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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.

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