Analytical intercomparison between flow injection‐chemiluminescence and flow injection‐spectrophotometry for the determination of picomolar concentrations of iron in seawater

Abstract

A lab-and ship-based analytical intercomparison of two flow injection methods for the determination of iron in seawater was conducted, using three different sets of seawater samples collected from the Southern Ocean and South Atlantic. In one exercise, iron was determined in three different size-fractions (<0.03 µm, <0.4 µm, and unfiltered) in an effort to better characterize the operational nature of each analytical technique with respect to filter size. Measured Fe concentrations were in the range 0.19 to 1.19 nM using flow injection with luminol chemiluminescence detection (FI-CL), and 0.07 to 1.54 nM using flow injection with catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (FI-DPD). The arithmetic mean for the FI-CL method was higher (by 0.09 nM) than the FI-DPD method for dissolved (<0.4 µm) Fe, a difference that is comparable to the analytical blanks, which were as high as 0.13 nM (CL) and 0.09 nM (DPD). There was generally good agreement between the FI-CL determinations for the <0.03 µm size fraction and the FI-DPD determinations for the <0.4 µm size fraction in freshly collected samples. Differences in total-dissolvable (unfiltered) Fe concentrations determined by the two FI methods were more variable, reflecting the added complexity associated with the analysis of partially digested particulate material in these samples. Overall, however, the FI-CL determinations were significantly (P = 0.05) lower than the FI-DPD determinations for the unfiltered samples. Our results suggest that the observed, systematic inter-method differences reflect measurement of different physicochemical fractions of Fe present in seawater, such that colloidal and/or organic iron species are better determined by the FI-CL method than the FI-DPD method. This idea is supported by our observation that inter-method differences were largest for freshly collected acidified seawater, which suggests extended storage (>6 months) of acidified samples as a possible protocol for the determination of dissolved iron in seawater.

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

“…However, it should be noted that only digestion including HF acid (which was not used here) ensures the recovery of the most refractory and crystalline particles determined in the TPFe fraction. These observations are in agreement with the study of Bowie et al (2004) who considered it useful to extend the storage of acidified samples to determine the total dissolved iron pool.…”

“…Unfiltered samples were collected in acid-cleaned 125 mL high density polyethylene (HDPE) bottles and immediately acidified with ultrapure hydrochloric acid (HCl, Merck, 250 µL, final pH 1.7). Samples were stored at room temperature and analysed 18 months later in the shore-based laboratory (LEMAR, Brest, France), in order to release the most refractory Fe species into the dissolved form (Löscher et al, 1997;Bowie and Sedwick, 2004).…”

“…That fraction represents the sum of the dissolved iron (i.e. the labile dissolved iron which is the fraction that is released after a few days of acidification) plus the refractory colloidal iron (i.e., that fraction that dissolved after more than a few days of acidification) plus the fraction of particulate material that dissolves during extended (>6 months) acid storage (iron adsorbed on lithogenic or biogenic particles, contained within biogenic particles and labile lithogenic particulate iron) (de Baar et al, 1999;Löscher et al, 1997;Bowie and Sedwick, 2004). In the literature, the set of TDFe observations is much smaller than that of DFe.…”

A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean

“…In the past decades, a large number of dFe measurements have been collected that provide seasonal variability data in the Southern Ocean (e.g., Martin et al, 1990;de Baar et al, 1995de Baar et al, , 1999Löscher et al, 1997;Lannuzel et al, 2011;Fitzwater et al, 2000;Sedwick et al, 1997Sedwick et al, , 1999Sedwick et al, , 2000Sedwick et al, , 2008Sedwick et al, , 2011Sohrin et al, 2000;Measures and Vink, 2001;Bowie et al, 2004Coale et al, 2005;Chever et al, 2010. However, despite these efforts, there are some months in different ocean regions and basins in which no dFe measurements have yet been made.…”

“…Advancements in measurement techniques and inter-laboratory comparisons and evaluation projects (e.g., Bowie et al, 2004Bowie et al, , 2006Johnson et al, 2007) have been of major importance. The first compilation of dFe data (354 observations at 30 stations) was performed by Johnson et al (1997) and then expanded by Parekh et al (2005), which aided in understanding the importance of scavenging of dFe in ocean models.…”