Overcoming the Challenges Associated with the Direct Analysis of Trace Metals in Seawater Using ICP-MS - - Spectroscopy
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Overcoming the Challenges Associated with the Direct Analysis of Trace Metals in Seawater Using ICP-MS


Special Issues


Seawater Analysis

As part of the experiment, undiluted seawater samples were loaded onto the autosampler and the internal standard containing diluent was added online using a T-piece located directly after the exit port from the Fast valve before the nebulizer. The ratio of seawater to diluent was 1:7. Online dilution is recommended to minimize unnecessary contamination. The NASS-5 and CASS-4 seawater reference material standards for trace metals (National Research Council Canada, Ottawa, Ontario) were selected to evaluate accuracy. Locally sourced filtered and acidified seawater was spiked to 10 ppb with a range of elements and used for the long term stability evaluation. A three-point external calibration was used for quantification with standard concentrations chosen relative to the expected concentration in the samples. Gallium (Ga), yttrium (Y), indium (In), and bismuth (Bi) were used as internal standards. All elements were measured in KED mode using the standard single gas mixture of 7% hydrogen (H2) in helium (He) for optimum sensitivity and interference suppression.

Results

Figure 2 shows the external calibration curves for the determination of trace metals in seawater by ICP-MS; the selection of elements includes iron, nickel, copper, and zinc. Figure 3 compares the raw count rates for the internal standards in both a 2% nitric acid (HNO3) and an undiluted seawater sample measured. Signal suppression of <20% was found in the undiluted seawater.


Figure 2: Calibration curves for Fe, Ni, Cu and Zn.

Figure 3: Internal standard count rates (counts per second) from Thermo Scientific PlasmaLab software. Suppression in the 1:7 diluted seawater is less than 20%.













Quantitative results for the seawater samples measured are shown in Table I. The findings indicate a good agreement between measured and certified values. To assess the stability of the analysis method, 180 samples of 10 ppb spiked seawater were analyzed using the described system over a 6-h period. The stability of the recovery for the 10 ppb spike is shown in Figure 4.


Table I: Comparison of measured with certified values for the NASS-5 and CASS-4 reference materials. Detection limits (ng/mL) were calculated from 3 standard deviations of 100 blank runs.

Figure 4: Measured values in 10 ppb spiked Bremerhaven seawater for 180 samples over a6-h period.















Conclusion

To understand the quality of seawater, the analysis of its chemical properties is vital. The ability to effectively undertake such analyses is invaluable for seawater environmental protection.

The method described in this work provides a rapid, contamination free sampling technology with full automation for high sample throughput. Through implementation of the new method, previous limitations on total dissolved solids with ICP-MS are eliminated, providing a sensitive and interference-free instrumental solution for trace-elemental determination even in difficult matrices such as seawater.

Shona McSheehy-Ducos is an ICP-MS application specialist at Thermo Fisher Scientific in Bremen, Germany.

References

(1) http://www.epa.gov/regulations/laws/cwa.html


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