Development and initial deployments of an autonomous in situ instrument for long‐term monitoring of copper (II) in the marine environment

Abstract

We developed an autonomous, in situ instrument to detect dissolved copper in seawater, suitable for deployment on time scales from weeks to a month. A commercially available in situ nitrate analyzer (YSI 9600) was adapted to measure copper (II) by chemiluminescence. Modifications included construction of a photomultiplier (PMT)-based detector and flow-cell, the use of more chemically resistant plastics for parts in contact with the reagents, addition of an online acidification step, and optimization of the method and flow parameters. Microsolenoid pumps take in sample and dispense reagent, standard, and blank solutions, which are stored in 1 L plastic bags. Filtration (0.45 μm) and acidification (pH ~1.7) permit a measurement of total dissolved Cu(II). Calibration is achieved by periodically analyzing ligand-stabilized seawater standard and blank solutions stored at pH 8 and acidified online. Averaged over all in situ deployments, the instrument had a detection limit of 0.8 ± 0.3 nM, a sample precision of 11% (n = 612), and an accuracy of 17% (compared to discrete samples n = 27). The instrument is capable of functioning autonomously for 25 d when sampling every hour and calibrating every 6 h. Reagent consumption is the limiting factor.

Microplate-reader method for the rapid analysis of copper in natural waters with chemiluminescence detection

“…Linearity was observed to 200 nM in pure water and 100 nM in seawater (data not shown). Sensitivity, represented by the slope of the calibration curve, was always greater in pure water then in seawater by a factor of 3–5, consistent with previous studies (Holm et al, 2008). It is therefore important to match the standard matrix to that of the samples.…”

“…Previous studies (Zamzow et al, 1998; Holm et al, 2008) have optimized reaction pH, and concentrations of 1,10-phenanthroline, TEPA, CDAB, and hydrogen peroxide for maximum signal intensity. We wanted to see whether the concentration of reagents injected into the cells had an impact on signal intensity.…”

“…UV irradiation is normally not used with flow-through copper detection via the luminescence reaction with 1,10-phenanthroline (Coale et al, 1992; Holm et al, 2008). An acidification step to pH ~1.7 for at least 24 h prior to analysis was assumed to release all ligand-bound copper to the solution.…”

“…Current methods for low-level copper analysis in natural waters include flow injection analysis with chemiluminescence detection (Coale et al, 1992) based on the luminescence produced by the complexation of copper with 1,10-phenanthroline (Yamada and Suzuki, 1984), in-situ analysis using chemiluminescence (Holm et al, 2008) and colorimetry (Callahan et al, 2003), ICP-MS (Field et al, 1999) and electrochemical methods (Achterberg and Braungardt, 1999; Wang, 2002). The complexity of these methods and their relatively large sample size requirements (generally at least 40 mL) has limited their use and contributes directly to the lack of regular monitoring of environmental copper concentrations.…”

Developing Autonomous Observing Systems for Micronutrient Trace Metals

“…In situ chemical analyzers based on flow techniques have been developed for Fe and Mn (e.g., Massoth, 1991;Chin et al, 1992;Okamura et al, 2001;Sarradin et al, 2005;Vuillemin et al, 2009) and Cu (Holm et al, 2008), but the reported limits of detection of these submersible analyzers (Table 3), which were originally designed for monitoring metal dynamics in the vicinity of hydrothermal vent systems or in coastal regions, are inadequate for open-ocean determinations.…”