Determination of Fe, Ni, Cu, Zn, Cd and Pb in seawater by isotope dilution automatic solid-phase extraction—ICP-MS
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Abstract: A thorough understanding of the biogeochemical cycling of trace metals in the ocean is crucial because of the important role these elements play in regulating metabolism in marine biotas and thus, the climate. However, a precise and accurate analysis of trace metals in seawater is difficult because they are present at extremely low concentrations in a high salt matrix. In this study, we report an analytical method for the preconcentration and separation of six trace metals, Fe, Ni, Cu, Zn, Cd and Pb, in seawater using a seaFAST automatic solid-phase extraction device, analysis by a triple quadrupole collision/reaction technique with inductively coupled plasma mass spectrometry (ICP-MS), and quantification by the isotope dilution technique. A small volume (10 mL) of seawater sample was mixed with a multi-element isotope spike and subjected to seaFAST procedures. The preconcentrate solution was then analyzed using the optimized collision/reaction cell mode of ICP-MS, with NH3 gas for Fe and Cd with a flow rate of 0.22 mL/min and He for Ni, Cu, Zn and Pb with a flow rate of 4.0 mL/min. The procedure blanks were 130 pmol/L, 3.0 pmol/L, 6.8 pmol/L, 37 pmol/L, 0.29 pmol/L and 0.42 pmol/L, for Fe, Ni, Cu, Zn, Cd and Pb, respectively. The method was validated using five reference materials (SLRs-6, SLEW-3, CASS-6, NASS-7 and GEOTRACE-GSC), and our results were consistent with the consensus values. The method was further validated by measuring full-water-column seawater samples from the subtropical Northwest Pacific Ocean, and our results demonstrated good oceanic consistency.
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Key words:
- seaFAST /
- trace metals /
- isotope dilution /
- seawater /
- ICP-MS
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Figure 1. Signal intensity and background equivalent concentration (BEC) of each element analyzed using ten different combinations of collision/reaction cell gases and single-quad/triple-quad modes. All modes are auto-tuned by the instrument before the experiment. N/A represents no gas; the cps means counts per second.
Figure 4. Vertical profiles of dissolved Fe, Zn, Pb, Ni, Cu and Cd in the subtropical Northwest Pacific (GEOTRACES GP09; Station K9, 11°00′N, 149°50′E). The comparison profiles were taken from the nearby GEOTRACES cross station (29°59′N, 165°00′E) for Zn, Ni, Cu, and Cd (Zheng et al., 2021), Fe and Pb were from the previous literature in the North Pacific reported by Boyle et al. (2005) and Jiang et al. (2021).
Table 1. ICP-MS/MS operating conditions
Parameter Operating condition Focus lens/V He mode: 1.25
NH3 mode: –0.5D1 lens/V –350 D2 lens/V He mode: –148
NH3 mode: –147.5CRC gas flow/(mL·min−1) He mode: 4.0
NH3 mode: 0.22Deflection lens/V –30 Spray chamber temperature/°C 2.7 Peristaltic pump speed/(r·min−1) 40 Cool flow/(L·min−1) 14 Sampling depth/mm 5 Plasma power/W 1550 Auxiliary flow/(L·min−1) 0.8 Extraction lens/V –120 Nebulizer flow/(L·min−1) 1.08 Note: CRC is abbreviation of collision/reaction cell. Table 2. Procedure concentration blanks (nmol/L) and detection limits of this method
Elements Procedure blank Method detection limits Fe 0.13 0.028 Ni 0.03 0.002 3 Cu 0.006 8 0.003 3 Zn 0.037 0.024 Cd 0.29 0.13 Pb 0.42 0.29 Table 3. Fe, Ni, Cu, Zn, Cd and Pb concentrations analysis of standard materials
GSC, n=10 NASS-7, n=10 CASS-6, n=10 Measured value/(nmol·L−1) Consensus value/(nmol·L−1) Measured value/(μg·L−1) Consensus value/(μg·L−1) Measured value/(μg·L−1) Consensus value/(μg·L−1) Fe 1.554±0.115 1.535±0.115 0.337±0.029 0.351±0.026 1.60±0.09 1.56±0.12 Ni 4.261±0.024 4.393±0.205 0.244±0.007 0.248±0.018 0.420±0.002 0.418±0.040 Cu 1.240±0.024 1.099±0.149 0.198±0.004 0.199±0.001 0.541±0.003 0.530±0.032 Zn 1.498±0.024 1.433±0.103 0.44±0.01 0.42±0.08 1.30±0.09 1.27±0.18 Cd 0.386±0.004 0.364±0.022 0.016 3±0.000 3 0.016 1±0.001 6 0.021 3±0.000 8 0.021 7±0.001 8 Pb 0.041±0.001 0.039±0.004 0.002 9±0.000 1 0.002 6±0.000 8 0.010 7±0.000 9 0.010 6±0.004 0 SLEW-3, n=10 SLRs-6, n=10 Measured value/(μg·L−1) Consensus value/(μg·L−1) Measured value/(μg·L−1) Consensus value/(μg·L−1) Fe 0.595±0.012 0.569±0.059 86.3±1.1 84.3±3.6 Ni 1.31±0.01 1.23±0.07 0.619±0.044 0.616±0.022 Cu 1.54±0.07 1.55±0.12 22.4±0.3 23.9±1.8 Zn 0.165±0.006 0.201±0.037 1.80±0.07 1.76±0.12 Cd 0.049±0.001 0.048±0.004 0.006 3±0.002 2 0.006 3±0.001 4 Pb 0.008±0.000 0.009 0±0.001 4 0.180±0.002 0.17±0.026 Note: GSC is the abbreviation of GEOTRACES intercalibration samples. -
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