Publication Date:
2022-05-26
Description:
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 98 (2006): 81-99, doi:10.1016/j.marchem.2005.07.002.
Description:
The first large-scale international intercomparison of analytical methods for the determination
of dissolved iron in seawater was carried out between October 2000 and December 2002. The
exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24
international laboratories. The comparison was based on a large volume (700 L), filtered
surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample),
which was acidified, mixed and bottled at sea. Two 1 L sample bottles were sent to each
participant. Integrity and blindness were achieved by having the experiment designed and
carried out by a small team, and overseen by an independent data manager. Storage,
homogeneity and time-series stability experiments conducted over 2.5 years showed that interbottle
variability of the IRONAGES sample was good (〈7%), although there was a decrease in
iron concentration in the bottles over time (from 0.8-0.5 nM) before a stable value was
observed. This raises questions over the suitability of sample acidification and storage.
For the complete dataset of 45 results (after excluding 3 outliers not passing the screening
criteria), the mean concentration of dissolved iron in the IRONAGES sample was 0.59±0.21
nM, representing a coefficient of variation (%CV) for analytical comparability (“community
precision”) of 36% (1s), a significant improvement over earlier exercises. Within-run precision
(5-10%), inter-run precision (15%) and inter-bottle homogeneity (〈7%) were much better than
overall analytical comparability, implying the presence of: (1) random variability (inherent to all
intercomparison exercises); (2) errors in quantification of the analytical blank; and (3)
systematic inter-method variability, perhaps related to secondary sample treatment (e.g.
measurement of different physicochemical fractions of iron present in seawater) in the
community dataset. By grouping all results for the same method, analyses performed using
flow injection – luminol chemiluminescence (with FeII detection after sample reduction)
[Bowie et al., 1998. Anal. Chim. Acta 361, 189] and flow injection – catalytic
3
spectrophotometry (using the reagent DPD) [Measures et al., 1995. Mar. Chem. 50, 3] gave
significantly (P=0.05) higher dissolved iron concentrations than analyses performed using
isotope dilution ICPMS [Wu and Boyle, 1998. Anal. Chim. Acta 367, 183]. There was,
however, evidence of scatter within each method group (CV up to 59%), implying that better
uniformity in procedures may be required. This paper does not identify individual data and
should not be viewed as an evaluation of single laboratories. Rather it summarises the status of
dissolved iron analysis in seawater by the international community at the start of the 21st
century, and can be used to inform future exercises including the SAFE iron intercomparison
study in the North Pacific in October 2004.
Description:
SCOR and NSF
(Grant No. OCE-0003700 to SCOR) kindly provided financial support for three workshops in
Amsterdam (1998), San Antonio (2000) and San Francisco (2002). The European Union
provided support for the fieldwork under the IRONAGES project (EVK2-1999-00031).
Laboratory studies were funded by the Australian Research Council (X00106765 and
DP0342826), ACROSS and the Australian Government’s Cooperative Research Centres
Programme through the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE
CRC). Final preparation of this manuscript was assisted by funding from NERC grant
NER/A/S/2003/00489.
Keywords:
Iron
;
Seawater
;
Determination
;
Intercomparison
;
IRONAGES
;
Large volume sample
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
53444 bytes
Format:
266863 bytes
Format:
application/pdf
Format:
application/pdf
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