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Determination of cadmium, mercury and lead in seawater by electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry
Spectrochimica Acta Part B 54 Ž1999. 1367]1375
Determination of cadmium, mercury and lead in seawater by electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry Hung-Wei Liu, Shiuh-Jen JiangU , Shin-Hung Liu Department of Chemistry, National Sun Yat-Sen Uni¨ ersity, Kaohsiung 804, Taiwan Received 9 April 1999; accepted 7 June 1999
Abstract Electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry ŽETV-ID-ICP-MS. has been applied to the determination of Cd, Hg and Pb in seawater samples. The isotope ratios of the elements studied in each analytical run were calculated from the peak areas of each isotope. Various modifiers were tested for the best signal of these elements. After preliminary studies, 0.15% mrv TAC and 4% vrv HCl were added to the sample solution to work as the modifier. The ETV-ID-ICP-MS method has been applied to the determination of Cd, Hg and Pb in NASS-4 and CASS-3 reference seawater samples and seawater samples collected from Kaohsiung area. The results for reference sample NASS-4 and CASS-3 agreed satisfactorily with the reference values. Results for other samples determined by isotope dilution and method of standard additions agreed satisfactorily. Detection limits were approximately 0.002, 0.005 and 0.001 ng mly1 for Cd, Hg and Pb in seawater, respectively, with the ETV-ICP-MS method. Precision between sample replicates was better than 20% for most of the determinations. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrothermal vaporization; Isotope dilution; Inductively coupled plasma-mass spectrometry ŽICP-MS.; Cd; Hg; Pb; Sea water
1. Introduction Cadmium, mercury and lead are used in a variety of products and industrial processes. The U
Corresponding author. Fax: q886-7-525-3908. E-mail address: [email protected] ŽS. Jiang.
determinations and monitoring of these elements in environmental and biological samples are extremely important because of the high toxicity of these elements. The determination of Cd, Hg and Pb in environmental samples is difficult, mainly because of its extremely low concentration therein. ICP-MS is a trace metal detection method with
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unique analytical capability, offering both exceptional sensitivity and multi-element capability compared with electrothermal AAS and ICP-AES. The majority of analysis by ICP-MS are carried out on solutions using a conventional pneumatic nebulizer. However, the type of analytical tasks that can be solved by ICP-MS can be extended using a number of other sample introduction techniques which can be easily adapted to ICPMS. Electrothermal vaporization ŽETV. is one of the sample introduction techniques, that is currently employed in ICP-MS w1]15x. Although ETV-ICP-MS was applied to several types of sample analysis, it still suffers from analyte loss during transportation and poor reproducibility problems. Furthermore, non-spectroscopic interference might occur when a high salt content sample was analyzed, which caused the suppression of analyte ion signals w15x. ETAAS and ETV-ICP-MS have been applied in many previous applications for the direct determination of trace metals in seawater w16]19x. In ETV-ICP-MS analysis, in order to alleviate non-spectroscopic interference, a relatively low vaporization temperature can be used which separates the volatile analyte from major matrix components and alleviates non-spectroscopic interference significantly. This strategy has been
applied to the determination of various trace elements in high salt content samples in several previous studies w19]22x. In this study, ETV-isotope dilution-ICP-MS was used to determine the concentration of Cd, Hg and Pb in several seawater samples. A relatively low vaporization temperature was used which separated the analyte from the major matrix components and improved the ion signals of Cd and Pb significantly. Isotope dilution ŽID. techniques were applied in several previous ICP-MS applications w23]26x. Isotope dilution is well recognized as a definitive analytical technique for the determination of the trace elements. Since another isotope of the same element represents the ideal internal standard for that element, isotope dilution results are expected to be highly accurate even when the sample contains high concentrations of concomitant elements andror loss in sample preparation or transport during sample introduction device and ICP. However, the isotope dilution method cannot be used for mono isotopic elements. Furthermore, the isotopes chosen must be free of isobaric and polyatomic interferences. In this study, the ETV-ID-ICP-MS method was applied to the determination of Cd, Hg and Pb in open ocean seawater reference sample NASS-4, nearshore seawater reference
Table 1 Equipment and operating conditions ICP mass spectrometer
Perkin-Elmer SCIEX ELAN 5000
Plasma conditions Outer gas flow rate Žl miny1 . Intermediate gas flow rate Žl miny1 . Carrier gas flow rate Žl miny1 . RF power ŽW. Samplerrskimmer
15.0 0.74 1.08 1100 Nickel
Mass spectrometer settings Dwell time Žms. Scan mode Sweeps per reading Readings per replicate Points per spectral peak Signal measurement mode Isotope monitored
10 Peak-hopping 1 350 1 Integrated 111 Cd, 113 Cd, 201 Hg, 202 Hg 204 Pb, 206 Pb
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sample CASS-3 and seawater samples collected from Kaohsiung area.
the peak areas of each isotope. Since 114 Sn interferes with 111 Cd, 113 Cd and 114 Cd were used for Cd isotope ratio determination.
2. Experimental
2.2. Reagents
2.1. Apparatus and conditions
Trace metal grade HNO3 Ž70% mrm. and HCl Ž35% mrm. were obtained from Fisher ŽFair Lawn, NJ, USA.. Element standard solutions were obtained from SPEX ŽEdison, NJ, USA.. Palladium powder was obtained from KOJUNDO Chemical Laboratory Co., Ltd. ŽSaitama, Japan.. Thioacetamide ŽTAC. was obtained from TCI Chemical ŽTokyo, Japan.. Diphosphorus pentasulfide ŽP2 S5 . was obtained from Merck ŽDarmstadt, Germany.. Isotopically enriched 111 CdO and 204 PbŽNO 3 . 2 were purchased from Oak Ridge National Laboratory ŽOak Ridge, TN, USA. and 201 HgO was obtained from Cambridge Isotope Laboratories ŽAndover, MA, USA.. Stock solutions of each enriched isotope were prepared by dissolution of an accurately weighed quantity of the material in HCl or HNO3 and diluted to volume. The concentration of the spike solution was verified by reversed spike isotope dilution ICP-MS. This was done by spiking the enriched isotope with a suitable amount of natural isotope of each element and measuring the isotope ratio altered with ICP-MS. The concentration of the enriched isotope was then calculated by the equation described in previous papers w25,26x.
A Perkin-Elmer Sciex ŽThornhill, Ontario, Canada. ELAN 5000 ICP-MS spectrometer equipped with a HGA-600MS electrothermal vaporizer was used. Pyrolytic coated graphite tubes and platforms were used throughout. The transfer line consisted of an 80 cm long, 6 mm i.d. PTFE tubing. The sample was introduced with a Model AS-60 autosampler. Teflon autosampler cups were used. The experimental conditions for the ICP-MS and ETV are described in Tables 1 and 2, respectively. ICP operating conditions were selected to maximize the sensitivity of the elements studied to get the best precision and accuracy for isotope ratio determination. The ICP conditions were selected to maximize ion signals while a solution containing 10 ng mly1 of Cd, Hg and Pb in 1% HCl was continuously introduced with a conventional nebulizer. The sensitivity of the instrument might vary slightly from day-to-day. The measurements were made by peak hopping rapidly from one mass to the other, staying only a short time Ždwell time. at each mass. For the best accuracy and precision for isotope ratio determination, a 10-ms dwell time was used in the following experiments w27x. Ion lenses voltages were set to get the best ion signals of the elements studied. The isotope ratios of the elements studied in each analytical run were calculated from
2.3. Sample preparation The applicability of the method to real samples was demonstrated by the analysis of several seawater samples. The seawater samples were
Table 2 HGA-600MS temperature programme for proposed method Žsample volume 20 ml. Step Condition TemperatureŽo C. Ramp time Žs. Hold time Žs. Signal acquisition
1500 1 10 ]
Cooling 2500 1 10 ]
20 5 5 ]
Drying 80 30 10 ]
120 10 10 ]
Vaporization
Cooling
1200 1 13 ON
20 5 5 ]
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determined by the isotope dilution and standard addition methods. Suitable amounts of HCl and TAC were added to 5 ml of the seawater samples to make the final solutions containing 4% vrv HCl and 0.15% mrv TAC. After suitable amounts of enriched isotope or various amounts of element standard solutions were spiked, the solutions were diluted to 10 ml with deionized water. For standard addition analysis, the seawaters were spiked with various amounts of Cd, Hg and Pb Ž0, 0.05, 0.1, 0.5, 1 and 2 ng mly1 in the final solutions. element standards. These solutions were then introduced into ETV-ICP-MS for Cd, Hg and Pb determinations. A seawater sample was passed through the Chelex-100 column three times to remove the trace analyte in the sample w28x. This solution was treated as the blank to correct any analyte contaminants in the reagents used for sample preparation. The analyte concentrations in the sample were then calculated by the equation described in previous papers w25,26x andror from the standard addition calibration curves. Owing to the mass bias effect, the isotopic compositions of Cd, Hg and Pb in both natural element and enriched isotope were obtained by determining the intensities of all isotopes by ICPMS with solution nebulization. The intensities of each isotope obtained during this measurement were used for the isotope ratios and atomic weights calculation of the elements studied.
3. Results and discussion 3.1. Selection of matrix modifier Fig. 1 shows the temporal behavour of Cd, Hg and Pb. The drying temperatures were set at 808C for 30 s and 1208C for 10 s; and the vaporization temperature was set at 10008C. The collected seawater sample was spiked with 2 ng mly1 of Cd and Pb and 5 ng mly1 of Hg and then injected into the furnace without any further pretreatment. As shown in Fig. 1a, all the elements studied evaporated at 10008C. However, a small fraction of Hg evaporated during the drying step when no modifier was used. Due to the volatility
of Hg, a chemical modifier is needed for the determination of Hg by the ETV-ICP-MS method. Modifiers are commonly used in ETV-ICP-MS in order to reduce losses of analyte due to condensation on different parts of the ETV cell or the transfer line that connects the ETV to the ICP-MS w1]7x. Although most effects of the modifiers used in ETV-ICP-MS analysis are believed to be physical, it is likely chemical effects could also be involved in specific instances w5x. The use of modifier would change the chemical or physical characteristics of the sample andror the atomizer surface in order to improve quantitation. In this study, several modifiers, including palladium, Pt, P2 S5 , TAC and several mixed modifiers, were tested for best signals of the elements studied. Fig. 1b,c show the effects of TAC and P2 S5 modifiers on ion signals. As shown in Fig. 1b, Hg was evaporated during the drying step when 0.5% mrv P2 S5 was used as the modifier. On the other hand, as shown in Fig. 1c, no significant mercury ion signal was detected in the drying step when 0.3% mrv TAC was added to the sample solution to work as the modifier. Moreover, as shown in Table 3, although the use of P2 S5 modifier could improve the ion signal of Cd, the signals of Hg and Pb were suppressed. Although not illustrated in the text, Pd Ž200 and 400 mg mly1 ., Pt Ž200 and 400 mg mly1 . and several other mixed modifiers could not improve ion signals of the elements studied. For the simultaneous determination of these elements, after evaluation, a mixture of TAC and HCl was selected as the modifier in this study. TAC has been used as the chemical modifier to improve the signal of Hg in previous ETVICP-MS and ETAAS applications w15,29x. Fig. 2 shows the effect of the amount of TAC on the ion signals. As shown, the signal of Hg increased with the increase of TAC concentration. However, the amount of TAC did not affect Cd and Pb ion signals significantly. After evaluation, 0.15% mrv of TAC was used as the modifier in the ETVICP-MS analysis. As described by Gregoire et al. ` w1x, the presence of mineral acid in ETV-ICP-MS analysis could affect ion signals somewhat. They found that analyte signals were enhanced by as much as a factor of 2 in the presence of 1% vrv
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Table 3 Effect of various modifiers on ion signals a,b Ž n s 7. Modifier
Peak arearcounts 111
Without modifier 0.3% TAC 0.3% TACq 2% HCl 0.5% P2 S5 0.5% P2 S5 q 2% HCl 0.3% TACq 0.5% P2 S5 0.3% TACq 0.5% P2 S5 q 2% HCl
Cd
1000" 100 1100 " 100 2200 " 100 3600 " 300 2900 " 300 2100 " 100 2300 " 100
202
Hg
2300 " 400 5300 " 800 10 000 " 500 2500 " 800 1800 " 200 4200 " 400 3800 " 700
208
Pb
50 000 " 5000 31 000 " 1000 88 000 " 5000 43 000 " 2000 46 000 " 3000 37 000 " 1000 34 000 " 1000
a
Values are means of seven measurements " S.D. An OmniRange setting of 3 was set for Pb measurement.
b
Fig. 2. Effect of TAC concentration on ion signals. The seawater sample was treated with 2% HCl and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various amounts of TAC. Vaporization temperature was set at 10008C.
Fig. 1. ETV-ICP-MS signals of Cd, Hg and Pb; Ža. without any modifier, Žb. with 0.5% mrv P2 S5 as modifier and Žc. with 0.3% mrv TAC as modifier. Drying temperatures were set at 808C and 1208C and vaporization temperature was set at 10008C. Injected seawater sample was spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb. 0.5% P2 S5 andror 0.3% TAC was added to the sample to work as modifier.
HNO3 . In this study, the effects of HCl and HNO3 on the ion signals of Cd, Hg and Pb were studied. Fig. 3 shows the effects of various acids
on the vaporization peaks of the elements studied. As shown, the addition of HNO3 suppressed the ion signals. Furthermore, the vaporization peak of Pb was split. Fig. 4 shows the effect of HCl concentration on ion signals. The collected seawater sample was spiked with 2 ng mly1 of Cd and Pb and 5 ng mly1 of Hg, 0.15% TAC and different amounts of HCl. As shown, the ion signals increased with the increase of HCl concentration. In the following experiments, 4% vrv HCl was added to all sample solutions to work as the modifier. In summary of previous studies, a mixed modifier of 0.15% mrv TAC and 4% vrv HCl was used in the following studies.
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Fig. 5. Effect of pyrolysis temperature on ion signal. The seawater sample was treated with 4% HCl, 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb. Vaporization temperature was set at 10008C. Fig. 3. Effect of Ža. HCl and Žb. HNO3 on Cd, Hg and Pb ion signals. The seawater sample was treated with 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various acids. Vaporization temperature was set at 10008C.
3.2. Selection of ETV operating conditions Fig. 5 shows the effect of pyrolysis temperature on ion signals. The ion signals of Cd and Pb did not change significantly when the temperature was increased. However, Hg ion signal decreased
Fig. 4. Effect of HCl concentration on Cd, Hg and Pb ion signals. The seawater sample was treated with 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various concentrations of HCl. Vaporization temperature was set at 10008C.
slightly with the increase of pyrolysis temperature even when TAC modifier was used, which could be due to the vaporization of Hg. In order to get the best Hg ion signal, in the following experiments no pyrolysis step was performed. Fig. 6 shows the effect of vaporization temperature on ion signals. As shown, the Hg ion signal did not change significantly with the increase of vaporization temperature. It could be due to the high volatility of Hg. On the other hand, the signals of Cd and Pb increased with the increase of vaporization temperature when the vaporiza-
Fig. 6. Effect of vaporization temperature on ion signal. No pyrolysis step was performed. The composition of the injected solution was the same as in Fig. 5.
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tion temperature was - 9008C, and then decreased gradually with the increase of vaporization temperature. An increase in vaporization temperature increased the amount of matrix vaporized with the analyte, which caused the gradual decrease in integrated peak area. However, as shown in Fig. 7, a more symmetrical peak could be obtained when a higher vaporization temperature was used; this could help to provide more reproducible integrated results. Furthermore, as shown in Fig. 8, a higher signal-to-background ratio could be obtained when the vaporization temperature was set at 12008C. In order to compensate for the symmetric of peak shape, peak area count, integrated blank signal and repeatability of signal, in the following experiments, 12008C was selected as the vaporization temperature. The summary of the ETV operating conditions are listed in Table 2. 3.3. Determination of Cd, Hg and Pb in seawater by ETV-ID-ICP-MS In order to validate the ETV-ID-ICP-MS method, concentrations of Cd, Hg and Pb were
Fig. 7. ETV-ICP-MS signals of Cd, Hg and Pb at various vaporization temperatures: Ža. 7008C; Žb. 12008C. The composition of the injected solution was the same as in Fig. 5.
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determined in the NRCC NASS-4 and CASS-3 seawater reference samples and seawater samples collected from Kaohsiung area. Since the vaporization behaviors of Cd, Hg and Pb and their ICP-MS signals quite depended on the sample matrix, the external calibration method could not be used for the quantification of analyte in this study. The isotope dilution method was used for the determination of Cd, Hg and Pb in seawater samples. For the seawater sample collected locally, an alternative verification process using the method of standard additions was conducted. Since the concentrations of the elements studied in the seawater were quite low, in order to improve the ion signals, five replicate injections and drying of a 20-ml sample was performed before the vaporization of the collected analyte. Analysis results are shown in Table 4. The determined concentrations of the reference sample are in good agreement with the reference values. Those data for which no reference values were provided were also found to agree with that obtained by the standard addition method. The precision between sample replicates is better than 20% for most of the determinations. This experiment indicates that Cd, Hg and Pb in seawater could be readily quantified by isotope dilution inductively coupled plasma mass spectrometry with electrothermal vaporization. Under the selected ETV-ICP-MS operating
Fig. 8. Effect of vaporization temperature on signal to background ŽSrB. ratio Ž n s 5..
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Table 4 Determination of Cd, Hg and Pb in seawater by ETV-ICP-MSa Ž n s 5. Sample
Methodb
Concentration Žng mly1 . Cd
Hg
Pb c
NASS-4
Certified Method 1 Method 2
0.016" 0.003 0.017" 0.001 0.018" 0.003
0.40" 0.04 0.51" 0.08 0.57" 0.10
0.013" 0.005 0.011" 0.002 0.013" 0.002
CASS-3
Certified Method 1 Method 2
0.030" 0.005 0.028" 0.003 0.028" 0.003
0.27 " 0.03c 0.29" 0.05 0.29" 0.02
0.012" 0.004 0.012" 0.003 0.019" 0.006
Seawater 1
Method 1 Method 2
0.027" 0.001 0.026" 0.001
0.12" 0.07 0.10" 0.06
0.034" 0.006 0.032" 0.005
Seawater 2
Method 1 Method 2
0.16" 0.03 0.16" 0.03
0.48" 0.02 0.51" 0.08
0.52" 0.03 0.49" 0.06
a
Values are means of five measurements " S.D. Method 1: isotope dilution method. Method 2: standard addition method. Certified: NRCC certified values Žvalues are given in 95% confidence limits.. c Determined by flow injection cold vapor generation isotope dilution ICP-MS w30x. b
conditions, standard addition calibration plots for Cd, Hg and Pb were linear with calibration coefficients better than 0.998. Detection limits, calculated from the calibration plots, were 0.002, 0.005 and 0.001 ng mly1 for Cd, Hg and Pb, respectively, and were based on the conventional definition as the concentration of the analyte yielding a signal equivalent to 3 S.D.s of the blank signal. Better detection limit would be expected with higher purity of HCl and TAC. Other applications of ETV-ICP-MS for the analysis of volatile trace elements are under investigation in our laboratory.
Acknowledgements This research was supported by a grant from the National Science Council of the Republic of China under Contract NSC 88-2113-M-110-017. References w1x D.C. Gregoire, D.M. Goltz, M.M. Lamoureux, C.L. ` Chakrabarti, Vaporization of acids and their effect on analyte signal in electrothermal vaporization inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 9 Ž1994. 919]926.
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Determination of cadmium, mercury and lead in seawater by electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry Hung-Wei Liu, Shiuh-Jen JiangU , Shin-Hung Liu Department of Chemistry, National Sun Yat-Sen Uni¨ ersity, Kaohsiung 804, Taiwan Received 9 April 1999; accepted 7 June 1999
Abstract Electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry ŽETV-ID-ICP-MS. has been applied to the determination of Cd, Hg and Pb in seawater samples. The isotope ratios of the elements studied in each analytical run were calculated from the peak areas of each isotope. Various modifiers were tested for the best signal of these elements. After preliminary studies, 0.15% mrv TAC and 4% vrv HCl were added to the sample solution to work as the modifier. The ETV-ID-ICP-MS method has been applied to the determination of Cd, Hg and Pb in NASS-4 and CASS-3 reference seawater samples and seawater samples collected from Kaohsiung area. The results for reference sample NASS-4 and CASS-3 agreed satisfactorily with the reference values. Results for other samples determined by isotope dilution and method of standard additions agreed satisfactorily. Detection limits were approximately 0.002, 0.005 and 0.001 ng mly1 for Cd, Hg and Pb in seawater, respectively, with the ETV-ICP-MS method. Precision between sample replicates was better than 20% for most of the determinations. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrothermal vaporization; Isotope dilution; Inductively coupled plasma-mass spectrometry ŽICP-MS.; Cd; Hg; Pb; Sea water
1. Introduction Cadmium, mercury and lead are used in a variety of products and industrial processes. The U
Corresponding author. Fax: q886-7-525-3908. E-mail address: [email protected] ŽS. Jiang.
determinations and monitoring of these elements in environmental and biological samples are extremely important because of the high toxicity of these elements. The determination of Cd, Hg and Pb in environmental samples is difficult, mainly because of its extremely low concentration therein. ICP-MS is a trace metal detection method with
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unique analytical capability, offering both exceptional sensitivity and multi-element capability compared with electrothermal AAS and ICP-AES. The majority of analysis by ICP-MS are carried out on solutions using a conventional pneumatic nebulizer. However, the type of analytical tasks that can be solved by ICP-MS can be extended using a number of other sample introduction techniques which can be easily adapted to ICPMS. Electrothermal vaporization ŽETV. is one of the sample introduction techniques, that is currently employed in ICP-MS w1]15x. Although ETV-ICP-MS was applied to several types of sample analysis, it still suffers from analyte loss during transportation and poor reproducibility problems. Furthermore, non-spectroscopic interference might occur when a high salt content sample was analyzed, which caused the suppression of analyte ion signals w15x. ETAAS and ETV-ICP-MS have been applied in many previous applications for the direct determination of trace metals in seawater w16]19x. In ETV-ICP-MS analysis, in order to alleviate non-spectroscopic interference, a relatively low vaporization temperature can be used which separates the volatile analyte from major matrix components and alleviates non-spectroscopic interference significantly. This strategy has been
applied to the determination of various trace elements in high salt content samples in several previous studies w19]22x. In this study, ETV-isotope dilution-ICP-MS was used to determine the concentration of Cd, Hg and Pb in several seawater samples. A relatively low vaporization temperature was used which separated the analyte from the major matrix components and improved the ion signals of Cd and Pb significantly. Isotope dilution ŽID. techniques were applied in several previous ICP-MS applications w23]26x. Isotope dilution is well recognized as a definitive analytical technique for the determination of the trace elements. Since another isotope of the same element represents the ideal internal standard for that element, isotope dilution results are expected to be highly accurate even when the sample contains high concentrations of concomitant elements andror loss in sample preparation or transport during sample introduction device and ICP. However, the isotope dilution method cannot be used for mono isotopic elements. Furthermore, the isotopes chosen must be free of isobaric and polyatomic interferences. In this study, the ETV-ID-ICP-MS method was applied to the determination of Cd, Hg and Pb in open ocean seawater reference sample NASS-4, nearshore seawater reference
Table 1 Equipment and operating conditions ICP mass spectrometer
Perkin-Elmer SCIEX ELAN 5000
Plasma conditions Outer gas flow rate Žl miny1 . Intermediate gas flow rate Žl miny1 . Carrier gas flow rate Žl miny1 . RF power ŽW. Samplerrskimmer
15.0 0.74 1.08 1100 Nickel
Mass spectrometer settings Dwell time Žms. Scan mode Sweeps per reading Readings per replicate Points per spectral peak Signal measurement mode Isotope monitored
10 Peak-hopping 1 350 1 Integrated 111 Cd, 113 Cd, 201 Hg, 202 Hg 204 Pb, 206 Pb
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sample CASS-3 and seawater samples collected from Kaohsiung area.
the peak areas of each isotope. Since 114 Sn interferes with 111 Cd, 113 Cd and 114 Cd were used for Cd isotope ratio determination.
2. Experimental
2.2. Reagents
2.1. Apparatus and conditions
Trace metal grade HNO3 Ž70% mrm. and HCl Ž35% mrm. were obtained from Fisher ŽFair Lawn, NJ, USA.. Element standard solutions were obtained from SPEX ŽEdison, NJ, USA.. Palladium powder was obtained from KOJUNDO Chemical Laboratory Co., Ltd. ŽSaitama, Japan.. Thioacetamide ŽTAC. was obtained from TCI Chemical ŽTokyo, Japan.. Diphosphorus pentasulfide ŽP2 S5 . was obtained from Merck ŽDarmstadt, Germany.. Isotopically enriched 111 CdO and 204 PbŽNO 3 . 2 were purchased from Oak Ridge National Laboratory ŽOak Ridge, TN, USA. and 201 HgO was obtained from Cambridge Isotope Laboratories ŽAndover, MA, USA.. Stock solutions of each enriched isotope were prepared by dissolution of an accurately weighed quantity of the material in HCl or HNO3 and diluted to volume. The concentration of the spike solution was verified by reversed spike isotope dilution ICP-MS. This was done by spiking the enriched isotope with a suitable amount of natural isotope of each element and measuring the isotope ratio altered with ICP-MS. The concentration of the enriched isotope was then calculated by the equation described in previous papers w25,26x.
A Perkin-Elmer Sciex ŽThornhill, Ontario, Canada. ELAN 5000 ICP-MS spectrometer equipped with a HGA-600MS electrothermal vaporizer was used. Pyrolytic coated graphite tubes and platforms were used throughout. The transfer line consisted of an 80 cm long, 6 mm i.d. PTFE tubing. The sample was introduced with a Model AS-60 autosampler. Teflon autosampler cups were used. The experimental conditions for the ICP-MS and ETV are described in Tables 1 and 2, respectively. ICP operating conditions were selected to maximize the sensitivity of the elements studied to get the best precision and accuracy for isotope ratio determination. The ICP conditions were selected to maximize ion signals while a solution containing 10 ng mly1 of Cd, Hg and Pb in 1% HCl was continuously introduced with a conventional nebulizer. The sensitivity of the instrument might vary slightly from day-to-day. The measurements were made by peak hopping rapidly from one mass to the other, staying only a short time Ždwell time. at each mass. For the best accuracy and precision for isotope ratio determination, a 10-ms dwell time was used in the following experiments w27x. Ion lenses voltages were set to get the best ion signals of the elements studied. The isotope ratios of the elements studied in each analytical run were calculated from
2.3. Sample preparation The applicability of the method to real samples was demonstrated by the analysis of several seawater samples. The seawater samples were
Table 2 HGA-600MS temperature programme for proposed method Žsample volume 20 ml. Step Condition TemperatureŽo C. Ramp time Žs. Hold time Žs. Signal acquisition
1500 1 10 ]
Cooling 2500 1 10 ]
20 5 5 ]
Drying 80 30 10 ]
120 10 10 ]
Vaporization
Cooling
1200 1 13 ON
20 5 5 ]
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determined by the isotope dilution and standard addition methods. Suitable amounts of HCl and TAC were added to 5 ml of the seawater samples to make the final solutions containing 4% vrv HCl and 0.15% mrv TAC. After suitable amounts of enriched isotope or various amounts of element standard solutions were spiked, the solutions were diluted to 10 ml with deionized water. For standard addition analysis, the seawaters were spiked with various amounts of Cd, Hg and Pb Ž0, 0.05, 0.1, 0.5, 1 and 2 ng mly1 in the final solutions. element standards. These solutions were then introduced into ETV-ICP-MS for Cd, Hg and Pb determinations. A seawater sample was passed through the Chelex-100 column three times to remove the trace analyte in the sample w28x. This solution was treated as the blank to correct any analyte contaminants in the reagents used for sample preparation. The analyte concentrations in the sample were then calculated by the equation described in previous papers w25,26x andror from the standard addition calibration curves. Owing to the mass bias effect, the isotopic compositions of Cd, Hg and Pb in both natural element and enriched isotope were obtained by determining the intensities of all isotopes by ICPMS with solution nebulization. The intensities of each isotope obtained during this measurement were used for the isotope ratios and atomic weights calculation of the elements studied.
3. Results and discussion 3.1. Selection of matrix modifier Fig. 1 shows the temporal behavour of Cd, Hg and Pb. The drying temperatures were set at 808C for 30 s and 1208C for 10 s; and the vaporization temperature was set at 10008C. The collected seawater sample was spiked with 2 ng mly1 of Cd and Pb and 5 ng mly1 of Hg and then injected into the furnace without any further pretreatment. As shown in Fig. 1a, all the elements studied evaporated at 10008C. However, a small fraction of Hg evaporated during the drying step when no modifier was used. Due to the volatility
of Hg, a chemical modifier is needed for the determination of Hg by the ETV-ICP-MS method. Modifiers are commonly used in ETV-ICP-MS in order to reduce losses of analyte due to condensation on different parts of the ETV cell or the transfer line that connects the ETV to the ICP-MS w1]7x. Although most effects of the modifiers used in ETV-ICP-MS analysis are believed to be physical, it is likely chemical effects could also be involved in specific instances w5x. The use of modifier would change the chemical or physical characteristics of the sample andror the atomizer surface in order to improve quantitation. In this study, several modifiers, including palladium, Pt, P2 S5 , TAC and several mixed modifiers, were tested for best signals of the elements studied. Fig. 1b,c show the effects of TAC and P2 S5 modifiers on ion signals. As shown in Fig. 1b, Hg was evaporated during the drying step when 0.5% mrv P2 S5 was used as the modifier. On the other hand, as shown in Fig. 1c, no significant mercury ion signal was detected in the drying step when 0.3% mrv TAC was added to the sample solution to work as the modifier. Moreover, as shown in Table 3, although the use of P2 S5 modifier could improve the ion signal of Cd, the signals of Hg and Pb were suppressed. Although not illustrated in the text, Pd Ž200 and 400 mg mly1 ., Pt Ž200 and 400 mg mly1 . and several other mixed modifiers could not improve ion signals of the elements studied. For the simultaneous determination of these elements, after evaluation, a mixture of TAC and HCl was selected as the modifier in this study. TAC has been used as the chemical modifier to improve the signal of Hg in previous ETVICP-MS and ETAAS applications w15,29x. Fig. 2 shows the effect of the amount of TAC on the ion signals. As shown, the signal of Hg increased with the increase of TAC concentration. However, the amount of TAC did not affect Cd and Pb ion signals significantly. After evaluation, 0.15% mrv of TAC was used as the modifier in the ETVICP-MS analysis. As described by Gregoire et al. ` w1x, the presence of mineral acid in ETV-ICP-MS analysis could affect ion signals somewhat. They found that analyte signals were enhanced by as much as a factor of 2 in the presence of 1% vrv
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Table 3 Effect of various modifiers on ion signals a,b Ž n s 7. Modifier
Peak arearcounts 111
Without modifier 0.3% TAC 0.3% TACq 2% HCl 0.5% P2 S5 0.5% P2 S5 q 2% HCl 0.3% TACq 0.5% P2 S5 0.3% TACq 0.5% P2 S5 q 2% HCl
Cd
1000" 100 1100 " 100 2200 " 100 3600 " 300 2900 " 300 2100 " 100 2300 " 100
202
Hg
2300 " 400 5300 " 800 10 000 " 500 2500 " 800 1800 " 200 4200 " 400 3800 " 700
208
Pb
50 000 " 5000 31 000 " 1000 88 000 " 5000 43 000 " 2000 46 000 " 3000 37 000 " 1000 34 000 " 1000
a
Values are means of seven measurements " S.D. An OmniRange setting of 3 was set for Pb measurement.
b
Fig. 2. Effect of TAC concentration on ion signals. The seawater sample was treated with 2% HCl and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various amounts of TAC. Vaporization temperature was set at 10008C.
Fig. 1. ETV-ICP-MS signals of Cd, Hg and Pb; Ža. without any modifier, Žb. with 0.5% mrv P2 S5 as modifier and Žc. with 0.3% mrv TAC as modifier. Drying temperatures were set at 808C and 1208C and vaporization temperature was set at 10008C. Injected seawater sample was spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb. 0.5% P2 S5 andror 0.3% TAC was added to the sample to work as modifier.
HNO3 . In this study, the effects of HCl and HNO3 on the ion signals of Cd, Hg and Pb were studied. Fig. 3 shows the effects of various acids
on the vaporization peaks of the elements studied. As shown, the addition of HNO3 suppressed the ion signals. Furthermore, the vaporization peak of Pb was split. Fig. 4 shows the effect of HCl concentration on ion signals. The collected seawater sample was spiked with 2 ng mly1 of Cd and Pb and 5 ng mly1 of Hg, 0.15% TAC and different amounts of HCl. As shown, the ion signals increased with the increase of HCl concentration. In the following experiments, 4% vrv HCl was added to all sample solutions to work as the modifier. In summary of previous studies, a mixed modifier of 0.15% mrv TAC and 4% vrv HCl was used in the following studies.
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Fig. 5. Effect of pyrolysis temperature on ion signal. The seawater sample was treated with 4% HCl, 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb. Vaporization temperature was set at 10008C. Fig. 3. Effect of Ža. HCl and Žb. HNO3 on Cd, Hg and Pb ion signals. The seawater sample was treated with 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various acids. Vaporization temperature was set at 10008C.
3.2. Selection of ETV operating conditions Fig. 5 shows the effect of pyrolysis temperature on ion signals. The ion signals of Cd and Pb did not change significantly when the temperature was increased. However, Hg ion signal decreased
Fig. 4. Effect of HCl concentration on Cd, Hg and Pb ion signals. The seawater sample was treated with 0.15% TAC and spiked with 5 ng mly1 of Hg and 2 ng mly1 of Cd and Pb and various concentrations of HCl. Vaporization temperature was set at 10008C.
slightly with the increase of pyrolysis temperature even when TAC modifier was used, which could be due to the vaporization of Hg. In order to get the best Hg ion signal, in the following experiments no pyrolysis step was performed. Fig. 6 shows the effect of vaporization temperature on ion signals. As shown, the Hg ion signal did not change significantly with the increase of vaporization temperature. It could be due to the high volatility of Hg. On the other hand, the signals of Cd and Pb increased with the increase of vaporization temperature when the vaporiza-
Fig. 6. Effect of vaporization temperature on ion signal. No pyrolysis step was performed. The composition of the injected solution was the same as in Fig. 5.
H. Liu et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1367]1375
tion temperature was - 9008C, and then decreased gradually with the increase of vaporization temperature. An increase in vaporization temperature increased the amount of matrix vaporized with the analyte, which caused the gradual decrease in integrated peak area. However, as shown in Fig. 7, a more symmetrical peak could be obtained when a higher vaporization temperature was used; this could help to provide more reproducible integrated results. Furthermore, as shown in Fig. 8, a higher signal-to-background ratio could be obtained when the vaporization temperature was set at 12008C. In order to compensate for the symmetric of peak shape, peak area count, integrated blank signal and repeatability of signal, in the following experiments, 12008C was selected as the vaporization temperature. The summary of the ETV operating conditions are listed in Table 2. 3.3. Determination of Cd, Hg and Pb in seawater by ETV-ID-ICP-MS In order to validate the ETV-ID-ICP-MS method, concentrations of Cd, Hg and Pb were
Fig. 7. ETV-ICP-MS signals of Cd, Hg and Pb at various vaporization temperatures: Ža. 7008C; Žb. 12008C. The composition of the injected solution was the same as in Fig. 5.
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determined in the NRCC NASS-4 and CASS-3 seawater reference samples and seawater samples collected from Kaohsiung area. Since the vaporization behaviors of Cd, Hg and Pb and their ICP-MS signals quite depended on the sample matrix, the external calibration method could not be used for the quantification of analyte in this study. The isotope dilution method was used for the determination of Cd, Hg and Pb in seawater samples. For the seawater sample collected locally, an alternative verification process using the method of standard additions was conducted. Since the concentrations of the elements studied in the seawater were quite low, in order to improve the ion signals, five replicate injections and drying of a 20-ml sample was performed before the vaporization of the collected analyte. Analysis results are shown in Table 4. The determined concentrations of the reference sample are in good agreement with the reference values. Those data for which no reference values were provided were also found to agree with that obtained by the standard addition method. The precision between sample replicates is better than 20% for most of the determinations. This experiment indicates that Cd, Hg and Pb in seawater could be readily quantified by isotope dilution inductively coupled plasma mass spectrometry with electrothermal vaporization. Under the selected ETV-ICP-MS operating
Fig. 8. Effect of vaporization temperature on signal to background ŽSrB. ratio Ž n s 5..
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Table 4 Determination of Cd, Hg and Pb in seawater by ETV-ICP-MSa Ž n s 5. Sample
Methodb
Concentration Žng mly1 . Cd
Hg
Pb c
NASS-4
Certified Method 1 Method 2
0.016" 0.003 0.017" 0.001 0.018" 0.003
0.40" 0.04 0.51" 0.08 0.57" 0.10
0.013" 0.005 0.011" 0.002 0.013" 0.002
CASS-3
Certified Method 1 Method 2
0.030" 0.005 0.028" 0.003 0.028" 0.003
0.27 " 0.03c 0.29" 0.05 0.29" 0.02
0.012" 0.004 0.012" 0.003 0.019" 0.006
Seawater 1
Method 1 Method 2
0.027" 0.001 0.026" 0.001
0.12" 0.07 0.10" 0.06
0.034" 0.006 0.032" 0.005
Seawater 2
Method 1 Method 2
0.16" 0.03 0.16" 0.03
0.48" 0.02 0.51" 0.08
0.52" 0.03 0.49" 0.06
a
Values are means of five measurements " S.D. Method 1: isotope dilution method. Method 2: standard addition method. Certified: NRCC certified values Žvalues are given in 95% confidence limits.. c Determined by flow injection cold vapor generation isotope dilution ICP-MS w30x. b
conditions, standard addition calibration plots for Cd, Hg and Pb were linear with calibration coefficients better than 0.998. Detection limits, calculated from the calibration plots, were 0.002, 0.005 and 0.001 ng mly1 for Cd, Hg and Pb, respectively, and were based on the conventional definition as the concentration of the analyte yielding a signal equivalent to 3 S.D.s of the blank signal. Better detection limit would be expected with higher purity of HCl and TAC. Other applications of ETV-ICP-MS for the analysis of volatile trace elements are under investigation in our laboratory.
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