Dataset: Trace Metals - 2004

Sampling method:
The samples were collected for trace metal determinations at 34 stations between 13 February and 23 March, 2004 from the ARSV Lawrence M. Gould using a custom-built trace metal clean rosette consisting of an epoxy painted Al rosette frame containing 12x12 L GO-FLO bottles (Measures et al., 2008).  The frame also housed an SBE 911 CTD system that included an SBE 43 dissolved oxygen sensor and a Wet labs FL1 fluorometer.  The rosette frame was lowered using a 4 conductor Kevlar cable coated with polyurethane which was passed over a Nylatron plastic block and wound on to a normal, but clean, hydraulic winch.  Sample bottles were closed while the rosette was moving upwards through clean water at ~5-10 m/minute.  Immediately after the package was recovered, the tops of the GO-FLOs were covered with plastic bags and the bottles were removed from the frame and carried into the clean van for sub-sampling.   The GO-FLO bottles were pressurized to 10 psi using 0.2 µm-filtered compressed air and samples filtered through 0.45 µm pore size acid washed, 47 mm polysulphone filters (Pall Supor 450 P/N 60173).  All sub-sampling was undertaken in the clean van using rigorous trace metal protocols.

Analytical method:
Samples obtained with this system and processed in this manner have been shown during the SAFe intercomparison cruise (Johnson et al., 2007) to produce trace metals (Al, Fe and Mn) that are, within analytical uncertainty, identical to those obtained using other currently accepted sampling methodologies for trace elements. 

1. Dissolved Al using FIA
Dissolved Al was determined using the Flow Injection Analysis (FIA) method of Resing and Measures (1994) within a few hours of sample collection on board.  Samples were drawn into pre-numbered 125 ml PMP bottles after three rinses and were stored in plastic bags in the dark at room temperature before determination which was usually within 12 -36 hours of collection. Prior to determination samples were acidified by the addition of 125 ul sub-boiling distilled 6N HCl (hereinafter 6N HCl) and were microwaved in groups of 4 for 3 minutes in a 900 W microwave oven to achieve a temperature of 60+/-10 ?C.  Samples were allowed to cool for at least 1 hour prior to determination.  Samples were determined in groups of 8. 

A shipboard mixed standard (Al and Fe) was prepared in the shore-based laboratory by serial dilution of commercial Al standards into distilled water which was acidified with the equivalent of 4 ml sub-boiled 6N HCl/L.  Standards for instrument calibration were prepared daily from filtered seawater by acidifying 1 L of low Fe seawater from a previous cast with 1 ml of 6N HCl and microwaving for 5 minutes.  After 1 hour, 200 +/- 2 ml of the cooled seawater was added to each of three 250 ml PMP bottles each of which had been rinsed three times with the microwaved seawater and shaken dry.  Working standards were prepared by adding spikes of the mixed standard to these bottles, to yield a standard curve. The system blank from the addition of the acid and buffer to samples was determined by double spiking a replicate sample i.e. by adding 2 x 125 ul 6N HCl and 5 ml of sample buffer to the replicate bottle and comparing the resulting signal to the original sample. Calculation of sample concentrations was by dividing the peak height derived from sample using the A/D software by the calculated slope of the standard curve.  A 1-minute pre-concentration of sample (~3 ml) onto an 8-hydroxyquinoline (8-HQ) resin column was used which yielded a detection limit of ~0.4 nM and a precision of 2.7% at 3 nM.

2. Dissolved Fe and Mn using ICPMS
Filtered sub-samples were taken for shore-based determination of dissolved Fe and Mn by Inductively Coupled Plasma Mass Spectrometry (ICP MS) using the protocol of Milne et al. (2010), which is briefly described here.  Stored samples were acidified to 0.024 M HCl by the addition of sub-boiled 6N HCl and kept for at least one year prior to analysis.  Aliquots of the acidified samples (15 ml) were spiked with 125 μL of an ⁵⁷Fe isotope solution (⁵⁶Fe: ⁵⁷Fe 0.063:0.936, 147 nM) and then left for >24 hours to equilibrate.  Pre-concentration and extraction of the samples was performed using a flow injection manifold with an in-line micro-column containing ~200 μL of Toyopearl AF Chelate-650M resin.  Prior to extraction of the sub-samples, the system was cleaned with sub-boiled HNO₃ (1 M).  The isotope spiked samples were buffered to pH 6 using 900 μL of 2 M ammonium acetate (pH 8.9) prepared from sub-boiled acetic acid and isopiestic distilled NH₃, and then pumped at 2 mL min^-1 through the column for 6 minutes.  The column was then rinsed with 1 mL DI water to remove the seawater matrix, and the adsorbed trace elements were eluted with 1 mL 1.5 M sub-boiled HNO₃.  Extracted samples were analyzed for their ⁵⁶Fe/⁵⁷Fe ratio and Mn concentration on an ICP MS (Thermo Scientific, Element 2, Medium-Resolution) with the desolvating apparatus (Elemental Scientific, Apex-Q) with self-aspirating nebulizer (Elemental Scientific, a 400 µL min^-1 PFA). Dissolved Fe concentrations were calculated by isotope dilution.  Mn concentrations were calculated using an external standard curve.  Fe and Mn sample concentrations were corrected for the blank that was measured in the acids and buffer that were used in the preconcentration and elution process.  The acid blank was 0.034 ± 0.003 nM (Fe, n=3) and 0.013 ± 0.005 nM (Mn, n=8).  Detection limits for Fe and Mn were calculated from 3 standard deviations of determinations of replicate ICP MS measurements and were approximately 0.028 (n = 75) and 0.027 (n = 161) nmol L^-1, for Fe and Mn respectively.  Determination of Fe and Mn concentrations in the SAFe open ocean reference material and GEOTRACES shallow standards were in good agreement with the inter-laboratory averages.
 

Related Files and References:
Johnson, K.S. et al. 2007.  Developing Standards for Dissolved Iron in Seawater, EOS, trans American Geophysical Union, 88, 131.

Measures, C.I. et al. 2008. A commercially available rosette system for trace metal clean sampling. Limnol. Oceanogr. Methods. 6, 384-394.

Milne, A. et al. 2010. Determination of Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in seawater using high resolution magnetic sector inductively coupled mass spectrometry (HR-ICP-MS). Analytica Chimica Acta. 665, 200-207.

Resing, J. Measures, C.I., 1994.  Fluorometric determination of A1 in seawater by FIA with in-line pre concentration. Anal. Chem. 66, 4105-4111.