Spectrochimica Acta Part B 2001 123128 Analytical note Detection of iron species using inductively coupled plasma mass spectrometry under cold plasma temperature conditions Li-Shing Huang a,b, King-Chuen Lin a,b, a Department of Chemistry, National Taiwan Uniersity, Taiwan b Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan Received 8 April 2000; accepted 7 October 2000 Abstract Under the conditions of low radio frequency rf. power of 600 W and aerosol gas flow rate of 1.251.35 lmin, Fe or Fe. ions can be detected from the isobaric interference of the ArO. matrix. Using this method, the detection limit of Fe can reach 16 ngl ppt., 60 times smaller than by normal plasma conditions at 1200 W rf power. The linear dynamic range of the analyte measurement extends to 1000 ngml ppb.. 2001 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma mass spectroscopy; Cold plasma; Iron; Isobaric interference 1. Introduction In natural waters, the iron cycle involves numerous reactions of electron-transfer catalysis 13. Iron detection is important to the research into atmospheric chemistry. However, the total iron content in the open ocean is 1 nm. For Corresponding author. Fax: 886-2-2362-1483. E-mail address: kclin@mail.ch.ntu.edu.tw K. Lin. determining such low limits of concentration, development of a sensitive analytical method becomes crucial. The present analytical methods for iron determination, and their limitations, have recently been reviewed by Pehkonen 4. Among these methods, e.g. Johnson and coworkers have employed a flow-injection analysis FIA. coupled with chemiluminescence detection, and successfully achieved a subnanomolar limit for Fe II. detection in seawater 5. King et al. have determined Fe II. at the nanomolar level 0584-8701$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4-8 5 4 7 0 0 00292-5
124 ( ) L.-S. Huang, K.-C. Lin Spectrochimica Acta Part B: Atomic Spectroscopy 2001 123128 with a spectrophotometric detector, using ferrozine FZ. immobilized on a C18 Sep-Pak cartridge for pre-concentration 6. In their treatment the Fe FZ. 3 2 complex was not separated from the excess FZ, the spectral absorption of FZ at the same wavelength as the complex might cause significant error to a quantitative measurement of Fe II. species. Brown and co-workers have employed a high-performance liquid chromatographic method to separate the Fe FZ. 2 3 complex from ferrozine and other contaminants, which were then monitored with a UV-vis absorption detector at 2 nm 7. A detection limit of 10 10 M was achieved, when the Fe II. species in rainwater or seawater was pre-concentrated 50100 times with a C18 Sep-Pak cartridge. The inductively coupled plasma mass spectrometer ICP-MS. is an alternative sensitive detector widely adopted in elemental trace analysis. ICP-MS provides advantages of simultaneous or sequential. multi-elemental analysis and extremely sensitive detection for the charged analyte species. The iron has a small ionization potential of 7.86 ev and its ionization efficiency is estimated to be 96% based on the Saha equation 8,9. ICP-MS appears to be a powerful tool for iron detection. However, the analyte signal is superimposed on top of the 40 ArO background noise, and thereby the analytical sensitivity is considerably reduced. For instance, Uchida and Ito obtained the iron detection limit of 2 ppb or 1 ppb. with Ar ICP-MS or airar mixed ICP- MS.. These results were higher than those values determined for other elements 10. Like ArO, the polyatomic matrix ions inherent in ICP-MS have been a severe interference to trace element analysis. In addition to the increased background noise, the resulting space charge effect may reduce the fraction of sample ions passing through the ion optics toward the detector 11,12. Thus far, several methods have been demonstrated in an attempt to eliminate these polyatomic matrix ions. E.g. with the use of membrane desolvation or cryogenic desolvation, the water may be removed prior to entering the ICP torch 13,14. The ICP may also be coupled with an electrothermal vaporization system, which enables the matrix to be removed prior to the sample vaporization 15. The addition of mixed gases into Ar ICP is an alternative method. The added gases such as N 2, Xe, and H 2 may attenuate the polyatomic ions 16,17. It has already been determined that the secondary discharge between the plasma and the grounded sampler cone can enhance the formation of some polyatomic ions such as ArO, ArH and other Ar-containing ions 11. Therefore, insertion of a grounded metal plate between the outer wall of the torch and the load coil may effectively diminish the secondary discharge, and in turn, substantially reduce the interference of these matrix ions 18,19. The decrease of radio frequency rf. forward power has led to similar effects in reducing the Ar-containing ions 20,21. Under the cold plasma conditions, Houk et al. have reported that the Ar and ArH ions may be suppressed and thus the obtained measurement of potassium isotope ratio of mz 39 and 41 is precise with a R.S.D. of 0.30.9% 20. In this work, we have employed an Ar ICP-MS to detect iron in the aqueous solution. The ICP- MS is performed at low rf power and large aerosol gas flow rate. Under the cold plasma condi- tions, the intensity ratio of Fe or Fe. ion to ArO. has been characterized. The dynamic range of a calibration curve for Fe III. concentration is obtained. The Fe III. can be monitored with a detection limit lower by approximately two orders of magnitude than the normal plasma conditions. 2. Experimental 2.1. ICP-MS apparatus A SCIEX ELAN Model 6000 ICP-MS Perkin- Elmer. was used for all data acquisition. It was run in sequential mode, peak hopping to masses of interest. A cross-flow nebulizer with a Scotttype double path spray chamber was used. To avoid the conductively coupling between the load coil and the plasma, both ends of the load coil were biased with a high voltage of equal amplitude but opposite phase. The plasma potential
( ) L.-S. Huang, K.-C. Lin Spectrochimica Acta Part B: Atomic Spectroscopy 2001 123128 125 may then be minimized. The sampling depth between the sampler tip and the top coil was fixed at 9 mm for all data acquisition. The coolant gas flow rate and the auxiliary gas flow rate were fixed at 15.0 and 1.2 lmin of Ar throughout the experiment, respectively, while the aerosol gas flow rate was varied. In this work the rf power was varied from 600 to 1200 W, taking advantage of plasmalok provided by SCIEX ELAN 6000. Even under the cold plasma operating conditions, the ICP remained very stable. 2.2. Reagents Twice deionized water Millipore. was used for the reagent preparation. Methanol HPLC grade. was purchased from Mallinckrodt. A 100 ppm Fe 3 solution was prepared from the NH Fe SO. 12H O sample Merck. 4 4 2 2 and then diluted to various concentrations in 0.1% HNO 3 Merck. solution. 3. Results and discussion 3.1. FeArO and FeArN ratios To optimize the ratio of FeArO, the rf power, aerosol gas flow rate and potential biased on the ion lens were varied, while the other parameters remained fixed. The coolant gas and auxiliary gas flow rate were fixed at 15 and 1.2 lmin, respectively. The aerosol gas flow rate was varied from 0.7 to 1.40 lmin at a step of 0.05 lmin, while rf power was varied from 600 to 1400 W at a step of 100 W at a fixed potential on the ion lens. The background counts per second cps of ArO and ArN in a blank solution were measured, as shown in Fig. 1. Data were averaged over five replicates. Similarly, 20 ppb Fe III. solution was measured at mz and. In Fig. 1, it is shown that either decreasing the rf power or increasing the aerosol gas flow rate may effectively reduce the cps of the ArO and ArN background ions. However, lowering the plasma temperature similarly reduces the ratio of metal ion to its neutral atom based on the Saha equation. The obtained cps of the Fe ion de-. Fig. 1. a rf power dependence of ArO mz and ArN mz. interference in the blank measurement at an aerosol gas flow rate 1.3 lmin. b. Aerosol gas flow rate dependence of ArO mz. and ArN mz. interference in the blank measurement at a rf power 600 W. creases also. As shown in Fig. 2, the net cps of the Fe ion is estimated by subtracting the ArO or ArN. background from the total cps of the signal at mz or. for the 20 ppb Fe III. solution. Thus, the ratio of FeArO or FeArN. may be determined as a function of rf power and aerosol gas flow rate. The results are plotted in Fig. 3. The ratios tend to increase with a low rf power and a large gas flow rate. In this experiment, the ratio is optimized at a rf power 600 W and aerosol gas flow rate 1.251.35 lmin. For the rf power at 600 W and the aerosol gas flow rate at 1.35 lmin, the ratio of Fe ion to ArO is plotted as a function of the biased potential. The optimized voltage is found at 3 V, smaller than that under the normal plasma conditions. The factor of biased voltage on the ion lens may be used to qualitatively evaluate the kinetic
126 ( ) L.-S. Huang, K.-C. Lin Spectrochimica Acta Part B: Atomic Spectroscopy 2001 123128 the ionization potential I.P.. of the sample. The Fe ion with a small I.P. decays more slowly than the matrix ions, as the plasma temperature decreases. Tanner has suggested that the cold plasma technique is most useful for elements having lower ionization potentials, but its sensitivity decreases markedly as the I.P. is above 8 ev 21. 3.2. Detection limit and dynamic range Under the conditions of rf power at 600 W and ion lens biased at 3.0 V, the Fe 3 ion in 0.1% HNO3 solution was detected in the concentration range from 0.05 to 1000 ppb. The calibration curve is plotted in Fig. 4. The detection limit is defined as three times the standard deviation. Fig. 2. a rf power dependence of Fe and Fe signal in the 20 ppb solution at an aerosol gas flow rate 1.3 lmin. b. Aerosol gas flow rate dependence of Fe and Fe signal in the 20 ppb solution at a rf power 600 W. energy of the ion species. The optimal biased voltage is, therefore, expected to be smaller under the cold plasma condition. Due to the isobaric interference by ArO and ArN, the detection limits of Fe and Fe with ICP-MS are higher than those determined for most elements 10. The ArO and ArN with weak bond energies are formed predominantly behind the sampler cone. It is found that the secondary discharge produced between the positive plasma potential and the grounded sampler may enhance the ArO and ArN formations via collision-induced chemical reaction 18,19. Lowering the plasma temperature is one of the effective methods to remove the secondary discharge 20,21, and subsequently eliminate these background interferences. However, one should note that the enhancement of the ratio of sample signal to matrix background is closely related to. Fig. 3. a rf power dependence of the ratios of FeArO and FeArN in the 20 ppb solution at an aerosol gas flow rate 1.3 lmin. b. Aerosol gas flow rate dependence of the ratios of FeArO and FeArN in the 20 ppb solution at a rf power 600 W.
( ) L.-S. Huang, K.-C. Lin Spectrochimica Acta Part B: Atomic Spectroscopy 2001 123128 127 Table 1 Comparison of detection limits obtained between cold plasma and normal plasma conditions Cold plasma Normal plasma RF power W. 600 600 1200 Ion lens voltage V. 600 3.0 7.0 Aerosol gas flow rate lmin. 1.30 1.35 1.05 Number of replicates 11 11 11 Mean blank signal cps. 1.6010 5.0910 3.4410 Standard deviation of blank signals cps. 1.1010 3.16 1.0310 Slope of calibration curve cpsppb. 9.2710 6.0510 3.4410 Detection limit ppb. 3.610 1.610 9.010 2 1 6 1 4 2 2 4 2 2 1 3. of a blank measurement. Given the slope of the calibration curve and value in Table 1, the detection limit reaches 16 and 36 ngl ppt. at flow rate of 1.35 and 1.30 lmin, respectively. The dynamic range of the analyte measurement extends to at least 1000 ppb, covering a very wide concentration range. When the rf power was changed to 1000 W, a normal ICP operating condition, the detection limit of Fe estimated similarly gave a value of 0.90 ppb, approximately 60 times larger than that under the cold plasma condition. Comparison of the data is also listed in Table 1. 4. Conclusion Suffered from the ArO. isobaric interference, the trace Fe or Fe. analysis by the ICP-MS is a difficult task without the use of a high resolution mass spectrometer 22. Our work provides an alternative method to minimize the ArO. interference by using a cold plasma technique. In this work, we have charac-. terized the ratio of Fe or Fe ion to ArO. as a function of rf power and aerosol gas flow rate and obtained the linear dynamic range of a calibration curve for the Fe III. concentration up to 1000 ppb. The detection limit of iron can reach 16 ppt, approximately 60 times lower than that by the normal plasma operating conditions. The ICP-MS with the use of cold plasma conditions may apparently provide a promising tool to detect the trace iron contents in seawater. Acknowledgements This work was supported by the National Science Council of the Republic of China under the Contract NSC 89-2113-M-002-027. References Fig. 4. Calibration curve for Fe III standard solution. 1 T.E. Graedel, C.J. Weschler, M.L. Mandich, Influence of transition metal complexes on atmospheric droplet acidity, Nature 317 1985. 240242. 2 G. Zhuang, Y. Zhen, R.A. Duce, P.R. Brown, Iron photoreduction and oxidation in an acidic mountain stream, Nature 355 1992. 537539. 3 P. Behra, L. Sigg, Evidence for redox cycling of iron in atmospheric water droplets, Nature 344 1990. 419421. 4 S. Pehkonen, Determination of the oxidation states of iron in natural waters, Analyst 120 1995. 26552663. 5 V.A. Elrod, K.S. Johnson, K.H. Coale, Determination of
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