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low-pressure inductively coupled plasma atomic emission spectrometry for trace elemental analysis in microliter samples
Microchemical Journal 78 (2004) 127 – 134 www.elsevier.com/locate/microc
Analytical evaluation of electrothermal vaporization/low-pressure inductively coupled plasma atomic emission spectrometry for trace elemental analysis in microliter samples Sang-Deuk Kim a, Jae-Min Lim a, Won Lee b, Young-Sang Kim c, Sang-Ho Nam d, Yong-Ill Lee a,* a
Department of Chemistry, Changwon National University, Changwon 641-773, South Korea b Department of Chemistry, Korea University, Jochiwon 339-700, South Korea c Department of Chemistry, Kyunghee University, Seoul 130-701, South Korea d Department of Chemistry, Mokpo National University, Mokpo 534-729, South Korea Received 12 February 2004; accepted 16 March 2004 Available online 14 May 2004
Abstract A novel method for the determination of trace elements in microliter samples using the tantalum filament electrothermal vaporization/lowpressure inductively coupled plasma (ETV/LP-ICP) atomic emission spectrometry has been developed. An improved tantalum filament ETV was directly coupled with LP-ICP system for efficient vaporization of microliter samples and further quantitative analysis. The experimental parameters including ETV current, rf power and mass flow rate of argon carrier gas were optimized using the copper emission signal produced by 5 Al of standard solution (5 Ag/ml). Under the optimized condition, the analytical performances including linearity, precision and detection limit for the developed system were investigated. Absolute detection limits in the range of 22 – 391 pg for selected eight elements (Fe, Cu, Cr, Mn, Pb, K, Zn and Mg) were obtained with satisfactory precision (<8.9% RSD). The feasibility of the developed system has been demonstrated by analyzing wheat gluten NIST standard sample. D 2004 Elsevier B.V. All rights reserved. Keywords: Electrothermal vaporization; Inductively coupled plasma atomic emission; Trace elemental analysis
1. Introduction Electrothermal vaporization (ETV) has been demonstrated a considerable utility as a direct sample introduction device for various atomic spectrometry including inductively coupled plasma atomic emission (ICP-AES) and mass spectrometry (ICP-MS) [1]. It has several advantages over the conventional pneumatic nebulizers, such as less sample amount consumption, high sample transport efficiency, improved sensitivity, lower detection limits and direct sample introduction of solid samples. Moreover, the analyte could be separated from the matrix components by using an appropriate temperature/time programming before the analyte is transported into the ICP torch [2]. Although some elements can form refractory carbides which lead to a decrease and sometimes complete suppression of analytical signals [3], graphite furnaces are still predominant type for * Corresponding author. Tel.: +82-55-279-7437; fax: +82-55-279-7439. E-mail address: [email protected] (Y.-I. Lee). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.03.003
ETV devices [4– 9]. Generally, two methods can be adapted in ETV/LP-ICP to prevent the formation of refractory carbides, by adding chemical modifiers such as halogenating reagents [10 –13] or using refractory metals (tungsten or tantalum) as furnace materials [14]. The attraction of the refractory metal filaments also lies in their relatively low price and high reproducibility of geometric shape and physical properties compared with the graphite furnace. A simple, inexpensive and easily connected to conventional atomic absorption spectrometry has been developed in this laboratory using a small Pyrex glass apparatus with tantalum boat filament [15 –17]. The system has been shown to have potential in the direct analysis of trace metal ions in microliter samples. Atmospheric ICP is the current plasma source of choice for a variety of elemental analyses. Recently, a low-pressure ICP (LP-ICP) has been developed as an efficient excitation source for atomic emission spectrometry due to its advantages over a conventional atmosphere-pressure ICP, such as low cost, low power, low gas consumption, a single gener-
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ator, plasma stability and being relatively free of interference [18 – 23]. Typically, an atmospheric ICP uses 17 l/min of argon with power consumption of 1.0 – 1.5 kW. LP-ICP power required is normally below 200 W, with support gas flow, being typically below 1 l/min. Self-ignition can also be made possible by not directing seed electrons to the plasma zone, which is used for the atmospheric ICP system. It was also found that low levels of elemental detection could be obtained by coupling LP-ICP with mass spectrometry [24 – 26], and a low-pressure plasma source could be capable of being operated in a tunable mode for the formation of atomic or molecular ions [27]. In this work, an improved ETV device equipped with tantalum boat filament was connected directly to LP-ICP system for sample introduction and then evaluated the analytical performances for trace elemental analysis in microliter samples. Several important experimental parameters of LP-ICP including gas flow rates and rf power and of ETV such as current programming were investigated to provide the optimum conditions for further application in elemental analysis of microliter samples. The developed method was also applied to determine the concentration of trace elements (K, Mn, Zn and Mg) in wheat gluten NIST standard with satisfactory results.
2. Experimental 2.1. Instrumentation A schematic diagram of plasma torch and ETV assembly that were designed and fabricated with quartz and Pyrex glass was shown in Fig. 1. The length of plasma torch is 220 mm that can provide a plasma that is usually 20– 30 mm in length.
A quartz nozzle with 2 mm i.d. in the center of tube was used for sample introduction to LP-ICP torch. An improved ETV consists of a Pyrex glass chamber, electrodes and tantalum boat filament. A tantalum boat was fabricated with 0.025mm-thick foils (510 mm, Aldrich Chemical, Milwaukee, WI, USA) and mounted on copper electrodes. The glass cone has a diameter of 3 cm at the base and a central height of 4 cm. The flow of argon carrier gas remained spiral in the vaporizer chamber by forcing argon to flow tangentially around the wall of the glass cone. The tangential flow cools the glass wall and enters the vaporized samples with the argon flow. On –off gas switching valves are equipped between the torch cell and ETV chamber for the interface and drainage during sample injections and drying procedures. The vaporized sample is directly penetrated from vaporization chamber to the torch cell through a quartz nozzle and then swept into the plasma by the stream of argon carrier gas. A DC power supply (Max. 50 V and 50 A, Korea-Switching, Korea) was connected to ETV electrode terminals to heat the filament. A detailed description of the design and optimization of the LP-ICP used in this work has been given previously [23]. In brief, the plasma generator unit consists of an rf generator (YSE-035, Young-Sin Engineering, Korea) and an impedance matching network (AMN-100). An rf generator with a working frequency of 13.56 MHz that provides a maximum power of 300 W was used. The water-cooled load coil was made of copper tube with 4 mm o.d. A coil of six turns was used for proper impedance matching. The load coil was well shielded by an iron cylinder equipped with a view port to observe the plasma condition. Mass flow controller (5850E Series, BROOK, Japan) was used for controlling the pressure between the plasma torch and vaporization chamber. A rotary vacuum pump (200 l/ min, Woo-Sung, Korea) was used to accomplish the evacu-
Fig. 1. Schematic diagram of the experimental setup for ETV/LP-ICP.
S.-D. Kim et al. / Microchemical Journal 78 (2004) 127–134
ation of ambient gas in a torch and a vacuum gauge (EYESYS ConvecTorr, Varian, USA) was connected at the other port of the torch for precise vacuum measurements. The spectrometer (SpectraPro-300i, Acton Research, USA) that has a 300 mm of focal length and 0.1 nm of resolution with a 1200 grooves/mm (300-nm blaze) grating was used for spectral evaluation of LP-ICP. The spectrometer is equipped with a thermoelectrically cooled photodiode array (PDA) detector (RY-1024, Princeton Instruments, USA) that consists of 1024 pixels and was controlled by WinSpec 1.6.2 software (Princeton Instruments). This PDA spectrometer system can view a 60-nm wavelength region with the 1200 grooves/mm grating simultaneously. One fused silica plano-convex lens of 60-mm focal length was used to collect the emission of LP-ICP to an optical fiber bundle. An UV – VIS fiber optic bundle (LG455, Acton Research) 1 m long consisted of a single row containing 19 fibers was used to deliver the plasma emission to spectrometer. The core diameter of the single fiber in silica is 200 Am and the fiber bundle transmission characteristic is about 45% through 190 –1100 nm. The other end of the fiber bundle was coupled to an entrance slit of spectrometer and the slit width was used at 10 Am. All emission signals were integrated within 1.2 s of exposure time and transferred to IBM computer for further data processing. 2.2. Reagents Stock standard solutions (1000 mg/l) of various metal ions (Cu, Cr, Zn, Mn, Pb, K, Fe and Mg) were purchased from the Aldrich. Working standard solutions were prepared daily by appropriate dilution of the respective stock standard solutions. All solutions were prepared by high purity deionized water (resistivity; 18 MV cm 1) obtained from Milli-Q water purification system (Millipore, Molsheim, France). The reference standard samples of NIST wheat gluten (RM8418) were used for the evaluation of accuracy for a developed ETV-LP-ICP system. 2.3. Operating procedures Before the generation of plasma, 5 Al of sample was loaded onto the filament through the sample injection port using a Hamilton syringe and initially desolvated by passage of 13-A current for 15 s through the filament. The desolvation step was set a typical temperature of 120 jC and no detectable losses of the analyte elements were observed. The vaporized solvent is swept by the argon carrier gas flow through the needle valve (B) for drainage. After the desolvation step, valve (B) was closed, and valve (A) and valve (C) were opened a little gradually to keep a proper pressure for self-ignition in the plasma torch and ETV chamber with constant gas flow. Normally, the extensive plasma that spread over the plasma torch was generated through selfignition at below 1 Torr. After the self-ignition of the plasma, argon flow was gradually increased for the formation of ‘‘core zone’’ [18] to achieve efficient atomization/
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excitation of metal ions. Further vaporization of the sample deposited on the filament was carried out by instantaneously raising the current up to 42 A for 4 s. The vaporized sample was transported and introduced into the plasma and finally analyzed by atomic emission spectrometry.
3. Results and discussion 3.1. Optimization of ETV/LP-ICP system Key operating parameters for ETV/LP-ICP such as ETV current, the flow rate of argon carrier gas and rf power were investigated to minimize the nonspectral interference effects. As the vaporization and the excitation of analytes were realized in two separate steps, the ETV and the ICP parameters have been optimized independently. The resistive heating of tantalum filament performed the volatilization of the sample. The filament was heated by supplying the electric current through copper electrodes by DC power supply operated in the constant voltage mode. The operating current was optimized with respect to the maximum peak heights of emission lines for various elements in aqueous standard solutions (5 Al of 5 ng/Al) according to the different volatility of elements, while rf power and argon carrier gas flow rate were kept constant at 200 W and 800 ml/min, respectively. Amount of sample that enters the plasma during the vaporization stage is critical. Introducing too much sample destabilizes the plasma and cools the plasma resulting in lower signal of the analyte than otherwise would have been obtained. The vaporization current has a critical effect on the analytical emission signal. Fig. 2 shows the effect of Cu emission intensity at 324.754 nm as a function of vaporization current from 25 to 45 A with 4 s holding time. As can be seen, the emission intensity of copper increased gradually with the increase of vaporization current and reached the maximum at 42 A of ETV current. However, the signal decreased when the vaporization current was larger than 42 A. The background signal at 323.086 nm was generally stable until 40 A of ETV current and then increased abruptly at 45 A. It might be due to the vaporization of impurities or tantalum filament itself. Furthermore, the highest signal-to-background ratio (S/B) could be obtained when the vaporization current was set at 42 A. Typical operating conditions for optical arrangement of the LP-ICP instrument and the ETV current program for Cu analysis are given in Table 1. The optimum vaporization currents for another seven elements, Fe, Cr, Mn, Pb, K, Zn and Mg, were also investigated for further studies. The emission lines used and the optimized ETV currents for eight elements are listed in Table 2. Optimum currents differ from one element to another because of their different physical and chemical properties. For multielement analysis, the selected ETV current should be compromised because only in some cases can common operating conditions be used for many ele-
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Fig. 2. Variation of copper emission intensity at 324.754 nm (A) and background at 323.086 nm (B), depending on the ETV current. Error bars represent standard deviations (r) of five measurements.
ments. The infrared thermometer was used to estimate the filament temperature. The temperature was reached about 1500 jC when the current was increased up to 42 A. However, the real filament temperature might be higher than the values estimated because IR thermometer integrates bright filament as well as dark surroundings in calculating temperature of the vaporizer. Representative emission profiles for Fe, Mn and Pb operated at an optimized ETV currents within selected wavelength regions are shown in Fig. 3. 3.2. Influences of flow rate of argon carrier gas and rf power The flow rate of argon carrier gas is one of the most important parameters in ETV/LP-ICP because transport efficiency and plasma pressure are of great concern to these parameters. The variation of argon flow rate from 300 to 1100 ml/min results in the increase of plasma pressure from 0.4 to 1.5 Torr, respectively. To optimize the flow rate of argon carrier gas, rf power was held constant at 200 W and ETV currents were set at 42 A (4 s). Fig. 4 shows the effect of argon flow rate in the range of 300– 1100 ml/min on copper emission signal on peak height at 324.754 nm. The copper emission intensity increased gradually with the increase of argon flow rate and was reached to the maximum with argon flow of 800 ml/min. Generally, very stable argon plasma was Table 1 The optimized experimental conditions for ETV/LP-ICP-AES system LP-ICP and spectrometer
ETV current program
rf power Reflected rf power Ar gas flow rate Integration time Slit width
Cu emission line Sample volume Sample conc. Dry Vaporization Clean
200 W 13 W 800 ml/min 1.2 s 10 Am
324.754 nm 5 Al 5 ng/Al 13 A (15 s) 42 A (4 s) 45 A (3 s)
generated at the range of 600 –1200 ml/min of argon flow rate. The typical relative standard deviation (RSD) of copper emission was <2.2% from five consecutive measurements for each point. However, the signal decreased when argon flow rate was larger than 900 ml/min because of lowering the residence time of individual atoms in the plasma at higher flow rate. For other investigations, the flow rate of argon gas was held constant at 800 ml/min for maximum collection of emission signals. At optimum experimental conditions mentioned above (argon carrier gas flow rate: 800 ml/min and ETV current for copper: 42 A and 4 s), the emission signal was gradually increased with increasing the rf power within the experimental forward power range of 100– 220 W. However, the plasma was unstable at the high rf power (over 200 W) because the reflected power also increased at the same time when the forward power increased. The rf power of 200 W was thus selected as recommended standard operating condition for the ETV-LP-ICP system. 3.3. Detection limits and precision The analytical performances including linearity of calibration, precision and detection limits of the developed Table 2 Emission lines used and optimized currents for selected eight elements in ETV/LP-ICP system Element
Wavelength (nm)
ETV currents (A)
Fe Cu Cr Mn Pb K Zn Mg
371.994(I) 324.754(I) 359.349(I) 257.610(II) 405.783(I) 766.491(I) 330.259(I) 279.504(II)
43 42 42 40 40 32 25 40
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Fig. 3. Typical LP-ICP emission spectra for (A) Fe, (B) Mn and (C) Pb.
ETV/LP-ICP system were evaluated by using the standard solutions for eight elements. All solutions were prepared by serial dilution of commercially available 100-ppm aqueous ICP standards. The figures of merit of the calibration curves for Fe, Cu, Cr, Mn, Pb, K, Zn and Mg are illustrated in Table 3. All calibration graphs developed by the use of selected wavelengths for eight elements were linear with a regression coefficient (r) of at least 0.9941 and the repro-
ducibility for five consecutive measurements (n=5) yielded the RSD (%) of 2.8– 8.9% for eight elements. The RSD values shown in Table 3 were the average values for each point in calibration graphs. The detection limits were calculated from three times the standard deviation of the blank measurements and the slopes of the calibration graphs. The calculated absolute detection limits for Cu, Mn and Cr are determined as 0.032, 0.033 and
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Fig. 4. Effect of argon flow rate on copper emission intensity at 324.754 nm by ETV/LP-ICP AES.
0.038 ng, respectively. The elements Zn, K, Pb and Fe also show low values of absolute detection limits, 0.081, 0.141, 0.227 and 0.391 ng, respectively. These detection limits are comparable to those achieved by a graphite furnace ETV/ ICP [4,5] and a tungsten coil ETV/ICP [14]. However, K has a little higher values (0.14 ng) compared to that of ETVFAAS developed in our laboratory previously because of lower boiling points of K (b.p./K=1047 K) [28] and too high excitation temperature of the plasma. In general, the detection limits in this work are largely improved over those obtained from ETV/FAAS [19] for six selected elements except for K, indicating the outstanding analytical capability for the determination of trace elements of the developed ETV/LP-ICP-AES. 3.4. Quantitative application of ETV/LP-ICP The determinations of minor and trace constituents in the certified reference materials of wheat gluten (NIST RM8418) were used to evaluate the accuracy and feasibility of ETV/LP-ICP-AES. Wheat gluten sample (0.4525 g) was digested with 3 ml of concentric nitric acid (70%), 3 ml of Table 3 Detection limits (3rblank, n=5), precision and correlated coefficient (r) of ETV/LP-ICP system Elements
Wavelength (nm)
%RSD
r
Detection limit (ng)
EVT-FAASa (ng)
Fe Cu Cr Mn Pb K Zn Mg
371.994(I) 324.754(I) 359.349(I) 257.610(II) 405.783(I) 766.491(I) 330.259(I) 279.504(II)
8.9 4.2 5.8 6.1 4.9 2.8 3.3 4.4
0.9982 0.9985 0.9999 0.9987 0.9977 0.9941 0.9986 0.9959
0.39 0.032 0.038 0.033 0.23 0.14 0.083 0.022
– 0.066 – 0.058 0.274 0.024 – –
a
Ref. [17].
hydrogen peroxide (30%) and 0.5 ml of hydrochloric acid under microwave digestion system. The residue was then diluted by distilled water to a final volume of 25 ml. For analysis of Zn and Mg, digestion solution was diluted by a factor of 2 with distilled water, and for K and Mg, was diluted by a factor of 4 and 10, respectively. Each solution was analyzed based on the analytical procedure under the optimized experimental conditions mentioned above. Standard addition method was adapted for accuracy assessment of the developed system. It is particularly useful to diminish the matrix effects for analyzing wheat gluten powder samples in which the likelihood of matrix effects are substantial. Although the calibration curves developed in this work can be used for the accuracy evaluation, the success of the calibration graph method is critically dependent upon how accurately the analyte concentration of the standards are known and how closely the matrix of standards resembles that of the samples to be analyzed. From the linear regression fit formula obtained from each standard addition curve, the concentration of the sample was calculated. The analyzed results are listed in Table 4, together with average %RSD values for each element. As can be seen, the results obtained were in good agreement with the certified values. The average accuracy of the developed Table 4 Analytical results for K, Mn, Zn and Mg in a wheat gluten NIST standard sample (NIST RM8418) by ETV/LP-ICP-AES compared with the certified concentrations Elements
Selected lines (nm)
%RSD
r
Measured conc. (Ag/ml)
Certified value (Ag/ml)
K Mn Zn Mg
766.496 257.610 330.259 279.504
6.1 9.2 6.3 8.4
0.9988 0.9945 0.9934 0.9978
442F33 14.3F1.6 54.1F4.4 543F56
472F61 14.3F0.8 53.8F3.7 510F47
Confidence interval at 95% confidence level (tstudent=2.776, n=5).
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system for determination of K, Mn, Zn and Mg falls in the range of 0.1– 6.8% with acceptable %RSD values (<9.2%). The total analysis time per sample was less than 2 min. No serious memory effects were observed even at high analyte concentrations as long as the plasma was left on for a period of 20 s. The filament and purge time optimization dealt with removing a molecular interferent and reducing the rate of erosion of the filament.
4. Conclusion A new design for ETV/LP-ICP system equipped with an improved tantalum-filament ETV system as a sample vaporizer has been developed in this work. Low carrier gas (16 ml/min) and low rf power consumption (200 W) for optimum operation were achieved for trace elemental analysis in microliter samples. The main advantages of the present tantalum-filament ETV system include a simple, small and inexpensive construction, high heating rates and microliter sample capability (only 5 Al). Moreover, plasma excitation processes of the LP-ICP were more efficient with an improved ETV owing to the removal of solvent before the sample is introduced. The creation of linear calibration curves for selected eight elements was constructed with good linearity and precision. Absolute detection limits achieved for eight elements were in sub-nanogram range. The standard addition method was applied to assess the accuracy of ETV/LP-ICP using a NIST wheat gluten standard sample. The concentration of K, Mn, Zn and Mg in the standard sample determined by ETV/LP-ICP is in good agreement with those of the certified value within the error range. The results show that ETV/LP-ICP method has a great potential for the determination of trace elements of parts-per-million (Ag/g) level in wheat powder samples with satisfactory analytical results. An improved ETV unit described here could be connected with any excitation source, which can tolerate vapors and dry aerosols.
Acknowledgements The authors gratefully acknowledge the financial support by the Korea Research Foundation (Grant No. 1998-001D00459) and the interdisciplinary research program of KOSEF (Grant No. 1999-1-124-001-3).
References [1] C.W. McLeod, M.W. Routh, M.W. Tikkanen, Introduction of Solids into Plasmas, in: A. Montaser, D.W. Golightly (Eds.), Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed., VCH, New York, 1992, p. 721, Chap. 16. [2] K.C. Ng, J.A. Caruso, Electrothermal Vaporization, in: J. Sneddon (Ed.), Sample Introduction in Atomic Spectroscopy, Elsevier, Amsterdam, The Netherlands, 1990, p. 165, Chap. 7.
133
[3] B. Hu, Z. Jiang, Y. Qin, Y. Zeng, Fluorination with polytetrafluoroethylene slurry in electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry, Anal. Chim. Acta 319 (1996) 255. [4] U. Schaffer, V. Krivan, A graphit furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry, Anal. Chem. 70 (1998) 482. [5] L. Moens, P. Verrept, S. Boonen, F. Vanhaecke, R. Dams, Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, Spectrochim. Acta, Part B 50 (1995) 463. [6] P. Verrept, R. Dams, U. Kurfurst, Electrothermal vaporization inductively coupled plasma atomic emission spectrometry for the analysis of solid samples: contribution to instrumentation and methodology, Fresenius’ J. Anal. Chem. 346 (1993) 1035. [7] T. Kantor, Gy. Zaray, Graphite furnace for alternative combination with d.c. arc of inductively coupled plasma. Introduction and analysis of solid samples, Fresenius’ J. Anal. Chem. 342 (1992) 927. [8] H. Nickel, Z. Zagorska, A new electrothermal vaporization device for direct sampling of ceramic powders for inductively coupled plasma optical emission spectrometry, Spectrochim. Acta, Part B 50 (1995) 527. [9] R.D. Ediger, S.A. Beres, The role of chemical modifiers in analyte transport loss interferences with electrothermal vaporization ICP-mass spectrometry, Spectrochim. Acta, Part B 47 (1992) 907. [10] F. Wang, Z. Jiang, B. Hu, T. Peng, Comparative studies on chemical modification of polytetrafluoroethylene slurry in ETV-ICP and ETAAS, J. Anal. At. Spectrom. 14 (1999) 1619. [11] S.Z. Chen, F. Li, Z.H. Liao, T.Y. Peng, Z.C. Jiang, Study on the direct analysis of solid powder biological samples using fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry with PTFE slurry modifier, Fresenius’ J. Anal. Chem. 364 (1999) 556. [12] S. Chen, T. Peng, Z. Jiang, Z. Liao, B. Hu, Direct determination of rare earth impurities in lanthanum oxide by fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry with slurry sampling, J. Anal. At. Spectrom. 14 (1999) 1723. [13] H. Xiong, B. Hu, Z. Jiang, Micro-column separation/preconcentration combined with fluorinating electrothermal vaporization-inductively couple plasma-atomic emission spectrometry for determination of ultratrace rare earth elements, J. Anal. At. Spectrom. 15 (2000) 759. [14] P. Barth, S. Hauptkorn, V. Krivan, Improved slurry sampling electrothermal vaporization system using a tungsten coil for inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom. 12 (1997) 1351. [15] P. Barth, S. Hauptkorn, V. Krivan, Improved slurry sampling electrothermal vaporization system using a tungsten coil for inductively coupled plasma atomic emission spectrometry, J. Anal. Atom. Spectrom. 12 (1997) 1351. [16] Z. Jiang, B. Hu, Y. Qin, Y. Zeng, Fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry: an effective method for direct determination of refractory elements in solid biological samples, Microchem. J. 53 (1996) 326. [17] Y.I. Lee, J.K. Kim, K.H. Kim, Y.J. Yoo, G.H. Back, S.C. Lee, Design and critical evaluation of improved electrothermal vaporization flame atomic absorption/emission spectrometry for direct determination of trace metals in microliter samples, Microchem. J. 60 (1998) 231. [18] E.H. Evans, J.A. Caruso, Low pressure inductively coupled plasma source for mass spectrometry, J. Anal. At. Spectrom. 8 (1993) 427. [19] X. Yan, T. Tanaka, H. Kawaguchi, Reduced-pressure inductively coupled plasma mass spectrometry for nonmetallic elements, Appl. Spectrosc. 50 (1996) 182. [20] J.W. Waggoner, L.S. Milstein, M. Belkin, K.L. Sutton, J.A. Caruso, H.B. Fannin, Application of a low power/reduced pressure helium ICP ionization source for mass spectrometric detection of organobro-
134
[21] [22]
[23]
[24]
S.-D. Kim et al. / Microchemical Journal 78 (2004) 127–134 mine compounds and derivatized organotin compounds, J. Anal. At. Spectrom. 15 (2000) 13. E.H. Evans, W. Pretorius, L. Ebdon, S. Rowland, Low-pressure inductively coupled plasma ion source, Anal. Chem. 66 (1994) 3400. Y. Sung, H.B. Lim, R.S. Houk, Diagnostic studies of a low-pressure inductively coupled plasma in argon using double Langmuir probe, J. Anal. At. Spectrom. 17 (2002) 565. J.-M. Lim, J.-K. Kim, C.-S. Lee, G.-H. Back, Y.-I. Lee, Development and characterization of directly connected laser ablation/low-pressure inductively coupled plasma atomic emission spectrometry for solid sample analysis, Appl. Spectrosc. 54 (2000) 1253. G. O’Connor, L. Ebdon, E.H. Evans, Qualitative and quantitative determination of tetraethyllead in fuel using low pressure ICP-MS, J. Anal. At. Spectrom. 14 (1999) 1303.
[25] C. Brede, S. Pedersen-Bjergaard, E. Lundanes, T. Greibrokk, Microplasma mass spectrometric detection in capillary gas chromatography, Anal. Chem. 70 (1998) 513. [26] X. Yan, T. Tanaka, H. Kawasuchi, Electrothermal vaporization for the determination of halogens by reduced pressure inductively coupled plasma mass spectrometry, Spectrochim. Acta B 51 (1996) 1345. [27] B. Rosenkranz, G. O’Connor, E.H. Evans, Low pressure inductively coupled plasma ion source for atomic and molecular mass spectrometry: investigation of alternative reagent gases for organomercury speciation in tissue and sediment, J. Anal. At. Spectrom. 15 (2000) 7. [28] J. Emsley, The Elements, 2nd ed., Clarendon Press, Oxford, UK, 1991.
Analytical evaluation of electrothermal vaporization/low-pressure inductively coupled plasma atomic emission spectrometry for trace elemental analysis in microliter samples Sang-Deuk Kim a, Jae-Min Lim a, Won Lee b, Young-Sang Kim c, Sang-Ho Nam d, Yong-Ill Lee a,* a
Department of Chemistry, Changwon National University, Changwon 641-773, South Korea b Department of Chemistry, Korea University, Jochiwon 339-700, South Korea c Department of Chemistry, Kyunghee University, Seoul 130-701, South Korea d Department of Chemistry, Mokpo National University, Mokpo 534-729, South Korea Received 12 February 2004; accepted 16 March 2004 Available online 14 May 2004
Abstract A novel method for the determination of trace elements in microliter samples using the tantalum filament electrothermal vaporization/lowpressure inductively coupled plasma (ETV/LP-ICP) atomic emission spectrometry has been developed. An improved tantalum filament ETV was directly coupled with LP-ICP system for efficient vaporization of microliter samples and further quantitative analysis. The experimental parameters including ETV current, rf power and mass flow rate of argon carrier gas were optimized using the copper emission signal produced by 5 Al of standard solution (5 Ag/ml). Under the optimized condition, the analytical performances including linearity, precision and detection limit for the developed system were investigated. Absolute detection limits in the range of 22 – 391 pg for selected eight elements (Fe, Cu, Cr, Mn, Pb, K, Zn and Mg) were obtained with satisfactory precision (<8.9% RSD). The feasibility of the developed system has been demonstrated by analyzing wheat gluten NIST standard sample. D 2004 Elsevier B.V. All rights reserved. Keywords: Electrothermal vaporization; Inductively coupled plasma atomic emission; Trace elemental analysis
1. Introduction Electrothermal vaporization (ETV) has been demonstrated a considerable utility as a direct sample introduction device for various atomic spectrometry including inductively coupled plasma atomic emission (ICP-AES) and mass spectrometry (ICP-MS) [1]. It has several advantages over the conventional pneumatic nebulizers, such as less sample amount consumption, high sample transport efficiency, improved sensitivity, lower detection limits and direct sample introduction of solid samples. Moreover, the analyte could be separated from the matrix components by using an appropriate temperature/time programming before the analyte is transported into the ICP torch [2]. Although some elements can form refractory carbides which lead to a decrease and sometimes complete suppression of analytical signals [3], graphite furnaces are still predominant type for * Corresponding author. Tel.: +82-55-279-7437; fax: +82-55-279-7439. E-mail address: [email protected] (Y.-I. Lee). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.03.003
ETV devices [4– 9]. Generally, two methods can be adapted in ETV/LP-ICP to prevent the formation of refractory carbides, by adding chemical modifiers such as halogenating reagents [10 –13] or using refractory metals (tungsten or tantalum) as furnace materials [14]. The attraction of the refractory metal filaments also lies in their relatively low price and high reproducibility of geometric shape and physical properties compared with the graphite furnace. A simple, inexpensive and easily connected to conventional atomic absorption spectrometry has been developed in this laboratory using a small Pyrex glass apparatus with tantalum boat filament [15 –17]. The system has been shown to have potential in the direct analysis of trace metal ions in microliter samples. Atmospheric ICP is the current plasma source of choice for a variety of elemental analyses. Recently, a low-pressure ICP (LP-ICP) has been developed as an efficient excitation source for atomic emission spectrometry due to its advantages over a conventional atmosphere-pressure ICP, such as low cost, low power, low gas consumption, a single gener-
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ator, plasma stability and being relatively free of interference [18 – 23]. Typically, an atmospheric ICP uses 17 l/min of argon with power consumption of 1.0 – 1.5 kW. LP-ICP power required is normally below 200 W, with support gas flow, being typically below 1 l/min. Self-ignition can also be made possible by not directing seed electrons to the plasma zone, which is used for the atmospheric ICP system. It was also found that low levels of elemental detection could be obtained by coupling LP-ICP with mass spectrometry [24 – 26], and a low-pressure plasma source could be capable of being operated in a tunable mode for the formation of atomic or molecular ions [27]. In this work, an improved ETV device equipped with tantalum boat filament was connected directly to LP-ICP system for sample introduction and then evaluated the analytical performances for trace elemental analysis in microliter samples. Several important experimental parameters of LP-ICP including gas flow rates and rf power and of ETV such as current programming were investigated to provide the optimum conditions for further application in elemental analysis of microliter samples. The developed method was also applied to determine the concentration of trace elements (K, Mn, Zn and Mg) in wheat gluten NIST standard with satisfactory results.
2. Experimental 2.1. Instrumentation A schematic diagram of plasma torch and ETV assembly that were designed and fabricated with quartz and Pyrex glass was shown in Fig. 1. The length of plasma torch is 220 mm that can provide a plasma that is usually 20– 30 mm in length.
A quartz nozzle with 2 mm i.d. in the center of tube was used for sample introduction to LP-ICP torch. An improved ETV consists of a Pyrex glass chamber, electrodes and tantalum boat filament. A tantalum boat was fabricated with 0.025mm-thick foils (510 mm, Aldrich Chemical, Milwaukee, WI, USA) and mounted on copper electrodes. The glass cone has a diameter of 3 cm at the base and a central height of 4 cm. The flow of argon carrier gas remained spiral in the vaporizer chamber by forcing argon to flow tangentially around the wall of the glass cone. The tangential flow cools the glass wall and enters the vaporized samples with the argon flow. On –off gas switching valves are equipped between the torch cell and ETV chamber for the interface and drainage during sample injections and drying procedures. The vaporized sample is directly penetrated from vaporization chamber to the torch cell through a quartz nozzle and then swept into the plasma by the stream of argon carrier gas. A DC power supply (Max. 50 V and 50 A, Korea-Switching, Korea) was connected to ETV electrode terminals to heat the filament. A detailed description of the design and optimization of the LP-ICP used in this work has been given previously [23]. In brief, the plasma generator unit consists of an rf generator (YSE-035, Young-Sin Engineering, Korea) and an impedance matching network (AMN-100). An rf generator with a working frequency of 13.56 MHz that provides a maximum power of 300 W was used. The water-cooled load coil was made of copper tube with 4 mm o.d. A coil of six turns was used for proper impedance matching. The load coil was well shielded by an iron cylinder equipped with a view port to observe the plasma condition. Mass flow controller (5850E Series, BROOK, Japan) was used for controlling the pressure between the plasma torch and vaporization chamber. A rotary vacuum pump (200 l/ min, Woo-Sung, Korea) was used to accomplish the evacu-
Fig. 1. Schematic diagram of the experimental setup for ETV/LP-ICP.
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ation of ambient gas in a torch and a vacuum gauge (EYESYS ConvecTorr, Varian, USA) was connected at the other port of the torch for precise vacuum measurements. The spectrometer (SpectraPro-300i, Acton Research, USA) that has a 300 mm of focal length and 0.1 nm of resolution with a 1200 grooves/mm (300-nm blaze) grating was used for spectral evaluation of LP-ICP. The spectrometer is equipped with a thermoelectrically cooled photodiode array (PDA) detector (RY-1024, Princeton Instruments, USA) that consists of 1024 pixels and was controlled by WinSpec 1.6.2 software (Princeton Instruments). This PDA spectrometer system can view a 60-nm wavelength region with the 1200 grooves/mm grating simultaneously. One fused silica plano-convex lens of 60-mm focal length was used to collect the emission of LP-ICP to an optical fiber bundle. An UV – VIS fiber optic bundle (LG455, Acton Research) 1 m long consisted of a single row containing 19 fibers was used to deliver the plasma emission to spectrometer. The core diameter of the single fiber in silica is 200 Am and the fiber bundle transmission characteristic is about 45% through 190 –1100 nm. The other end of the fiber bundle was coupled to an entrance slit of spectrometer and the slit width was used at 10 Am. All emission signals were integrated within 1.2 s of exposure time and transferred to IBM computer for further data processing. 2.2. Reagents Stock standard solutions (1000 mg/l) of various metal ions (Cu, Cr, Zn, Mn, Pb, K, Fe and Mg) were purchased from the Aldrich. Working standard solutions were prepared daily by appropriate dilution of the respective stock standard solutions. All solutions were prepared by high purity deionized water (resistivity; 18 MV cm 1) obtained from Milli-Q water purification system (Millipore, Molsheim, France). The reference standard samples of NIST wheat gluten (RM8418) were used for the evaluation of accuracy for a developed ETV-LP-ICP system. 2.3. Operating procedures Before the generation of plasma, 5 Al of sample was loaded onto the filament through the sample injection port using a Hamilton syringe and initially desolvated by passage of 13-A current for 15 s through the filament. The desolvation step was set a typical temperature of 120 jC and no detectable losses of the analyte elements were observed. The vaporized solvent is swept by the argon carrier gas flow through the needle valve (B) for drainage. After the desolvation step, valve (B) was closed, and valve (A) and valve (C) were opened a little gradually to keep a proper pressure for self-ignition in the plasma torch and ETV chamber with constant gas flow. Normally, the extensive plasma that spread over the plasma torch was generated through selfignition at below 1 Torr. After the self-ignition of the plasma, argon flow was gradually increased for the formation of ‘‘core zone’’ [18] to achieve efficient atomization/
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excitation of metal ions. Further vaporization of the sample deposited on the filament was carried out by instantaneously raising the current up to 42 A for 4 s. The vaporized sample was transported and introduced into the plasma and finally analyzed by atomic emission spectrometry.
3. Results and discussion 3.1. Optimization of ETV/LP-ICP system Key operating parameters for ETV/LP-ICP such as ETV current, the flow rate of argon carrier gas and rf power were investigated to minimize the nonspectral interference effects. As the vaporization and the excitation of analytes were realized in two separate steps, the ETV and the ICP parameters have been optimized independently. The resistive heating of tantalum filament performed the volatilization of the sample. The filament was heated by supplying the electric current through copper electrodes by DC power supply operated in the constant voltage mode. The operating current was optimized with respect to the maximum peak heights of emission lines for various elements in aqueous standard solutions (5 Al of 5 ng/Al) according to the different volatility of elements, while rf power and argon carrier gas flow rate were kept constant at 200 W and 800 ml/min, respectively. Amount of sample that enters the plasma during the vaporization stage is critical. Introducing too much sample destabilizes the plasma and cools the plasma resulting in lower signal of the analyte than otherwise would have been obtained. The vaporization current has a critical effect on the analytical emission signal. Fig. 2 shows the effect of Cu emission intensity at 324.754 nm as a function of vaporization current from 25 to 45 A with 4 s holding time. As can be seen, the emission intensity of copper increased gradually with the increase of vaporization current and reached the maximum at 42 A of ETV current. However, the signal decreased when the vaporization current was larger than 42 A. The background signal at 323.086 nm was generally stable until 40 A of ETV current and then increased abruptly at 45 A. It might be due to the vaporization of impurities or tantalum filament itself. Furthermore, the highest signal-to-background ratio (S/B) could be obtained when the vaporization current was set at 42 A. Typical operating conditions for optical arrangement of the LP-ICP instrument and the ETV current program for Cu analysis are given in Table 1. The optimum vaporization currents for another seven elements, Fe, Cr, Mn, Pb, K, Zn and Mg, were also investigated for further studies. The emission lines used and the optimized ETV currents for eight elements are listed in Table 2. Optimum currents differ from one element to another because of their different physical and chemical properties. For multielement analysis, the selected ETV current should be compromised because only in some cases can common operating conditions be used for many ele-
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Fig. 2. Variation of copper emission intensity at 324.754 nm (A) and background at 323.086 nm (B), depending on the ETV current. Error bars represent standard deviations (r) of five measurements.
ments. The infrared thermometer was used to estimate the filament temperature. The temperature was reached about 1500 jC when the current was increased up to 42 A. However, the real filament temperature might be higher than the values estimated because IR thermometer integrates bright filament as well as dark surroundings in calculating temperature of the vaporizer. Representative emission profiles for Fe, Mn and Pb operated at an optimized ETV currents within selected wavelength regions are shown in Fig. 3. 3.2. Influences of flow rate of argon carrier gas and rf power The flow rate of argon carrier gas is one of the most important parameters in ETV/LP-ICP because transport efficiency and plasma pressure are of great concern to these parameters. The variation of argon flow rate from 300 to 1100 ml/min results in the increase of plasma pressure from 0.4 to 1.5 Torr, respectively. To optimize the flow rate of argon carrier gas, rf power was held constant at 200 W and ETV currents were set at 42 A (4 s). Fig. 4 shows the effect of argon flow rate in the range of 300– 1100 ml/min on copper emission signal on peak height at 324.754 nm. The copper emission intensity increased gradually with the increase of argon flow rate and was reached to the maximum with argon flow of 800 ml/min. Generally, very stable argon plasma was Table 1 The optimized experimental conditions for ETV/LP-ICP-AES system LP-ICP and spectrometer
ETV current program
rf power Reflected rf power Ar gas flow rate Integration time Slit width
Cu emission line Sample volume Sample conc. Dry Vaporization Clean
200 W 13 W 800 ml/min 1.2 s 10 Am
324.754 nm 5 Al 5 ng/Al 13 A (15 s) 42 A (4 s) 45 A (3 s)
generated at the range of 600 –1200 ml/min of argon flow rate. The typical relative standard deviation (RSD) of copper emission was <2.2% from five consecutive measurements for each point. However, the signal decreased when argon flow rate was larger than 900 ml/min because of lowering the residence time of individual atoms in the plasma at higher flow rate. For other investigations, the flow rate of argon gas was held constant at 800 ml/min for maximum collection of emission signals. At optimum experimental conditions mentioned above (argon carrier gas flow rate: 800 ml/min and ETV current for copper: 42 A and 4 s), the emission signal was gradually increased with increasing the rf power within the experimental forward power range of 100– 220 W. However, the plasma was unstable at the high rf power (over 200 W) because the reflected power also increased at the same time when the forward power increased. The rf power of 200 W was thus selected as recommended standard operating condition for the ETV-LP-ICP system. 3.3. Detection limits and precision The analytical performances including linearity of calibration, precision and detection limits of the developed Table 2 Emission lines used and optimized currents for selected eight elements in ETV/LP-ICP system Element
Wavelength (nm)
ETV currents (A)
Fe Cu Cr Mn Pb K Zn Mg
371.994(I) 324.754(I) 359.349(I) 257.610(II) 405.783(I) 766.491(I) 330.259(I) 279.504(II)
43 42 42 40 40 32 25 40
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Fig. 3. Typical LP-ICP emission spectra for (A) Fe, (B) Mn and (C) Pb.
ETV/LP-ICP system were evaluated by using the standard solutions for eight elements. All solutions were prepared by serial dilution of commercially available 100-ppm aqueous ICP standards. The figures of merit of the calibration curves for Fe, Cu, Cr, Mn, Pb, K, Zn and Mg are illustrated in Table 3. All calibration graphs developed by the use of selected wavelengths for eight elements were linear with a regression coefficient (r) of at least 0.9941 and the repro-
ducibility for five consecutive measurements (n=5) yielded the RSD (%) of 2.8– 8.9% for eight elements. The RSD values shown in Table 3 were the average values for each point in calibration graphs. The detection limits were calculated from three times the standard deviation of the blank measurements and the slopes of the calibration graphs. The calculated absolute detection limits for Cu, Mn and Cr are determined as 0.032, 0.033 and
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Fig. 4. Effect of argon flow rate on copper emission intensity at 324.754 nm by ETV/LP-ICP AES.
0.038 ng, respectively. The elements Zn, K, Pb and Fe also show low values of absolute detection limits, 0.081, 0.141, 0.227 and 0.391 ng, respectively. These detection limits are comparable to those achieved by a graphite furnace ETV/ ICP [4,5] and a tungsten coil ETV/ICP [14]. However, K has a little higher values (0.14 ng) compared to that of ETVFAAS developed in our laboratory previously because of lower boiling points of K (b.p./K=1047 K) [28] and too high excitation temperature of the plasma. In general, the detection limits in this work are largely improved over those obtained from ETV/FAAS [19] for six selected elements except for K, indicating the outstanding analytical capability for the determination of trace elements of the developed ETV/LP-ICP-AES. 3.4. Quantitative application of ETV/LP-ICP The determinations of minor and trace constituents in the certified reference materials of wheat gluten (NIST RM8418) were used to evaluate the accuracy and feasibility of ETV/LP-ICP-AES. Wheat gluten sample (0.4525 g) was digested with 3 ml of concentric nitric acid (70%), 3 ml of Table 3 Detection limits (3rblank, n=5), precision and correlated coefficient (r) of ETV/LP-ICP system Elements
Wavelength (nm)
%RSD
r
Detection limit (ng)
EVT-FAASa (ng)
Fe Cu Cr Mn Pb K Zn Mg
371.994(I) 324.754(I) 359.349(I) 257.610(II) 405.783(I) 766.491(I) 330.259(I) 279.504(II)
8.9 4.2 5.8 6.1 4.9 2.8 3.3 4.4
0.9982 0.9985 0.9999 0.9987 0.9977 0.9941 0.9986 0.9959
0.39 0.032 0.038 0.033 0.23 0.14 0.083 0.022
– 0.066 – 0.058 0.274 0.024 – –
a
Ref. [17].
hydrogen peroxide (30%) and 0.5 ml of hydrochloric acid under microwave digestion system. The residue was then diluted by distilled water to a final volume of 25 ml. For analysis of Zn and Mg, digestion solution was diluted by a factor of 2 with distilled water, and for K and Mg, was diluted by a factor of 4 and 10, respectively. Each solution was analyzed based on the analytical procedure under the optimized experimental conditions mentioned above. Standard addition method was adapted for accuracy assessment of the developed system. It is particularly useful to diminish the matrix effects for analyzing wheat gluten powder samples in which the likelihood of matrix effects are substantial. Although the calibration curves developed in this work can be used for the accuracy evaluation, the success of the calibration graph method is critically dependent upon how accurately the analyte concentration of the standards are known and how closely the matrix of standards resembles that of the samples to be analyzed. From the linear regression fit formula obtained from each standard addition curve, the concentration of the sample was calculated. The analyzed results are listed in Table 4, together with average %RSD values for each element. As can be seen, the results obtained were in good agreement with the certified values. The average accuracy of the developed Table 4 Analytical results for K, Mn, Zn and Mg in a wheat gluten NIST standard sample (NIST RM8418) by ETV/LP-ICP-AES compared with the certified concentrations Elements
Selected lines (nm)
%RSD
r
Measured conc. (Ag/ml)
Certified value (Ag/ml)
K Mn Zn Mg
766.496 257.610 330.259 279.504
6.1 9.2 6.3 8.4
0.9988 0.9945 0.9934 0.9978
442F33 14.3F1.6 54.1F4.4 543F56
472F61 14.3F0.8 53.8F3.7 510F47
Confidence interval at 95% confidence level (tstudent=2.776, n=5).
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system for determination of K, Mn, Zn and Mg falls in the range of 0.1– 6.8% with acceptable %RSD values (<9.2%). The total analysis time per sample was less than 2 min. No serious memory effects were observed even at high analyte concentrations as long as the plasma was left on for a period of 20 s. The filament and purge time optimization dealt with removing a molecular interferent and reducing the rate of erosion of the filament.
4. Conclusion A new design for ETV/LP-ICP system equipped with an improved tantalum-filament ETV system as a sample vaporizer has been developed in this work. Low carrier gas (16 ml/min) and low rf power consumption (200 W) for optimum operation were achieved for trace elemental analysis in microliter samples. The main advantages of the present tantalum-filament ETV system include a simple, small and inexpensive construction, high heating rates and microliter sample capability (only 5 Al). Moreover, plasma excitation processes of the LP-ICP were more efficient with an improved ETV owing to the removal of solvent before the sample is introduced. The creation of linear calibration curves for selected eight elements was constructed with good linearity and precision. Absolute detection limits achieved for eight elements were in sub-nanogram range. The standard addition method was applied to assess the accuracy of ETV/LP-ICP using a NIST wheat gluten standard sample. The concentration of K, Mn, Zn and Mg in the standard sample determined by ETV/LP-ICP is in good agreement with those of the certified value within the error range. The results show that ETV/LP-ICP method has a great potential for the determination of trace elements of parts-per-million (Ag/g) level in wheat powder samples with satisfactory analytical results. An improved ETV unit described here could be connected with any excitation source, which can tolerate vapors and dry aerosols.
Acknowledgements The authors gratefully acknowledge the financial support by the Korea Research Foundation (Grant No. 1998-001D00459) and the interdisciplinary research program of KOSEF (Grant No. 1999-1-124-001-3).
References [1] C.W. McLeod, M.W. Routh, M.W. Tikkanen, Introduction of Solids into Plasmas, in: A. Montaser, D.W. Golightly (Eds.), Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed., VCH, New York, 1992, p. 721, Chap. 16. [2] K.C. Ng, J.A. Caruso, Electrothermal Vaporization, in: J. Sneddon (Ed.), Sample Introduction in Atomic Spectroscopy, Elsevier, Amsterdam, The Netherlands, 1990, p. 165, Chap. 7.
133
[3] B. Hu, Z. Jiang, Y. Qin, Y. Zeng, Fluorination with polytetrafluoroethylene slurry in electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry, Anal. Chim. Acta 319 (1996) 255. [4] U. Schaffer, V. Krivan, A graphit furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry, Anal. Chem. 70 (1998) 482. [5] L. Moens, P. Verrept, S. Boonen, F. Vanhaecke, R. Dams, Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, Spectrochim. Acta, Part B 50 (1995) 463. [6] P. Verrept, R. Dams, U. Kurfurst, Electrothermal vaporization inductively coupled plasma atomic emission spectrometry for the analysis of solid samples: contribution to instrumentation and methodology, Fresenius’ J. Anal. Chem. 346 (1993) 1035. [7] T. Kantor, Gy. Zaray, Graphite furnace for alternative combination with d.c. arc of inductively coupled plasma. Introduction and analysis of solid samples, Fresenius’ J. Anal. Chem. 342 (1992) 927. [8] H. Nickel, Z. Zagorska, A new electrothermal vaporization device for direct sampling of ceramic powders for inductively coupled plasma optical emission spectrometry, Spectrochim. Acta, Part B 50 (1995) 527. [9] R.D. Ediger, S.A. Beres, The role of chemical modifiers in analyte transport loss interferences with electrothermal vaporization ICP-mass spectrometry, Spectrochim. Acta, Part B 47 (1992) 907. [10] F. Wang, Z. Jiang, B. Hu, T. Peng, Comparative studies on chemical modification of polytetrafluoroethylene slurry in ETV-ICP and ETAAS, J. Anal. At. Spectrom. 14 (1999) 1619. [11] S.Z. Chen, F. Li, Z.H. Liao, T.Y. Peng, Z.C. Jiang, Study on the direct analysis of solid powder biological samples using fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry with PTFE slurry modifier, Fresenius’ J. Anal. Chem. 364 (1999) 556. [12] S. Chen, T. Peng, Z. Jiang, Z. Liao, B. Hu, Direct determination of rare earth impurities in lanthanum oxide by fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry with slurry sampling, J. Anal. At. Spectrom. 14 (1999) 1723. [13] H. Xiong, B. Hu, Z. Jiang, Micro-column separation/preconcentration combined with fluorinating electrothermal vaporization-inductively couple plasma-atomic emission spectrometry for determination of ultratrace rare earth elements, J. Anal. At. Spectrom. 15 (2000) 759. [14] P. Barth, S. Hauptkorn, V. Krivan, Improved slurry sampling electrothermal vaporization system using a tungsten coil for inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom. 12 (1997) 1351. [15] P. Barth, S. Hauptkorn, V. Krivan, Improved slurry sampling electrothermal vaporization system using a tungsten coil for inductively coupled plasma atomic emission spectrometry, J. Anal. Atom. Spectrom. 12 (1997) 1351. [16] Z. Jiang, B. Hu, Y. Qin, Y. Zeng, Fluorination assisted electrothermal vaporization inductively coupled plasma atomic emission spectrometry: an effective method for direct determination of refractory elements in solid biological samples, Microchem. J. 53 (1996) 326. [17] Y.I. Lee, J.K. Kim, K.H. Kim, Y.J. Yoo, G.H. Back, S.C. Lee, Design and critical evaluation of improved electrothermal vaporization flame atomic absorption/emission spectrometry for direct determination of trace metals in microliter samples, Microchem. J. 60 (1998) 231. [18] E.H. Evans, J.A. Caruso, Low pressure inductively coupled plasma source for mass spectrometry, J. Anal. At. Spectrom. 8 (1993) 427. [19] X. Yan, T. Tanaka, H. Kawaguchi, Reduced-pressure inductively coupled plasma mass spectrometry for nonmetallic elements, Appl. Spectrosc. 50 (1996) 182. [20] J.W. Waggoner, L.S. Milstein, M. Belkin, K.L. Sutton, J.A. Caruso, H.B. Fannin, Application of a low power/reduced pressure helium ICP ionization source for mass spectrometric detection of organobro-
134
[21] [22]
[23]
[24]
S.-D. Kim et al. / Microchemical Journal 78 (2004) 127–134 mine compounds and derivatized organotin compounds, J. Anal. At. Spectrom. 15 (2000) 13. E.H. Evans, W. Pretorius, L. Ebdon, S. Rowland, Low-pressure inductively coupled plasma ion source, Anal. Chem. 66 (1994) 3400. Y. Sung, H.B. Lim, R.S. Houk, Diagnostic studies of a low-pressure inductively coupled plasma in argon using double Langmuir probe, J. Anal. At. Spectrom. 17 (2002) 565. J.-M. Lim, J.-K. Kim, C.-S. Lee, G.-H. Back, Y.-I. Lee, Development and characterization of directly connected laser ablation/low-pressure inductively coupled plasma atomic emission spectrometry for solid sample analysis, Appl. Spectrosc. 54 (2000) 1253. G. O’Connor, L. Ebdon, E.H. Evans, Qualitative and quantitative determination of tetraethyllead in fuel using low pressure ICP-MS, J. Anal. At. Spectrom. 14 (1999) 1303.
[25] C. Brede, S. Pedersen-Bjergaard, E. Lundanes, T. Greibrokk, Microplasma mass spectrometric detection in capillary gas chromatography, Anal. Chem. 70 (1998) 513. [26] X. Yan, T. Tanaka, H. Kawasuchi, Electrothermal vaporization for the determination of halogens by reduced pressure inductively coupled plasma mass spectrometry, Spectrochim. Acta B 51 (1996) 1345. [27] B. Rosenkranz, G. O’Connor, E.H. Evans, Low pressure inductively coupled plasma ion source for atomic and molecular mass spectrometry: investigation of alternative reagent gases for organomercury speciation in tissue and sediment, J. Anal. At. Spectrom. 15 (2000) 7. [28] J. Emsley, The Elements, 2nd ed., Clarendon Press, Oxford, UK, 1991.