A bulk extraction method to determine the stable isotope ratios of iron, nickel, copper, zinc, and cadmium in seawater using multi-collector inductively coupled plasma mass spectrometry
School of Oceanography, Shanghai Jiao Tong University, Shanghai 200030, China
2.
Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519015, China
3.
Shanghai Key Laboratory of Polar Life and Environment Sciences, Shanghai Jiao Tong University, Shanghai 200030, China
4.
Key Laboratory of Polar Ecosystem and Climate Change of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200030, China
5.
Laboratory for Polar Science, Polar Research Institute of China, Ministry of Natural Resources, Shanghai 200136, China
Funds:
The National Key Research and Development Program of China under contract No. 2022YFE0136500; the National Nature Science Foundation of China under contract Nos 41890801 and 42076227; the Shanghai Pilot Program for Basic Research-Shanghai Jiao Tong University under contract No. 21TQ1400201.
The oceanic trace metals iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) are crucial to marine phytoplankton growth and global carbon cycle, and the analysis of their stable isotopes can provide valuable insights into their biogeochemical cycles within the ocean. However, the simultaneous isotopic analysis of multiple elements present in seawater is challenging because of their low concentrations, limited volumes of the test samples, and high salt matrix. In this study, we present the novel method developed for the simultaneous analysis of five isotope systems by 1 L seawater sample. In the developed method, the NOBIAS Chelate-PA1 resin was used to extract metals from seawater, the AG MP-1M anion-exchange resin to purify Cu, Fe, Zn, Cd, and the NOBIAS Chelate-PA1 resin to further extract Ni from the matrix elements. Finally, a multi-collector inductively coupled plasma mass spectroscope (MC-ICPMS) was employed for the isotopic measurements using a double-spike technique or sample-standard bracketing combined with internal normalization. This method exhibited low total procedural blanks (0.04 pg, 0.04 pg, 0.21 pg, 0.15 pg, and 3 pg for Ni, Cu, Fe, Zn, and Cd, respectively) and high extraction efficiencies (100.5% ± 0.3%, 100.2% ± 0.5%, 97.8% ± 1.4%, 99.9% ± 0.8%, and 100.1% ± 0.2% for Ni, Cu, Fe, Zn, and Cd, respectively). The external errors and external precisions of this method could be considered negligible. The proposed method was further tested on the seawater samples obtained from the whole vertical profile of a water column during the Chinese GEOTRACES GP09 cruise in the Northwest Pacific, and the results showed good agreement with previous related data. This innovative method will contribute to the advancement of isotope research and enhance our understanding of the marine biogeochemical cycling of Fe, Ni, Cu, Zn, and Cd.
Iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd), the trace metals essential for marine organisms, have a significant influence on the oceanic biogeochemical cycles (Morel and Price, 2003). These metals in their dissolved phases exhibit nutrient-like vertical profiles. They are taken up in surface waters and subsequently released into deep waters through biogenic particle decomposition. Over the past decades, numerous studies along with the GEOTRACES program (an international study of the marine biogeochemical cycles of trace elements and their isotopes) have extensively examined the oceanic distributions of various elements (SCOR Working Group, 2007; Tagliabue et al., 2017; Weber et al., 2018; Roshan et al., 2018; Middag et al., 2018; Richon and Tagliabue, 2019; John et al., 2022). Dissolved Fe concentrations as low as 0.02 nmol/kg in surface waters can limit oceanic primary productivity, thereby affecting the global carbon cycle and climate change (Boyd and Ellwood, 2010; Falkowski et al., 1998; Morel and Price, 2003). Zn is the second most abundant micronutrient in phytoplankton biomass (Twining and Baines, 2013). Meanwhile Ni plays a crucial role in various biogeochemical processes, including nitrogen and carbon fixation, nitrogen uptake by plants, and methanogenesis (Alfano and Cavazza, 2020). Depending on their concentrations, both Cd and Cu can function as the nutrients or toxins of phytoplankton (Price and Morel, 1990; Payne and Price, 1999; Lane and Morel, 2000).
Analyzing the isotopes of Fe, Ni, Cu, Zn, and Cd in seawater using samples of limited volumes presents several challenges. First, the concentrations of these metals in seawater are extremely low (Ni: 2–10 nmol/kg, Cu: 0.2–5 nmol/kg, Fe: 0.02–2 nmol/kg, Zn: 0.01–10 nmol/kg, and Cd: 1–1 000 pmol/kg) (de Baar et al., 1994; Moore and Braucher, 2007; Roshan et al., 2018; Richon and Tagliabue, 2019; John et al., 2022), increasing the potential for contamination and limiting the analytical precision (John and Adkins, 2010). Second, a high salt matrix content generates interferences and suppresses the signal voltages. Third, the unavailability of the double-spike technique for Cu requires a 100% yield to prevent isotopic fractionation (Hou et al., 2016).
Pretreatment and measurement methods for determining isotope ratios of Fe, Ni, Cu, Zn and Cd in seawater have been reported (Conway et al., 2013; Takano et al., 2017; Yang et al., 2020). The pretreatment procedure involved bulk extraction and purification steps. Bulk extraction in which the NOBIAS Chelate-PA1 resin is placed in a sample bottle to quantitatively capture transition metals, could concentrate metals in 1 L of seawater by up to 1 000 times (John and Adkins, 2010) with low procedural blanks. Isotope ratio measurements using multi-collector inductively coupled plasma mass spectroscopy (MC-ICPMS) employed the double-spike technique for Ni, Fe, Zn, and Cd and combined standard-sample bracketing with internal normalization method (C-SSBIN) for Cu. Besides correcting instrumental mass bias, the double-spike method can correct element fractionation caused by nonquantitative yields and accurately recalculate element concentrations through isotope dilution. The C-SSBIN method has improved the precision of isotopic ratios by at least two times comparing to the standard-sample bracketing method (Hou et al., 2016). However, there were only purification methods available for the simultaneous determination of, at most, three isotope systems (Conway et al., 2013; Takano et al., 2017). Conway et al. (2013) simultaneously separated Fe, Zn, and Cd using a single AG MP-1M resin microcolumn; However, Cu recovery was low and Ni elution occurred alongside the salt matrix with this chromatography. Yang et al. (2020) quantitatively separated Cu by employing a mixed solvent consisting of 11 mol/L acetic acid and 4 mol/L hydrochloric acid (HCl) to dissolve samples for loading columns and elute salts. This solvent markedly enhanced Cu recovery (Yang et al., 2019) compared to previous methods utilizing only HCl as the reagent (Maréchal et al., 1999; Takano et al., 2013; Hou et al., 2016; Paredes et al., 2018). Additionally, Yang et al. (2020) introduced a NOBIAS Chelate-PA1 resin column step to eliminate interfering elements from the Ni fraction. However, the purification column with a normal volume used above would elevate the blanks for Fe and Zn.
To address this, our novel method refined the existing methodologies, leading to the development of a streamlined and efficient technique for the isotopic analysis of Fe, Ni, Cu, Zn, and Cd in 1 L seawater. Additionally, we employed miniaturized polytetrafluoroethylene (PTFE) teflon microcolumns filled with 20 ng of AG MP-1M anion-exchange resin (Bio-Rad) to minimize the purification blanks. The developed technique involved a single chelating extraction process, followed by ion-exchange-based purifications, and isotopic analysis conducted using the Thermo Neptune MC-ICPMS.
2.
Materials and method development
2.1
Reagents, standards, and samples
All samples were prepared using flow benches equipped with ultralow particulate air filters and located in the class-1000 clean laboratory of the School of Oceanography, Shanghai Jiao Tong University (SJTU). The reagents used in the study included Fisher OptimaTM hydrobromic acid (HBr), hydrogen peroxide (H2O2), and ammonium hydroxide (NH3·H2O). Fisher TraceMetalTM nitric acid (HNO3), acetic acid (HAc) and Merck Suprapur® hydrochloric acid (HCl) were purified using a polyfluoroalkoxy (PFA) distillation system (Savillex® DST-1000) that employed sub-boiling distillation. Ultrapure water (>18.2 MΩ·cm, Milli-Q) obtained from a Q-POD element system (Merck) was used in the experiments. All acid-cleaned PFA teflon filtration apparatus and vials were sourced from Savillex. Acid-cleaned VWR metal-free 15 mL centrifuge tubes made of low-density polyethylene (LDPE) were used for eluent collection, isotopic analysis, and concentration analysis. The acid cleaning protocols used with all plastic labware were described in Zhang et al. (2015a). The Hitachi NOBIAS Chelate-PA1 resin and Bio-Rad AG MP-1M resin (100–200 mesh) were cleaned by soaking them three times over a week in 3 mol/L HNO3 and 10% HCl, respectively, with extensive rinsing between the soakings. Before their use, the Hitachi NOBIAS Chelate-PA1 resin and Bio-Rad AG MP-1M resin were stored in 3 mol/L HNO3 and ultrapure water, respectively.
A multielement standard (InorganicTM, IV-ICPMS-71A, 10 µg/mL 43 Element ICP Calibration/Quality Control Standard, including Al, Ba, Cd, Ce, Co, Dy, Eu, Ga, Fe, Pb, Mg, Nd, P, Pr, Sm, Ag, Sr, Tl, Tm, V, Zn, B, As, Be, Ca, Cr, Cu, Er, Gd, Ho, La, Lu, Mn, Ni, K, Rb, Se, Na, S, Th, U, Yb, Cs) was utilized to evaluate the pretreatment method, which included assessing the extraction efficiency, matrix element removal efficiency and purification yield.
Seawater samples obtained from the Chinese GEOTRACES subtropical Northwest Pacific (GP09) cruise and the Yellow Sea cruise were employed for method development and validation. Samples of a full seawater column at Station K9 (11.00°N, 149.83°E) were collected by the R/V Tan Kah Kee during the GEOTRACES GP09 cruise (from April 25 to June 6, 2019) and the related data of Fe, Ni, Cu, Zn, and Cd concentrations have been reported by the study of Ge et al. (2022). Seawater samples were collected using the GEOTRACES trace element Carousel Sampling System equipped with 24 teflon-coated OTE Niskin-X bottles mounted on a trace-metal rosette system, deployed using a teflon-coated external spring (Ocean Test Equipment, USA). After its collection, seawater was filtered from the OTE Niskin-X bottles through Acropak capsules (0.2 μm) into precleaned 1 L LDPE bottles (Nalgene) in a class-1000 trace-metal clean van. All operations were performed according to the standard procedures recommended by the GEOTRACES program. The sample obtained during the Yellow Sea cruise (March/April 2022) was collected at 34.88°N, 121.68°E aboard the R/V Xuelong. It was obtained using X-Vane sampling assemblies (Zhang et al., 2015b) by mounting a 5 L Niskin-X bottle (comprising a teflon-coated inner bottle and teflon-coated external springs) at a depth of ~15 m upstream of a metal hydrowire with a weathering vane installed downstream of the bottle. The seawater sample was filtered through an Acropak capsule (0.2 μm) filter in a class 100 flow bench installed at the wet laboratory of the R/V Xuelong.
2.2
Metal extraction from seawater
The procedure used in sample handling and analysis is depicted in Tables 1 and 2. The seawater samples were acidified to a pH of ~2 at the clean laboratory of the SJTU by adding distilled HCl within 1–2 months after sample collection. The acidified samples were then stored for at least three months before performing the concentration and isotope analyses. The concentration of Cu was measured prior to the isotope analyses to determine the amounts of Zn internal standard to be added to each sample, and adjusted the Cu isotope standard concentration to match sample concentration using C-SSBIN method. The concentrations of Ni, Fe, Zn, and Cd were measured prior to the isotope analyses to determine the amounts of Ni, Fe, Zn, and Cd double spikes to be added to each sample using double-spike technique. Briefly, after pretreatment using a seaFAST automatic solid-phase extraction device, metal concentrations were quantified by the isotope dilution technique measured using a PerkinElmer PE-5000 triple quadrupole collision/reaction cell along with inductively coupled plasma mass spectrometry (ICP-MS), as described by Ge et al. (2022).
Table
1.
Protocol for preconcentration of Ni, Cu, Fe, Zn and Cd by NOBIAS Chelate-PA1 resin
Step
Collection
a. Add 61Ni–62Ni, 57Fe–58Fe, 64Zn–67Zn, 110Cd–111Cd double spikes to 1 L seawater sample (pH = 2; 1 mmol/L H2O2) 72 h before extraction
−
b. Add 3 mL pre-cleaned resin in sample; shake for >5 h (Fe)
−
c. Adjust pH to 6.15 ± 0.2 with CH3CHOONH4 and NH3·H2O; shake for >5 h (Ni, Cu, Zn, Cd)
−
d. Load sample through filter to separate resin
salts
e. Rinse resin with 125 mL ultrapure water
salts
f. Elute metals with (5 × 5) mL 3 mol/L HNO3
metals
g. Evaporate samples at 200℃; redigest using 100 µL 16 mol/L HNO3 and 100 µL 10 mol/L HCl to dissolve organics for >2 h
−
h. Evaporate samples at 200℃; redissolve in 200 μL 11 mol/L acetic acid + 4 mol/L HCl + 0.003% H2O2 for purification
Based on Conway et al. (2013), metals were extracted from seawater using a bulk extraction technique that employed the NOBIAS Chelate-PA1 cation-exchange resin. Along with lower blank results, the NOBIAS Chelate-PA1 resin demonstrates higher affinities to Zn and Cd compared with the IDA resin, and its affinity to Zn is also higher than that of the NTA resin (Conway et al., 2013). The concentration-matched double spikes containing 61Ni–62Ni, 57Fe–58Fe, 64Zn–67Zn, 110Cd–111Cd, and 100 μL of 30% H2O2 were added to 1 L of the acidified seawater samples and allowed to stand for at least 72 h. The samples were then treated with 3 mL of precleaned NOBIAS Chelate-PA1 resin and agitated for >5 h on a shaker table. To bring up the seawater pH to 6.15 ± 0.2, an ammonium acetate (NH4Ac) buffer and NH3·H2O were added to the seawater, which was then continuously shaken for >5 h. The resin was subsequently passed through an acid-washed, 5-μm polycarbonate filter (Whatman), rinsed with ~125 mL of ultrapure water to eliminate salts, while metals were eluted with ~25 mL of 3 mol/L HNO3 and collected in a 30-mL PFA beaker. The solution was then evaporated at 200℃ to dryness on a hot plate, digested using 100 µL 16 mol/L HNO3 and 100 µL 10 mol/L HCl for at least 2 h, and evaporated to dryness once again. Finally, the samples were redissolved in 200 µL of 11 mol/L HAc + 4 mol/L HCl.
2.3
Sample purification
Following metal extraction, the samples underwent purification via microcolumn chromatography. The polytetrafluoroethylene (PTFE) teflon microcolumns were employed for minimizing the purification blanks (John and Adkins, 2010). These columns were fashioned using 5 mm inner diameter 4:1 heat-shrink PTFE tubing for a funnel shape. The final configuration of the column had a funnel tubing with a diameter of 1.25 mm and a height of approximately 20 mm, while the unheated funnel reservoir was about 10 mm in height. In this study, two anion-exchange purification methods from previously studies (Conway et al., 2013; Yang et al., 2020) were integrated to enable the concurrent purification of Ni, Cu, Fe, Zn, and Cd at low concentrations (0.2–200 ng/L). The full column purification protocols of the Ni, Cu, Fe, Zn, and Cd are provided in detail in Table 2. In brief, the samples were loaded onto 20 μL of precleaned and conditioned AG MP-1M resin in PTFE teflon microcolumns. Subsequently, the Ni + salts, Cu, Fe, Zn, and Cd were sequentially eluted using different reagents (AG MP-1M purification step; Table 2). The Ni fraction was then dried, redissolved in 200 µL of 0.05 mol/L NH4Ac, and further purified on the NOBIAS Chelate-PA1 microcolumn to separate Ni from the matrix salts (NOBIAS Chelate-PA1-Ni step; Table 2). Following microcolumn chromatography, the final fractions of Ni, Cu, Fe, Zn, and Cd were dried in 7 mL PFA vials. The samples were then redigested using 200 µL of 16 mol/L HNO3 and 100 µL of 30% H2O2 at 160℃ for about 6 h to decompose any residual organic matter, as described by Yang et al. (2019). The samples were dried again and redissolved in 0.1 mol/L HNO3, then contained in 15-mL LDPE tubes for the isotopic analysis.
2.4
Blanks and recovery efficiencies
The procedural blanks for extraction were estimated by processing five replicates of 1 L of 0.01 mol/L HCl (pH = 2), only using the extraction procedure applied to the samples. The purification blanks were estimated from the five replicates by loading columns with 200 µL of 11 mol/L HAc + 4 mol/L HCl + 0.003% H2O2 for the AG MP-1M step and with 200 µL of 0.05 mol/L NH4Ac for the NOBIAS Chelate-PA1-Ni step, followed by their purification processing as samples. The total procedural blanks were determined by processing five replicates of 1 L of 0.01 mol/L HCl (pH = 2) using the complete procedure applied to samples. The details will be discussed in Section 3.1.
The extraction efficiencies of the proposed method for Ni, Cu, Fe, Zn, and Cd were assessed by adding 200 µL of 1 µg/mL multielement standards (200 ng for all 43 elements) to ultrapure water, and extracting them as samples. The purification recoveries of the AG MP-1M step and NOBIAS Chelate-PA1-Ni step were determined by drying the multielement standards (200 ng for all 43 elements) and redissolving them in 200 µL of 11 mol/L HAc + 4 mol/L HCl, and 200 µL of 0.05 mol/L NH4Ac respectively, and purifying them as samples. The original standard (untreated by either NOBIAS Chelate-PA1 or AG MP-1M) was used to calculate the recovery of the respective step. The recovered metal concentrations were determined using isotope dilution. We also obtained the total procedural recovery efficiencies from natural seawater samples by comparing the weights of samples before and after pretreatment. The metal weights of samples after the entire protocol were calculated based on the isotope dilution method, using isotope ratios from Neptune and the quantities of added spikes. The details will be discussed in Section 3.2.
2.5
Isotopic measurement
The Thermo Neptune Plus MC-ICPMS, equipped with a Ni Jet sample cone and an Al x-skimmer cone (Thermo Scientific), was employed for the analysis of δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd, performed at the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai). In all isotope analyses performed, a PFA nebulizer with a flow rate of 100 μL/min and a quartz dual cyclonic spray chamber were employed for sample introduction. The isotopes of Cd and Cu were measured in the low-resolution mode (LR), while the isotopes of Fe, Zn, and Ni were measured in the high-resolution mode (HR) to circumvent potential polyatomic interferences (e.g., 40Ar14N+ on 54Fe, 40Ar16O+ on 56Fe, 40Ar16O1H+ on 57Fe, 40Ar18O+ on 58Fe and 58Ni, 40Ar27Al+ on 67Zn, and 40Ar28Si+ on 68Zn). These isotopes were resolved from their respective polyatomic interferences by measuring the signal voltages on the left flat shoulder of the combined metal-argide peak (Weyer and Schwieters, 2003). The proposed method achieved resolutions >7 000 (m/Δm; 5%–95% of the side of the peak) for Fe, Ni, and Zn. Typical representative sensitivities for 56Fe (HR), 58Ni (HR), 63Cu (LR), 66Zn (HR), and 114Cd (LR) were 44 V, 32 V, 160 V, 15 V, and 110 V per ppm (10−6), respectively.
To correct isobaric interferences, we measured their signal intensities for each isotope system, as indicated in the relevant cup configuration presented in Table 3. The 54Cr and 58Ni isobaric interferences on 54Fe and 58Fe were corrected by measuring the abundances of 53Cr and 60Ni, and the 58Fe isobaric interference on 58Ni was corrected by measuring the abundances of 57Fe. Similarly, 64Ni isobaric interference on 64Zn was corrected by measuring the abundances of 60Ni; 110Pd, 112Sn, and 114Sn isobaric interferences on 110Cd, 112Cd, 114Cd were corrected by measuring 105Pd and 117Sn (Table 3). Because Zn isotopes were used as the internal standard to correct for instrumental mass bias, 63Cu, 65Cu, 66Zn and 64Zn were measured simultaneously during Cu isotope analyses.
Table
3.
Cup configurations for Ni, Cu, Fe, Zn, Cd, and Fe isotopic analysis by Neptune MC-ICPMS
Faraday cup position
L4
L3
L2
L1
C
H1
H2
H3
H4
Ni (HR)
−
57Fe
58Ni
−
60Ni
61Ni
62Ni
−
−
Cu (LR)
−
−
63Cu
64Zn
65Cu
66Zn
67Zn
68Zn
−
Fe (HR)
−
53Cr
54Fe
−
56Fe
57Fe
58Fe
−
60Ni
Zn (HR)
−
62Ni
−
64Zn
−
66Zn
67Zn
68Zn
−
Cd (LR)
−
105Pd
−
110Cd
111Cd
112Cd
114Cd
−
117Sn
Note: Isotopes used in isotope ratios are bolded, while spiked isotopes are underlined. − represents no data.
For δ56Fe, δ60Ni, δ66Zn, and δ114Cd, a double-spike technique was employed to correct for instrumental mass bias and any potential isotope fractionation during sample pretreatment. The data reduction scheme employed adhered to the algebraic approach described by Rudge et al. (2009). The Fe, Ni, Zn, and Cd double spikes primarily comprised 50% 57Fe and 50% 58Fe, 50% 61Ni and 50% 62Ni, 80% 64Zn and 20% 67Zn, and 33% 110Cd and 66% 111Cd, respectively to minimize errors occurring during data reduction (Rudge et al., 2009). A sample-spike ratio of 1:2 for Fe and 1:1 for Ni, Zn, and Cd were adopted to enhance analytical precision and reduce the potential impact of isobaric interference (John, 2012). Double spikes were prepared from monoisotopic solutions obtained from Isoflex USA, with concentrations determined by isotope dilution using the purchased concentration standards. Because Cu has only two stable isotopes, the double-spike technique cannot be used with Cu. Instead, C-SSBIN was used to correct for mass bias on the Cu isotope ratios (Takano et al., 2013, 2017). Zn isotope standard was added to the Cu isotope samples to serve as the internal standard, and the data reduction scheme followed was based on Hou et al. (2016) and Sullivan et al. (2020).
We used the solutions of the isotope standards IRMM-524a for Fe, NIST-986 for Ni, NIST-647 for Cu, NIST-3702 for Zn, and NIST-3108 for Cd. Isotope ratios of Fe, Cu, and Zn are traditionally expressed relative to IRMM-014, NIST-976, and Lyon JMC, respectively, which cannot be obtained now. IRMM-014 Fe δ56Fe, NIST-976 Cu δ65Cu, and Lyon JMC Zn δ66Zn were −0.004‰ (2SD = 0.014‰), +0.2‰ (2SD = 0.02‰), and +0.28‰ (2SD = 0.02‰), respectively (Doucet et al., 2018; De Vega et al., 2020; Kidder et al., 2020) compared to IRMM-524a Fe, NIST-647 Cu, NIST-3702 Zn. Accordingly, corrections were applied to all data expressed relative to IRMM-014 Fe, NIST-976 Cu, and Lyon JMC Zn. In conclusion, we expressed Fe, Ni, Cu, Zn, and Cd isotope ratios using delta notation in per mil (‰) relative to the isotope standards IRMM-014 Fe, NIST-986 Ni, NIST-976 Cu, Lyon JMC Zn, and NIST-3108 Cd, as described in Eqs (1)−(5):
The sequences required for the isotopic analysis using MC-ICPMS were conducted following the methodology outlined in Conway et al. (2013). At the commencement of each analytical session, the solutions of the isotope standards and pure double spikes were analyzed twice. Mixtures of the isotope standards and double spikes, matched for concentration, were used to bracket groups made of five samples. During the Cu isotopic measurement using the C-SSBIN approach, δ65Cu values obtained by being bracketed by NIST-647 itself were utilized for monitoring instrumental stability, which exhibited an external precision of 0.02‰ (1SD, n = 83). Washout times were set as follows: rinsing in 0.5 mol/L HNO3 for 2 min and in 0.1 mol/L HNO3 for 1 min. Signal intensity was recorded over 50 cycles (each duration of 4.2 s), and the cycles exceeding three times the standard deviation were discarded. To correct for instrumental background, the average blank signal in a 0.1 mol/L HNO3 solution was measured over 50 cycles. Each group of five samples was bracketed using two blank measurements.
3.
Results and discussion
3.1
Procedural blanks
The procedural blanks for each step of the extraction and purification processes are listed in Table 4. The total procedural blanks of Ni, Cu, Fe, Zn, and Cd were (0.04 ± 0.05) ng, (0.04 ± 0.02) ng, (0.21 ± 0.20) ng, (0.15 ± 0.10) ng, and (3.0 ± 3.0) pg, respectively. The combined reagent blanks for sample extraction and subsequent purification, obtained by calculation, were less than the total blanks for each element, as revealed by the analysis of concentrated reagents that had been dried. Overall, our procedural blanks for Ni, Cu, Fe, Zn, and Cd were lower than those reported by John and Adkins (2010), Yang et al. (2020), and Takano et al. (2017) and were similar to those reported by Conway et al. (2013). These total procedural blanks are negligible when compared to the Ni, Cu, and Cd contents in 1 L of seawater obtained from the open ocean, which fall in the ranges of 100–600 ng Ni, 15–300 ng Cu, and 0.1–100 ng Cd. However, the total procedural blanks could potentially affect Fe and Zn, which have extremely low concentrations at the surface. In that case, a larger volume for preconcentration is recommended for Fe and Zn isotope measurements.
Table
4.
The blanks (ng) in pretreatment steps of NOBIAS Chelate-PA1 resin extraction, AG MP-1M resin purification, NOBIAS Chelate-PA1 resin Ni purification, and total procedure (±1SD, n = 5)
The extraction recoveries of Ni, Cu, Fe, Zn, and Cd are presented in Table 5, demonstrating the quantitative recovery of the elements. The quantitative recovery is required for Cu to avoid potential isotope fractionation during its extraction using the C-SSBIN method. The Fe, Zn, and Cd recoveries of bulk extraction from the study of Conway et al. (2013) were 78%, 101%, and 102%, respectively. Here we extended the shaking time for Fe from 2 h to 5 h to achieve an enhanced Fe extraction efficiency of 97.8% ± 1.4% (1SD, n = 5). We also could achieve a higher Cu recovery during bulk extraction than what was achieved by Yang et al. (2020), who reported a Cu extraction efficiency of 86% ± 2% (1SD) without adding any H2O2 prior to bulk extraction. This difference in the extraction efficiencies could be due to the addition of H2O2 to the samples (1 mmol/L H2O2 in each seawater sample), which destroyed the strong copper-binding ligands. The total procedural recovery efficiencies for Ni, Cu, Fe, Zn, and Cd were determined during the MC-ICPMS analysis of the seawater samples obtained from the Northwest Pacific and Yellow Sea. The total procedural recoveries for Ni, Cu, Fe, Zn, and Cd in the natural seawater samples were 98.3% ± 3.0%, 96.8% ± 1.6%, 92.8% ± 5.6%, 101.5% ± 3.8%, and 99.1% ± 3.3%, respectively. Besides, the seawater sample collected from the Yellow Sea was split into 14 samples and separately pretreated following procedure. Their extraction efficiencies for Ni, Cu, Fe, Zn, and Cd were 99.2% ± 2.1%, 99.8% ± 1.2%, 98.8% ± 1.6%, 100.2% ± 1.9%, and 99.5% ± 1.7%, respectively. These results showed comparable extraction efficiencies for seawater samples compared with ultrapure water supplemented with multielement standards.
Table
5.
The recoveries in pretreatment steps of NOBIAS Chelate-PA1 resin extraction, AG MP-1M resin purification, and NOBIAS Chelate-PA1 resin Ni purification (±1SD, n = 5)
Ni was purified using a two-step process (Table 2), with purification yields for each step listed in Table 4. Elution processes for interest metals (Fe, Ni, Cu, Zn, and Cd), as well as interfering elements in both the AG MP-1M step and the NOBIAS Chelate-PA1-Ni step, were examined alongside purification recovery testing using multielement standards, as shown in Fig. 1. In the AG MP-1M purification step, Cu, Fe, Zn, and Cd fractions were separated from interfering elements, while Ni was eluted alongside with salts (Fig. 1). The purification yields for Ni, Cu, Fe, Zn, and Cd in the AG MP-1M purification step were 99.8% ± 0.2%, 99.5% ± 0.6%, 99.1% ± 1.1%, 99.5% ± 0.6%, and 99.7% ± 0.6% (1SD, n = 5), respectively (Table 4). Figure 1 demonstrated the quantitative separation of salts from Cu, and salts, Cr, Ni from Fe, and salts, V, Cr, Ni from Zn, which avoided potential polyatomic interferences (e.g. 23Na40Ar +, 24Mg40Ar +, 44Ca16O+, 50V16O+, 51V16O+, 50Cr16O+, 52Cr16O+) (Gall et al., 2012; John and Adkins, 2010; Mason et al., 2004) and isobaric interferences (Fig. 5). Recent studies showed that Ti would be proportionally eluted into Cu and Ni fractions (Takano et al., 2017; Yang et al., 2020), resulting in polyatomic interferences (e.g. 47Ti16O+, 49Ti14N+, 49Ti16O+, 46Ti16O+). Also, Mn was eluted with Cu (Fig. 1), causing polyatomic interferences of 55Mn14N+, 55Mn16O+ on Ga (May and Wiedmeyer, 1998) when using Ga for mass bias correction measuring Cu isotopes. However, for our purification protocol, low concentration (0–300 pmol/L) of dissolved Ti in seawater samples (Orians et al., 1990) and using Zn for mass bias correction avoided further purification of Cu fraction after the AG MP-1M step, contrasting with purification protocols by Yang et al. (2020). Following the AG MP-1M purification step, Ni was effectively separated from the salts in the NOBIAS Chelate-PA1-Ni purification step (Fig. 1), which yielded 99.7% ± 0.3% (1SD, n = 5) recovery for Ni. Our results demonstrated the quantitative recovery of Ni, Cu, Fe, Zn, and Cd with low procedural blanks in purification steps.
Figure
1.
Elution scheme for sample purification using an AG MP-1M resin microcolumn (a) and a NOBIAS Chelate-PA1 resin microcolumn (b). For the detailed protocols, see Table 2.
3.3
Analytical accuracy and isotopic analysis precision
To evaluate the external error or accuracy of the proposed method, a series of isotope standards and double spikes were added to ultrapure water to simulate natural sample concentrations. The doped ultrapure water had to go through the entire process. The results obtained for δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd were within the 1σ internal error obtained for all the concentrations by calculation (Fig. 2). Thus, the external errors could be considered negligible, and the internal error could be considered to represent the main error caused by Johnson noise and counting statistics (John, 2012; Conway et al., 2013).
Figure
2.
δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd values in ultrapure water doped with varying concentrations of natural isotope standards (IRMM-524a Fe, NIST-986 Ni, NIST-647 Cu, NIST-3702 Zn, and NIST-3108 Cd) measured according to the proposed method. Each concentration measurement included five replicates. Error bars represent the internal errors (1σ).
Figure 3 presents the typical internal errors of Fe, Zn, Cd, and Ni isotope analyses using the double-spike method, which used seawater samples and natural isotope standard solutions. The measured internal errors (1σ) of Fe (n = 141), Ni (n = 56), Zn (n = 82), and Cd (n = 52) were fitted to the theoretical lines obtained from Monte Carlo simulation (John, 2012) using the double-spike toolbox (Rudge et al., 2009). Thus, we could use the fitted theoretical lines to estimate the internal error of a specific mass sample or the required sample volume. The fitted theoretical lines in Fig. 3 demonstrate that our proposed method can reliably determine δ56Fe, δ60Ni, δ66Zn, and δ114Cd in a 1 L seawater sample with concentrations as low as 0.2 nmol/kg (11 ng), 0.2 nmol/kg (12 ng), 0.4 nmol/kg (26 ng), and 0.1 nmol/kg (11 ng), respectively, with an accuracy <0.1‰ (1σ). Higher concentrations can yield higher accuracies for isotope analyses. However, in the surface ocean where Fe was 0.02 nmol/kg, the measured 1σ errors of a 2 L sample was 0.6‰. With regard to Cd in the surface ocean, which could be 0.01 nmol/kg, a 2 L sample was measured with 1σ errors of 0.5‰. The lowest surface concentration of Cd in the Northwest Pacific samples was 0.004 nmol/kg, which would result in an error (1σ) >1‰ for δ114Cd using a 2 L sample, indicating that a large sample volume was required for a reliable δ114Cd. The internal errors (1σ) of δ65Cu obtained using 1 L of seawater sample were 0.02‰, 0.01‰, and 0.01‰ for Cu concentrations of 25 ng/kg, 50 ng/kg, and 100 ng/kg, respectively (Fig. 2), which are lower than the Cu concentrations in natural seawater samples. In summary, the 1σ internal errors for all five isotopes measured across the seawater profiles (δ56Fe 0.02‰–0.6‰, δ60Ni 0.01‰–0.03‰, δ65Cu < 0.02‰, δ66Zn 0.02‰–0.06‰, and δ114Cd 0.03‰–1‰) were considerably smaller than the isotopic variability of each element in seawater (1‰–3‰) except for Cd and Fe in most of the surface ocean samples, which required larger sample volumes.
Figure
3.
Theoretical (gray line) versus measured (black points) 1σ internal errors of all samples for δ56Fe, δ60Ni, δ66Zn, and δ114Cd as compared to the MC-ICPMS signal. Theoretical lines were drawn based on Monte Carlo simulations. Vertical gray bars denote the seawater concentrations corresponding to 0.1‰ and 0.05‰ 1σ internal errors. Light blue rectangles indicate the range of signal intensities that correspond to 1 L nature seawater samples following their pretreatment based on the proposed method (de Baar et al., 1994; Moore and Braucher, 2007; Roshan et al., 2018; Richon and Tagliabue, 2019; John et al., 2022).
To examine the external precision of the proposed method when analyzing Fe, Ni, Cu, Zn, and Cd isotopes, the seawater sample collected from the Yellow Sea (34.88°N, 121.68°E) was split into 14 samples and each of those samples was separately pretreated as set out in the proposed method. The average δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd values with one standard deviation of the Yellow Sea seawater sample were −1.03‰ ± 0.03‰, 1.14‰ ± 0.03‰, 0.79‰ ± 0.02‰, 0.01‰ ± 0.03‰, and 0.63‰ ± 0.02‰, respectively, for element concentrations of (1.35 ± 0.02) nmol/kg, (7.75 ± 0.02) nmol/kg, (11.0 ± 0.02) nmol/kg, (1.98 ± 0.03) nmol/kg, and (0.32 ± 0.00) nmol/kg, respectively (Fig. 4). The results suggest that the external precisions of the method when analyzing Fe, Ni, Cu, Zn, and Cd isotopes are 0.03‰, 0.03‰, 0.02‰, 0.03‰, and 0.02‰, respectively, based on conservative estimations. The values obtained for the external precisions of the proposed method are comparable to the corresponding analytical internal errors.
Figure
4.
External precisions for δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd based on repeated pretreatment and analysis of a single seawater sample. The seawater sample was obtained at 34.88°N, 121.68°E from a depth of ~15 m during the Yellow Sea cruise (March/April 2022). Error bars represent the internal errors (1σ), while lines and gray bars depict the average isotope values and external precisions (average ± 1 SD), respectively.
3.4
Method validation using GEOTRACES GP09-K9 profile samples
To further validate the pretreatment method, we extracted and purified seawater samples obtained during the Chinese GEOTRACES GP09 cruise, and the results of dissolved δ56Fe, δ60Ni, δ66Zn, and δ114Cd isotopes and recalculated metal concentrations are presented in Fig. 5. The recalculated metal concentrations were determined based on the isotope dilution method, using isotope ratios from the Neptune and the quantities of added spikes. These recalculated metal concentrations showed good agreement with historical measurements by Ge et al. (2022), as their differences were less than 3%. Accordingly, these high-quality data are reasonable to be used for further elucidating the vertical distribution characteristics of Fe, Ni, Zn, and Cd in the subtropical Northwest Pacific. Furthermore, the concentrations of the trace metals in the Yellow Sea were consistent with those obtained in previous observations (Zhang et al., 2022).
Figure
5.
Concentrations and stable isotope profiles of the samples collected at Station K9 (11°N, 150°E) along the GEOTRACES cruise GP09 in the Northwest Pacific compared with the GR19 and GR21 profiles obtained from the GEOTRACES Intermediate Data Product 2021 during the GP19 cruise (https://www.bodc.ac.uk/geotraces/data/idp2021/), SAFe Station (30°N, 140°W) in the Northeast Pacific (Conway and John, 2015a), and GR03 Station (15°N, 165°E) in the Northwest Pacific (Takano et al., 2022). Error bars represent the 1σ internal errors of the isotopic analysis.
Fe concentrations and isotopes from the K9 vertical profiles were similar to GR19 and GR21 profiles during the GP19 cruise which are available in the GEOTRACES Intermediate Data Product 2021 (https://www.bodc.ac.uk/geotraces/data/idp2021/). The heavy Fe isotopic composition in the chlorophyll maximum layer (at a depth of 80–180 m) indicated the influence of biological fractionation (König et al., 2021), while the light Fe isotope ratios at a depth of ~300 m characterized the contribution of North Pacific Intermediate Water (NPIW) (Conway and John, 2015a). The NPIW was indicated by salinity minimum (salinity < 34.5) lying around σθ = 26.7 (Fine et al., 1994). Positive Fe isotopes characteristic in deep water (750–5 000 m) revealed the deterministic role of dust supply on deep-ocean Fe of the Northwest Pacific, as the dissolved Fe released from dust always complexed with the organic ligand and showed a positive isotope signal (0.4‰–0.8‰) (Conway and John, 2014a; Fitzsimmons et al., 2015; John et al., 2018). The heavy δ56Fe values of ~0.8‰ in the near-bottom (at a depth below 5 000 m) reflected a substantially nonreductive input of lithogenic Fe from the Lower Circumpolar Deep Water (LCDW) (Conway and John, 2014b; Abadie et al., 2017; John et al., 2018).
Ni concentrations and isotope ratios were comparable to their corresponding values in the GR03 Station (15°N, 165°E) in the Northwest Pacific (Takano et al., 2022). The increase in the δ60Ni content from ~500 m toward the surface with a maximum value of 1.8‰ reflected the preferential phytoplankton uptake of light Ni isotopes in the surface water followed by the remineralization of biogenic particulates in deeper water (Takano et al., 2017; Archer et al., 2020; Yang et al., 2020, 2021). The Ni isotope composition in deeper water at depths below 500 m became nearly uniform, with mean values of 1.34‰ ± 0.02‰ (1SD, n = 12, Fig. 5), consistent with the deeper water δ60Ni values observed in the global ocean (Takano et al., 2017, 2022; Wang et al., 2019; Archer et al., 2020; Yang et al., 2020, 2021).
In this study, we developed a new method for the simultaneous determination of five isotope systems (δ56Fe, δ66Zn, δ114Cd, δ60Ni, δ65Cu) in a single 1 L seawater sample by incorporating and refining the methods previously proposed. The procedure involves only three pretreatment steps, including NOBIAS Chelate-PA1 resin extraction, AG MP-1M anion-exchange resin purification for Cu, Fe, Zn, and Cd, and NOBIAS Chelate-PA1 resin refinement to enable the separation of Ni from matrix elements. The separated elements were measured using the Thermo Neptune MC-ICPMS, employing the double-spike technique for Ni, Fe, Zn, Cd isotopes and the C-SSBIN method for Cu isotopes. The total procedural blanks for Ni, Cu, Fe, Zn, and Cd were 0.04 ng, 0.04 ng, 0.21 ng, 0.15 ng, and 3 pg, respectively, which are considered negligible in most seawater isotope analyses. High extraction efficiencies were achieved for Ni, Cu, Fe, Zn, and Cd at 100.5% ± 0.3%, 100.2% ± 0.5%, 97.8% ± 1.4%, 99.9% ± 0.8%, and 100.1% ± 0.2%, respectively. Quantitative recovery, low procedural blanks, coupled with double-spike and C-SSBIN techniques guaranteed the precision and accuracy of the method. The method accuracy was examined by analyzing Milli-Q water doped with various concentrations of isotope standards, demonstrating isotope values within the 1σ internal error for all the concentrations. Repeated analyses using 1 L of Yellow Sea seawater samples indicated an external precision of 0.03‰, 0.02‰, 0.03‰, 0.04‰, and 0.03‰ (1SD; n = 14) for δ60Ni, δ65Cu, δ56Fe, δ66Zn, and δ114Cd, respectively. Additionally, the proposed method was validated using the seawater samples of the Station K9 vertical profiles obtained during a Chinese GEOTRACES GP09 cruise in the Northwest Pacific. The profiles of δ56Fe, δ66Zn, δ114Cd, and δ60Ni demonstrated good agreement with previous related data or studies. Furthermore, we reported the δ56Fe vertical profiles in the Northwest Pacific for the first time, revealing influences for Fe from biological fractionation (80–180 m), NPIW (200–500 m), Fe resulting from dust deposition chelated by organic ligand (750–5 000 m), and LCDW (>5000 m) from the surface ocean to the deep ocean. The stable isotopes of Ni, Zn, and Cd were consistent with typical oceanic distributions, influenced by biological activity, reversible scavenging, and large-scale water mass circulations. We believe that the proposed method will encourage researchers to handle the chromatography of systems with a large number of isotopes, enhancing our understanding of the oceanic biogeochemical cycles of Ni, Cu, Fe, Zn, and Cd.
Acknowledgements:
We would like to thank the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) for the instrumental support for isotopic determination. This work was also supported by the Shanghai Frontiers Science Center of Polar Science.
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Table
4.
The blanks (ng) in pretreatment steps of NOBIAS Chelate-PA1 resin extraction, AG MP-1M resin purification, NOBIAS Chelate-PA1 resin Ni purification, and total procedure (±1SD, n = 5)
Table
5.
The recoveries in pretreatment steps of NOBIAS Chelate-PA1 resin extraction, AG MP-1M resin purification, and NOBIAS Chelate-PA1 resin Ni purification (±1SD, n = 5)
Figure 1. Elution scheme for sample purification using an AG MP-1M resin microcolumn (a) and a NOBIAS Chelate-PA1 resin microcolumn (b). For the detailed protocols, see Table 2.
Figure 2. δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd values in ultrapure water doped with varying concentrations of natural isotope standards (IRMM-524a Fe, NIST-986 Ni, NIST-647 Cu, NIST-3702 Zn, and NIST-3108 Cd) measured according to the proposed method. Each concentration measurement included five replicates. Error bars represent the internal errors (1σ).
Figure 3. Theoretical (gray line) versus measured (black points) 1σ internal errors of all samples for δ56Fe, δ60Ni, δ66Zn, and δ114Cd as compared to the MC-ICPMS signal. Theoretical lines were drawn based on Monte Carlo simulations. Vertical gray bars denote the seawater concentrations corresponding to 0.1‰ and 0.05‰ 1σ internal errors. Light blue rectangles indicate the range of signal intensities that correspond to 1 L nature seawater samples following their pretreatment based on the proposed method (de Baar et al., 1994; Moore and Braucher, 2007; Roshan et al., 2018; Richon and Tagliabue, 2019; John et al., 2022).
Figure 4. External precisions for δ56Fe, δ60Ni, δ65Cu, δ66Zn, and δ114Cd based on repeated pretreatment and analysis of a single seawater sample. The seawater sample was obtained at 34.88°N, 121.68°E from a depth of ~15 m during the Yellow Sea cruise (March/April 2022). Error bars represent the internal errors (1σ), while lines and gray bars depict the average isotope values and external precisions (average ± 1 SD), respectively.
Figure 5. Concentrations and stable isotope profiles of the samples collected at Station K9 (11°N, 150°E) along the GEOTRACES cruise GP09 in the Northwest Pacific compared with the GR19 and GR21 profiles obtained from the GEOTRACES Intermediate Data Product 2021 during the GP19 cruise (https://www.bodc.ac.uk/geotraces/data/idp2021/), SAFe Station (30°N, 140°W) in the Northeast Pacific (Conway and John, 2015a), and GR03 Station (15°N, 165°E) in the Northwest Pacific (Takano et al., 2022). Error bars represent the 1σ internal errors of the isotopic analysis.
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