 shows the Boltzmann plots in the ICP at rf forward powers of 1.2 and 0.8 kW. In a similar way to the result for the Ar±GDP, the Boltzmann plots depart positively from a linear relationship at the point of about 9 eV.However, the effect is smaller than with the Ar±GDP. The excitation temperature calculated from the linear parts of the plots is also shown in Fig. 2(b). The excitation temperatures rise with the rf forward powers. Mechanisms Fig. 4 shows a simpliÆed energy diagram for the vanadium ion, together with the metastable levels of argon. In order to consider energy-transfer collisions occurring in the plasma, it is convenient to use values of the total energy (ionization and Table 2 Observed emission lines of singly-ionized vanadium Emission line nm Assignment17 (ionic excitation energy) gA value186108 Emission intensity Upper (eV) Lower (eV) ICPa Ar±GDPb V II 252.792 3d3(a 2H)4p 3H6 (6.4784) 3d4 3H6 (1.5753) 138 57 60 V II 252.884 3d3(a 2H)4p 3H5 (6.4661) 3d4 3H5 (1.5648) 97 48 50 V II 264.433 3d3(a 4F)4d 5H4 (8.9950) 3d3(a 4F)4p 5G3 (4.3078) 92 8 2100 V II 265.565 3d3(a 4F)4d 5H6 (9.0305) 3d3(a 4F)4p 5G5 (4.3633) 108 14 3200 V II 265.898 3d3(a 2H)4p 3H5 (6.4661) 3d4 3G4 (1.8047) 21 6 V II 265.961 3d3(a 2H)4p 1H5 (6.8809) 3d4 1G4 (2.2206) 30 5 7 V II 267.201 3d3(a 4F)4p 5D3 (4.6519) 3d4 5D2 (0.0132) 5 210 200 V II 267.780 3d3(a 4F)4p 5D2 (4.6331) 3d4 5D1 (0.0045) 6.3 220 230 V II 267.857 3d3(a 4F)4p 5D4 (4.6532) 3d4 5D3 (0.0259) 4.9 140 160 V II 267.932 3d3(a 4F)4p 5D3 (4.6519) 3d4 5D3 (0.0259) 6.6 290 290 V II 268.797 3d3(a 4F)4p 5D4 (4.6532) 3d4 5D4 (0.0421) 19 1000c 1000c V II 268.872 3d3(a 4F)4p 5D3 (4.6519) 3d4 5D4 (0.0421) 2.9 130 130 V II 268.988 3d3(a 4F)4p 5D0 (4.6123) 3d4 5D1 (0.0045) 2.4 140 150 V II 269.026 3d3(a 4F)4p 5D2 (4.6331) 3d4 5D3 (0.0259) 3.8 220 230 V II 269.079 3d3(a 4F)4p 5D1 (4.6195) 3d4 5D2 (0.0132) 3.9 230 230 aSample: 0.8 mg mL21 V, rf: 1.0 kW, PMT: 570 V.bSample: 99.8% V, discharge voltage and current: 425 V and 64.5 mA, PMT: 600 V. cInternal standard for estimating the relative intensity. Fig. 1 Boltzmann plots of several V II lines emitted by the Ar±GDP at the discharge voltage and current of 475 V and 67.8 mA (circle) and 275 V and 44.0 mA (square).Plasma gas: Ar530 Pa; sample: vanadium disk (99.8%). Fig. 2 Variations in the excitation temperature of the Ar±GDP (a) and the ICP (b), estimated from Boltzmann plots by using the V II lines of which excitation energy ranges from 4.61 to 6.88 eV, as a function of the discharge voltage (a) and of the rf forward power (b). 1728 J. Anal. At. Spectrom., 1999, 14, 1727±1730excitation), i.e., a scale whose origin is the level of the vanadium ground state coinciding with that of the argon atom. Total excitation energy is deÆned as the sum of the Ærst ionization energy (6.78 eV)17 and excitation energies in singly-ionized vanadium.It should be noted that the total excitation energy of the 3d3(a 4F)4d 5H levels is almost the same as that of the argon ion ground state (3p5 2P3/2). From good matching in their total excitation energies, we can consider the following chargetransfer collision, for instance, for the V II 265.565 nm line: VÖ3d34s2 4F3=2; 0:00 eVÜzArzÖ3p5 2P3=2; 15:76 eVÜ ?Vzâ3d3Öa 4FÜ4d 5H6; 15:77 eVäzArÖ3p6 1S1=2; 0:00 eVÜ zDEÖ{0:001 eVÜ The small difference in the total energies could contribute to resonance energy transfer,10 thus leading to the increased density of the 3d3(a 4F)4d 5H6 excited level.This reaction could explain the intensity enhancement of the V II 265.565 nm line, and therefore the non-linear Boltzmann plots shown in Fig. 1 and 3. Table 3 gives the relative intensities of V II emission lines assigned to the 3d3(a 4F)4d 5H±3d34p 5G transition in the Ar± GDP as well as in the ICP. Apart from the V II 364.433 nm and the V II 265.565 nm lines, the corresponding gA values are lacking in the literature;18 thus, the data could not be included in the Boltzmann plots. Their emission intensities have similar characteristics: the intensities obtained with the Ar±GDP are much larger than those with the ICP, which indicates that the charge-transfer collisions occur in the Ar±GDP more dominantly.This effect is probably due to longer lifetime of argon ions because the Ar±GDP is operated at reduced pressures. However, Fig. 3 indicates that excitation processes occurring through charge-transfer collision also cannot be ignored in the ICP. Conclusions The Boltzmann plots using vanadium ionic lines suggest that the LTE conditions are not established in either the Ar±GDP or the ICP.We consider that the major reason is a selective excitation of particular energy levels through charge-transfer collisions with argon ions, thus leading to the non-Boltzmann distribution. The studies using the Boltzmann plot provide useful knowledge of the excitation mechanisms in such analytical excitation sources. References 1 H. R. Griem, Plasma Spectroscopy, McGraw-Hill, New York, USA, 1964. 2 J. M. Mermet, in Inductively Coupled Plasma Emission Spectroscopy, Part 2, ed.P. W. J. M. Boumans, John Wiley, New York, USA, 1987. 3 J. M. Mermet, Spectrochim. Acta, Part B, 1975, 30, 383. 4 H. Uchida, K. Tanabe, Y. Nojiri, H. Haraguchi and K. Fuwa, Spectrochim. Acta, Part B, 1981, 36, 711. 5 N. Furuta and G. Horlick, Spectrochim. Acta, Part B, 1982, 37, 53. 6 M. Marichy, M. Mermet and J. M. Mermet, J. Anal. At. Spectrom., 1987, 2, 561. 7 T. M. Bricker, F. G. Smith and R. S. Houk, Spectrochim. Acta, Part B, 1995, 50, 1325. 8 J. Vicek, Spectrochim. Acta, Part B, 1997, 52, 599. 9 M. W. Blades, in Inductively Coupled Plasma Emission Spectros- Fig. 3 Boltzmann plots of several V II lines emitted by the ICP at an rf powers of 1.2 kW (circle) and 0.8 kW (square). Sample: 0.8 mg mL21 V, 1.4 M HNO3 solution. Fig. 4 Schematic energy level diagram for vanadium ion, together with the metastable levels of argon. Table 3 Relative intensities of the V II lines assigned to the 3d3(a 4F)4d 5H±3d3(a 4F)4p 5G transition Emission line nm Assignment17 (ionic excitation energy) Relative emission intensity Upper (eV) Lower (eV) ICPa Ar±GDPb Ar±GDPc V II 264.082 3d3(a 4F)4d 5H3 (8.9823) 3d3(a 4F)4p 5G2 (4.2889) 7 1800 980 V II 264.433 3d3(a 4F)4d 5H4 (8.9950) 3d3(a 4F)4p 5G3 (4.3078) 8 2100 1100 V II 264.933 3d3(a 4F)4d 5H5 (9.0111) 3d3(a 4F)4p 5G4 (4.3327) 12 2700 1500 V II 265.565 3d3(a 4F)4d 5H6 (9.0305) 3d3(a 4F)4p 5G5 (4.3633) 14 3200 1800 V II 266.321 3d3(a 4F)4d 5H7 (9.0532) 3d3(a 4F)4p 5G6 (4.3993) 17 3800 2100 V II 266.676 3d3(a 4F)4d 5H5 (9.0111) 3d3(a 4F)4p 5G5 (4.3633) 1 220 120 V II 267.629 3d3(a 4F)4d 5H6 (9.0305) 3d3(a 4F)4p 5G6 (4.3993) 1 130 70 V II 268.797 3d3(a 4F)4p 5D4 (4.6532) 3d4 5D4 (0.0421) 1000d 1000d 580 aSample: 0.8 mg mL21 V, rf: 1.0 kW, PMT: 570 V.bSample: 99.8% V, discharge voltage and current: 425 V and 64.5 mA, PMT: 600 V. cSample: 99.8% V, discharge voltage and current: 275 V and 44.0 mA, PMT: 600 V. dInternal standard for estimating the relative intensity. J. Anal. At. Spectrom., 1999, 14, 1727±1730 1729copy, Part 2, ed. P. W. J. M. Boumans, John Wiley & Sons, New York, USA, 1987. 10 O. S. Duffendach and J. G. Black, Phys. Rev., 1929, 34, 35. 11 E. B. M. Steers and R. J. Fielding, J. Anal. At. Spectrom., 1987, 2, 239. 12 K. Wagatsuma and K. Hirokawa, J. Anal. At. Spectrom., 1989, 4, 525. 13 K. Wagatsuma and K. Hirokawa, Spectrochim. Acta, Part B, 1991, 46, 269. 14 E. B. M. Steers and F. Leis, Spectrochim. Acta, Part B, 1991, 46, 527. 15 W. Grimm, Spectrochim. Acta, Part B, 1968, 23, 443. 16 K. Wagatsuma and K. Hirokawa, Surf. Interface Anal., 1984, 6, 167. 17 C. E. Moore, Atomic Energy Levels, NBS Circular 467, Washington, DC, USA, 1949, vol. 1. 18 C. H. Corliss and W. R. Bozman, Experimental Transition Probabilities for Spectral Lines of Seventy Elements, NBS Monograph 53, Washington, DC, USA, 1962. Paper 9/01072F 1730 J. Anal. At. Spectrom., 1999, 14, 1727±1730

ISSN:0267-9477    

DOI:10.1039/a901072f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Direct solid sampling ETAAS determination of lead in muscle tissue contaminated by gun-shot residues{ Ernst Lu»cker Institute of Veterinary Food Science, Justus-Liebig University, Frankfurter Str. 92, D-35392 Giessen, Germany. E-mail: ernst.h.luecker@vetmed.uni-giessen.de Received 17th May 1999, Accepted 17th August 1999 An increase in analyte content during pre-analytical steps such as sampling or sample preparation is still a serious source of error in trace and ultra-trace elemental analysis.In this study, the determination of Pb in muscle tissue of game containing gun-shot residues was used as an extreme model for such a `secondary contamination'. For muscle samples free of gun-shot residues it was shown that direct solid sampling electrothermal atomization atomic absorption spectrometry (dSS-ETAAS) allows the determination of Pb without any sample preparation. The total analytical error in dSS-ETAAS mean value estimation could be considerably reduced by increasing the number of microsamples and by taking all microsamples from different sites of the sample.Thereby, repetition of analysis did not considerably increase the analytical time and cost. Results of dSS-ETAAS (y) showed good agreement with a conventional compound method as reference (x) (y~1.18x0.96, r~0.92; t-test: Pw0.05) down to 7 ng g21 fresh substance. In non-homogenized muscle samples heavily contaminated with gun-shot residues, dSS-ETAAS made it possible to discern between primary (original Pb content) and secondary contamination (gun-shot residues).Secondary contamination was indicated by an increase of several orders of magnitude in the analyte content of microsamples contaminated by gun-shot residues. Pertaining to the low analytical masses, the ability of dSS-ETAAS to discern between samples with and without gun-shot residues was estimated to be up to four orders of magnitude higher than in conventional compound procedures.Introduction At present, the determination of toxic trace elements is usually performed by means of electrothermal atomization atomic absorption spectrometry (ETAAS) after sample homogenization and matrix decomposition.1 With regard to its widespread use and high quality instrumental performance, this compound procedure has been stipulated as a reference.1,2 Moreover, homogenization and sample decomposition are prescribed by German meat hygiene law.3 However, many heavy metals are ubiquitously present in the environment in relevant analytical concentrations. Thus, all levels of analysis, including the sampling, are prone to contamination. Such `secondary' contamination is reported to be one of the remaining and most important errors in modern instrumental trace and ultratrace analysis.4,5 The term `secondary' is used in this paper for a contamination which occurs during the analytical process including the acquisition of the sample.`Primary' contamination is caused by environmental exposure, e.g., of the living animal, thus happening prior to sampling.The problem of secondary contamination becomes most obvious in the case of the determination of Pb in tissues of wild game.6 The original Pb content of tissues of avian and mammalian game (caused by primary contamination) is predominantly reported to be very low (v50 ng g21 fresh substance) and/or often found to be in the range near the limit of detection.1,7 Thus, even very small particles as derived from Pb-containing gun-shots may conspicuously increase the original Pb content.To give an example, the secondary contamination of a 10 g muscle sample with an original Pb content of 10 ng g21 by a 1 mg Pb particle will lead to a 10 000- fold increase in the analytical result. In the literature, the maximum Pb content of game is often reported to reach or even to exceed the mg g21 range.6±8 In bulk analysis, this result will not be representative owing to the heterogeneous distribution of gun-shot residues.Furthermore, the result will not reØect the actually low hazard for the consumer as there is no relevant absorption of Pb from gun-shot residues in the gut.6 Previous studies have shown that the determination of various heavy metals in a variety of biological tissues, especially the liver,9 renal cortex10 and muscle tissue,11 can be performed without preliminary sample homogenization and sample decomposition.In direct solid sampling ETAAS (dSSETAAS), low masses of the sample in the range of usually 0.02±10 mg (microsamples) are introduced without any sample preparation directly into the AA±graphite system. The main analytical prerequisites are thus (i) suitable and convenient sample carrier systems, (ii) large volume graphite systems, (iii) high capacity background correction systems and (iv) high grade microbalances. Solid sampling ETAA spectrometers have been commercially available since 198012 and several suppliers of conventional AA spectrometers provide modiÆed graphite systems for solid sampling, e.g., the cup-in-tube13 or the solid sampling autoprobe technique.14 In dSS-ETAAS, the low analytical sample masses (microgram to milligram range) and short analytical cycle duration (several minutes) make it possible to replace homogenization by the determination of the Pb content of several microsamples.This approach was shown, by means of multifactorial hierarchical analysis of variance, to be highly representative and at the same time to provide information about the analyte distribution and thus the uncertainty of analysis.15 The present study was designed to show whether dSSETAAS is a suitable method to discern between original analyte content (primary contamination) and secondary contamination.For this purpose, the distribution of Pb was analysed in muscle tissue of wild mallards heavily contaminated by Pb-containing gun-shot residues (secondary contamination).In addition, studies were performed to obtain {Dedicated to Professor Dr. E. F. Kaleta on the occasion of his 60th birthday. J. Anal. At. Spectrom., 1999, 14, 1731±1735 1731 This Journal is # The Royal Society of Chemistry 1999information about the representativity, accuracy and analytical limits of dSS-ETAAS. Experimental Direct solid sampling ETAAS (dSS-ETAAS) Analytical parameters and the procedure of dSS-ETAAS have already been described in detail.9,11,16,17 In short, the actual analytical sample (`microsample') is taken directly from the laboratory sample without any sample preparation.Microsampling is performed rapidly by means of micro-tweezers (Kretschmer, Giessen, Germany) in repetition, either by taking each microsample from different anatomical locations (comparison of methods) or the same sampling site (distribution analysis). The microsample is then transferred on to a graphite platform, already tared on the microbalance.Following weight determination, the platform and microsample are introduced into the graphite furnace. Then the analytical cycle (drying, ashing, atomizing) is started. The analytical parameters used were as follows: solid sampling ETAA spectrometer with direct Zeeman-effect background correction SM20 and SM1 (Gru»n, AMS, Ehringshausen, Germany); resonance line, 283.3 nm; graphite system, pyrolytic graphite boat-shaped L'vov platform and pyrolytic graphite-coated (or uncoated) graphite tube (Schunk, Heuchelheim, Germany); and furnace parameters as given in Table 1.Calibration was achieved by use of aqueous solutions of 0.5, 1.0, 2.0 and 4.0 ng per 0.1 ml. Reference material was used to optimize the furnace conditions, to check upon calibration and to control the analytical performance throughout analysis. The reference materials used in this study were: NIST SRM 1577a Bovine Liver (National Institute of Standards and Technology, Washington, DC, USA) and LIS-G01 laboratory internal bovine liver.17 Weight determination was performed by means of a 4503MPS or M2P microbalance (Sartorius, Go» ttingen, Germany).Distribution of Pb in contaminated muscle tissue Organs and tissues of mallards were derived from previous studies (distribution analysis)18 or obtained in retail outlets (n~36, comparison of methods). In addition, four mallards (Nos. 1±4) were shot in the vicinity of Giessen, Germany. These animals were X-rayed in order to localize the gun-shot residues.The heavily contaminated cervical muscle tissue of animal No. 1 was analysed in nine sampling sites on both lateral sides of the fourth to the twelfth vertebral bones. Each sampling site was composed of three microsamples (mass range 0.05±14 mg). Thus 54 microsamples were analysed individually (cervical muscle A in Table 2). In addition, three repetitions of analysis were performed taking 8±10 microsamples from sampling site No. 5 of the right body side (cervical muscle B, C, D in Table 2). The Pb content of the tongue muscles of the same animal contaminated by a Pb pellet in the proximal region was analysed in eight sampling sites (three microsamples per sampling site) starting in the distal region of the tongue and progressing with a 2 mm distance between the sampling sites towards the pellet. In addition, the distribution of Pb in three muscles and one liver of mallards was studied with respect to the distance of the sampling site from the gun-shot track.Distribution of Pb in non-contaminated muscle tissue Information about the representativity of dSS-ETAAS was obtained in a hierarchical structured distribution analysis of the following model: 3 animals63 muscles (M. pectoralis, M. semitendinosus, M. Æbularis longus)62 body sides (left, right)64 sampling sites (totally randomized)66 microsamples. Thus, the Pb content of 432 microsamples was determined.The observed variances were discerned by means of a multifactorial analysis of variance of the mixed model (BMDP8V19) into variance between sampling sites (heterogeneity) and within sampling sites (residual analytical variance).10,18 A logarithmic transformation of data was performed in order to minimize heteroscedasticity. Note that the resulting distribution factors (e.g., df~1.12°1) are expressed in this study as relative standard deviation (RSD~12%), which is a good approximation and more illustrative.In addition, the distribution of Pb in heart muscle and muscle tissue of the stomach of each animal was analysed. Accuracy, lower analytical limits In order to obtain information on the accuracy of dSS-ETAAS results, a comparison of methods was performed. Following the determination of Pb in samples of 36 non-contaminated Mm. pectorales, the same samples were analysed by means of a compound reference method including the homogenization of about 200 g material and matrix decomposition of an aliquot of 10 g in the open system using puriÆed concentrated HNO3.16,17 The data from this comparison of methods were also used to calculate the lower limit of correspondence of dSSETAAS results with regard to the results obtained by the reference method.20 In addition, the limits of detection and quantiÆcation were determined according to the 3s model.12 Results and discussion Table 2 shows total mean values and variances (RSD) obtained from dSS-ETAAS analyses of different tissues from four wild mallards. The extreme increase in Pb content of the tongue and cervical muscle tissue of animal No. 1 is obvious. The mean Pb contents of these samples range from 600 to 3700 ng g21 FS (fresh substance), whereas those of the other muscle samples range from 5 to 44 ng g21 FS. Additional information with regard to contamination of muscle tissue can be obtained by the analytical variance observed in dSS-ETAAS.The mean Pb content of the cervical muscle tissue A of animal No. 1 is 44 ng g21 FS. This is well within the range of the Pb content of the cervical muscle of the other three animals. However, the observed RSD of 275% indicates heterogeneity. Fig. 1 shows Table 1 Instrumental conditions for direct solid sampling ETAAS determination of Pb in fresh muscle tissue at 283.3 nm using solid sampling atomic absorption spectrometers with direct Zeeman-effect background correction12 Furnace temperaturea/�C Time/s Phase Step Pyrolytic graphite coatedb Uncoated Minimum Maximum I Dry 200 400 24 24 II Ash 350 700 15 150 III Atomize 2200 2600 7 20 IV Clean w3000 w3000 1 5 V Cool 20 20 15 15 aThe temperature is measured through conductivity and depends on the graphite material applied.bMatrix modiÆcation (0.5% v/v NH4H2PO4) is recommended when using highly resistant graphite. 1732 J. Anal. At. Spectrom., 1999, 14, 1731±1735this to be correlated with distinctly increased Pb contents of sampling sites Nos. 5 and 7, which are well within the vicinity of gun-shot residues of the right cervical side. Repetition of analysis from sampling site No. 5 (samples `cervical muscle B, C, D' in Table 2) conÆrms these results. Comparison of the Pb content of the left with that of the right cervical muscle tissue (sample `cervical muscle A') shows that the mean Pb content of the microsamples from the left side is 10 ng g21 FS (n~27), whereas that of the right side is 78 ng g21 FS (n~27).The respective maxima are 39 and 705 ng g21 FS. A sudden and distinct increase in the Pb content (and partly in the variances) with respect to the distance from the source of secondary contamination is demonstrated in Fig. 2 for a Pb pellet in the tongue and in Fig. 3 for gun-shot tracks in muscle and liver tissue of mallards. The Pb contents in the muscle tissue of a tongue contaminated by a Pb pellet (Fig. 2) are found to vary around a median value of 37 ng g21 FS.The variances showed no indication of considerable heterogeneity when compared with previous results derived from nonhomogenized tissues9,18 or results obtained in distribution analyses as performed in this study (see below). This applies to all microsamples which are taken at a distance from the gunshot residue of more than 2 mm. In close vicinity to the source of contamination (2±1 mm from pellet) the Pb content abruptly increases 276-fold. In this case, the variances are found to increase only slightly, from RSD~43% (n~18) to 66% (n~6).The distinct increase in Pb content but slight increase in variance might be due to the fact that the microsamples were taken from within the gun-shot track and from closely circumscribed sampling sites (n~3). In the case of gun-shot tracks, the sudden increase in Pb content becomes obvious only when the tissue from the channel track itself is analysed (Fig. 3). Note that X-ray examination of these samples did not reveal secondary contamination by gun-shot residues in most cases (data not shown).In muscle tissue of game an extreme Pb content such as 532 mg g21 FS21 or 3151 mg g21 FS22 can easily be attributed to contamination by gun-shot residues. However, slightly increased results are difÆcult to classify when compound procedures are applied. Not even extremely careful sampling and X-ray examination of samples will guarantee freedom from contamination.It has been suggested that outlier tests be used for the identiÆcation of suspicious samples.23 This approach is well suited for ecotoxicological studies where a great number of Fig. 1 Pb content in cervical muscle tissue of a mallard heavily contaminated with gun-shot residues (above) as determined by dSSETAAS. Location of sampling sites Nos. 4±8 at the left side of the vertebral muscles is indicated in the inverse scanned X-rays (below). Fig. 2 Pb content in microsamples of a mallard's tongue with regard to their distance from a gun-shot pellet.Median (slashed line) and 95% conÆdence range (dotted region) are given as calculated for Pb content of microsamples from a 4 to 14 mm distance from the Pb pellet. Fig. 3 Pb content in samples of three muscles and one liver of mallards with regard to the distance of sampling site from gun-shot track. Table 2 Results of Pb determination by means of dSS-ETAAS in different tissues of mallards (m~number of microsamples, x~arithmetic mean) Animal No.Matrix m x/ng g21 RSD (%) 1 Liver 6 91 10 Heart muscle 4 5 73 Tongue 24 1205 209 Cervical muscle Aa 54 44 275 Cervical muscle Bb 10 589 119 Cervical muscle Cb 8 2823 51 Cervical muscle Db 8 3705 39 Stomach muscle 6 27 32 Stomach content 6 177 55 2 Liver 6 270 18 Heart muscle 6 9 70 Ceical muscle 6 44 33 Stomach muscle 6 33 33 Stomach content 6 1091 95 3 Liver 6 56 29 Cervical muscle 6 24 30 4 Liver 6 175 14 Cervical muscle 6 62 21 aDistribution analysis including sampling sites Nos. 1±9. bRepetition of analysis in sampling site No. 5 (right body side). J. Anal. At. Spectrom., 1999, 14, 1731±1735 1733different samples are analysed. It is totally unsuited, however, in the scope of meat hygiene where single cases have to be decided upon with regard to legal limits.24 In this case the best approach to identify suspicious samples with regard to secondary contamination (e.g., gun-shot residues) is the manifold repetition of the whole analytical procedure.In compound procedures this would be too expensive and time consuming. In dSS-ETAAS the sample masses used are 3±4 orders of magnitude lower than those used in compound procedures. Hence the sensitivity of dSS-ETAAS with regard to secondary contamination is accordingly increased, as shown in the examples given above. Furthermore, the whole process of analysis can be repeated many times in dSS-ETAAS easily and without a distinct increase in analytical time and cost.Fig. 4 shows the results of the distribution analysis in muscles of mallards without secondary contamination (i.e., gun-shot residues). Lead appears to be distributed fairly homogeneously within and between the muscles of every animal with the exception of heart (animals B and C) and stomach muscle (animal A). Analysis of variance of the dSSETAAS determination of Pb in skeletal muscles shows that the heterogeneity (RSD~29%) is lower than the residual analytical variance (35%), albeit to a minor extent.As shown in Table 3, the error of mean value estimation can most effectively be reduced when each microsample is taken from a different site of the sample. The total analytical variance (RSD) of 48% (one microsample) can be reduced to only 33% when six microsamples, all taken from one sampling site, are analysed. When the six microsamples are each taken from a different sampling site, however, the total variance is reduced to 17%.In the case of 12 microsamples from different sampling sites, the total variance is further reduced to 12%. This appears to be acceptable for a mean value estimation with regard to the low analytical range as in non-homogenized muscle tissue. Comparison of dSS-ETAAS results obtained for Pb in nonhomogenized fresh muscle tissue samples (n~36) in the range 3±62 ng g21 FS with the results of ETAAS obtained after sample homogenization and digestion shows good agreement between the methods.The regression curve is characterized by y~1.18x0.96 (y~compound procedure, x~solid sampling) with r~0.92. Statistically relevant differences cannot be demonstrated (t-test, Pw0.05). Limits of detection and quantiÆcation of Pb in dSS-ETAAS are found to be 4 and 12 ng g21 FS, respectively. The lower limit of correspondence20 with respect to the reference compound method is 7 ng g21 FS. Overall, these results show that dSS-ETAAS determination in muscle tissue without secondary contamination such as gunshot residues, i.e., with its usually very low content of Pb, is comparable to compound methods.Hence dSS-ETAAS can be applied not only as a screening method but also as a method of reference with regard to the original Pb content in critical samples, such as muscle tissue of game contaminated by gunshot residues. Moreover, these Ændings might illustrate, albeit by way of an extreme example, the general problem posed by secondary contamination and the application of in praxi nonrepeatable analytical compound procedures.Acknowledgements The valuable technical assistance of E. Hornung and the assistance with sampling of R. and C. Gerbig and R. M. Hadlok are kindly acknowledged. This study was Ænancially supported in part by the German Research Foundation (DFG Lu394/1). References 1 H. Hecht, Fleischwirtschaft, 1993, 73, 240. 2 Commission of the European Communities, Commission Decision 90/515/EEC, ABl, EC 1990, Off.J. Eur. Commun., 1990, L286, 33. 3 Ministry of Health, Allgemeine Verwaltungsvorschrift zur Durchfu »hrung der amtlichen Untersuchungen nach dem Fleischhygienegesetz, Bonn, Germany, Bundesanzeiger, 1986, No. 238a. 4 G.To» lg, Labo, 1987, 9, 9. Fig. 4 Distribution of Pb in muscles of mallards. Arithmetic means and standard deviation of dSS-ETAAS results with respect to different muscles: 1, M. pectoralis dexter; 2, M. pectoralis sinister; 3, M. semitendinosus dexter; 4, M.semitendinosus sinister; 5, M. Æbularis longus dexter; 6, M. Æbularis longus sinister; 7, heart muscle (M. cordis); 8, muscle of stomach (M. gastricus). Number of microsamples per muscle: m~24. Fig. 5 Comparison between direct solid sampling and sample digestion ETAAS determination of Pb in musclesn of mallards (n~36) with y~dSS-ETAAS, non homogenized samples, six microsamples per sample and x~SD-ETAAS, homogonized, two digestions per sample, two determinations per digestions.Table 3 Reduction of error in mean value estimation by sampling strategy in direct solid sampling ETAAS determination of Pb in muscle tissue of mallards Number of sampling sites Number of microsamples per sampling site Total variance (RSD, %) 1 1 48 1 6 33 2 3 24 6 1 17 12 1 12 1734 J. Anal. At. Spectrom., 1999, 14, 1731±17355 P. M. Gy, Microchim Acta, 1991, II, 457. 6 F. Moreth and H. Hecht, Fleischwirtschaft, 1981, 61, 1326. 7 B. Glu» ck and J. Hahn, Fleischwirtschaft, 1991, 71, 160. 8 R. Klein and M. Nentwich, Richtlinie zur Probenahme und Probenvorbereitungen–Reh (Capreolus capreolus), Federal OfÆce of Environment and Institute of Biogeography, University of Saarland, Saarbru» cken, Germany, 1995. 9 B. Klu» ssendorf, A. Rosopulo and W. Kreuzer, Fresenius' Z. Anal. Chem, 1985, 322, 721. 10 E. Lu» cker, A. Rosopulo and W. Kreuzer, Fresenius' Z. Anal Chem., 1987, 328, 370. 11 E. Lu» cker, J. Anal. At. Spectrom., 1999, 14, 583. 12 U. Kurfu» rst, Solid Sample Analysis, Springer, Berlin, 1998. 13 U. Vo» llkopf, Z. Grobenski, R. Tamm and B. Welz, Analyst, 1985, 110, 573. 14 E. Lu» cker, W. Kreuzer and C. Busche, Fresenius' Z. Anal. Chem., 1989, 335, 176. 15 E. Lu» cker, Applied Spectrosc., 1997, 51, 1031. 16 A. Rosopulo and W. Kreuzer, in Fortschritte in der Atomspektrometrischen Spurenanalytik, ed. B. Welz, Verlag Chemie, Weinheim, 1986, vol. 2, pp. 455±463. 17 E. Lu» cker, A. Rosopulo and W. Kreuzer, Fresenius' J. Anal. Chem., 1991, 340, 234. 18 E. Lu» cker, C. Gerbig and W. Kreuzer, Fresenius' J. Anal. Chem., 1993, 346, 1062. 19 W. J. Dixon, BMDP Statistical Software Manual, University of California Press, Berkeley, CA, 1992. 20 E. Lu» cker, K. Failing and T. Schmidt, Fresenius' J. Anal. Chem., in the press. 21 H. Hecht, Fleischwirtschaft, 1987, 67, 1511. 22 Residues in Venison, Hessian Ministry of Agriculture, Wiesbaden, 1986. 23 Hecht, personal communication, 1998. 24 Ministry of Health, Fleischhygieneverordnung, Bonn, Germany, Bundesgesetzblatt I, 1997, 1138. Paper 9/903922H J. Anal. At. Spectrom., 1999, 14, 1731±1735 1735

ISSN:0267-9477    

DOI:10.1039/a903922h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of Ag, Pb and Sn in aqua regia extracts from sediments by electrothermal atomic absorption spectrometry using Ru as a permanent modiÆer Jose� Bento Borba da Silva,a Ma�rcia Andreia Mesquita da Silva,b Adilson Jose� Curtius*b and Bernhard Welzb aDepto. de Quý�mica da Universidade Estadual de Maringa�, 87020-900 Maringa�, P.R., Brazil bDepto. de Quý�mica da Universidade Federal de Santa Catarina, 88040-900 Floriano�polis, S.C., Brazil. E-mail: curtius@qmc.ufsc.br Received 5th July 1999, Accepted 1st September 1999 Ruthenium, deposited on a L'vov platform, is proposed as a permanent modiÆer for the determination of Ag, Pb and Sn in aqua regia extracts from sediments by electrothermal atomic absorption spectrometry.The coating process is simple: a solution containing Ru is pipetted repeatedly on to the platform inserted in a graphite tube and is submitted to a temperature program. In a 50% v/v aqua regia solution, high pyrolysis temperatures could be used: 1200 �C for Ag and Pb, and 1500 �C for Sn.At these temperatures, similar characteristic masses to those found for a nitric acid medium, using a Pd±Mg modiÆer, were obtained, showing that the high concentration of chloride does not interfere with the determination. In the aqua regia medium, the permanent modiÆer is much superior in comparison with Pd or PdzMg, modiÆers applied as a solution, which could not stabilize the analytes satisfactorily. Very long tube lifetimes, around 1700 cycles, were obtained for Pb and Sn in this medium.Three sediment reference materials were partially dissolved using a mixture of aqua regia and hydrogen peroxide in a microwave oven. The results for Ag and Pb were in agreement with the recommended values, demonstrating the efÆciency of the extraction. However, for Sn, the precison was less satisfactory, indicating that the extraction may be less efÆcient and reproducible for this analyte. Other advantages of the permanent Ru modiÆer are the low blanks due to in situ cleaning of the modiÆer and the shorter analysis time in comparison with the modiÆers in solution.Introduction The total analyte concentration in a solid environmental sample, such as a sediment or a soil, can only be determined after a fusion with sodium carbonate1 or lithium metaborate, 2±4 or after an acid digestion in the presence of hydroØuoric acid and frequently also perchloric acid.5 Such digestion procedures are relatively time consuming and not without problems for the following analysis. Hence, instead of a total digestion, it is usual in environmental analysis to perform an extraction with boiling aqua regia.This is justiÆed on the assumption that heavy metals that are so strongly bound to the mineral matrix that they do not go into solution with such a digestion will also not be taken up by plants and cannot be dissolved by water and bacteria. In the meantime, reference materials have been produced for which, in addition to the certiÆed total concentration, information is also provided on the fraction that can be extracted in aqua regia.Aqua regia must be considered a difÆcult matrix for electrothermal atomic absorption spectrometry (ETAAS) because, due to its high chloride content, it may cause analyte loss during the pyrolysis stage, gas phase interferences in the atomization stage, as well as pronounced corrosion of graphite tubes, so that all aspects of the stabilized temperature platform furnace (STPF) concept6 have to be considered, particularly the use of appropriate chemical modiÆers.Chemical modiÆers have been used routinely in the determination of a great number of analytes by ETAAS. Generally, the use of modiÆers allows high pyrolysis temperatures, reducing or eliminating volatilization and vapor phase interferences and minimizing background signals.7 Solutions containing Pd with or without Mg have been employed for a wide range of elements, especially for the more volatile elements,8 and are recommended in the software of the instruments and in the manuals from the instrument manufacturers.9 The most common way of applying the modiÆer is by pipetting its solution together with or after the sample and calibration solutions.More recently, the modiÆer has been applied as a metal deposit on the graphite tube surface or on the L'vov platform, acting as a ``permanent'' modiÆer, making possible between 50 and more than 1000 atomization cycles10,11 without repeating the treatment of the tube or platform.Platinum group metals (PGMs), such as Pd, Pt, Ir, Ru and Rh, as well as carbide-forming elements, such as Zr, Nb, Ta and W, have been used as permanent modiÆers for the determination of volatile elements by ETAAS.7,11±15 In this sense, Ir and Ru, due to their high melting-points, should allow higher pyrolysis temperatures than Pd.14 On the other hand, Tsalev et al.16 did not Ænd a pronounced correlation between the maximum pyrolysis temperature and the melting-point of the PGMs using Pd, Rh and Ru chlorides in the determination of 18 analytes.The fact that the carbide-forming elements and the PGMs have a similar behavior as chemical modiÆers has also been attributed to their catalytic action.17 The high boiling-points of Ir and Ru, as compared with that of Pd, allows their use as permanent modiÆers, while Pd is lost at relatively low temperatures (1800 �C), and cannot be employed for this purpose.18 In spite of the wide use of PGMs as chemical modiÆers, Pd alone or mixed with Mg as solutions, and Ir or Rh as permanent modiÆers, have been preferred while Ru has rarely been used.Sturgeon et al.19 employed Ru, Pt, Pd and Rh to sequester and concentrate the hydrides of Bi, Se, As, Sn and Sb in a graphite tube. Palladium showed the best performance, leading to higher signal appearance temperatures, which was attributed J.Anal. At. Spectrom., 1999, 14, 1737±1742 1737 This Journal is # The Royal Society of Chemistry 1999to its afÆnity for H2. Only for Se was the same signal appearance temperature obtained for Pd and Ru. Mixtures of PdzPtzRhzRu were also used as thermal stabilizers in the hydride collection.20 Using PGM chlorides as modiÆers for 18 analytes, Tsalev et al.16 observed that the maximum pyrolysis temperature decreased in the order RuwRhwPd. In another study, using different metals as modiÆers, their efÆciency diminished in the order PdwRuwCewAgwPt.21 Cai and McDonald22 investigated Ru as a potential chemical modiÆer for Pb and proposed the formation of an intermetallic compound PbRu2 as the stabilizing mechanism.In this work, the determination of Ag, Pb and Sn by ETAAS after partial dissolution with aqua regia was investigated. High pyrolysis temperatures are desirable to eliminate chlorides from the matrix effectively, without losing the analytes.This situation seems to be very challenging to test Ru, deposited on a L'vov platform, as a permanent modiÆer. Experimental Apparatus An Aanalyst 100 atomic absorption spectrometer (Perkin- Elmer, Norwalk, CT, USA), equipped with an HGA-800 graphite tube atomizer, an AS-72 autosampler and a deuterium-arc background corrector, was operated under the conditions recommended by the manufacturer. Integrated absorbance (peak area) was used exclusively for signal evaluation.The hollow cathode lamp for Ag (Hitachi, Mitorika, Ibaraki, Japan) was operated at 15 mA and the hollow cathode lamps for Pb and Sn (Perkin-Elmer) at 10 and 25 mA, respectively. The volume pipetted into the graphite tube was 20 mL for the test sample and calibration solutions. The volume of the chemical modiÆer when added in solution was 10 mL. Argon, 99.996% (White Martins, Saƒo Paulo, S.P., Brazil), was used as sheath gas. Pyrolytic graphite coated graphite tubes (Perkin-Elmer, Part No.B010-9322) with a total pyrolytic graphite platform (Perkin-Elmer, Part No. B010- 9324) were used. The platform was pre-treated with Ru, in a similar way to that described previously forapplying 40 mL of a 500 mg mL21 Ru solution on to the platform and submitting the tube to the temperature program given in Table 1. This procedure was repeated 25 times in order to obtain a deposit of 500 mg of Ru, as a permanent modiÆer.This temperature program also served to remove volatile contaminants, and hence to ensure low blank values in the Ænal analysis.11,23 The absolute blank values for Ru as well as for PdzMg, when applied in solution, and also for Ru when applied as a permanent modiÆer, are shown in Table 2, demonstrating the puriÆcation effect of the permanent modiÆers. The graphite furnace temperature program for each analyte was optimized and is shown in Table 3. A microwave oven, MLS-1200 MEGA (Milestone, Sorisole, B.G., Italy), was used to dissolve the samples. Reagents and solutions All chemicals used were of analytical-reagent grade, unless otherwise speciÆed.Water was de-ionized in a Milli-Q system (Millipore, Bedford, MA, USA). Hydrochloric acid (Merck, Darmstadt, Germany, No. 334) and nitric acid (Carlo Erba, Milan, Italy, No. 408015) were further puriÆed by sub-boiling distillation in a quartz still (Ku» rner Analysentechnik, Rosenheim, Germany). Hydrogen peroxide, 30% v/v (Merck, No. 507016), was used as supplied. The following 1000 mg mL21 stock solutions were used: ruthenium (Fluka, Buchs, Switzerland, No. 84033) in 1 mol L21 hydrochloric acid; silver (Fluka, No. 85137) in 0.5 mol L21 nitric acid; lead (Spex, Edison, NJ, USA, No. PLK10-Pb) in 0.3 mol L21 hydrochloric acid; and tin (Spex, No. PLK10-Sn) in 1 mol L21 hydrochloric acid. For the modiÆer in solution the following 10.0°0.2 g L21 stock solutions were used: magnesium nitrate solution, modiÆer for graphite furnace AAS (Merck, No.B593213 431); and palladium nitrate solution, modiÆer for graphite furnace AAS (Merck, No. B936989 710). The calibration solutions for Ag, 1±5 mg L21, for Pb, 10± 50 mg L21, and for Sn, 20±80 mg L21, were obtained by dilution with 0.2% v/v nitric acid. The modiÆers in solution were also diluted with 0.2% v/v nitric acid. Samples Three certiÆed reference materials were analysed, one from the National Research Council of Canada (Ottawa, Canada): Marine Sediment (MESS-2), and two from Canadian CertiÆed Reference Material Project (Ottawa, Canada): Stream Sediment (STSD-2) and Lake Sediment (LKSD-3).To each aliquot of about 250 mg of the material, weighed directly in the PTFE Øask of the microwave system, 5 mL of aqua regia and 1 mL of hydrogen peroxide were added. A four-step program was used in the microwave oven: 5 min at 250 W, 5 min at 400 W, 5 min at 650 W and 5 min at 250 W. The volume of the Ænal solution was made up to 10 mL with aqua regia used to wash the Øask.A solid residue remained on the bottom of the calibrated Øask. These solutions were diluted 1z19 for Pb, 1z19 for Ag in LKDS-3 and in STDS-2 and 1z9 for Sn in STDS-2 with 50% v/v aqua regia. The other solutions were not further diluted. An aliquot of the supernatant was transferred into the autosampler cup. Before use, all glassware was kept in an Extran solution (Merck) for 12 h and in an ultrasonic water-bath for 30 min and was then rinsed with Milli-Q water.Then, it was kept in a 20% aqua regia solution for at least 48 h and washed four times with Milli-Q water. The PTFE Øasks of the microwave system and the autosampler cups were kept in a warm 50% v/v nitric acid solution for 4 h and were then washed several times with Milli-Q water. Results and discussion Pyrolysis temperature Fig. 1(a)±(c) shows the pyrolysis curves for Pb, Ag and Sn, respectively, in 50% v/v aqua regia solution using A–Ru as a permanent modiÆer; B–PdzMg added in solution as a modiÆer; C–no modiÆer; and D–Pd alone in solution as a modiÆer. For all three elements the Ru-treated platforms provide the best stabilization of the analyte up to fairly high Table 1 Temperature program for the Ru coating of the L'vov platform Step Temperature/�C Ramp/s Hold/s Ar Øow rate/mL min21 1 90 5 15 250 2 140 5 15 250 3 1000 10 10 250 4 2000 0 5 0 5 20 1 10 250 Table 2 Blank values obtained for different modiÆers, applied directly in 0.2% v/v nitric acid (20 mL of modiÆers), and as permanent modiÆers.(n~3) ModiÆer Blank signal/s Ag Pb Sn 15 mg Pdz10 mg Mg 0.007°0.002 0.029°0.005 0.021°0.005 10 mg Ru 0.155°0.008 0.056°0.013 0.006°0.002 500 mg Ru permanent 0.001°0.001 0.001°0.001 0.001°0.002 1738 J. Anal. At. Spectrom., 1999, 14, 1737±1742pyrolysis temperatures. The only element that exhibits a fairly strong dependence of the integrated absorbance signal on the pyrolysis temperature is Sn.This is most likely due to the low sublimation point of SnCl2 of 650 �C, which coincides satisfactorily with the beginning of the drop of the pyrolysis curve at 600 �C. This loss mechanism has previously been proposed by Rayson and Holcombe,24 who also pointed to the high thermal stability of this gaseous molecule. The performance of PdzMg as a modiÆer, added in solution, is clearly inferior to that of the Ru-treated platform in the presence of this high chloride matrix. This may be explained through the stabilization mechanism of this modiÆer, which was investigated in detail by Ortner et al.25 The Pd penetrates to a depth of 10 mm into the pyrolytic graphite surface, and is activated by covalent bonding to the graphite lattice (intercalation compound).This ``activated Pd'' then forms a covalent bond with the analyte element during the drying stage.25 Although this mechanism was established only for As, it is most likely valid also for other elements.A penetration of Pd into the graphite structure has been described earlier by Majidi and Robertson,26 and it does undoubtedly play the key role in the stabilization of analytes by Pd. Other proposed mechanisms, such as a retention of the analyte in particles on the surface at temperatures w800 �C, are not possible from considerations on the diffusion of analytes in the corresponding matrices.25 Likewise, the formation of intermetallic compounds between the analyte and Pd does not occur due to the extreme concentration ratio of Pd : analyte w1000 : 1.The interference of chloride could then be explained as a competition with the Pd for the formation of intercalation compounds with graphite, i.e. less Pd can be intercalated in the presence of high chloride concentrations. This intercalation of chloride under similar conditions has been clearly demonstrated for total pyrolytic graphite tubes27 and is likely to occur here as well.Palladium alone, added in solution, although proposed by other groups as a modiÆer,28,29 is signiÆcantly less efÆcient than PdzMg in the high chloride matrix for all three elements investigated here. This is according to expectation as the magnesium nitrate can contribute to the stabilization of the analyte in at least three different ways. Firstly, the magnesium chloride which is undoubtedly formed under these high chloride conditions, hydrolyzes with the formation of MgO(s) and HCl(g).30 Secondly, the MgO can at least in part prevent analyte molecules, such as chlorides, from being lost in the pyrolysis stage by imbedding them in an oxidizing atmosphere.31 Thirdly, several workers19,32±34 reported the formation of a Pd±M±O bond (where M is the analyte) in the presence of the PdzMg modiÆer, a bond that is most likely not formed in a high chloride matrix in the absence of magnesium nitrate. For Tl it could be shown that ``reduced Pd'', i.e.Pd that was pipetted onto the platform Ærst and pyrolyzed at 1000 �C before the sample solution was introduced, could prevent a chloride interference effectively, but not Pd added in solution.31 This further supports the above-discussed competition between chloride and Pd for intercalation as the most likely mechanism of interference and the low stabilizing power d modiÆers under these conditions. The use of reduced Pd as a modiÆer was not investigated in this work as it was considered too time consuming, particularly in comparison with the use of a permanent modiÆer.We have no explanation for the somewhat higher sensitivity without modiÆer in comparison with Pd alone as the modiÆer for Ag, and in part also for Pb. In essence, this demonstrates that Pd, added in solution, has very little stabilizing power for these elements under the conditions of this experiment. In other words, Pd cannot be intercalated into the graphite lattice under these conditions or the ``activated Pd'' cannot form a stable covalent bond with the analyte, whereas this is apparently possible, at least in part, in the presence of magnesium nitrate.It is unlikely that the condensation of matrix vapor,35,36 and the trapping of the analyte on such clusters is responsible for the observed effect, as in that situation, the interference should be more pronounced in the presence of Mg in addition to Pd, compared with Pd alone, which is not the case.For Sn, in the absence of a modiÆer, only a very small integrated absorbance signal of about 0.05 s could be measured at low pyrolysis temperatures, which disappeared completely at Table 3 Temperature program for the determination of Ag, Pb and Sn using the Ru-treated platform Step Temperature/�C Ramp/s Hold/s Ar Øow rate/mL min21 1 90 5 10 250 2 140 5 20 250 3 1200 (Ag,Pb); 1500 (Sn) 10 30 250 4a 1800 (Ag, Pb); 2300 (Sn) 0 5 0 5 2650 1 5 250 6 20 1 10 250 aReading in this step.Fig. 1 Pyrolysis temperature curves for (a) 1 ng Pb; (b) 0.1 ng Ag; and (c) 1.6 ng Sn, all in 50% v/v aqua regia. A: Ru-coated platform; B: 15 mg Pdz10 mg Mg as modiÆer in solution; C: without modiÆer; and D: J. Anal. At. Spectrom., 1999, 14, 1737±1742 1739w800 �C. This observation is in agreement with the work of Brown and Styris,37 who investigated the atomization of SnCl2 by mass spectrometry and observed only SnO(g) and SnCl2(g), but no free Sn(g).These workers concluded that atomic Sn, which can be detected by AAS, is only formed on the graphite surface. Tube lifetime Fig. 2(a) and (b) shows the average integrated absorbance and relative standard deviation (RSD; n~20) for Pb and Sn, respectively, in 50% v/v aqua regia solution over the lifetime of a tube with a platform treated with Ru as described under Experimental. No long-term measurements were made for Ag, as a similar behaviour can be anticipated, because it is certainly not the analyte, but the aggressive matrix and the atomization and clean-out temperatures that determine the tube lifetime.38 Most striking is the very long lifetime of the tubes in the presence of this high acid concentration without the need for any recoating. The Pb experiment was terminated after 1700 atomization cycles, as the integrated absorbance started to drop signiÆcantly.A visual inspection showed that the platform was still in good condition, but some corrosion appeared at the inner and outer tube walls.The tube from the Sn experiment broke after about 1750 atomization cycles. For comparison, Rohr et al.,39 in an extensive lifetime test for graphite tubes in the presence of various matrices, found the most severe corrosion and tube breakage after only 240 atomization cycles for a 6 mol L21 HCl matrix. The signal stability for Sn was excellent over the entire lifetime of the tube without any signiÆcant drift in sensitivity, and for the majority of the individual measurement intervals of 20 atomization cycles each, the RSD was around 2.5%; only for a few of them–more pronounced towards the end of the tube lifetime–was the RSD between 5 and 10%.The integrated absorbance values for Pb appear to be slightly less consistent over the lifetime of the tube with an overall drop in sensitivity of about 25%, and some irregularities around 400 and 1250 atomization cycles. It must be kept in mind, however, that these experiments were carried out over a period of about 2 weeks each, most of the time unattended, and with a repeated change of the measurement solution, so that these inconsistencies should not be over-interpreted.Similar to the Sn experiment, the RSD was mostly around 2.5%, and only in a few series were values between 5 and 10% obtained. Fig. 3 shows a series of atomization pulses for 1 ng Pb in 50% v/v aqua regia over the lifetime of the tube, using a Ru-treated platform.Over more than 1000 atomization cycles the signal shape is almost symmetrical, which is unusual for an endheated graphite tube atomizer, and is probably due to the high stabilizing power of Ru. Around 900 atomization cycles a small early peak appeared, which became more and more prominent with increasing tube age. This is an indication that some of the Pb is no longer stabilized to the same degree, most likely because the Ru layer breaks, and some Pb is atomized directly from the exposed graphite surface.It should be noted, however, that there was essentially no change in the integrated absorbance recorded between atomization cycle 900 and 1600, which means that no Pb is lost in the pyrolysis stage. Towards the end of the tube lifetime, the second peak started to shift to even later appearance, and a shoulder appeared where the original peak maximum used to be. This is a further indication of a breakdown of the Ru layer. Most likely parts of the layer are peeling off, and hence lose contact with the platform, resulting in a further delay of the Pb atomization. Rademeyer et al.,7 for an Ir-coated graphite tube, also observed the appearance of a second early peak for Pb after 220 Ærings, Fig. 2 Average integrated absorbance and RSDs of 20 consecutive measurements each for (a) 1 ng Pb and (b) 1.6 ng Sn, both in 50% v/v aqua regia, using a Ru-treated platform. Fig. 3 Atomization pulses for 1 ng Pb in 50% v/v aqua regia, using a Ru-treated platform, after 1, 900, 1200, 1400 and 1600 atomization cycles. 1740 J. Anal. At. Spectrom., 1999, 14, 1737±1742which they attributed to an atomization from a different surface. On inspection by scanning electron microscopy they found that there was no visible residue of Ir in the center of the tube where the sample was deposited, which had apparently migrated to the cooler ends of the tube. It should be stressed again that the lifetime of the tube and the long-term performance of the Ru permanent modiÆer found in this work are far better than previous results obtained in our laboratory before11 or published in the literature, particularly considering the matrix.The only comparable published lifetime of about 1500 atomization cycles for a W±Rh treated platform40 was obtained for Cd in 0.2% v/v HNO3, and the platform had to be recoated repeatedly after every 300±350 atomization cycles. In later work the same group reported a lifetime of only about 600 atomization cycles for Cd in Æsh slurry, also with one recoating after 300 cycles.41 Analytical application The Ægures of merit for the determination of Ag, Pb and Sn in 50% v/v aqua regia are shown in Table 4.The obtained characteristic masses (the analyte mass that produces an integrated absorbance of 0.0044 s) were similar to those reported in the literature8 and by the instrument manufacturer9 for the analytes in 0.02% v/v nitric acid, using PdzMg (15 mgz10 mg) in solution as modiÆer.As is also shown in Table 4, using a Ru-treated platform, the obtained characteristic masses in aqua regia medium (50% v/v) and in 0.2% v/v nitric acid were similar. Hence, the Ru-treated platform allows sufÆciently high pyrolysis temperatures to eliminate efÆciently the effect of the chloride resulting from the high concentration of aqua regia. The limits of detection (LOD), deÆned as the mass (absolute) or the concentration (relative) that gives an integrated absorbance equal to three times the standard deviation of ten measurements of a solution close to the blank, based on 20 mL, are also shown in Table 4.The results for the analysis of the certiÆed reference materials arewn in Table 5. Since the characteristic masses in the aqua regia medium, as already discussed, were similar to those in 0.2% v/v nitric acid, the calibration solutions were prepared in the latter medium. The concentrations obtained for Ag and Pb using aqua regia extraction agreed well with the certiÆed or recommended values for the total content, which is not so much a proof of the accuracy of the proposed method, but an indication of the efÆciency of the aqua regia extraction, at least for the samples investigated in this study.For these analytes, the RSDs were below 10%. For Sn, the agreement was also good; however, for this analyte, the RSDs were fairly high. It was observed that the absorption pulse for Sn in the sample extract was different from the absorption pulses for this analyte in the calibration solution in 0.2% v/v nitric acid and in 50% v/v aqua regia.While the former was noisy, and not symmetrical, the latter was smoother and more symmetrical. Also, the background signals were much higher in the extracts, probably surpassing the correction capability of the continuum source background corrector. It was observed that the differences in the absorption pulses and in the background signals were not due to the presence of the aqua regia in the extract, but to some concomitants of the samples.Conclusions The use of a Ru coating on a L'vov platform, as a permanent modiÆer for Ag, Pb and Sn, is advantageous in comparison with the use of Pd alone or mixed with Mg in solution. The coating procedure is simple and efÆcient and the modiÆer is cleaned in situ by applying the conditioning temperature program, resulting in much lower blank signals in comparison with the use of the cited modiÆers in solution.Since the modiÆer does not have to be pipetted into the tube, the analysis time is shorter. High pyrolysis temperatures could be used, allowing the determination of the analytes in an aqua regia medium using calibration solutions in dilute nitric acid without matrix-matching. Particularly outstanding was the lifetime of the Ru-treated platform in the aggressive medium of aqua regia, making this permanent modiÆer a potential candidate for the stabilization of other volatile elements, and particularly for a routine analysis of aqua regia extracts using ETAAS.The aqua regia was very efÆcient in extracting Ag and Pb from the sediment materials, but less efÆcient for Sn. Acknowledgements The authors are grateful to Conselho Nacional de Pesquisas e Desenvolvimento Tecnolo�gico (CNPq) and to CoordenacÀaƒo de AperfeicÀoamento de Pessoal de Ný�vel Superior (CAPES). M. A. Mesquita da Silva has a scholarship from CAPES and J.B. Borba da Silva and B. Welz from CNPq. We also thank Dr. C. Gre�goire of the Canadian Geological Survey for providing the certiÆed reference materials. We dedicate this paper to the memory of our colleague, the late Dr. Eduardo Stadler. Table 4 Figures of merit for the determination of the analytes in 50% v/v aqua regia, using the Ru-treated platform Characteristic mass/pg Limit of detection (k~3, n~10) Analyte Founda Foundb Literaturec Absolute/pg Relative/mg L21 Ag 1.8°0.2 1.9°0.1 1.5 10 0.5 Pb 9.5°1.2 10.0°0.5 10.0 10 0.5 Sn 10.0°0.1 10.0°0.5 10.0 35 1.7 aIn 50% v/v aqua regia medium, using the Ru-treated platform.bIn 0.2% nitric acid, using the Ru-treated platform. cIn 0.2% nitric acid using 15 mg Pdz10 mg Mg in solution, as modiÆer.9 Table 5 Concentrations of Ag, Pb and Sn in sediment reference materials (n~3) Sample Concentration in the sample/mg L21 Ag Pb Sn Found CertiÆed Found CertiÆed Found CertiÆed LKDS-3 3.06°0.01 2.8a 32°1.3 29a 2.3°1 3a MESS-2 0.18°0.002 0.18°0.02 22.7°0.1 21.9°1.2 1.9°0.5 2.27°0.42 STDS-2 0.58°0.01 0.5a 69°7.0 66a 4.8°1.3 5a aRecommended value.J. Anal. At. Spectrom., 1999, 14, 1737±1742 1741References 1 P. J. Potts, Handbook of Silicate Rock Analysis, Blackie, Glasgow, 1987. 2 A. A. 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ISSN:0267-9477    

DOI:10.1039/a905415d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Investigation of low molecular weight Al complexes in human serum by fast protein liquid chromatography (FPLC)-ETAAS and electrospray (ES)-MS-MS techniques Tjasœa Bantan,a Radmila Milacœ icœ,*a Bojan Mitrovic�b and Boris Pihlarc aDepartment of Environmental Sciences, Jozœef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. E-mail: radmila.milacic@ijs.si bLek Pharmaceutical and Chemical Company d.d., Celovsœka 135, 1000 Ljubljana, Slovenia cFaculty of Chemistry and Chemical Technology, Asœkercœeva 5, 1000 Ljubljana, Slovenia Received 25th May 1999, Accepted 15th September 1999 Speciation of low molecular weight (LMW) Al complexes was performed in human serum from eight healthy volunteers in order to investigate the individual variability in the percentage and composition of LMW-Al species.Spiked samples (100±120 ng cm23 Al3z) were microultraÆltered through a membrane Ælter (cut-off 30 000 Da) to separate Al bound to transferrin from LMW-Al complexes.A 0.5 cm3 volume of the Æltrate was injected onto an anion-exchange fast protein liquid chromatography (FPLC) column and aqueous 4 mol dm23 NH4NO3 linear gradient elution was applied for 10 min to separate LMW-Al complexes. Fractions of 0.2 cm3 were collected throughout the chromatographic run and Al was determined `off-line' by electrothermal atomic absorption spectrometry (ETAAS). The characterisation of LMW-Al species in spiked serum was performed not only on the basis of the retention time (ETAAS detection), but also by electrospray (ES)-MS-MS analysis.A tandem quadrupole mass spectrometer equipped with a Z spray ion source as LC-MS interface was used for the identiÆcation of LMW ligands eluted under the chromatographic peaks. It was found experimentally that the amount of LMW-Al species in spiked serum ranged from 14 to 55%. On the basis of FPLC-ETAAS and ES-MS-MS analysis, it was found that the main LMW-Al species present in serum were Al-citrate, Alphosphate and ternary Al-citrate-phosphate complexes.The distribution of these species varied among particular individuals. In some of them Al-citrate and Al-phosphate were the main LMW-Al species in serum, while in others the ternary Al-citrate-phosphate complex was also present. The serum of some other individuals did not contain Al-phosphate and the main LMW-Al species were either Al-citrate and Al-citrate-phosphate complexes or Al-citrate species alone. The limit of detection for the separated Al species on the FPLC column was 5.0 ng cm23, while the RSD was found to be 8%.Introduction Aluminium toxicity, particularly in patients with chronic renal failure, is related to many clinical disorders. Its accumulation in the brain and bone is associated with dialysis encephalopathy1 and osteomalacia.2 It has been demonstrated that the gastrointestinal absorption of Al is greatly increased by the presence of low molecular weight (LMW) ligands,3,4 most intensively by citrate, although its role in Al accumulation in the body is not yet clear.5±8 It has been demonstrated that the dominant Al species in serum is Al-transferrin.9±14 The remaining Al is bound to LMW ligands, presumably citrate, phosphate and hydroxide.There are contradictory reports on speciation of LMW-Al complexes in serum.15±20 Martin21,22 and O» hman and Martin23 theoretically predicted that citrate is the most likely LMW binder of Al in human serum.Jackson24 calculated that 50% of Al in serum is bound to transferrin and the remaining 50% as Al(HPO4)OH, [Al(citrate)OH]2 and [Al(HPO4)citrate]22. Clevette and Orvig25 presumed Al-citrate to be the dominant LMW species in serum, while in the model of Harris26 it was predicted that 81% of Al is bound to transferrin and the remainder exists primarily as Al(PO4)(OH)2 with minor amounts of Al-citrate and Alhydroxide species. Another study20 proposed Al(OH)3 and Al(PO4) as the main LMW species in serum.A qualitative proton NMR study27 indicated that Al3z added to plasma ultraÆltrate was initially bound to citrate. Kiss and co-workers investigated the interaction of Al with phosphate28 and also the Al-citrate-phosphate interaction29 by potentiometry and 31P NMR techniques. It was found that at physiological pH the negatively charged binary species Al-citrate and Al-phosphate, as well as ternary Al-citrate-phosphate complexes, are present.29 For quantitative determination of LMW-Al species, microultraÆltration was used to fractionate high molecular weight (HMW) from LMW Al complexes in serum.30,31 Reported data indicate that 9±19% of Al in spiked pooled serum samples of healthy volunteers corresponded to ultraÆltrable LMW-Al species.Speciation of Al-citrate was also investigated by employing the HPLC-ETAAS method, but only moderate recoveries of Al-citrate were achieved.32 A study of the speciation of Al in biological Øuids by size-exclusion chromatography and ETAAS detection33 indicated the presence of two LMW fractions of Al in the spiked haemoÆltrate of uremic patients.In our group, a systematic study of Alcitrate by anion-exchange fast protein liquid chromatography (FPLC)-ICP-AES34 and anion-exchange FPLC-ETAAS31 was performed. It was demonstrated that LMW-Al present in spiked serum was quantitatively eluted under the peak of Alcitrate, 31 but the existence of this species was not deÆnitely proved.Since in addition to citrate, phosphate is also considered to be an important LMWligand for binding of Al in serum,20,24,26 the aim of our work was to provide more detailed information on the presence of Al species eluted under the chromatographic peak.31 For this purpose the characterisation of LMW-Al species in spiked serum was performed not only on the basis of the retention time, but also by electrospray (ES)-MS-MS analysis of theLMWligands eluted under the chromatographic J.Anal. At. Spectrom., 1999, 14, 1743±1748 1743 This Journal is # The Royal Society of Chemistry 1999peak. The study was performed on spiked serum samples from eight healthy volunteers in order to estimate individual variability in the percentage and composition of LMW-Al species. Experimental Instrumentation A strong anion-exchange FPLC column of Mono Q (Pharmacia, Uppsala, Sweden) was employed for the separation of negatively charged Al species. The column was connected to a Merck±Hitachi (Darmstadt, Germany) 6200 gradient highpressure pump equipped with a Rheodyne (Cotati, CA, USA) Model 7161 injector (0.5 cm3 loop).Total Al in human serum as well as the concentration of Al in separated species was determined by ETAAS on a Hitachi (Hitachi, Tokyo, Japan) Z-8270 polarized Zeeman atomic absorption spectrometer equipped with an autosampler at 309.3 nm. A Micromass (Micromass UK, Manchester, UK) Quatro LC tandem quadrupole mass spectrometer equipped with a Z spray ion source as LC-MS interface, employing negative electrospray ionisation, was used for the identiÆcation of LMW ligands in separated Al species.A Heraeus (Osterode, Germany) Model 17S Sepatech biofuge was used in the microultraÆltration procedure. Reagents Merck (Darmstadt, Germany) suprapur acids and water doubly distilled in quartz were used for the preparation of samples and standard solutions. All other reagents were of analytical-reagent grade.A stock Al3z solution (100 mg cm23 Al) was prepared in a 100 cm3 calibrated Øask by dissolving 0.1388 g of Al(NO3)3?9H2O (Riedel-de Hae»n, Hannover, Germany) in water. A stock Al-citrate solution (100 mg cm23 Al) was made weekly by mixing 0.0694 g of Al(NO3)3?9H2O and 3.5 g of citric acid (Merck) in a 50 cm3 Øask (100 : 1 citric acid to Al molar ratio). Imidazole (C3H4N2, 0.2 mol dm23) (Merck) buffer solution with the addition of an appropriate amount of hydrochloric acid (0.1 mol dm23) was used to adjust the pH of synthetic samples to 7.4.The 4 mol dm23 ammonium nitrate eluent was prepared by dissolving 320.16 g of NH4NO3 in 1 dm3 of water. Chelex 100 (Naz form, 100±200 mesh) chelating ion-exchange resin (Sigma, St. Louis, MO, USA) and a silica-based LiChrosorb -18 HPLC column (15064.6 mm id) were used for cleaning of reagents.31 Centricon 30 concentrators (Amicon, Witten, Germany) with a nominal cut-off of 30 000 Da were used in the ultramicroÆltration of human serum.Sample preparation In order to study the behaviour of LMW-Al complexes on an anion-exchange column at pH 7.4, synthetic solutions of LMW-Al species were prepared daily in 50 cm3 TeØon calibrated Øasks. A synthetic solution of Al-citrate (100 ng cm23 Al) was prepared by mixing 0.1 cm3 of stock Al-citrate solution in imidazole-HCl buffer solution with a pH of 7.4 (100 : 1 citrate to Al molar ratio). A synthetic solution of Al-phosphate (100 ng cm23 Al) was made by mixing 0.1 cm3 of stock Al3z solution and 0.1 cm3 of 2 mol dm23 H3PO4 in imidazole-HCl buffer solution (pH~7.4, 1000 : 1 PO4 32 to Al molar ratio).The Al to citrate and/or phosphate molar ratios were the same as in human serum. A synthetic solution of Al- ATP (100 ng cm23 Al) was prepared by mixing 0.023 g of adenosine triphosphate (ATP) (Merck) and 0.1 cm3 of stock Al3z solution in imidazole-HCl buffer solution (pH~7.4). The Al to ATP molar ratio was 1 : 100 to ensure complex formation.Blood from healthy volunteers was collected into Al-free Becton±Dickinson vacutainers without additives. Samples were centrifuged for 10 min at 3000 rpm. Serum was transferred into a TeØon Øask with a polyethylene pipette and was analysed within 12 h. Total Al was Ærst determined. In order to study the distribution of LMW-Al species, 2 cm3 of serum were spiked with 0.05 cm3 of Al3z solution, so that the Ænal concentration of Al in the spiked serum ranged from 100 to 120 ng cm23.Spiked serum was left to equilibrate for 4±6 h, after which it was microultraÆltered (cut-off 30 000 Da) to separate HMWfrom LMW-Al species. Speciation of Al was then performed in the serum Æltrate by the recommended analytical procedure. Recommended procedures Sample preparation, chromatographic separations and determination of Al by ETAAS were carried out under clean-room conditions (Class 10 000). To avoid contamination by extraneous Al, polyethylene or TeØon ware was treated with 10% HNO3 for 24 h, rinsed well with water and dried at room temperature.In order to lower the blank in FPLC separations, the NH4NO3 eluent and the FPLC columns were puriÆed by the cleaning procedure reported previously.31 A cleaning procedure was also applied to remove trace amounts of Al from the microultracentrifugation membranes of the Centricon 30 concentrators.31 Negatively charged LMW-Al complexes were separated on a Mono Q strong anion-exchange FPLC column.A 0.5 cm3 volume of sample was injected onto the column and aqueous (0±100% 4 mol dm23 NH4NO3) linear gradient elution was applied for 10 min at a Øow rate of 1 cm3 min21. Eluate was collected in 0.2 cm3 fractions and diluted to 0.5 cm3 with water in Eppendorf polyethylene cups. The concentration of Al was determined `off-line' by ETAAS under optimum measurement conditions as described previously.31 For the identiÆcation of LMW ligands eluted under the chromatographic peak, fractions were diluted 1z9 with water and analysed by the ES-MS-MS technique employing a Z spray ion source.A 20 mm3 volume of sample was injected into the Micromass Quatro LC mass spectrometer. The mobile phase was acetonitrile±0.005 mol dm23 ammonium acetate±formic acid (600z399z1, v/v/v). The electrospray probe voltage and sample cone voltage were set at 2.5 kV and 35 V, respectively. The source temperature of the mass spectrometer was held at 80 �C, while the desolvation temperature was 350 �C.The MS analyses were performed by scanning negative ions. The Q1- scan represented the pseudo-molecular ions (M2H)2 in the mass range m/z 50±1000. MS-MS collision-induced dissociation (CID) experiments were performed by introducing argon (2.061023 mbar) into the collision cell and setting a collision energy of 18 eV. The Ærst quadrupole analyser was set to transmit only the user-selected precursor ion. Results and discussion Distribution of Al-citrate, Al-phosphate and Al-ATP on an anion-exchange FPLC column A method for quantitative determination of Al-citrate in a wide pH range has been developed and validated by our group previously.31 The procedure was applied to the determination of LMW-Al species in spiked human serum (pooled sample).It was proved experimentally that LMW-Al species in serum were quantitatively eluted under the chromatographic peak which corresponded to Al-citrate.Since there are reports in the literature on the possibility of co-existence of aluminium phosphate and citrate species in the LMW serum fraction, the chromatographic peak was investigated more carefully. For this purpose, 0.2 cm3 fractions were collected and the behaviour of Al-citrate, Al-phosphate and Al-ATP at pH 7.4 was Ærst examined in synthetic solutions containing 100 ng cm23 of total Al. Separation of these species by 1744 J. Anal. At. Spectrom., 1999, 14, 1743±1748anion-exchange FPLC with ETAAS detection is presented in Fig. 1. It is evident that Al-citrate was quantitatively eluted from 3.0 to 3.8 min with two maximum peaks at 3.2 and 3.6 min, respectively. This indicates the presence of different negatively charged Al-citrate species, which is in agreement with the theoretical calculations of Martin.21 The Al-phosphate system is difÆcult to study because of Al-phosphate precipitation. However, at ppb concentration levels, the soluble species exist and it was found that 21°4% of Al-phosphate was eluted as negatively charged species from 2.2 to 3.2 min.The remaining 79% was strongly adsorbed on the resin column and did not disturb further separations. Data from Fig. 1 further indicate that 50% of Al-ATP was eluted from 2.8 to 3.2 min. The reproducibility of measurement was tested for six consecutive separations of Al-citrate, Al-phosphate and Al- ATP (100 ng cm23 Al, pH~7.4). The relative standard deviation (RSD) was found to be 2% for Al-citrate and Al- ATP, while for Al-phosphate it was 15%.The limit of detection (LOD) for determination of separated Al species on the FPLC column was found to be 5.0 ng cm23. IdentiÆcation of LMW-Al ligands by the ES-MS-MS technique In order to identify LMW-Al ligands, the ES-MS-MS technique using a Z spray ion source was applied. The conÆguration of this ionisation source permitted the analysis of samples with a high salt content, such as samples of eluted fractions after the chromatographic separation.First, mass spectra of standard solutions of Al-citrate, Al-phosphate and Al-ATP were recorded. The MS analysis was performed by scanning negative ions. Since ES is a `soft' ionisation technique very little fragmentation was observed and the most intense ion in the mass spectra of the investigated Al species was the deprotonated ligand ion (M2H)2. This precursor (parent) ion was selected for a further CID experiment and the product (daughter) ion mass spectra were recorded under the optimum conditions.The mass spectra and the corresponding MS-MS scans of Al-citrate, Al-phosphate and Al-ATP standard solutions are presented in Fig. 2. MS analysis of the blank indicated that m/z 81, 91, 108, 127, 137 and 154 corresponded to signals from the eluent used in the ES-MS procedure. It is evident (Fig. 2A) that in the mass spectrum of Al-citrate the peak with m/z 191 is the most intense and corresponds to deprotonated citric acid.This peak was selected as a parent ion for the CID experiment. After fragmentation, masses of 111, 87 and 85 were present in the resulting daughter ion mass spectrum. In the mass spectrum of Al-phosphate (Fig. 2B), a peak at m/z 97, which was selected as a parent ion for CID analysis, appeared. In the daughter ion mass spectrum, two peaks were present (m/z 97 and 79). The Al-ATP mass spectrum (Fig. 2C) resulted in a peak at mass 506.In the daughter ion mass spectrum of m/z 506, fragment ions of m/z 159, 177, 273, 408 and 426 appeared. The same standard solutions were also injected onto the FPLC coland ESMS- MS scans recorded for the separated fractions eluted under the chromatographic peaks. It was found experimentally that in the ES-MS scans additional peaks appeared (m/z 80, 103, 125, 142, 147, 171 and 188) which all resulted from the eluent of the chromatographic run (NH4NO3). ES-MS-MS peaks at m/z 191, 97 and 506 were the same as in standard solutions before chromatographic separation.Determination of LMW-Al species in spiked human serum In order to investigate individual variability in the percentage and distribution of LMW-Al species in serum, eight healthy volunteers were involved in the study. After sampling, total Al was Ærst determined by ETAAS using the standard additions calibration method. To reduce the matrix effects of proteins,35 5 ml of 32% nitric acid were added to the graphite tube before each determination and Al was determined under the optimum measurement conditions.31 The accuracy of the determination of total Al was checked by the determination of Al in Seronom‘ Trace Elements serum certiÆed reference material obtained from Nycomed Pharma.Good agreement between determined Al (61.4°0.3 ng cm23) and the reported certiÆed value (63°4 ng cm23) was obtained. The results for total Al concentrations in the serum of eight healthy volunteers are presented in Table 1.It can be seen that natural Al concentrations in serum ranged from 5 to 11 ng cm23. Caroli et al.36 reported that the normal value of Al in serum ranged from 0.5 to 8 ng cm23. Since the results in Table 1 are higher than the reported normal Al concentrations,36 the problem of contamination cannot be completely ruled out under the adopted working conditions. However, the natural Al concentrations were too low to perform speciation analysis.Therefore, the samples were spiked with Al3z solution, so that the Ænal concentration of Al in the spiked serum ranged between 100 and 120 ng cm23 Al. The concentrations of Al in spiked serum were similar to those that could be found in the serum of some haemodialysis patients. After equilibration (4± 6 h), serum was microultraÆltered (cut-off 30 000 Da) and the Æltrate injected onto the anion-exchange FPLC column. Speciation was performed as described under Recommended procedures.Fractions were collected throughout the chromatographic run and Al was determined by ETAAS. The results are presented in Table 2; data represent the average of three successive separations. It can be seen that the concentration of microultraÆltrable Al which was separated on the column ranged from 14 to 55% of the total Al in spiked serum samples. In the literature there are some reports on the determination of microultraÆltrable Al in spiked human serum.30,31 Data indicate that the amount of microultraÆltrable Al ranged from 10 to 20%.Since these studies were performed on pooled serum samples it was not possible to follow individual variability. In the present study, data from Table 2 further indicate that three LMW-Al species are separated on the column and that the distribution of these species varies among particular individuals. The Al species that was eluted from 0.8 to 1.2 min represents 3±37% of the LMW-Al species separated on the column.On the basis of our previous investigations,31,34 the presence of Al(OH)4 2 species, which at higher pH values was eluted at the same retention time, can be presumed. However, on performing mass spectrometric analysis it was not possible to identify Al(OH)4 2 species, nor other Al binding ligands with a mass up to 1000. Al species that were eluted from 2.4 to 2.6 min have the same retention time as Al-phosphate while Al-species eluted from 3.0 to 3.4 min appeared at the retention time of Al-citrate.In order to identify the Al binding ligands, ES-MS-MS analysis was performed on each separated fraction containing Al. Typical mass spectra and the corresponding MS-MS scans for Sample No. I are presented in Fig. 3. From the data of Fig. 3 it is evident that in the mass spectrum of the fraction eluted from 2.4 to 2.6 min a Fig. 1 Typical chromatograms of Al-citrate, Al-ATP and Al-phosphate (100 ng cm23 Al) at pH7.4. Separation was performed on an anion-exchange FPLC Mono Q HR 5/5 column and separated species were detected by ETAAS.Sample volume, 0.5 cm3; aqueous NH4NO3 (4 mol dm23) linear gradient elution; Øow rate, 1 cm3 min21; fraction collection, 0.2 cm3; n~3. J. Anal. At. Spectrom., 1999, 14, 1743±1748 1745characteristic peak at m/z 97 is present. In the daughter ion spectrum two masses with m/z 97 and 79 were observed, which conÆrmed the presence of phosphate as a binding ligand. This is in agreement with the Ændings of Kiss and co-workers,28,29 Jackson,24 Harris26 and Dayde et al.,20 who, on the basis of computer-aided speciation studies, predicted that phosphate is also an Al binding ligand in serum.The amount of Alphosphate in the LMW-Al fraction of serum sample No. I was 21%. The mass spectrum of the fraction eluted from 3.0 to 3.2 min indicates the presence of a characteristic peak with m/z 191. The corresponding daughter ion spectrum with characteristic masses of 111, 87 and 85 conÆrmed that the binding ligand was citrate.The same spectra were found for the fraction eluted from 3.2 to 3.4 min. The total amount of Al-citrate in sample No. I represented 55% of the LMW-Al species present in the serum. On the basis of the retention times and ES-MS-MS analysis it was found that in sample Nos. II and III, the Al species eluted on the column corresponded to Al-phosphate and Al-citrate. The amount of Al-phosphate in the LMW-Al fraction was 61% for sample No.II and 40% for sample No. III, while for Al-citrate it was 29 and 44%, respectively. In Fig. 4 typical mass spectra and the corresponding MS-MS scans are presented for sample No. IV. It is evident that in the mass spectrum of the fraction eluted from 2.4 to 2.6 min a characteristic peak at m/z 97 is present. The daughter ion spectrum of m/z 97 (masses 97 and 79) conÆrmed the presence of phosphate as the binding ligand. The amount of Alphosphate in the LMW fraction of serum sample No. IV was 18%.The mass spectrum of the fraction eluted from 3.0 to 3.2 min indicates the presence of characteristic peaks with m/z 191 and 97. The corresponding daughter ion spectrum of m/z 191 resulted in characteristic masses of 111, 87 and 85, while in the daughter ion spectrum of m/z 97, peaks with m/z 97 and 79 are present. These data conÆrm the presence of citrate and phosphate binding ligands. The same spectra were found for the fractions eluted from 2.8 to 3.0 and from 3.2 to 3.4 min.On the basis of these data it can be presumed that the Al which is eluted from 2.8 to 3.4 min is present as Al-citrate and ternary Al-citrate-phosphate complexes. The presence of Al-phosphate in this chromatographic peak could be excluded on the basis of the retention time. The ternary Al-citrate-phosphate complex has also been predicted as one of the possible LMW-Al species in serum by computer-aided speciation.24,29 The amount of Alcitrate and ternary Al-citrate-phosphate complexes in sample No.IV represented 78% of the LMW-Al species. With the Fig. 2 ES-mass spectra and corresponding daughter ion mass spectra for synthetic solutions of Al-citrate (A), Al-phosphate (B) and Al-ATP (C) (100 ng cm23 Al) at pH 7.4. Table 1 Concentrations of total Al (ng cm23) in serum from eight healthy volunteers determined by ETAAS (n~3). Serum sample (No.) Total concentration of Al/ng cm23 I 9.0°1.0 II 9.0°1.0 III 6.0°0.5 IV 7.0°1.0 V 7.5°1.0 VI 6.0°0.5 VII 5.0°0.5 VIII 11.0°1.0 1746 J. Anal.At. Spectrom., 1999, 14, 1743±1748speciation procedure described it was not possible to distinguish quantitatively between these two Al species. On the basis of the retention times and ES-MS-MS analysis, it was found that in sample Nos. V and VI, the Al species eluted from 2.4 to 2.6 min corresponded to Al-phosphate (26 and 43% of the LMW-Al species, respectively). The LMW-Al species eluted from 2.8 to 3.6 min corresponded to Al-citrate and ternary Al-citrate-phosphate complexes.The amount of these species in the LMW-Al fraction was 62% for sample No. V and 47% for sample No. VI. In contrast to the other samples analysed, sample Nos. VII and VIII did not contain Al-phosphate. The Al peak that was eluted from 2.8 to 3.2 min (Table 2) corresponded, on the basis of ES-MS-MS analysis, to Al-citrate and ternary Al-citrate- Table 2 Separation of microultraÆltrable (LMW) Al in spiked human serum from eight healthy volunteers on an anion-exchange FPLC column and determination of Al (ng cm23) in separated fractions by ETAAS (n~3) Concentration of Al in separated fractions/ng cm23 Sample No.Time/min I II III IV V VI VII VIII 0±0.2 a a a a a a a a 0.2±0.4 a a a a a a a a 0.4±0.6 a a a a a a a a 0.6±0.8 a a a a a a a a 0.8±1.0 5.7°0.3 1.0°0.2 a a 1.5°0.2 1.0°0.2 6.9°0.3 a 1.0±1.2 13.4°0.4 1.0°0.2 1.0°0.2 1.0°0.2 1.8°0.2 1.0°0.2 a 5.9°0.3 1.2±1.4 a a a a a a a a 1.4±1.6 a a a a a a a a 1.6±1.8 a a a a a a a a 1.8±2.0 a a a a a a a a 2.0±2.2 a a a a a a a a 2.2±2.4 a a a a a a a a 2.4±2.6 12.6°0.4 12.2°0.4 5.9°0.4 6.2°0.3 7.1°0.3 9.6°0.4 a a 2.6±2.8 a 1.0°0.2 1.0°0.2 a a a a a 2.8±3.0 a 1.0°0.2 a 6.5°0.3 a 1.0°0.2 1.5°0.2 1.5°0.2 3.0±3.2 24°0.5 5.0°0.3 7.0°0.3 16.9°0.4 6.6°0.3 6.8°0.4 15.4°0.4 8.5°0.4 3.2±3.4 6.0°0.4 a 1.2°0.2 3.4°0.2 5.0°0.4 2.8°0.3 a a 3.4±3.6 a a a a 5.6°0.4 a a a 3.6±3.8 a a a a a a a a 3.8±4.0 a a a a a a a a 4.0A10.0 a a a a a a a a LMW Al species (%) 55 17.5 14.50 30 24 20 21 14.5 aBelow instrumental LOD (0.5 ng cm23) Fig. 3 ES-mass spectra and corresponding daughter ion mass spectra of m/z 191 and 97 for eluted fractions from 2.4±2.6 and 3±3.2 min, respectively, on an anion-exchange FPLC column for serum sample No. I. J. Anal. At. Spectrom., 1999, 14, 1743±1748 1747phosphate complexes (sample No. VII), and to the Al-citrate complex alone (sample No. VIII). The amounts of these LMWAl species eluting from 2.8 to 3.2 min were 71 and 63%, respectively. On the basis of this study it is evident that the percentage and distribution of LMW-Al species in spiked human serum from healthy volunteers varies between particular individuals involved in the study.Acknowledgements This work was Ænancially supported by the Ministry of Science and Technology of Slovenia. References 1 A. I. Arieff, Am. J. Kidney Dis., 1995, 6, 317. 2 D. M. Grekas, H. A. Ellis, M. K. Ward, A.M. Martin, I. Parkinson and D. N. S. Kerr, Uremia Investig., 1984, 8, 9. 3 J. W. Coburn, M. G. Mischel, W. G. Goodman and I. B. Salusky, Am. J. Kidney Dis., 1991, 16, 708. 4 J. S. Lindberg, J. B. Copley, K. G. Koenig and H. M. Cuhner, South. Med. J., 1993, 86, 1385. 5 B. Quartley, G. Esselmont, A. Taylor and M. Dobrota, Food Chem. 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Martin, Inorg. Chem., 1996, 35, 7089. 29 A. Lakatos, T. Kiss and I. Banyai, COST D8 and ESF Workshop on Biological and Medicinal Aspects of Metal Ion Speciation, Szeged, Hungary, 1998, p. L26. 30 J. Pe�rez Parajo�n, E. Blanco Gonza� lez, J. B. Cannata and A. Sanz-Medel, Trace Elem. Med., 1989, 6, 41. 31 T. Bantan, R. Milacœicœ and B. Pihlar, Talanta, 1998, 47, 929. 32 A. K. Datta, P. J. Wedlund and R. A. Yokel, J. Trace Elem. Electrolytes Health Dis., 1990, 4, 107. 33 H. Keirsse, J. Smeyers-Verbeke, D. Verbeelen and D. L. Massart, Anal. Chim. Acta, 1987, 196, 103. 34 T. Bantan, R. Milacœicœ and B. Pihlar, Talanta, 1998, 46, 227. 35 J. Sœ cœ ancœar, R. Milacœicœ, M. Benedik and P. Bukovec, Clin. Chim. Acta, 1999, 283, 139. 36 S. Caroli, A. Alimonti, E. Coni, F. Petrucci, O. Senofonte and N. Violante, Crit. Rev. Anal. Chem., 1994, 24, 363. Paper 9/904213J Fig. 4 ES-mass spectra and corresponding daughter ion mass spectra of m/z 191 and 97 for eluted fractions from 2.4±2.6 and 3±3.2 min, respectively, on an anion-exchange FPLC column for serum sample No. IV. 1748 J. Anal. At. Spectrom.,

ISSN:0267-9477    

DOI:10.1039/a904213j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

On-line preconcentration system for Øame atomic absorption spectrometry using unloaded polyurethane foam: determination of zinc in waters and biological materials Ricardo Jorgensen Cassella,a,b Denise Teixeira Bitencourt,b Aline Garcia Branco,b Se�rgio Luis Costa Ferreira,c Djane Santiago de Jesus,c Marcelo Souza de Carvalhod and Ricardo Erthal Santellib aEscola Te�cnica Federal de Quý�mica, Rio de Janeiro, RJ 20270-021, Brazil bDepartamento de Geoquý�mica, Universidade Federal Fluminense, Nitero� i, RJ 24020-007, Brazil cInstituto de Quý�mica, Universidade Federal da Bahia, Salvador, BA 40170-290, Brazil dInstituto de Engenharia Nuclear–CNEN, Rio de Janeiro, RJ, Brazil Received 22nd June 1999, Accepted 3rd September 1999 An on-line procedure for the preconcentration and determination of zinc in waters and biological materials was developed.Zinc is preconcentrated from acidic medium (pH 3.0) as its thiocyanate complex onto a polyurethane foam mini-column placed in the loop of a four-way valve.The elution step is performed with a stream of 30% acetone in 2% HNO3 and the zinc displaced is introduced directly into the nebuliser of a Øame atomic absorption spectrometer. The system was operated under two different preconcentration times since the concentration of zinc in the two types of samples was different. For biological samples, 1 min was used and a detection limit of 3.0 mg L21 was achieved with a throughput of 40 samples per hour.At a concentration of 20 mg L21, an RSD of 2.5% was obtained. For the determination of zinc in natural water, a preconcentration time of 3 min was employed and a detection limit of 0.85 mg L21 was obtained. In this case, the system was slower with a throughput of 17 samples per hour. The RSD at 10 mg L21 was 6.0%. The continuous Øow system was applied to the analysis of several biological reference materials and natural water samples. 1. Introduction Analytical chemists frequently have to apply different previous treatments to samples that cannot be analysed in their natural state due to matrix interferences or low sensitivity of the methodology employed.Separation and preconcentration procedures have been performed in order to solve these problems. Thus, on-line preconcentration systems have proved to be the most interesting way to improve the performance of Øame atomic absorption spectrometry (FAAS) in trace ion determinations, allowing high preconcentration factors with limited amounts of sample and also increasing the precision and speed of analysis.Moreover, the possibilities of contamination and losses are considerably reduced. The process involved in continuous liquid±solid separation preconcentration always take place in two steps that involve retention and elution. The active solid phase (polyurethane foam in this study) is a permanent part of the continuous system involving mini-columns. Several Øow conÆgurations can be found in the literature employing classical ionexchangers, chelating resins, functionalized silica and cellulose, activated alumina and charcoal.1 With regard to zinc as an analyte, Burguera et al.in 19812 were the Ærst to study its retention and elution from an ion-exchange column with chemiluminescence detection. Employing AAS, Olsen et al.3 were the pioneers in coupling mini-column Øow systems with such detection. Also, more recent papers dealing with this goal can be found in the literature, especially those of Purohit and Devi4 and Greenway and Townshend,5 although neither dealt with real sample analysis.Advances in this synergistic coupling (FAAS and mini-column continuous Øow systems) are well documented in books by Fang6 and Sanz-Medel.7 Solid-phase extraction (SPE) can be considered the most important technique for performing preconcentration in Øow systems. The application of polyurethane foam (PUF) as a solid phase for analytical purposes was Ærst reported by Bowen8 in his pioneering research.Braun and co-workers9±11 have reviewed this Æeld. Several papers have appeared in the literature employing loaded and unloaded PUF for metal sorption from aqueous medium.12±20 Unloaded PUF has been extensively studied as an extractant for several metal cations from thiocyanate medium, with zinc being cited in some of these papers. Chow et al.21 described cobalt retention from a medium containing 1.0 mol L21 NH4SCN under different conditions of ionic strength.They found that Ni(II) and Pb(II) were not extracted by the foam and Fe(III), Zn(II) and Cu(II) were simultaneously extracted with Co(II). Braun and Abbas22 studied the sorption of several metal cations, including Zn(II), from different types of foam and concluded that polyether as well as polyester foams are efÆcient in removing metals from thiocyanate solutions. Maloney et al.23 also reported that Fe(III), Co(II), Cd(II) and Zn(II) are extracted by polyether PUF as their thiocyanate complexes.Applications of PUF as a solid extractant in Øow injection analysis (FIA) systems for the preconcentration and separation of metals have been described by our research group. Recently, de Jesus et al.24 have performed the separation of zinc and cadmium in matrices containing high concentrations of cadmium by using PUF. In the Ærst work describing the use of PUF in a Øow system, Zn(II) was preconcentrated on a PUF mini-column from thiocyanate solution as the Zn±SCN2 complex.25 After elution, effected with water, the metal was measured spectrophotometrically. Several biological certiÆed reference materials were analysed with good precision and accuracy.Ferreira et al.26 employed an FIA-PUF system to improve the selectivity in the spectrophotometric determination of nickel in alloys and silicates. Cassella et al.27 also investigated the use of an FIAJ. Anal. At.Spectrom., 1999, 14, 1749±1753 1749 This Journal is # The Royal Society of Chemistry 1999PUF system to enhance the selectivity of the spectrophotometry. They applied this system to the spectrophotometric determination of aluminum with MTB (Methyl Thymol Blue) in silicate samples containing large amounts of iron. The aim of this work was to develop an analytical system capable of determining zinc at mg L21 levels by FAAS using zinc preconcentration, exploring the high selectivity of the FAAS technique and improving the global performance of the system.Some advantages of this new application are: very low cost of the solid phase (the PUF employed was of the same type as the domestic PUF used for cleaning purposes); very low overpressure due to the inherent physical properties of the foam (e.g. high resilience level) without swelling providing high sample Øow rates without clogging; and very effective extraction of metals from thiocyanate solutions, removing the matrix, with very high breakthrough capacity (4.5 mg of Zn per gram of PUF). 2 Experimental 2.1 Apparatus A Perkin-Elmer 3100 atomic absorption spectrometer (Perkin- Elmer, Norwalk, CT, USA) equipped with a zinc hollow cathode lamp was used. The instrument was connected to an RB-201 recorder (ECB-Equipamentos CientiÆcos do Brasil, Saƒo Paulo, Brazil) and was operated according to standard conditions recommended by the manufacturer. The FIA system was constructed using an Alitea XV peristaltic pump (Seattle, WA, USA), furnished with Tygon tubes, to propel all solutions. Both preconcentration and elution steps were switched by using a Rheodyne 5041 (Cotati, CA, USA) fourway injection valve.All connections were made using Ættings, unions and tees made from plastic and PEEK materials. The manifold was constructed with PTFE tubes of 0.5 mm id. 2.2 Reagents and solutions All solutions were prepared with water obtained from a Milli-Q water puriÆcation system (Millipore, Bedford, MA, USA) and by using analytical-reagent grade reagents. Zinc(II) standard solutions were prepared daily by appropriate dilution of a stock solution containing 1000 mg L21 Zn.This solution was obtained by dissolving 2.4696 g of dried ZnSO4 in 1000 mL o21 HNO3 and was standardized by complexometric titration with EDTA.28 A reagent solution of 1.0 mol L21 potassium thiocyanate was prepared daily by dissolving 24.30 g of KSCN in water. The pH of this solution was adjusted to 3.0°0.2 with 0.01 mol L21 HNO3.Eluent solution was prepared by carefully mixing 300.0 mL of acetone with 500.0 mL of water and 20.0 mL of concentrated HNO3, after which the volume was made up to 1000 mL. Polyurethane foam (PUF), open cell, polyether type, was obtained as a commercial product (Vulcan of Brazil–VCON 202, 42% resilience and 10±12 cells cm21). In order to use PUF as sorbent, the foam was comminuted in a blender with Milli-Q water and washed several times with 6 mol L21 HCl to ensure elimination of metallic species.A mini-column was prepared by packing 50 mg of PUF in a small plastic tube (3.0 cm6 3 mm id). 2.3 Flow injection system A schematic diagram of the developed Øow system is depicted in Fig. 1. In this system a sample solution pumped at 6.7 mL min21 merges with a 0.43 mL min21 stream of 1.0 mol L21 KSCN. The mixture generated percolates through the PUF mini-column where the Zn±thiocyanate complex is retained.The remaining solution goes directly to waste. After a suitable preconcentration time, the valve is switched and the eluent solution (30% acetonez2% HNO3) Øows at 2.0 mL min21 through the mini-column, displacing Zn(II) ions to the spectrometer where absorbance signals are monitored. Peak heights were used for all calculations. After the elution step, which takes about 30 s, the mini-column is ready for a new preconcentration cycle. 2.4 Sample preparation The Øow system was used to determine zinc in several certiÆed reference materials and water samples (sea-water and well water).The biological certiÆed reference materials analysed were Copepoda (MA-A-1/TM), Fish Tissue (MA-B-3/TM), Fish Flesh (MA-A-2/TM) and Tuna Homogenate (IAEA-350) from the International Atomic Energy Agency (IAEA), Monaco, and Rice Flour–Unpolished Sample No. 10-a from the National Institute of Environmental Studies (NIES), Japan. Firstly, all samples were dried overnight at 110°5 �C, after which they were dissolved by the usual treatment with HNO3 and HClO4.For this purpose, different amounts of the samples (between 0.1 and 0.3 g depending on the concentration of zinc) were placed in a PTFE beaker and mixed with 10 mL of concentrated HNO3. After standing overnight in contact with the acid as recommended by Icbinoki and Yamazaki,29 the mixtures were heated until total dissolution of the samples had occurred and a clear pale yellow solution was obtained.Then, 4 mL of 70% HClO4 were carefully added in two 2 mL portions. After evaporating to fumes of HClO4, the remaining residue was cooled and further dissolved with 50 mL of water. The pH was then adjusted to 3.0°0.2 and the volume was made up to 100 mL. The solution samples were stored in poly(propylene) Øasks and were analysed according to the developed procedure. At least one blank solution was run for each sample to control reagent contamination. Natural sea-water was taken from the Guanabara bay region near to a shipyard and to a sewage discharge, both located at Nitero� i, RJ.The well water was sampled at Itaipu balneary region near to Nitero� i city. For these samples, the only pretreatment was acidiÆcation to pH 1.8, which was performed immediately after collection, in order to prevent adsorption of the Zn(II) ions on the poly(propylene) Øask walls. In the laboratory, the samples were Æltered and 100 mL of the Æltrate were taken and the pH was adjusted to 3.0°0.2.The volume was then made up to 200 mL. Also, at least one blank solution Fig. 1 Flow system manifold for zinc(II) determination after its preconcentration on a PUF mini-column. (a) Preconcentration step and (b) elution step. S~Sample, 6.7 mL min21; R~reagent, 1.0 mol L21 KSCN, 0.43 mL min21; E~eluent solution, 30% acetonez2% HNO3 (v/v), 2.0 mL min21; C~PUF mini-column, 50 mg; D~detector, Øame atomic absorption spectrometer; W~waste. 1750 J. Anal. At. Spectrom., 1999, 14, 1749±1753was run for each sample in order to evaluate zinc contamination by the reagents used. 3. Results and discussion 3.1 Flow system optimization The Ærst procedure adopted in the development of this new methodology was the optimisation of the chemical and Øow variables of the system in order to improve its performance. In batch procedures, the pH of the sample solution was not an important parameter since the extraction was quantitative over a wide range of pH.24 Despite this fact, the pH of the sample solution was studied since it controls the concentration of free thiocyanate ion in the medium, thereby inØuencing the formation of the Zn±SCN2 complex.The pH was studied between 1.0 and 6.0 in the presence of 0.5% sodium citrate as masking agent for possible interferents. The highest signals were obtained for pH values up to 3.9; an abrupt decrease was observed when the pH was higher than 4.0 probably due to the high concentration of free citrate with consequent complexing of the analyte. Hence, a pH of 3.0°0.2 was chosen for all further experiments.Different results from batch procedures were observed probably due to kinetic differences between continuous and batch modes. Another chemical variable tested was the thiocyanate concentration, which also controls the formation of the Zn± SCN2 complex. In this case, it was decided to Æx the pH and to change only the concentration of the reagent, since the variation of pH could modify the concentration of free thiocyanate ions.The pH of this solution was kept at 3.0 and the concentration of SCN2 was tested between 0.05 and 2.0 mol L21. The best results were observed for concentrations above 0.8 mol L21. Thus, a thiocyanate concentration of 1 mol L21 was selected. In earlier work, water was used as an eluent24,25 to desorb Zn(II) only from the solid phase by depletion of the SCN2 concentration inside the mini-column, thereby dissociating the Zn±SCN2 complex. In the Øow system, where detection was effected spectrophotometrically with PAR [2-(2-pyridylazo)- resorcinol], the use of organic eluents disturbed the colorimetric measurement.By using FAAS, this effect was minimized or eliminated. Hence, several solutions that could be used as eluent were tested. Nitric acid at 0.1 mol L21, 50% ethanol in 2% HNO3 and 30% acetone in 2% HNO3 were initially tested, and the signal obtained with water was used as a reference value. The performance of the organic eluents was much better than that observed with water and nitric acid, as can be seen in Fig. 2. Moreover, experiments were carried out with different compositions of these solutions in order to choose the best eluent solution for this system. Several concentrations of organic solvent and acid were investigated, such as ethanol and acetone concentrations between 10 and 50% in acid solutions containing from 1 to 5% v/v HNO3.For ethanol, the best combination between acid and solvent was 50% ethanol in 4% v/v HNO3. However, this high concentration of ethanol caused instability of the baseline, probably due to alterations in Øame transparency. For acetone, the best results were observed in solutions of 30% acetone in 2% v/v HNO3 with absorbance signals of the same magnitude as that observed for the ethanol solution. Good stability of the baseline was found, in contrast to ethanol solutions.Hence, even though the signals had the same magnitude, the acetone-based eluent was chosen due to the lower noise generated and to preserve the Tygon pump tubes by using lower organic solvent concentrations. The sample Øow rate was investigated from 2.0 to 12.8 mL min21 and the best response was obtained at a Øow rate of 6.7 mL min21. However, the best ratio of absorbance signal : sample volume was observed at a Øow rate of 3.0 mL min21. Therefore, 6.7 mL min21 was chosen in order to reach a higher sample throughput and to improve sensitivity. Increasing the saOslash;ow rate to higher values leads to a decrease of the analytical signal due to slow sorption kinetics which does not allow quantitative sorption of Zn(II) by the PUF mini-column.The thiocyanate Øow rate was studied between 0.28 and 1.69 mL min21 and the best analytical signals were found at 0.43 mL min21. As this Øow rate maintains low sample dilution, it was chosen for further experiments.The eluent Øow rate was tested from 1.69 to 5.8 mL min21. Its inØuence must be analysed from two points of view: (1) desorption kinetics and (2) dispersion of the sample plug. The best results were observed at 2.0 mL min21 and under this condition there is a compromise between the two parameters cited above. However, by analysing the results obtained it is clear that the Ærst parameter has little inØuence on the signal generated since the eluent solution provided a high desorption rate.This fact can be attested to by the short time required to complete the analytical signal. Hence, dispersion is the most important parameter in the control of the analytical signal. From 1.69 until 2.0 mL min21 the signals increased because low dispersion occurred, and above 2.0 mL min21 the signals decreased, probably due to the high Øow rate which merely enhanced the sample dispersion and made the signals smaller. Therefore, an eluent Øow rate of 2.0 mL min21 was chosen.According to our previous studies,25 the mini-columns were constructed with 50 mg of PUF packed in mini-tubes (3 cm63 mm id). Under the optimized experimental conditions, very low overpressure was observed, in contrast to other sorbents employed in FIA systems. The PUF mini-column retention capacity was evaluated by a simple experiment in which a deÆned volume (50 mL) of several solutions containing different amounts of zinc was pumped Fig. 2 Signals obtained with different eluent solutions for ZnII preconcentration solutions containing 100 ng mL21 during 1 min. J. Anal. At. Spectrom., 1999, 14, 1749±1753 1751through the mini-column. Zinc was determined in the collected efØuent and the amount retained was calculated by difference. Table 1 shows the behaviour of the system when these different solutions are pumped into the mini-column in terms of amount and per cent. of Zn(II) retained. From these data, the maximum retention capacity can be estimated as 4.5 mg of Zn per gram of PUF.However, as can also be seen in Table 1, in solutions with concentrations higher than 5 mg mL21 of Zn(II), the Zn(II) retained decreases abruptly, decreasing the process efÆciency. 3.2 Interference study AAS has an inherent characteristic that is its selectivity. However, when working with PUF mini-columns, the presence of metal cations can affect the zinc signal by competition of their thiocyanate complexes for the PUF active sites.Hence, several possible interferent species, especially those which form thiocyanate complexes, were tested in order to verify the selectivity of the procedure: Fe(III), Cu(II), Co(II), Mn(II) and Cd(II). However, by using a mixed solution containing thiocyanate and 0.5% sodium citrate as masking agent, all interferents could be suppressed. Higher concentrations of sodium citrate were not used because they could affect the zinc signal by complexation with the metal.26 Al(III) and Ni(II) were not tested because the rate of formation of their thiocyanate complexes is negligible.Several zinc : interferent ratios were tested such as 1 : 50, 1 : 100, 1 : 200 and 1 : 500 and no interferences were observed for any ratio for any of the species studied. Interference was considered to have occurred when a signal difference greater than 10% was observed. 3.3 Analytical features The Øow system was operated employing two different preconcentration times due to differences in the concentration of zinc in the two types of samples analysed.For the determination of zinc in the biological samples, a 1 min preconcentration time (6.7 mL of sample) was used. Under this condition, the system shows linearity for concentrations of Zn(II) between 20 and 100 ng mL21, which can be represented by the equation: A~0.0045 [Zn(II) (ng mL21)]z0.003, r~0.999. The detection limit, calculated as three times the standard deviation, was 3.0 ng mL21.The RSD was calculated by taking ten measurements of a 20 ng mL21 solution and was always better than 2.5%. The quantiÆcation limit found was 10 ng mL21. The throughput achieved under the optimized experimental conditions was 40 samples per hour. In the analysis of water samples, a longer preconcentration time must be used since the Zn(II) was below the concentration range used. A preconcentration time of 3 min (20.1 mL of sample) was sufÆcient to reach a measurable analytical signal.Calibration graphs were constructed from 10 to 50 ng mL21, the characteristic equation being A~0.0085 [Zn(II) (ng mL21)]z0.002, r~0.999. The detection limit, calculated as described above, was 0.85 ng mL21. The RSD, assessed by ten measurements of a 10 ng mL21 solution, was 6.0%. In this case, the quantiÆcation limit was 5 ng mL21 and the throughput was 17 samples per hour. Preconcentration factors were calculated by comparing the calibration graphs constructed with the Øow system with that obtained by direct aspiration of Zn(II) solutions. For a preconcentration time of 1 min, the preconcentration factor was 8 and for 3 min it was 15. 3.4 Applications The developed Øow system showed good performance in the preconcentration and determination of zinc in different matrices such as biological materials and natural waters. Table 2 shows the results obtained in the analysis of biological reference materials and compares them with the certiÆed values.In Table 3, the results obtained in the analysis of natural saline and non-saline waters and the recoveries found are presented. The results are expressed as the mean of three determinations and their conÆdence limits. As can be seen, good agreement was observed between the certiÆed values and those obtained by the FIA method, and good recoveries were obtained from spiked natural waters. 4. Conclusions An increase in the speed of the analytical process and a reduced sample manipulation were possible by coupling an on-line preconcentration procedure to a Øame atomic absorption spectrometer.PUF can be considered an effective extractant for zinc from thiocyanate solutions under Øow conditions. Under the described conditions, it is possible to retain large amounts of Zn(II), viz., 4.5 mg per gram of PUF. The PUF provides a useful means of concentrating zinc from different matrices by the continuous mode. Also, very low overpressure is observed inside the mini-column, in contrast to other sorbents frequently used as solid phases in Øow systems.Other advantages in the Table 1 Retention capacity of the PUF mini-column (50.6 mg) for Zn(II) under the optimized conditions; 50 mL of sample solution containing Zn(II) Zn(II) solution/mg mL21 Zn retained/mg Zn retained (%) 1 43.8 89.8 5 225 90.0 10 345 69.0 20 540 54.1 50 800 32.0 100 1485 29.7 200 2780 28.0 400 4388 21.9 600 4536 15.1 Table 2 Results obtained for the analysis of biological reference materials.Results in mg g21 dry mass Reference sample Obtained value CertiÆed value Tuna Homogenate, IAEA, 350 16.9°0.8 17.4°0.8 Fish Tissue, IAEA, MA-B-3/TM 118.1°6.9 109.2°2.8 Rice Flour, Unpolished, High Cd Level, No. 10c, NIES 22.8°1.5 23.1°0.8 Copepoda, IAEA, MA-A-1/TM 156.6°6.7 158°2 Fish Flesh, IAEA, MA-A-2/TM 33.1°2.4 33°1 Table 3 Results obtained for the analysis of natural water samples, expressed as mean and standard deviation of three independent determinations, and recoveries after spiking with 20 mg L21 of Zn(II) Sample Obtained value/mg L21 Recovery (%) Sea-watera 63.0°0.5 106 Sea-waterb 40.2°0.6 94 Underground water 30.0°0.6 96 aSample collected near to the sewage discharge of a Domestic Waste Treatment Plant.bSample collected near to a shipyard. 1752 J. Anal. At. Spectrom., 1999, 14, 1749±1753use of PUF as sorbent are the simplicity and the very low cost of constructing the Øow system.The main objective of this work was to improve both the selectivity and analytical throughput of the system developed previously25 as well as to enhance sensitivity. The selectivity was excellent, making it possible to detect zinc in the presence of concentrations of interferents 500 times higher. The analytical throughput was improved, reaching a frequency of 40 samples per hour, but the sensitivity was virtually the same when determining zinc at the same concentration levels as those determined by FIA-spectrophotometry.25 The detection limits achieved were satisfactory for the samples studied, and can be improved by using more sensitive detectors such as ICP-AES or ICP-MS employing a similar manifold to that used in this work.The results obtained for the reference materials analysed were in good agreement with the certiÆed values. Fresh and saline water samples can be analysed accurately by this methodology. Acknowledgements The authors are grateful to Conselho Nacional de Desenvolvimento Cientý�Æco e Tecnolo� gico (CNPq), Financiadora de Estudos e Projetos (FINEP) and Comissaƒo Nacional de Energia Nuclear (CNEN) for grants and fellowships.References 1 M. Valca� rcel and M. D. Luque de Castro, Non-chromatographic Continuous Separation Techniques, Royal Society of Chemistry, Cambridge, 1991. 2 J. L. Burguera, M. Burguera and A. Townshend, Anal. Chim. Acta, 1981, 127, 199. 3 S. Olsen, L. C. R. Pessenda, J.Ruzicka and E. H. Hansen, Analyst, 1983, 108, 905. 4 R. Purohit and S. Devi, Analyst, 1991, 116, 825. 5 G. M. Greenway and A. Townshend, Anal. Proc., 1991, 30, 438. 6 Z. Fang, Flow Injection Separation and Preconcentration, VCH, Weinheim, 1993. 7 A. Sanz-Medel Flow Injection with Atomic Spectrometric Detectors, Elsevier, Amsterdam, 1999. 8 H. J. M. Bowen, J. Chem. Soc., 1970, 1082. 9 T. Braun and A. B. Farag, Talanta, 1975, 22, 699. 10 T. Braun and A. B. Farag, Anal.Chim. Acta, 1978, 99, 1. 11 T. Braun, J. D. Navratil and A. B. Farag, Polyurethane Foam Sorbents in Separation Science, CRC Press, Boca Raton, FL, 1985. 12 T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 73, 301. 13 D. Wildhagen and V. Krivan, Anal. Chim. Acta, 1993, 274, 257. 14 S. L. C. Ferreira, V. A. Lemos, A. C. S. Costa, D. S. de Jesus and M. S. Carvalho, J. Braz. Chem. Soc., 1998, 9, 151. 15 J. J. Oren, K. M. Gough and H. D. Gesser, Can. J. Chem., 1979, 57, 2032. 16 M. S. Carvalho, I. C. S. Fraga, K. C. M. Neto and E. Q. S. Filho, Talanta, 1996, 43, 1675. 17 M. S. Carvalho, M. L. F. Domingues, J. L. Mantovano and E. Q. S. Filho, Spectrochim. Acta, Part B, 1998, 53, 1945. 18 T. Braun and M. N. Abbas, Anal. Chim. Acta, 1980, 119, 113. 19 T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 69, 85. 20 D. W. Lee and M. Halmann, Anal. Chem., 1976, 48, 2214. 21 A. Chow, G. T. Yamashita and R. F. Hamon, Talanta, 1981, 28, 437. 22 T. Braun and M. N. Abbas, Anal. Chim. Acta, 1982, 134, 321. 23 M. P. Maloney, G. J. Moody and J. D. R. Thomas, Analyst, 1980, 105, 1087. 24 D. S. de Jesus, M. S. Carvalho, A. C. S. Costa and S. L. C. Ferreira, Talanta, 1998, 46, 1525. 25 D. S. de Jesus, R. J. Cassella, S. L. C. Ferreira, A. C. S. Costa, M. S. Carvalho and R. E. Santelli, Anal. Chim. Acta, 1998, 366, 263. 26 S. L. C. Ferreira, D. S. de Jesus, R. J. Cassella, A. C. S. Costa, M. S. Carvalho and R. E. Santelli, Anal. Chim. Acta, 1999, 378, 287. 27 R. J. Cassella, R. E. Santelli, A. G. Branco, V. A. Lemos, S. L. C. Ferreira and M. S. Carvalho, Analyst, 1999, 124, 805. 28 H. A. Flaschka, EDTA Titrations–An Introduction to Theory and Practice, Pergamon Press, Oxford, 2nd edn., 1964. 29 S. Icbinoki and M. Yamazaki, Anal. Chem., 1985, 57, 2219. Paper 9/904997E J. Anal. At. Spectrom., 1999, 14, 1749±175

ISSN:0267-9477    

DOI:10.1039/a904997e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

ModiÆer effects in the determination of arsenic, antimony and bismuth by electrothermal atomic absorption spectrometry Leon Pszonicki* and Jakub Dudek Institute of Nuclear Chemistry and Technology, Dorodna 16, 01-231 Warsaw, Poland Received 23rd July 1999, Accepted 14th September 1999 The effect of palladium, mixed palladium±magnesium and magnesium modiÆers on the determination of arsenic, antimony and bismuth was tested. It was found that palladium modiÆer works correctly only in nitric acid solution.If the sample solution contains hydrochloric acid then palladium becomes a strong interferent and causes signiÆcant losses of the analyte. For arsenic and antimony, but not for bismuth, this shortcoming may be eliminated by use of mixed palladium±magnesium or magnesium modiÆer. Magnesium modiÆer was found to be superior to the mixed modiÆer since it is able to eliminate also the negative effects of perchloric acid and of the iron group elements. It is, however, completely ineffective in relation to bismuth when used individually or in a mixture with palladium.Palladium may be used as a modiÆer for the determination of antimony and bismuth in hydrochloric acid solution only when it is preliminarily reduced to the metal form. The mechanism of the modiÆer activity is discussed. A very effective technique for arsenic, antimony and bismuth determination by atomic absorption spectrometry is hydride generation.1 It is necessary, however, to keep the elements in solution in a low oxidation state, it is sensitive to many interfering sample components and, in the case of organic samples, it requires very good mineralization of the organic matrix.All these shortcomings have led to various methods using electrothermal atomic absorption spectrometry for the determination of these elements. Since the above elements and many of their compounds are volatile, chemical modiÆers must always be added to the samples to prevent losses during the preliminary phases of the atomization process.Frech2 applied chromium and nickel salts as modiÆers for the determination of antimony. Ediger3 used nickel nitrate for the determination of arsenic and Gladney4 for the determination of bismuth. Later, when palladium became popular as a very versatile modiÆer, it was applied to the determination of arsenic,5 antimony6 and bismuth.7 Welz and co-workers8±10 suggested improving the activity of the palladium modiÆer by using a mixture with magnesium nitrate.Qiao and Jackson11 showed, using scanning electron microscopy, that an increase in pyrolysis temperature is accompanied by the formation of large palladium droplets, by increasing migration of palladium towards the edges of the platform and by broadening of the absorption peaks of the analyte. Addition of magnesium nitrate, owing to its very low melting temperature, eliminates all these effects. During the pyrolysis phase, magnesium causes a homogeneous distribution of palladium and the analyte in the form of small droplets in the centre of the platform.These results suggest the physical nature of the activity of magnesium used in a mixed modiÆer. On the other hand, the application of magnesium nitrate individually as modiÆer for the determination of many elements in sea-water12 and a comparison of its activity with that of palladium and palladium±magnesium mixture suggests that its chemical activity should also be considered.Kildahl and Lund13 found palladium nitrate modiÆer to be superior to nickel nitrate and nickel sulfate in the determination of arsenic and antimony. Various workers have used palladium modiÆer either as the chloride or the nitrate.14,15 In general, however, the mechanism of its action is not discussed and remains unknown. The effect of chloride was usually investigated by using sodium chloride and in most cases it was found to be negligible.Bermejo- Barrera et al.12 did not Ænd any effect of sodium chloride on the determination of arsenic in sea-water in the presence of palladium nitrate, mixed palladium and magnesium nitrates and reduced palladium up to a concentration of 20 g l21. The investigation of the effect of hydrochloric acid, which may be present in the sample solution, has been little studied although this acid is often used in a mixture with other acids for the dissolution of samples and is not always carefully removed.Such an effect on the determination of lead with palladium modiÆer16 indicates that palladium in the presence of hydrochloric acid may be partially transformed into the chloride. Palladium chloride decomposes during the pyrolysis phase and evolves free chlorine atoms, causing losses of lead. This observation affects the suggestion that palladium chloride may also be used as a modiÆer since in some situations, when during the drying of the sample a sufÆcient amount of hydrochloric acid is present, it can become an interferent.In general, one still observes many inconsistencies in the description of modiÆer effects. The mechanism of their action is usually unknown and seldom discussed. Most observations have a purely empirical character and concern a given element, a given type of sample or given atomisation conditions. Therefore, they are hardly transferable to the other analytical systems. The aim of these studies was to test systematically the behaviour of arsenic, antimony and bismuth during atomization in a graphite tube with a platform in the presence of palladium and palladium±magnesium modiÆers.The effect of nitric, hydrochloric and perchloric acid and their mixtures was also tested. Experimental Apparatus A Thermo Jarrell Ash (Franklin, MA, USA) SH 4000 atomic absorption spectrometer, equipped with a Model 188 controlled furnace atomizer (CTF 188) and Smith±Hieftje background correction system, was used.A Visimax II (Thermo Jarrell Ash) hollow-cathode lamp for arsenic, antimony and bismuth and pyrolytic graphite-coated graphite tubes with a pyrolytic graphite platform were used for all measurements. J. Anal. At. Spectrom., 1999, 14, 1755±1760 1755 This Journal is # The Royal Society of Chemistry 1999Reagents All solutions were prepared from high purity analytical-reagent grade compounds using ultra-pure water (resistivity 18 MV cm21) obtained with a Milli-Q water puriÆcation system (Millipore, Bedford, MA, USA).Hydrochloric and nitric acid used to prepare all solutions were puriÆed by sub-boiling distillation using a quartz subboiling apparatus (Kuerner Analysentechnik, Rosenheim, Germany). Perchloric acid (70%, Suprapur) was obtained from Merck (Darmstadt, Germany). Arsenic, antimony and bismuth stock standard solutions (10 mg ml21) were prepared by dissolution of As2O3, Sb2 O3 and Bi2O3 in nitric or hydrochloric acid and dilution with water to 1 mol l21 acid concentration. Palladium chloride solution (20 mg ml21 Pd) was prepared by dissolution of palladium chloride in hydrochloric acid and dilution with water to 1 mol l21 acid concentration.Palladium nitrate solution (20 mg ml21 Pd) was prepared by dissolution of palladium metal sponge in nitric acid and dilution with water to 1 mol l21 acid concentration. Magnesium, iron(III), calcium, nickel and cobalt nitrate stock standard solutions (50 mg ml21 of the element) were prepared by dissolution of the oxides in nitric acid and dilution with water to 1 mol l21 acid concentration.Magnesium and iron(III) chloride solutions (50 mg ml21 of the element) were prepared by dissolution of the oxides in water and dilution with hydrochloric acid to 1 mol l21 acid concentration. Sodium chloride solution (50 mg ml21 Na) was prepared by dissolution of sodium chloride in water. Procedure In all experiments, the integrated absorbance of arsenic, antimony and bismuth was measured.The applied measurement parameters are presented in Table 1 and the atomisation time±temperature program in Table 2. In the experiments, if not indicated otherwise, one of the following two types of solutions were always used: type 1, 50 ng ml21 of an analyte (arsenic, antimony or bismuth) in 1 mol l21 nitric or hydrochloric acid or in their 1z1 mixture (solutions without modiÆers); type 2, 50 ng ml21 of an analyte (arsenic, antimony or bismuth) and 300 mg ml21 of a modiÆer [palladium, magnesium, iron(III) or a mixture of palladium and magnesium] in 1 mol l21 nitric or hydrochloric acid or in their 1z1 mixture (solutions with modiÆers).When a mixture of palladium and magnesium was applied then the concentration of each of them was equal to 300 mg ml21. ModiÆers were introduced into hydrochloric acid solution in the form of chlorides and into nitric or mixed nitric±hydrochloric acid solution in the form of nitrates.Using an Eppendorf pipette, 10 ml aliquots were always injected into the tube, corresponding to amounts of 0.5 ng of analyte and 3 mg of modiÆer. Results and discussion Although arsenic, antimony and bismuth belong to the same group of the periodic system and for determination of the Ærst two of them almost identical procedures are often proposed, their volatility and behaviour in the presence of modiÆers during atomisation in a graphite tube show signiÆcant differences.Therefore, the individual discussion of the observed phenomena for each of these elements is justiÆed. Arsenic Arsenic in nitric acid solution dropped on the platform in the graphite tube is thermally stable up to almost 600 �C (Fig. 1, curve A). Above this temperature one observes losses, increasing gradually with increase in temperature. This suggests that arsenic, present in the solution in the form of arsenic acid, is decomposed during the drying and early pyrolysis stages to oxide and successively reduced to elemental arsenic that sublimes at temperatures above 600 �C.Palladium nitrate added to the solution extends the range of arsenic stability up to 1100 �C (curve D). Such behaviour agrees with commonly known facts and justiÆes the application of palladium as modiÆer. In hydrochloric acid solution arsenic is present in the form of chloride and it is lost almost completely during the drying stage below 200 �C (curve B). This result corresponds to the previous observations of Galban et al.17 Krivan and Arpadian,18 using radiotracers and uncoated graphite tubes, obtained different results.They found that arsenic is stable during the drying of the sample and it is lost during pyrolysis in the temperature range 200± 600 �C. Their results suggest that the arsenic solution in hydrochloric acid contained some amount of nitrate ions, and this is discussed below. In our investigations, the addition of palladium modiÆer (in the chloride form) to the hydrochloric acid solution did not improve the situation (curve E), indicating that palladium in hydrochloric acid is not able to form any stable compound with arsenic at low temperature.The situation becomes much more complex when arsenic is in a mixed solution of nitric and hydrochloric acid (Fig. 1, curve C). In this solution arsenic is partially transformed into the chloride and lost during the evaporation and drying of the sample below 200 �C.The residue is in the form of arsenic acid and, therefore, the shape of curve C is similar to that of curve A. An unexpected phenomenon occurs when palladium modiÆer is added to the mixed solution (curve F). Instead of stabilisation of the measured arsenic signals at higher temperatures, one observes their signiÆcant suppression. The Table 1 Parameters for the determination of arsenic, antimony and bismuth Parameter Arsenic Antimony Bismuth Wavelength/nm 193.70 217.60 223.10 Bandpass/nm 2.0 0.4 0.4 Background correction Smith±Hieftje Smith±Hieftje Smith±Hieftje Signal pulse lamp current/mA 5.0 6.0 6.0 Background pulse lamp current/mA 3.5 2.5 3.0 Table 2 Temperature±time program Atomization Drying Pyrolysis 1 Pyrolysis 2 As Sb Bi Cleaning Temperature/�C 120 900 200 2100 2200 2100 2400 Ramp/s 40 10 10 0 0 0 – Hold/s 10 5 0 4 4 4 2 Purge Ar Low Medium Medium Off Off Off Medium 1756 J.Anal. At. Spectrom., 1999, 14, 1755±1760difference between curves C and F is particularly large above 400 �C. This range corresponds to the temperature of the decomposition of palladium chloride to elemental palladium and free chlorine (about 500 �C).It suggests that the evolved chlorine forms the volatile chloride with arsenic, which is lost during the pyrolysis stage. The relatively small difference between the curves below 400 �C is due to the fact that palladium chloride is not decomposed during the pyrolysis stage but at the beginning of the atomization stage.Under these conditions, however, within a very short time of operation and with the `gas stop' function switched on, only a small amount of the arsenic chloride formed is able to leave the tube and be lost. The rest is decomposed again to free atoms and measured. Curve A in Fig. 2 demonstrates the effect of various amounts of hydrochloric acid added to a solution of arsenic in 1 Mnitric acid in the presence of palladium. It shows that even a very low concentration of hydrochloric acid (0.05 M) added to the solution causes the loss of almost all the arsenic during pyrolysis.Curve B was obtained under identical conditions, except that palladium nitrate was not added to the sample solution but introduced directly onto the platform in the tube and pyrolysed at 900 �C to the form of elemental palladium. Then the sample was dropped into the tube and atomized in the usual way. In this case, the losses caused by hydrochloric acid during the drying stage are relatively small, up to about 30% (compare also the difference between curves A and C at 200 �C in Fig. 1). They achieve a constant level at a concentration of hydrochloric acid of 0.2 M. The difference between curves A and B in Fig. 2 is due to the formation of palladium chloride in the sample solution containing free hydrochloric acid. Free chlorine, evolved in the decomposition of this chloride during pyrolysis, causes almost total loss of arsenic.The results obtained in the presence of preliminarily pyrolysed palladium chloride provide further conÆrmation of the mechanism proposed above (Fig. 3). When the sample without modiÆer is introduced into the tube on the palladium chloride preliminarily pyrolysed at temperatures below 600 �C, one observes signiÆcant suppression of the signal. This suppression decreases with increase in the temperature of preliminary pyrolysis and it disappears at 600 �C when all the palladium has been reduced to the metal.A similar suppression effect occurs when arsenic is preliminarily pyrolysed with palladium from nitric acid solution at 900 �C. Then the process is stopped, an additional amount of palladium in the form of chloride is added to the tube and the normal atomization process is carried out from the beginning (Fig. 4, curve A). The suppression effect is proportional to the amount of added palladium chloride up to 3 mg and above that level it quickly approaches saturation.When the additional palladium is added in the form of palladium nitrate the suppression is not observed (curve B). The last two experiments indicate unambiguously that palladium chloride, present in the sample during the pyrolysis stage, always causes losses of arsenic even if the latter is already bound in a refractory arsenic±palladium compound. It should be emphasised that the above effects are observed only in the presence of palladium chloride or palladium nitrate and free hydrochloric acid.Addition of chlorides of various metals to the arsenic and palladium nitrate modiÆer solution in nitric acid is without effect, even if the amount of the introduced chloride ions is several times larger than that introduced in the form of free acid. This probably results from the fact that the afÆnity of chloride ions to these metals is higher than that to palladium and, therefore, during the drying Fig. 1 Pyrolysis curves for arsenic: A, in nitric acid; B, in hydrochloric acid; C, in a mixture of nitric and hydrochloric acid (1z1); D, in nitricE, in hydrochloric acid with palladium nitrate; and F, in a mixture of nitric and hydrochloric acid with palladium nitrate.Fig. 2 Effect of hydrochloric acid concentration on the arsenic signal: A, in nitric acid with palladium nitrate; B, in samples with various concentrations of HCl introduced into the tube on palladium preliminarily pyrolysed at 900 �C; and C, in nitric acid with palladium and magnesium nitrate. Fig. 4 Effect of additional portions of palladium added to the preliminarily pyrolysed arsenic with palladium: A, palladium added as chloride; and B, palladium added as nitrate. Fig. 3 Effect of preliminary pyrolysis temperature of PdCl2 on arsenic solution in nitric acid. J. Anal. At. Spectrom., 1999, 14, 1755±1760 1757stage, when water and free acids are being removed, palladium chloride is not formed.Moreover, all these chlorides are either volatile as chloride or decompose at high temperatures during atomization stage. The only exception was found for iron(III) chloride, which at 315 �C is transformed into iron(II) chloride with evolution of free chlorine. Its effect in the presence of hydrochloric acid is similar to that of palladium chloride. In this case, however, the situation is complex since iron, at least in the nitrate form, is also a good modiÆer for arsenic determination.However, when it is present together with palladium it causes serious suppression of the arsenic signal. This last problem will be discussed later. It was mentioned above that arsenic±palladium compounds, formed during the pyrolysis stage, are sensitive to the action of chlorine evolved from palladium chloride. The exact investigation of these compounds was carried out in the following way. Arsenic together with palladium nitrate in the nitric acid solution was dried and pyrolysed in the tube at various temperatures in the usual way.At the end of the pyrolysis stage the process was stopped and the gaseous hydrochloride or gaseous chlorine was introduced to the tube using a plastic syringe with a quartz needle. Then the pyrolysis was repeated at a constant temperature of 400 �C with the `gas stop' function switched on, followed by the atomization stage. The results obtained in the presence of gaseous hydrochloride are represented by curve A in Fig. 5.The arsenic± palladium compound (probably palladium pyroarsenite, PdAs2O5) obtained during the drying stage below 200 �C is almost completely lost during the second pyrolysis in the atmosphere of gaseous hydrochloride. When the temperature of the Ærst pyrolysis is increased, the compound is transformed into a form more resistant against gaseous hydrochloride and above 600 �C losses of arsenic are not observed. As the second pyrolysis is carried out in an atmosphere of gaseous chlorine, the losses of arsenic are very high and independent of the type of arsenic±palladium compound formed (curve B).These results conÆrm in an indirect way the previous hypothesis that the losses of arsenic occurring during the pyrolysis stage at temperatures above 400 �C can be caused only by free chlorine evolved in the decomposition of palladium chloride. All problems resulting from the losses of arsenic in the presence of some amount of hydrochloric acid can be completely removed by using the mixed palladium±magnesium nitrate modiÆer, as demonstrated by curve C in Fig. 2. Comparison of the pyrolysis curves obtained with palladium and palladium±magnesium modiÆers shows that with the latter arsenic is stabilised towards higher temperature only to an insigniÆcantly greater extent (Fig. 6, curves A and B) than with palladium alone. The above data seem to conÆrm the recommendation of a mixed palladium±magnesium modiÆer as being very versatile. 8±10 However, the question arises of what the role of magnesium nitrate is. Does it support only the activity of palladium or does it play a more independent role? The pyrolysis curve for arsenic (Fig. 6, curve C) obtained in the presence of magnesium nitrate alone, without palladium, has the same character as that obtained with palladium alone (curve A). An identical curve is obtained for arsenic in hydrochloric acid solution when magnesium nitrate is used.The addition of palladium improves only insigniÆcantly the stability of arsenic compounds in the range above 1000 �C, as demonstrated by the difference between curves B and C. These experiments prove that magnesium nitrate plays the role of an independent modiÆer for arsenic and its activity is comparable to that of the mixed modiÆer in all types of solution. The identical behaviour of the magnesium modiÆer and the mixed magnesium±palladium modiÆer indicates that, at least in the mixed nitric±hydrochloric acid medium, magnesium plays the main role.It forms with arsenic, during the evaporation of the sample solution, a compound resistant to gaseous hydrochloride and chlorine during the pyrolysis stage in the temperature range up to 1000 �C (Fig. 5, curves C and D). Moreover, the excess of magnesium nitrate decomposes at a low temperature to magnesium oxide, which can act as a trap for free chlorine evolved during pyrolysis. Magnesium nitrate alone is superior to palladium used individually as modiÆer when the sample solution contains hydrochloric acid.It should be emphasized, however, that magnesium is active only as the nitrate and, therefore, it may be applied only when an amount of nitrate ions is present in the sample solution. The dependence of magnesium activity on the concentration of nitrate ion is shown in Fig. 7 and indicates that magnesium achieves its full activity as modiÆer already at a low concentration of nitrate ions of about 0.05 mol l21.Exactly the opposite situation is observed for palladium, which loses its modifying properties and becomes an interferent in the presence of a small amount of hydrochloric acid (Fig. 1, curve F and Fig. 2, curve A). Another acid often used in the preparation of samples for analysis, particularly for the mineralization of organic matter, is perchloric acid. It is well known as a strong interferent in electrothermal AAS. Its effect on the arsenic signal cannot be eliminated by application of either palladium (Fig. 8, curve A) or the mixed palladium±magnesium modiÆer (curve B). Application of magnesium nitrate removed this effect completely (curve C). It was mentioned earlier that in nitric acid medium iron is also a good modiÆer for arsenic. The pyrolysis curve (Fig. 6, curve D) obtained in its presence is similar to those in the presence of palladium, magnesium and the mixture of palladium and magnesium.However, if some amount of Fig. 5 Effect of the preliminary pyrolysis temperature on the arsenic signal obtained in the presence of gaseous hydrochloride or chlorine at 400 �C: A, arsenic with palladium in the presence of hydrochloride; B, arsenic with palladium in the presence of chlorine; C, arsenic with magnesium in the presence of hydrochloride; and D, arsenic with magnesium in the presence of chlorine. Fig. 6 Pyrolysis curves for arsenic in nitric acid: A, with palladium nitrate; B, with palladium nitrite and magnesium nitrate; C, with magnesium nitrate; and D, with iron(III) nitrate. 1758 J. Anal. At. Spectrom., 1999, 14, 1755±1760hydrochloric acid is present in the sample solution, one observes the suppression of the measured signals and the atomization peaks become widened and deformed, which makes them difÆcult to interpret. This effect may be eliminated by addition of magnesium nitrate. If iron modiÆer is applied in combination with palladium, then always, independently of the type of sample solution, a signiÆcant suppression and widening of the atomization peaks occur and the magnitude of the observed deformation is proportional to the amount of iron used.Addition of magnesium nitrate does not remove this effect. It may be partially eliminated only by increasing the atomization temperature (Fig. 9), but its total elimination does not occur. This suggelusmn;iron±palladium triple compounds are formed and they are much more refractory than those of arsenic±palladium.An identical effect is observed in the presence of cobalt and nickel. The above phenomenon indicates the limitation of the application of the palladium and mixed palladium±magnesium modiÆers to samples containing the metals of the iron group. This limitation does not concern the magnesium modiÆer. Antimony The behaviour of antimony is similar to that of arsenic, with one exception. Without modiÆers it is stable at a low temperature in hydrochloric acid medium and its pyrolysis curves obtained for nitric and hydrochloric acid solutions are almost identical (Fig. 10, curves A and B). This suggests that antimony exists in both media in the form of antimonous acid. In nitric acid the addition of palladium stabilizes it up to 1000 �C (curve C). Addition of palladium to hydrochloric acid solution causes signiÆcant losses of antimony at temperatures above 400 �C (curve D). The small losses observed in this case for pyrolysis at 200 �C are due to the activity at the beginning of the atomization stage of the palladium chloride that was not decomposed during pyrolysis.Curve E obtained with palladium in the mixed nitric and hydrochloric acid solution is similar, but the losses of antimony at 200 �C are smaller since only part of the palladium was transformed into chloride. All observed interferences may be removed by the use of either magnesium nitrate or mixed palladium±magnesium modiÆer.Unlike arsenic (compare Fig. 2, curve B), antimony is stable during the evaporation of hydrochloric acid solution up to 400 �C (Fig. 10, curve B). Therefore, it may be determined in such a solution with palladium modiÆer pyrolysed preliminarily at a temperature above 600 �C and reduced to the metal. Palladium in this form is resistant to hydrochloric acid introduced into the tube with the sample and palladium chloride, which causes the losses of antimony, cannot be formed.All this concerns, however, only the pure hydrochloric acid medium. If antimony is introduced into the tube on the pyrolysed palladium in the mixed nitric and hydrochloric acid medium, the palladium metal may be partially dissolved. Then some amount of palladium chloride is formed, giving losses of antimony proportional to the concentration of hydrochloric acid in the mixed medium. Perchloric acid and the iron group elements interfere in the determination of antimony with palladium or mixed palladium ±magnesium modiÆer in the same way as described for arsenic. These interferences do not occur when magnesium nitrate alone is applied. Bismuth The effect of the investigated modiÆers on bismuth is radically different from their effects on arsenic and antimony.Bismuth without modiÆers is stable during pyrolysis up to 600 �C Fig. 7 Effect of nitric acid concentration on the signal of arsenic atomized with magnesium from hydrochloric acid.Fig. 8 Effect of perchloric acid on the arsenic signal in nitric acid: A, with palladium nitrate; B, with palladium and magnesium nitrate; and C, with magnesium nitrate. Fig. 9 Atomization peaks of arsenic with palladium in nitric acid with 1 mg ml21 Fe(III) as nitrate: A, without iron at 2100 �C; B, with iron at 2100 �C; C, with iron at 2300 �C; and D, with iron at 2500 �C. Fig. 10 Pyrolysis curves for antimony: A, in nitric acid; B, in hydrochloric acid; C, in nitric acid with palladium; D, in hydrochloric acid with palladium; and E, in a mixture of nitric and hydrochloric acid (1z1) with palladium.J. Anal. At. Spectrom., 1999, 14, 1755±1760 1759(Fig. 11, curves A and B). Palladium added to the nitric acid solution suppresses its signal by about 30% but the atomization peaks obtained are very well shaped and reproducible. This indicates the formation of a refractory bismuth±palladium compound stable up to above 1100 �C (curve C).The observed suppression of the bismuth signal cannot be explained on the basis of our earlier experiments. The fact that the peaks obtained do not change their shape or magnitude with increase in the atomization temperature up to 2500 �C indicates that it cannot be explained simply as the result of a trapping effect, as described by Frech et al.19 Addition of palladium to hydrochloric acid solution causes losses of bismuth in the range between 400 and 600 �C (curve D), similarly as was observed for antimony.For bismuth, however, the atomization peaks are very badly shaped and irreproducible. Exactly the same phenomenon is observed for mixed nitric±hydrochloric acid solution. Unlike arsenic and antimony, the application of the mixed palladium±magnesium modiÆer or magnesium nitrate alone for bismuth does not improve the situation. Bismuth may be determined in hydrochloric solution with palladium modiÆer only when palladium is preliminarily pyrolysed to the metal form, as described for antimony.This does not concern, however, mixed nitric±hydrochloric acid solution. Addition of perchloric acid or the elements of the iron group to bismuth solution in nitric acid containing palladium modiÆer gives the same interferences as those observed for arsenic and antimony. For bismuth, however, these effects cannot be eliminated by replacement the palladium modiÆer with magnesium. In general, it may be stated that magnesium nitrate, applied either alone or in combination with palladium, is absolutely inert in the relation to bismuth. Conclusion Palladium is a really good modiÆer for the determination of arsenic, antimony and bismuth only when the sample to be analysed is dissolved in nitric acid.If the solution contains some amount of hydrochloric acid it may become a strong interferent and, in extreme cases, it can cause the total loss of these elements during the pyrolysis stage. This effect is due to the transformation of palladium, at least partially, into the chloride that decomposes at about 500 �C, evolving free chlorine atoms.At this temperature chlorine is able to form very volatile chlorides with the analyte atoms, even if they are already bound in analyte±palladium compounds. In relation to arsenic and antimony this effect can be easily eliminated by use of a mixed palladium±magnesium modiÆer or magnesium nitrate modiÆer instead of palladium. Both of them work correctly when only some amount of nitrate ions is present in the sample solution. However, they are ineffective for bismuth in the mixed nitric±hydrochloric acid solution.The fact that magnesium alone is an equally good modiÆer as the palladium±magnesium mixture indicates that it works by itself. The suggestion of some workers that its role is limited to supporting the palladium activity seems to be only marginally correct. The protection of arsenic against its losses during the evaporation of the sample solution containing a large excess of hydrochloric acid indicates that magnesium forms with arsenic a compound (probably magnesium arsenite) already in the solution.This compound is resistant to hydrochloric acid and later, during the pyrolysis stage, it is transformed into a more refractory form (probably mixed magnesium arsenic oxide) that is resistant to chlorine gas evolved in the decomposition of palladium chloride. The completely inert behaviour of magnesium towards bismuth shows that the hypothesis of the action of magnesium oxide as a trap for chlorine atoms is unjustiÆed. In such a case magnesium should protect also the elements with which it is not able to form compounds in solution, e.g., bismuth.Magnesium alone used as modiÆer for the determination of arsenic and antimony is superior to the mixture of magnesium with palladium. It is able to protect these elements not only against the effect of hydrochloric acid but also against the effect of perchloric acid and the iron group elements when they are present in the sample. Antimony and bismuth can be determined in hydrochloric acid solution using palladium modiÆer only innitrate or chloride should be prelimi narily pyrolysed in the tube to the metal form, then the sample solution should be added and atomized in the usual way.Acknowledgement Financial support of this research by the Committee for ScientiÆc Research as Project No. 3 TO9A141 11 is greatly appreciated. References 1 D. L. Tsalev, J. Anal. At. Spectrom., 1999, 14, 147. 2 W. Frech, Talanta, 1974, 21, 565. 3 R. D. Ediger, At. Absorpt. Newsl., 1975, 14, 127. 4 E. S. Gladney, At. Absorpt. Newsl., 1977, 16, 114. 5 X.-Q. Shan, Z.-M. Ni and L. Zhang, Anal. Chim. Acta, 1983, 151, 179. 6 X.-Q. Shan and Z.-M. Ni, Acta Chim. Sin., 1981, 39, 575. 7 L.-Z. Jin and Z.-M. Ni, Can. J. Spectrosc., 1981, 26, 219. 8 B. Welz, G. Schlemmer and J. R. Mudakavi, J. Anal. At. Spectrom., 1988, 3, 93. 9 B. Welz, G. Schlemmer and J. R. Mudakavi, J. Anal. At. Spectrom., 1988, 3, 695. 10 B. Welz, G. Schlemmer and J. R. Mudakavi, J. Anal. At. Spectrom., 1992, 7, 1257. 11 H. Qiao and K. W. Jackson, Spectrochim. Acta, Part B, 1991, 46, 1841. 12 P. Bermejo-Barrera, J. Moreda-Pineiro, A. Moreda-Pineiro and A. Bermejo-Barrera, J. Anal. At. Spectrom., 1998, 13, 777. 13 B. T. Kildahl and W. Lund, Fresenius' J. Anal. Chem., 1996, 354, 93. 14 T. M. Rettberg and L. M. Beach, J. Anal. At. Spectrom., 1989, 4, 427. 15 V. I. Slaveykowa, F. Rastegar and M. J. F. Leroy, J. Anal. At. Spectrom., 1996, 11, 997. 16 L. Pszonicki and A. M. Essed, Chem. Anal. (Warsaw), 1993, 38, 759. 17 J. Galban, E. Marcos, J. Lamana and J. R. Castillo, Spectrochim. Acta, Part B, 1993, 48, 53. 18 V. Krivan and S. Arpadian, Fresenius' Z. Anal. Chem., 1989, 335, 743. 19 W. Frech, L. Ke, M. Berglund and D. C. Baxter, J. Anal. At. Spectrom., 1992, 7, 141. Paper 9/905984I Fig. 11 Pyrolysis curves for bismuth: A, in nitric acid; B, in hydrochloric acid; C, in nitric acid with palladium; and D, in hydrochloric acid with palladium. 1760 J. Anal. At. Spectrom., 1999, 14, 1755±1760

ISSN:0267-9477    

DOI:10.1039/a905984i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Continuous extraction with acidiÆed subcritical water of arsenic, selenium and mercury from coal prior to on-line derivatisation-atomic Øuorescence detection V. Ferna�ndez-Pe�rez, M. M. Jime�nez-Carmona and M. D. Luque de Castro Analytical Chemistry Division, Faculty of Sciences, University of Co�rdoba, E-14004 Co�rdoba, Spain. E-mail: qa1lucam@uco.es Received 27th July 1999, Accepted 9th September 1999 AcidiÆed subcritical water is proposed for the continuous extraction of minor pollutants (namely, selenium, arsenic and mercury) from coal prior to continuous derivatisation (by hydride formation for Se and As, and cold vapor formation for Hg) and determination by atomic Øuorescence.Coal samples (3 g) were subjected to a 15 min static extraction followed by a 90 min dynamic extraction with water modiÆed with 4% (v/v) HNO3 for both steps. An in-depth study of the variables affecting the continuous leaching step, as well as those referring both to preconcentration (for mercury), derivatisation and detection (all of them) was performed.The linear ranges of the calibration curves for all analytes were at the ng ml21 level, with correlation coefÆcients, r2, better than 0.999 for Se and As and better than 0.99 for Hg. The method was validated by using a bituminous coal reference material (NIST SRM 1635). The good precision of the method [RSDs (n~6) of 12.0, 4.7 and 6.5% for As, Se and Hg, respectively], together with its safety and rapidity make it a good alternative for the determination of these analytes in coal.Introduction Coal combustion has been identiÆed as a potential source of pollution from volatile trace metals. Studies1 on the fate of such elements during combustion have shown that up to 85, 60 and 55% of the mercury, arsenic and selenium, respectively, originally present in the coal could not be accounted for in the waste streams examined. Other research2 has suggested that almost all of the arsenic and selenium present in the coal appears in the Øy ash in the exhaust gases.Arsenic is a metalloid used in many industrial processes and applications, thus making possible the contamination of water, soil and food.3 The toxic effect of mercury compounds has for long been recognised. Selenium is a metalloid present in the environment, mainly as a result of human activity, whose content in environmental materials has had to be established because of its ambivalent character (both toxic and essential). 4,5 Thus, due to the high toxicity and proved inØuence in the environment of these analytes, it is mandatory to search for a reliable quantitative determination for them in coal. Several methods have been applied for arsenic, selenium and mercury determination in coal. Spectrophotometry,6,7 X-ray Øuorescence spectrometry,8 colorimetry9 and atomic absorption spectrometry10 have been applied to the determination of arsenic in coals. Selenium is present in coal as both organic and inorganic compounds and it has been determined by Øuorimetry, 11,12 activation analysis13 and atomic absorption spectrometry. 14,15 In the case of mercury, the methods described have been based on colorimetry±Øuorimetry,16 gas chromatography, 17 neutron activation18 and atomic absorption spectrometry. 19 Other methods for the analysis of coals have been developed, such as slurry sampling with graphite furnace AAS.20,21 The methods thus developed for the analysis of trace elements in coal involve pretreatment of the matrix prior to the measurement step, consisting mainly of an ashing procedure, used especially by standard methods.22 In recent years, the digestion step has been carried out under very drastic conditions, this being thought the best way to achieve quantitation.23 The environmentally aggressive character of these methods, as well as their slowness, makes the search for analytical alternatives urgent.Subcritical water extraction, the technique based on the use of water as an extractant, at temperatures between 100 and 374 �C and pressures high enough to maintain the liquid state, is emerging as a powerful alternative for solid sample extraction.Thus, subcritical water extraction has been used for the extraction of organic pollutants within a wide range of polarities from environmental samples.24±28 The aim of this research has been to develop a rapid, clean and efÆcient method for extraction of trace metals from coal, based on continuous subcritical water29±31 extraction modiÆed if required, and the joint use of static and dynamic extraction.Experimental Apparatus The acidiÆed subcritical water extractions were performed using a prototype extractor (designed by Salvador and Mercha�n32) consisting of a stainless steel cylindrical extraction chamber (150611 mm id), closed with screws at either end, that permit the circulation of the leaching Øuid through them. Both screw caps contain stainless steel Ælter plates (2 mm in thickness and 1/4 in id) to ensure that the sample remains in the extraction chamber.The chamber, together with a stainless steel preheater, is located in an oven designed to work up to 300 �C and controlled by a Toho TC-22 temperature controller. A cooler system (consisting of a loop made from 1 m length stainless steel tubing and cooled with water at room temperature) was used to cool the extract from the oven to a temperature close to 25 �C.A Shimadzu (Tokyo, Japan) LC10AD pump with digital Øow-rate and pressure readouts was used to impel the extractant through the system. An Excalibur-Merlin atomic Øuorescence detector (PS Analytical, Orpington, Kent, UK) Ætted with boosted discharge hollow cathode lamps for Se, As and Hg, two Gilson Minipuls-3 peristaltic pumps (Gilson, Middleton, WI, USA) Ætted with rate selectors, two gas±liquid separators (for hydride J. Anal. At. Spectrom., 1999, 14, 1761±1765 1761 This Journal is # The Royal Society of Chemistry 1999generation and mercury separation), two Rheodyne (Cotati, CA, USA) 504 injection valves and PTFE tubing of 0.5 mm inner diameter were used to build the Øow injection (FI) manifold for the detection step. A Knauer recorder (Bad Harzburg, Germany) was used in order to record the signals. Reagents and solutions Ultrapure water from a Milli-Q system (Millipore, Bedford, MA, USA) was used throughout.Water modiÆed with 4% v/v nitric acid was used as the extractant. A 2% w/v NaBH4 solution (Sigma-Aldrich, Deisenhufen, Germany) in 0.1 mol l21 NaOH (Merck, Darmstadt, Germany) was used for selenium and arsenic derivatisation and 5% SnCl2 was used as the reagent in the mercury derivatisation step; 6.0, 3.0 and 0.3 mol l21 HCl solutions were used for Se and As reduction, hydride generation and mercury reduction, respectively. Solutions 561022% of 1-pyrrolidinecarbodithioic acid ammonium salt (APDC, Aldrich, Milwaukee, WI, USA) in 30 mmol l21 ammonium acetate (Merck)±acetic acid (Panreac, Barcelona, Spain) buffer, pH 6.5, and ethanol were used as complexing agent and eluent, respectively, for the mercury preconcentration step.The preconcentration column was packed with C18 bonded phase, obtained from Bond Elut cartridges (Varian, Harbor City, CA, USA). Argon (Carburos Meta� licos, Barcelona, Spain) was used to Øush the analytes to the detector.A stock solution of 1 g l21 AsIII was prepared by dissolving 1.320 g of As2O3 (Merck) in 25 ml of 20% m/v KOH solution, neutralizing with 20% H2SO4 and diluting to 1 l with 1% v/v H2SO4. One g l21 solution of SeIV (Merck) and HgII (Aldrich) were prepared in 3 mol l21 HCl and 1% HNO3, respectively. All reagents were of analytical grade and prepared daily. Procedure Extraction step. Coal (3 g) was placed in the extraction cell. After assembling the cell and locating it in the oven, this was brought up to the work temperature (180 �C) and pressurised with y50 bar by opening the inlet valve from the pump.The valve was then closed and static extraction was developed for 15 min. Both the inlet and outlet valves were then opened. AcidiÆed water oven at a Øow-rate of 2.5 ml min21 and the extract was collected in a vial after being cooled in the refrigerant at 25 �C. For kinetic experiments, volumes of 40 ml of extracts were collected at intervals of 16 min.Derivatisation and detection steps. Two aliquots from each extraction were subjected to different treatments, one for selenium and arsenic and the other for mercury. Reduction step (AsV and SeVI). These analytes are in their highest oxidation states as a consequence of the presence of nitric acid in the extractant. They were reduced to AsIII and SeIV from which the corresponding hydride was generated. The reduction step was performed by adding 12.5 ml of 6.0 mol l21 HCl to 12.5 ml of extract and heating the mixture at 75 �C for 45 min.33 Determination of AsIII and SeIV.Once the sample had been reduced, it was injected into a 3.0 mol l21 HCl carrier stream which then merged with a 2% NaBH4 stream [see Fig. 1(a)]. The volatile hydride was formed and swept out of the gas± liquid separator by an argon stream into the chemically generated hydrogen diffusion Øame, which was maintained by the excess of hydrogen produced in the reaction between NaBH4 and HCl.The hydride was atomised in the Øame and detected by atomic Øuorescence spectrometry. Preconcentration±detection step (Hg). The low concentration of mercury made necessary the development of a preconcentration34,35 step in an on-line Øow injection (FI) system. The manifold used in this step is shown in Fig. 1(b). A solution of 561022% chelating agent (APDC) was prepared in 30 mmol l21 ammonium acetate adjusted to pH 6.5 with 2 mmol l21 acetic acid.APDC (2.5 ml) was added to 100 ml of extract. The sample (or the standard solution) was aspirated and allowed to pass through the preconcentration column for 15 min. Then, the two injection valves were simultaneously switched to the injection position and the retained complex was eluted with an ethanol stream. The HgII was reduced to HgO by an SnCl2 stream, swept out of the gas±liquid separator by an argon stream into the atomic Øuorescence detector, and the signal recorded.The volume of sample injected was in all cases the inner volume of the injection valve (50 ml). Results and discussion Optimisation The experimental variables were optimised with the following aims: (a) to achieve quantitative extraction of the analytes; (b) to shorten the extraction time as much as possible; (c) to reduce both extractant consumption, in order to save reagent (nitric acid), and waste; and (d) to decrease analyte dilution. With these aims a hybrid discontinuous/continuous extraction method was intended.The univariate method was used in all instances. Ranges over which the effect of the variables was studied and the optimum values found are shown in Table 1. Extraction variables. The variables concerning leaching (namely temperature, concentration and composition of the leacher) were studied in order to Ænd the optimum conditions for this step. Three g of coal, a static extraction time of 10 min and a dynamic extraction time of 30 min were used for optimisation experiments.Since the extractor was not Ætted with a pressure controller, a pressure of 50 bar (created in the system by the working conditions) was used in all instances. The temperature of the extraction chamber was studied by performing a 30 min dynamic extraction with water modiÆed with nitric acid to 4% (v/v). The stainless steel extractor allowed the extraction to be performed up to 180 �C. An increase in the extraction efÆciency was observed when the temperature increased.Temperatures higher than 180 �C did not permit Fig. 1 Flow injection manifolds for the derivatisation±determination of arsenic and selenium (a), and for preconcentration±derivatisation of mercury (b). PP, peristaltic pump; IV, injection valve; W, waste; PC, preconcentration column; MC, mixing coil; RC, reaction coil; GLS, gas±liquid separator; AFD, atomic Øuorescence detector; R, recorder. 1762 J. Anal. At. Spectrom., 1999, 14, 1761±1765us to maintain the acidiÆed water in liquid state because the pressure was not high enough.A temperature of 180 �C was selected as optimum for further experiments. The leacher composition was studied in previous work29 and HNO3 was selected as extractant. The leacher concentration, ranging from 1 to 5% (v/v), was studied too. The efÆciency of the extraction increased with more acidic media, but nitric acid concentrations higher than 4% gave rise to overpressure in the system which hindered extraction, so 4% nitric acid concentration was Æxed for subsequent experiments.The hydrodynamic variables, namely Øow-rates and static and dynamic extraction times, were also evaluated. The Øowrate of the leaching agent was studied, with an extraction time of 16 min and values of the variable from 0.5 to 5.0 ml min21 were investigated. The efÆciency increased with the Øow-rate up to 2.5 ml min21, and a drop in yield occurred for higher values. This trend could be due to a higher compactness with increased Øow-rates that inhibited sample±extractant contact. Thus, a Øow-rate of 2.5 ml min21 was selected as the optimum.Quantitative leaching of the analytes was attained by a static extraction time of 15 min followed by a dynamic extraction time of 90 min. Derivatisation±detection variables. The efÆciency of hydride generation was studied for different Øow-rates of HCl and NaBH4. The HCl stream was Æxed at 4.5 ml min21 to ensure a sufÆciently acidiÆed medium.A NaBH4 Øow-rate lower than 2.5 ml min21 gave rise to Øame extinction owing to the low hydrogen concentration produced. Therefore, a HCl Øow-rate of 4.5 ml min21 and a NaBH4 Øow-rate of 2.5 ml min21 were selected for further experiments. The NaBH4 concentration is an important parameter for hydride generation. This concentration was studied in an interval between 0.5 and 4% w/v in 0.1 mol l21 NaOH solution. A concentration higher than 2% w/v was required in order to obtain both an appropriate hydride formation and hydrogen generation; for a lower concentration than this the Øame was extinguished, while for NaBH4 concentration higher than 3.5% the instability of the Øame caused by an excess of hydrogen gave a poor reproducibility and the signal to noise ratio was smaller.When the FI system was optimised for mercury determination, the Øow-rates of SnCl2 and HCl were Æxed at 1 and 2.5 ml min21, respectively. The best results for the mercury determination were found for 5% SnCl2 and 0.3 mol l21 HCl.Different manifolds were tested for the mercury preconcentration step, and the maximum recovery was obtained with the values of the variables showed in Table 1. Discontinuous±continuous extraction versus continuous extraction. A comparison between hybrid discontinuous± continuous extraction (consisting of a prior static extraction step, followed by a dynamic extraction step in which the extractant was continuously passed through the extraction chamber) and continuous extraction (in which only the latter step was applied) was performed.Different static extraction times were tested with the aim both of increasing the efÆciency and minimising dilution of the sample, thus reaching quantitative extraction in a substantially lower extract volume. As an example, the inØuence of this step on the extraction kinetics of the analytes is shown in Fig. 2, where the kinetic curves for As and Se with and without application of a static extraction step can be observed.A range from 0 to 30 min static extraction time was studied. The Table 1 Optimisation of the method Type of variable Variable Range studied Selected value Extraction variables Temperature/�C 80±250 180 Pressure/bar – 50 Nitric acid (%) 1±5 4 Flow-rate/ml min21 0.5±5 2.5 Static extraction time 0±30 15 Dynamic extraction time 15±180 90 Derivatisation±deteles for As and Se.HCl Øow-rate/ml min21 0±5.0 4.5 NaBH4 Øow-rate/ml min21 0.6±4.5 2.5 [NaBH4] (%) 0±5 2 [HCl]/mol l21 0.5±4 3 Derivatisation±detection variables for Hg HCl Øow-rate/ml min21 1.8±5 3.5 SnCl2 Øow-rate/ml min21 0.5±3.5 1.0 [HCl]/mol l21 0±3.5 0.3 [SnCl2] (%) 0±10 5 Type of eluent H2O Acetonitrile±H2O Acetonitrile Ethanol Ethanol Type of complexing agent DDC APDC APDC [APDC] (%) 561024±0.5 561022 Preconcentration time/min 0±30 15 Eluent volume/ml 0.2±2 1.5 Fig. 2 Kinetic extraction curves obtained by acidiÆed subcritical water with and without static extraction for As (–+– 0 min and –r– 15 min of static extraction), and Se (–6– 0 min and –&– 15 min static extraction).J. Anal. At. Spectrom., 1999, 14, 1761±1765 1763efÆciency increased with time up to 15 min, observing no signiÆcant improvement in the kinetics for longer times. Thus, a static extraction time of 15 min was selected as it provided the best analytical performance. The application of a 90 min dynamic extraction time after 15 min static extraction time yielded quantitative extraction of the analytes.The absence of static extraction time hindered quantitative extraction of the analytes even after 2 h, which demonstrates the necessity of applying this hybrid extraction mode in order to achieve completeness of extraction in a relatively short time. Features of the method Calibration curves were obtained using eight individual standard solutions of the analytes.Table 2 shows the equation for the calibration curves, the linear range for each analyte at the ng ml21 level. The sensitivity of the detector was changed in order to cover a wide concentration range. For this reason, two different slopes of the calibration curve (one for each value of the sensitivity) were obtained for each analyte. The standards and samples were injected in triplicate into the FI system in all instances. The method shows a good linearity with correlation coefÆcients, r2, better than 0.999 for Se and As, and better than 0.99 for Hg.The precision of the derivatisation±detection step for each analyte, expressed as RSD%, is also shown in Table 2. As can be seen in the table, the repeatibility study (for n~11 and concentrations of 40 ng ml21 for selenium and arsenic and 0.1 ng ml21 and 100 ng ml21 for mercury with and without preconcentration, respectively), yielded values lower than 3.50 in all instances, but for HgII without preconcentration the value was 5.9 owing to the difÆculty in measuring small signals.Validation of the method The accuracy of the method was evaluated by analysing a bituminous coal reference material with a sulfur content of approximately 0.3% (NIST-SRM 1635). As can be seen in Table 3, the results obtained are in good agreement with the certiÆed values. Finally, the method was applied to three coals from different locations. The study of the precision of the whole process (including the extraction and detection steps), expressed as RSD, was carried out performing extraction of 3 g of coal under the optimum working conditions.The RSDs (n~6) for arsenic, selenium and mercury were 12.0, 4.7 and 6.5, respectively. All these results are shown in Table 3. Conclusions A clean and rapid method for the determination of trace pollutants (namely, selenium, arsenic and mercury) in coal based on continuous subcritical extraction with acidiÆed water (in a static±dynamic mode) as a step prior to preconcentration± derivatisation (for mercury), on-line derivatisation based on hydride formation for As and Se and atomic Øuorescence detection (for all) is proposed.The establishment of a static extraction step prior to the continuous dynamic extraction is the key to both avoiding the dilution effect caused by continuous passage of the extractant through the chamber and shortening the time required for complete extraction. The proposed method is thus quicker, cleaner and safer than other methods established with this aim.Acknowledgements The Spanish Comisio�n Interministerial de Ciencia y Tecnologý�a (CICyT) is thanked for Ænancial support (Project No. PB96- 1265). References 1 N. E. Bolton, J. A. Carter, J. F. Emery, C. Felman, W. Fulkerson, L. D. Hulett and W. S. Lyon, in Trace Elements in Fuel, Advances in Chemistry Series, ed. S. P. Babu, American Chemical Society, Washington, DC, USA, 1975. 2 G. T.Moore and V. J. Elia, 71st Annual Meeting of Pollution Control Association, Houston, Texas, 1978, 78-34.2. 3 R. M. Harrison and S. Rapsomanakis, Environmental Analysis Using Chromatography Interfaced with Atomic Spectrometry, Wiley, Chichester, UK, 1989. 4 H. A. Schroder, D. V. Frost and J. Balassa, J. Chronic Dis., 1970, 23, 227. 5 W. O. Robinson, J. Assoc. Off. Anal. Chem., 1993, 16, 423. 6 N. A. Kevin and V. E. Marincheva, Zavod. Lab., 1970, 36, 1061. Table 2 Features of the method Analyte Equation Linear range/ng ml21 Regression coefÆcient/r2 Repeatability RSD (%) (n~11) Detection limit/ng ml21 AsIII Range 1 y~45.9z1.40x 1±20 0.997 3.3 0.22 Range 2 y~62.9z0.43x 20±100 0.9993 SeIV Range 1 y~4.12z4.80x 1.5±30 0.9990 3.44 0.46 Range 2 y~87.1z0.82x 30±150 0.998 HgII ay~22.8z0.15x 0.01±1 0.992 5.9 0.015 by~7.20z0.25x 5±250 0.98 2.7 4.5 aWith preconcentration.bWithout preconcentration. Table 3 Application of the method to natural samples and CRM Sample Se/mg kg21 As/mg kg21 Hg/mg kg21 Coal A 2.12°0.09 3.15°0.34 0.128°0.006 Coal B 0.88°0.09 3.54°0.28 0.096°0.002 Coal C 2.26°0.07 6.27°0.31 0.059°0.003 RSD (%) 4.7 12 6.5 NIST-RSM 1635 CertiÆed value/mg kg21 Found value/mg kg21 Selenium 0.9°0.3 0.755°0.005 Arsenic 0.42° 0.15 0.350°0.056 Mercury 0.02 0.022°0.002 1764 J.Anal. At. Spectrom., 1999, 14, 1761±17657 G. I. Spielholtz and R. B. Diehl, Talanta, 1996, 13, 991. 8 E. Hafter, D. Schmidt, P. Freimann and W. Gerwinski, Spectrochim.Acta, Part B, 1997, 52, 935. 9 J. Messerschmidt, A. Von-Bohlen, F. Alt and R. Klockenka»mper, J. Anal. At. Spectrom., 1997, 12, 1251. 10 T. Kubota, T. Yamaguchi and T. Okutani, Talanta, 1998, 46, 1311. 11 W. H. Gutenmann, G. J. Doss and D. J. Lisk, Chemosphere, 1998, 37, 389. 12 I. S. Palmer and N. Thiex, J. AOAC Int., 1997, 80, 469. 13 A. M. Yusof, Z. ManaÆah and A. K. H. Wood, Sci. Total Environ., 1998, 214, 247. 14 L. C. Robles and A. J. Aller, Anal. Sci., 1996, 12, 783. 15 G. J. Kumar and N. N. Meeravali, At. Spectrom., 1996, 17, 27. 16 I. Vedrina-Dragojevic, D. Dragojevic and S. Cadez, Anal. Chim. Acta, 1997, 355, 151. 17 K. C. Bowles and S. C. Apte, Anal. Chem., 1998, 70, 395. 18 L. J. Blanchard and J. D. Robertson, Radioanal. Nucl. Chem., 1998, 235, 255. 19 M. W. Hinds, Spectrochim. Acta, Part B, 1998, 53, 1063. 20 L. Ebdon and W. C. Pearce, Analyst, 1982, 107, 942. 21 D. Bradshaw and W. Slavin, Spectrochim. Acta, Part B, 1989, 44, 1245. 22 Australian Standard 1038, Part 14.2, Analysis of Higher Rank Coal Ash and Coke, Standard Association of Australia, Sydney, Australia, 1985. 23 E. Ikavalko, T. Laitinen and H. Revitzer, Fresenius J. Anal. Chem., 1999, 363, 314. 24 R. W. Shaw, T. B. Brill, A. A. Clifford, C. A. Ackert and E. U. Franck, Chem. Eng. News, 1991, 69, 26. 25 V. K. Jam, Environ. Sci. Technol., 1993, 27, 806. 26 S. B. Hawthorne, Y. Yang and D. J. Miller, Anal. Chem., 1994, 66, 2912. 27 Y. Yang, S. B. Hawthorne, S. Bowardt and D. J. Miller, Anal. Chem., 1995, 67, 4571. 28 Y. Yang, S. B. Hawthorne and D. J. Miller, J. Environ. Sci. Technol., 1997, 31, 430. 29 M. M. Jimenez-Carmona, J. J. Manclus, A. Montoya and M. D. Luque de Castro, J. Chromatogr., 1997, 785, 329. 30 M. M. Jimenez-Carmona, V. Ferna�ndez-Pe�rez and M. M. Luque de Castro, Anal. Chim. Acta, in the press. 31 V. Ferna�ndez-Pe�rez, M. M. Jime�nez-Carmona and M. D. Luque de Castro, J. Chromatogr., submitted for publication. 32 US Pat. 5,400,642, 1995. 33 L. Pitts, P. J. Worsfold and S. J. Hill, Analyst, 5. 34 R. Falter and H. F. Scho» ler, Fresenius J. Anal. Chem., 1995, 353, 34. 35 M. Ferna�ndez-Garcý�a, R. Pereiro-Garcý�a, N. Berdel-Garcý�a and A. Sanz-Mendel, Talanta, 1994, 41, 1833. Paper 9/906094D J. Anal. At. Spectrom., 1999, 14, 1761±1

ISSN:0267-9477    

DOI:10.1039/a906094d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

INTER-LABORATORY NOTE Furnace-fusion system for the direct determination of cadmium in biological samples by inductively coupled plasma atomic emission spectrometry using tungsten boat furnace±sample cuvette technique Yasuaki Okamoto Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashihiroshima 739-8526, Japan. E-mail: Yasuaki.Okamoto@sci.hiroshima-u.ac.jp Received 8th June 1999, Accepted 25th August 1999 A solid sampling technique using an electrothermal vaporisation device is described.To a small sample cuvette made of tungsten, a solid mixture of a biological sample and diammonium hydrogenphosphate powder as a fusion Øux was added. The cuvette was superposed on a tungsten boat furnace, then tetramethylammonium hydroxide solution was injected as a sample decomposition reagent. By resistance heating of the tungsten boat furnace, the cuvette temperature was maintained at a wet-digestion temperature sufÆcient to decompose the solid sample.After the on-furnace digestion was complete, the temperature was successively elevated up to maximum to generate analyte vapour. The transient cloud of vapour was introduced into the plasma. Since any solid samples could be decomposed to ash completely on the cuvette, the sensitivity was the same as that of aqueous standards. The method was successfully applied to the direct determination of cadmium in biological certiÆed reference materials. The determination of trace elements by inductively coupled plasma atomic emission spectrometry (ICP-AES) is an established technique.In most ICP applications, a pneumatic nebuliser is used to convert an aqueous sample solution into a Ænely dispersed aerosol. In this manner, only 3±12% of the sample is eventually introduced into the plasma.1 However, most of the samples analysed using the ICP are not in liquid form. Therefore, extensive sample preparation procedures are necessary to convert them into liquid form.Moreover, the performance of the nebulisation system often depends on the physical properties of the resulting solution.2 The advantages of the direct analysis of solid samples by ICPAES are the time saved by avoiding dissolution and dilution steps as well as the high sensitivity. However, the solid sampling method has not found wide application, although various approaches have been described,3±15 including the application of a pelletized solid insertion device,3 a graphite cup direct insertion device,3,4,9±11 in-torch vaporisation,12 an electrically vaporised thin-Ælm plasma technique,5 and the electrothermal vaporisation (ETV) technique,6,13±15 as well as its modiÆcation to a graphite furnace arc (halonarc) system8 and to a pelletized ETV system.7 Using electrothermal vaporisation for sample introduction into the ICP, micro-amounts of solid samples can be vaporised by resistance heating and carried into the plasma by an argon gas stream.The major difÆculties inherent in this technique are the introduction of pre-weighed small amounts of powdered samples into the ETV device and the convenient removal of residues from it. In order to keep the plasma stable during measurements, the design of a special manifold device is necessary, otherwise poor accuracy and precision will be obtained. The other shortcoming of solid sampling is the necessity of Ænding an accurate calibration method. A conventional calibration curve prepared by using aqueous standards was not always applicable to direct determination by solid sampling.Calibration using certiÆed reference materials (CRMs) as solid standards has most frequently been used. However, the blank signal cannot be quantiÆed in the conventional way. The purpose of this paper is to describe a newly conceived method for the direct determination of biological samples by ICP-AES using a tungsten boat furnace (TBF)±sample cuvette technique.Fusion techniques are often used to dissolve silicacontaining materials, etc. Generally, the mixture of the Ænely ground sample and a Øux is placed in a nickel or platinum crucible and heated until the Øux melts. The resulting melt can dissolve easily either in water or in dilute acid. In this experiment, the fusion technique was combined with the TBF± sample cuvette technique for the direct analysis of solid samples with detection by ETV-ICP-AES. It can be abbreviated as the furnace±fusion method.Thus, small amounts of both the biological sample and diammonium hydrogenphosphate, as a Øux in this instance but which also acts as a chemical modiÆer in a vaporisation stage, are mixed. The mixture is placed into the removable small sample cuvette made of tungsten. The sample cuvette is superposed on the TBF, then the temperature of the cuvette is maintained at a fusion temperature by electrical heating of the TBF. After the sample has been wetdigested and dried in the furnace, the cuvette is successively heated up to vaporisation temperature to carry the analyte vapour into the plasma by use of an argon carrier gas stream.Experimental Apparatus A Kyoto Koken (Kyoto, Japan) Model UOP-1S inductively coupled plasma high resolution atomic emission spectrometer was used. The spectrometer, with a modiÆed Czerny±Turner conÆguration, incorporated an echelle grating and an oscillating quartz refractor plate. By using the refractor plate, the second derivative emission intensity was measured.The reciprocal linear dispersion of the spectrometer was, for example, 0.031 nm mm21 at 200 nm, and the continuum background was corrected by the wavelength modulation± J. Anal. At. Spectrom., 1999, 14, 1767±1770 1767 This Journal is # The Royal Society of Chemistry 1999second derivative system. Background corrected emission-time responses were recorded with a strip-chart recorder. Integrated emission intensities were estimated by using a Yokogawa Analytical Systems (Tokyo, Japan) Model HP3396II integrator.For electrothermal sample introduction, a Seiko II (Chiba, Japan) Model SAS-705V metal furnace atomiser for AAS, equipped with a tungsten boat (large-volume type, 10660 mm) was used after modiÆcation. This TBF vaporisation device has been described in detail previously.16 The sample cuvettes (10615 mm) were shaped by cutting both edges of the tungsten boats.Each cuvette was placed on the TBF. The TBF vaporiser is shown in Fig. 1, together with a schematic experimental procedure. A PTFE tube (4 mm id650 cm long) was used for the connection of the inlet port of the ICP torch and the outlet port of the TBF vaporiser. Lids of PTFE±perØuoroalkoxy (PFA) Tuf-Tainer vials (Pierce Chemical, Rockford, IL, USA) were utilised as carriers for the sample cuvettes. A pair of tweezers (made of titanium, Iuchi-Seieido, Osaka, Japan) was used for handling the cuvettes.A Microman M-250 and Pipetman P-20 (both Gilson Medical Electronics, Villiers-le-Bel, France) digital pipettes were used for reagent and aqueous standards injections, respectively. A Mettler Toledo (Zurich, Switzerland) Model AT261 microbalance (readability, 10 mg) was used for weighing samples. Agate mortars were used for grinding samples and reagents. Reagents Water from an Advantec Toyo (Tokyo, Japan) Model GSU- 901 water puriÆcation system was used.All chemicals were of analytical-reagent grade. A cadmium stock standard solution for AAS (1000 mg dm23; Hayashi Pure Chemical, Osaka, Japan) was diluted with 0.1 mol dm23 nitric acid as required. Biological certiÆed reference materials were purchased from the NIST (National Institute of Standards and Technology, US Department of Commerce) and the NIES (National Institute for Environmental Studies, Environmental Agency of Japan). These certiÆed reference materials were dried at 85 �C for 4 h and stored in a desiccator with a dried silica gel.Extra-pure grade tetramethylammonium hydroxide (TMAH, 25% solution, Tama Chemical, Tokyo, Japan) and Suprapur-grade diammonium hydrogenphosphate (Merck, Darmstadt, Germany) were used as chemical modiÆers. The diammoum hydrogenphosphate was powdered by means of an agate mortar before use. The resulting particle diameter was approximately 10 mm. Procedure An aliquot of a pre-dried sample was weighed accurately and was ground to a Æne powder (approximately 10 mm diameter) with the agate mortar. To the powdered sample, diammonium hydrogenphosphate, the amount of which was equal to one half of the sample, was added and mixed with the mortar.Routinely, 1 g of the sample and 0.5 g of diammonium hydrogenphosphate were taken, respectively. A 10 mg amount of the mixture was weighed accurately into the sample cuvette by means of the microbalance. For the standard batch, used for construction of a calibration curve, a 3.30 mg amount of the diammonium hydrogenphosphate was weighed into the sample cuvette and then an aliquot of the cadmium standard solution was injected into it.The cuvette was next located on the TBF and 80 mm3 of the TMAH solution were pipetted into the cuvette. The cuvette was heated for 30 s at 130 �C. During this heating stage, the strongly basic environment created by TMAH facilitated the decomposition of the organic matrix, especially proteins.The temperature was ramped up and maintained at 250 �C for 60 s to remove the TMAH and to pyrolyse the dry matrix. After the temperature had been heated at 550 �C to remove a large portion of the diammonium hydrogenphosphate, the sample insertion port was closed. At a Ænal temperature elevation of 2000 �C, a momentary cloud of analyte vapour was generated and transported into the ICP by the carrier gas stream through the PTFE tube. The transient emission peak was recorded, and the peak area was measured by means of the integrator.The instrument operating conditions are listed in Table 1. Results and discussion Optimisation of operating conditions In order to optimise the operating conditions for the decomposition of solid sample on the cuvette and for the vaporisation of cadmium, the effects of the heating program for the drying, ashing and vaporisation stages, of Øow rates of argon and hydrogen carrier gases, and of the amounts of Fig. 1 Experimental procedure and schematic diagram of the TBF vaporiser system. A, Carrier gas inlet port; B, Bakelite plate; C, O-ring; D, tungsten boat furnace; E, sample cuvette; F, furnace electrode; G, quartz dome; H, sample insertion port; I, silicone rubber stopper; and J, port from which carrier gas Øows to the plasma torch sample introduction oriÆce. Table 1 Instrument operating conditions ICP atomic emission spectrometer (UOP-1S)– Rf incident power 1.5 kW Argon gas Øow rate Plasma gas 15 dm3 min21 Auxiliary gas 1.6 dm3 min21 Analytical line Cd II 214.438 nm TBF vaporiser (modiÆed SAS-705V)– Powdered sample ca. 10 mg [(NH4)2HPO4, 33%] TMAH (25%) 80 mm3 Ashing-1 130 �C for 30 s (ramp 0 s) Ashing-2 250 �C for 60 s (ramp 30 s) Ashing-3 550 �C for 180 s (ramp 30 s) Vaporisation 2000 �C for 4 s (ramp 11 s) Carrier gas Øow rate Argon gas 300 cm3 min21 Hydrogen gas 50 cm3 min21 1768 J. Anal. At. Spectrom., 1999, 14, 1767±1770modiÆers used, were investigated. Examinations were carried out by recording signals from each aliquot of the NIES CRM No.10-c, Rice Flour, for the sample.The strongly basic reagents were useful for the decomposition of protein. In particular, sodium hydroxide and potassium hydroxide, as well as a mixture of them, have been used successfully as alkali Øuxes in the fusion technique. However, these reagents were not recommended for atomic spectrometry because of the high salt content in the resulting solutions and their spectral interferences during the measuring procedure. TMAH is basic enough to apply the alkali fusion technique.Moreover, it is completely removable by heating at approximately 200 �C or above. In this experiment, extra-pure grade 25% TMAH aqueous solution, purchased from Tama Chemicals, was used. In our previous papers, we reported that the diammonium hydrogenphosphate was a suitable modiÆer for the TBF ICP atomic emission spectrometric determination of cadmium, lead17 and boron18 in aqueous samples.Especially for cadmium, a clear difference was observed in the presence and absence of phosphate ion. In the case of solid samples, injected phosphate modiÆer solution tended to be repelled by a hydrophobicity of the surface of solid samples. Therefore, in this experiment, solid diammonium hydrogenphosphate and powdered sample were premixed in the optimum weight ratio by means of the agate mortar. With increase in the weight ratio of diammonium hydrogenphosphate to more than 80%, the plasma also became unstable.The sensitivity was a maximum at a weight ratio of 20±35%. Hence, a ratio of 33% was selected. In order to accelerate the decomposition of organic matrix by TMAH, the temperature program for the ashing-1 stage was set to 130 �C and 30 s. To expel any excess of the added TMAH and to pyrolyse the matrix with diammonium hydrogenphosphate, 250 �C for 60 s (ramp 30 s) was selected for the ashing-2 stage.During the ashing-3 stage, diammonium hydrogenphosphate was gradually degraded and the white fumes began to appear. Approximately 150 s were required to remove the excess phosphate. A 2000 �C vaporisation temperature was suitable to obtain sensitive and quantitative atomic emission signals. The effect of ramp time with maximum vaporisation temperature on the sensitivity was investigated. The atomic emission peak height signal was maximum at a ramp time of 10±12 s, while its peak area was maximum and constant within the tested range of 5±18 s.To prevent the deterioration of the tungsten cuvette and TBF, hydrogen gas was added to the carrier argon gas.19 The sensitivity and precision for cadmium increased as the argon gas Øow rate decreased within the tested range of 300±500 cm3 min21. In line with the above-mentioned results, the optimum conditions were adopted as in Table 1. Application to the direct determination of the mg g21 levels of cadmium The vastly different matrixes of solids and liquids often result in different vaporisation, transportation and plasma conditions.Therefore, calibration using CRMs as solid standards has been used frequently. In such cases, selection of the CRMs for calibration is very important. CRMs with matrixes similar to those of the samples should be used. Moreover, the certiÆed analyte concentration of the CRM used as a calibrant should be in the range of interest. Even if suitable CRMs were available, a disadvantage of the methods still remained, viz., a blank cannot easily be quantiÆed and corrected.Eames and Matousek proposed a new standard additions method for AAS, in which the same volume of standard aqueous solution was added to various accurately known weights of solid sample.20 Atsuya et al. proposed a three-point estimation standard additions method for the analysis of biological materials by solid sampling AAS.21 For more accurate determination, Boonen et al.demonstrated a kind of standard additions method named simpliÆed generalizing standard addition method. Their method led, by varying both the mass of the solid sample and the amount of analyte (standard solution) added, to a response surface from which the analyte concentration was able to be deduced by multiple linear regression.22 In our method, before the vaporisation stage, solid samples were chemically pre-digested with TMAH and (NH4)2HPO4 in the sample cuvette: at the same time, cadmium reacted with phosphate ion to form Cd3(PO4)2 (m.p. 1500 �C, b.p. not measured23), a compound thermally more stable than other cadmium species such as Cd(NO3)H2O (m.p. 59.5 �C, b.p. not measured23), CdCl2 (m.p. 568 �C, b.p. 960 �C23) and Cd (m.p. 321 �C, b.p. 765 �C23). By adding the phosphate, cadmium can be retained up to a higher ashing temperature, 600 �C, otherwise only up to 300 �C. Moreover, interferences caused by foreign ion were greatly suppressed in the presence of phosphate ion.17 Therefore, the biological solid samples were successfully analysed directly byy use of aqueous standard solutions.Analytical results for the direct determination of cadmium in some environmental and biological samples by TBF±ICP-AES with the small sample cuvette technique are listed in Table 2. These results are in satisfactory agreement with the certiÆed values. The 3s detection limit of the proposed method was estimated to be 5.6 pg of cadmium using an aqueous standard solution. This value corresponds to 0.84 ng g21 of cadmium in solid samples.The relative standard deviation was estimated to be 2.6% (n~6) when 40 ng of cadmium were loaded into the cuvette. A linear calibration graph for cadmium intersecting the origin of the coordinate axes and covering absolute amounts of up to 200 ng was established. In conclusion, to develop a simple, rapid, practical and widely applicable method for the direct determination of cadmium in biological samples, a newly conceived electrothermal vaporisation system using TBF±sample cuvette was designed.The TBF±sample cuvette technique overcomes problems such as weighing small amounts of powdered samples, introduction of the samples into the TBF device and removal of the residues from it. The proposed technique allows an easier standardisation procedure by using aqueous standard solutions. The technique makes it possible to measure a number of samples sequentially by preparing a lot of sample cuvettes previously.Moreover, the proposed method can be applied to the determination of cadmium even in more complex matrixes. This work was partially supported by Grant-in-Aid for ScientiÆc Research, No. 09740556, from the Ministry of Education, Science and Culture, Japan. References 1 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 939. 2 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 613. 3 L. Blain, E. D. Salin and D. W. Boomer, J. Anal. At. Spectrom., 1989, 4, 721. Table 2 Determination of cadmium in certiÆed reference materials Cadmium/mg g21 Sample Founda,b CertiÆed Pepperbush (NIES No.1) 6.58°0.35 6.7°0.5 Rice Flour (NIES No.10-b) 0.40°0.09 0.32 Rice Flour (NIES No.10-c) 1.89°0.02 1.95 Oyster Tissue (NIST SRM 1566a) 4.22°0.05 4.15°0.4 aCalibration curve method (Cd aqueous standards). bMean°average deviation, n~3. J. Anal. At. Spectrom., 1999, 14, 1767±1770 17694 V.Karanassios, G. Horlick and M. Abdullah, Spectrochim. Acta, Part B, 1990, 45, 105. 5 K.M. Trivedi, S. W. Brewer Jr. and R. D. Sacks, Appl. Spectrosc., 1990, 44, 367. 6 I. Atsuya, T. Itoh and T. Kurotaki, Spectrochim. Acta, Part B, 1991, 46, 103. 7 V. Karanassios, J. M. Ren and E. D. Salin, J. Anal. At. Spectrom., 1991, 6, 527. 8 T.Ka�ntor and Gy. Za�ray, Fresenius'J.Anal.Chem., 1992, 342, 927. 9 M. Umemoto, K. Hayashi and H. Haraguchi, Anal. Chem., 1992, 64, 257. 10 V. Karanassios and T. J. Wood, Appl. Spectrosc., 1999, 53, 197. 11 C. D. Skinner, M. Cazagou, J. Blaise and E. D. Salin, Appl. Spectrosc., 1999, 53, 191. 12 H. R. Badiei and V. Karanassios, J. Anal. At. Spectrom., 1999, 14, 603. 13 P. Verrept, R. Dams and U. Kurfu» rst, Fresenius' J. Anal. Chem., 1993, 346, 1035. 14 L. Moens, P. Verrept, S. Boonen, F. Vanhaecke and R. Dams, Spectrochim. Acta, Part B, 1995, 50, 463. 15 A. Golloch, M. Haveresch-Kock and F. Plantikow-Vo°ga» tter, Spectrochim. Acta, Part B, 1995, 50, 501. 16 K. Fujiwara, Y. Okamoto, M. Ohno and T. Kumamaru, Anal. Sci., 1995, 11, 829. 17 Y. Okamoto, H. Kakigi and T. Kumamaru, Anal. Sci., 1993, 9, 105. 18 Y. Okamoto, K. Sugawa and T. Kumamaru, J. Anal. At. Spectrom., 1994, 9, 89. 19 Y. Okamoto, H.Murata, M. Yamamoto and T. Kumamaru, Anal. Chim. Acta, 1990, 239, 129. 20 J. C. Eames and J. P. Matousek, Anal. Chem., 1980, 52, 1248. 21 H. Minami, Q. Zhang, H. Itoh and I. Atsuya, Microchem. J., 1994, 49, 126. 22 S. Boonen, P. Verrept, L. J. Moens and R. F. J. Dams, J. Anal. At. Spectrom., 1993, 8, 711. 23 The Merck Index, ed. S. Budavari, M. J. O'Neil, A. Smith and P. E. Heckelman, 11th edn., Merck, West Point, PA, USA, 1989. Paper 9/904570H 1770 J. Anal. At. Spectrom., 1999, 14, 1767±

ISSN:0267-9477    

DOI:10.1039/a904570h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

COMMUNICATION Determination of trace cerium in doped crystal Ce :K3.0Li2.0Nb5.0O15.0 by ICP-MS Hong-Jun Shi*a and Hu-Sheng Liub aInstitute of Physics, Chinese Academy of Sciences, Beijing, 100080, China. E-mail: shihj@aphy.iphy.ac.cn bSchool of Public Health, Beijing Medical University, Beijing, 100083, China Received 24th June 1999, Accepted 1st September 1999 In this paper, an accurate and simple method is described for the determination of trace cerium in doped potassium lithium niobate (Ce : KLN) crystals by inductively coupled plasma mass spectrometry (ICP-MS).The KNbO3 multiplying frequency of an AsAlGa diode may be replaced with a K3.0Li2.0Nb5.0O15.0 (KLN) crystal laser1 in order to obtain blue light. The storage time of blue light is 4 times better than that of red light. Therefore, KLN crystals offer excellent application. However, the photorefractive properties of Ce :K3.0Li2.0Nb5.0O15.0 (Ce : KLN) crystals depend upon the doping concentration of cerium.Samples were digested with concentrated sulfuric acid and hydrogen peroxide. The determination of cerium was carried out using external calibration with matrix matched standards and high-purity spiking material. The detection limit of Ce was 7 pg ml21. The recovery was 85 to 107%. The relative standard deviation was 3.0 to 3.8%. The doping concentration of cerium in KLN crystals was 97.6 mg g21. Experimental Instrument and operating conditions A Perkin-Elmer (Norwalk, CT, USA) SCIEX Elan 5000 inductively coupled argon plasma mass spectrometer equipped with a peristaltic pump and an IBM PS/2 Model 70 computer for data handling and storage was used.2,3 A closed loop water chiller was used for cooling, with 10% (v/v) ethylene glycol to achieve an approximate coolant temperature of 15 �C.The instrumental operating conditions, optimized for the measurement of cerium isotope m/z 140 are given in Table 1. Reagents Sulfuric acid, hydrogen peroxide (Beijing HuaGong-Chang, Beijing, China) and KLN crystals were of high purity grade.Deionized water was further puriÆed in a clean room by subboiling distillation in a quartz vessel. Cerium standard stock solution (1000 mg l21, CeO2, specpure) was obtained from Johnson Matthey Chemicals Limited (Royston, Hertfordshire, UK). A working standard solution of cerium was prepared by diluting the cerium standard stock solution to 30 ng ml21. A standard solution of 30 ng ml21 Ce±0.25 mg ml21 KLN was used as the high standard.A standard solution of 0 ng ml21 Ce±0.25 mg ml21 KLN was used as the low standard. Sample preparation Preparation of the sample solutions for ICP-MS was as follows: 0.0125 g Ce :KLN was weighed and placed in a 50 ml beaker, 1.5 ml of concentrated sulfuric acid was added followed by mixing for homogeneity, and digestion on a hot plate to dissolve the sample. After cooling, 2.5 ml of 10% (v/v) hydrogen peroxide solution were added and the solution was transferred into a 50 ml calibrated Øask, followed by dilution to volume with 1% nitric acid.The sample solutions were found to be stable for more than 12 h. Detection limit The concentration of cerium in KLN crystals is usually very low and free from interference. In order to obtain a lower detection limit, a more abundant isotope of cerium was chosen. The sensitivity of Ce was 50 MCPS ppm21. The limit of detection (LOD) was calculated as 3 times the standard deviation of twenty determinations of the low standard solution.The isotope of cerium selected, abundance and LOD are shown in Table 2. Results of actual KLN crystal sample analysis Seven actual samples were digested for the ICP-MS determination of cerium. Data are listed in Table 3. Table 1 ICP mass spectrometer operating conditions RF power 1025W Coolant Øow rate (Ar) 15 l min21 Auxiliary Øow rate (Ar) 0.8 l min21 Nebulizer Øow rate (Ar) 0.84 l min21 Sample uptake rate (Ar) 1 ml min21 Sampling cone oriÆce (Ni) 1.1 mm Skimmer cone oriÆce (Ni) 0.89 mm Sample depth above load coil 10 mm Ion lens setting (optimum for Ce) B 65, E1 50, P 55, S2 33 CEM voltage 3.4 kV MS operating pressure 1.1561023 Pa Resolution (10% peak height) 0.8 u Dwell time 30 ms Points per peak 3 Number of replicates 10 Table 2 Abundance and detection limit Element m/z Abundance (%) LOD (3s)/pg ml21 Ce 140 88.48 7 J.Anal. At. Spectrom., 1999, 14, 1771±1772 1771 This Journal is # The Royal Society of Chemistry 1999Recoveries of cerium A certain amount of cerium standard solution was added to the samples before digestion and the samples were prepared for analysis as detailed above.Table 3 lists the recovery of cerium. It can be seen that recoveries obtained were from 85 to 107% with a precision of 3.0 to 3.8% RSD. Conclusions The determination of trace cerium in doped KLN crystals by ICP-MS is a very fast and effective method. References 1 M. Ferriol, G. Foulon, A. Brenier, M. T. Cohen-Adad and G. Boulon, J. Cryst. Growth, 1997, 173, 226. 2 J. G. Sen Gupta and N. B. Bertrand, Talanta, 1995, 42, 1947. 3 H. Sh. Liu, N. F. Wang, M. Liu and X. Y. Wang, Spectrosc. Spectral Anal. (Beijing), 1996, 16, 66. Paper 9/905074D Table 3 Results obtained for cerium-spiked samples (n~7) Element Added/ng ml21 Found/ng ml21 Recovery (%) RSD (%) Ce 0 (base) 24.4 – 4.2 Ce 10 35.1 107 3.8 Ce 15 37.1 85 3.0 1772 J. Anal. At. Spectrom., 1999, 14, 1771±17

ISSN:0267-9477    

DOI:10.1039/a905074d

出版商:RSC    

年代:1999

数据来源: RSC